Pollution characteristics of organic, elemental carbon and water soluble organic carbon in PM2.5 a Tropical City Tiruchirappalli, Tamil Nadu, India

Airborne particulate matter has now become an issue in the global environment. In this study, the atmospheric PM2.5, organic carbon (OC), elemental carbon (EC) and water-soluble organic carbon (WSOC) were measured in the urban city of Tiruchirappalli, Tamil Nadu, India. The annual average concentration of PM2.5 was observed in 76.1 μg/m 3 , Similarly, the seasonal average of PM2.5 were 49.4 μg/m 3 in summer, 73.3μg/m 3 in Pre-monsoon, 86.1 μg/m 3 in winter and 87.9 μg/m -3 during monsoon respectively. The present study found that the annual average concentrations of PM2.5 in Tiruchirappalli was significantly higher than the limit of 40 μg/m 3 prescribed in the National Ambient Air Quality Standards (Indian-NAAQS) and 10 μg/m 3 of that of the World Health Organization (WHO). While, the annual concentrations of OC, EC and WSOC were 8.9 μg/m 3 , 4.1 μg/m 3 and 3.1 μg/m 3 respectively. On seasonal average, the OC and EC concentrations ranked in the order of monsoon> winter>pre-monsoon > summer, which could be attributed to the combined effects of changes in local emissions and seasonal meteorological conditions. The secondary organic carbon (SOC) estimated by EC-tracer method was the highest in pre-monsoon (7.94 μg/m 3 ) followed by monsoon (6.60 μg/m 3 ), winter (6.04 μg/m 3 ) and summer (4.57 μg/m 3 ). Overall, results revealed that the mass concentration of PM2.5 bound carbonaceous fractions, and their contributions were varies by seasons. This study provides baseline information that can be exploited for policy formulation and mitigation strategies to control air pollution in south Indian urban cities.


I. INTRODUCTION
In recent decades, the earth's atmosphere is remarkably polluted due to industrialization and urbanization occurring largely in developing countries (1) (2). Urbanization activities involving road transport and construction are widely recognized as a key source of atmospheric particulate matter (PM) pollution in urban cities (3) (4). Atmospheric fine particulate matter (PM 2.5 ) has acquired a worldwide attention for its adverse effects on public health and climate change (5) (6). Exposure to PM 2.5 has been found to correlate with an enhanced risk of cardio-respiratory diseases (7) (8). On the other hand, the composition and the the atmosphere from anthropogenic combustion as well as biogenic sources (10). EC is found to possess a strong capability of absorbing solar radiation, and it is the second most important contributor to global warming, after CO 2 (11). In-addition, watersoluble organic carbon (WSOC) is another component, significantly contributing to carbonaceous aerosol, which may potentially influence the number density of cloud condensation nuclei (12). WSOCs are therefore recognized for altering the radiation balance of the atmosphere (13) (14).
The composition of these carbonaceous fractions of PM 2.5 (EC, OC and WSOC) varies significantly based on the spatial distribution and seasonal variations in a given area. Therefore, it is important to understand the influence of these factors in determining the trend of the carbonaceous aerosols in the lower atmosphere. There are several studies that have focused on the carbonaceous aerosols and lower atmospheric air quality in metro cities; whereas, information regarding the variation of carbonaceous aerosols in the urban cities is limited (15) (16) (17) (18). Also, most of the previous research studies reported in the literatures focused on the temporal and spatial variability of trace particle concentrations in urban cities; whereas, the information available on the corresponding trends of carbonaceous aerosols in urban cities is inadequate (19) (20). In this context, the current study focused on the assessment of the influence of spatial and temporal variation on the chemical characteristics and carbonaceous species of PM 2.5 , in an urban city like Tiruchirappalli. Tiruchirappalli was chosen as the sampling location as it is listed as one of the most polluted cities of Tamil Nadu, with a global ranking of 370 according to the new global urban ambient air pollution ranking released by WHO (21); It is also the fourth largest city of South India, rapidly growing in terms population density and vehicular pollution; Tiruchirappalli is currently facing the challenge of severe air pollution, which has already led to numerous adverse impacts on the atmospheric environment and public health. The main objective of the study was to assess airborne PM 2.5 fraction and carbonaceous species in Tiruchirappalli city with emphasis on seasonal variations.

Study area
The Tiruchirappalli city (10.5°N, 78.43°E,78.8 MSL) is situated on the banks of the River Cauvery ( Figure 1) Tamil Nadu, South India. Total geographical area of the city is 164.70 km 2 with the total population of one million as per 2011 census within its municipal corporation. Tiruchirappalli city has the population in 2017 was around 1.2 million and the total number of vehicles registered with the Regional Transport Authority exceeded 0.7million in 2017. Four major highways NH 45, NH 67, NH 210 and NH 277 pass through the city. The heavy traffic on these highways has significantly contributed to air pollution in the city (19). Sampling was conducted at five sampling sites, located in distinctly different over the Tiruchirappalli city and Figure 1 shows the location of all sampling sites. The sampling locations were selected at Jamal Mohamed College -TVS Tollgate (JMC), Orchard school-KK.Nagar (OrdS), Central Bus stand (CeBS), Thillai Nagar (ThN) and Chathiram Bus Stand (ChBS) ( Figure. 1).
The station JMC (10°47'11.72"N, 78°41'40.85"E) is surrounded by educational and residential buildings, National High way (NH45). In the OrdS station, samples were collected from the Orchard School building (10°45'34.70"N 78°40'52.02"E) and this station was located at the heart of KK Nagar residential area. Also OrdS lies on the outer border of the Tiruchirappalli thus, the new building construction will be the main source of pollution. At the CeBS station, samples were collected at Shri Sangeetha tower which is located near the Central bus stand (10°47'57.63"N, 78°41'1.22"E). It is surrounded by scattered inhabited residences and heavy traffic. The ThN station samples were collected from the Cethar Hospital in Thillai Nagar (10°49'26.43"N, 78°41'3.89"E). This site is surrounded by high-rise residential buildings, many hospitals and traffic. Similarly, in ChBS station samples were collected from Hotel Chitra of Chathiram Bus stand (10°50'0.65"N, 78°41'34.96"E). This site is densely populated with commercial sites and heavy traffic being the city's second bus station next to central bus satation.The samplers were placed on the rooftop of buildings in all sampling sites, about 15m height from the ground level. October -December were defined as winter, summer, pre-monsoon and monsoon, respectively. Samples were collected for 24 hours continuously in each station and the flow rate was maintained at 100 ± 2 L min -1 . In this study, quartz filters -Whatman, QM-A quartz filters 90 mm (Quartz microfiber filters: Whatman, GE Healthcare Life Sciences, UK.) were used for the analyses of gravimetric, carbon, and WSOC. All blank quartz filters were prebaked at 600°C for at least 8 hours in a muffle furnace to remove impurities, and polypropylene filters were used without further treatment (32). PM 2.5 mass concentration was obtained by gravimetric method with an analytical microbalance (Mettler Toledo AE420, ± 0.01 mg) after being conditioned under constant temperature and relative humidity. Each filter was stored in a separate sealed Petri dish.

Chemical Analysis
Carbonaceous species including OC and EC were analyzed by the Thermal/Optical transmittance analyzer (DRI Model 2001 Desert Research Institute, Reno, Nevada, USA) following the temperature program outlined in the NIOSH 5040 method (22). Around 10% of the samples were randomly selected and measured twice to check the analytical precision. Field blanks were used to quantify procedure detection limits. Trace OC (average 0.98 μg C per punch) and EC (average 0.02 μg C per punch) were detected on the field blank filters. The limits of detection (LOD), calculated as three times of the standard deviation (3σ) of the field blanks, were 0.474 and 0.015 μg C m -3 for OC and EC, respectively, based on 140 m 3 of a typical sampling volume per sample. Both carbonaceous species were significantly higher than the LOD in all samples collected. The final carbonaceous mass on each filter was corrected with the field blank. The remaining half of the merged filter was extracted with 20mL organic free Milli-Q water (Millipore Co., Ltd, Shanghai, China) under ultrasonication for 20 min. The extracts were then filtered into a 50mL comparison tube using a 0.45 mm PTFE Acrodisc syringe filter (13mm, Pall Co., Ltd, Beijing,China).WSOC in the extract (10mL) was quantified using a Total Organic Carbon Analyzer (TOC-VCPH, Shimadzu Co., Ltd, Beijing, China).The instrument was calibrated using a series of sucrose 300 solutions every day through the analysis period.

Concentration Levels of PM 2.5
The annual average concentrations of PM 2.5 in Tiruchirappalli was observed in the range of 74.1±23.3µg/m 3 and the summary of results is presented in Table-1 Particularly, the CeBS and ChBS sampling stations are the main transportation hub in the whole city and hence also mark large vehicular movements. In addition, the sampling sites of OrdS, ThN and JMC various local sources including vehicular exhaust, road side restaurants, waste incineration and building construction activities affect the atmosphere. Overall results shows that concentrations of PM 2.5 ranked in the order of CeBS>ChBS>JMC>ThN>OrdS. The present higher concentrations of PM 2.5 would cause potential adverse effects to human health (23). Generally, the PM 2.5 concentration varies considerably with time, location meteorological conditions and source emissions rate (37) (38). The details of meteorological data during the sampling period are presented in Table- Fig. 2 (a), (b), (c) and (d) presents the wind rose diagram for winter, summer, pre-monsoon and monsoon seasons, respectively. During winter and monsoon, air masses flow from NE, whereas, summer and pre-monsoon the wind flow from SW directions. During summer and pre-monsoon similar wind speed recorded 4 to 7 m/s, whereas monsoon and winter the wind speed varied from 7 to 11, 11 to 17m/s respectively. Similarly, wind speed was found to be low during summer and pre-monsoon seasons and gradually increased during monsoon and winter. Frequent changes in wind force increases the atmospheric turbulence during summer months, thereby increasing the dispersion of PM 2.5 emissions. The present study has shows that, significant seasonal variations of PM 2.5 are controlled by meteorological factors.  at the five stations revealed a typical seasonality with higher values in winter, monsoon followed by pre-monsoon and lower values in summer. This seasonal variation could be attributed to the effects of changes in emissions rate and seasonal metrological conditions. It is clear that prevailing wind direction is southsouthwest in summer and pre-monsoon and north-northeast in other seasons. In addition to these anthropogenic emissions, the steady meteorological conditions during winter season (high RH and low wind speed) favor the accumulation of particles (24). Furthermore, in winter, due to lower ambient temperatures, lower mixing depths, temperature inversion and foggy conditions, low, calm condition and higher consumption of fuel augments the pollution (25). The results of this study the PM 2.5 concentrations were directly compared with those measured in other cities in India, the PM 2.5 concentration (Table 3) in Tiruchirappalli city was at a moderately higher level.   Fig.3 In fine-mode OC concentration varied 5.8 µg / m 3 in summer, 7.7 µg / m 3 in pre-monsoon, 9.7 µg / m 3 in winter and 12.6 µg / m 3 in monsoon. The concentration of EC range varied from 1.9 µg C /m 3 in summer, 3.0 µg / m 3 in pre-monsoon, 4.7 µg /m 3 in winter to 6.8 µg / m 3 in monsoon. The concentrations of WSOC varied from 2.6 µg /m 3 in pre-monsoon and summer, 3.2 µg/ m 3 in winter to 3.7 µ g/ m 3 in monsoon. During summer, OC and EC concentrations are nearly 2-3 times lower than those during winter and monsoon. Seasonal variations of OC, EC and WSOC concentrations were significant, with the descending order: monsoon > winter > pre-monsoon > summer. The lowest concentrations of OC, EC and WSOC occurred in summer. The highest concentrations of carbonaceous species appeared in monsoon and winter, and the mean OC, EC and WSOC concentrations in monsoon were 2.2 times, 3.6 times and 1.3 times as high as that in summer, respectively. In general, mass concentrations of OC and EC, are higher in monsoon and winter and significantly lower in summer and pre-monsoon. The higher concentrations found in monsoon and winters could be attributed to the biomass burning after harvest and low atmospheric boundary layer height conditions and low dispersion rate in Tiruchirappalli. The results are in agreement with those observed that the abundance of WSOC relative to OC could be employed as an indicator to decipher whether organic aerosol is primary or secondary, because SOC usually tends to be more water soluble than primary organic matter (38).

OC/EC and WSOC/OC Ratios and Source Characteristics
Organic and elemental carbon ratio (OC/EC) provides an idea to identify the possible sources of carbonaceous aerosols. Higher OC/EC ratios are used as an indicator of biomass burning sources while lower ratio are linked with fossil fuel combustions, hence OC/EC ratios could be used to make out different source and transformation characteristics (39). Sandradewi et al., reported an average OC/EC ratio of 7.3 for wood burning and 1.1 from vehicular emissions. Similarly, Watson et al., (40) found an OC/EC ratio of 2.7 for coal combustion, 9.0 for biomass burning. The present study, the ratio of OC/EC was in the range of 1.21 -3.05, 1.18-4.55, 1.68-4.08 and 1.14-2.3, with average of 2.1, 3.2, 2.9, 1.9 in winter, summer, pre-monsoon and monsoon respectively. Figure 4 illustrate the ratios of OC/EC in different seasons along the study area. It shows that the OC/EC ratios in summer, pre-monsoon and monsoon were greater than 2, and winter has the OC/EC ratio slightly lower than 2. Summer had the highest ratio and winter had the lowest ratio, which matches the findings by Zhao et al. (4). Usually, the SOC can be formed when the ratio of OC/EC is over 2 (41). In this study, the OC/EC ratio varied from 1.21 to 4.55 within an average of 2.51. These ratios clearly indicate that the impact of vehicular emission and biomass burning was more prominent in the present samples.  Figure 4 illustrate the ratios of WSOC/OC in different seasons along the study area. A large spread in WSOC/OC ratios further suggests temporal variability in emission sources, their strength, and contribution from SOC sampling site. The WSOC/OC ratios are a unique tracer to understand secondary organic carbon (SOC) formation mechanism (43) (. It has been postulated that relatively high WSOC/OC ratios during summer months indicate contribution from SOC due to increased photochemical activity and/or aging of aerosols during the long-range atmospheric transport. As expected, higher OC/EC and WSOC/OC ratios were found in summer than in winter (Table, 4). The higher WSOC/OC slope in the summer than in winter suggests that secondary organic carbon formation processes produce significant amounts of WSOC during summer. Information on the WSOC partitioning between its primary and secondary fraction can be derived by means of the EC tracer method.

Estimation of the Secondary Organic Carbon (SOC)
The present study, contribution of secondary organic carbon (SOC) was estimated using an EC tracer method (44) (4) (45). SOC = OC-EC x (OC/EC) pri Where (OC/EC) pri is the ratio for primary sources contributing to PM 2.5 . Following the method in Feng et al. (46) and Li et al., (45), the values of (OC/EC) pri were simplified as the smallest observed ratio in the range of 1.21-4.55. These (OC/EC) pri ratio were comparable with 2.2 -5.3 in Feng et al (46) but higher than 1.27 -1.85 in Li et al., (45).
Concentration of secondary organic carbon (SOC) retrieved from EC-tracer method was 6.4±4.2 μgm -3 in winter, 4.6±2.8 μg.m -3 in summer, 7.9±3.2 μg.m -3 in pre-monsoon and 6.6±3.3 μg.m -3 in monsoon. Using this method SOC/OC ratios accounted for 53.4 to 78.4% of the OC, suggesting that SOC is an important component of PM 2.5. It illuminated that SOC was an important component of OC mass in Tiruchirappalli city, presenting a significant trend of secondary transformation. Present high SOC/OC ratios in pre-monsoon were mainly due to the increased emission of volatile organic precursors and the adsorption and condensation of semi-volatile organic compounds onto existing solid particles under low temperature conditions, as well as enhanced SOC formation under the stagnated atmospheric condition (6) (44) .

IV. CONCLUSIONS
The mass concentrations of PM 2.5 were 86.1±13.6 µg/m 3 in winter, 45.9 ±25.6 µg/m 3 in summer, 73.3 ± 27.5 µg/m 3 in in premonsoon and 87.9 ± 25.3 µg/m 3 in monsoon, respectively, indicating the serious circumstances of fine particle pollution in Tiruchirappalli. Similarly, the mass concentrations of OC, EC and WSOC were observed in 8.95µg/m 3 , 4.10µg/m 3 and 3.1µg/m 3 respectively. On seasonal average, PM 2.5, OC, EC and WSOC concentrations ranked in the order of monsoon > winter > pre-monsoon > summer. In comparison with other cities in India the PM 2.5 concentrations in Tiruchirappalli were at moderately high level. The annual average value of OC/EC ratios in this study observed 2.5, which indicates that carbonaceous fractions in Tiruchirappalli were emitted from mixed sources such as Vehicular exhaust, biomass burning and road side restaurants etc. Annual average OC concentrations accounted about 70 % in TCA, which illuminated that OC was the main fraction of TCA. Moreover, annual average SOC concentrations accounted for about 74% in OC suggesting that SOC is an important component of PM 2.5. The source characteristics of PM 2.5 and carbonaceous species in different seasons were examined using OC/EC, WSOC/OC ratio methods. The present assessment of PM 2.5 and carbonaceous species of PM 2.5 is the first step to determine the health and safe limit on human beings, which will be evaluated in future and this kind of studies are yet to be developed for Tiruchirappalli city. A need for inventory of air trajectory within canyon and implementation of good practices such as land use policy and vehicular emission control is imperative.