Characterization of Particulate Matter and Carbonaceous Aerosol over Two Urban Environments in Northern India

Monitoring and simultaneous sampling of Particulate matter (PM10 and PM2.5) was carried out for the first time over two urban sites in Northern India (Jabalpur and Udaipur) during December 2010–November 2012 (up to August 2012 over Udaipur). The samples of PM2.5 were analyzed for elemental carbon (EC) and organic carbon (OC) using advanced DRI Thermal optical carbon Analyzer. The monthly mean PM10 values were as high as 149 ± 44 μg m over Jabalpur (JBL) and 171 ± 42.2 μg m over Udaipur (UDPR). PM2.5 mass over JBL varied between 25–79 μg m and over UDPR between 24–82 μg m. The monthly mean OC concentration varied from 12.5 ± 7.3 μg m to 28.4 ± 10.7 μg m over JBL and from 7.8 ± 2.9 to 39.7 ± 11.6 μg m over UDPR. The variation of monthly mean EC concentration was from 3.9– 10.3 μg m over JBL and from 3–10.9 μg m over UDPR. The contribution of TC to PM2.5 was in the range of 31–75% over JBL and 30–83% over UDPR. The EC showed higher concentration in winter and minimal values in monsoon. The OC/EC ratio showed low variation over JBL compared to that over UDPR suggesting spectrum of sources responsible for EC and OC components over UDPR. Formation of secondary organic carbon (SOC) was also recognized as a potential component altering OC/EC ratio. Up on extracting the sub fractions, it is found that OC2 and OC3 are the major component over both the sites contributing up to 51% to the total OC in different months. EC1 (EC component derived at 580°C) was found to be the major EC component contributing up to 79% over the sites.


INTRODUCTION
Aerosols influence the climate directly and indirectly through radiative forcing.Out of several aerosol species, carbonaceous aerosols are important in air pollution as well as in climate change perspectives.Carbonaceous aerosols are generated from vehicular exhaust, power generation biofuels, and biomass burning.Significant contribution from biogenic sources also was reported for the production of OC (Saarikoski et al., 2008a).Elemental carbon (EC) also known as Black carbon (BC) (Rosen and Novakov, 1977) has been regarded as a strong climate forcing agent after CO2 (IPCC, 2007).Organic carbon (OC) is another carbonaceous species, which also has considerable effect on climate.OC generally is a scattering aerosol species and it also acts as a potential cloud condensation nuclei (Saxena et al., 1995;Sahu et al., 2011;Novakov and Penner, 1993).However EC is a strong absorbent of solar radiation and warms the earth atmosphere system (Panicker et al., 2013a;Park et al., 2010).BC (EC) is found to be a global climate forcing agent, contributing significantly to positive radiative and thus enhancing global warming (IPCC., 2013).The BC effect over snow and ice cover found to be enhancing a heating by 0.04 Wm -2 (0.02 to 0.09 Wm -2 ) (IPCC., 2013).EC aerosols along with its direct impact also influence the climate indirectly by modifying cloud microphysics, known as indirect effect (Kumar et al., 2011).BC inside and around the cloud enhances evaporation and cloud dissipation by solar absorption and it is known as semi direct effect (Hansen et al., 1997;Panicker et al., 2014).Hence it is essential to characterize OC and EC aerosols to unravel its role in climate change.In this scenario, this paper aims to report the concentrations, temporal variations and source characteristics of these two important climate forcing agents (EC and OC) over two urban sites in Northern India, Viz.Jabalpur (JBL) and Udaipur (UDPR).This two regions are characterized with extreme weather events with temperature reaching up to 45°C in summer and with cold winters, inducing high air pollution issues.Many studies report the variation of EC and OC across different Indian region (e.g., Ram et al., 2008Ram et al., , 2012;;Satsangi et al., 2012).However this study is believed to be the first of its kind over both the regions, which simultaneously address PM and carbonaceous aerosol characteristics.

Sampling Methods and Data Analysis
Particulate Matter (PM) has been continuously monitored using a Met one Instrument Beta Attenuation Monitor (Model BAM-1020) over Jabalpur (23.16N; 79.93E) and Udaipur (24.5N; 73.6E).Location maps of both the observation sites are shown in Fig. 1.Udaipur in located in Northwest India, and the region experiences frequent dust storm events from the adjacent desert regions.Jabalpur is an industrialized area in Northern India.The detailed Monthly mean meteorological features of the sites are provided in Fig. 2. The monthly mean temperature was as low as 14.5°C over Udaipur in December and it was maximum of 34.3°C in April.The monthly mean temperature over Jabalpur varied between minimum of 13.5°C to maximum of 34.5°C.Rainfall recorded high during monsoon, peaking up to 11mm day -1 over Udaipur and 17 mm day -1 over Jabalpur.BAM-1020 automatically measures and records airborne PM concentration levels using the principle of beta ray attenuation.In this technique, a simple determination of concentration in micrograms of particulate per cubic meter of air is carried out.A small 14C (carbon 14) element in the analyzer emits a constant source of high energy electrons known as beta particles.These beta particles are detected and counted by a sensitive scintillation detector.An external pump pulls a measured amount of air through a filter tape.After the filter tape is loaded with dust, it is automatically placed between the source and the detector thereby causing an attenuation of the beta particle signal.The degree of attenuation of the beta particle signal is used to determine the mass concentration of PM on the filter tape, and hence the volumetric concentration of PM in ambient air.Span check of the instrument is automatic and measurement cycle is verified hourly (Ali et al., 2013).The mass concentration was observed at every 5 min interval and then stored in data repository as 1-h average.
Apart from above measurement, PM  compounds and EC at different temperatures.The instrument uses IMPROVE_A protocol (Chow et al., 2011) to detect EC and OC concentrations.OC is extracted by volatilizing it from the sample deposit in non-oxidizing helium (He) atmosphere, while EC must be combusted by an oxidizer.The analyzer uses a small sample punch (0.496 cm 2 ) taken from a quartz-fiber filter and converts the carbonaceous compounds to carbon dioxide (CO 2 ) by passing the volatilized compounds through an oxidizer (heated manganese dioxide, MnO 2 ).Further CO 2 is reduced to methane (CH 4 ) by passing the flow through a methanator (hydrogen-enriched nickel catalyst) and CH 4 equivalents of OC and EC are quantified with a flame ionization detector (FID) (Holm, 1999).The reflectance and transmittance component used in the analyzer corrects the pyrolysis charring of OC compounds into EC.Without this correction, the OC fraction of the sample might be underestimated and the EC fraction might include some pyrolyzed OC.Subtractions of EC (EC1, EC2, EC3) and OC (OC1, OC2, OC3, OC4) are also extracted in this method.OC 1 , OC 2 , OC 3 , OC 4 are obtained respectively at 140, 280 and 480, 580°C temperatures in non-oxidizing helium (He) atmosphere and EC 1 , EC 2 and EC 3 are obtained respectively at 580, 740 and 840°C temperatures in an oxidizing atmosphere of 10% oxygen in a balance of helium.
The error introduced in the quantification of mass concentration of particulate OC due to adsorption of organic vapor on to and inside the filter paper during sampling is corrected by field blank sampling method as explained in Ali et al. (2015).One passive sample of 24-hour duration was collected nearly in the middle of every month.The passive sampling was made by loading the quartz filter paper in the sampler and keeping the sampler at the usual sampling point for 24 hours without running the pump.The organic vapor present in the ambient air diffused to the quartz filter surface and adsorbed there.The sampling duration was kept 24 hours in order that the adsorbed organic vapor may reach to its saturation level.Thermal analysis of these samples could yield amount of OCs (OC1, OC2, OC3 and OC4) in unit of µg(C) cm -2 and then in µg(C) filter -1 by multiplying the values in unit of µg(C) cm -2 with the exposed area of the filter paper.Monthly average amount of OCs was prepared based on collection of all passive samples over the entire period considered in the present study.These average amounts of individual OCs obtained for each month represent probable sampling artifact of that month due to adsorption of gaseous organic carbon in the filter paper.Thus the sampling artifact of the OCs of the active samples of a month has been removed by subtracting from it the month's average OCs of the passive samples.This method is described in detail in Ali et al. (2015).

Variation of Particulate Matter
The variation of PM over JBL and UDPR were studied during February 2011-November 2012.The diurnal pattern of PM 10 and PM 2.5 showed a bimodal peak with morning peak between 8-10 Hrs and evening peak during 18.00-21 Hrs over both the regions (Fig. 3).The highest PM 10 peak was up to 215 µg m -3 over UDPR and it was up to 160 µg m -3 over JBL.PM 2.5 showed highest peak of 142 µg m -3 over UDPR and was up to 112 µg m -3 over JBL.The morning peak mainly is attributed to meteorology, as the particles gets lifted up due to the breakup of nocturnal boundary layer by solar heating (Safai et al., 2007).The evening peak could be a double way effect by enhanced traffic emissions due to after work hours and the trapping of pollutants as a result of the temperature inversions and lowering of boundary layer.Similar diurnal variation of PM 2.5 /BC was reported by many investigators over different Indian regions (Ramachandran et al., 2007;Reddy et al., 2007;Safai et al., 2007).
The seasonal variation of PM is depicted in Fig. 4. PM 10 showed an average variation of 48-149 µg m -3 during different months over Jabalpur.PM 2.5 Values varied between 25-79 µg m -3 during the same periods over JBL (Fig. 4).The variation of PM 2.5 was found to be 24 to 82 µg m -3 over UDPR.Highest value of PM 10 (149 ± 17.8 µg m -3 ) was observed in March 2012 and lowest value in September 2012 (48 ± 7.4 µg m -3 ) over JBL.Similarly, highest PM 10 also were observed in March 2012 (171 ± 40 µg m -3 ) over  UDPR and lowest in August 2011(54 ± 7.5 µg m -3 ).It may be noted that PM 10 values found to have shown higher concentrations in pre-monsoon (March-May) followed by winter (December-February) over both the sites (Table 1).However, PM 2.5 over both the sites showed higher concentration in winter (December-February) followed by post-monsoon (October, November) seasons (Table 1).This discrepancy in PM 2.5 concentrations compared to PM 10 trends could mainly be associated with the temperature patterns.Another reason could be associated with high biomass burning during these times as suggested by fire counts over the region (Fig. 5(c)).The autumn and winter season in Northern India is associated with high temperature inversions, which leads to trapping of particles with small sizes in lower atmosphere (Ramachandran et al., 2006;Safai et al., 2007).The lower concentrations of PM 10 and PM 2.5 during Monsoon seasons (June-September) could mainly be associated with the rain out and washout during monsoon precipitation.The higher concentration of PM 10 in pre-monsoon season may be associated with the higher temperature during this season leading to excessive convection and hence lifting of dust over the regions (Panicker et al., 2013b).Another reason could be associated with the influx of aerosols from adjacent areas.To confirm the air mass path, we conducted the back trajectory analysis for enhanced PM loading periods during winter and premonsoon periods over JBL and UDPR (As PM 10 was highest in March, bi weekly day back trajectory was extracted in mid March; and in mid November and mid January as a representative of Post-monsoon and winter seasons).It is evident from Figs. 5(a) and 5(b), that, air mass back trajectories are directed from far west subcontinents during winter and pre-monsoon over JBL, carrying desert dust aerosols to the region.However, major aerosol sources in UDPR were found to be from/through arid Thar Desert region in March, and from Arabian subcontinent in January, confirming influx of desert dust during this period could be the reason for high values in PM, especially PM 10 .

Temporal Variation of EC and OC
The variation of OC, EC and its components are depicted in Fig. 6.EC and OC concentrations were analyzed for different seasons during 2011-2012.The average OC concentrations were found to be as high as 39.7 ± 11.1 µg m -3 over Udaipur (In May 2012).Jabalpur showed relatively lower concentrations than that observed over Udaipur.Highest OC values were found to be 28.4 ± 10.7 µg m -3 (February 2011).Fig. 6(a) depicts the monthly variation of OC and EC over Jabalpur and Udaipur.The concentration of EC varied between 3.9-10.3µg m -3 over JBL and from 3-10.9 µg m -3 over UDPR in different seasons.JBL showed higher OC and EC values during winter months.Unlike this pattern, OC and EC showed higher mass over UDPR in pre-monsoon (In 2012), post-monsoon and also in winter (In 2011) seasons.The high concentration in winter could be mainly associated with the high bio mass burning (Especially wood burning for heating the surroundings in winter) along with meteorological factors such as lower temperature; mixing layer height and wind speed influencing the pollutant transport.It may be noted from Fig. 2. That the temperature and wind speed were low in winter, leading to lower ventilation and lower dispersion of pollutants, leading to trapping of the pollutants in near surface and lower troposphere.The OC concentrations found to decrease in pre-monsoon and reaching to minimal values in monsoon season over JBL in 2011.Again from Fig. 2, it may be noted that rain fall was higher in monsoon seasons over both the regions.Hence, the lower OC and EC values in monsoon season over both the sites are mainly associated with the wet removal of aerosols by precipitation.
The contribution of Total Carbonaceous aerosols (TC) to the PM were analyzed and are depicted in Table 1.The contribution of TC to PM 2.5 was found to be 30-83% over UDPR and was 31-75% over JBL.The high contribution of TC to PM 2.5 in Jabalpur could be associated with the high number of industries and high crop residue burning present over the region.It may be noted that there were significant biomass burning in winter, pre-monsoon and post-monsoon seasons as depicted by fire count in Fig. 5(c).High contribution of PM 2.5 in TC over Udaipur could also be associated with biomass burning (depicted by fire count in Fig. 5(c)).Another reason could be frequent dust storms   in Fig. 6(b).Carbon content in each of these fractions differs by carbon sources (Watson et al., 1994).The reason for lower concentrations of OC1 could be associated with the highly volatile nature of this component.OC2 and OC3 found to be the major contributor to OC concentrations over Udaipur.OC2 was found to contribute from 27 to51% during different months.However the contribution of OC3 was found to vary from 31 to 44% on different months.The contribution of OC3 was found to be high during winter months.However the contribution of OC2 dominated in winter and monsoon months.The contribution of OC2 was found to be 19-41% over JBL.However, the contribution of OC3 was found to be 26-42% over the site.The major source for OC3 was found to be enriched in cooking exhaust (43%) as well as in vegetative burning and motor vehicle exhaust (> 10%) (Chow et al., 2004).Paved dust and unpaved road dust contributes 1to 10% of the OC3 mass (Chow et al., 2004).In northern India, biofuel has been used for cooking and biomass burning is common during winter.
Apart from these sources, vehicular exhaust and road dust could be other reasons for enhanced OC3 contribution in winter.The major contribution to OC4 are derived from soil, Cooking composite, cement kiln composite, motor vehicle composite and paved as well as unpaved dust.EC1 was found to be the major contributor of EC fraction in both the sites.It contributed 37-79% over Udaipur and 45-76% over Jabalpur.The contribution of EC1 found to be less in monsoon months (36-50%) and the contribution of EC2 and EC3 found high in this period.The major source of EC1 is reported to be motor vehicle exhaust and vegetative burning (Chow et al., 2004;Cao et al., 2005).Other major contributing factors for EC1 are coal burning and cement Kline.EC2 and EC3 are also contributed from oil combustion and vehicular exhausts (Yu et al., 2002).Hence this suggests that major EC subtractions over both the sites are vehicular exhaust.To confirm the major sources contributing to OC and EC, we have conducted the OC/EC ratio method and which is discussed in the next section.

OC/EC Ratio and Source Characteristics
OC/EC ratio has been used as a methodology to estimate the source characteristics of OC and EC (Ram et al., 2008).OC/EC ratio showed wide variation in UDPR, compared to JBL (Fig. 7).The ratio varied between 1.5 to 21 over UDPR suggesting contribution from discrete sources for carbonaceous aerosols.However, the variation of ratio found to be less over Jabalpur (2.7-11) suggesting moreover constant sources of contribution of OC and EC.The OC/EC ratios for 1 to 4.2 correspond to diesel and gasoline-powered vehicular exhaust (Schauer et al., 1999(Schauer et al., , 2002)).2.5 to 10.5 suggest high residential coal burning (Chen et al., 2006).Watson et al. (2007) suggested a value of 4.2 for residential wood burning.OC/EC ratio 0.71 represents traffic, 0.8 heavy duty diesel vehicles and 2.2 light duty gasoline vehicles (Saarikoski et al., 2008b).For biomass burning OC/EC ratio reported was 7.7 (Zhang et al., 2007;Feng et al., 2009) and it was 12 for long range transport (Saarikoski et al., 2008b).Hence the values above 14 over Udaipur suggest the influx of carbonaceous aerosols by long range transport.However residential biofuel burning, biomass burning and vehicular exhaust found to be the dominant contributors of EC and OC over Jabalpur on different months.The Seasonal mean OC/EC ratio over JBL and UDPR are shown in Table .2. Seasonal mean OC/EC ratio range between 2.3-6.4 over UDPR and between3.93-4.85 over JBL.The annual mean OC/EC ratio over UDPR found to be 4.36 and 3.36 respectively in 2011 and 2012.The annual mean OC/EC ratio over JBL was 4.3.High OC/EC ratio represents high value of OC and low EC, which majorly happens during (a)    (Ram and Sarin. 2012).Hence this relatively low OC/EC ratio over both the sites reveals that the major contributor of OC and EC over JBL and UDPR are vehicular exhaust (Schauer et al., 1999(Schauer et al., , 2002)).The OC/EC values obtained here found to be comparable with those reported over other Indian regions (Singh et al., 2002;Venketaraman et al., 2002;Rengarajan et al., 2007;Ram et al., 2008).
The formation of secondary organic carbon (SOC) attributed to oxidation and gas to particle conversion of volatile organic compounds (VOCs) are reported to be an important factor influencing the OC/EC ratio and hence source characteristics.Generally OC/EC ratios above 2 indicate formation of SOC (Satsangi et al., 2012).
We have estimated the SOC using the minimum OC/EC ratio method as suggested by Ram et al. (2008); Satsangi et al. (2012).SOC has been calculated using the equation SOC = (OC) tot -Primary Organic Carbon (POC) (1) where POC = (OC/EC) min .(EC) tot The (OC/EC) min in each season , calculated Primary Organic Carbon (POC),SOC concentrations and its contribution to total OC concentration are depicted in Table 3.
SOC contribution found to be 24-60.5% over UDPR in different seasons.However it found to be 14-39% over JBL.The contribution found to be highest in monsoon season over UDPR in 2011.However it was during post-monsoon in 2012.The highest concentration of SOC was found in post-monsoon season over JBL.This high SOC over JBL and UDPR results in High OC/EC ratios over these regions.
Several studies report variation of OC and EC across different Indian regions.Singh et al. (2007) and Satsangi et al. (2010) reported high OC values over Delhi and Agra (60.9-68.6 µg m -3 ).The EC value found very high over Delhi (38.5 µg m -3 ).Venketaraman et al. (2002) reported OC concentrations of 25 µg m -3 over Mumbai.Rengarajan et al. (2007) found an OC concentration of 33 µg m -3 over Hissar in Northern India.The EC concentrations found to be 12 µg m -3 over Mumbai and were 3.8 µg m -3 and 7.5 µg m -3 over Hissar and Agra respectively.Mandal et al. (2014) also reported high PM 10 and EC, OC over Delhi during winter months.They showed a very high mass of OC (Up to 159 µg m -3 ) and EC (up to 47 µg m -3 ) over Delhi.The Carbonaceous aerosol contribution found to be up to 66% over Delhi in winter, indicating huge pollution levels over the region.The OC concentrations found to be less over mountainous locations.It found to be respectively 3.7 µg m -3 and 4.9 µg m -3 over mount Abu and manora peak (Rengarajan et al., 2007;Ram et al., 2008).However, Ram et al. (2010) reported an increase in EC and OC concentrations during dust storm events over high altitude sites.On an extensive study, Rajput et al. (2013) has shown that, paddy residue burning is the major contributing factor for higher OC mass and PM 2.5 over Indogangetic planes.However, the dominant source for EC was found to be wheat residue burning over the region.Pavuluri et al. (2011) reported high EC mass over winter season.However OC did not show much seasonal difference in winter and summer seasons.A comparison study of OC and EC over different environments in India found that, EC and OC are highest in Allahabad site in Northern India compared to other urban, high altitude and oceanic regions (Ram and Sarin, 2012).

SUMMARY
1. Continuous monitoring and sampling of PM (PM 10 and PM 2.5 ) were carried out over two urban sites (Jabalpur and Udaipur) over Northern India.Carbonaceous aerosol species (Organic Carbon and Elemental carbon) were extracted from the sampled PM 2.5 concentrations.2. PM 10 showed mass as high as 171 ± 40 µg m -3 over UDPR and 149 ± 17.8 µg m -3 over JBL.PM 2.5 concentration ranged between 24 to 82 µg m -3 over UDPR and 25-79 µg m -3 over JBL. 3. The EC and OC concentration in both the sites found to be higher during winter majorly contributed by trapping of pollutant by temperature inversions leading to lower mixing layer height and less pollutant dispersion.The washout and rain out during monsoon could be the reason for lower carbonaceous concentrations during this season.4. The contribution of TC to PM 2.5 was found to be 30-83% over UDPR and was 31-75% over JBL 5.In the sub components, it is found that OC2 and OC3 are the major OC components, contributing up to 51% of total OC concentration.EC1 was found to be major EC component contributing to the EC concentration (up to 79%).6.The source characteristics of carbonaceous aerosols on different seasons were examined using OC/EC ratio method.Biomass burning and vehicular exhaust found to be the dominant contributors of EC and OC over JBL; however divergent sources (biomass burning/road dust/ fossil fuel and long range transport) are responsible for origin of carbonaceous species over UDPR.7. SOC contribution found to be 24-60.5% over UDPR in different seasons.However it found to be 14-39% over JBL.Hence formation of SOC is identified as a reason for High OC/EC ratios during Monsoon in JBL and premonsoon in UDPR.
Fig. 1.Location Map of Jabalpur and Udaipur.

Fig. 4 .
Fig. 4. Monthly variation of PM 10 and PM 2.5 over Jabalpur and Udaipur.

Table 1 .
Seasonal variation of Particulate Matter (PM 10 and PM 2.5 ); Contribution of carbonaceous aerosols to PM over JBL and UDPR.

Table 2 .
Seasonal mean OC/EC ratio over JBL and UDPR.

Table 3 .
OC/EC ratio, POC, SOC and contribution of SOC to OC over JBL and UDPR.