Atmospheric ozone (O₃) concentration is impacted by a number of factors, such as the amount of solar radiation, the composition of nitrogen oxides (NOx) and hydrocarbons, the transport of pollutants and the amount of particulate matter in the atmosphere. The oxidative potential of the atmosphere and the formation of secondary organic aerosols (SOAs) as a result of atmospheric oxidation are influenced by the prevalent O₃ concentration. The formation of secondary aerosols from O₃ depends on several meteorological, environmental and chemical factors. The relationship between PM_2.5 and O₃ in different urban environmental regimes of India is investigated in this study during the summer and winter seasons. A relationship between PM_2.5 and O₃ has been established for many meteorological and chemical variables, such as RH, WS, T and NOx, for the selected study locations. During the winter season, the correlation between PM_2.5 and O₃ was found to be negative for Delhi and Bengaluru, whereas it was positive in Ahmedabad. The city of Bengaluru was seen to have a positive correlation between PM_2.5 and O₃ during summer, coinciding with the transport of marine air masses with high RH and low wind speed (as evident from FLEXPART simulations), leading to the formation of SOAs. Further, O₃ concentrations are predicted using a Recurrent Neural Network (RNN) model based on the relation obtained between PM_2.5 and O₃ for the summer season using NOx, T, RH, WS and PM_2.5 as inputs.

The COVID-19 pandemic resulted in changed emission regimes all over the world. India also imposed complete lockdown on all modes of travel and industrial activities for about 2 months from 25-March-2020 and later unlocked these activities in a phased manner. Here, we study signatures of emissions changes on levels of atmospheric trace gases and aerosols contributing to air pollution over multiple sites in India’s capital Delhi covering various lockdown and unlock phases using satellite data and in-situ observations. The resulting changes in the levels of these species were compared with respect to their average of 2015–2019 to attribute for year to year and seasonal changes. A clear impact of lockdown was observed for AOD, PM, NO₂, CO, and SO₂ as a result of emission changes, while changed precursor levels led to a change in O₃ chemical regimes impacting its concentrations. A detailed analysis of FLEXPART trajectories revealed increased PM levels over Delhi in north-westerly air masses sourced to Punjab region all the way up to Pakistan. Changes in aerosols and NO₂ were not only restricted to the surface but transcended the total tropospheric column. The maximum decrease in PM, NO₂, CO, and SO₂ was observed during the month of total lockdown in April. The lockdown impact varied with species e.g., PM₁₀ and PM_2.5 as well as locations even within the periphery of Delhi. While surface level aerosols and NO₂ showed significant and almost similar changes, AOD showed much lower decrease than tropospheric column NO₂.

Most of the published articles which document changes in atmospheric compositions during the various lockdown and unlock phases of COVID-19 pandemic have made a direct comparison to a reference point (which may be 1 year apart) for attribution of the COVID-mediated lockdown impact on atmospheric composition. In the present study, we offer a better attribution of the lockdown impacts by also considering the effect of meteorology and seasonality. We decrease the temporal distance between the impacted and reference points by considering the difference of adjacent periods first and then comparing the impacted point to the mean of several reference points in the previous years. Additionally, we conduct a multi-station analysis to get a holistic effect of the different climatic and emission regimes. In several places in eastern and coastal India, the seasonally induced changes already pointed to a decrease in PM concentrations based on the previous year data; hence, the actual decrease due to lockdown would be much less than that observed just on the basis of difference of concentrations between subsequent periods. In contrast, northern Indian stations would normally show an increase in PM concentration at the time of the year when lockdown was effected; hence, actual lockdown-induced change would be in surplus of the observed change. The impact of wind-borne transport of pollutants to the study sites dominates over the dilution effects. Box model simulations point to a VOC-sensitive composition.

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.