Stabilized Criegee Intermediates are important nocturnal oxidants in the summertime air of the Indo-Gangetic Plain.

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Night-time oxidation significantly affects concentrations of both primary and secondary air pollutants but is poorly constrained over South Asia. Here, we investigate the chemistry, formation and abundance of Stabilized Criegee Intermediates (SCI) in the summertime air of the Indo-Gangetic Plain using measurements of its precursors and sinks. This includes ethene, propene, 1-butene, cis-2-butene, trans-2-butene, 1-pentene, cis-2-pentene, trans-2-pentene, and 1-hexene for which this work also reports the first summertime dataset from the IGP. Ethene, propene, and 1-butene were the highest ambient alkenes in both summer and winter. Morning and noon-time concentrations in summer were ~5.6 and ~3.3 times higher relative to winter, suggesting stronger alkene emission sources in summer. Applying chemical steady-state to the measured precursors, the average calculated SCI concentrations were 4.5 (± 3.8) × 103 molecules cm-3, with Z-CH3CHOO (55 %) as the major SCI. SCI production rates drove ambient SCI with Z-RCHOO (35 %) and α-pinene derived PINOO (34 %) as the largest contributors to the SCI production rate of 7.8 × 105 molecules cm-3 s-1. Peak SCI occurred during evenings. All SCI loss was dominated (>70 %) by unimolecular decomposition or reactions with water vapour. Pollution events influenced by crop biomass fires resulted in significantly elevated SCI production (2.1 times higher relative to non-polluted periods) reaching as high as (7.4 ± 2.5) × 105 molecules cm-3 s-1. Among individual SCI species, Z-CH3CHOO was highest in all the plume events with a contribution of at least ~41 % and among alkenes, trans-2-butene was the highest contributor to P(SCI) in plume events with values ranging from 22-32 %. SCIs dominated the night-time oxidation of sulphur dioxide with rates as high as 1.5 (± 1.3) × 104 molecules cm-3 s-1at midnight, suggesting this pathway could be a significant source of fine mode sulphate aerosols over the Indo-Gangetic Plain, especially during summertime pollution episodes.