Characterization, sources, and formation processes of dicarboxylic acids, oxocarboxylic acids and α-dicarbonyls in PM2.5 aerosols in New Delhi

Abstract

Daytime and nighttime fine aerosol (PM2.5) samples were collected during a heavy pollution episode in November 2020, January 2021, and also during clean event (August 2020) in New Delhi. These samples (n = 20) were analyzed for water-soluble dicarboxylic acids (diacids) and related compounds, carbonaceous components, and inorganic ions to identify their sources and atmospheric processing in different episodes/seasons. Among the measured organics, oxalic acid (C2) was found to be the most abundant species followed by terephthalic (tPh), succinic (C4), phthalic (Ph), and azelaic (C9) acids. Such high abundance of tPh were different from those in other Asian cities where C3 is generally the second or third dominant species. tPh very well correlated with OC, Cl− and K+, indicating combustion of biomass and plastic-containing waste may be the possible sources. Significant high contents of diacids, oxoacids, and dicarbonyls were observed during heavy pollution episode, implying an enhanced emission and/or atmospheric processing of these species. Additionally, two important secondary formation pathways for C2 diacid were discussed in New Delhi, i.e. (i) biomass burning derived (biogenic) volatile organic precursors lead to formation of C2 diacid through transformation of oxoacids and dicarbonyls by oxidation, and (ii) oxidative degradation of the unsaturated fatty acids results in formation of C5 and C4 which subsequently decompose to C2 diacid. This study demonstrated that both primary emissions in clean event and secondary production during polluted event are important sources of aerosols in New Delhi.