Investigation of factors that drive the extremely high summertime ozone pollution over the north-west Indo-Gangetic Plain using in-situ measurements and a chemical box model

Every year in May, the north-west Indo-Gangetic Plain experiences its worst ozone pollution with ambient hourly ozone frequently exceeding 100 ppb. Till date however, a mechanistic study of the oxidant and radical chemistry during these periods has been lacking. Here using a novel in-situ dataset of measured ozone precursors, including isoprene and acetaldehyde and nitrogen oxides, we investigate three contrasting ambient ozone periods experienced in May 2023. While two periods had lower NOx (~10 ppb) but contrasting ozone levels of ~50 ppb (period I) and 90 ppb (period III), respectively, period II was impacted strongly by wheat residue-fire plumes and associated with high ambient average daytime ozone (~90 ppb) and higher NOx (~20 ppb). Using a detailed chemical box model, we investigated these three periods in terms of the Leighton ratio, total OH reactivity, radical concentrations and photo-chemically formed oxidation products for mechanistic insights. Furthermore, the ozone production regime and rates were investigated for all three periods. With climate change likely to increase regional temperatures, our results will also present insights on implications for ozone pollution with future climate change.