Discovery of reactive chlorine, sulphur and nitrogen containing ambient volatile organic compounds in the megacity of Delhi during both clean and extremely polluted seasons

Abstract. Volatile organic compounds significantly impact the atmospheric chemistry of polluted megacities. Delhi is a dynamically changing megacity and yet our knowledge of its ambient VOC composition and chemistry is limited to few studies conducted mainly in winter before 2020 (all pre-covid). Here, using a new extended volatility range high mass resolution (10000–15000) Proton Transfer Reaction Time of Flight Mass Spectrometer10K, we measured and analyzed ambient VOC-mass spectra acquired continuously over a four-month period covering “clean” monsoon (July–September) and “polluted” post-monsoon seasons, for the year 2022. Out of 1126 peaks, 111 VOC species were identified unambiguously. Averaged total mass concentrations reached ~260 µgm-3 and were >4 times in polluted season relative to cleaner season, driven by enhanced emissions from biomass burning and reduced atmospheric ventilation (~2). Among 111, 56 were oxygenated, 10 contained nitrogen, 2 chlorine, 1 sulphur and 42 were pure hydrocarbons. VOC levels during polluted periods were significantly higher than most developed world megacities. Surprisingly, methanethiol, dichlorobenzenes, C6-amides and C9-organic acids/esters, which have previously never been reported in India, were detected in both the clean and polluted periods. The sources were industrial for methanethiol and dichlorobenzenes, purely photochemical for the C6-amides and multiphase oxidation and partitioning for C9-organic acids. Aromatic VOC/CO emission ratio analyses indicated additional biomass combustion/industrial sources in post-monsoon season, alongwith year-round traffic sources in both seasons. Overall, the unprecedented new information concerning ambient VOC speciation, abundance, variability and emission characteristics during contrasting seasons significantly advances current atmospheric composition understanding of highly polluted urban atmospheric environments like Delhi.