Variation between near-surface and columnar aerosol characteristics during the winter and summer at Delhi in the Indo-Gangetic Basin

Journal of Atmospheric and Solar-Terrestrial Physics 77 (2012) 57–66

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Variation between near-surface and columnar aerosol characteristics during the winter and summer at Delhi in the Indo-Gangetic Basin A.K. Srivastava a, Sachchidanand Singh b,n, S. Tiwari a, V.P. Kanawade c, D.S. Bisht a a

Indian Institute of Tropical Meteorology (Branch), Prof. Ramnath Vij Marg, New Delhi, India Radio & Atmospheric Sciences Division, National Physical Laboratory, CSIR, New Delhi, India c Department of Chemistry, Kent State University, Kent, OH 44240, USA b

a r t i c l e i n f o

abstract

Article history: Received 29 July 2011 Received in revised form 5 November 2011 Accepted 20 November 2011 Available online 30 November 2011

Aerosol characteristics were studied over Delhi, a typical urban station in the Ganga basin in Northern India, during two contrasting weather conditions: winter and summer, to explain the changes in columnar and surface aerosol characteristics with the help of ground based measurements and CALIPSO satellite data. The near-surface mean aerosol mass concentrations of PM10 and PM2.5 ( 7 standard deviation) were observed to be  200 ( 7 24) and 118 ( 733) mg m  3, respectively, during the winter and  168 ( 7 31) and 55 (7 12) mg m  3, respectively, during the summer. PM2.5 was found to be about two times higher than the PM10 concentration during the winter period. Aerosol mass size distribution showed bi-modal nature during both the periods, with relative dominance of fine-particle mass concentrations during the winter, having low Reff value (0.63 7 0.05 mm) and coarse-particle mass concentrations during the summer, having large Reff value (1.52 7 0.60 mm). The concurrent measurement of columnar aerosol optical depth (AOD) showed high values (4 0.60 at 500 nm) during both the ˚ ¨ exponent (a) over the station, however, also suggests relatively large contribuperiods. The Angstr om tion of fine-mode particles during the winter (a  1.02) and coarse-mode dust particles during the summer (a  0.51). The observed features in the surface and columnar measured aerosol characteristics during two different seasons are explained using the vertical winds coupled with the vertical profile of aerosols. & 2011 Elsevier Ltd. All rights reserved.

Keywords: PM10 PM2.5 Aerosol optical depth Vertical winds (omega) Aerosol backscatter

1. Introduction Atmospheric aerosols influence the Earth-atmosphere system directly by scattering and absorbing sunlight (Schwartz et al., 1995) and indirectly by changing radiative properties and modifying microphysical properties, and lifetime of clouds (Twomey, 1991). They are considered to be one of the largest uncertain components, estimating the radiative forcing in the assessments of global climate change (IPCC, 2007). Recent research reveals that assessing aerosol effects on climate requires knowledge of not only the regional and global distribution of aerosol amount, but also the various properties of aerosols such as physical (e.g. size distribution), chemical (e.g. composition) and optical properties, including especially the absorption (Myhre, 2009 and references therein). The Indo-Gangetic Basin (IGB) region, encompassing most of the northern part of India, is the world’s most populated river basin where more than 700 million people are living and exposed

n

Corresponding author. Tel.: þ911145608243; fax: þ 911145609310. E-mail address: [email protected] (S. Singh).

1364-6826/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jastp.2011.11.009

to the enormous pollution from various anthropogenic and natural sources. The region extensively experiences two extreme weather conditions every year such as winter and summer and is of particular research interest due to large seasonal heterogeneity in aerosol characteristics including aerosol load during these periods (Dey et al., 2004; Jethva et al., 2005; Dey and Di Girolamo, 2010; Srivastava et al., 2011a). The heterogeneity in various aerosol characteristics may be different for near-surface and columnar measured aerosol characteristics, which is highly associated with the surface synoptic conditions over the region, apart from the variability in aerosol emission sources in and around the region. During the winter, low surface convection occurs due to low surface temperature and high pressure system persists over the region, which results in shallow atmospheric boundary layer (ABL) condition due to its highly stable nature (Stull, 1999). On the other hand, during the summer, strong surface convection occurs due to high surface temperature and low pressure system over the region, which results in deep ABL condition due to its highly unstable nature. Studies undertaken so far, over the IGB region, indicate large aerosol emissions from different anthropogenic and natural sources in and around the region (Reddy and Venketaraman, 2002a, 2002b; Ramanathan

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and Ramana, 2005; Rengarajan et al., 2007; Tiwari et al., 2009; Ram and Sarin, 2010), which are found to have considerable radiative impacts over the region (Srivastava et al., 2011b). Most of the studies in the IGB, as mentioned earlier, are either from winter or of summer/pre-monsoon periods and study separately the surface and/or columnar characteristics of aerosols. It is for the first time (in terms of the wide spectrum of measurements), we studied not only the changes in aerosol characteristics during the winter and summer periods at a typical station in Delhi over the IGB, but also explained possible association between near-simultaneously measured surface and columnar aerosol characteristics with vertical wind (omega) and aerosol vertical profiles. In addition, we have also reported in detail the general aerosol characteristics over the station during winter and summer periods.

2. Experimental site and measurement details The experimental site, Delhi (28.61N, 77.21E and  240 m above mean sea level), is the capital city of India and located in the western part of the IGB in the northern India. Delhi is situated near the Thar Desert region in western India, which is the single largest contributor to the mineral dust aerosols over the northwest Indian region (Todd et al., 2007). Delhi is one of the highly polluted mega cities in Asia, which experiences two extreme weather conditions every year such as winter and summer having different scenario in aerosol characteristics. Delhi, apart from being a major source region for various aerosols, is mainly bordered by densely populated and industrialized areas from where different aerosol species such as soil dust, soot, nitrate, sulfate particles and organics are produced and thus making it aerosol hot spot (Tiwari et al., 2009; Singh et al., 2010). Although vehicular and industrial activities are the largest contributor to the total particulate matter in the atmosphere of Delhi, significant contributions from other sources like soil originated particles and re-suspended dust associated with the strong winds and construction activities also enhance the ¨ ¨ total particulate matter (Monkk onen et al., 2004; Srivastava and Jain, 2007). The experimental site is, however, located close to large green area in the heart of Delhi and no major industries are located within 5 km radius. Further, the sampling was done 15 m above the ground to ensure the free flow of the air and to avoid any local contaminants. In the present study, near-surface aerosol samples were collected using single stage PM10 and PM2.5 aerosol samplers, which provide information about the aerosol mass concentrations of sizes up to 10 and 2.5 mm, respectively. Both aerosol samples were collected on Whatmann, Teflon Micro fibre filter papers (2 mm PTFE) of the size 46.2 mm using APM 550 and APM 541 samplers (Envirotech Pvt. Ltd., India) for PM2.5 and PM10, respectively. The sampling cycle was for 24 h with a flow rate of one cubic meter per hour, collecting sufficient mass of aerosols. The filter papers, used for aerosol sampling, were subject to 24 h desiccation before and after the sampling, to remove moisture content of the filter papers. The desiccated filter papers were weighted using electronic microbalance (Model GR202, A&D Company Ltd., Japan) with 0.01 mg resolution. The particle concentrations were determined gravimetrically by the difference in their weights before and after the sampling. More details about the sampling are available elsewhere (Tiwari et al., 2009). Measurements of near-surface particle sizes using an optical aerosol analyzer, also known as optical particle counter (OPC, Model 1.108 from GRIMM, Germany) have been carried out over the station on regular basis. It is a portable unit, used for the continuous measurements of different size of particles in the air in 15 channels in the size range between 0.3 and 20 mm

(http://www.grimmaerosol.com). The instrument enables realtime measurements of ambient aerosol mass or number concentrations under varying environmental conditions (Tiwary and Colls, 2004). Measurements were carried out by operating it in the mass mode with an operational flow rate of 1.2 l min  1. This instrument uses the laser scattering technology for single particle counts, whereby a semiconductor laser serves as the light source. The scattered signal from the particle passes through the laser beam and is collected at approximately 901 by a mirror and transferred to a recipient diode. The major limitation of the instrument is that it cannot be operated during the foggy/hazy periods when ambient relative humidity (RH) becomes higher than 75%. To overcome this limitation, instrument is placed inside a hut and the inlet passes through a jar containing silica gel, which absorbs moisture coming along with the particulate matter. In addition, columnar aerosol characteristics (i.e. aerosol optical depth, AOD) were also measured with recently calibrated MICROTOPS-II sunphotometer (Solar Light Co., USA) at five discrete wavelengths such as 340, 500, 675, 870 and 1020 nm (full width at half maximum, FWHM: 72–10 nm). The MICROTOPS-II observations are susceptible to filter degradation errors and needs periodic filter calibration (Morys et al., 2001). The pointing accuracy of the instrument is better than 0.11 and long-term stability of the filter is better than 0.1 nm year  1. The accuracy of measurements for precision and consistency of the MICROTOPS-II instruments are discussed in detail by Srivastava et al. (2006), which was found within 1.8%. The spectral variation of AOD provides useful information on columnar size distribution and ˚ ¨ can be best represented by Angstr om power law relationship, ˚ ¨ (1964): given by Angstr om

ta ðlÞ ¼ bla

ð1Þ

where ta(l) is the aerosol optical depth at wavelength l (in mm), b is the turbidity coefficient, indicating aerosol loading, which equals ˚ ¨ exponent, which ta at l ¼1 mm and a is widely known as Angstr om is a good indicator of aerosol particle size (Eck et al., 1999). The ˚ ¨ exponent (a) depends on the size distribution of aerosols Angstr om and is a measure of the ratio of coarse- to fine-mode aerosols, with higher values representing abundance of fine-mode aerosols and lower values representing increased coarse-mode aerosols, whereas, b depends on the total aerosol loading in the atmosphere (Satheesh and Moorthy, 1997; Srivastava et al., 2008). Aerosol sampling with 24 h sampling cycle, using PM10 and PM2.5 samplers, was done once a week during winter (December, January and February, hereinafter referred as DJF) and summer (March, April and May, hereinafter referred as MAM) for the year 2007–2008. The other aerosol measurements like, aerosol size distribution and columnar AOD have been carried out simultaneously on a regular basis. As the columnar AOD measurements can be done only during day time, AOD was measured between 09:00 and 16:00, every half an hour. The size distribution data by OPC were obtained every minute and averaged for the period for which PM samplings were done. The near-surface aerosol measurements were carried out inside the campus of the Indian Institute of Tropical Meteorology, Delhi, which is  250 m away from columnar aerosol measurement site at the National Physical Laboratory (NPL), Delhi. Also, the vertical profile of omega over Delhi during the measurement periods was obtained from reanalysis data of National Center for Environmental Prediction (NCEP) and National Center for Atmospheric Research (NCAR) whereas the vertical aerosol distribution was obtained from Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP)—a space-borne lidar instrument onboard the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellite.

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Synoptic meteorology (wind pattern, air temperature and specific humidity) over the IGB region (measurement station is shown by a star) along with its surroundings is shown in Fig. 1a and b for winter and summer periods, respectively. The European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis monthly data of weather parameters such as wind, air temperature and specific humidity at 850 hPa pressure level were used to study the synoptic meteorological conditions over the station. In both the figures, winds are shown with arrows pointing towards the wind direction, where length of arrows defines the magnitude of wind speed (in ms  1), line contour represents air temperature (in 1C) and shaded color contour represents specific humidity (in kg kg  1) (showing blue color for low and red color for high magnitude). Results reveal that the study region over IGB during the winter period is relatively drier than during the summer. The persistence of low temperature and the westerlies (with low intensity) can be seen over the station during the winter whereas during the summer, relatively high temperature with intense southwesterly winds was observed to dominate over the station. These winds are

found to pass through arid regions of the western India (particularly from the Thar Desert) and bring dry air mass over the station (Pandithurai et al., 2008; Srivastava et al., 2011a, c). The general aerosol characteristics over the IGB region have been shown in Fig. 2 as mean AOD values at 550 nm for (a) winter and (b) summer in color codes, obtained from the Moderate Resolution Imaging Spectroradiometer (MODIS). The location of measurement site (Delhi) is marked by a star. Large spatial heterogeneity in AOD can readily be noticed over the IGB region during both winter (Fig. 2a) and summer (Fig. 2b) periods. Relatively large magnitude in AOD was observed throughout the IGB region, including at Delhi during the summer, which was mainly due to frequent occurrence of dust storms over the Thar Desert region that caused large amount of dust particles to be transported over the station (showing the highest AOD). However, large AOD during the winter is confined mostly over the eastern part of IGB. Results obtained, although, include the impacts of aerosol emissions from various natural and anthropogenic sources and the prevailing meteorology over the region, it also encourages to further investigate the plausible causes and impacts over the radiation budget as well as on weather and climate during these two extreme weather conditions.

Fig. 1. Synoptic meteorological conditions derived from ECMWF over the entire IGB region during (a) winter (mean from December 2007 to February 2008) and (b) summer (mean from March to May 2008). The location of measurement site Delhi (28.61N, 77.21E) is marked by the star.

Fig. 2. MODIS AOD values at 550 nm for (a) winter and (b) summer over the entire IGB region. The location is marked by the star.

3. Synoptic conditions and aerosol characteristics over IGB

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4. Results and discussion Various microphysical and optical characteristics of aerosols have been studied over Delhi during the winter when intermittent foggy, hazy and clear-sky conditions prevailed over the station along with the highly stable atmospheric conditions. Also, the above characteristics of aerosol were studied during the summer when station is highly influenced by the transported Desert dust aerosols along with the highly convective and/or unstable atmospheric conditions. In this section, we have described the important features observed in the characteristics of near-surface and columnar aerosols measured during these two weather conditions along with their possible association with each other. 4.1. Near-surface aerosol characteristics 4.1.1. PM10 and PM2.5 measurements We examined PM10 and PM2.5 daily aerosol mass concentrations measured at Delhi from December 2007 to May 2008, comprising the winter and summer periods. During the winter, PM10 and PM2.5 values were found to vary between 98 and 341 mg m  3 (with mean value of 200 724 mg m  3) and from 17 to 235 mg m  3 (with mean value of 118 733 mg m  3), respectively. On the other hand, these values were found to be between 83 and 301 mg m  3 (with mean value of 168732 mg m  3) and 25 and 116 mg m  3 (with mean value of 55 712 mg m  3), respectively, for PM10 and PM2.5 during the summer. The monthly mean

Fig. 3. Monthly mean variations in PM2.5, PM10 and PM10  2.5 aerosol mass concentrations along with the annual Indian National Ambient Air Quality Standard (NAAQS) levels of PM2.5 and PM10. Vertical bars show standard deviations from their monthly mean values.

values of PM10, PM2.5 and PM10  2.5 along with the annual Indian National Ambient Air Quality Standards (NAAQS) for PM10 (solid open blue box) and PM2.5 (dotted open red box) are shown in Fig. 3, and are also given in Table 1. About two times increase in PM2.5 (  54%) with respect to PM10 (  16%) was observed during the winter as compared to the summer. Result suggests an enhanced production of secondary aerosol particles over the station during the winter, which largely associated with the enhanced anthropogenic activities coupled with the prevailing meteorology over the station (Tiwari et al., 2009). The other possible reason, which may be responsible for the enhancement in PM2.5 mass concentration over the station could be the hygroscopic nature of these particles and their likely growth due to condensation of water vapor and other vapor phase species coupled with the prevailing meteorology over the station during the winter period. During the winter, ABL becomes shallow and highly stable in nature due to low surface convection, which does not allow aerosol particles to disperse much into the atmosphere and get trapped within the boundary layer near to the surface. On the other hand, during the summer, an increase in the vertical extension of the ABL occurs due to the fact that strong convection persists near the surface, which results in an increase dilution of surface level aerosol concentrations. The observed levels of monthly mean PM10 and PM2.5 were found to be higher than their annual NAAQS levels (PM10 ¼60 mg m  3 and PM2.5 ¼40 mg m  3) during the winter. However, during the summer, level of PM2.5 was found to be by and large equal to their annual NAAQS level. Further, if we compare the same with their daily NAAQS levels (PM10 ¼100 mg m  3 and PM2.5 ¼60 mg m  3), we find that the observed monthly mean of PM10 is relatively higher than the daily NAASQ level during both the seasons whereas the PM2.5 values are above the NAASQ level during the winter and nearly comparable during the summer. This may be largely due to the higher contribution of dust aerosols, which is one of the major natural sources over the station during summer (Singh et al., 2005; Pandithurai et al., 2008; Srivastava et al., 2011c) along with the thermodynamic conditions in the planetary boundary layer, which influence the pollutants dispersion. The mass concentrations of particulate matter of size ranges between 10 and 2.5 mm (PM10 2.5), vary according to the variations in PM10 and PM2.5, and is also shown in Fig. 3. The monthly mean variations of PM10 2.5 provides a clear picture of the fraction of coarse aerosol particles over the station, which shows relatively higher fraction during the summer and lower fraction during the winter months. To understand the contribution of PM2.5 in PM10, the monthly mean ratios of PM2.5/PM10 are also given in Table 1. The mean ratio was found to be  0.60 (70.17) during the winter, which clearly indicates the large fraction of fine particles in PM10. Result confirms the previous assumption that these fine particles are developed due to low level inversion and large emissions from various anthropogenic sources, which include biomass burning in open fields, domestic fuel in rural settings, emissions from thermal power plants, fossil fuel burning and vehicular exhausts (Venkataraman et al., 2005; Tripathi et al., 2006). On the other

Table 1 Monthly mean surface measured aerosol parameters over Delhi during the winter and summer periods. Months

PM2.5 (lg m  3)

PM10 (lg m  3)

PM10  2.5 (lg m  3)

PM2.5/PM10

Reff (lm)

Dec 2007 Jan 2008 Feb 2008 Mar 2008 Apr 2008 May 2008

147.42 82.85 122.38 61.87 41.40 61.37

217.96 207.69 172.72 202.86 140.12 162.21

70.54 124.84 50.34 141.00 98.72 100.83

0.68 0.40 0.71 0.31 0.30 0.38

0.63 0.58 0.67 0.95 1.48 2.14

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hand, during the summer, the ratio was found to be  0.33 ( 70.05), which is about half of the value found during the winter. Result indicates the lower fine particle fraction in PM10 and suggests the relative dominance of coarse dust particles over the station, which are mostly transported from the adjacent Desert regions apart from the dust raised from local soils. 4.1.2. Aerosol mass size distribution Aerosol size distribution is an important parameter in radiative forcing estimation (Ricchiazzi et al., 1998), which was measured from OPC in 15 channels during the winter and summer periods. Ensemble of monthly mean aerosol size distribution is shown in Fig. 4, which shows bi-modal size distribution during both the periods. The seasonal mean size distribution of aerosols during the winter and summer is shown in the inset of Fig. 4. Fine-mode particle mass concentration was found to be relatively high during the winter period as compared to the summer because of the existence of shallow and stable ABL and possible contribution to pollutant loads from anthropogenic activities mainly biomass burning. The increased fine-mode contribution may be due to the increase in the accumulation mode (size range with the longest life-time) due to weakened removal processes during the winter period (Hyv¨arinen et al., 2010). Recently, Das et al. (2008) have also found an increase in aerosol mass concentration in both fine- (nucleation, accumulation) and coarse-modes (relatively large increase in fine-mode as compared to coarse-mode) during the winter period over Hissar—another polluted urban region situated  200 km northwest of Delhi in the IGB region. On the other hand, during the summer, coarse-mode particle mass concentrations were found to be slightly higher as compared to those observed during the winter, which is mainly due to relative dominance of coarse dust particles (transported from the adjacent Desert regions). Relatively large variability (in term of standard deviation) in aerosol mass concentration in each size channel was observed during the summer as compared to that during the winter. The effective radius (Reff) of aerosol is a useful measure of average aerosol particle size in a poly-disperse aerosol population, which can be calculated from the OPC measurements using the formula: R di d V c ðDp Þ Reff ¼ R d1 ð2Þ i d1 ac ðDp Þ

Fig. 5. Monthly mean aerosol effective radius (Reff).

where Vc (Dp) is the volume concentration of particles as a function of particle diameter (Dp), ac (Dp) is the area concentration of particles as a function of particle diameter. The monthly mean values of Reff were calculated and shown in Fig. 5 (also given in Table 1). It showed a minimum of  0.58 mm during January (winter) and a maximum of  2.14 mm during May (summer), increasing systematically. The Reff values were found to be nearly constant during the winter months, and were found to increase about three folds from the month of February to May, showing an impact of coarse-size dust particles over the station with the advancement of summer. The seasonal mean Reff values of aerosol over Delhi during the winter and summer periods are found to be  0.63 ( 70.05) and 1.52 ( 70.60) mm, respectively. Results clearly show dominance of fine-size particles during the winter and coarse-size during the summer. Aerosol loading over Delhi are mostly contributed either by transported mineral dust or by various anthropogenic aerosol sources, in which the important one being vehicular exhausts, smokes from various kinds of factories are common during the summer and winter periods (Singh et al., 2010). The major anthropogenic factors, which contribute significantly to the production of fine-mode aerosols in and around the observing station, during the winter, are disposal of waste by burning, such as burning of dry leaves, wood, paper or other solid wastes by population dwelling in the city slums, to keep themselves warm during cold winter nights (Ganguly et al., 2006) combined with the ability of the basin region to trap these pollution within the ABL during winter period. The anthropogenic and industrial particles contribute to the fine particle mode (particle diameter Dp o2 mm) and dust particles mainly contribute to the coarse particle mode (Dp Z2 mm). Using the above conditions based upon the diameter of the particle, aerosol mass concentration was divided into fine- and coarse-modes and shown in Fig. 6. As expected, fine-mode aerosol mass concentration was found to be relatively higher (  25%) than the coarse-mode during the winter period while during the summer, coarse-mode aerosol mass concentration was found to be  62% higher (more than two fold) than that of the fine-mode. 4.2. Columnar aerosol characteristics

Fig. 4. Monthly mean aerosol mass size distributions (mean mass size distribution during the winter and summer is shown in the inset).

˚ 4.2.1. Aerosol optical depth and Angstr¨ om exponent Monthly mean values of AOD (at 500 nm) along with a (in the wavelength range of 340–870 nm) are given in Table 2 and are

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Fig. 6. Monthly mean fine- and coarse-mode aerosol mass concentrations.

Table 2 Monthly mean columnar measured aerosol parameters over Delhi during the winter and summer periods. Months

AOD500

a340–870

Reff,c (lm)

Dec 2007 Jan 2008 Feb 2008 Mar 2008 Apr 2008 May 2008

0.66 0.78 0.58 0.75 0.63 0.82

1.04 1.10 1.08 0.68 0.46 0.37

0.46 0.44 0.48 0.79 0.96 1.05

resulting in very low ventilation coefficients (Bano et al., 2011). In some of the earlier studies, monthly mean AOD at Delhi was reported as 0.9170.48 during the winter (Ganguly et al., 2006), whereas it was found to be about 0.7770.29 at Kanpur (Tripathi et al., 2006) and 0.4670.18 at Hissar (Das et al., 2008). On the other hand, corresponding a value shows relatively higher magnitude (about two fold) during the winter (1.02 70.07) than the summer (0.5170.16), indicating high spectral variability in AOD during the winter as compared to that during the summer. Relatively large variations in the magnitude of a (in term of standard deviation from the monthly mean values) was observed during the summer, which suggest large mixing of fine- to coarsemode particles due to enhanced convective activities over Delhi. Results suggest that the high AOD over Delhi during the winter is largely influenced by accumulation or fine-mode particles from various anthropogenic sources as discussed earlier. However, high AOD during the summer is mainly influenced by the coarse-mode dust particles transported from the Thar Desert region. Fig. 8 shows monthly mean spectral AODs at Delhi for winter and summer. Significant spectral dependence in AOD was observed during all the considered months showing a decrease in AOD with increasing wavelength. AOD was found to be relatively higher at lower wavelength and smaller at larger wavelength during the winter as compared to the summer months, which can be seen clearly in the seasonal mean spectral AODs, as shown in the inset of Fig. 8. The seasonal mean AODs were found to be relatively smaller at all the wavelengths (except at 340 nm) in winter as compared to the summer. The difference in the magnitude of AOD is increased from ultra-violet to near-infrared wavelength regions. High AODs at larger wavelength during the summer is associated with the enhanced coarse dust loading over the station. As spectral characteristics of AOD have an imprint of the columnar aerosol size distribution, it is possible to estimate the same from the spectral AOD measurements. Out of several available methods for inverting the aerosol columnar size distributions, the constrained linear inversion technique suggested by King et al. (1978) and King (1982) has been used in the present case. More details of the application of this technique to the spectral AOD have been discussed in the literatures (e.g. Saha and Moorthy, 2004; Gogoi et al., 2009). Plots for the columnar aerosol number size distribution during the winter and summer periods have been shown in Fig. 9a and b, respectively (with open circles), along with the surface aerosol number size distribution (with

Fig. 7. Monthly mean AOD and a.

shown in Fig. 7. AOD was found to be high ( 40.60) during all the months; however, it was observed to be slightly higher during the summer (0.73 70.10) than the winter period (0.6770.10). The surface measurements, described earlier, show opposite features, i.e., near-surface aerosol mass loading measured during the winter is higher than those measured during the summer. This is understandable that during the winter, atmospheric boundary layer is usually low and the winds are also relatively slow,

Fig. 8. Monthly mean spectral AOD (mean spectral AOD during the winter and summer is shown in the inset).

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Fig. 9. Comparison of aerosol number size distributions derived from surface (solid squares) and columnar (open circles) measurements during (a) winter and (b) summer.

solid squares) derived from the OPC mass size distribution data for comparison purposes. To convert aerosol mass size distribution data to the number size distribution, particle density of 1.66 g cm  3 was used. Slightly bi-modal feature can be seen in the aerosol number size distribution estimated from the OPC data; however, the same retrieved from the columnar spectral AODs, shows power-law distributions (probably due to relatively coarse resolution of the wavelength) during both winter and summer periods. The estimated parameters differed, primarily because those retrieved from the AOD data pertained to the column, while those from the OPC pertained to the ambient atmosphere near the surface. As such, total aerosol number concentrations would be higher in the column as compared to those near the surface, which is clearly reflected in the figure during both the periods. Further, these monthly columnar aerosol size distributions were used to estimate effective radius of columnar aerosols (Reff,c) using Eq. (2) and the monthly mean values are given in Table 2. It is notable that the trend of monthly mean Reff,c was found to be similar to that observed for Reff, estimated for surface measured aerosols during all the considered months; however, their magnitude differ. The magnitude of Reff was found to be relatively higher than the magnitude of Reff,c during all the considered months. The difference in the magnitude was found to be relatively higher during the summer and lower during the winter months, which are largely associated with the surface meteorology. This is due to the fact that the particles near the surface are comparatively larger in size than those higher up in the atmosphere. The observed increase and/or decrease in the various columnar aerosol characteristics over the station are not necessarily accompanied with the simultaneous changes in the near-surface aerosol characteristics, particularly during the summer. Relatively large discrepancy in near-surface and columnar aerosol characteristics during the summer may be caused due to dispersion of aerosols in a large volume of atmosphere away from the surface. Results arise due to large vertical extent of the boundary layer caused by the persistence of strong surface convection associated with increased surface temperature during the summer (as seen in Fig. 1b). Higher temperature over the station during summer creates low pressure at the surface, which makes surface more convective in nature causing upward vertical winds to be persistant. On the other hand, relatively high pressure persists at the surface due to low surface temperature during the winter, which creates strong subsidence near the surface causing persistence of downward vertical winds. The surface concentrations in the IGB typically exhibit a large diurnal variation ¨ ¨ (Monkk onen et al., 2004; Hyvarinen et al., 2010). Though AOD measurements are conducted only during the daytime, discrepancy

Fig. 10. Mean omega profiles from NCEP–NCAR during winter (solid line with stars) and summer (solid line with open squares) periods, centered over Delhi (26.61N–30.61N, 75.21E–79.21E).

in the AOD and surface aerosol concentration may also be contributed partially due to the temporal difference in surface and columnar measurements. An association between PM (surface measured) and AOD (satellite derived columnar measurement) was studied previously by Kumar et al. (2007) at Delhi. Though they have found a significant positive association between these two, it is relatively weaker than that reported in the other parts of ¨ ¨ the world (Chu, 2006). In another study, Monkk onen et al. (2004) have studied variations and relationship between aerosol number and PM10 mass concentrations over Delhi. They have found an increase in the number concentration with the mass concentration up to 300 mg m  3 and a decrease in the number concentration afterwards. To analyze vertical wind characteristics over the experimental site at Delhi, omega (vertical wind) data at different pressure level was obtained from NCEP–NCAR and shown in Fig. 10a and b, respectively, for the winter and summer. Omega values were averaged at 7 different pressure levels (1000, 925, 850, 700, 600,

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500 and 400 mb) in a latitude bin from 26.61N to 30.61N and longitude bin from 75.21E to 79.21E, centered at Delhi (28.61N, 77.21E) for both winter and summer periods. Significantly different vertical wind pattern was observed during these two periods. Completely downdraft was observed during the winter throughout the altitude levels; however, opposite (updraft) was observed during the summer at all the altitude levels. The observed downdraft and updraft winds, respectively, during the winter and summer may play a significant role in the dispersal/distribution of aerosols vertically, starting from the surface. Thus, columnar aerosol characteristics over the station may be largely affected and associated with the aerosols dispersed vertically. In order to see such an effect on aerosol vertical distributions, vertical aerosol characteristics during the two seasons are described in the next section. 4.3. Vertical aerosol characteristics Apart from the surface and columnar information of atmospheric aerosols, vertical profile of aerosols is also crucial to understand the vertical distribution of aerosols over the region, mainly during the summer when aerosols are lofted to elevated altitudes in the troposphere due to enhanced surface convection activities and also to understand their effect on columnar aerosol characteristics. These elevated aerosol layers into the free troposphere can impact heating of the atmosphere and change the radiation balance at the elevated heights (Srivastava et al., 2011d). CALIOP provides global vertical distribution of aerosols and clouds in the atmosphere (Winker et al., 2007; Hunt et al., 2009); however, we have analyzed Level 2 (Version 3.01) retrieved aerosol total backscatter (sr  1 km  1) and depolarization ratio (DPR) profiles at 532 nm wavelength over Delhi during the winter and summer periods. The aerosol profile products are at a spatial resolution of 120-m vertically and 40-km horizontally. The total backscatter and DPR profiles were averaged in the same latitude and longitude bins as used to obtain omega profiles over Delhi. The total number of profiles obtained over Delhi was 1532 and 1125, respectively, during the winter and summer periods. Fig. 11(a) shows mean backscatter profile of aerosols over Delhi during the winter (line connected with blue stars) and summer (line connected with red open squares). For better visualization, smoothing has been done with a five-point moving average filter

and shown with solid blue (winter) and red (summer) lines. An enhancement in aerosol backscatter can be seen throughout the altitude region (except at the lower altitude of below 1 km) during the summer as compared to the winter. The large differences in aerosol backscatter during the winter (at lower altitude) and the summer (at higher altitude) can be attributed to the prevailing synoptic meteorology over the station during these two extreme seasons. Contrary to the summer, aerosol profile during the winter was characterized by relatively higher values of backscatter within first few hundred meters from the surface and followed by a sharp decrease with an increasing altitude. Results are well compared with the earlier such measurements, done using ground-based lidar system at Delhi (Ganguly et al., 2006) and Gual Pahari, close to Delhi (Komppula et al., 2010). Such aerosol behavior in the winter can be attributed mainly due to the prevailing synoptic meteorology over the station. During the winter, ABL is highly stable and shallow over the station, posed aerosols to get trapped within the boundary layer and do not allow them to disperse much into the free troposphere, which results in an enhancement in aerosol concentrations within the boundary layer near the surface. On the other hand, during the summer, ABL is highly unstable due to the fact that strong surface convection persists over the station near the surface, which results in an increase in the vertical extension of the ABL and provides large space to disperse aerosol particles in a larger vertical column (consequently increase the dilution of surface aerosol concentrations). An enhancement in aerosol loading at the higher altitude levels during the summer (Fig. 11a) was also found to be comparable with the aerosol structure reported during the spring season by Komppula et al. (2010) at Gual Pahari. Such aerosol behavior in the summer can thus largely affect the columnar aerosol characteristics (AOD is relatively higher during the summer than the winter) and lead to more discrepancy when compared with near-surface aerosol characteristics. Fig. 11(b) shows mean DPR profile for aerosols during the winter (solid blue line) and summer (solid red line) periods. The DPR indicates the type of particles and is a good proxy to distinguish between spherical (e.g. sulfate) and non-spherical (e.g. dust) aerosols (Gautam et al., 2009a). It is defined as the ratio of perpendicular and parallel components of the attenuated backscatter signal (Liu et al., 2008). The magnitude of DPR, in the

Fig. 11. Mean aerosol profile of (a) total backscatter (km  1 sr  1) at 532 nm and (b) DPR from CALIOP during winter (blue stars) and summer (red open squares) periods, centered over Delhi (26.61N–30.61N, 75.21E–79.21E). Smoothed profile after a five-point moving average filter for winter (solid blue line) and summer (solid red line). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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present study, was observed to be relatively higher at all the altitude levels during the summer as compared to the winter, suggesting the presence of large amount of non-spherical particles (such as dust) over Delhi. Although smoke and pollution aerosols dominate mostly during the winter period, also contain non-spherical soot particles, the magnitude of DPR for these types of aerosol particles are usually small due to small size of the soot particles (Gautam et al., 2009b). However, the detailed analysis of aerosol shape and size parameter is not being undertaken here.

5. Conclusions Near-surface and columnar aerosol characteristics at a typical urban location at Delhi, situated in the western part of the Ganga basin in the northern India, were studied during the winter and summer periods using wide spectrum of measurements. Nearsurface aerosol mass concentrations of PM10 and PM2.5 over Delhi were found to be  200 ( 724) and 118 ( 733) mg m  3, respectively, during the winter and  168 ( 732) and 55 ( 712) mg m  3, respectively, during the summer. Both PM10 and PM2.5 concentrations were found to be relatively higher during the winter as compared to the summer. PM2.5 concentration was about two times higher when compared with PM10 during the winter, which may be due to an enhanced production of secondary aerosols largely associated with the enhanced anthropogenic emissions coupled with shallow and stable nature of the atmospheric boundary layer. Columnar AOD was found to be high ( 40.60) during both winter and summer periods; however, relatively large magnitude of a during the winter (1.02) than the summer (0.51) clearly suggests large contribution of fine-mode particles during the winter and coarse-mode dust particles during the summer. The surface aerosol concentrations were found to be higher during the winter as compared to the summer whereas vice-versa was observed for columnar AODs during these periods. The Reff values increase about three folds from winter to summer, showing an impact of coarse dust particles over the station with the advancement of summer. The effective radius of columnar aerosols (Reff,c) are, however, found to be in large association (similar trend) with the Reff for the surface aerosols. The observed differences in the characteristics of near-surface and columnar aerosols may be due to the vertical winds (omega) that show downdraft during the winter and updraft during the summer, and coupled with the prevailing surface meteorology over the station.

Acknowledgments Authors would like to thank Prof. B.N. Goswami, Director, IITM and Dr. P.C.S. Devara, Head, PM&A Division for their support and encouragement. We acknowledge the use of ECMWF, MODIS, NCEP–NCAR and CALIPSO data in this study. Authors are grateful to the anonymous reviewers for their constructive comments and suggestions, which helped to improve the manuscript. References ˚ ¨ Angstr om, A., 1964. The parameters of atmospheric turbidity. Tellus 16, 64–75. Bano, T., Singh, S., Gupta, N.C., Soni, K., Tanwar, R.S., Nath, S., Arya, B.C., Gera, B.S., 2011. Variation of aerosol black carbon concentration and its emission estimates at the mega-city Delhi. International Journal of Remote Sensing 32 (21), 6749–6764. Chu, D.A., 2006. Analysis of the relationship between MODIS aerosol optical depth and PM2.5 over the summertime US. Proceedings of SPIE 6299, 29903. doi:10.1117/12.678841. Das, S.K., Jayaraman, A., Misra, A., 2008. Fog-induced variations in aerosol optical and physical properties over the Indo-Gangetic Basin and impact to aerosol radiative forcing. Annales Geophysicae 26, 1345–1354.

65

Dey, S., Tripathi, S.N., Singh, R.P., Holben, B.N., 2004. Influence of dust storm on the aerosol optical properties over the Indo-Gangetic Basin. Journal of Geophysical Research 109, D20211. doi:10.1029/2004JD004924. Dey, S., Di Girolamo, L., 2010. A climatology of aerosol optical and microphysical properties over the Indian subcontinent from 9 years (2000–2008) of Multiangle Imaging Spectroradiometer (MISR) data. Journal of Geophysical Research 115, D15204. doi:10.1029/2009JD013395. Eck, T.F., Holben, B.N., Reid, J.S., Dubovik, O., Smirnov, A., O’Neill, N.T., Slutsker, I., Kinne, S., 1999. Wavelength dependence of the optical depth of biomass burning, urban, and desert dust aerosols. Journal of Geophysical Research 104, 31333–31349. Ganguly, D., Jayaraman, A., Rajesh, T.A., Gadhavi, H., 2006. Wintertime aerosol properties during foggy and non-foggy days over urban center Delhi and their implications for shortwave radiative forcing. Journal of Geophysical Research 111, D15217. doi:10.1029/2005JD007029. Gautam, R., Hsu, N.C., Lau, K.-M., Kafatos, M., 2009a. Aerosol and rainfall variability over the Indian monsoon region: distributions, trends and coupling. Annales Geophysicae 27, 3691–3703. Gautam, R., Liu, Z., Singh, R.P., Hsu, N.C., 2009b. Two contrasting dust-dominant periods over India observed from MODIS and CALIPSO data. Geophysical Research Letters 36, L06813. doi:10.1029/2008GL036967. Gogoi, M.M., Moorthy, K.K., Babu, S.S., Bhunyan, P.K., 2009. Climatology of columnar aerosol properties and the influence of synoptic conditions—first time results from the northeastern region of India. Journal of Geophysical Research 114, D08202. Hunt, W.H., Winker, D.M., Vaughan, M.A., Powell, K.A., Lucker, P.L., Weimer, C., 2009. CALIPSO lidar description and performance assessment. Journal of Atmospheric and Oceanic Technology 26, 1214–1228. ¨ Hyvarinen, A.-P., et al., 2010. Aerosol measurements at the Gual Pahari EUCAARI station: preliminary results from in-situ measurements. Atmospheric Chemistry and Physics 10, 7241–7252. Intergovernmental Panel on Climate Change (IPCC), 2007. Climate Change 2007: The Physical Science Basis: Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Chapter 2, p. 129. Jethva, H., Satheesh, S.K., Srinivasan, J., 2005. Seasonal variability of aerosols over the Indo-Gangetic basin. Journal of Geophysical Research 110, D21204. doi:10.1029/2005JD005938. King, M.D., 1982. Sensitivity of constrained linear inversion to the selection of the lagrange multiplier. Journal of the Atmospheric Sciences 39, 1356–1369. King, M.D., Byrne, D.M., Herman, B.M., Reagan, J.A., 1978. Aerosol size distributions obtained by inversion of spectral optical depth measurements. Journal of the Atmospheric Sciences 35, 2153–2167. Komppula, M., et al., 2010. One year of Raman-lidar measurements in Gual Pahari EUCAARI site close to New Delhi in India: seasonal characteristics of the aerosol vertical structure. Atmospheric Chemistry and Physics Discussions 10, 31123–31151. doi:10.5194/acpd-10-31123-2010. Kumar, N., Chu, A., Foster, A., 2007. An empirical relationship between PM2.5 and aerosol optical depth in Delhi Metropolitan. Atmospheric Environment 41, 4492–4503. Liu, Z., et al., 2008. Airborne dust distributions over the Tibetan Plateau and surrounding areas derived from the first year of CALIPSO lidar observations. Atmospheric Chemistry and Physics Discussions 8, 5957–5977. ¨ ¨ Monkk onen, P., et al., 2004. Relationship and variations of aerosol number and PM10 mass concentrations in a highly polluted urban environment—New Delhi, India. Atmospheric Environment 38, 425–433. Morys, M., Mims III, F.M., Hagerup, S., Anderson, S.E., Baker, A., Kia, J., Walkup, T., 2001. Design, calibration, and performance of Microtops II handheld ozone monitor and sun photometer. Journal of Geophysical Research 106, 14573–14582. Myhre, G., 2009. Consistency between satellite-derived and modeled estimates of the direct aerosol effect. Science 325, 187–190. Pandithurai, G., Dipu, S., Dani, K.K., Tiwari, S., Bisht, D.S., Devara, P.C.S., Pinker, R.T., 2008. Aerosol radiative forcing during dust events over New Delhi, India. Journal of Geophysical Research 113, D13209. doi:10.1029/2008JD009804. Ram, K., Sarin, M.M., 2010. Spatio-temporal variability in atmospheric abundances of EC, OC and WSOC over Northern India. Journal of Aerosol Science 41, 88–98. Ramanathan, V., Ramana, M.V., 2005. Persistent, widespread, and strongly absorbing haze over the Himalayan foothills and the Indo-Ganges plains. Pure and Applied Geophysics 162, 1609–1626. Reddy, M.S., Venketaraman, C., 2002a. Inventories of aerosols and sulphur dioxide emissions from India: I. Fossil fuel combustion. Atmospheric Environment 36, 677–697. Reddy, M.S., Venketaraman, C., 2002b. Inventories of aerosols and sulphur dioxide emissions from India: II. Biomass combustion. Atmospheric Environment 36, 699–712. Rengarajan, R., Sarin, M.M., Sudheer, A.K., 2007. Carbonaceous and inorganic species in atmospheric aerosols during wintertime over urban and highaltitude sites in North India. Journal of Geophysical Research 112, D21307. doi:10.1029/2006JD008150. Ricchiazzi, P., Yang, S., Gautier, C., Sowle, D., 1998. SBDART: a research and teaching tool for plane-parallel radiative transfer in the Earth’s atmosphere. Bulletin of the American Meteorological Society 79, 2101–2114. Saha, A., Moorthy, K.K., 2004. Impact of precipitation on aerosol spectral optical depth and retrieved size distributions: a case study. Journal of Applied Meteorology 42, 902–914.

66

A.K. Srivastava et al. / Journal of Atmospheric and Solar-Terrestrial Physics 77 (2012) 57–66

Satheesh, S.K., Moorthy, K.K., 1997. Aerosol characteristics over coastal regions of the Arabian Sea. Tellus B 49, 417–428. Schwartz, S.E., et al., 1995. Group Report: Connections Between Aerosol Properties and Forcing of Climate. John Wiley, Hoboken, NJ (pp. 251–280). Singh, S., Nath, S., Kohli, R., Singh, R., 2005. Aerosols over Delhi during premonsoon months: characteristics and effects on surface radiation forcing. Geophysical Research Letters 32, L13808. doi:10.1029/2005GL023062. Singh, S., Soni, K., Bano, T., Tanwar, R.S., Nath, S., Arya, B.C., 2010. Clear-sky direct aerosol radiative forcing variations over mega-city Delhi. Annales Geophysicae 28, 1157–1166. Srivastava, A.K., Devara, P.C.S., Rao, Y.J., Bhavanikumar, Y., Rao, D.N., 2008. Aerosol optical depth, ozone and water vapor measurements over Gadanki, a tropical station in peninsular India. Aerosol Air Quality Research 8 (4), 459–476. Srivastava, A., Jain, V.K., 2007. A study to characterize the suspended particulate matters in an indoor environment in Delhi, India. Building and Environment 42, 2046–2052. Srivastava, A.K., et al., 2011a. Pre-monsoon aerosol characteristics over the IndoGangetic Basin: implications to climatic impact. Annales Geophysicae 29, 789–804. doi:10.5194/angeo-29-789-2011. Srivastava, A.K., Singh, S., Tiwari, S., Bisht, D.S., 2011b. Contribution of anthropogenic aerosols in direct radiative forcing and atmospheric heating rate over Delhi in the Indo-Gangetic Basin. Environment Science and Pollution Research. doi:10.1007/s11356-011-0633-y. Srivastava, A.K., Tiwari, S., Bisht, D.S., Devara, P.C.S., Goloub, P., Li, Z., Srivastava, M.K., 2011c. Optical characteristics of aerosols and radiative forcing for the coolest June month over Delhi, India. International Journal of Remote Sensing 32 (23), 8463–8483. Srivastava, A.K., Pant, P., Hegde, P., Singh, S., Dumka, U.C., Naja, M., Singh, N., Bhavanikumar, Y., 2011d. Influence of south Asian dust storm on aerosol

radiative forcing at a high-altitude station in central Himalayas. International Journal of Remote Sensing 32 (22), 7827–7845. Srivastava, M.K., Singh, S., Saha, A., Dumka, U.C., Hegde, P., Singh, R., Pant, P., 2006. Direct solar ultraviolet irradiance over Nainital, India, in the central Himalayas for clear-sky day conditions during December 2004. Journal of Geophysical Research 111, D08201. doi:10.1029/2005JD006141. Stull, R.B., 1999. An Introduction to Boundary Layer Meteorology. Springer, New York (p. 620). Tiwari, S., Srivastava, A.K., Bisht, D.S., Bano, T., Singh, S., Behura, S., Srivastava, M.K., Chate, D.M., Padmanabhamurty, B., 2009. Black carbon and chemical characteristics of PM10 and PM2.5 at an urban site of North India. Journal of Atmospheric Chemistry 62 (3), 193–209. Tiwary, A., Colls, J.J., 2004. Measurements of atmospheric aerosol size distributions by collocated optical particles. Journal of Environmental Monitoring 6, 734–739. Todd, M.C., Washington, R., Martins, J.V., Dubovik, O., Lizcano, G., M’Bainayel, S., Engelstaedter, S., 2007. Mineral dust emission from the Bode´le´ Depression, northern Chad, during BoDEx 2005. Journal of Geophysical Research 112, D06207. doi:10.1029/2006JD007170. Twomey, S., 1991. Aerosols, clouds, and radiation. Atmospheric Environment 25 (A), 2435–2442. Tripathi, S.N., et al., 2006. Measurements of atmospheric parameters during Indian Space Research Organization Geosphere Biosphere Programme Land Campaign II at a typical location in the Ganga Basin: 1. Physical and optical properties. Journal of Geophysical Research 111, D23209. doi:10.1029/2006JD007278. Venkataraman, C., Habib, G., Figuren-Fernandez, A., Miguel, A.H., Friedlander, S.K., 2005. Residential biofuels in south Asia: carbonaceous aerosol emissions and climate impacts. Science 307, 1454–1456. Winker, D.M., Hunt, W.H., McGill, M.J., 2007. Initial performance assessment of CALIOP. Geophysical Research Letters 34, L19803. doi:10.1029/2007GL030135.