Variabilities of δ13C and carbonaceous components in ambient PM2.5 in Northeast India: Insights into sources and atmospheric processes.

A year-long sampling campaign of ambient PM2.5 (particulate matter with aerodynamic diameter ≤2.5 mm) at a regional station in the North-Eastern Region (NER) of India was performed to understand the sources and formation of carbonaceous aerosols. Mass concentration, carbon fractions (organic and elemental carbon), and stable carbon isotope ratio (δ13C) of PM2.5 were measured and studied along with cluster analysis and Potential Source Contribution Function (PSCF) modelling. PM2.5 mass concentration was observed to be highest during winter and post-monsoon seasons when the meteorological conditions were relatively stable compared to other seasons. Organic carbon (OC) concentration was more than two times higher in the post-monsoon and winter seasons than in the pre-monsoon and monsoon seasons. Air mass back trajectory cluster analysis showed the dominance of local and regional air masses during winter and post-monsoon periods. In contrast, long-range transported air masses influenced the background site in pre-monsoon and monsoon. Air mass data and PSCF analysis indicated that aerosols during winter and post-monsoon are dominated by freshly generated emissions from local sources along with the influence from regional transport of polluted aerosols. On the contrary, the long-range transported air masses containing aged aerosols were dominant during pre-monsoon. No significant variability was observed in the range of δ13C values (-28.2‰ to -26.4‰) during the sampled seasons. The δ13C of aerosols indicates major sources to be combustion of biomass/biofuels (C3 plant origin), biogenic aerosols, and secondary aerosols. The δ13C variability and cluster/PSCF modelling suggest that aged aerosols (along with enhanced photo-oxidation derived secondary aerosols) influenced the final δ13C during the pre-monsoon. On the other hand, lower δ13C in winter and post-monsoon is attributed to the freshly emitted aerosols from biomass/biofuels.

PM2.5 pollution exceeding Indian standard over a semi-urban region at eastern IGP: Chemistry, meteorological impact, and long-range transport.

A year-long study (January-December 2019) on the chemical characterization and meteorological impact on PM2.5 was conducted over a semi-urban station, Shyamnagar, in the easternmost part of the Indo-Gangetic Plains (IGP). PM2.5 concentrations (Mean = 81.69 ± 66.27 µgm-3; 7.10-272.74 µgm-3), the total carbonaceous aerosols (TCA) (Mean = 22.85 ± 24.95 µgm-3; 0.77-102.97 µgm-3) along with differential carbonaceous components like organic carbon (OC) (Mean = 11.28 ± 12.48 µgm-3; 0.48-53.01 µgm-3) and elemental carbon (EC) (Mean = 4.83 ± 5.28 µgm-3; 0.1-22.13 µgm-3) exhibited prominent seasonal variability with the highest concentrations during winter, followed by post-monsoon, pre-monsoon and lowest during monsoon. A similar seasonal variation was observed for the total water-soluble ionic species (Mean = 31.91 ± 20.12 µgm-3; 0.1-126.73 µgm-3). We observed that under the least favorable conditions (low ventilation coefficient), high PM2.5 pollution (exceeding Indian standard) was associated with a high increase in secondary components of PM2.5. Eastern, central and western parts of IGP, as well as Nepal, were the major long-distant source regions whereas the northern part of West Bengal and parts of Bangladesh were the major regional source region for high PM2.5 pollution over Shyamnagar. The ratios like char-EC/soot-EC, non-sea-K+/EC and non-sea-SO42-/EC strongly indicated the dominance of fossil fuel burning over biomass burning. Compared with other studies, we observed that the PM2.5 pollution over this semi-urban region was comparable (and even higher in some cases) with other parts of IGP. The high exceedance of PM2.5 over the Indian standard in Shyamnagar strongly demands an immediate initiation of systematic and regular based air pollution monitoring over semi-urban/non-urban regions in India, especially IGP, in addition to the polluted cities.

Wintertime aerosol properties of urban desert region of western India: Implications in regional climate assessment.

This study assessed the inter-relation between physiochemical and optical characteristics of aerosols measured at a desert-urban region affected by anthropogenic sources and desert dust during October 2020 to January 2021. Based on horizontal visibility and measured PM2.5 concentration, clear (37 %), light (33 %) and high (31 %) pollution periods were identified. Elemental and organic carbon (50 ± 15 µgm-3; 31 %) and secondary inorganics (53 ± 21 µgm-3; 33 %) dominated the PM2.5 mass (160 ± 4 µgm-3) during high pollution period with low dust (14 ± 7 µgm-3; 8 %) content. Interestingly, the clear pollution period was also influenced by carbonaceous fraction (19 ± 8 µgm-3; 32 %) and secondary inorganics (19 ± 5 µgm-3; 32 %), but the PM2.5 concentrations (59 ± 9 µgm-3) were ∼ one-third as compared to high pollution period. High scattering coefficients were observed which were comparable to highly polluted Indian city like Delhi. An exponential increase in non-absorbing material was observed and showed clear influence on light absorption capacity of EC and dust due to coating/mixing. High absorption Ångström exponent (AAE) >0.6 was observed for the ratio of non-absorbing to light absorbing components (LAC) in the range of 1-2.5 and EC/PM2.5 fraction of 7-14 %. While further increase in non-absorbing to absorbing components ratio > 4 and low amount of EC (<4 %) tend to decrease AAE below 0.4. Higher mass absorption cross-section (>30 m2g-1 of EC) was observed when 4-10 % EC fraction of PM2.5 associated with 1.5-3.5 times non-absorbing components to total absorbing components. Likewise, absorption enhanced by three to five folds compared to uncoated EC for low EC fraction (3-6 %) in PM2.5, but high non-absorbing to absorbing component ratio (>2.5). Interestingly, absorption was minimally amplified for nominal coating fraction associated with significant core materials or vice-versa. These findings have implications not only in regional climate assessment but also for other regions with comparable geography and source-mixes.

Investigation of sources and atmospheric transformation of carbonaceous aerosols from Shyamnagar, eastern Indo-Gangetic Plains: Insights from δ13C and carbon fractions.

This study reports the chemical characterization of the carbonaceous component of PM2.5 (particulate matter with aerodynamic diameter ≤2.5 µm) collected over a year-long campaign from a regional site in Shyamnagar, West Bengal, in the Indo-Gangetic Plains (IGP), India. The carbonaceous fractions (elemental and organic carbon), mass concentrations, and stable carbon isotopic composition (δ13C value) of aerosols were measured and utilized to characterize the sources and understand the atmospheric processing of aerosols. Cluster analysis, Potential Source Contribution Function (PSCF) modeling, and fire count data were analyzed to decipher the pattern of air masses, source contributions, and extent of burning activities. The PM2.5 mass concentrations were significantly higher during winter (168.3 ± 56.3 µg m-3) and post-monsoon (109.8 ± 59.1 µg m-3) compared to the monsoon (29.8 ± 10.7 µg m-3) and pre-monsoon (55.1 ± 23.0 µg m-3). Organic carbon (OC), elemental carbon (EC), and total carbon (TC) concentrations were also several factors higher during winter and post-monsoon compared to monsoon and pre-monsoon. The winter and post-monsoon experienced the impact of air masses from upwind IGP. On the other hand, long-range transported air masses from the South-West direction dominated during monsoon and pre-monsoon, which are also relatively cleaner periods. The average δ13C during post-monsoon and winter was ∼1‰ higher compared to monsoon and pre-monsoon. The vehicular exhaust and biomass/biofuel burning contributed dominantly in winter and post-monsoon. In comparison, lower δ13C in pre-monsoon and monsoon might be attributed to the dominance of biomass/biofuel combustion. Photochemical-induced aging of the anthropogenic aerosols resulted in a higher δ13C of TC in winter and post-monsoon, whereas the mixing of different local sources in pre-monsoon and monsoon resulted in lower δ13C values. These findings benefit policymakers in strategizing proper and effective management of biomass/biofuel burning in the IGP to minimize air pollution.

Wintertime chemical characteristics of aerosol and their role in light extinction during clear and polluted days in rural Indo Gangetic plain.

This paper reports the chemical and light extinction characteristics of fine aerosol (PM2.5) during the winter period (2017-18) at Lumbini, Nepal, a rural site on the Indo Gangetic Plains. A modified IMPROVE algorithm was employed to reconstruct light extinction by chemical constituents of aerosol. The fine aerosol levels impacted visibility adversely during daytime, but during nighttime visibility was controlled by fog droplets rather than by aerosols. The PM2.5 chemical constituents showed varying characteristics during clear and polluted days. The average NO3-/SO42- concentration ratio was 0.57 during clear and 1.36 and polluted days, signifying a change in secondary inorganics and formation processes mainly due to decreasing photochemical production and due to increased partitioning of nitrate particles at a lower temperature. The increased secondary organics contribution and the higher OM/OC ratio (2.2) during polluted days showed the vital role of aqueous processing and biomass burning emissions in determining the concentration of organics. Total light extinction was 2.3 times higher on polluted days compared to clear days, while the PM2.5 mass concentration was 1.5 times higher. This variation in mass and extinction order signifies that various chemical components in fine particles have a more considerable impact on light extinction. On clear days we found that carbonaceous particles (OM and EC) made a major contribution to light extinction. In contrast, the extinction contribution by secondary inorganic (especially NH4NO3) increased significantly during polluted days, with hygroscopic growth and enhanced scattering efficiency at higher RH conditions playing a major role. The comparison between clear and polluted days altogether suggests that regulating the nitrate sources can help significantly in improving the visibility levels and restrict fog haze development during wintertime in rural IGP.