Aerosol and Air Quality Research, 16: 1615–1624, 2016 Copyright © Taiwan Association for Aerosol Research ISSN: 1680-8584 print / 2071-1409 online doi: 10.4209/aaqr.2016.01.0023

Size-Segregated Characteristics of Carbonaceous Aerosols over the Northwestern Indo-Gangetic Plain: Year Round Temporal Behavior Atinderpal Singh1*, Neeraj Rastogi2, Anil Patel2, R.V. Satish2, Darshan Singh1 1 Department of Physics, Punjabi University, Patiala, Punjab, India 2 Geosciences Division, Physical Research Laboratory, Ahmedabad, Gujarat, India ABSTRACT

Size-segregated aerosol samples (PM<0.95, PM0.95–1.5, PM1.5–3.0, PM3.0–7.2 and PM>7.2) were collected over Patiala (30.33°N, 76.40°E; 250 m amsl), a semi-urban city located in northwestern Indo-Gangetic Plain (IGP) during October, 2012 to September, 2013. These samples were analyzed for carbonaceous aerosols (organic carbon (OC), elemental carbon (EC) and water-soluble organic carbon (WSOC)) to study their temporal variation, prevailing emission source (s) and secondary formation processes. Annual average of total suspended particulates (TSP) concentration, estimated by adding the aerosol concentrations in different size ranges, was found to be 199 ± 82 µg m–3, varying from 88–387 µg m–3 with majority of particulate mass found in submicron size (PM<0.95). OC3.0 and WSOC3.0 ranged from 5.4 to 70 µg m–3 (23 ± 14 µg m–3) and 2.5 to 37 µg m–3 (12 ± 8.7 µg m–3), respectively over an annual cycle. Highest mass fraction of OC (> 75%) and WSOC (> 80%) was observed in submicron size, suggesting that OC mainly comes from combustion and/or secondary source (s). It has been observed that almost half of OC is secondary. On the other hand, climate forcing agent EC in PM<0.95 varied from 1.1 to 9.8 µg m–3 (4.8 ± 2.2 µg m–3). High mass ratios of OC/EC (~1.5–7.2) and WSOC/OC (~0.33–0.68) indicate the relative dominance of biomass burning emission over the study region. Total carbonaceous aerosols account for ~10–59% of submicron particulates mass (PM<0.95), indicating that fine particulates are enriched with carbonaceous species. These results have implications to regional climate model development and validation. Keywords: Organic carbon; Water-soluble organic carbon; Elemental carbon; Secondary organic aerosols; Biomass burning emissions. INTRODUCTION

Carbonaceous aerosols consist of both light-scattering organic carbon (OC) and light-absorbing elemental carbon (EC) (Bond et al., 2004; Kondo et al., 2011). EC is emitted directly into atmosphere from incomplete combustion of fossil fuel and biomass burning, while OC can either be released directly into atmosphere or formed through gas-to-particle conversion in the atmosphere. Organic aerosols are documented to be the major component (20–90%) of non-refractory submicron particulate matter (Jimenez et al., 2009). Carbonaceous aerosols directly affect air quality and Earth’s radiative balance (Gustafsson et al., 2009 and references therein). Additionally, they also affect microphysical properties of clouds by acting as a cloud condensation nuclei (Haywood and Boucher, 2000). Biomass burning is among prominent source of ambient carbonaceous aerosols * Corresponding author.

Tel.: +91-94177 82219 E-mail address: atinderastro@gmail.com

that may have significant impact on the radiative balance and human health (Kharol and Badrinath, 2006; Singh et al., 2016). On global scale, aerosols from biomass burning emissions significantly reduce the solar radiation reaching Earth’s surface and their contribution to radiative forcing was estimated to be +0.04 ± 0.07 (IPCC, 2007). The atmospheric aerosols produced from fossil fuel combustion and biomass burning has been reasonably studied and reported (Simoneit, 2002; Venkataraman et al., 2005; Wang et al., 2010). On the other hand, the assessment of aerosols emitted from agriculture waste burning emission is rather lacking in the literature (Hays et al., 2005). In Indian sub-continent, northwest part of India is affected by two distinct post-harvest agriculture waste (paddy and wheat residue) burning emissions (Rajput et al., 2011). The extensive paddy residue burning takes place during October–November and wheat residue burning during the first half of May every year. It makes the northwest part of India as important region to study carbonaceous aerosols produced from agricultural waste burning emissions. Further, these emissions get transported to the Bay of Bengal and Arabian Sea through the IGP and thus, affect regional air quality and marine atmospheric chemistry (Rastogi and Sarin, 2008; Sarin et al., 2010; Kaskaoutis et

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al., 2014). There are a few studies in literatures which have reported the chemical characteristics of PM2.5 from the northwest part of India (Rajput et al., 2011; Rastogi et al., 2014; Rajput et al., 2014; Rastogi et al., 2016; Srinivas et al., 2016); however, the study on size-segregated chemical composition of carbonaceous aerosols over northwestern part of the IGP is still lacking in literature. Size distribution of aerosols is a strong indicator in understanding their primary and secondary sources and atmospheric formation process. The natural aerosols, produced by physical mechanism, are relatively large in size as compared to aerosols emitted from anthropogenic sources (Gerasopoulos et al., 2007). Furthermore, the size distribution of carbonaceous aerosols greatly affects the climate forcing as suggested by the previous model investigations (Gong et al., 2003; Park et al., 2011). Therefore, there is important to perform systematic studies on the characterization of size-segregated carbonaceous aerosols. In this context, present study aims to understand size-segregated PM mass, OC and WSOC along with temporal variability of light absorbing EC over the study region, and compare these values with those documented from downwind regions and other Asian countries. SITE DESCRIPTION AND METEOROLOGY

The present study has been carried out at the source region of biomass burning emissions (Patiala; 30.33°N, 76.40°E, 250 m above mean sea level; Fig. 1) of the northwest part of the IGP. The farmers of this region and nearby states (Haryana and Uttar Pradesh) grow both wheat and rice crop in rotation in a year, and the harvesting leaves large quantities of straw in the fields. The large fraction of wheat residue, after harvesting, is used for animal feed but the rice

stubble, being unfit for animal feed, is burnt in the open fields, which emits large amount of submicron particle and trace gases to the atmosphere (Sahai et al., 2007). The present study examines characteristics of size-segregated carbonaceous aerosols with temporal record of a yearlong data (October, 2012–September, 2013). Depending upon prevailing meteorological conditions, the data was grouped into five seasons named autumn (October–November), winter (December–February), spring (March–April), summer (May–June) and wet (July–September). Seasonal average 5-days air mass back trajectories ending at Patiala at the altitude of 1500 m have been computed using Hybrid Single-Particle Langrangian Integrated Trajectory (HYSPLIT) model. The prevailing winds were northwesterly during autumn to spring, and changed to southwesterly during summer. During wet season, the study site receives moisture from the Arabian Sea and the Bay of Bengal, and the prevailing winds remain south-easterly or south westerly (Fig. 1). More details about the site description have been provided in our earlier publications (Rastogi et al., 2014; Singh et al., 2015; Rastogi et al., 2016). The meteorological parameters such as ambient temperature, relative humidity and wind speed have been measured using automatic weather station of Indian Meteorological Department (IMD) observatory, Patiala as shown in Fig. 2. The average values of these meteorological parameters and total rainfall are summarized in Table 1. SAMPLING AND ANALYSES

Size-segregated aerosol samples (n = 54, where one sample is set of 5 filters) have been collected on tissuquartz and glass-fiber filters using four stage high volume sampler (Staplex, USA; Model: 234) at the flow rate of 1.1 m3 min–1

Fig. 1. Map showing sampling location Patiala (PTA) and their nearby locations Delhi (DLH) and Kanpur (KNR) with seasonal average 5-days air mass back trajectories at altitude of 1500 m at Patiala.

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Fig. 2. Day to day variation of daily average temperature (temp), relative humidity (RH) and wind speed (WS) over Patiala during October, 2012 to September, 2013.

Table 1. Seasonal mean value of temperature (Temp), relative humidity (RH), wind speed (WS) and total rainfall (RF).

Seasons Temp (°C) (Avg ± 1σ) RH (%) (Avg ± 1σ) WS (m s–1) (Avg ± 1σ) RF (mm) (Total) Autumn 27 ± 3 56 ± 10 1.0 ± 0.8 4 Winter 17 ± 4 71 ± 15 4.0 ± 3.0 140 Spring 30 ± 3 49 ± 11 6.0 ± 4.0 25

Summer 38 ± 3 54 ± 13 4.0 ± 2.0 148 Wet 34 ± 2 76 ± 09 2.0 ± 1.0 597

with sampling time from 09:00 to 18:00 Indian standard time (IST). First, second, third and fourth stages of cascade impactor collect the particles with aerodynamic diameter > 7.2, in between 7.2–3.0, 3.0–1.5 and 1.5–0.95 µm, respectively. Cascade impactor is facilitated with back up filter which collected the particles having size less than 0.95 µm. After collection, filters were packed and stored in deep freezer (–20°C) until analysis. The mass of size-segregated aerosols was determined gravimetrically on a high precision analytical balance (Sartorius, model LA130S-F). The concentrations of EC and OC were determined on EC-OC analyzer (Sunset laboratory, USA) using thermal-optical method and National Institute of Occupational Health and Safety (NIOSH)-5040 protocol (Birch and Cary, 1996). In EC-OC analyzer, OC and EC oxidized to CO2 separately, by heating the aerosol sample in inert (helium) and oxidizing condition (helium + oxygen), respectively. Subsequent conversion of CO2 to methane by methanator facilitates the quantification of OC and EC using flame ionization detector (FID). To set the OC-EC split point and correction for pyrolyzed carbon (PC), transmittance of light from a He-Ne laser source (λ = 678 nm), passing through the filter-

punch was monitored continuously. At the end of every analysis, fixed volume of methane (5% CH4 + 95% He; methane in Helium as an internal standard) was injected for quantification of OC and EC. A known amount of sucrose solution was used as an external standard to monitor the response of the detector. The average ratio of measured carbon to expected carbon in standard was 1.05 ± 0.05. The concentration of water-soluble organic carbon (WSOC) was determined using TOC analyzer (Shimadzu, TOC-5000A). For WSOC analysis, 1 punch (6 cm2) of back up filter (and 1 strip of slotted filter, 12.5 cm2 each) was taken in 14 mL Milli-Q (Resistivity: 18.2 MΩ cm) water and sonicated twice for 10 minutes. The measurement protocol includes two steps. During first step, water extract was injected into the furnace (at 680°C) and oxidized to CO2 using Platinum catalyst. The evolved CO2 was measured using a non-dispersive infrared (NDIR) detector to assess water-soluble total carbon (WSTC) content. In second step, another aliquot of the water-extract was acidified with 25% phosphoric acid (25% H3PO4, vol/vol) and evolved CO2 was measured using a NDIR detector and inferred as water-soluble inorganic carbon (WSIC). The concentration of WSOC was calculated from

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the difference between these two sets of measurements (WSOC = WSTC – WSIC). The response of NDIR detector was checked using standard solution of potassium hydrogen phthalate (KHP). The average ratio of measured carbon to expected carbon in standards was 0.99 ± 0.05. Several blank filters were analyzed for OC and WSOC concentration, and their concentration in each samples were corrected for blank concentration. Based on replicate analysis of samples, the precision of measurements for OC, EC and WSOC was found to be 8%, 6% and 4% respectively. The samples of PM<0.95, PM0.95–1.5, PM1.5–3.0 have been used for OC and WSOC analysis, and PM<0.95 samples for EC analysis due to constraint of sample deposition pattern on slotted filter substrate and instrument (EC-OC analyzer) specification. RESULTS AND DISCUSSION Size-Segregated PM Mass Concentration

During the observation period, the mass concentration of total suspended particulates (TSP) varied from 88–387 µg m–3 with highest mass concentration during autumn (267 ± 69 µg m–3), followed by summer (227 ± 77 µg m–3), winter (179 ± 55 µg m–3), spring (137 ± 38 µg m–3), and minimum in wet season (130 ± 62 µg m–3). Here, TSP was estimated by adding the particle mass concentration of PM<0.95, PM0.95–1.5, PM1.5–3.0, PM3.0–7.2 and PM>7.2 sample filters. Mass concentration of PM<0.95 varied from 25 to 237 µg m–3, PM0.95–1.5 from 2 to 47 µg m–3, PM1.5–3.0 from 2 to 29 µg m–3,

PM3.0–7.2 from 6 to 82 µg m–3 and PM>7.2 from 8 to 71 µg m–3, respectively. These results suggest that majority of particulate mass confined within the submicron size (PM<0.95) as compared to coarser size (PM3.0–7.2). These results further indicate the overall dominance of anthropogenic source (s) over the natural source (s) over northwestern part of the IGP. It is noteworthy that the absolute mass concentration of PM<0.95 was increased by ~200% during paddy residue burning relative to background mass concentration of PM<0.95

(during wet season), which has several climatic and societal implications. These implications are visible in day-to-day life of the people living in downwind region (e.g., New Delhi), where poor air quality has become major concern in autumn and winter, and thoughtful mitigation strategies are immediately required.

Carbonaceous Species in Size-Segregated Aerosols

Temporal variability in mass concentration of OC3.0 and WSOC3.0 is presented in (Fig. 3), where, OC3.0 and WSOC3.0 represent the total concentration of organic species in PM<0.95, PM0.95–1.5 and PM1.5–3.0 samples. During the entire observation period, the mass concentration of OC3.0 ranged from 5.4 to 70 µg m–3 with annual average of 23 ± 14 µg m–3. The mass concentration of OC3.0 was significantly higher during October–November as compared to rest of study period, which is attributable to extensive paddy residue burning in the open fields. During this period, OC3.0 attains the highest value of 69 µg m–3 on 31st October and then

Fig. 3. Temporal variation of OC3.0, WSOC3.0 and their relative mass distribution in different size ranges.

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gradually decreases till February. Afterwards, it decreased sharply during March–April due to relatively less anthropogenic activities, higher temperature and stronger winds (Table 1). Again during the beginning of May, steep increases in mass concentration of OC3.0 has been observed, which is attributed to wheat residue burning in the open fields. During July–September, frequent rain spells are responsible for lower abundance of OC3.0. On the other hand, the mass concentration of WSOC3.0 ranged from 2.5 to 37 µg m–3 (12 ± 8.7 µg m–3) with the similar temporal variability as observed for OC3.0 (Fig. 3). The average contribution of WSOC3.0 to OC3.0 was ~50% with mass ratio of WSOC3.0/OC3.0 ranging from 0.33–0.68 (0.52 ± 0.09) during entire study period. A significant linear relationship has been observed between OC and WSOC in different size ranges with relatively higher slope for PM<0.95 and lower for PM0.95–1.5 (Fig. 4). These observations also suggest that almost half of OC is secondary, as WSOC is often considered as a measure of secondary organic aerosols (SOA) (Ervens et al., 2011; Rastogi et al., 2015). Recently, Rastogi et al. (2015) has documented that ~80% of WSOC in PM2.5 are of secondary origin over the study region during winter. In literature, reported WSOC to OC mass ratios are relatively low for vehicular emission than those for biomass burning emission because of production of water insoluble organic species during combustion of liquid fuel. The ratio of 0.27 has been reported for vehicular emission at the site in Northern Europe (Saarikoski et al., 2008) whereas the ratio of 0.60 has been reported over northwestern IGP during the peak of agricultural waste burning emission (Rastogi et al., 2014). This also suggests that biomass burning emission is prominent source of organic aerosols as compared to vehicular emission over the study region as average mass ratio of WSOC3.0 to OC3.0 is 0.52.

Further, the abundance of OC and WSOC in different size ranges has been investigated (Fig. 3). Throughout the study period, the majority of OC (> 75%) and WSOC (> 80%) were found in PM<0.95. The contribution of OC to PM decreases

significantly as the size of PM increases, suggesting the contribution from soil deflation to OC was relatively insignificant. Soil deflation is among significant possible sources of coarse mode OC (Wan et al., 2015). Majority of OC in PM<0.95 also indicates that the dominant part of ambient OC comes from combustion source (s) and/or of secondary origin. Due to frequent removal of atmospheric species by rains, wet season can be considered as representative of background concentrations of carbonaceous species. During wet season, the mass fraction of OC to OC3.0

was distributed almost equally in submicron size (56%) and other size (PM0.95–3.0; 44%), however, absolute concentrations of OC were very low. OC contribution in submicron size was increased by 24% during autumn as compared to wet season due to extensive paddy residue burning in autumn. In case of WSOC also, maximum concentrations was associated with PM<0.95 and it decreases as the size of particulates increases, suggesting significant formation of SOA in submicron size (Fig. 3). A significant correlation (p < 0.0001, r ≥ 0.86) between WSOC and OC has been observed for all size ranges, suggesting OC and precursors of SOA come from the same source (Fig. 4).

On seasonal scale, OC3.0 shows highest mass concentration (38 ± 17 µg m–3) during autumn, followed by winter (25 ± 6.7 µg m–3), summer (19 ± 7.0 µg m–3), spring (14 ± 2.2 µg m–3), and minimum during wet season (9.4 ± 2.7 µg m–3) (Fig. 5). WSOC3.0 exhibited the similar seasonal variability as showed by OC3.0 with mass concentration of 23 ± 9.0, 13 ± 3.7, 5.9 ± 1.4, 9.8 ± 3.4 and 4.4 ± 1.5 µg m–3 during autumn, winter, spring, summer and wet seasons, respectively (Fig. 5). The boundary layer dynamics could play crucial role along with source (s) and source strength in controlling the seasonal variability of measured organic species. The average mass ratios of WSOC3.0 to OC3.0 remained between 0.51 and 0.61 during autumn, winter and summer seasons whereas it was 0.42 and 0.46 during spring and wet seasons (Fig. 5). Higher mass ratios during autumn, winter and summer seasons as

Fig. 4. Scatter plot between WSOC and OC in size range of < 0.95 (~1), 0.95–1.5 and 1.5–3.0 micron.

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Fig. 5. Seasonal average value of (a) OC, (b) WSOC, (c) WSOC/OC in PM3.0 and, (d) EC in PM<0.95.

compared to spring and wet indicates the relative dominance of biomass burning emissions over vehicular emissions to the loading of organic species during these seasons. However, the mass ratio during spring (0.42) and wet seasons (0.46) are still significantly higher than that reported for vehicular emissions (0.27) by Saarikoski et al. (2008).

Light absorbing potential of EC has drawn the attention of scientific community and recognition as a driver of global warming. This necessitates the demands of EC’s spatial distribution and temporal variability record. During present study, the mass concentration of EC in PM<0.95 varied from 1.1 to 9.8 µg m–3 with annual average of 4.8 ± 2.2 µg m–3. Similar factors are responsible for temporal and seasonal variability of EC as that for OC (discussed above). On seasonal scale, EC exhibited the highest mass concentration during autumn (6.7 ± 2.0 µg m–3), followed by summer (5.3 ± 2.1 µg m–3), winter (4.2 ± 1.1 µg m–3), spring (3.7 ± 1.2 µg m–3) and minimum in wet season (2.4 ± 1.0 µg m–3) (Fig. 5). The mass fraction of EC to PM<0.95 was found to be 4.8, 4.4, 4.9, 3.8 and 5.1% during autumn, winter, spring, summer and wet seasons, respectively. The earlier study by Rastogi et al. (2014) over study region has reported the similar mass concentration of EC (6.9 µgm–3) bound with PM2.5 during autumn. Similarity in absolute concentrations of EC in PM2.5 in documented studies and in PM<0.95 (this study) suggests that majority of EC resides in PM<0.95.

Table 2 presents observed mass concentrations of OC, EC and WSOC over Patiala as well as other sites located in the downwind region of the IGP and some other Asian countries. During autumn, observed mass concentrations of OC, EC and WSOC are somewhat lower as compared to that reported from other locations of the IGP (Delhi and Kanpur) and their mass concentrations remained also low over Patiala as compared to these locations during winter.

However, EC mass concentration is observed to be higher over Patiala as compared to Kanpur during winter (Table 2). Kanpur represents the typical urban atmosphere influenced by biomass and fossil fuel burning emissions. Industrial activities are higher over Delhi and Kanpur than that over Patiala. Further during winter season, the mass concentration of carbonaceous aerosols over the study region exhibit lower values as compared to those over Beijing (China) and Kathmandu (Nepal) whereas, it has significantly higher as compared to background site of China (Nam Co). During agricultural waste (rice straw) burning emissions period (autumn), level of carbonaceous aerosols was significantly high over study region than that over Gwangju (Korea). These differences are attributable to variation in source (s) and source strength along with difference in size of collected samples over these locations (Table 2).

As discussed earlier, the study site experiences emissions from two distinct type of agricultural waste burning every year: the paddy residue burning (PRB) generally takes place during middle of October to middle November, and the wheat residue burning (WRB) during the first half of May. The mass concentration of EC and OC (in PM<0.95) found to be relatively high during PRB (7.4 and 37 µg m–3) as compared to WRB (6.9 and 21 µg m–3) and the difference is significant in the case of organic carbon. Inter period variation in mass concentration of carbonaceous aerosols can be attributed to several factors such as difference in amount and type of biomass burnt during these two periods, boundary layer height, and efficiency of combustion. Significant amount of wheat residue after harvesting is used for cattle feed but paddy residue is unfit for animal feed. Consequently, larger amount of paddy residue is burnt. Thicker boundary layer is also responsible for low concentration of these species during WRB as compared to that during PRB.

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Table 2. Mass concentration of particulate matter (PM), elemental carbon (EC), organic carbon (OC) and water-soluble organic carbon (WSOC) over different geographical regions of Indo-Gangetic Plain and other Asian countries.

Location Sampling period Size EC (µg m–3) OC (µg m–3) WSOC (µg m–3)

Patiala, Indiaa

Autumn PM< 0.95 6.7 ± 2.0 31 ± 15 20 ± 7.6 Winter PM< 0.95 4.2 ± 1.1 18 ± 5.5 9.8 ± 2.6 Spring PM< 0.95 3.7 ± 1.2 10 ± 1.7 4.7 ± 1.2

Summer PM< 0.95 5.3 ± 2.1 14 ± 6.2 8.3 ± 3.1 Wet PM< 0.95 2.4 ± 1.0 5 ± 1.8 3.3 ± 1.2

Kanpur, Indiab Autumn PM2.5 7.4 ± 1.8 53 ± 21 39 ± 15 Winter PM2.5 3.6 ± 1.7 29 ± 15 21 ± 9.3

Delhi, Indiac, d Autumn PM2.5 8.3 ± 2.9 47 ± 12 N.A. Winter PM2.5 10 ± 5 54 ± 39 N.A.

Nam Co, Chinae Post-monsoon PM1.1 0.09 ± 0.05 1.9 ± 0.5 N.A. Beijing, Chinaf Winter PM2.5 5.6 ± 1.4 33 ± 17 11 ± 3

Kathmandu, Nepalg Winter TSP 4.5 ± 1.2 20 ± 7 10 ± 3.6 Gwangju, Koreah Autumn PM2.5 2.6 ± 1.0 21 ± 8.7 N.A.

a Present study. b Ram et al.(2012). c Tiwari et al.(2013). d Bisht et al.(2015). e Wan et al. (2015). f Yan et al. (2015). g Shakya et al. (2010). h Ryu et al. (2004).

OC to EC mass ratio is often used to characterize the source (s) of ambient carbonaceous aerosols and reported studies have suggested relatively high OC/EC ratio in particles from biomass burning emission as compared to those from vehicular emission (fossil fuel emission) (Sandradewi et al., 2008). During present study, relatively high OC/EC mass ratio (annual average = ~4) suggests the relative dominance of biomass burning emission over the vehicular emission at study region. However it was relatively low as compared to OC/EC ratio reported in earlier studies over the IGP region (Ram et al., 2012; Singh et al., 2014; Rastogi et al., 2016). This observation is attributed to difference in the size of collected samples in this study (PM<0.95) and other documented studies (PM2.5). Virtually, there is no difference in the absolute mass concentration of EC bounded with PM2.5 and PM<0.95 as EC confined mainly within submicron range (as discussed earlier), whereas OC is distributed in several size modes. Thus, OC/EC mass ratio is higher in PM2.5 samples as compared to PM<0.95 samples. On seasonal scale, the scatter plot has been made between EC and OC (in PM<0.95) to investigate their major source (Fig. 6). A statistically good correlation (p < 0.05), except during spring (p = 0.06), has been observed between EC and OC during all the seasons, however, slopes were different. Slopes show high magnitude (> 2.5) during autumn, winter and summer, indicating the dominant source of carbonaceous aerosols is from biomass burning and/or bio-fuel combustion, or from SOA formation. On the other hand, the magnitude of slopes was relatively low during spring (0.90) and wet (1.60) season suggesting the enhanced contribution of fossil fuel combustion over biomass burning. The OC to EC mass ratio of greater than 2 can also indicate the formation of SOA (Chow et al., 1996). Our observations suggest that SOA were abundant

throughout the year as OC/EC ratio centered at ~4.0 over study region. Further, the fraction of SOA to OC (in PM<0.95) has been quantified using widely used EC tracer method, suggested in literature (Turpin and Huntzicker, 1995; Castro et al., 1999). Method calculations are based on the measured EC, OC mass concentrations and minimum OC/EC mass ratio. The different minimum OC/EC mass ratios have been used for different seasons (autumn: 3.0, winter: 2.6, spring: 1.9, summer: 2.0 and wet: 1.5) as major sources of carbonaceous aerosols differ seasonally. The contribution of SOA to PM<0.95 was relatively high during winter (38%) and autumn (35%) in comparison to other seasons of study (spring: 31%, summer: 25% and wet: 33%).

Further, the scattering and absorption of incoming solar radiation depends upon the relative distribution of scattering and absorbing aerosols in aerosol mixture. During observation period, the mass fraction (to PM<0.95) of scattering aerosols (OC: ~17%) found to be higher than absorbing aerosols (EC: ~5%). Recently reported study from the sampling site shows that contribution of brighter aerosols (SO4

2– + NO3–

+ OC) is more than 50% whereas the absorbing aerosol is less than 10% (Rastogi et al., 2014). The contribution of primary and secondary organic matter (POM and SOM) to PM<0.95 were estimated using OM/OC ratio of 1.2 for non-polar (considered as primary) and 2.2 for polar organic compound (considered as secondary) (Rajput and Sarin, 2013; Rastogi et al., 2016). However, the estimated POM and SOM contribution may not be very precise due to uncertainty in conversion factor but it likely very close to accurate value. POM contribution exhibits weak temporal variability (6–23%) with high contribution during autumn (17%) and winter (13%) as compared to spring (11%), summer (9%) and wet season (9%). On the other hand, there is relatively large

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Fig. 6. Scatter plot between EC and OC during different seasons.

temporal variability (0–41%) in SOM contribution with considerably high SOM contribution during autumn (17%) and winter (15%) as compared to other seasons (spring: 9%, summer: 5% and wet: 8%). The present approximate estimation of POM and SOM contribution shows that the contribution of POM and SOM were comparable during autumn, and little increase in SOM contribution during winter. It decreases in the subsequent seasons and reverse is true for POM contributions. The contribution of total carbonaceous aerosols TCA = OM (i.e., POM + SOM) + EC to PM<0.95

showed the highest fraction (37%) during autumn, and subsequently, showed decreasing trend with fraction of 33% during winter, 25% during spring and 18% during summer. However, there was little increase in the contribution of TCA to PM<0.95 (23%) during wet as compared to summer. Annually, TCA contributed around ~28% to submicron particulate (PM<0.95) which shows that fine particulate are enriched with carbonaceous species. During study period, the contribution of SOM to OM varied from 0 to 74% (in submicron size) which is comparable (0–84%) to recently reported for the fine particulate (PM2.5) over the same site (Rastogi et al., 2016). The high mass ratios (OC/EC) along with significant amount of WSOC indicate the dominance of biomass burning emissions and significant formation of SOA over the IGP. SUMMARY AND CONCLUSIONS

The present study reports year round (October, 2012–September, 2013) measurements of size-segregated particulate matter mass as well as carbonaceous aerosols (OC, WSOC and EC) concentrations over Patiala, located in the northwestern part of the IGP. Mass concentration of total suspended particulates (TSP) varied from 88–387 µg m–3 with highest mass concentration during autumn (267 ± 69 µg m–3) and minimum in wet season (130 ± 62 µg m–3). Distribution of aerosol mass concentration in different size range indicates that majority of particulate mass concentration

corresponds to submicron size (PM<0.95) as compared to coarser size (PM3.0–7.2). These observations suggest the dominance of anthropogenic source (s) over the natural source (s).

During autumn (representing the paddy residue burning emission), OC and WSOC shows significantly high mass concentration as compared to rest of the study period. Further, it has been observed that the dominant part of ambient OC comes from combustion source (s) as it was predominantly found in submicron size, and significant fraction of OC was SOA. On annual time scale, WSOC accounts for ~50% of OC, suggesting almost half of OC is secondary as WSOC is often considered as a measure of SOA. The absolute mass concentration of light absorbing carbonaceous species (EC) was highest during paddy residue burning emission (autumn; 6.7 ± 2.0 µg m–3) and minimum in wet season (2.4 ± 1.0 µg m–3). Significant differences in absolute mass concentration of carbonaceous aerosols were found during two distinct type of agricultural waste burning emission (paddy and wheat residue), and attributed to several factors such as difference in amount and type of biomass burnt during these two periods, boundary layer height, and efficiency of combustion.

The scatter plot between OC and EC in different seasons suggests that the dominance of biomass burning emission over the study region with highest contribution during autumn followed by winter and summer. Enhanced contribution from fossil fuel combustion was observed during spring and wet season. The same has been observed for WSOC/OC mass ratio. These results also highlight the relative contribution of natural and anthropogenic source (s) to ambient particulate matter and carbonaceous aerosols. The information provided on secondary aerosol formation as a function of size has implications in designing and validating air quality models. ACKNOWLEDGEMENT

The work is supported under ISRO-GBP research project

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and authors are grateful to ISRO for financial support. We also thank to NOAA Air Resources Laboratory (ARL) for availability of HYSPLIT model online. The meteorological data for Patiala station provided by IMD is duly acknowledged. We would like to thank both the Reviewers for their valuable comments and suggestions to improve the quality of our manuscript. REFERENCES Birch, M.E. and Cary, R.A. (1996). Elemental carbon-

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Received for review, January 17, 2016 Revised, April 4, 2016

Accepted, April 18, 2016