JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112, D06211, doi: /2006jd007488, 2007

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112,, doi: /2006jd007488, 2007 Black carbon aerosol mass concentrations over Ahmedabad, an urban location in western India: Comparison with urban sites in Asia, Europe, Canada, and the United States S. Ramachandran 1 and T. A. Rajesh 1 Received 8 May 2006; revised 13 October 2006; accepted 3 November 2006; published 29 March [1] Black carbon (BC) aerosol mass concentrations measured using an aethalometer at Ahmedabad, an urban location in western India, from September 2003 to June 2005 are analyzed. BC mass concentrations are found to show diurnal and seasonal variations. Diurnal evolution in BC is marked with two peaks, one in the morning hours, just after the sunrise, and the other in the late evening hours. The peaks occur due to fumigation effect of boundary layer, gradual increase in the anthropogenic activities, and rush hour traffic. January BC values are about a factor of 5 higher than July mass concentrations. During winter the surface boundary layer is shallow resulting in trapping of pollutants in a lesser volume which leads to higher BC concentrations. In July an increase in boundary layer height, surface temperature, convective activity, and rainfall result in lower BC values. BC mass concentrations are about 0.8 mg m 3 in July 2004 (southwest monsoon), while BC was higher than 5 mg m 3 in January 2004 (northeast monsoon). Ahmedabad BC mass concentrations are higher than those measured over central, western India and Hyderabad, an urban city in south India during a land campaign in February BC values measured over Ahmedabad are found to be higher than those measured over various locations representing different environments in Europe. Seasonal variations are less pronounced in urban locations in Europe. BC mass concentrations at east St. Louis, Illinois, an urban site are found to be less than 2 mg m 3 during September 2003 to June 2005, with less pronounced seasonal variations. BC mass concentrations at various land locations in India, Beijing, and Seoul are higher than those measured over various locations in Europe, Canada, and the United States. Citation: Ramachandran, S., and T. A. Rajesh (2007), Black carbon aerosol mass concentrations over Ahmedabad, an urban location in western India: Comparison with urban sites in Asia, Europe, Canada, and the United States, J. Geophys. Res., 112,, doi: /2006jd Introduction [2] Atmospheric aerosols can have significant local, regional, and global climate impacts. Emissions from vehicles, burning of wood, and industries that can cause urban air pollution are of local importance. Aerosols from regional sources can be transported from areas of high emissions to clean remote locations. On a global scale aerosols can affect the Earth s climate by scattering and absorption of sunlight and serving as cloud condensation nuclei. Radiative effects of different kinds of aerosols exhibit the largest uncertainty in constraining radiative 1 Space and Atmospheric Sciences Division, Physical Research Laboratory, Ahmedabad, India. Copyright 2007 by the American Geophysical Union /07/2006JD forcing of aerosols. The current emphasis is on to quantify the climate forcing due to manmade changes in the composition of the atmosphere. A better understanding of the formation, chemical composition, and transformation of aerosols in the atmosphere is of crucial importance to better quantify these effects. It is now well realized that sulfate aerosols cool the Earth s atmosphere while soot (black carbon, BC) aerosols warm the atmosphere. Black carbon aerosols are produced as primary particles from incomplete combustion processes such as fossil fuel and biomass burning, and hence most of the BC in the atmosphere originate from manmade activities. This paper describes the diurnal and monthly mean variations in aerosol black carbon mass concentrations measured over an urban location in western India during The BC mass concentrations measured over Ahmedabad are compared with other land locations in India, Asia, Europe, Canada, and the United States. A detailed comparison is made with 1of19

2 BC mass concentrations measured over east St. Louis, an urban site in Illinois, USA, and the results are discussed. 2. Instrument, Attenuation Cross Section, and Uncertainties [3] Black carbon aerosol mass concentration measurements began in Ahmedabad from September 2003 using a 7-wavelength Aethalometer (AE-47 of Magee Scientific, USA). AE-47 measures BC mass concentrations in the wavelength range of 370 to 950 nm. The measurements are made from an altitude of about 20 m above the ground using its inlet tube and pump. The BC mass concentrations are estimated using the optical method of measurement of the attenuation of a beam of light transmitted through the sample collected on a filter, which is proportional to the amount of BC mass loading in the filter deposit [Hansen et al., 1982, 1984]. This is a filter based technique that measures the light attenuation due to particles deposited onto a filter. The yielded attenuation absorption coefficient is then converted into BC mass concentration. The conversion of attenuation absorption coefficient into BC mass concentration is done using appropriate absorption efficiency values. The absorption efficiency varies as a function of wavelength. [4] The light transmission is detected using a set of two photo diodes, one through the sample spot and the other through a blank or an unsampled portion of the filter which is called the reference spot. The change in attenuation is obtained as a function of time and is related to the BC mass concentrations in g m 3 as BlackCarbonMass ¼ A100 ln ð i 2=i 1 Þ kqðt 2 t 1 Þ where i 1 and i 2 are the ratios of the intensities of sample beam to the reference beam at times t 1 and t 2 (seconds) respectively, Q in m 3 s 1 is the sample flow rate through the filter, A is the area of the exposed spot on the filter and is in units of m 2 and k (m 2 g 1 ) is referred to as the absorption efficiency. [5] Two values of spectral absorption efficiencies for aethalometer based BC measurements are provided by the manufacturers of aethalometers, namely, Magee Scientific. The Classic BC calibration by Magee Scientific at 880 nm is given as 16.6 m 2 g 1, while the calibration factor derived using thermal/oxidation reflectance (TOR) technique was found to be 12.6 m 2 g 1. These absorption efficiencies were derived by operating aethalometers and quartz fiber filter samples side by side in Harvard School of Public Health for several years. It was found that the EC (elemental carbon) measured by TOR thermal analyses tracked well the aethalometer BC data but the numbers were higher. When particles are collected on a filter paper, EC is the material that remains after heating the filter to 700 C, while BC is the part of the particulate matter that absorbs light. It was reasoned that the higher EC concentration indicates either BC is a subset of EC or alternatively the TOR method produces higher EC. It should be mentioned that most of the measurements treat EC as equal to BC. In addition to the above calibration, aethalometer measured BC mass concentrations have been compared with several ð1þ other instruments such as Particle Soot Absorption Photometer, Laser integrating plate method, thermal EC analyzer etc. and the comparisons were found to be good ( pdf). [6] Several investigations in different environments have been conducted using aethalometer and other analytical techniques to calibrate and obtain site specific absorption efficiencies for aethalometer, including Allen et al. [1999], Ballach et al. [2001], Clarke et al. [2004], Im et al. [2001], Martins et al. [1998], and Sharma et al. [2002], to name a few. It has been shown that the absorption efficiency which relates particle light absorption to the BC measured with an aethalometer changes with age, type, and composition of the aerosol, which varies with time and space [Liousse et al., 1993; Lavanchy et al., 1999]. Sharma et al. [2002] found that the absorption efficiency showed variations, from remote continental to rural, urban, suburban locations and also from summer to winter. Aethalometer and PSAP were found to agree well with each other [Sharma et al., 2002]. The range of median site specific absorption efficiencies observed varied from a low of 6.4 m 2 g 1 to 20.1 m 2 g 1 for the BC mass concentration measurements made at different locations in Canada. Sharma et al. suggested that the variability is related to the distribution of sources and processes contributing to the carbonaceaous aerosols at different sites. Absorption efficiencies were found to be in the range of 9.9 m 2 g 1 to 19.5 m 2 g 1 for the measurements made over Munich and Berlin [Petzold et al., 1997]. The measurements in Munich and Berlin were conducted in residential areas without high traffic impact and in the immediate vicinity of main streets. [7] Martins et al. [1998] determined black carbon mass absorption efficiency of smoke particles for various types of biomass fires during the Smoke, Clouds, and Radiation- Brazil (SCAR-B) experiment from thermal evolution measurements for black carbon and optical absorption methods. The efficiency values ranged between 5.2 and 19.3 m 2 g 1 with an average value of 12.1 ± 4.0 m 2 g 1. It was noted that unrealistically high values of black carbon efficiencies were linked to high concentrations of K, which in turn influenced the volatilization of BC at lower temperatues than usual, leading to possible artifacts in the determination of BC by thermal technique. Although the aethalometer uses a standard absorption efficiency, the aethalometer measured BC mass concentrations are found to satisfactorily describe the concentration levels and trends in the urban atmosphere [Hansen and Novakov, 1990; Liousse and Cachier, 1992]. As the BC mass concentration is inversely proportional to the absorption efficiency, a higher value of absorption efficiency would yield low BC mass concentrations. [8] It is to be noted that hematite (iron oxide, Fe 2 O 3 )is the other strong absorber found commonly in atmospheric aerosol [Fialho et al., 2005]. It has been pointed out that about 200 times as much hematite (mass) as BC is needed for equivalent absorption [Bodhaine, 1995]. It has been pointed out [Bodhaine, 1995] that the optics of aethalometer allows detection of changes in light intensity of 1 in 10 4 which corresponds to a noise level of about m 1 in absorption coefficient. This translates into 1.5 ng m 3 ( mg m 3 ) BC for a 1-hour collection period. It is to 2of19

3 Figure 1. Monthly mean surface wind fields over Ahmedabad, where BC mass concentration measurements are made, in (a) January 2004 (northeast winter monsoon) and (b) July 2004 (southwest summer monsoon). (c) The city map of Ahmedabad showing the measurement location (PRL) and surrounding urban areas. be noted that the lowest mean BC mass reported in this study is 0.21 mg m 3 and the highest is mg m 3 (Table 2). These two values are more than an order of magnitude higher than the noise level mass concentration of BC. In this study, an absorption efficiency value of 16.6 m 2 g 1 corresponding to 880 nm is used to determine the BC mass concentrations over an urban location. As the value of absorption efficiency used is either comparable or higher 3of19

4 Table 1. Number of Days of Black Carbon Mass Concentration Observations During Over Ahmedabad Number of Days Months Jan 19 6 Feb 2 21 Mar Apr May Jun Jul 26 Aug 8 Sep Oct Nov 19 7 Dec 5 Total than the absorption efficiencies determined in different environments, the concentrations reported in this study represent the minimum values for BC concentrations. The other sources of uncertainty in BC mass concentrations using an aethalometer arise from instrumental artifacts such as flow rate, filter spot area and detector response. The flow rate can be checked by the flowmeter and value at which the center of the float lies. This has been checked on a daily basis during the measurement period and the flow rate was found to be quite stable. Taking into account all these effects, the overall uncertainty in the reported BC mass concentrations is estimated to be about a maximum of 10%. 3. Wind Patterns, Site Description, and BC Observations [9] Black carbon mass concentration measurements using a 7-wavelength Magee aethalometer were made in Physical Research Laboratory (PRL), Ahmedabad (23.03 N, 72.5 E), an urban, densely populated (population in excess of 5 million) and industrialized city in western India. Figure 1 shows the monthly mean surface wind speeds during the months of January and July 2004 respectively over the Indian subcontinent. During January (Figure 1a) shown as representative of the wind pattern during northeast monsoon (November March), the winds are calm, north/ northeasterly and are from the polluted northern hemisphere. In July (Figure 1b) which represents the southwest monsoon season of June September, the winds are stronger, moist, and are from the marine and western regions surrounding India. During October wind patterns start shifting in direction from southwest to northeast. By November, when the northeast monsoon sets in the winds are entirely from north/northeast. [10] PRL, which is located on the western part of Ahmedabad in Navrangpura, is surrounded by a number of residential and business complexes, within a radius of 10 km (Figure 1c). This results in a heavy movement of vehicular traffic with a distinct diurnal variation. Navrangpura and the other surrounding areas are prime urban locations and are densely populated (Figure 1c). At about 20 km southeast of the measurement location small-scale industrial complexes are located. In addition the emissions from two coal-based power plants situated in the northeast at a distance of about 10 km (Sabarmati) and 25 km (Gandhinagar) are found to significantly contribute to the airborne particle loading [Rastogi and Sarin, 2005]. The number of days in each month on which BC measurements were made and used in the study during are given in Table 1. More than 400 days of BC measurements are available spread over a period of 22 months. [11] Aethalometer was operated at the flow rate of 3lmin 1, for 24 hours a day at a time resolution of 5 min. The BC measured at 880 nm wavelength is considered to represent a true measure of BC in the atmosphere as at this wavelength BC is the principal absorber of light while the other aerosol components have negligible absorption at this wavelength [Bodhaine, 1995]. Though hematite (Fe 2 O 3 )in dust is the other strong absorber in the atmosphere, as the absorption cross section of dust is smaller than BC by more than 100 times, to obtain comparable absorption in this wavelength the amount of dust in the atmosphere has to be 100 times higher than the amount of BC. The measurement location is an urban location and is dominated by local sources, mainly anthropogenic. As at 880 nm the major absorbing species is BC, in this work, BC mass concentrations measured at 880 nm from September 2003 to June 2005 in PRL, Ahmedabad are analyzed and reported. 4. Results and Discussion 4.1. Diurnal Variations in BC Mass Concentrations [12] In Figure 2 the diurnal variations in BC aerosol mass concentrations measured over Ahmedabad during January and July 2004 are plotted. The monthly mean concentrations as a function of time are plotted in the figure. The monthly average sunrise and sunset times corresponding to January and July 2004 over Ahmedabad are plotted as dotted lines in the figure. At the outset, the BC mass concentrations are found to show distinctly different diurnal variations in the months of January and July. BC aerosol mass concentrations are found to exhibit extreme temporal variability over Ahmedabad owing to the two contrasting airflows during northeast and southwest monsoons. The ratios of BC mass concentrations between January and July also exhibit strong diurnal variability (Figure 2c). The January BC concentrations are about a factor of 5 10 higher than those measured in July during the morning and late evening hours. During the afternoon hours, this ratio decreases to about 1 to 2 (Figure 2c). The BC mass concentration during January starts to increase about an hour before sunrise, peaks in an hour s time and starts decreasing from 0830 hours. Note that the Sun rises only after 0700 hours during winter and sets at around 1800 hours. In July the BC concentration starts increasing just after sunrise, reaches its peak around 0700 hours and starts decreasing. In January the BC mass concentrations increase just after sunset, reach a peak at around 2000 hours and then decrease to about 5 mg m 3. During the local afternoon hours the BC concentrations are found to be in the 1 2 mg m 3 range during January and July. The diurnal variation in BC mass concentration is more pronounced during winter when the diurnal amplitude is higher by a factor of 5 or more which reduces to 3 during summer. 4of19

5 Figure 2. Diurnal variations in BC mass concentrations over Ahmedabad during (a) January and (b) July Dotted vertical lines represent the monthly mean sunrise and sunset times in Ahmedabad during January and July (c) BC mass concentration ratios (January/July 2004) from the monthly mean BC obtained in January and July (d) Monthly mean diurnal temperature variations for January and July 2004 at Ahmedabad. [13] The BC mass concentrations are constant from midnight to 0600 hours. The values are 5 mgm 3 in January which decreases to 0.5 mg m 3 in July. The building up of BC mass concentrations start after 0600 hours. The peak BC concentrations in the morning hours occur at around 0700 hours and then the values decrease. The rates of decrease are different, it is slower during winter than in summer. The BC mass concentrations are constant and at background levels from 1000 to 1700 hours when there is much less vehicular movement in an urban location. The peak BC concentrations are found in the evening hours at around 2100 hrs after which the BC concentrations decrease. Similar features in diurnal variations in BC mass concentrations have been observed at a suburban site in Maryland, USA [Chen et al., 2001]. Diurnal variations in BC mass concentrations over a tropical coastal station, Trivandrum (8.55 N, 77 E) in January 2001 showed different features when compared to those seen over Ahmedabad. The BC mass concentrations were about 10 mgm 3 or more in Trivandrum during midnight which decreased to about half by 0600 hours. Then the BC gradually builds up resulting in a peak between 0700 and 0900 hours [Babu and Moorthy, 2002]. [14] Over Ahmedabad, an urban location the BC mass concentration exhibit variations in league with variations in diurnal anthropogenic activities and local dynamics. The BC mass concentrations are found to be less in the afternoon hours when there is much less anthropogenic sources 5of19

6 of BC. Monthly mean diurnal variations in temperature over Ahmedabad in January and July 2004 are plotted in Figure 2d. Temperatures are around 15 C or less during January from midnight to 0600 hours after which the temperature increases and reaches a maximum of 27 C at 1500 hours. During nighttime the temperature decreases and is around 18 C at 2200 hours. While in July, the temperatures are high throughout the day and exceed 33 C in the afternoon hours. BC mass concentrations are lower in July when compared to January though the temperatures are higher in July. During July due to rain aerosols are removed from the atmosphere through wet deposition leading to a lower BC mass concentration. It is to be noted that the diurnal variation (factor of 3 or higher) in BC mass concentration over Ahmedabad is much higher than the uncertainty that would arise using a different absorption efficiency known for urban location. [15] Aerosol concentration is affected by the stability of boundary layer, which is active during the daytime due to surface temperature increase and stable at night [Stull, 1988]. It has been shown that the nocturnal boundary layer is shallower than its daytime counterpart by a factor of about 3 [Kunhikrishnan et al., 1993]. Also, as the wind speeds, in general, are lower during night the ventilation coefficient rapidly reduces, resulting in the confinement of aerosols during nighttime. This results in an increase in the BC mass concentration during early nighttime [Babu and Moorthy, 2002]. The BC mass concentration reduces as the night advances due to reduction in anthropogenic activities and loss of particles closer to the surface by sedimentation. The gradual buildup of the BC mass concentration from the morning hours and the occurrence of peak between 0700 and 0900 hours about an hour after the sunrise during both winter and summer is attributed to the combined effects of fumigation effect of the boundary layer and morning increase in the anthropogenic activities in an urban environment. The fumigation effect in the boundary layer brings in aerosols from the nocturnal residual boundary layer in a short time after the sunrise [Stull, 1988]. The increased solar heating as the day advances (Figure 2d) produces a deeper and more turbulent boundary layer which leads to a faster dispersion resulting in a dilution of BC near the surface [Babu and Moorthy, 2002]. The surface aerosol loading in urban areas is typically found to be the smallest from late night to early morning ( hours local time) and increases to the first maximum in a day at around 1000 local time which drops in the afternoon until the occurrence of the second peak around 2000 local time [Jin et al., 2005]. The peaks were attributed to the early morning and late afternoon vehicle combustion resulting from the rush hour traffic. [16] In India, diesel and petrol are the major fossil fuels used for road transportation [Reddy and Venkataraman, 2002a]. Diesel vehicles are known to produce much more black carbon particles than the vehicles which run on gasoline [Weingartner et al., 1997]. More than 80% of particulate emissions from heavy duty diesel vehicles were found to be of particulate carbon [Lowenthal et al., 1994]. Heavy duty (e.g., trucks, buses etc.) diesel vehicle can contribute an average 42% BC to particulate matter [Reddy and Venkataraman, 2002a, and references therein]. The contribution of BC to particulate matter from light duty (e.g., cars) diesel vehicle is higher at 67%. Vehicles which run on leaded petrol and unleaded petrol (which do not have catalytic convertors) can give rise to 6 and 23% of BC in particulate matter. These include two-wheelers such as scooters, motor bikes in addition to cars. The carbonaceous aerosol fraction was also reported to be higher for light duty vehicles at 95% than the heavy duty vehicles at 88%. In Ahmedabad, a mixture of all these vehicles ply on the roads and contribute to BC. It was estimated that during in India, diesel oil and coal account for 59% and 40% of BC emissions from fossil fuel combustion [Reddy and Venkataraman, 2002a]. Transportation (58%) was found to be the major source of BC emissions followed by emissions from brick kilns (24%), utilities (15%), and domestic usage (1%). Aerosol emissions (SO 2,PM 2.5, Organic Matter and BC) from coal, petrol, and high-speed diesel oil are estimated to have increased by about 10% or more per year during over India [Ramachandran and Jayaraman, 2003]. [17] The biomass burning sources over India include biofuels (wood, crop waste, and dung cake) and forest fires (accidental, shifting cultivation, and controlled burning) [Reddy and Venkataraman, 2002b]. It was found that among biomass, fuel wood and crop waste were primary contributors to BC emissions, and northern and east coast India exhibited high densities of biomass comsumption [Reddy and Venkataraman, 2002b]. These estimates are based on annual average emissions. It has been shown that during northeast monsoon (November March) the anthropogenic sources contribute in excess of 70% to the aerosol optical depths measured over land and oceanic locations in and around India [Ramachandran, 2004]. The high anthropogenic influence to the aerosol optical depths in and around India were due to manmade submicron aerosols from India and surrounding regions which get transported to these locations during the winter monsoon season. As Ahmedabad is an urban, industrialized and densely populated location the diurnal and seasonal variations in BC mass concentrations can mainly be attributed to the vehicular emissions and transport from the surrounding regions, while the contribution from biomass consumption could be minimal throughout the year. The diurnal peaks in BC occur throughout the year (Figure 2), the absolute BC mass concentrations are higher in winter due to the shallow boundary layer which results in trapping of pollutants. The local sources in urban location, mainly vehicular traffic, contributes to the diurnal peaks in BC concentration and the effect gets enhanced during winter due to boundary layer dynamics Effects of Local Festivities on BC Mass Concentrations [18] In India, two local festivals (1) Navratri (festival of nine nights) and (2) Diwali (festival of lights) are celebrated during fall every year. In 2003, Navratri was celebrated from 26 September to 5 October 2003, while Diwali was celebrated on 25 October In the western state of Gujarat, where Ahmedabad is located, Navratri celebrations are accompanied by people dancing in open grounds at several locations in the city during nighttime. This results in an increase in vehicular traffic during nighttime due to people traveling to different locations. In 6of19

7 Figure 3. BC mass concentrations measured during (a) September and (b) October Effects of festivities causing an increase in BC mass concentrations measured over Ahmedabad compared with normal days of BC measured in September and October Figure 3 the diurnal BC mass concentrations measured during September and October 2003 are plotted. To delineate the effects local festivities have on measured BC mass concentrations, the data collected during September and October 2003 were analyzed in a different manner. The BC mass data collected from 4 to 25 September 2003 were averaged and used as background (normal). While BC mass concentrations measured from 26 September to 5 October were averaged and drawn to represent the effect of Navratri (Figure 3a). [19] The southwest monsoon still continues in September though it is mild when compared to June July, resulting in few rains in September. BC mass concentrations are found to be the lowest during the southwest monsoon season mainly due to removal of BC from the lower atmosphere during rain (Table 2). During September the BC mass concentrations start showing an increase. In September 2003 the BC mass concentrations in the background (normal) period of 4 25 are found to be less than about 1 2 mg m 3 or less throughout the day. During Navratri the increase in BC concentration is clearly seen. The increase is about 4 times and is found after about 2200 hours and extends up to early morning hours (0400 hours). This pattern is in stark contrast to those seen during January and July, when BC starts decreasing (Figure 2) after 2030 hours and remains at a constant level from midnight to 0600 hours though the magnitudes of this constant BC level are different. It is to be noted that not only the BC mass concentration increases during Navratri but the diurnal evolution also changes mainly due to sustenance and movement of vehicles which extend till early morning hours. [20] Diwali is celebrated with bursting of firecrackers and fireworks both during day and night in Gujarat for a period of 3 4 days; 2 days each before and after the day of the festival. The bursting of firecrackers and fireworks are found to be potent sources of soot [Babu and Moorthy, 2001]. The non-diwali (normal) mean in BC concentrations is obtained using the data collected during 6 15 October The global average lifetime of BC is about a week in the lower atmosphere. Taking the lifetime of BC into account, the BC data from 1 to 5 October is excluded because of Navratri festival discussed earlier, while calculating the background BC mass concentrations of October [21] During October the wind patterns shift and winds start coming from northeast. In addition to the local BC sources in Ahmedabad due to transport from polluted northern hemisphere BC mass concentrations are higher than those measured during September. The background BC mass concentration during October is in the 1 4 mg m 3 range from midnight to about 1800 hours. The evening increase due to vehicular traffic is more clearly seen when Table 2. Monthly Mean Black Carbon Mass Concentration With ±1 Standard Deviation During Over Ahmedabad Mean ± s Months Jan 5.29 ± ± 5.29 Feb 3.86 ± ± 5.69 Mar 2.01 ± ± 3.71 Apr 1.23 ± ± 2.67 May 1.69 ± ± 0.75 Jun 0.84 ± ± 0.57 Jul 0.77 ± 0.30 Aug 0.55 ± 0.26 Sep 0.75 a ± ± 0.58 Oct 3.72 a ± ± 1.16 Nov 7.34 ± ± 0.16 Dec 3.80 ± 1.61 a Monthly mean BC mass concentrations in September and October 2003 are derived from BC mass concentrations measured during 4 25 September and 6 15 October, respectively. The other days in these months were effected by festivities which gave rise to increased BC mass concentrations and are excluded (Figure 3). 7of19

8 Figure 4. (a) Monthly mean BC mass concentrations over Ahmedabad from September 2003 to June (b) Monthly mean wind speeds, (c) maximum, minimum temperatures, DT (difference between the maximum and minimum temperatures in a day) and (d) rainfall over Ahmedabad are plotted from September 2003 to June Vertical bars indicate ±1s from the mean. The abscissa on the x axis denote the midpoint of each month. BC mass concentrations are about 15 mg m 3 at around 2030 hours while afterwards BC concentration decreases. BC mass concentrations measured during Diwali indicate that a higher or similar amount of BC is contributed entirely by the bursting of firecrackers and due to fireworks. The diurnal pattern is almost similar during Diwali and normal days but BC concentrations are higher throughout the day during Diwali. A case study made in Trivandrum, a tropical coastal station, showed similar increases in BC mass concentrations [Babu and Moorthy, 2001] which were associated with Diwali Monthly Mean Variations in BC Aerosol Mass Concentrations [22] BC aerosol mass concentrations measured for 24 hours a day in a particular month are averaged and the monthly mean BC mass concentrations are obtained. In Figure 4a the monthly mean BC aerosol mass concentrations measured over Ahmedabad from September 2003 to June 2005 are plotted. Vertical bars indicate ±1s from the mean of measured BC mass concentrations and indicates the variability in the BC mass measured during that particular month (Table 2). The BC concentrations are found to increase from September 2003 to November The mean BC concentrations for September (4 25 September) and October 2003 (6 15 October) excluding the festive days which are referred to as normal are only plotted. During 2004 BC mass concentration measurements were not available during the festive days of Navratri and Diwali. The variations in BC mass concentrations are higher during the October February (northeast monsoon season) while during premonsoon (March-April-May) and monsoon (June-July-August-September) both BC mass concentrations and variations are less. November 2003 BC mass concentration is about 7 mg m 3 while in July 2004 the mean BC mass is less than 1 mg m 3. The BC concen- 8of19

9 trations are higher during January March 2005 when the mean BC mass values are higher than 5 mg m 3, with peak of about 10 mg m 3 in February. [23] In Figures 4b 4d the monthly mean wind speeds (ms 1 ), maximum (T max ), minimum (T min ) temperatures of the day, DT ( C) (difference between the maximum and minimum temperatures), and rainfall (mm) (amount of rainfall during the entire month) over Ahmedabad are plotted from September 2003 to June The monthly mean wind speeds are low and are in the range of 1 2 ms 1 during October March which increases to about 4 ms 1 during May July. A regression of monthly mean BC mass concentrations with wind speeds yielded a negative correlation coefficient (quality of regression) of 0.54, while the regression coefficient (slope) yielded These fitting parameters indicate that with increase in wind speeds the BC mass concentrations would decrease as seen. It is probable that due to higher wind speeds BC produced from local sources in Ahmedabad are transported to other locations. It was found from BC measurements made in urban sites in Toronto that low wind speeds lead to much less dispersion of BC and the traffic emissions remain concentrated around the emission site [Sharma et al., 2002]. It was also noted that as the traffic density was relatively constant through the measurement period, higher wind speeds will have a dilution effect on BC. A clear negative correlation between BC and wind speed at Evans Avenue (urban, r 2 = 0.77) and Downsview (suburban, r 2 = 0.64) were found while in Egbert, a rural location (r 2 = 0.05) no clear pattern was found [Sharma et al. 2002]. It was reasoned that a clear correlation between wind speeds and BC mass concentrations gives an indication of the proximity of BC sources at the measurement site, while a not so significant correlation shows that the BC originates from distant sources. [24] The maximum temperatures over Ahmedabad are about 30 C during January February after which the values start increasing (Figure 4c). The maximum temperatures are about 40 C during the summer months of April May June. During the summer monsoon months (July August September) and in October November the maximum temperatures decrease. The monthly mean maximum temperatures are in the C range. The temperatures are in the 30 C range during December. The minimum temperatures are around 12 C during January and increase slowly thereafter. The minimum temperatures are in the range of C from April to September. The values start decreasing from October and are in the 10 C range during January DT is less than 19 C during November March after which it decreases to about 10 C during May June. The DT values are at the lowest (about 6 C) during August after which they increase. The monthly mean BC mass concentrations versus DT values plot had a positive correlation coefficient of While over Trivandrum, the monthly averaged BC mass concentrations were found to exhibit a higher positive correlation coefficient of 0.77 with DT [Babu and Moorthy, 2002]. It has been seen at Maryland, USA, a suburban site, that the BC emissions, in particular, from heavy duty diesel engines, increase with increase in ambient temperatures [Chen et al., 2001]. It was suggested based on the temperature dependence of DEC/ DCO (carbon monoxide) that the EC emission increases once it exceeds a threshold value. This would become important during the dry months when the ambient temperature becomes higher. The correlations between BC mass and other meteorological parameters over Ahmedabad were not very strong, which indicates that in Ahmedabad BC mass concentrations arise both due to local sources and long range transported pollutants. [25] The BC mass concentrations are found to be higher in winter than summer. The seasonal mean BC concentrations over Ahmedabad during 2004 are 3.43 mg m 3 (January February March mean), 1.30 (April May June), 0.90 (July August September) and 1.30 (October November December). The lowest BC mass concentrations are seen during the southwest monsoon months of July- August-September. The 2004 July August September season rainfall over Ahmedabad in 2004 was 672 mm, with a peak in August at 432 mm. June 2004 witnessed a rainfall of about 100 mm. The lows and highs in monthly mean BC mass concentrations are not highly correlated with the rainfall amounts over Ahmedabad (Figure 4d), perhaps due to the fact that almost the entire rainfall over Ahmedabad occurs during the months of July-August-September. A correlation coefficient of 0.35 has been found between monthly mean BC mass concentrations and rainfall over Ahmedabad. The monthly averaged BC mass concentrations over Trivandrum were found to exhibit a stronger negative correlation with a coefficient of 0.74 with rainfall [Babu and Moorthy, 2002]. [26] The higher negative correlation coefficient over Trivandrum occurs as Trivandrum is a tropical, coastal station and rainfall occurs throughout the year during both southwest and northeast monsoons (Figure 2) [Babu and Moorthy, 2002], while the rainfall over Ahmedabad is restricted only to the southwest monsoon. The average atmospheric residence times of BC varies from about 7 to 10 days (during dry conditions) to about 5 days or less during wet periods [Reddy and Venkataraman, 1999]. It is to be noted that longer residence time results in higher concentrations. It is seen that high BC concentrations occur during the dry winter months when DT is high and rainfall is scanty over Ahmedabad. While low BC concentrations occur in the summer monsoon months of July August September when rainfall is high and when DT is about 10 C or less. In addition, the surface boundary layer is shallow over Ahmedabad during winter which results in trapping the pollutants in a smaller volume leading to higher BC mass concentrations. While in summer with an increase in surface temperatures and convective activity the pollutants are easily dispersed into a larger volume as the boundary layer height increases, resulting in lower BC mass concentrations. [27] BC mass concentrations for continental average aerosol model is about 0.5 mgm 3 [Hess et al., 1998] which increases by more than an order of magnitude to 7.8 mgm 3 in urban aerosol model. Continental polluted BC lies in between the two models with a value of about 2 mg m 3. Continental average aerosol model indicates continental areas influenced by manmade activities. Continental polluted aerosol type represents sites highly polluted by anthropogenic activities, while urban aerosol model refers to urban locations with strong pollution. The aerosol models defined in OPAC are for average conditions at a certain location [Hess et al., 1998]. At Ahmedabad 95% of days 9of19

10 Table 3. Mean Black Carbon Mass Concentrations Measured at Ahmedabad (Figure 4) in Comparison With BC Mass Concentrations (Along With ±1s or Their Range) Over Other Land Locations in India, Asia, Europe, Canada, and United States, Including East St. Louis, Illinois (Figure 7) Location Type of Location Period Mean BC ± s/bc Mass Range, mg m 3 India Ahmedabad (23.03 N, 72.5 E) Urban, industrialized Sep 2003 Jun Trivandrum (8.55 N, 77 E) Tropical, coastal Aug 2000 Oct Hyderabad (17.47 N, E) Urban Jan Jul Bangalore (13 N, 77 E) Continental, urban Nov Goa (15.75 N, E) Coastal Jan Mar ± 0.7 Mumbai (19.38 N, E) Urban, industrialized Jan Mar ± 5.1 Western, Central India ( N, E) Rural Feb Hyderabad (17.47 N, E) Urban Feb Kanpur (26.43 N, E) Urban, continental Dec China Beijing (40 N, 116 E) Urban Jul1999 Sep Korea Seoul Polluted, urban Jun Cheju island Clean Jul Aug Japan Rishiri (45.12 N, E) Remote, island Mar Sado (38.25 N, E) Remote, island Apr Hachijo (33.15 N, E) Remote, island 24 Mar 30 Apr Chichijima (27.07 N, E) Remote, island 28 Mar 30 Apr Amami-Oshima (28.44 N, E) Remote, island 3 29 Apr Europe Sevettijarvi (69.35 N, E) Natural Nov 1993 Jan Skreadalen (58.82 N, 6.72 E) Natural Feb 1991 Feb Birkenes (58.38 N, 8.25 E) Natural Feb 1991 Feb Chaumont (47.05 N, 7.58 E) Rural Jan 1998 Mar Illmitz (48.23 N, E) Rural Oct 1999 Oct Waasmunster (51.12 N, 4.08 E) Near city Jul 1994 Nov Zürich (47.37 N, 8.53 E) Urban Jan 1998 Mar Basel (47.53 N, 7.58 E) Urban Jan 1998 Mar Gent (51.02 N, 3.73 E) Urban May 1993 Jul Sep 1999 Oct 2000 Bologna (44.53 N, E) Urban Jan Dec Barcelona (41.37 N, 2.12 E) Kerbside Jun 1999 Jun Bern (46.95 N, 7.43 E) Kerbside Jan 1998 Mar Wien (47.75 N, E) Kerbside Oct 1999 Oct Paris, France High traffic road Aug Oct ± 7 Jungfraujoch, Switzerland High-alpine Jul 1995 Jun Canada Alert (82.45 N, W) Remote, continental Nov 1998 Jul Egbert (44.23 N, W) Rural Jul Downsview (northwest of Toronto) Suburban 8 Jan 11 Feb Evans Avenue (Central Toronto) Urban (near highway) Mar Winchester Road (Central Toronto) Urban (school yard) Aug Palmerston Road (Central Toronto) Urban (school yard) Aug United States Barrow, Alaska Natural Mauna Loa, Hawaii Midtroposphere Fort Meade, Maryland Suburban Jul 1999 Jul ± 0.17 Uniontown, Philadelphia Semiurban Summer East St. Louis (38.61 N, W) Urban Sep 2003 Jun during the study period of September 2003 to June 2005 the BC mass concentrations are found to be higher than the BC mass value of continental average. 45% of days the BC mass values over Ahmedabad exceed continental polluted value of 2 mg m 3 while 9% of days the BC values are above even the urban aerosol type Comparison of BC Mass Concentrations Over Other Land Locations in India, Asia, Europe, Canada, and the United States India [28] BC mass concentrations measured in different land locations varying from clean, natural to highly polluted, urban sites from various parts of the world are discussed in 10 of 19

11 Table 4. Details of Locations in Europe Where BC Mass Concentration Measurements Were Made During the Data of Which Are Utilized in the Study a Designation N1 N2 N3 R1 R2 NC U1 U2 U3 U4 K1 K2 K3 Natural Rural Location Sevettijarvi (69.35 N, E) Skreadalen (58.82 N, 6.72 E) Birkenes (58.38 N, 8.25 E) Chaumont (47.05 N, 7.58 E) Illmitz (48.23 N, E) Near City Waasmunster (51.12 N, 4.08 E) Urban Kerbside Zürich (47.37 N, 8.53 E) Basel (47.53 N, 7.58 E) Gent (51.02 N, 3.73 E) Bologna (44.53 N, E) Barcelona (41.37 N, 2.12 E) Bern (46.95 N, 7.43 E) Wien (47.75 N, E) a The locations are classified as natural, rural, near city, urban, and kerbside. this section and are listed in Table 3. This comparison, though not comprehensive, is aimed at illustrating the nature of variations in BC mass concentration that occurs in different environments. Also, the nature of BC variation in the same type of location, for example, urban, in different parts of the globe brings out the importance of local sources, meteorology, vis-a-vis the long-range transport of BC over a particular location. (Table 3). [29] The daily mean BC mass concentrations over Trivandrum measured during August 2000 to October 2001 using an aethalometer are found to vary from a low of 2 m gm 3 in August to a high of 8 mg m 3 in January [Babu and Moorthy, 2002]. Over Ahmedabad in August 2004 the monthly mean BC mass concentrations was found to be 0.55 mg m 3 (Table 2) which reaches a high of 9 mg m 3 during January February The monthly total rainfall during August 2004 over Ahmedabad is found to be in excess of 400 mm while during August 2001 over Trivandrum the rainfall was about 200 mm, which could have led to a lower value of BC over Ahmedabad when compared to Trivandrum. The measurement location in Trivandrum is not highly industrialized and not subjected to seasonal anthropogenic activities. It was conjectured that the observed seasonal changes in BC over Trivandrum would be more associated with the synoptic meteorology and long-range transport [Babu and Moorthy, 2002]. Unlike Trivandrum, Ahmedabad is a highly industrialized, densely populated urban location and the BC mass concentration variations are more dominated by local anthropogenic sources, while long-range transport of pollutants over Ahmedabad cannot also be ruled out. [30] The 4 hour average BC mass concentrations measured using an aethalometer at Hyderabad (17.47 N, E), an urban location in south India during January July 2003 were found to be in the mg m 3 range [Latha and Badarinath, 2003]. Over Bangalore (13 N, 77 E) a continental urban location in south India, BC mass concentrations measured using an aethalometer were found to lie in the range of mg m 3 during November 2001 [Babu et al., 2002]. The mean BC mass concentrations over Mumbai (19.38 N, E), an urban, densely populated and industrialized city on the west coast of India, during January March 1999 was 12.4 ± 5.1 mgm 3 [Venkataraman et al., 2002]. BC concentrations over Mumbai were obtained by carrying out the carbon analysis of samples collected on quartz fiber filters at the Environmental Quality Laboratory, California Institute of Technology using a thermal-optical method [Venkataraman et al., 2002]. [31] BC mass concentrations were measured in western and central India employing the same aethalometer used in the current study, as part of a land campaign experiment between Ahmedabad and Hyderabad from 7 to 29 February 2004 [Jayaraman et al., 2006]. Observations of BC mass concentrations were made at selected rural sites, more than 3 km away from any major industry and highway between 1100 and 1700 hrs. The average BC mass concentration during the campaign was about 2 mg m 3. Moorthy et al. [2004] reported near surface BC mass concentration of 3.5 mg m 3 over Hyderabad, higher than the average value at several locations in central and western India. The average BC mass concentrations obtained using an aethalometer during January March 1999 at Goa (15.75 N, E), a coastal station bounded by Arabian Sea on its west and Western Ghats on its east, was about 3 mg m 3 [Leon et al., 2001]. At Kanpur (26.43 N, E) an urban continental location in northern India, BC concentrations measured by an aethalometer were found to be in the 6 20 mg m 3 range during December 2004 [Tripathi et al., 2005a]. BC concentrations measured using an aethalometer onboard an aircraft over Kanpur during January 2005 were found to be 7 mg m 3, a factor of 2 higher mass than those measured over Hyderabad [Tripathi et al., 2005b]. It is clear that wintertime BC mass concentrations are higher in India than summer due to shallow boundary layer, higher DT and transport of BC from other polluted regions under favorable wind conditions Asia: China, Korea, and Japan [32] The average BC mass concentrations at Chegongzhuang, Beijing (40 N, 116 E) from PM 2.5 measurements from July 1999 to September 2000 were found to be 6.27, 10.23, 11.08, and 6.67 mg m 3 during summer, fall, winter, and spring, respectively [He et al., 2001]. The annual average BC was in the mg m 3 range (Table 3). BC mass concentrations were obtained by the thermal/ optical reflectance method. The BC mass concentration values are higher in fall and winter in Beijing while spring and summer concentrations are almost the same. [33] EC mass concentrations in PM 2.5 aerosols were analyzed in Cheju, an island station and Seoul, an urban location in Korea [Kim et al., 1999]. PM 2.5 aerosol particles were collected using a modified SCAQS (Southern California Air Quality Study) sampler in Seoul during June 1994 while in Cheju island PM 2.5 measurements were made using a low volume sampler in July August The samples were analyzed by selective thermal oxidation 11 of 19

12 method using MnO 2 for analysis [Kim et al., 1999]. EC mass concentrations were in the range of mgm 3 (Table 3) in Seoul while it was lower over Cheju and in the range of mg m 3 [Kim et al., 1999]. The higher EC mass concentrations in Seoul were attributed to the primary emissions of carbonaceous aerosols. It was noted that the measurement period in Cheju could be divided into two periods, higher EC mass concentrations when EC was comparable or lower than those found over other clean areas in the world. The higher EC mass at Cheju could have resulted due to atmospheric transformation/transport of manmade and biogenic emissions while the lower EC values represent marine background concentrations in the location [Kim et al., 1999]. [34] BC mass concentrations were measured at five islands during Asian-Pacific Regional Aerosol Characterization Experiment (ACE-Asia) field study which was conducted during March May 2001 in the vicinity of Japan, Korea, China, and Chinese Taipei. The five islands in which BC measurements were made during ACE Asia are (1) Rishiri (45.12 N, E), (2) Sado (38.25 N, E), (3) Hachijo (33.15 N, E), (4) Chichjima (27.07 N, E), and (5) Amami-Oshima (28.44 N, E) (Table 3). BC measured during ACE-Asia were found to be low and in the range of mg m 3 during March April 2001 [Uno et al., 2003]. Particulate elemental carbon concentrations were measured by thermal analysis method using ambient carbon particulate monitors (Rupprecht and Patashnick Co. Inc., Model 5400) at 4-hour intervals in Rishiri, Sado, Hachijo and Chichijima [Uno et al., 2003]. Amami-Oshima BC concentrations were measured by collecting the fine particles on quartz fiber filters which were analyzed by combustion and gas chromatography [Uno et al., 2003]. The BC values are lower as they are measured during the spring season. The average BC concentrations from aethalometer measurements made at Saint- Denis, an urban site of La Réunion island (21.5 S, 55.5 E) in the Indian Ocean in November 1996, April and September 1998 were found to be in the mg m 3 range [Bhugwant et al., 2000]. The BC mass concentrations over Maldives were found to be high and in the mg m 3 range during February 1999 [Chowdhury et al., 2001]. BC mass concentrations over Maldives were obtained by using the thermal-optical carbon analysis method Europe [35] A phenomenology of aerosols over Europe [Putaud et al., 2003] was obtained from measurements made over various locations for 10 years from 1991 to The sites have been classified into six categories: (1) natural background, (2) rural background, (3) near city background, (4) urban background, (5) free troposphere, and (6) kerbside. Natural background sites are located at >50 km away from large pollution sources. Rural background sites are in the km range from pollution sources. Near city sites are in the 3 10 km range from large pollution sources. Urban background locations are located within a radius of 50 m in an urban area where the number of vehicles per day are <2500. Kerbside locations indicate stations which are within street canyons [Putaud et al., 2003]. BC mass concentrations obtained from the following locations in natural, rural, near city, urban, and kerbside over Europe and their seasonal variations are compared with those found over Ahmedabad. The sites are natural : (1) Sevettijarvi (69.35 N, E) (N1), (2) Skreadalen (58.82 N, 6.72 E) (N2), (3) Birkenes (58.38 N, 8.25 E) (N3); rural (4) Chaumont (47.05 N, 7.58 E) (R1), (5) Illmitz (48.23 N, E) (R2); near city (6) Waasmunster (51.12 N, 4.08 E) (NC); Urban (7) Zürich (47.37 N, 8.53 E) (U1), (8) Basel (47.53 N, 7.58 E) (U2), (9) Gent (51.02 N, 3.73 E) (U3), (10) Bologna (44.53 N, E) (U4); kerbside (11) Barcelona (41.37 N, 2.12 E) (K1), (12) Bern (46.95 N, 7.43 E) (K2), and (13) Wien (47.75 N, E) (K3). Carbonaceous aerosol analysis (organic carbon, elemental carbon) in the above mentioned sites in Europe were done by thermo-optical evolved gas analysis, light reflectance, coulometry, and nondispersive infrared techniques [Putaud et al., 2003]. The uncertainty in the determination of total carbon was estimated to be less than 10%. [36] Seasonal average BC mass concentrations for the above locations for winter (December, January, February (DJF)), spring (March, April, May (MAM)), summer (June, July, August (JJA)), fall (September, October, November (SON)) and the annual averages (AVG) are plotted in Figure 5. The BC mass concentrations are less than 1 mg m 3 at all natural sites which increases to about 2 or more in rural sites. BC mass concentrations at a rural site Illmitz are comparable to near city location of Waasmunster. The urban sites show a variation in BC mass concentration from 2 to 4 mg m 3. Kerbside locations exhibit a higher range of variation in BC mass concentration varying from about 2 mgm 3 to 11 mg m 3. No seasonal variations in BC mass concentrations are seen in natural background sites in Europe. In Chaumont, a rural site also there is no seasonal variation while at another rural site Illmitz a factor of 4 variation in BC mass concentrations is seen which vary from a low of 0.66 mg m 3 in summer to a high of 2.74 mg m 3 in winter. BC mass concentrations are more or less the same with a value of around 1 mg m 3 in Waasmunster, a near city location. However, the BC mass concentration increases to 2.2 mg m 3 during fall (SON) in Waasmunster. Seasonal variations in BC mass concentrations are seen in all the urban locations and the BC mass concentrations are higher in fall and winter than during spring and summer. Seasonal variations are highly pronounced in all the kerbside sites. In kerbside locations also, the BC mass concentrations during spring and summer are lower than those measured in fall and winter. BC mass concentrations in Wien, a kerbside site are higher than 6 mg m 3 during all seasons and the annual average is higher than 9 mg m 3.It was noted that the natural background sites were also contaminated by long-range transport of anthropogenic aerosol and vehicular traffic was responsible for the high concentrations of BC seen at urban and kerbside locations in Europe. [37] The mean BC mass concentrations measured using an aethalometer during August October 1997 in Paris nearby a high traffic road were found to be high at about 14 ± 7 mg m 3 [Ruellan and Cachier, 2001]. The high values of BC were attributed to the high proportion of diesel engines in the vehicle fleet in France. The density of the truck traffic was also high on the ring motorway which is one of the main transit roads over the Paris area. As diesel vehicles emit more BC than gasoline vehicles [Weingartner 12 of 19