Seasonal Inhomogeneity of Soot Particles over the Central Indo-Gangetic Plains,India:Influence of Meteorology

SINGH B.P.,TIWARI S.,HOPKE Philip K.,SINGH R.S.,BISHT D.S.,SRIVASTAVA A.K., SINGH R.K.,DUMKA U.C.,SINGH A.K.,RAI B.N.,and SRIVASTAVA Manoj K.∗

1 Department of Geophysics,Banaras Hindu University,Varanasi 221005,India

2 Indian Institute of Tropical Meteorology(New Delhi Branch),Delhi 110060,India

3 Clarkson University,Box 5708,Potsdam,NY 13699-5708,USA

4 Department of Chemical Engineering,Banaras Hindu University,Varanasi 221005,India

5 Aryabhatta Research Institute of Observational Sciences,Manora Peak,Nainital 263002,India

6 Department of Physics,Banaras Hindu University,Varanasi 221005,India

Seasonal Inhomogeneity of Soot Particles over the Central Indo-Gangetic Plains,India:Influence of Meteorology

SINGH B.P.1,TIWARI S.2,HOPKE Philip K.3,SINGH R.S.4,BISHT D.S.2,SRIVASTAVA A.K.2, SINGH R.K.1,DUMKA U.C.5,SINGH A.K.6,RAI B.N.4,and SRIVASTAVA Manoj K.1∗

1 Department of Geophysics,Banaras Hindu University,Varanasi 221005,India

2 Indian Institute of Tropical Meteorology(New Delhi Branch),Delhi 110060,India

3 Clarkson University,Box 5708,Potsdam,NY 13699-5708,USA

4 Department of Chemical Engineering,Banaras Hindu University,Varanasi 221005,India

5 Aryabhatta Research Institute of Observational Sciences,Manora Peak,Nainital 263002,India

6 Department of Physics,Banaras Hindu University,Varanasi 221005,India

Black carbon(BC)particles play a unique and important role in earth's climate system.BC was measured (in-situ)in the central part of the Indo-Gangetic Plains(IGP)at Varanasi,which is a highly populated and polluted region due to its topography and extensive emission sources.The annual mean BC mass concentration was 8.92±7.0µg m−3,with 34%of samples exceeding the average value.Seasonally,BC was highest during the post-monsoon and winter periods(approximately 18µg m−3)and lower in the premonsoon/monsoon seasons(approximately 6µg m−3).The highest frequency(approximately 46%)observed for BC mass was in the interval from 5 to 10µg m−3.However,during the post-monsoon season,the most common values(approximately 23%)were between 20 and 25µg m−3.The nighttime concentrations of BC were approximately twice as much as the daytime values because of lower boundary layer heights at nighttime. The˚Angstr¨om exponent was significantly positively correlated(0.55)with ground-level BC concentrations, indicating the impact of BC on the columnar aerosol properties.The estimated mean absorption˚Angstr¨om exponent was 1.02±0.08µg m−3,indicating that the major source of BC was from fossil fuel combustion. Significant negative correlations between BC mass and meteorological parameters indicate a pronounced effect of atmospheric dynamics on the BC mass in this region.The highest mean BC mass concentration (18.1±6.9µg m−3)as a function of wind speed was under calm wind conditions(38%of the time).

black carbon,Indo-Gangetic Plains,absorption˚Angstr¨om exponent,biomass,fossil fuel

1.Introduction

Black carbon(BC)is produced by incomplete combustion of natural as well as anthropogenic substances,including the burning of biofuels and fossil fuels(Bond and Bergstrom,2006;Hyv¨arinen et al., 2010;Praveen et al.,2012;Srivastava et al.,2012a; Safai et al.,2013;Tiwari et al.,2013).The presence of BC in the atmosphere causes absorption of solar radiation in the visible and near infrared wavelengths.It exerts positive radiative forcing(i.e.,heating)at the top of the atmosphere,negative forcing(i.e.,cooling) at the surface,and heating in the lower troposphere. Because of these effects,it is an important constituent of the atmosphere that can cause global and regional warming(Satheesh and Ramanathan,2000;Jacobson,2001;IPCC,2007;Ramanathan and Carmichael, 2008;Moosm¨uller et al.,2009;Srivastava et al.,2012b;

Bond et al.,2013).According to the recent IPCC report(Stocker et al.,2014),the contribution of BC to atmospheric radiative forcing(direct impact)is 0.4 (0.05–0.8)W m−2.However,Bond et al.(2013)reported in an extensive review that the BC radiative forcing is 1.1(0.17–2.1)W m−2. It is the second strongest contributor to current global warming,after carbon dioxide(Ramanathan and Carmichael,2008; Bond et al.,2013).

Apart from its direct impact on the atmosphere, indirect impacts of BC are also apparent;for example, its influence on cloud droplet number concentrations and related alterations of cloud properties(Kaufman et al.,2002).The presence of higher concentrations of BC in the atmosphere makes it an important ingredient in the analysis of regional climate(Wang, 2004;Liao and Shang,2015),and instrumental in the alteration of the heat regime of regional air as well as regional atmospheric stability/vertical motion.It can also cause alterations in regional atmospheric hydrodynamics(Menon et al.,2002;Ramanathan and Carmichael,2008;Bond et al.,2013;Wang et al., 2014).

It has been hypothesized that BC may also impact upon the precipitation patterns over the Indian subcontinent.The precipitation over the past several decades,with increased rainfall in South Asia and increased drought in North Asia,is attributed to regional,post-industrial era emissions(Menon et al.,2002;Hyv¨arinen et al.,2010).Specifically for the Indian climate,increased particulate loadings,which can absorb radiation,are believed to have caused the advancement of rainy periods as well as intensification of the Indian monsoon system(Lau and Kim, 2006).Existence of BC in the atmosphere is,however, a complicated issue,because it can be generated locally as well as transported regionally.Although BC exhibits a short atmospheric lifetime of around one week to 10 days,compared with hundreds of years for carbon dioxide(Jacobson,2002),the continuous impingement of BC due to anthropogenic emissions is of concern for climate change estimations.Furthermore,the BC mass concentration has been increasing in the recent past because of the changing socioeconomic status and rapid industrialization in many parts of the globe,especially in developing countries.High temporal resolution and real-time BC data are useful to estimate the impact of BC on climate,environment,and human health.However,BC measurements at most global locations,including the Indo-Gangetic Plains(IGP),are limited.The sources and mechanisms that affect their concentrations in the air are also mostly unknown.The scarcity of real-time BC data over the IGP is especially important since it is one of the most populated and polluted regions of the world.The common finding of studies carried out in the IGP region is one of high variability in BC concentrations(Tripathi et al.,2005;Safai et al.,2008; Beegum et al.,2009;Tiwari et al.,2009;Hyv¨arinen et al.,2010;Ramachandran and Kedia,2010;Bano et al., 2011;Moorthy and Satheesh,2011;Raju et al.,2011; Praveen et al.,2012).These findings suggest the need for continuous long-term monitoring of BC at multiple locations around the IGP,so as to estimate the impacts of BC effectively and assist with mitigation plans.To the best of our knowledge,this paper is the first report of long-term,in-situ measurements of BC over Varanasi(25.3°N,83.0°E;76 m above mean sea level),which is located in the central part of the IGP region.The major objectives of the present study were to:(1)assess the variability of BC concentrations on different timescales;(2)understand the impact of BC concentrations on columnar aerosol optical properties; and(3)examine the effect of surface meteorology on the distribution of BC over this study region.

2.Site description and experimental setup

2.1Sampling location and meteorological conditions

In-situ measurements of BC mass concentrations were made on the campus of Banaras Hindu University,Varanasi,during the period 1 January to 31 December 2009.The data were gathered as part of a national program called the“Aerosol Radiative Forcing Initiative Network”funded by the Indian Space Research Organization,over the IGP region.Varanasi (25.28°N,82.95°E)is located in the central Ganges

Valley of North India,in the eastern part of the state of Uttar Pradesh,along the left crescent-shaped bank of the Ganges River.The population of Varanasi City was approximately 1.2 million in 2011.The land surrounding the city is very fertile and supports substantial agricultural activity.The city also possesses many small-scale industries.Air quality in Varanasi is very poor due to emissions from badly maintained automobiles and heavily loaded transport vehicles,as well as from small-scale industries,domestic heating,and large-scale construction activities(Kumar et al.,2015; Tiwari et al.,2015a,b).

There are four major seasons:winter(December to next February),the pre-monsoon season(March to June),the monsoon season(July to September), and the post-monsoon season(October and November)(Sharma et al.,2010;Mishra and Shibata,2012). During winter,the wind generally moves eastward with the passage of extratropical low pressure systems (called westerly disturbance),bringing occasional rain or fog over the IGP and snow over the Himalayan region.During the pre-monsoon season,the temperature rises rapidly(up to 49℃)from March to June, when the winds blow from the west.The region receives heavy rainfall during the monsoon season,when depressions and moderate low pressure systems form over the northern Bay of Bengal and move northwestward.

Meteorological variables,such as temperature (TEM),relative humidity(RH),wind direction,wind speed(WS),and rainfall were obtained from an automatic weather station installed in an open field at a distance of about 250 m from the measurement site.Mixing height(MH)and visibility(VIS)data were obtained from the National Oceanic and Atmospheric Administration(NOAA)Air Resources Laboratory Hybrid Single-Particle Lagrangian Integrated Trajectory(HYSPLIT)model using NCEP FNL(final)analysis data.The daily variations of the abovementioned meteorological variables are shown in Fig. 1.During the study period,the mean temperature was approximately 27±2.1℃,varying from 17.6℃(January)to 35.8℃(June).WS varied from 1.72(December)to 3.61 m s−1(May),with a mean of about 2.5±0.8 m s−1.In addition,the means(±standard deviation)of the MH,RH,rainfall,and VIS were 750 (±172.5)m,60(±10)%,864.3(±198.7)mm,and 4.1(±0.6)km,respectively.

Fig.1.Daily variations of meteorological parameters over Varanasi.(a)Temperature(TEM),(b)relative humidity (RH),(c)visibility(VIS),and(d)wind speed(WS).

2.2BC monitoring instrument

BC mass concentrations were measured with an Aethalometer(Model:AE-31;Magee Scientific Co., USA,http://www.mageesci.com)at a flow rate of 4.0 L min−1.This instrument measures the optical absorption at seven wavelengths(370,470,520,590,660, 880,and 950 nm)during a discrete time interval(2 min).The BC mass concentration is calculated assuming that the attenuation is linearly proportional to the amount of BC on the filter,and calculated in accordance with earlier studies(Ramachandran and Rajesh,2007;Tiwari et al.,2009,2013;Srivastava et al.,2012b,2014;Safai et al.,2013)to convert the filter transmittance in inverse mega meters(Mm−1;1 Mm−1=10−6m−1)to BC concentrations.The min-

imum detection limit of the Aethalometer,defined as twice the standard deviation of the noise,is 5 ng m−3(Virkkula et al.,2007).The attenuation coefficientis calculated by multiplying the measured BC mass concentration(µg m−3)at 880 nm and constant cross-sectional value of attenuation(i.e.,

The attenuation coefficient obtained from Eq.(1)is corrected for the particle loading effect,also known as the shadowing effect(R),and the multiple scattering effect(C),in order to obtain the aerosol absorption coefficient(babs;Mm−1):

As a filter-based measurement,the BC measured by the Aethalometer may possess some artifacts due to the loading/shadowing effect,matrix effect and scattering effect(Weingartner et al.,2003; Arnott et al.,2005;Virkkula et al.,2007;Collaud et al.,2010).Weingartner et al.(2003)found that the loading/shadowing effect is more prominent for freshly emitted BC,while it is almost negligible for aged particles.The uncertainties in the estimates of BC mass concentration were around 7%.In another study,Dumka et al.(2010b)reported uncertainties of approximately 5%over the high altitude station at Manora Peak in the central Himalaya.In addition, literature also exists on the uncertainties of BC measurements by the Aethalometer(Weingartner et al., 2003;Sheridan et al.,2005;Corrigan et al.,2006). To obtain information on the sources of the BC particles,the wavelength exponent called the absorption ˚Angstr¨om exponent(AAE,α)was estimated from the absorption at the seven wavelengths using the power law relationship(Kirchstetter et al.,2004)as previously used by Dumka et al.(2010b,2013):

where K is a constant and λ is the wavelength(inµm). The wavelength exponent(α)is estimated by the linear regression of lnβabsand lnλ,which is a measure of the spectral dependence of aerosol absorption.An α value of approximately 1 denotes that the major BC source is from fossil fuel combustion.For biomass burning,it is between 1.5 and 3,and for dust it may be 2 to 3(Bergstrom et al.,2004;Kirchstetter et al., 2004;Sandradewi et al.,2008a,b;Russell et al.,2010).

3.Results and discussion

3.1BC aerosol characteristics over Varanasi

Day-to-day and monthly variability in BC mass concentrations over Varanasi during the entire study period are shown in Fig.2.The daily mean hourly average BC concentration is 8.9±7.0µg m−3,varying from 34.6(7 January 2009)to 2.1(2 July 2009) µg m−3during January–December 2009.BC was relatively higher during the winter months and lower values were found in monsoonal months.The BC concentrations exceeded the average value on approximately 34%of the days.Monthly averages were highest in January(24.7µg m−3),followed by:November(24.6 µg m−3),December(20.4µg m−3),October(11.6µg m−3),February(10.0µg m−3),September(9.2µg m−3),March(8.6µg m−3),June(6.5µg m−3),July (4.8µg m−3),April(4.6µg m−3),August(4.5µg m−3),and May(4.2µg m−3).

Fig.2.Daily,monthly,running mean,and annual mean BC mass concentrations over Varanasi during January–December 2009.

Over the past decade,BC monitoring studies have been conducted in urban,rural,coastal,marine,and high altitude environments over the Indian subcontinent(Table 1)(Babu and Moorthy,2002;Tripathi et al.,2005;Pant et al.,2006;Safai et al.,2007,2008,

2013;Awasthy et al.,2010;Dumka et al.,2010a;Pani and Verma,2010;Ramachandran and Kedia,2010; Tiwari et al.,2013;Bisht et al.,2015;Singh et al., 2015)and other global locations(Table 2),representing different environments and reflecting the variation in magnitude of BC concentrations from different regions.In India,significant spatial variability in BC was reported for higher concentrations(＞10µg m−3)over 33 locations in the IGP region(Moorthy and Satheesh,2011).Lower BC concentrations(approximately 5µg m−3)were found in southern India (Hyderabad,Bangalore,Pune,Trivandrum,etc.).In mountainous regions(Nainital,Mukteshwar,and Sinhagad),concentrations have been found to be around 2µg m−3(Pant et al.,2006;Hyv¨arinen et al.,2009; Dumka et al.,2010b;Raju et al.,2011),except Kullu, where the BC mass concentration was approximately 4.6µg m−3(Kuniyal,2010).The much higher BC mass concentration at Kullu is likely due to vehicular emissions because of the area's popular tourist spots.Meanwhile,BC mass concentrations have been observed to be less than 2µg m−3at several locations, such as Godavari(Nepal),Uto(Finland),Granada (Spain),´Evora(Portugal),and Millan(Italy)–levels that are around six times lower than in Varanasi.However,in Lahore(Pakistan),BC has been measured to be approximately 2.5 times higher than the values reported in the present study(Husain et al.,2007).

Seasonally,BC was higher during the postmonsoon and winter seasons(approximately 18µg m−3)and lower in the pre-monsoon/monsoon seasons (approximately 6µg m−3).Srivastava et al.(2012a) reported that open burning of crop residues is common practice in northern India during the pre-monsoon season,resulting in the transport of BC across the region. During most of the winter,the wind is calm,with lower temperatures,thick fog,and a low MH(Tiwari et al., 2013).Under such conditions,pollutants are not well dispersed,resulting in poor VIS and high local pollutant concentrations(Mohan and Bhati,2009).The frequency distribution of BC was classified into eight different concentration bins(in intervals of 5µg m−3from＜5µg m−3to＞40µg m−3)during the different seasons(Fig.3).The distribution was positively skewed,except for the bin ranging from 5 to 10µg m−3,which contributed approximately 46%of the observed concentrations.During the pre-monsoon and monsoon seasons,about 94%and 90%of the levels of BC mass were＜10µg m−3,whereas only 30% and 15%were＜10µg m−3in the post-monsoon and winter seasons,respectively.During the winter period, which often features deep smog/foggy conditions over the IGP,BC concentrations were found to be largely (26%)within 15–20µg m−3,while only 6%of the con-centrations was within 30–35µg m−3.A very high contribution of BC mass(39%)in the winter was observed within 20–30µg m−3.Similar characteristics were apparent during the post-monsoon season,withthe highest frequency(23%)within 20–25µg m−3. Fewer BC values(1%)were within 35–40µg m−3.

Table 1.The mean mass concentration of BC over Varanasi in the present study compared with other earlier reported concentrations elsewhere in India

Table 2.The mean mass concentration of BC over Varanasi in the present study compared with other earlier reported concentrations from various locations worldwide

Fig.3.Seasonal and annual frequency distribution of BC mass concentrations over Varanasi.

3.2 Diurnal variability of BC mass over Varanasi

The BC concentrations in Varanasi were sorted into four different seasons above mentioned.Their seasonal mean diurnal variations are shown in Fig.4, along with their standard deviations.Different diurnal patterns were apparent during the different seasons.The variations were more pronounced during the winter and post-monsoon seasons compared with the pre-monsoon and monsoon seasons.BC showed a gradual rise at around 0700 LT(local time),peaking at around 0800–0900 LT.Low values were observed in the afternoon(1200–1900 LT),and there was a secondary maximum between 2100 and midnight(0100 LT).This large diurnal variability in BC mass was likely due to the combined effect of the diurnal variation in emissions and surface meteorology,including associated boundary layer dynamics.BC concentrations(maximum)during the pre-monsoon,monsoon,post-monsoon,and winter seasons in the morning hours(0800–0900 LT)were approximately 9,8, 25,and 21µg m−3,respectively,more than two to three times higher than the afternoon hours(1500–1700 LT).For the post-monsoon season,the rise in BC started at 0500 LT and continued until 0800 LT (24.1µg m−3),followed by a sudden drop in concentration until there incremental rises in values from 2000 LT until the next morning.In the morning,the surface inversion coupled to layers above a few hours after sunrise,resulting in the vertical mixing of ground-level pollutants with the free troposphere and causing the surface BC concentrations to suddenly decrease.Increased wind speeds after sunrise also increased mixing,thereby facilitating dispersion and dilution.After 0800 LT,BC emissions-primarily from vehicular traffic-caused increasing BC concentrations.During winter mornings,BC concentrations rose after 0800 LT(21.4µg m−3)and continued until 1000 LT.After that time,they dropped until 1700 LT.The mean BC mass concentrations at night(1800–0600 LT)were approximately twice the daytime values(0700–1700 LT).

At night,the highest mean BC concentration was during the post-monsoon season(26.4µg m−3),followed by the winter(24.3µg m−3),monsoon(8.6µg m−3) and pre-monsoon(8.5µg m−3)seasons.The corresponding night/day ratios were 2.4,1.8,2.4,and 1.3, respectively.The BC concentration reached its minimum between 1400 and 1600 LT in every season due to the increased MH.After 1800 LT,the evening rush hour commences,as well as an increase in cooking activity and domestic heating use.Also,the surface inversion begins to form trapping pollutants.Subsequently,the BC mass concentration starts increasing from the evening onwards.The earlier onset of the morning rise and the later onset of the evening accumulation during the pre-monsoon season,compared with winter,correspond to the earlier sunrises and later sunsets in the pre-monsoon season.

Fig.4.Seasonal diurnal variability of BC mass concentrations over Varanasi.

Simpson and McGee(1996)described the marked effect of the local climate on the diurnal variation of pollutants due to fumigation effects,which greatly increase daily averages,particularly during winter.The diurnal BC variations in Varanasi have been observed at a variety of other locations,including a suburban site in Maryland,USA(Chen et al.,2001),the tropical coastal station of Trivandrum(Babu and Moorthy, 2002),and other inland sites in India(Sreekanth et al., 2007;Tiwari et al.,2013;Bisht et al.,2015).

3.3 BC mass concentration in relation to aerosol optical properties

Being highly light absorbent,BC particles affect various optical properties,such as aerosol optical depth(AOD),the˚Angstr¨om exponent(AE),and single scattering albedo,which are responsible for aerosol radiative forcing(Pandithurai et al.,2008).AE is a good indicator of aerosol particle size and largely depends on the aerosol size distribution.It measures the comparative contributions of coarse-and finemode particles,where higher values of AE represent a higher contribution of fine-mode particles and lower values represent a higher contribution of coarse-mode particles(Pandithurai et al.,2008;Srivastava et al., 2011a;Singh et al.,2014).

Fig.5.Seasonal values of(a)BC and(b)aerosol optical depth(AOD 550 nm)and˚Angstr¨om exponent(AE)during January–December 2009.

During the study period,AOD and AE were retrieved from MODIS(http://gdata1.sci.gsfc.nasa.gov/ daac-bin/G3/gui.cgi?instance−id=MODIS−DAILY−L3).The mean AOD(550 nm)was 0.65±0.3,varying from 0.39±0.2(February)to 1.01±0.4(August). The mean AE was 0.83±0.3,varying from 1.19±0.4 (January)to 0.56±0.1(July).Its seasonal variability

is depicted in Fig.5.Prasad et al.(2005)reported an AOD over Varanasi during the monsoon period that was similar(＞0.6)to the present study.The monthly AOD increased from April(0.43±0.2)to June(1.01 ±0.4),and decreased in July(0.74±0.3),followed by a sudden increase in August(0.98±0.2).The increase may be due to a long dry period/break phase of the monsoon during this month.AOD increased further in October(0.44±0.2)and continued increasing until December(0.77±0.3).

AOD represents the attenuation of direct solar irradiance.It may be higher in situations where either coarse-or fine-mode particles,or both,increase.Along with AE,delineation of a higher contribution of fine-or coarse-mode particles may be deduced.The AOD values were negatively correlated(–0.27)with AE and,in general,the higher AOD values were associated with lower AE during the pre-monsoon season.This situation suggests that the dominance of coarse-mode particles was likely due to transported dust(Singh et al.,2005;Srivastava et al.,2011a).Conversely,high AODs were associated with high AE values during the winter and post-monsoon periods,indicating the dominance of fine-mode particles.Fine particles are mostly produced by enhanced anthropogenic activities(Srivastava et al.,2011b).Regression analysis between the AE and BC daily data revealed a significant positive correlation(0.55),indicating that fine particles, including BC,dominated during the study period.

Reddy and Venkataraman(2002)reported that fuel wood and crop wastes were the primary contributors to biomass-based BC emissions in northern India.These estimates were,however,based on annual average emissions.It has been shown that,during November and next March,anthropogenic source contributions exceed 70%of the measured AODs in and around India(Ramachandran,2004).This high anthropogenic influence on AOD is due to the man-made submicron aerosols from local and regional sources. Regional transport is greater during the winter season.AE showed a decreasing trend from January(1.24 ±0.30)to May(0.55±0.10),and then increased from June onwards,up to a value of 1.20±0.40 in December.

Seasonally averaged AE(Fig.5)was observed to be higher during winter(1.16±0.20),followed by the post-monsoon(0.97±0.20),monsoon(0.76±0.10), and pre-monsoon(0.58±0.10)seasons.Higher BC and AE during the post-monsoon and winter seasons indicate the dominance of fine particles in this location.A long-term analyses in Delhi by Lodhi et al. (2013),and episodic observations by Pandithurai et al. (2008),showed the lowest AE values in June(approximately 0.4)and highest values(approximately 1.0) in winter,gradually decreasing from winter months to a peak during the pre-monsoon months because of the increasing importance of soil-derived coarse-mode particles(dust transport from the nearby Thar desert) (Singh et al.,2005)along with the possible mixing of boundary layer particles with free tropospheric material caused by surface heating during the pre-monsoon period.The observation in Varanasi confirms the dominance of fine-mode particles,largely BC,during the winter and post-monsoon seasons.

3.4 Spectral aerosol absorption characteristics

BC particles are largely produced by the burning of fossil fuels from transport,industrial activity,the power sector,etc.,and the burning of biomass/biofuels (home heating/cooking,agricultural burning,and wild fires).As noted above,the AAE can help to identify the possible sources of BC.The AAE values were estimated from BC measurements at seven discrete wavelengths,and its variability is depicted in Fig.6.The mean value of AAE in Varanasi was 1.02±0.08,varying from 0.73 to 1.32 over the observation period,indicating a strong influence of fossil fuel combustion. Seasonal AAE was generally higher during the postmonsoon and winter seasons,and lower in the premonsoon/monsoon season,following the order:postmonsoon season(1.11±0.10),winter(1.02±0.10), monsoon season(1.01±0.10),and pre-monsoon season(0.98±0.10).These results suggest fossil fuel combustion as the major source of BC(Soni et al., 2010)during the pre-monsoon and winter periods in Varanasi. In Delhi,a similar AAE value(1.03±0.09)was observed with relatively similar seasonal values(Ganguly et al.,2005).However,in central India,

Ganguly et al.(2005)reported the AAE value as 1.52, suggesting biofuel/fossil fuel emissions.Bond(2001) reported values of AAE between 1 and 2.9 and suggested that the BC source is residential biofuel burning.Aruna et al.(2013)reported a value of AAE around 1.1(varying from 0.9 to 1.1)for a tropical location near Chennai.The diurnal variability of AAE for different seasons was examined to separate the possible BC sources in Varanasi(Fig.7).Two major peaks were observed:morning(0800–1100 LT)and evening (2000–2300 LT),confirming the combined effect as the burning of fossil fuels and biomass during the morning and evening period.In addition,as a rural environment is very close to the city,the people burn anything (e.g.,wood,cow dung,leaves,crop residue,etc.)for heating purposes and for cooking food(during morning and evening),which produces large amounts of BC.

Fig.6.Daily variation of absorption˚Angstr¨om exponent, along with the running and annual mean,over Varanasi.

Fig.7.Diurnal variations of absorption˚Angstr¨om exponent over Varanai during January–December 2009.

3.5Influence of meteorology on BC

The impact of changes in surface meteorological conditions on the BC concentrations was investigated.Regression analysis between daily BC concentrations and surface meteorological parameters in Varanasi during the study period is presented in Fig.8.WS yielded a negative correlation coefficient (–0.50),indicating that increasing WS increased the dilution of local emissions.WS was found to be highest during the pre-monsoon season(11.1 km h−1)and lowest during winter(7.0 km h−1),and the corresponding mean BC values were 6.64 and 18.61µg m−3, respectively.A similar relationship was observed in Ahmadabad(Ramachandran and Rajesh,2007)and Delhi(Srivastava et al.,2012a).Cao et al.(2009) observed a strong inverse relationship(–0.66)between BC and WS at Xi'an,China,from September 2003 to August 2005,implying a local origin for BC.These studies suggested important contributions from locally generated BC that accumulate at low WS.Sharma et al.(2002)reported that low WS led to poorer dispersion such that BC remained concentrated around the emissions source.They also found that higher WS was the driving force for dilution of the BC concentration near the surface due to constant traffic density.These dilution conditions are a general feature over urban and suburban sites,and largely show negative correlation between BC and WS.The strength of the correlation between WS and BC is an indicator of the proximity of BC sources to the measurement site. Low correlation coefficients suggest that BC originates from distant sources.The mean monthly temperature in Varanasi was approximately 27℃.It was lower(approximately 19℃)during winter and higher(approximately 32℃)during the pre-monsoon period.

Low correlation coefficients suggest that BC originates from distant sources.The mean monthly temperature in Varanasi was approximately 27℃.It was lower(approximately 19℃)during winter and higher (approximately 32℃)during the pre-monsoon season. An inverse relationship(–0.32)was observed between temperature and BC(Fig.8),similar to central and southern parts of India at Ahmedabad(Ramachandran and Rajesh,2007)and Trivendrum(Babu and

Moorthy,2002).The high BC emissions during the late post-monsoon and winter seasons,from agricultural burning and biomass burning for heating and cooking,along with the lower MH at that time,led to the negative correlation between temperature and BC. During the study period,the seasonal MHs were 1277, 628,556,and 370 m during the pre-monsoon,monsoon,winter,and post-monsoon seasons,respectively. The MH was shallower during the post-monsoon and winter seasons over Varanasi,resulting in the trapping of locally emitted pollutants and higher BC concentrations.The increase in surface temperature with associated convective activity during the pre-monsoon season improved the dispersion,causing lower surface BC concentrations.Given the volume into which groundlevel emissions are dispersed,a negative relationship between BC and MH was observed(–0.66).Sloane and White(1986)suggested that the loss of VIS is an easily measured manifestation of air pollution,arising from the loss of contrast between the object and the background and attenuation of the light signal from the object due to scattering and absorption of light by fine particles and other atmospheric pollutants.Regression analysis between VIS and BC yielded a large negative correlation(–0.84).Horvath(1995)suggested the absorption of sunlight by BC contributes to reduce VIS in polluted regions.Xu et al.(2012)reported a negative effect of BC on VIS,with a significant correlation of–0.79 between atmospheric VIS and optical properties(σscatand σabs),at an urban site in Shanghai,China.

Fig.8.Correlation coefficients between BC and the meterological parameters of MH,TEM,RH,VIS,and WS during the study period over Varanasi.

A positive correlation between BC and RH was 0.41.RH is high during the monsoon season,and low during the post-monsoon and winter seasons when there are higher emissions;thus,this correlation likely reflects the seasonal variation as discussed previously.

Regression analysis between BC and annual precipitation was studied and a significant negative correlation(–0.35)was found between them(annual rainfall of 876 mm in 2009).Rainfall is high during the monsoon season,so part of this negative correlation may again reflect the winter/monsoon seasonal differences already presented.In addition,the precipitation will remove some particles from the air through washout. A similar correlation(–0.35)between BC and rainfall was also reported in Ahmadabad(western India) (Ramachandran and Rajesh,2007).A strong correlation(–0.74)between these variables at the Trivandrum coastal station(southern India)was reported because of the heavy rainfall(Babu and Moorthy,2002). To analyze the transport of BC,the surface wind direction data along with the BC mass concentration over Varanasi was separated and five major wind categories were identified:east(comprising northeasterly, east-northeasterly,easterly,and north-northeasterly wind);west(comprising westerly,west-southwesterly, south-southwesterly,and southwesterly wind);north (comprising northerly,north-northwesterly,northwesterly,and west-northwesterly wind);south(comprising southerly,southeasterly,east-southeasterly, and south-southeasterly wind);and calm(meaning no wind).The observed percentage frequencies of wind during the study period resulted in the order:calm (38%),west(31%),east(18%),south(10%),and north(3%);however,the BC mass concentrations were highest(38%;18.1±6.9µg m−3)during calm conditions,confirming that the major source of BC was localized.

4.Summary

In-situ measurements of BC mass concentrations were made in Varanasi(an urban environment)in the central IGP region of India.For the year 2009,the results suggest the importance of BC at this location

and the need for extensive mitigation activities to reduce the concentrations.The annual mean mass concentration of BC was 8.92±6.98 g m−3,with 34% of the days exceeding this average value.The highest (approximately 46%)mass BC frequency was in the range 5–10µg m−3;however,during the post-monsoon season,the higher contribution(23%)was between 20 and 25 g m−3.The BC mass showed gradual buildup at around 0700 LT,peaking at around 0800–0900 LT,with low values in the afternoon(1200–1900 LT) and secondary maxima between 2100 LT and midnight (0100 LT).Overall,the nighttime concentrations of BC were approximately two times higher than that during the day,due to the impact of boundary layer conditions.AE was significantly positively correlated (0.55)with surface BC,indicating the impact of BC on columnar aerosols.The estimated mean AAE was 1.02±0.08,indicating the combustion of fossil fuel sources.The significant negative correlation(＞0.4) between BC mass and meteorological parameters indicated a pronounced effect on atmospheric dynamics of the enhancement of BC mass over this region.The highest concentration(38%;18.1±6.9µg m−3)of BC mass was found under calm wind conditions.

In view of the above,urgent action is needed in reducing the current level of BC in Varanasi.Concentrations of BC in the atmosphere over Varanasi are much higher than that in other parts of India and elsewhere around the globe.

Acknowledgments.The authors are grateful for the financial support of the ISRO-ARFI program. They also gratefully acknowledge the NOAA Air Resources Laboratory for the provision of the HYSPLIT transport and dispersion model and ready website(http://www.arl.noaa.gov/ready.html)used in the current study.Also acknowledged are the data from the Giovanni online data system,and the comments from the anonymous reviewers.

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∗Corresponding author:mksriv@gmail.com.

©The Chinese Meteorological Society and Springer-Verlag Berlin Heidelberg 2015

(Received April 30,2015;in final form September 22,2015)