大气成分与环境气象

大气成分与环境气象研究进展

Advances in Research on Atmospheric Composition and Environmental Meteorology

1 大气成分及相关特性变化观测研究

1 Observational studies of atmospheric composition and related properties

1.1 Aerosol and gaseous pollutant characteristics during the heating season (winter-spring transition) in the Harbin-Changchun megalopolis,Northeast China

Serious pollution events caused by residential heating activities occur frequently in Northeast China,and these increased particulate matter levels and gaseous pollutants could contribute to both local emissions and regional climate change.In this study,meteorological data,air trajectories and aerosol vertical distributions were used to study the pollutant sources and movement paths during the heating season (winter-spring transition) in the Harbin-Changchun megalopolis,Northeast China.Higher concentration of fine-mode particles (PM2.5/PM10,0.70–0.80) and the trace gas SO2(35–45 μg m−3) could be seem as an indicator of the air quality in the Harbin-Changchun megalopolis during the residential heating seasons,caused by local emissions.The apparent changes of meteorological elements in the high latitude and cold regions over Northeast China could have important effects on the pollutant formation,secondary transformation,aerosol accumulation and vertical transport over the heating season.The PM2.5concentrations were highly associated with the SO2and NO2from both fossil fuel combustion and biomass burning.The obvious diurnal pollutant variation shown thermodynamic conditions could be considered one of the important environmental elements responsible for residential heating processes.The mixture effect of local emissions and aerosol convective activities were found to contribute to the aerosol extinction and air pollution in the Harbin-Changchun megalopolis.These findings could help to improve our understanding of the association between aerosol pollutants influenced by the anthropogenic activities and the aerosol regional climate effects during the heating season in Northeast China.(Che Huizheng )

1.2 Five-year observation of aerosol optical properties and its radiative effects to planetary boundary layer during air pollution episodes in North China:Intercomparison of a plain site and a mountainous site in Beijing.

The aerosol microphysical,optical and radiative properties of the whole column and upper planetary boundary layer (PBL) were investigated from 2013 to 2018 based on long-term sun-photometer observations at a surface site (～106 m a.s.l.) and a mountainous site (～1225 m a.s.l.) in Beijing.Raman-Mie lidar data combined with radiosonde data were used to explore the aerosol radiative effects on PBL during dust and haze episodes.The results showed size distribution exhibited mostly bimodal pattern for the whole column and the upper PBL throughout the year,except in July for the upper PBL,when a trimodal distribution occurred due to the coagulation and hygroscopic growth of fine particles.The seasonal mean values of aerosol optical depth at 440 nm for the upper PBL were 0.31±0.34,0.30±0.37,0.17±0.30 and 0.14±0.09 in spring,summer,autumn and winter,respectively.The single-scattering albedo at 440 nm of the upper PBL varied oppositely to that of the whole column,with the monthly mean value between 0.91 and 0.96,indicating weakly to slightly strong absorptive ability at visible spectrum.The monthly mean direct aerosol radiative forcing at the Earth’s surface and the top of the atmosphere varied from −40±7 to −105±25 and from −18±4 to −49±17 W m−2,respectively,and the maximum atmospheric heating was found in summer (～66±12 W m−2).From a radiative point of view,during dust episode,the presence of mineral dust heated the lower atmosphere,thus promoting vertical turbulence,causing more air pollutants being transported to the upper air by the increasing PBLH.In contrast,during haze episode,a large quantity of absorbing aerosols (such as black carbon) had a cooling effect on the surface and a heating effect on the upper atmosphere,which favored the stabilization of PBL and occurrence of inversion layer,contributing to the depression of the PBLH.(Che Huizheng )

1.3 Characterization of vertical distribution and radiative forcing of ambient aerosol over the Yangtze River Delta during 2013–2015

As the central part of eastern China,the Yangtze River Delta (YRD) region,with its rapid economic growth and industrial expansion,has experienced severe air quality issues.In this study,the monthly variation and interaction between aerosol direct radiative forcing (ADRF) and aerosol vertical structure during 2013–2015 over the YRD were investigated using ground-based observations from a Micro Pulse Lidar (MPL) and a CE-318 sun-photometer.Combining satellite products from MODIS and CALIPSO,and reanalysis wind fields,an integrated discussion of a biomass burning episode in Hangzhou during August 2015 was conducted by applying analysis of optical properties,planetary boundary layer (PBL),spatial-temporal and vertical distributions,backward trajectories,Potential Source Contribution Function (PSCF),and Concentration Weighted Trajectory (CWT).The results reveal that a shallower PBL coincides with higher scattering extinction at low altitude,resulting in less heating to the atmosphere and radiative forcing to the surface,which in turn further depresses the PBL.In months with a deeper PBL,the extinction coefficient decreases rapidly with altitude,showing stronger atmospheric heating effects and ADRF to the surface,facilitating the turbulence and vertical diffusion of aerosol particles,which further reduces the extinction and raises the PBL.Because of the hygroscopic growth facilitated by high relative humidity,June stands out for its high scattering extinction coefficient and relatively low PBL and the reduced ADRF at the surface,and the enhanced cooling effect on near-surface layer in turn depresses the PBL.Absorptive aerosols transported from biomass burning events located in Zhejiang,Jiangxi,and Taiwan provinces at 1.5 km,result in high ADRF efficiency for atmospheric heating.And the enhanced heating effect on near-surface layer caused by absorptive particles facilitates PBL development in August over the YRD.(Che Huizheng )

1.4 Impact of biomass burning in South and Southeast Asia on background aerosol in Southwest China

Biomass burning (BB) in Southeast Asia is particularly pronounced during the dry season.However,the complex topography and long-range transport inherent to Southeast Asia have limited local research on pollution resulting from BB.In this study,the monthly variation in aerosol optical properties at six sites in Southeast Asia (Chiang Mai,Mukdahan,Bac-Lieu,Penang,Singapore,and Bandung) and the fire-point distribution have been analyzed in detail.The Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model was used to simulate the 72-hour back-trajectory from the Shangri-La atmospheric background station in Yunnan Province,China.Our results showed that BB was more common on the Indochinese Peninsula from March to May,whereas it was more common on the Malay Archipelago from August to October due to the latitudinal difference and crop harvest season.Significant BB activity on the Indochinese Peninsula in March resulted in a high surge in extinction (AODt440nm=1.32±0.69,AODf440nm=1.24±0.59) by particles with a smaller diameter (AE =1.68±0.13) in Chiang Mai.Mapping the longrange transport of BB aerosols reveals that Shangri-La’s pollution was primarily affected by emissions from the northern-central India (accounting for 45.2%),and Bangladesh and the northern Myanmar (accounting for 38.7%),which indicates that the aerosol pollution on the Yunnan-Guizhou Plateau in springtime could have originated on the southern periphery of the Tibetan Plateau.The results also indicate that BB emission in Southeast Asia had a limited impact on pollution in Southwest China but a relatively large effect on local areas.This study is the first to analyze the trend of aerosols produced from BB in Southeast Asia via ground-based observation,which deepens our understanding of the potential effects of BB aerosols transported long-range from outside Southwest China.(Che Huizheng )

1.5 Spatial distribution of aerosol microphysical and optical properties and direct radiative effect from the China Aerosol Remote Sensing Network

Multi-year observations of aerosol microphysical and optical properties,obtained through ground-based remote sensing at 50 China Aerosol Remote Sensing Network (CARSNET) sites,were used to characterize the aerosol climatology for representative remote,rural,and urban areas over China to assess effects on climate.The annual mean effective radii for total particles (ReffT) decreased from north to south and from rural to urban sites,and high total particle volumes were found at the urban sites.The aerosol optical depth at 440 nm (AOD440nm) increased from remote and rural sites (0.12) to urban sites (0.79),and the extinction Ångström exponent (EAE440-870nm) increased from 0.71 at the arid and semi-arid sites to 1.15 at the urban sites,presumably due to anthropogenic emissions.Single-scattering albedo (SSA440nm) ranged from 0.88 to 0.92,indicating slightly to strongly absorbing aerosols.Absorption AOD440nmvalues were 0.01 at the remote sites versus 0.07 at the urban sites.The average direct aerosol radiative effect (DARE) at the bottom of atmosphere increased from the sites in the remote areas (−24.40 W m−2) to the urban areas (−103.28 W m−2),indicating increased cooling at the latter.The DARE for the top of the atmosphere increased from −4.79 W m−2at the remote sites to −30.05 W m−2at the urban sites,indicating overall cooling effects for the Earth-atmosphere system.A classification method based on SSA440nm,fine-mode fraction (FMF),and EAE440-870nmshowed that coarse-mode particles (mainly dust) were dominant at the rural sites near the northwestern deserts,while light-absorbing,fine-mode particles were important at most urban sites.This study will be important for understanding aerosol climate effects and regional environmental pollution,and the results will provide useful information for satellite validation and the improvement of climate modelling.(Che Huizheng )

1.6 Long-term validation of MODIS C6 and C6.1 Dark Target aerosol products over China using CARSNET and AERONET

This study provided a comprehensive evaluation of the Moderate Resolution Imaging Spectroradiometer (MODIS) Collection 006 (C6) and 061 (C6.1) Dark Target (DT) 10 km aerosol optical depth (AOD) over China during 2002–2014.Considering that sparse Aerosol Robotic Network (AERONET) sites are available in China,18 sites from China Aerosol Remote Sensing Network (CARSNET) were also used to conduct this validation.The results showed that C6.1 DT outperform C6 with 59.03% of the retrievals falling within the expected error (EE) compared to C6 (54.94%).Meanwhile,C6.1 DT achieved a reduced RMSE of 0.171,a higherRof 0.901 and a bias closer to 0 relative to C6 (RMSE:0.185;R:0.890).When the validation was conducted over different underlying surfaces,C6 DT overestimated AOD by 19.8%,with only 45.01% of the retrievals within the EE over urban sites,whereas C6.1 showed clear improvements,with 11.8% more data falling within the EE.Hardly any improvement was observed in C6.1 over forest,cropland,and grassland sites.The C6.1 DT exhibited more significant improvements over Beijing area and the northern China than southern China.The highest retrieval accuracy of 61.05% among the four Beijing sites was achieved at Beijing_CARSNET,but the improvements were lower than other Beijing sites.The extent of the improvements was positively correlated with the percentage of urban pixels over the sites in Beijing and the northern China in terms of the retrieval accuracy.Moreover,C6.1 DT had a little effect on improvements over the southern China and showed reduced collocation over coastal cities.(Che Huizheng )

1.7 Aerosol vertical distribution and optical properties of different pollution events in Beijing in autumn 2017

As one of the largest metropolitan areas in the world,Beijing has a high aerosol loading.Although the government has taken various measures to reduce emissions,the large-scale and intermittent occurrence of pollution is still a major issue.To understand the mechanisms and characteristics of pollution,Raman-Mie lidar and CE-318 sun photometer were used to study the aerosol vertical distribution and optical properties during dust,haze and clean periods in Beijing that occurred in September to October,2017.Combined with meteorological data,the sources and transmission paths of the pollution events were analyzed.The results are as follows.For the dust event,irregular particles with a large linear particle depolarization ratio played a leading role.The aerosol boundary layer height reached 3.5 km.The volume concentration of coarse particles was obviously larger in the volume size distribution.Strong northwest winds at high altitudes cause highaltitude long-distance transmission of pollutants.And this process can exacerbate air quality of Beijing.For the haze event,high-spherical water-soluble particles were dominant.The extinction coefficient exceeded 1 km−1while the linear particle depolarization ratio was relatively small.The aerosol boundary layer height was＜1 km.The range of aerosol optical depth at 440 nm varied between 0.8 and 2.4.Compared with the dust process,meteorological conditions contribute differently to the haze pollution.Aerosols from the south-west is one of the major resources of haze in Beijing.Combined with static weather such as low wind speed,the pollution level can be continuously increased.During the clean period,favorable weather conditions,such as high wind speed,provided good conditions for the diffusion of local particles.The extinction coefficient and the linear particle depolarization ratio were significantly lower.The single-scattering albedo fluctuated between 0.75 and 0.85,the scattering of light was weak and the visibility was generally high.(Che Huizheng )

1.8 Aerosol vertical mass flux measurements during heavy aerosol pollution episodes at a rural site and an urban site in the Beijing area of the North China Plain

Due to excessive anthropogenic emissions,heavy aerosol pollution episodes (HPEs) often occur during winter in the Beijing-Tianjin-Hebei (BTH) area of the North China Plain.Extensive observational studies have been carried out to understand the causes of HPEs; however,few measurements of aerosol vertical fluxes exist,despite them being the key to understanding vertical aerosol mixing,specifically during weak turbulence stages in HPEs.In the winter of 2016 and the spring of 2017,based on the light propagation theory and surfacelayer similarity theory,aerosol vertical mass fluxes were measured by combining large aperture scintillometer (LAS) observations,surface PM2.5and PM10mass concentrations,and meteorological observations,including temperature,relative humidity (RH),and visibility,at a rural site in Gucheng (GC),Hebei Province,and an urban site at the Chinese Academy of Meteorological Sciences (CAMS) in Beijing located 100 km to the northeast.The near-ground aerosol mass flux was generally lower in winter than in spring and weaker in rural GC than in urban Beijing.This finding provides direct observational evidence from the perspective of vertical aerosol fluxes for a weakened turbulence intensity in winter and in polluted areas such as GC.The HPEs included a transport stage (TS),an accumulative stage (AS),and a removal stage (RS).During the HPEs from January 25,2017 to January 31,2017,in Beijing,the mean mass flux decreased by 51% from 0.0049 mg m−2s−1in RSs to 0.0024 mg m−2s−1in the TSs.During the ASs,the mean mass flux decreased further to 0.00087 mg m−2s−1,accounting for approximately 1/3 of the flux in the TSs.A similar reduction from the TSs to ASs was observed in the HPE from December 16,2016 to December 22,2016 in GC.The weakened mass flux indicates that the already weak turbulence would be further weakened by aerosol pollution to a certain extent,which would further facilitate aerosol accumulation.（Zhang Xiaoye）

1.9 Spatial and temporal distribution of open bio-mass burning in China from 2013 to 2017

Open bio-mass burning plays an important role in the formation of heavy pollution events during harvest seasons in China by releasing the trace gases and particulate matters into the atmosphere.A better understanding of spatial-temporal variations of open bio-mass burning in China is required to assess its impacts on the air quality and especially on the heavy haze pollution.The MODIS fire spots data and the calculated burned areas were used in this research,which shows the varying number of fire spots in China from 2013 to 2017,with the highest in 2014 and the lowest in 2016.Meanwhile,the fire spots were found mainly concentrated in three key periods (March–April,June and October–November) and two zones (Zone 1 and Zone 2) with inter-annual variations of burned areas.In addition,the contribution of major vegetation types burning was studied,the cropland occupied the largest proportion of burned area of more than 70% in any period time,followed by forest.Finally,from the perspective of climate and human activities,the causes of inter-annual variations were discussed.By comparing the average temperature and precipitation in the two zones from 2013 to 2017,it was found that the burned forest area is positively correlated with the average temperature of the zones and negatively correlated with the average precipitation.Meanwhile,the relationship between the El Niño events and the bio-mass burning was discussed.（Gong Sunling）

1.10 Water-soluble ion components of PM10 during the winter-spring season in a typical polluted city in Northeast China

From January 1 to April 22,2014,an online analyzer for monitoring aerosols and gases (MARGA) was used to measure and analyze water-soluble ions in inhalable particulate matter with a diameter less than 10 µm (PM10) during winter-spring in Shenyang,China.The results yielded three main findings:(1) During the entire observation period and in 7 pollution episodes,SO42−,NO3−,and NH4+(SNA) accounted for 84.4%–93.1% of the total water-soluble ions (TWSIs).TWSIs accounted for 32% of PM10mass during the entire observation period,and the contribution of TWSIs in PM10ranged from 33.4%–43.1% in the 7 pollution episodes.The contribution of TWSIs components increased during the pollution episodes,but certain differences were observed in different pollution episodes.In terms of ionic equilibrium,the total concentration of negative ions was slightly greater than that of positive ions and the difference was 3.1% of total ion load on average,indicating that local aerosols are mainly neutral.The water-soluble ions show clear diurnal variation with the high concentration around 09:00 for SO42−,NH4+and Cl−which is consistent with the high heating grade index.(2) Pollution episodes often occur in Northeast China,especially during the winter period.Due to the low temperature in winter,the local coal burning for heating is one of the main sources of pollution besides vehicle exhaust and industrial pollution,which is supported by the higher NO3−/SO42−ratio in April than that in Jan.−March.Sometimes,under the prevailing wind directions (W and SSW),the long-distance transport of pollutants from the Beijing-Tianjin-Hebei region and Shandong Province superimposed on local pollution leads to the most severe pollution,such as Ep3 and Ep5.(3) SO42−concentrations are closely related to ambient water vapor pressure (e*),with increase as e* increased depending on temperature.NO3−concentration showed a linear relationship of excess NH4+,which suggests that homogeneous gas-phase reaction of ammonia and nitric acid is possibly an important pathway of nitrate formation in the haze pollution process in Shenyang.In addition,our results also suggest that the nighttime liquid-phase reaction may cause large increases of nitrate in the haze pollution process.(Sun Junying )

1.11 Observational study of aerosol hygroscopic growth on scattering coefficient in Beijing:A case study in March of 2018

A humidified nephelometer system was deployed to measure the aerosol scattering coefficients at RH ＜30% and RH in the range of 40% to 85% simultaneously in megacity Beijing in March 2018.The aerosol optical properties and aerosol hygroscopicity of two sizes (PM10and PM1) during the pollution period,dust period and a new particle formation event (NPF) were analyzed.During the pollution period,the scattering and absorption coefficients increased dramatically with the accumulation of pollutants,while scattering Ångström exponent (SAE),submicron scattering fraction (Rsp),submicron absorption fraction (Rap) decreased,as well as single scattering albedo (SSA) rose slightly,which indicated the increasing contribution of larger particle to scattering and absorption,and enhanced the scattering ability of aerosols.The average PM10mass scattering efficiency is 3.86±1.19 m2g−1with a range of 2.05−5.74 m2g−1during the pollution period,and 0.40±0.05 m2g−1during the dust period.Rspat wavelength of 550 nm varied from 55.8% to 89.3% during the measurement period,with the average of 64.8%±5.2% and 73.1%±6.8% during the pollution period and dust period,respectively,which suggests that the aerosol scattering coefficient is mainly affected by fine particles.The average PM10and PM1aerosol scattering hygroscopic growth factorsf(80%) are 1.75±0.05 and 1.75±0.04 during the pollution period,1.14±0.09 and 1.15±0.06 during the dust period,1.59±0.05 and 1.60±0.06 during the NPF event period,respectively.Aerosol scattering hygroscopic growth factors showed a strong correlation with the scattering Ångström exponent which suggests that the hygroscopicity is much stronger for fine particles (SAE＞1.5) than for the coarse particles (SAE＜1.0).(Sun Junying)

1.12 Variations in submicron aerosol liquid water content and the contribution of chemical components during heavy aerosol pollution episodes in winter in Beijing

The aerosol liquid water content (ALWC) of submicron particles (PM1) was calculated in this work by three methods based on the aerosol physical and chemical property measurement campaigns in winter in Beijing,including (a) the PM1volume difference between the ambient and dry states by applying the particle number size distribution and particle hygroscopicity measurement; (b) the thermodynamic equilibrium model (ISORROPIA II) based on the chemical composition; and (c) the κ-Köhler theory of chemical composition with a volume mixing scheme.The three methods agreed well with reasonable uncertainties.The ALWC showed an exponential trend depending on the relative humidity (RH),and an abundant ALWC was also favored by the high PM1mass loading.The contributions of different chemical components to the ALWC are evaluated by the κ-Köhler method,which revealed that during the measurement,the inorganics and organics could contribute to～80% and～20%,respectively,under ambient RH conditions,with the largest contributor of ammonium nitrate.When the RH was above 85%,the mass concentration of ALWC was comparable to,or even larger than that of the dry PM1.（Shen Xiaojing）

1.13 NH3-promoted hydrolysis of NO2 induces explosive growth in HONO

The study of atmospheric nitrous acid (HONO),which is the primary source of OH radicals,is crucial with respect to understanding atmospheric photochemistry and heterogeneous chemical processes.Heterogeneous NO2chemistry under haze conditions has been identified as one of the missing sources of HONO on the North China Plain,and also produces sulfate and nitrate.However,controversy exists regarding the various proposed HONO production mechanisms,mainly regarding whether SO2directly takes part in the HONO production process and what roles NH3and the pH value play.In this paper,never before seen explosive HONO production was reported and evidence was found—for the first time in field measurements during fog (usually with 4＜ pH ＜6) and haze episodes under high relative humidity (pH ≈4)—that NH3was the key factor that promoted the hydrolysis of NO2,leading to the explosive growth of HONO and nitrate under both high and relatively lower pH conditions.The results also suggest that SO2plays a minor or insignificant role in HONO formation during fog and haze events,but was indirectly oxidized upon the photolysis of HONO via subsequent radical mechanisms.Aerosol hygroscopicity significantly increased with rapid inorganic secondary aerosol formation,further promoting HONO production as a positive feedback.For future photochemical and aerosol pollution abatement,it is crucial to introduce effective NH3emission control measures,as NH3-promoted NO2hydrolysis is a large daytime HONO source,releasing large amounts of OH radicals upon photolysis,which will contribute largely to both atmospheric photochemistry and secondary aerosol formation.（Xu Wanyun）

1.14 A comprehensive study about the hygroscopic behavior of mixtures of oxalic acid and nitrate salts:Implication for the occurrence of atmospheric metal oxalate complex

The hygroscopic behavior of particles is important for assessing their environmental and climate effect.The hygroscopic behavior of individual particles has been widely investigated; however,the hygroscopic behavior of mixed particles is still not well-known.Although the Zdanovskii-Stokes-Robinson (ZSR) method could be applied to predict the hygroscopicity of some aerosol mixtures,it is invalid to describe the humidification process of mixed particles when chemical reactions take place.In the present study,the hygroscopic growth factors (GF) of individual oxalic acid (H2C2O4),nitrate (Ca(NO3)2,NaNO3,Zn(NO3)2),and the mixed H2CvO4/nitrate particles were measured using a hygroscopic tandem differential mobility analyzer (H-TDMA).The GFs of these mixtures are much smaller than those predicted by the ZSR method.The chemical composition of individual and mixed particles was characterized by a Raman spectrometer.It was found that oxalate dominated the constituents of internal mixtures.Further comparison of the humidification of externally mixed oxalic acid and nitrate revealed that the solubility of oxalate plays a critical role in the replacement of nitrate by oxalate.These results could be helpful for understanding the occurrence of metal oxalate complex in the atmospheric aerosol.( Liu Chang )

1.15 Aqueous-phase reactions occurred in the PM2.5 cumulative explosive growth during the heavy pollution episode (HPE) in 2016 Beijing wintertime

A heavy pollution episode (HPE) that lasted for seven days occurred over the North China Plain in December 2016.An in situ Ambient Ion Monitor was applied to analyze the chemical composition of PM2.5(fine particulate matter with diameters less than 2.5 µm) and gaseous HONO concentration during that event.A representative explosive growth in the pollution cumulative stage was selected to investigate the pollution mechanism during the HPE in Beijing.PM2.5cumulative explosive growth processes were observed to occur commonly under high relative humidity (RH) condition.Our results demonstrated that the aqueous-phase oxidation of SO2by NO2to sulfate could contribute to the cumulative explosive growth.Nitrate produced by secondary formation was another factor in the growth of PM2.5.Depending on the relative humidity,temperature,and chemical species,the deliquescence relative humidity was calculated to 82%,81%,and 83% for (NH4)2SO4,NH4NO3,and NH4Cl,respectively.The preexisting PM2.5surface changed from solid to liquid when RH＞81%.Coincidentally,both the sulfur oxidation ratio (SOR) and reaction product HONO displayed an evident exponential relationship with RH and increased more quickly when RH was larger than 80%.In addition,sufficiently excessive NO2made the aqueous-phase oxidation of SO2efficiently proceed even at relative low SO2concentrations (below 15 µg m−3).Potential H+in the reactions was neutralized by NH3,resulting in fully neutralized PM2.5during HPE.The chemical evolution of these reactions was discussed in detail in this study.(Wu Lingyan)

1.16 Mixing layer transport flux of particulate matter in Beijing,China

Quantifying the transport flux (TF) of atmospheric pollutants plays an important role in under standing the causes of air pollution and in making decisions regarding the prevention and control of regional air pollution.In this study,the mixing layer height (MLH) and wind profile were measured by a ceilometer and Doppler wind radar,respectively,and the characteristics of the atmospheric dilution capability were analyzed using these two datasets.The ventilation coefficient (VC) appears to be the highest in the spring (3940±2110 m2s−1) and lower in the summer (2953±1322 m2s−1),autumn (2580±1601 m2s−1) and winter (2913±3323 m2s−1).Combined with the backscatter measured by the ceilometer,vertical profiles of the PM2.5concentration were obtained and the PM2.5TF in the mixing layer was calculated.The TF was the highest in the spring at 4.33±0.69 mg m−1s−1and lower in the summer,autumn and winter,when the TF values were 2.27±0.42,2.39±0.45 and 2.89±0.49 mg m−1s−1,respectively.Air pollutants transport mainly occurs between 14:00 and 18:00 LT.The TF was large in the pollution transition period (spring:5.50±4.83 mg m−1s−1; summer:3.94±2.36 mg m−1s−1; autumn:3.72±2.86 mg m−1s−1; winter:4.45±4.40 mg m−1s−1) and decreased during the heavy pollution period (spring:4.69±4.84 mg m−1s−1; summer:3.39±1.77 mg m−1s−1; autumn:3.01±2.40 mg m−1s−1; winter:3.25±2.77 mg m−1s−1).Our results indicate that the influence of the air pollutants transport in the southern regions should receive more focus in the transition period of pollution,while local emissions should receive more focus in the heavy pollution period.(Liu Yusi)

1.17 Climatology of mixing layer height in China based on multi-year meteorological data from 2000 to 2013

Atmospheric aerosol levels are high in different regions of China.Changes in atmospheric stability and mixing layer height (MLH) may affect the vertical aerosol distribution during pollution events.Here,we continuously studied the spatiotemporal distribution of MLH in China over 2000–2013 to characterize the regional-scale properties of atmospheric boundary layers.MLH was larger in the southern Coastal and southeastern China (1241.5±73.5 and 632.7±61.8 m,respectively),but lower in the eastern and southwestern China (518.1±63.6 and 462.7±88.5 m,respectively).We divided the monitoring stations into 11 regions; these regions could be classified into three types with an increasing linear trend (95% confidence level),with a decreasing linear trend (95% confidence level),and with fluctuation changes.In all regions,MLH was higher in spring and summer than in autumn and winter.The spatial distribution of the diurnal MLH increased significantly at 14:00 BJT (Beijing time) in all regions,with the strongest radiation being noted during the daytime.Higher levels of PM2.5with corresponding lower MLH were noted during winter,indicating that lower MLH could confine the pollutant diffusion to near the surface.By contrast,in spring,higher MLH could be potentially due to the dynamic condition.In general,the poor air quality days occurred more frequently in winter than in other seasons.As the air quality worsened,most MLH showed a decreasing trend in almost all regions.This work provides information that aids in further understanding not only MLH distribution in China but also specific PM-MLH interaction to facilitate regional atmospheric environment monitoring and prediction.(Che Huizheng )

1.18 Spatial and temporal distribution of the cloud optical depth over China based on MODIS satellite data during 2003–2016

The cloud optical depth (COD) is one of the important parameters used to characterize atmospheric clouds.We analyzed the seasonal variations in the COD over East Asia in 2011 using cloud mode data from the AERONET (Aerosol Robotic Network) ground-based observational network.The applicability of the MODIS (Moderate Resolution Imaging Spectroradiometer) COD product was verified and compared with the AERONET cloud mode dataset.There was a good correlation between the AERONET and the MODIS.The spatial and temporal distribution and trends in the COD over China were then analyzed using MODIS satellite data from 2003 to 2016.The seasonal changes in the AERONET data and the time sequence variation of the satellite data suggest that the seasonal variations in the COD are significant.The result shows that the COD first decreases and then increases with the season in the northern China,and reaches the maximum in summer and minimum in winter.However,the spatial distribution change is just the opposite in the southern China.The spatial variation trend shows that the COD in China decreases first with time and gradually increases after 2014.And the trend of COD in the western and central China is consistent with that in China.While the trend of COD shows a continuously increasing over time in Northeast China and the Pearl River Delta.(Che Huizheng )

1.19 Synoptic patterns and sounding-derived parameters associated with summertime heavy rainfall in Beijing

Heavy precipitation in Beijing is modulated by both synoptic forcings and local thermodynamic characteristics of troposphere,which has yet to be well known.This study investigated the large-scale synoptic patterns and local sounding features associated with the summertime heavy precipitation in Beijing,based on long-term surface meteorological observations,radiosonde measurements,in combination of reanalysis data from 2008 to 2017.The results show that the heavy rainfall occurs more frequently in late July,which is associated with the movement of subtropical anticyclone.The sounding parameters during the heavy rainfall days are examined as well.The heavy rainfall is often related to favourable convective conditions characterized by abundant water vapour and high unstable energy.The soundings for around 45% of heavy rainfall days in Beijing exhibit the pattern of“thin tube”(TT),and those for～25% and～20% of heavy rainfall days show the patterns of“loaded gun”(LG) and“inverted V”(IV),respectively.On average,the rainfall amount of TT is 55.8 mm day−1,which is～3 mm day−1(～15 mm day−1) higher than that of LG (IV).The more frequent and heavier rainfall observed for TT pattern is due to the high values of precipitable water and wind shear.On the large scale,three dominant synoptic patterns associated with heavy rainfall in Beijing have been identified using the T-mode principle component analysis.These synoptic patterns are all characterized by prevailing southerly winds within the lower troposphere,resulting in water vapours being easily transported from the southern regions to Beijing,which in turn favours the occurrence of heavy precipitation.These dominant synoptic patterns and thermodynamic characteristics associated with the heavy rainfall in Beijing revealed in this study have important implications for better understanding of heavy rainfall in the North China Plain.（Miao Yucong）

2 环境气象数值预报模式及大气成分与天气气候相互作用

2 Development of environmental meteorological numerical prediction model and studies of interactions between atmospheric compositions and weather/climate

2.1 Detection of new dust sources in Central/East Asia and their impact on simulations of a severe sand and dust storm

Information regarding dust concentrations and size distributions is very important for determining air quality and aerosol-radiation-cloud interactions.Only by using a correct erosion database can the sectional dust emission schemes resolve detailed size distributions and determine where and how dust will be emitted.In this paper,the bias and reasons for dust emission in the China Meteorological Administration Unified Atmospheric Chemistry Environment-Dust,an operational dust forecasting model,are analyzed using a heavy sand and dust storm episode.We used 18 years of routine sand and dust storm phenomena recorded at meteorological stations to retrieve and update the desertification details in the MBA sectional dust emission scheme adopted in China Meteorological Administration Unified Atmospheric Chemistry Environment-Dust.New desertification details include decreased erodibility in the area adjacent to Uzbekistan,Turkmenistan,and the southern Kazakhstan,where Kyzylkum,Karakum,and Aralkum are located in central Asia,and in the Chinese deserts of Onqin Daga,Mu Us,and Gurbantungut.New desertification also results in increased erodibility in the northern Mongolia.Comparisons show that the new desertification database decreases the overestimation of dust emission in central Asia,including the western Mongolia.It improves the underestimation of dust emission in the northern Mongolia and the Gobi desert in southeast Mongolia,and the Taklimakan desert in China.Consequently,it corrects the overestimated dust clouds in the source area and in areas impacted by dust transportation.The timing,quantitative mass concentrations,and dust size distributions determined here are all more reasonable and rational than those of the original case.(Zhou Chunhong)

2.2 Lidar data assimilation method based on CRTM and WRF-Chem models and its application in PM2.5 forecasts in Beijing

A three-dimensional variational (3DVar) lidar data assimilation method is developed based on the Community Radiative Transfer Model (CRTM) and Weather Research and Forecasting model coupled to Chemistry (WRF-Chem) model.A 3DVar data assimilation (DA) system using lidar extinction coefficient observation data is established,and variables from the Model for Simulating Aerosol Interactions and Chemistry (MOSAIC) mechanism of the WRF-Chem model are employed.Hourly lidar extinction coefficient data from 12:00 to 18:00 UTC on March 13,2018 at four stations in Beijing are assimilated into the initial field of the WRF-Chem model; subsequently,a 24 h PM2.5concentration forecast is made.Results indicate that assimilating lidar data can effectively improve the subsequent forecast.PM2.5forecasts without using lidar DA are remarkably underestimated,particularly during heavy haze periods; in contrast,forecasts of PM2.5concentrations with lidar DA are closer to observations,the model low bias is evidently reduced,and the vertical distribution of the PM2.5concentration in Beijing is distinctly improved from the surface to 1200 m.Of the five aerosol species,improvements of NO3−are the most significant.The correlation coefficient between PM2.5concentration forecasts with lidar DA and observations at 12 stations in Beijing is increased by 0.45,and the corresponding average RMSE is decreased by 25 μg m−3,which respectively compared to those without DA.(Cheng Xinghong)

2.3 Regional air quality forecast using machine learning method based and WRF model over the Yangtze River Delta,East China

A statistical forecasting model system based on the mesoscale simulation of meteorology and the BP network was established for the air compound pollutants PM2.5.In consideration of the driving role of the large-scale meteorological fields,the mesoscale meteorological model WRF was used to predict the fine meteorological elements of the atmospheric boundary layer.Utilizing the meteorological elements simulated in the high spatiotemporal resolution by WRF model,a short-term air quality forecasting system was established based on the BP neural network model to predict air quality changes in the next 72 hours over the Yangtze River Delta (YRD) region of East China.Conducting short-term statistical forecasting of air quality in major cities in the YRD region,this study evaluated the forecast accuracies of the main air combined pollutants PM2.5.The results showed that the short-term air quality forecasting system based on the BP neural network has a good ability of forecasting PM2.5in the major cities in the Yangtze River Delta region.The average IAs of PM2.5in four seasons are from 74% to 77%,and the RMSEs are between 15.25 μg m–3and 33.03 μg m–3.Especially in cases of heavy haze pollution with the poor performance of model forecasting,our statistical forecasting model with the specializing heavy haze pollution presented an excellent air quality forecasting capability.The RMSE of EXP-Polluted simulation results increased by 44.1%.(Cheng Xinghong)

2.4 The“two-way feedback mechanism”between unfavorable meteorological conditions and cumulative PM2.5 mass existing in polluted areas south of Beijing

In winter,heavy aerosol pollution episodes (HPEs) occur frequently on the North China Plain (NCP) under unfavorable meteorological conditions.After the aerosol pollution with fine aerosol particles less than 2.5 μm (PM2.5) accumulates to a certain extent in Beijing and its vicinity (BIV),boundary-layer (BL) meteorological conditions worsen due to radiative cooling effects of aerosols,which further facilitate PM2.5accumulation.This“two-way feedback mechanism”between unfavorable metrological conditions and cumulative PM2.5,particularly PM2.5explosive growth with mass concentration doubled in several to 10 h,has been found in BIV.To investigate whether the two-way feedback exists on the southern NCP,we selected four representative cities with radiosonde observations,which lie south of Beijing and used PM2.5data,radiation observations,and radiosonde data from December 2016 to the beginning of January 2017.We found that the two-way feedback existed in these four cities.In each city,HPEs included transport stages (TSs),during which relative strong winds transport pollutants into different regions,and cumulative stages (CSs),in which temperature inversions occured and became striking due to the radiative cooling effects of elevated aerosols under slight or calm winds.This would further cause an increase of PM2.5concentration.During the CSs,growth of PM2.5concentration was governed by the two-way feedback,which explained over 70% of the increase.During HPEs,warm and humid advection,topographic wind,longwave radiation at night were conducive to the inversion at the beginning of HPEs,but not dominant with respect to anomalous inversion in CSs.In addition,with the presence of supersaturated layers,aerosol would enter the cloud-fog system,which would reduce observed PM2.5mass concentration to a certain degree.(Zhang Xiaoye)

2.5 The effects of the“two-way feedback mechanism”on the maintenance of persistent heavy aerosol pollution over areas with relatively light aerosol pollution in Northwest China

Persistent heavy aerosol pollution episodes (HPEs) often occur in the central and eastern regions of China during the wintertime.When PM2.5mass concentration cumulates to a certain extent that normally exceeds 100 μg m−3,the PM2.5pollution will then further worsen the meteorological conditions in the boundary layer (BL),resulting in significant two-way feedback between the cumulative PM2.5pollution and worsened meteorological conditions.In the less polluted northwest area of China,whether there is significant two-way feedback phenomenon mentioned above after the accumulation of PM2.5mass concentration has always been an issue of concern for understanding the full picture of meteorological causes of HPEs in China.Here,measurements of PM2.5concentration,meteorological data and an integrated pollution-linked meteorological index (PLAM) are used to characterize the relationship between meteorological elements and PM2.5concentration in persistent HPEs from 1 December 2016 to 10 January 2017 in Lanzhou and Urumqi.In the early stage of the HPEs formation,the general 500 hPa circulation situations are normally highpressure ridge or zonal westerly airflow patterns,associating with worsened meteorological conditions.At this time,with the decrease of BL height,the mass concentration of PM2.5also increases as the pollutants mix in a relatively small space as compared to the clean period.When PM2.5cumulates to a certain extent (above 75 μg m−3in Lanzhou and above 100 μg m−3in Urumqi),there are often obvious inversion and humidification in the near-surface.The inversion is mainly caused by the accumulation of the PM2.5concentration,which indicates that even in Northwest China,where anthropogenic activities are less affected and PM2.5pollution is less serious,and the cumulative PM2.5can nonetheless worsen meteorological conditions.The worsened meteorological conditions,which are quantified by the PLAM index,dominate the changes in PM2.5in the explosive growth of persistent HPEs (accounted for 50%–85%).(Zhang Xiaoye)

2.6 Reflections on the threshold for PM2.5 explosive growth in the cumulative stage of winter heavy aerosol pollution episodes (HPEs) in Beijing

Particulate matter smaller than 2.5 μm in diameters (PM2.5) often experiences explosive growth (mass concentration at least doubled in several to 10 h) in winter aerosol pollution episodes (HPEs) in Beijing.Whether a threshold value exists for such PM2.5explosive growth is uncertain.Here,we used PM2.5mass concentration,surface and vertical meteorological factors including winds,temperature,relative humidity (RH),and radiation in winter (Jan.,Feb.,and Dec.) from 2013 to 2017 to confirm the existence of the threshold for PM2.5explosive growth and determine the threshold value.We found that the positive feedback from aerosols to near-ground radiative cooling to anomalous inversion is effectively triggered under slight or calm winds when the near-ground PM2.5mass reaches a certain threshold.A threshold value for PM2.5explosive growth (100 μg m−3) is determined by analyzing 30 cumulative stages (CSs) during 28 HPEs.Keeping the PM2.5mass concentration transported from the south of Beijing below this value would avoid the majority of PM2.5explosive growth during the 30 CSs.As a reference for the government to further set the emission reduction target,a more stringent threshold value (～71 μg m−3) is established by differentiating the lighter HPEs from the heavier HPEs.(Zhang Xiaoye)

2.7 The two-way feedback mechanism between unfavorable meteorological conditions and cumulative aerosol pollution in various haze regions of China

Accompanied by unfavorable meteorological conditions with stable stratification in various haze regions of China,persistent heavy aerosol pollution episodes (HPEs) lasting more than 3 consecutive days frequently occur,particularly in winter.In the North China Plain (NCP),explosive growth of fine particulate matter smaller than 2.5 µm in diameter (PM2.5),which occurs during some HPES,is dominated by a two-way feedback mechanism between more unfavorable meteorological conditions and cumulative aerosol pollution.However,the existence of a two-way feedback mechanism such as this in other key haze regions in China is uncertain; these regions include the Guanzhong Plain (GZP),the Yangtze River Delta (YRD) region,the Two Lakes Basin (TLB; a large outflow basin connected to Hubei Province and Hunan Province),the Pearl River Delta (PRD) region,the Sichuan Basin (SB),and the Northeast China Plain (NeCP).In this study,using surface PM2.5and radiation observations,radiosonde observations,and reanalysis data,we observed the existence of a two-way feedback mechanism in the six abovementioned regions.In the SB,this two-way feedback mechanism is weak due to the suppression of cloudy mid-upper layers.In the more polluted NCP,the GZP,and the NeCP,the feedback is more striking than that in the YRD,the TLB,and the PRD.In these regions,the feedback of worsened meteorological conditions on PM2.5explains 60 %–70 % of the increase in PM2.5during the cumulative stages (CSs).For each region,the low-level cooling bias becomes increasingly substantial with increasing aerosol pollution and a closer distance to the ground surface.With PM2.5mass concentrations greater than 400 µg m−3,the near-ground bias exceeded −4 in Beijing and reached up to approximately −4 in Xi’an; this result was caused by accumulated aerosol mass to some extent.In addition to the increase in PM2.5caused by the two-way feedback,these regions also suffer from the regional transport of pollutants,including inter-regional transport in the GZP,trans-regional transport from the NCP to the YRD and the TLB,and southwesterly transport in the NeCP.(Zhang Xiaoye)

2.8 The impact of meteorological changes from 2013 to 2017 on PM2.5 mass reduction in key regions in China

In 2013,China issued the“Action Plan for the Prevention and Control of Air Pollution”(“Ten Statements

of Atmosphere”) and implemented a series of pollution reduction measures from 2013 to 2017.In key regions of China,the mass concentrations of particulate matter with aerodynamic equivalent diameters less than 2.5 µm (PM2.5) have dropped significantly.However,the contributions of meteorological changes to PM2.5reduction are largely uncertain,which has attracted particular concern from the government and the public.Here,we investigated the impact of large-scale and boundary layer (BL) meteorological conditions on aerosol pollution and estimated the contributions of meteorological changes to PM2.5reduction based on in-depth analysis and diagnosis of various observed meteorological elements and an integrated pollution-linked meteorological index (PLAM,which is approximately and linearly related to PM mass concentration).In this study,we found that the meteorological conditions worsened in 2014 and 2015 and improved in 2016 and 2017 relative to those in 2013 in key regions in China.In 2017 relative to 2013,only～5% (approximately 13% of the total PM2.5decline) of the 39.6% reduction in PM2.5mass concentrations can be attributed to meteorological changes in the Beijing-Tianjin-Hebei (BTH) region,and only～7% (approximately 20% of the total PM2.5decline) of the 34.3% reduction can be attributable to meteorological changes in the Yangtze River Delta (YRD) region.Overall,the PM2.5reduction due to meteorological improvement is much lower than the observed PM2.5reduction in these areas,which indicates that emission reduction during the five-year implementation of the“Ten Statements of Atmosphere”is the dominant factor in the improvement in air quality.The changes in meteorology and climate are conducive to PM2.5reduction but do not dominate the substantial improvement in air quality.Similar to the above regions,in the Pearl River Delta (PRD) region,the impact of meteorological changes on the annual averaged PM2.5concentration from 2013 to 2017 was relatively weak,and the PM2.5reduction was mainly due to emission reductions.During winter 2017 (January,February,and December of this year),the meteorological conditions improved～20% in the BTH region (observed total PM2.5reduction:40.2%) and～30% in the YRD region (observed total PM2.5reduction:38.2%) relative to those in 2013,showing the meteorological factors played more important role in the decrease of PM2.5in winter of these years in the two regions,respectively.The meteorological conditions in winter 2016 were 14% better than those in winter 2017,but the PM2.5reduction in winter 2016 was still less than that in winter 2017,reinforcing the significant contributions of the increasing efforts to reduce PM2.5emissions in 2017.The substantial progress of strict emission measures was also confirmed by a comparison of several persistent heavy aerosol pollution episodes (HPEs) with similar meteorological conditions.It is found that the decrease of PM2.5mass caused by emission reduction increases year by year,especially the decrease of PM2.5concentration in 2016 and 2017.In China,HPEs mainly occur in winter,when meteorological conditions are approximately 40%–100% worse than those in other seasons.This worsening is partly due to the harbor effect of high topography,including downdrafts and the weak wind zone,and partly due to the increasingly stable regional BL structure caused by climate warming.For the formation of HPEs,it occurred under regional stagnant and stable conditions associated with upper-level circulation patterns,including the zonal westerly winds type and high-pressure ridges.After pollution formation,PM2.5with mass accumulated to a certain degree can further worsen the BL meteorological conditions.The feedback effect associated with worsening conditions dominates PM2.5mass explosive growth.In the context of high air pollutant emissions in China,unfavorable meteorological conditions are the necessary external conditions for the formation and accumulation of HPEs.Therefore,reducing aerosol pollution significantly during the earlier transport stage is critical in reducing persistent HPEs.Currently,even under favorable meteorological conditions,allowing emissions without restriction is also not advisable because aerosol pollution allowed to accumulate to a certain extent will significantly worsen the BL meteorological conditions and close the“meteorological channels”available for pollution dispersion.(Zhang Xiaoye)

2.9 Relatively weak meteorological feedback effect on PM2.5 mass change in winter 2017/2018 in the Beijing area:Observational evidence and machine-learning estimations

Heavy aerosol pollution episodes (HPEs) in Beijing are worsened by the two-way feedback mechanism between unfavorable meteorological conditions and cumulative aerosols.In winter 2017/2018,mean PM2.5mass concentration substantially decreased by 62% from 113 μg m−3in winter 2016/2017 to 43 μg m−3.With reduced PM2.5levels,the meteorological feedback on PM2.5was relatively weak in winter 2017/2018.However,the weakening degree and its contributions to PM2.5reduction are still uncertain.In this study,we investigated the change in the aerosol-induced modification of atmospheric stratification by combining PM2.5data,radiosonde observations,and ERA-Interim reanalysis data,and then estimated the weakened meteorological feedback effect on PM2.5change using machine learning.During polluted days,near-ground cooling bias,specific humidity (SH) increase,and relative humidity (RH) enhancement in winter 2017/2018 merely account for 38%,65%,and 36% of the meteorological modification caused by aerosols in winter 2016/2017,respectively.Using machine learning algorithms with three most related variables,we found that during polluted days,the PM2.5increase due to the meteorological feedback in winter 2017/2018 was merely 49% of that in winter 2016/2017.Effective pollution control and more favorable meteorological conditions have resulted in an additional benefit in PM2.5reduction.(Zhang Xiaoye)

2.10 Contribution of meteorological conditions to the variation in winter PM2.5 concentrations from 2013 to 2019 in Middle-Eastern China

Severe air pollution events accompanied by high PM2.5concentrations have been repeatedly observed in Middle-Eastern China since 2013 and decreased in recent years.The reason for this caused widespread attention.The month of January was selected to represent the winter season annual changes in the winter PM2.5and meteorological conditions—including the upper-air meridional circulation index (MCI),winds at 700 and 850 hPa levels and surface meteorology—from 2013 to 2019.These conditions were analyzed to study the contribution of meteorology changing to the annual PM2.5changing on the regional scale.Results show that,based on values of upper-level MCI,the years 2014,2015,2017,and 2019 were defined as meteorologyhaze years and the years 2016 and 2018 were defined as meteorology-clean years.A change in meteorological conditions may lead to a 26% change in PM2.5concentration between 2014 and 2013 (two meteorology-haze years) and 16%–20% changes in PM2.5concentration between meteorology-haze years and meteorology-clean years.Changes in pollutant emissions may cause 21%–47% changes in PM2.5concentration between each two meteorology-haze years.A comparison of two meteorology-clean years and pollutant emissions in 2018 may be reduced by 40% compared with 2016.Overall,changes in emissions had a greater influence on changes in PM2.5compared with meteorological conditions.(Wang Hong)

2.11 The impacts of the meteorology features on PM2.5 levels during a severe haze episode in Central-East China

The most polluted urban agglomeration including 13 cities in Central-East China (112°–122°E,34°–42°N) were selected to study the impacts of meteorology features on PM2.5levels during the severe haze episode by using observational PM2.5concentration,surface and balloon sounding meteorology data.The study results showed that the temporal changing of PM2.5in the 13 cities showed well correlation at the haze beginning,maintenance,and ending period due to the similar 500 hPa circulation and surface sea level pressure pattern.The increasing of surface relative humidity (RH) and temperature preceded PM2.5accumulation when haze began.RH usually reached up to 90%–95% during the period of PM2.5peak,suggesting the possible contribution of high humidity to extreme PM2.5values.In contrast to the similar circulation of upper air and surface pressure pattern,the divergences of local PBL meteorology,especially their vertical structure,were very obvious,which was the major meteorological cause for the different PM2.5levels in these cities.The temperature rise at 850 hPa layer was higher than that at 925 hPa,which was higher than that at 1000 hPa,leading to the formation of temperature inversion.This was the most important trigger factor for haze.PM2.5maximum generally occurred within 12 h after the formation of the strongest inversion in each city.PM2.5levels in the 13 cities strongly depend on their reverse intensity:for the cities where the inversion was strong and long lasting,their PM2.5was often the highest.Stable inversions are more likely to form in the transitional area from the northwestern mountains to the southeastern plains because of the mountain’s blocking of cold air and the warming of the boundary layer by sinking airflow from the mountaintop.This is the major meteorology cause for the frequent occurrence of the extreme PM2.5levels in the middle-south plain of Hebei Province.This study reminds us that the local boundary layer and inversion conditions,closely related with geographical location and local topography,contributes greatly to local PM2.5levels and should be fully considered in the emission reduction and industrial layout policy by government.(Wang Hong)

2.12 Large contribution of meteorological factors to inter-decadal changes in regional aerosol optical depth

Aerosol optical depth (AOD) has become a crucial metric for assessing global climate change.Although global and regional AOD trends have been studied extensively,it remains unclear what factors are driving the inter-decadal variations in regional AOD and how to quantify the relative contribution of each dominant factor.This study used a long-term (1980–2016) aerosol dataset from the Modern-Era Retrospective Analysis for Research and Applications,version 2 (MERRA-2) reanalysis,along with two satellite-based AOD datasets (MODIS/Terra and MISR) from 2001 to 2016,to investigate the long-term trends in global and regional aerosol loading.Statistical models based on emission factors and meteorological parameters were developed to identify the main factors driving the inter-decadal changes of regional AOD and to quantify their contribution.Evaluation of the MERRA-2 AOD with the ground-based measurements of AERONET indicated significant spatial agreement on the global scale (r= 0.85,root-mean-square error=0.12,mean fractional error=38.7%,fractional gross error=9.86% and index of agreement=0.94).However,when AOD observations from the China Aerosol Remote Sensing Network (CARSNET) were employed for independent verification,the results showed that MERRA-2 AODs generally underestimated CARSNET AODs in China (relative mean bias=0.72 and fractional gross error =−34.3%).In general,MERRA-2 was able to quantitatively reproduce the annual and seasonal AOD trends on both regional and global scales,as observed by MODIS/Terra,although some differences were found when compared to MISR.Over the 37-year period in this study,significant decreasing trends were observed over Europe and the eastern United States.In contrast,the eastern China and southern Asia showed AOD increases,but the increasing trend of the former reversed sharply in the most recent decade.The statistical analyses suggested that the meteorological parameters explain a larger proportion of the AOD variability (20.4%–72.8%) over almost all regions of interest (ROIs) during 1980–2014 when compared with emission factors (0–56%).Further analysis also showed that SO2was the dominant emission factor,explaining 12.7%–32.6% of the variation in AOD over anthropogenic-aerosol-dominant regions,while black carbon or organic carbon was the leading factor over the biomass-burning-dominant (BBD) regions,contributing 24.0%–27.7% of the variation.Additionally,wind speed was found to be the leading meteorological parameter,explaining 11.8%–30.3% of the variance over the mineral-dust-dominant regions,while ambient humidity (including soil moisture and relative humidity) was the top meteorological parameter over the BBD regions,accounting for 11.7%–35.5% of the variation.The results of this study indicate that the variation in meteorological parameters is a key factor in determining the inter-decadal change in regional AOD.(Che Huizheng )

2.13 Satellite-derived PM2.5 concentration trends over the eastern China from 1998 to 2016:Relationships to emissions and meteorological parameters.

Fine particulate matter (PM2.5) pollution in the Eastern China (EC) has raised concerns due to its adverse effects on air quality,climate,and human health.This study investigated the long-term variation trend in satellite-derived PM2.5concentrations and how it was related to pollutant emissions and meteorological parameters over EC and seven regions of interest (ROIs) during 1998–2016.Over EC,the annual mean PM2.5increased before 2006 due to the enhanced emissions of primary PM2.5,NOxand SO2,but decreased with the reduced SO2emissions after 2006 evidently in response to China’s clean air policies.In addition,results from statistical analyses indicated that in the North China Plain (NCP),Northeast China (NEC),Sichuan Basin (SCB) and Central China (CC),the planetary boundary layer height (PBLH) was the dominant meteorological driver for the PM2.5decadal changes,and in the Pearl River Delta (PRD) wind speed is the leading factor.Overall,the variation in meteorological parameters accounted for 48% of the variances in PM2.5concentrations over EC.The population-weighted PM2.5over EC increased from 36.4 μg m−3in 1998–2004 (P1) to 49.4 μg m−3in 2005–2010 (P2),then decreased to 46.5 μg m−3in 2011–2016 (P3).In the NCP and NEC,the percentages of the population living above the World Health Organization (WHO) Interim Target-1 (IT-1,35 μg m−3) have risen steadily over the past 20 years,reaching maxima of 97.3% and 78.8% in P3,respectively,but decreases of～30% from P2 to P3 were found for the SCB and PRD.(Che Huizheng)

2.14 Interaction between planetary boundary layer and PM2.5 pollution in megacities in China:A review

During the past decades,the number and size of megacities have been growing dramatically in China.Most of Chinese megacities are suffering from heavy PM2.5pollution.In the pollution formation,the planetary boundary layer (PBL) plays an important role.This review is aimed at presenting the current state of understanding of the PBL-PM2.5interaction in megacities,as well as to identify the main gaps in current knowledge and further research needs.The PBL is critical to the formation of urban PM2.5pollution at multiple temporal scales,ranging from diurnal change to seasonal variation.For the essential PBL structure/process in pollution,the coastal megacities have different concerns from the mountainous or land-locked megacities.In the coastal cities,the recirculation induced by sea-land breeze can accumulate pollutants,whereas in the valley/basin,the blocking effects of terrains can lead to stagnant conditions and thermal inversion.Within a megacity,although the urbanization-induced land use change can cause thermodynamic perturbations and facilitate the development of the PBL,the increases in emissions outweigh this impact,resulting in a net increase of aerosol concentration.Moreover,the aerosol radiative effects can modify the PBL by heating the upper layers and reducing the surface heat flux,suppressing the PBL and exacerbating the pollution.This review presented the PBL-PM2.5interaction in 13 Chinese megacities with various geographic conditions and elucidated the critical influencing processes.To further understand the complicated interactions,long-term observations of meteorology and aerosol properties with multi-layers in the PBL need to be implemented.(Miao Yucong)

2.15 Linkages between aerosol pollution and planetary boundary layer structure in China

China suffers from high levels of PM2.5pollution,which is often exacerbated by unfavorable planetary boundary layer (PBL) structures.Partly due to a lack of appropriate observations,the PBL-aerosol linkages in China are not clearly understood.Thus,we investigated these linkages from a national perspective using sounding data collected from 2014 to 2017.Correlation analyses revealed a significant anti-correlation between monthly boundary layer height (BLH) and aerosol pollution that was ubiquitous across China,indicating the important role of the PBL in regulating the seasonal variations of pollution in China.Besides,the day-to-day variations in pollution were modulated by the daily variabilities in the PBL structure.During winter,highly polluted days in most of the Chinese cities studied were associated with a low BLH,strong thermal stability,and weak PBL winds.In the North China Plain and Northeast China,the wintertime heavy pollution was often related to southerly winds and moister PBL.This study has important implications for understanding the crucial role that the PBL plays in modulating aerosol pollution in China.(Miao Yucong)

2.16 Synoptic pattern and planetary boundary layer structure associated with aerosol pollution during winter in Beijing,China

The day-to-day variations in the planetary boundary layer (PBL) structure and air quality are closely governed by large-scale synoptic forcings.Partly due to the lack of long-term PBL observations during the winter in Beijing,the complex relationships between the large-scale synoptic patterns,local PBL structures/processes,and PM2.5pollution have not been fully understood.Thus,this study systematically investigated these linkages by combining aerosol measurements,surface meteorological observations,radiosonde data,reanalysis,long-term three dimensional meteorological simulations,and idealized meteorology-chemistry coupled simulations.Based on the validated long-term simulation results,the boundary layer height (BLH) in Beijing during two winters from 2013 to 2015 was calculated and compared with PM2.5measurements.A significant anti-correlation was found between the daily BLH and PM2.5concentration in Beijing,indicating the importance of the PBL structure on the variations in the aerosol pollution levels.Those days with low BLHs are often accompanied by a strong elevated thermal inversion layer.Based on the daily 900 hPa geopotential height fields,seven synoptic patterns were identified using an objective approach,in which two types were found to be associated with heavy PM2.5pollution in Beijing.One pattern was characterized by weak northwesterly prevailing winds and a strong elevated thermal inversion layer over Beijing,and the local emissions of aerosols played a decisive role in the formation of heavy pollution.The other pattern was associated with southerly prevailing winds,which could transport the pollutants emitted from southern cities to Beijing.According to the meteorology-chemistry coupled simulations,southerly regional transportation can contribute approximately 56% of the PM2.5in Beijing.The results of this study have important implications for understanding the crucial roles that multiscale meteorological factors play in modulating the aerosol pollution in Beijing during the winter.(Miao Yucong)

2.17 Influence of boundary layer structure and low-level jet on PM2.5 pollution in Beijing:A case study

Beijing experiences frequent PM2.5pollution,which is influenced by the planetary boundary layer (PBL) structure/process.Partly due to a lack of appropriate observations,the impacts of PBL on PM2.5pollution are not yet fully understood.Combining wind-profiler data,radiosonde measurements,near-surface meteorological observations,aerosol measurements,and three-dimensional simulations,this study investigated the influence of PBL structure and the low-level jet (LLJ) on the pollution in Beijing from 19 to 20 September 2015.The evolution of the LLJ was generally well simulated by the model,although the wind speed within the PBL was overestimated.Being influenced by the large-scale southerly prevailing winds,the aerosols emitted from the southern polluted regions could be easily transported to Beijing,contributing to～68% of the PM2.5measured in Beijing on 20 September.The relative contribution of external transport of PM2.5to Beijing was high in the afternoon (≥80%),which was related to the strong southerly PBL winds and the presence of thermallyinduced upslope winds.On 20 September,the LLJ in Beijing demonstrated a prominent diurnal variation,which was predominant in the morning and after sunset.The occurrence of the LLJ could enhance the dilution capacity in Beijing to some extent,which favors the dilution of pollutants at a local scale.This study has important implications for better understanding the complexity of PBL structure/process associated with PM2.5pollution in Beijing.(Miao Yucong)

2.18 Influence of high relative humidity on secondary organic carbon:Observations at a background site in East China

To investigate the impacts of relative humidity (RH) on secondary organic aerosol (SOA) concentrations and chemical reactions,carbonaceous aerosol components,i.e.,organic carbon (OC) and element carbon (EC),were quantified in daily PM2.5samples collected at a background site of East China during summer season.Based on the method of EC-tracer,SOC (secondary organic carbon) demonstrated an obvious negative relationship with RH higher than 60%.Moreover,the ratio of SOC/EC also exhibited obvious decreasing trends with increasing RH,indicating negative effects for chemical production of SOA under high RH condition.Due to high RH condition exerting weakening of photochemistry and gaseous oxidant concentrations,such as significantly decreased O3levels,production rates of SOA were relatively low.On the other hand,because of more water uptake under higher RH condition,the aerosol droplet acidity was reduced and enhancement of SOA formation by acidity was accordingly absent.In addition,high RH also plays an important role on the effect of the viscosity of pre-existing aerosol coatings,which can affect reactive uptake yield of SOA.Overall,these results from this study imply that SOA production may be more associated with photochemical processes,while aqueous-phase chemistry is not very important for some SOA formation in a moist ambient environment.In the ambient atmosphere,oxidant concentrations,reaction rates,airborne species,etc.,are highly variable.How these factors affect SOA yields under given ambient environment warrants further detailed investigations.(Liang Linlin)

2.19 Comparing the impact of strong and weak East Asian winter monsoon on PM2.5 concentration in Beijing

We investigate the relationship between the wintertime PM2.5concentration in Beijing from 2005 to 2016 and the East Asian winter monsoon (EAWM) strength based on PM2.5observations and the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis data.The large-scale atmospheric circulations are compared and the differences in spatial distribution and source areas of Beijing PM2.5concentration are studied in depth combined with GRAPES-CUACE atmospheric chemistry model and its aerosol adjoint model in strong and weak EAWM years.The results show that the monthly Beijing PM2.5concentration in winter is significantly anti-correlated with the individual EAWM indices of winter month.According to three EAWM indices,December 2014 and 2016 are selected as the representatives of strong and weak EAWM years for further comparison.In December 2014,the strong EAWM circulation was characterized by the strengthened subtropical westerly jet stream at 300 hPa,the deepened 500 hPa East Asian trough,the strengthened northwesterly wind at 850 hPa in North China,and the developed Siberian High.Conversely,these anomalous patterns in December 2016 were much weaker,resulting in the lower boundary layer height,more stable atmosphere stratification and more frequent inversion.The simulated PM2.5concentrations in the northern and southern parts of Beijing were affected by the EAWM to different extents,with the values of 55–75 μg m−3in the north and 95–125 μg m−3in the south.There was no obvious difference in the locations of sensitive source areas between December 2014 and 2016.However,compared to December 2014,the accumulated sensitivity coefficients of Beijing,Tianjin,Hebei and Shanxi sources at 72 h ahead of the objective time period were 2.2,5.9,1.6,and 1.4 times higher,and the key discharge period was 2.6 times longer in December 2016.The contributions of Beijing and Tianjin sources had greater increase rates,stressing the importance of sensitive source areas in the direction of southeast Beijing in weak EAWM year.(An Xingqin)

2.20 Impacts of different types and intensities of El Niño events on winter aerosols over China

El Niño is a strong signal of interannual climate variability.The occurrence of an El Niño event may have an important impact on atmospheric pollutant concentrations in China through changes to the regional climate.This study quantitatively examines the differences among the effects of different types and intensities of El Niño events on winter mean aerosol concentration and severe haze days in China.It is found that Central Pacific (CP) El Niño events lead to larger increases in the aerosol burden over the southern China relative to the same intensity of Eastern Pacific (EP) events.For El Niño events of the same type but with different intensities,moderate events induce large increases in surface aerosol concentrations over the entire eastern China,while strong and weak events lead to obvious decreases in the surface aerosol concentration over the northern China.These differences are mainly attributed to changes in atmospheric circulation,with the resulting changes in aerosol mass transport occurring in response to the different types of El Niño events.Except for moderate CP events,all kinds of El Niño events lead to an increase in severe haze days in winter over the northern China.Strong and weak EP El Niño events and weak CP El Niño events lead to an increase in severe haze days in winter over the southern China.A change in the south-north transport of aerosols caused by El Niño events also play a dominate role in changing the occurrence of winter haze days in China.(Wang Zhili)

2.21 The role of anthropogenic aerosol forcing in inter-decadal variations of summertime uppertropospheric temperature over East Asia

Observations show a significant interdecadal decline of July and August (JA) mean upper-tropospheric temperature (UTT) over East Asia during the second half of the twentieth century,which is likely responsible for the weakening of the East Asian summer monsoon and the“southern flood and northern drought”phenomenon observed in the eastern China.This study investigates the role of anthropogenic aerosol forcing in the interdecadal cooling for the period 1951–2001 and the physical mechanisms involved using a coupled Earth system model,combined with reanalysis data.Our results show that the aerosol forcing leads to a drop of 0.14 per decade in JA mean UTT averaged over East Asia (30°–45°N,90°–130°E),which significantly contributes to the observed cooling of 0.2 per decade.The interdecadal cooling may be attributed to the weakening of interhemispheric differences in JA mean sea surface temperature in response to aerosols.The asymmetric hemispheric change in sea surface temperature alters the local eddy flux,thereby weakening upper-tropospheric westerlies and the transport of warm air from over the Tibetan plateau into downstream regions between 30°N and 40°N.We further find that changes in the geographic distribution of SO2emissions are the dominant cause of differences in JA mean UTT trends over East Asia between decades.The projected decrease in anthropogenic aerosols will lead to a rise of 0.05 per decade in JA mean UTT over East Asia during 2010–2050 under the Representative Concentration Pathways 8.5 scenario.This will likely have large impacts on the distribution of summer precipitation in East Asia in the future.(Wang Zhili)

2.22 Acute and cumulative effects of haze fine particles on mortality and the seasonal characteristics in Beijing,China,2005–2013:A time-stratified case-crossover study

We observed significant effects of particulate matter (PM2.5) on cause-specific mortality by applying a time-stratified case-crossover and lag-structure designs in Beijing over a nine-year study period (2005–2013).The year-round odds ratio (OR) was 1.005 on the current day with a 10 μg m−3increase in PM2.5for allcause mortality.For cardiovascular mortality and stroke,the ORs were 1.007 and 1.008 on the current day,respectively.Meanwhile,during a lag of six days,the cumulative effects of haze on relative risk of mortality,respiratory mortality and all-cause mortality was in the range of 2%–11%.Moreover,we found a significant seasonal pattern in the associations for respiratory mortality:significant associations were observed in spring and fall,while for all-cause mortality,cardiovascular mortality,cardiac and stroke,significant associations were observed in winter.Moreover,increasing temperature would decrease risks of mortalities in winter taking fall as the reference season.We concluded that in summer,temperature acted as a direct enhancer of air pollutants; while in winter and spring,it was an index of the diameter distribution and composition of fine particles.(Li Yi)

2.23 Associations between meteorological factors and visceral leishmaniasis outbreaks in Jiashi County,Xinjiang Uygur Autonomous Region,China,2005–2015

Although visceral leishmaniasis disease is controlled overall in China,it remains a serious public health problem and remains fundamentally uncontrolled in Jiashi County,Xinjiang Uygur Autonomous Region.During 2005–2015,there were two outbreaks in Jiashi County.Assessing the influence of meteorological factors on visceral leishmaniasis incidence is essential for its monitoring and control.In this study,we applied generalized estimating equations to assess the impact of meteorological factors on visceral leishmaniasis risk from 2005 to 2015.We also compared meteorological factors among years with Kruskal-Wallis test to explore possible reasons behind the two outbreaks that occurred during our study period.We found that temperature and relative humidity had very significant associations with visceral leishmaniasis risk and there were interactions between these factors.Increasing temperature or decreasing relative humidity could increase the risk of visceral leishmaniasis events.The outbreaks investigated might have been related to low relative humidity and high temperatures.Our findings will support the rationale for visceral leishmaniasis control in China.(Li Yi)

2.24 Temporal disparity of the atmospheric systems contributing to interannual variation of wintertime haze pollution in the North China Plain

Previous studies indicated that the meridional (northerly or southerly) wind anomalies over East China play an important role in modulating interannual variation of the winter haze pollution in the North China Plain (NCP) mainly via changing surface wind speed and humidity.Here,we report that the factors for the formation of the meridional wind anomalies over East China related to interannual variation of winter haze pollution experienced a significant interdecadal change around the mid-1990s.Before the mid-1990s,two upstream atmospheric wave trains contribute to generation of the meridional wind anomalies over East China via inducing significant geopotential height anomalies over Northeast Asia.The first occurred over mid-latitude Eurasia and propagated eastward into East Asia,resembling the East Atlantic-west Russia (EAWR) pattern.The second propagated eastward along the subtropical Asian jet.Furthermore,during this period,the change in the intensity of the East Asian trough (EAT) was closely linked with interannual variation of the winter haze variation in the NCP.By contrast,after the mid-1990s,the atmospheric wave train along the Asian subtropical jet was not observed.Furthermore,the connection between the EAT intensity and the winter time NCP haze variation was weak.The mid-latitude EAWR-like pattern and the El Niño-Southern Oscillation-related sea surface temperature anomalies in the tropical Pacific were possible factors that explain the meridional wind anomalies over East China.Understanding the change in the atmospheric anomalies contributing to interannual variation of the haze is essential for the prediction of haze in the NCP.(Chen Shangfeng,Guo Jianping)

2.25 Inter-annual variation of the spring haze pollution over the North China Plain:Roles of atmospheric circulation and sea surface temperature

Haze pollution is a serious air quality issue in China.Previous studies over the North China Plain (NCP) mainly focused on analysing the haze during boreal winter.However,the variation in haze during spring and the related factors remain unclear.This study investigates inter-annual variation of the spring haze,which is represented by the humidity-corrected dry extinction coefficient (DEC) over the NCP.During high DEC years,pronounced positive DEC anomalies appear over the NCP and its surrounding regions.Correspondingly,a notable anticyclonic anomaly is observed over Northeast Asia,inducing significant southeasterly wind anomalies over the NCP.The anomalous southeasterly winds reduce wind speed and increase relative humidity,which provides favourable meteorological conditions for accumulation and growth of aerosol pollutants.Further analysis shows that the North Atlantic Oscillation (NAO) contributes to the formation of anticyclonic anomalies over Northeast Asia via downstream propagating atmospheric wave trains.Additionally,positive sea surface temperature (SST) anomalies over the subtropical northeastern Atlantic Ocean may also impact the DEC variation.The linear barotropic model experiment further confirms the important role of subtropical northeastern Atlantic SST anomalies in contributing to the anomalous anticyclone over Northeast Asia and southerly wind anomalies over the NCP region,which are triggered via eastwards propagating atmospheric wave trains.This study gives support to the idea that the NAO and the North Atlantic SST may be potential predictors for the frequently observed springtime NCP haze variation.(Chen Shangfeng,Guo Jianping)

2.26 Tropopause trend across China from 1979 to 2016:A revisit with updated radiosonde measurements

The long-term trend in tropopause has profound implications for the expansion of tropical zone and the variation of large-scale circulation.However,the changes of tropopause in China have not been explicitly investigated as yet.In this study,the trend of lapse rate tropopause (LRT) height over China has been comprehensively revisited for the period of 1979−2016,using the newly released quality-controlled radiosonde data from China Meteorological Administration.Results show that the LRT height in most parts of China shows a significant upwards trend with a rate of 370 m/decade,most likely due to global warming.The fastest increase occurs in northwest regions,followed by the low-latitude regions (15°−25°N),while the slowest increase occurs in the high-latitude regions (45°−55°N).Overall,the LRT height varies with latitudes,exhibiting a south high and north low pattern.In particular,the high LRT height over low latitudes is found to be expanding rapidly polewards in recent years,in contrast to the almost constant LRT height over mid and high latitudes.In terms of the seasonality,the tropopause height reaches the peak in summer and bottom in winter.The frequency distribution in the vertical direction exhibits a bimodal pattern with the major peak mostly occurring at around 15 km and a secondary peak occurring between 8 and 12 km.This bimodal distribution is similar to the findings revealed in previous studies.Our findings offer important circumstantial observational evidences for the polewards expansion of the Tropics under global warming.(Chen Xinyan,Guo Jianping,Yin Jinfang)

2.27 Layer-wise formation mechanisms of an entire-troposphere-thick extratropical cyclone that induces a record-breaking catastrophic rainstorm in Beijing

A rarely seen entire-troposphere-thick (ETT) extratropical cyclone that induced a record-breaking catastrophic rainstorm in Beijing (the largest since 1963) on 20 July 2016 was reproduced reasonably by a convection-permitting Weather Research and Forecasting Model.This cyclone was the result of a vertical coupling of a lower,a middle,and an upper tropospheric cyclone,which formed at different times and locations.Dry air descending from the stratosphere through a tropopause folding process capped moist air to the middle and lower troposphere,making convection associated with the cyclone relatively shallow.Overall,cyclonic vorticity associated with the ETT cyclone enhanced much rapider than its wind kinetic energy (KE),and formation mechanisms of the ETT cyclone were characterized by layer-wise features:(i) The formation of the middle and upper tropospheric cyclones was dominated by cyclonic vorticity transport,instead of cyclonic vorticity production,whereas the lower tropospheric cyclone was generated through both cyclonic vorticity transport and convergence-related cyclonic vorticity production.(ii) Consistent with the cyclonic vorticity intensification,the formation of the lower and middle tropospheric cyclones featured an enhancement of rotational wind KE (in the western section of the cyclone,this was mainly due to the work done by the pressure gradient force,but in the eastern section,it was mainly due to the inward transport of wind KE by rotational wind),whereas the formation of the upper tropospheric cyclone featured a decrease in the rotational wind KE (particularly in its eastern section),which was in contrast to its increasing cyclonic vorticity.The export of wind KE by a southwesterly wind within the eastern section of the cyclone governed the rotational wind attenuation.(Li Wanli,Xia Rudi,Sun Jianhua)

2.28 Non-monotonic aerosol effect on precipitation in convective clouds over tropical oceans

Aerosol effects on convective clouds and associated precipitation constitute an important open-ended question in climate research.Previous studies have linked an increase in aerosol concentration to a delay in the onset of rain,invigorated clouds and stronger rain rates.Here,using observational data,we show that the aerosol effect on convective clouds shifts from invigoration to suppression with increasing aerosol optical depth.We explain this shift in trend (using a cloud model) as the result of a competition between two types of microphysical processes:cloud-core-based invigorating processes vs.peripheral suppressive processes.We show that the aerosol optical depth value that marks the shift between invigoration and suppression depends on the environmental thermodynamic conditions.These findings can aid in better parameterizing aerosol effects in climate models for the prediction of climate trends.(Liu Huan,Guo Jianping,Koren Ilan)

2.29 On the relationship between aerosol and boundary layer height in summer in China under different thermodynamic conditions

The observed relationships between boundary layer height (BLH) and PM2.5on a national scale remain unclear due to the dearth of observations.Here we investigated this relationship from a unique perspective of thermodynamic stability in the planetary boundary layer (PBL),using summertime (June-August) soundings from China for the period from 2014 to 2017.For all three times of soundings (08:00,14:00,and 20:00 Beijing time [BJT]),positive (negative) PM2.5concentrations anomalies were found to correlate with negative (positive) BLHs anomalies relative to daily means.The negative correlation was strongest at 14:00 BJT,followed by 20:00 BJT and 08:00 BJT.Overall,the PM2.5was found to nonuniformly anticorrelate with BLH across China at 08:00 and 20:00 BJT.The strongest anticorrelation occurred in the North China Plain at 14:00 BJT,in sharp contrast to the much weaker correlation in other regions characterized by much less polluted regions.The averaged PM2.5in neutral boundary layers was higher than that in convective boundary layers (CBLs).The CBL,where the anticorrelation was the strongest,was conducive to dissipating more aerosol in the heavily polluted area in China than neutral boundary layer.The higher CBL formed under low cloud cover,low surface humidity,and strong wind speed was favorable for the dispersion of aerosol,in contrast to the stable boundary layers that happen under the highest cloud cover.Also,positive correlation was seen between stable boundary layer and PM2.5.The findings call for attention that the thermodynamical condition of PBL should be considered when examining the aerosol-PBL interactions.(Lou Mengyun,Guo Jianping,Wang Lingling)

2.30 喀拉海和巴伦支海海冰对中高纬异常纬向环流暨中国东部重霾天气形成的作用

采用1981年1月至2017年2月国家气象信息中心雾、霾数据集资料、同期NCEP/NCAR再分析资料以及哈德来中心的海冰资料，分析了秋冬季喀拉海和巴伦支海海冰变化与东亚冬季风暨中国东部冬季雾和霾日数变化特征之间的关系。研究结果表明，喀拉海和巴伦支海海冰对亚洲区中高纬纬向环流有重要影响，秋季海冰异常偏少是冬季亚洲区中高纬异常纬向环流形成的诱因之一。该地区秋季海冰偏少年，冬季亚洲中高纬地区纬向环流异常偏强，东亚大槽偏弱，影响我国东部地区的东亚冬季风减弱，这为大气污染物在水平方向上的聚集提供了有利条件。同时在海冰偏少年，对流层从中层向下均为正温度距平，与地表温差减小，不利于对流发展，使得大气的状况变得更加稳定，不利于大气污染物在垂直方向上的扩散，水平和垂直方向上的共同作用导致中国东部地区易发生霾天气过程。虽然喀拉海和巴伦支海海冰是影响中国东部地区冬季霾过程发生的重要因子之一，但其对冬季中国东部雾天气发生日数多寡的影响并不显著。亚洲区纬向环流指数相比经向环流指数更能反映中国东部地区冬季雾—霾日数的变化，冬季亚洲中高纬纬向环流越强，中国东部地区雾—霾日数越多。（牛涛）

2.31 海口地区2018年2月持续低能见度过程的气象条件分析

利用海口地区的能见度资料、地表气象观测、探空数据和再分析资料，对2018年2月发生在海口和琼州海峡地区的持续低能见度过程的气象学成因进行分析。结果表明，海口地区冬季的低能见度事件通常出现在暖湿条件下; 能见度与相对湿度、温度和热力稳定度呈显著的负相关关系，与风速和边界层高度呈显著的正相关关系。海口地区空气质量良好，其能见度与PM2.5浓度间无显著的相关性，这一特点与我国其他污染较重的地区有所不同。在925 hPa 高度上，当海口处于高压系统的南侧，受到东北风控制时，能见度状况较好; 当受东南风控制时，来自海上的暖湿气流有利于低能见度事件的出现和维持。（缪育聪）

2.32 北京地区大气颗粒物输送路径及潜在源分析

利用TrajStat软件和全球资料同化系统数据，计算了2005—2016年北京市逐日72 h气流后向轨迹，采用聚类分析方法，结合北京同期PM2.5逐日质量浓度数据，分析北京市年及四季后向气流轨迹特征及其对北京市颗粒物浓度的影响，运用潜在源贡献因子分析法（PSCF）和浓度权重轨迹分析法（CWT），探讨研究时期内不同季节影响北京市颗粒物质量浓度的潜在源区以及不同源区对北京颗粒物质量浓度的贡献。结果表明，全年而言，西北输送气流占总轨迹的比例最高，达59.97%，且其输送距离最远、输送高度最高、移速最快。输送高度最低、距离最短、移速最慢的东南气流占比次之，为27.64%。东北气流占比最低为12.40%，其移速和输送距离介于前两者之间。主要污染轨迹来自山东、河北，其次为来自俄罗斯、蒙古国和内蒙古荒漠戈壁地区的西北气流。PSCF和CWT分析发现，蒙中、晋中、冀西南、豫北及鲁西是影响北京PM2.5的主要潜在区域。而不同季节、不同输送路径对北京PM2.5污染影响的差异显著：春季主要受来自蒙晋交界区域的短距离输送气流影响，潜在源区位于冀南、鲁西、豫东和皖西北地区；夏季污染轨迹来自鲁、晋地区，潜在源区为豫东北、皖北和苏北地区；秋季主要受来自冀南地区的短距离气流影响，潜在源区为晋北、冀南、豫北和鲁西地区；冬季主要受来自蒙古中西部和中地区的远距离输送气流影响，潜在源区主要在冀南、鲁西、豫北、晋和蒙西地区。（安兴琴）

2.33 多尺度气象条件对济南PM2.5污染的影响

基于长时间序列空气质量数据和气象数据分析济南大气污染与气象条件关系的研究相对较少。利用2010—2016年济南市环境空气质量监测数据、气象再分析和观测数据，分析了济南市PM2.5污染特征、PM25浓度与2 m温度（T）、2 m相对湿度（RH）、10 m高度U和V风速（U和V）、10 m风速（WS）、K指数（K）、A指数（A）和边界层高度（BLH）的相关性、天气类型对PM2.5浓度的影响，并基于逐步回归分析方法构建统计模型，利用解释方差量化气象条件对PM2.5浓度变化的影响。分析发现，济南PM2.5浓度存在显著的季节变化和年际变化特征，年均PM2.5浓度呈下降趋势；近地面PM2.5浓度与T、RH、K和A显著正相关，与WS和BLH显著负相关，U和V与PM2.5浓度相关性不显著（p＜0.05）;不同天气类型对应的PM2.5浓度均值存在显著差异；基于回归模型分析发现气象条件可以解释10%～40%的PM2.5浓度逐日变化，气象条件的影响有明显的季节变化。（何建军）

2.34 济南地区逆温层特征及其对颗粒物质量浓度的影响

利用L波段探空雷达数据、温度廓线仪数据、微脉冲激光雷达边界层数据、PM2.5（质量浓度数据），结合常规地面气象观测资料，分析济南2008—2017年逆温层参数特征以及逆温对PM2.5（质量浓度）的影响。结果表明，2008—2017年济南逆温发生频率为47.2%，且呈逐年下降趋势，气候倾向率为0.44%/a。逆温频率、逆温强度和逆温层厚度在冬季较高，夏季较低，但不同类型逆温季节变化不同。济南地区贴地逆温平均厚度为134 m，逆温强度为2.46 ℃/（100 m）；脱地逆温平均厚度为212 m，逆温强度为1.34 ℃/（100 m）。逆温是影响空气质量的重要因子之一，逆温的存在导致济南近地面PM2.5质量浓度升高24.8%。PM2.5质量浓度与逆温层厚度呈显著正相关，而与逆温强度的相关性未通过显著性检验。（何建军）

2.35 2016年11—12月北京及周边重污染过程PM2.5特征

对2016年11—12月北京及周边地区不同站点重污染期间PM2.5质量浓度变化特征进行分析，并结合地面和探空气象要素及化学组分等对重污染成因进行深入探讨，比较了其中两次持续3 d及以上重污染过程的异同。结果表明：重污染期间北京及周边地区PM2.5质量浓度较高，北京上甸子站、顺义站、朝阳站的PM2.5质量浓度分别为73.1、130.8和226.0 µg m−3，河北保定站和石家庄站分别为357.8和346.9 µg m−3。12月17—21日重污染过程比11月3—5日持续时间更长且PM2.5质量浓度更高。通过对11—12月所有重污染过程分析发现，北京颗粒物重污染发生的主要气象条件是静稳天气。在排放源相对稳定情况下，逆温层的结构、演变和持续时间决定了重污染的程度，其中污染持续时间和污染期间的主导逆温层类型演变对重污染程度有较好的指示作用。较低的水平风速、逆温层的持续出现及更多的燃煤和机动车尾气排放是12月17—21日污染偏重的原因。（孟昭阳）