Tracing the Boundary Layer Sources of Carbon Monoxide in the Asian Summer Monsoon Anticyclone Using WRF-Chem

YAN Renchang and BIAN JianchunKey Laboratory of Middle Atmosphere and Global Environment Observation,Institute of Atmospheric Physics, Chinese Academy of Sciences,Beijing 100029

Tracing the Boundary Layer Sources of Carbon Monoxide in the Asian Summer Monsoon Anticyclone Using WRF-Chem

YAN Renchang and BIAN Jianchun∗Key Laboratory of Middle Atmosphere and Global Environment Observation,Institute of Atmospheric Physics, Chinese Academy of Sciences,Beijing 100029

The Asian summer monsoon(ASM)anticyclone is a dominant feature of the circulation in the upper troposphere–lower stratosphere(UTLS)during boreal summer,which is found to have persistent maxima in carbon monoxide(CO).This enhancement is due to the upward transport of air with high CO from the planetary boundary layer(PBL),and conf i nement within the anticyclonic circulation.With rapid urbanization and industrialization,CO surface emissions are relatively high in the ASM region,especially in India and East China.To reveal the transport pathway of CO surface emissions over these two regions,and investigate the contribution of these to the CO distribution within the ASM anticyclone,a source sensitivity experiment was performed using the Weather Research and Forecasting(WRF)with chemistry model(WRFChem).According to the experiment results,the CO within the ASM anticyclone mostly comes from India,while the contribution fromEastChinaisinsignif i cant.Theresult ismainlycaused by thedifferent transportationmechanisms.InIndia, CO transportation is primarily affected by convection.The surface air with high CO over India is directly transported to the upper troposphere,and then conf i ned within the ASM anticyclone,leading to a maximum value in the UTLS region.The CO transportation over East China is affected by deep convection and large-scale circulation,resulting mainly in transportation to Korea,Japan,and the North Pacif i c Ocean,with little upward transport to the anticyclone,leading to a high CO value at 215 hPa over these regions.

Asian summer monsoon,anticyclone,surface emission,convection

1.Introduction

In boreal summer,the Asian summer monsoon(ASM) anticyclone is a dominant feature of the circulation in the upper troposphere–lowerstratosphere(UTLS).The anticyclone is a closedcirculation,whichis primarilya responsetothediabatic heating associated with the presence of persistent deep convection(Hoskins and Rodwell,1995).Analyses of satellite data show that the anticyclone also has persistent maxima(or minima)in various trace constituents in the UTLS region(Rosenlof et al.,1997;Randel et al.,2001;Park et al., 2004;Li et al.,2005;Vernier et al.,2011).The extrema of these constituents are attributed to the trapping effect of the strong winds and closed streamlines associated with the anticyclone,which act to conf i ne air within the anticyclone for a few weeks(Li et al.,2005;Randel and Park,2006).

Asia isthoughttocurrentlyhavethefastest growingeconomy and be the most densely populated region in the world. Thus,anthropogenic emissions over the region are relatively high.Research has shown that the ASM is a key pathway for water vapor and pollutants at low levels to transport into the stratosphere(Randel et al.,2010).However,the source region and upward transport pathway of surface pollutants over the ASM regions remains an issue of uncertainty(Li et al.,2005;Berthet et al.,2007;Park et al.,2009;Randel et al.,2010;Wright et al.,2011;Bergman et al.,2013).Li et al. (2005)suggested that the surface air from northeastIndia and southwest China is lifted to the upper troposphere by deep convection and then trapped within the anticyclone.Other research has revealed that planetary boundary layer(PBL) particles from India and East Asia are transported to the anticyclone and to the tropics,respectively(Chen et al.,2010; Chen et al.,2012).Bergman et al.(2013)indicated that the PBL source from the Tibetan Plateau is also very important forthedistributionofatmosphericcompositionwithinthe anticyclone.

Tropospheric pollutants,including black carbon,reactive nitrogen species(NOx),sulfur dioxide(SO2),and carbon monoxide(CO),entering the stratosphere,have a signif i cant inf l uence on stratospheric ozone chemistry,microphysics,radiation balance,and the distribution of atmospheric compo-sition.Based on observed data,an increasing trend of stratospheric aerosols was found from 2000 to 2010(Hofmann et al.,2009;Solomon et al.,2011).During the same period, SO2emissions from coal combustion in China increased by about 60%,which are believed to one of the most important reasonsforthe increasein stratosphericaerosols(Hofmannet al.,2009).But Neely III et al.(2013)offereda differentopinion on this.Their simulation results showed that moderate volcanic eruptions are the primary source of the observed increases in stratospheric aerosol.Satellite observations reveal the existence of a layer of aerosol extending vertically from about 13 km to 18 km in the ASM region,which has been termed the Asian tropopause aerosol layer(ATAL)(Vernier et al.,2011;Thomason and Vernier,2013).The existence of the ATAL is most likely due to anthropogenicemissions.The aerosol microphysical model simulation indicates that Chinese and Indian SO2emissions contribute about 30%of the sulfate aerosol extinction in the ATAL during volcanically quiescent periods(Neely III et al.,2014).

Pollutants from the ASM region are rapidly transported to the stratosphere by the vigorous summertime circulation patterns associated with the Asian monsoon.The huge Asian monsoon system can be divided into two subsystems:the Indian(SouthAsian)andtheEastAsianmonsoonsystem(Ding and Chan,2005).These two subsystems have signif i cant differences,dictated by the contrasting sea–land distributions (Wang et al.,2005).In summer,the former features prevailing southwesterly winds,while the latter features prevailing southeasterly winds.India and East China,the two primary sources of pollutionemissions,are respectively located in the South Asian and East Asian monsoon system.Thus,the pollutants in these two regions may have different upward transport pathways due to the differences of their monsoon circulation.Using theMOZART-4(ModelforOzoneandRelatedchemical Tracers,version 4),Park et al.(2009)indicated that the pollutionsourcesoverIndiaand East Chinaare the significant source of CO at 100 hPa.But the horizontal resolution they used was 2.8°×2.8°,meaning that small and mediumscale convection systems may not have been reproduced in the model.The resolution of the Weather Research and Forecasting(WRF)model with chemistry(WRF-Chem)can be customized as required.In this study,we use WRF-Chem to examine the importance of deep convective dynamical transport and the contribution of emissions from India and East China to the CO distribution within the ASM anticyclone. We chose to focus on CO because of the availability of highresolution global observation products in the UTLS for this gas.Furthermore,CO emissions,whose photochemical lifetime is 1–2 months in the troposphere(Xiao et al.,2007),can act as an index of all anthropogenic emissions.

2.Method and data

2.1.Model description

Version 3.3.1 of WRF-Chem was used in this study.The dynamic scheme and microphysical processes of the WRF model is coupled with a chemistry model in WRF-Chem. The WRF(Skamarock et al.,2008)model uses terrainfollowing hydrostatic pressure as the vertical coordinate and the Arakawa-C grid for grid staggering.In this work,the simulation domain covers the entire Asia region,with a 60 km horizontal resolution,and there are 41 vertical levels in the model from the surface to about 10 hPa.The model used theThompsonmicrophysicsscheme(Thompsonetal.,2004), the Dudhia shortwave radiation algorithm(Dudhia,1989), the Rapid Radiative Transfer Model(RRTM)(Mlawer et al., 1997)for longwave radiation,the Grell–D´ev´enyi ensemble convective parameterization(Grell and D´ev´enyi,2002),the Noah land-surface scheme(Chen and Dudhia,2001),and the Yonsei University(YSU)atmospheric boundary layer(ABL) scheme(Hong and Lim,2006).The initial and lateral boundary conditions for meteorological variables were obtained from National Centers for EnvironmentalPrediction(NCEP) Final analysis(FNL)f i elds,available every 6 h at the spatial resolution of 1°×1°.

Inthisstudy,weattemptedtosimulatethetransportofCO surface emissions over India and East China,and the chemical reactions were ignored in the model.Initial and boundary conditions for the chemical species in WRF-Chem were extracted from the output of the MOZART-4 global chemical transport model(Emmons et al.,2010).Anthropogenic emissions came from the Intercontinental Chemical Transport Experiment-Phase B inventory,with a horizontal resolution of 0.5°×0.5°(Zhang et al.,2009).The Reanalysis of Tropospheric Chemical Composition(RETRO)inventories(http://retro.enes.org/data emissions.shtml),with a horizontal resolution of 0.5°×0.5°,were used for the regions where the INTEX-B inventory does not provide data.The INTEX-B emissions are representative of the year 2006,and RETRO emissions are representative of the year 2000.The biomass burning emissions came from the Global Fire Emissions Database,version 3,with a horizontal resolution of 0.5°×0.5°.

2.2.MLS database

The microwave limb sounder(MLS)instrument aboard the Aura spacecraft,one of the National Aeronautics and Space Administration(NASA)Earth Observing System (EOS)platforms,has been measuring atmospheric parameters since August 2004(Schoeberl et al.,2006).The MLS fi eld of view vertically scans the limb in the orbit plane and gives 82°S–82°N latitudinal coverage in each orbit(Waters et al.,2006).The horizontal resolution is 3°along the orbit, with 14 orbits per day.Vertical pro fi les of CO were obtained from version 2.2/level 2 MLS products.The available vertical coverage for CO varies from 215 hPa to 1 hPa(Livesey et al.,2007).We constructed gridded data on 4°(lat)×8°(lon)grids by averagingpro fi les.The quality screening of individual pro fi les was conducted according to the instructions given by Livesey et al.(2007),and about 80%of the data were retained.

3.Experimental design

It is known that there are persistent maxima of CO within the anticyclonic circulation in the UTLS over the ASM region(Li et al.,2005;Park et al.,2007).A global climatological distribution of CO averaged in June–August from 2005 to 2012 in the UTLS is shown in Figure 1.CO has a broad maximum at 215 hPa,146 hPa,100 hPa and 68 hPa in the ASM region.At 215 hPa,the distribution of CO reveals three peak areas corresponding to the Asian,North American,and Africanmonsoons,respectively.IntheASMregion,theareas of enhanced CO cover the Indian peninsula,East China,and Korea,and the maximum concentration is about 217 ppbv over Southeast China(Fig.1a).These peaks are associated with the transport of near surface air with high CO,which was transported to the height by the persistent deep convection(Gettelman et al.,2004).From 146 hPa to 68 hPa,the spatial structure of CO is somewhat different to that of 215 hPa.At 146 hPa,100 hPa,and 68 hPa,the distribution of CO has only a highconcentrationregionlinked to the structureof the ASM anticyclonic circulation,and the center of the maximum is located in the southeast of the anticyclone.The CO distribution differences between these regions are primarily due to the difference of the monsoon circulation(Gettelman et al.,2004).The Asian monsoon circulation is higher and deeper,which results in more tropospheric air with high CO being transported to the upper levels.And the isolation of the ASM anticyclonepreventsmixing between the inside and outside of the anticyclone,leading to a high CO concentration within the anticyclone.The relatively high CO in the UTLS over the ASM region is evidence of transport from near-surface levels(Li et al.,2005).The North American and African monsoons are smaller,do not reach well into the stratosphere,and are not isolated circulations.At the upper levels,there is more signif i cant mixing of tropospheric air with stratospheric air containing low CO(Gettelman et al., 2004).Therefore,a maximum of CO cannot be formed in these two areas.

Based on the above analysis,the ASM circulation can transport more surface air with high CO to the UTLS and conf i ne it inside the anticyclone.The CO comes from INTEX-B anthropogenic emissions,and Global Fire Emissions Database(http://www.globalf i redata.org/)during the summer in 2006 are shown in Fig.2.There are two main surface emission sources—one over India and one over East China.In this study,we address the transport pathway of CO surface emissions over these two regions using WRF-Chem to isolate the transport of specif i c surface sources of CO. The model simulation was performed with CO tagged in the two regions:India(10°–35°N,65°–100°E)and East China (20°–40°N,100°–120°E)(see Fig.2).The MOZART-4 CO results,which were used for the chemical species initial and boundaryconditions,alreadycontainedCO globalemissions. To ref l ect the individual contribution of CO emissions over India and East China,the CO emissions over these two regions were additionallysuperimposedin the model.The contributions of CO emissions were calculated using the model results forIndiaorEast China CO emissions minusthemodelresults without these sources.Since it took some time for the surface emissions to be transported to the upper troposphere, during which the CO concentration in the upper troposphere did not achieve a quasi-steady state,the simulated results for the f i rst 15 days are not included in the analysis.

4.Results

Figures 3a and b present the horizontal structure of MLS and WRF-Chem CO at 100 hPa from 15 June to 15 July 2006.Note that the CO surface emissions in Fig.3b come from the entire simulation region.The two f i gures show that the results of the model correspond well with the MLS observations.The MLS results show an area with enhanced CO over the ASM anticyclone at 100 hPa(Fig.3a),and the model presents the same results.The magnitude in the model result is slightly reduced due to the MLS-observed CO appearing to be a bit higher in the UTLS(Livesey et al., 2008).Figures 3c and d show the mean CO concentration at 100 hPa from CO surface emissions over India and East China,respectively.Thehorizontalwinds forthecorresponding pressure levels are plotted in Figs.3a and b,which come fromtheNCEP/NCAR(NationalCenterforAtmosphericResearch)reanalysis data and WRF results,respectively.To compare the CO concentration inside and outside the ASM anticyclone,we chose two regions according to the horizontal structure of winds.In this paper,the area of(15°–30°N, 60°–100°E)is def i ned as“inside the anticyclone”,and that of(15°–30°N,150°–180°E)is def i ned as“outside the anticyclone”(see Fig.3).Here,we calculate the mean CO concentration differences between these two regions by“inside minus outside”.The results are shown in Table 1.The differ-ence in the MLS observations is about 30 ppbv.The difference in the model results is about 19.8 ppbv,of which about 13 ppbv(～67%)comes from Indian emissions(Fig.3c)and about 2 ppbv(～11%)from East China emissions(Fig.3d). Thedistributiondifferencebetweeninsideandoutsidetheanticycloneis due to the trappingeffect of the strong and closed circulation of the anticyclone(Li et al.,2005).The MLS observations feature a much larger difference of CO concentrations between the inside and outside of the anticyclone than the simulation,which may be caused by many factors,such as the simulated transport f l ux,source emissions given in the model,CO retrieval uncertainty,and so on.The calculation results suggest the relatively high CO concentration within the anticyclone is from Indian emissions,while the contribution from East China emissions is insignif i cant.In addition, the CO surface emissions from other regions are also important,contributing～22%of the distribution of CO within the anticyclone.

Table 1.Difference of mean CO concentrations between the inside and outside of the anticyclone.The area of(15–30N,60–100E)is def i ned as“inside the anticyclone”,and that of(15°–30°N,150°–180°E)is def i ned as“outside the anticyclone”.

TheCOconcentrationdifferencebetweenIndianandEast China emissions reaches up to 100 hPa,and is thought to be the result of differenttransport pathways.The CO concentrations at 700 hPa,500 hPa,215 hPa,and 146 hPa that come from India and East China are shown in Fig.4 and Fig.5, respectively.In the summer of the Northern Hemisphere, the strongest deep convection behavior mainly occurs over North India.The Indian emissions are transported to the upper troposphere(～200 hPa)by deep convection,where the South Asian High(SAH)anticyclone circulation has formed. Theanticycloniccirculationcenteris locatedovertheTibetan Plateau.The CO coming from India is conf i ned effectively within the anticyclone,and results in a high concentration center in the UTLS.Compared with India,the convection overEast Chinahas lowerfrequencyanda lowerlevelofconvection outf l ow.Thus,the transport of CO is mainly affected by large-scale circulation.Eastern China is located on the east edge of the anticyclone.The southwest airf l ow at lower levels transports CO toward the northeast(Fig.5),leading to a maximum over Korea,Japan,and the North Pacif i c Ocean. The air is then transported southward above 200 hPa due to thenortheastairf l owontheeastside oftheanticycloniccirculation.According to this result,it is concluded that the high CO concentrations at 215 hPa(Fig.1a)over Korea,Japan,and the North Pacif i c mainly come from East China emissions.

To isolate the effect of convection,a sensitivity experiment was conducted in which latent heating in the model microphysics was turned off.The development of deep convection was subsequently suppressed.Figure 6 shows the vertical crosssections of mean CO from 75°–95°E and 100°–120°E with and without convection.With the effect of deep convection(Figs.6a andb),there is a plume of highCO from the surface up to around 16 km over 75°–95°E,and the CO extremum center is located at around 14km.On the eastern side(100°–120°E),there is also a plume of high CO from the surface to the upper troposphere.The highest height is about 14 km,which is lower than the region of 75°–95°E,and the CO extremum center is located at about 12 km.Without the effect of deep convection(Figs.6c and d),both the high CO plumeand CO highvalue centerin the uppertropospheredisappear over the region 75°–95°E(Fig.6c).But there is still a slightly higher plume over100°–120°E(Fig.6d),and the CO extremum center is located at about 10 km.

The high value plume of CO shown in Fig.6a correspondsto the strongestdeepconvectionoverNorthIndia,and the high value plume of CO shown in Fig.6b corresponds to the deep convection activity of East China.The convection sensitivity experiment shows that the CO surface emissions over India are mainly transported via convection.In India, the strongest deep convection can transport more air to the upper troposphere.The anticyclone circulation over the region then prevents the air from mixing with the air outside. This is the main reason for the high CO concentration distribution over India at the height of between 12 km and 16 km.Whereas,the deep convective activity is relatively weak in East China,and the CO transport is affected by both deep convection and the large-scale circulation.Therefore,even though without convection behavior,there is still a slightly higher plume of CO.

5.Conclusion

The ASM anticyclone is a dominant feature of the circulation in the UTLS during summer in the Northern Hemisphere.Observed data show that there are persistent maxima in tropospheric trace constituents,such as HCN,CH4,and CO.Owing to rapid industrialization and urbanization,surface emissions are relatively higher in the ASM region,especially India and East China.The present study used the WRF-Chem model to examine the importance of deep convective dynamical transport and the contributions of Indian and East China emissions to the CO distribution within the ASM anticyclone.To analyze the dynamical transport,we ignored the chemical reaction processes in the model.

The model results show that most of the CO within the ASM anticyclone comes from India,while the contribution from East China is much smaller.According to the horizontal structure of the wind f i eld at 100 hPa,we def i ned the area (15°–30°N,60°–100°E)as“inside the anticyclone”.Inside this area,about 67%CO comes from Indian emissions.East China emissions contribute～2%of the CO concentration within the anticyclone.

Deep convection over North India is much stronger than that over eastern China,which can transport more air to the upper troposphere.In India,the air with high CO is transported to the upper tropospherevia deep convection and confi ned within the ASM anticyclone,leadingto a maximumCO in the UTLS.The CO surface emissions over East China are affected by both deep convection and large-scale circulation. The CO emissions are mainly transported to Korea,Japan, and the North Paci fi c Ocean,with little upward transport to theanticyclone,leadingto highCO concentrationsat 215hPa over these regions.

This paper focuses mainly on the transport of CO coming from Indian and East China emissions.Furthermore,chemical reaction processes were not considered in the model. There are in fact a number of different transport pathways for different chemical compositions.Thus,more analysis of the transport of near-surface emissions should be carried out, especially short-lived chemical species.

Acknowledgements.This work was supported by theNational Basic Research Program of China(Grant No.2010CB428602) and the National Natural Science Foundation of China(Grant Nos. 41175040 and 91337214).

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(Received 23 Jun 2014;revised 7 November 2014;accepted 4 December 2014)

∗Corresponding author:BIAN Jianchun Email:bjc@mail.iap.ac.cn