Extreme Summertime Ozone Pollution Over the North-west Indo-gangetic Plain Driven by Amplified Peroxy-radical Chemistry Due to Precursor Emissions

Every year in May, the north-west Indo-Gangetic Plain experiences its worst ozone pollution with ambient ozone frequently exceeding 100 ppb, due to a combination of warm and dry weather and agricultural biomass burning emissions. 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 calculate the Leighton ratios and evaluate daytime peroxy radical budgets for three contrasting periods experienced in May 2023. While two periods had lower NOx (~10 ppb) but contrasting ozone levels of ~50 ppb (I) and 90 ppb (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). Leighton ratios were much higher than 1, with average values of 3, 4.5 and 7.5, for period I, II and III, respectively, suggesting a significant role of peroxy-radicals in the ozone formation chemistry. Peroxy radical mixing ratios, namely the sum of hydroperoxy and alkyl peroxy radicals (RO2*=HO2+RO2) more than doubled to ~1.4 ppb during period II, relative to periods I (RO2*=0.5) and II (RO2*=0.6), respectively, suggesting strong amplification and perturbation of radical chemistry contributing to the high ambient ozone in period II. The ozone production regime was found to be VOC limited for all three periods, with warmer conditions generally associated with higher ambient ozone. Isoprene and acetaldehyde were the highest contributors to the VOC OH reactivity across all periods. With climate change likely to increase regional temperatures, our results and insights suggest that ozone pollution may exacerbate even as efforts to mitigate the currently more serious particulate matter pollution bear fruit.Every year in May, the north-west Indo-Gangetic Plain experiences its worst ozone pollution with ambient ozone frequently exceeding 100 ppb, due to a combination of warm and dry weather and agricultural biomass burning emissions. 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 calculate the Leighton ratios and evaluate daytime peroxy radical budgets for three contrasting periods experienced in May 2023. While two periods had lower NOx (~10 ppb) but contrasting ozone levels of ~50 ppb (I) and 90 ppb (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). Leighton ratios were much higher than 1, with average values of 3, 4.5 and 7.5, for period I, II and III, respectively, suggesting a significant role of peroxy-radicals in the ozone formation chemistry. Peroxy radical mixing ratios, namely the sum of hydroperoxy and alkyl peroxy radicals (RO2*=HO2+RO2) more than doubled to ~1.4 ppb during period II, relative to periods I (RO2*=0.5) and II (RO2*=0.6), respectively, suggesting strong amplification and perturbation of radical chemistry contributing to the high ambient ozone in period II. The ozone production regime was found to be VOC limited for all three periods, with warmer conditions generally associated with higher ambient ozone. Isoprene and acetaldehyde were the highest contributors to the VOC OH reactivity across all periods. With climate change likely to increase regional temperatures, our results and insights suggest that ozone pollution may exacerbate even as efforts to mitigate the currently more serious particulate matter pollution bear fruit.