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 mu gm-3; 31 %) and secondary inorganics (53 +/- 21 mu gm-3; 33 %) dominated the PM2.5 mass (160 +/- 4 mu gm-3) during high pollution period with low dust (14 +/- 7 mu gm-3; 8 %) content. Interestingly, the clear pollution period was also influenced by carbonaceous fraction (19 +/- 8 mu gm-3; 32 %) and secondary inorganics (19 +/- 5 mu gm-3; 32 %), but the PM2.5 concentrations (59 +/- 9 mu gm-3) were similar to one-third as compared to high pollution period. High scattering coefficients were observed which were com-parable 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 Angstrom 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 find-ings have implications not only in regional climate assessment but also for other regions with comparable geography and source-mixes.

Automated summary

This study investigates the relationship between physiochemical and optical characteristics of aerosols in a desert-urban region impacted by anthropogenic sources and desert dust. The results show that during high pollution periods, elemental and organic carbon and secondary inorganics dominate PM2.5 mass, while during clear pollution periods, carbonaceous fraction and secondary inorganics also play a significant role. The findings also reveal the impact of coating/mixing on the light absorption capacity of EC and dust.