Variabilities of? 13C and carbonaceous components in ambient PM 2.5 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 (delta 13C) 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 613C values (-28.2%o to -26.4%o) during the sampled seasons. The 613C of aerosols indicates major sources to be combustion of biomass/biofuels (C3 plant origin), biogenic aerosols, and secondary aerosols. The 613C variability and cluster/PSCF modelling suggest that aged aerosols (along with enhanced photo-oxidation derived secondary aerosols) influenced the final 613C during the pre-monsoon. On the other hand, lower 613C in winter and post-monsoon is attributed to the freshly emitted aerosols from biomass/ biofuels.

Automated summary

A year-long sampling campaign was conducted in the North-Eastern Region of India to study the sources and formation of carbonaceous aerosols in ambient PM2.5. The study found that PM2.5 mass concentration was highest in winter and post-monsoon seasons, with organic carbon concentration also higher during these periods. Local and regional air masses dominated during winter and post-monsoon, while long-range transported air masses influenced the background site during pre-monsoon and monsoon. The carbon isotope ratio and cluster/PSCF modeling suggested that aged aerosols influenced the carbon isotope ratio during pre-monsoon, while freshly emitted aerosols from biomass/biofuels were responsible for the lower ratio in winter and post-monsoon.