Evaluating urban ozone dynamics in two Indian megacities using ground data and predictive ozone modelling: role of AVOC – NOx regime and influence on secondary PM levels

Ozone (O₃) in ambient air acts as a greenhouse gas and has harmful effects on human health and vegetation. Short-term exposure to elevated surface O₃ is linked to increased risks of respiratory and cardiovascular mortality. The emission of volatile organic compounds (VOCs) and nitrogen oxides (NO_x) into the atmosphere can trigger chemical reactions influenced by solar radiation (SR), resulting in O₃ formation in the troposphere. This study focuses on a few locations within Delhi and Mumbai using publicly available data. O₃ concentrations peak in the afternoon and decrease subsequently. During winter, NO_x concentrations were higher, while O₃ concentrations were lower, possibly due to reduced solar radiation and altered atmospheric VOC-NO_x regimes. The HCHO/NO₂ ratios in both Delhi and Mumbai are less than 1, indicating VOC-limited conditions. The secondary fraction (SA) of PM_2.5 at select locations was estimated using the Approximate Envelope Method (AEM). SA values derived from AEM exhibited diurnal trends consistent with field studies and established knowledge. This analysis demonstrated that SA can constitute up to 85% of total PM_2.5, highlighting its significant contribution to overall particulate matter levels. An evaluation of the AVOC-NOx-O₃-SA relationship revealed that elevated O₃ concentrations predominantly occur at higher AVOC/NOx ratios, often leading to increased SA levels to some extent. To predict O₃, a multiple linear regression model was employed, incorporating various parameters. The model achieved a coefficient of correlation when compared to measured data of over 0.90, indicating its effectiveness in predicting O₃ levels. This research provides valuable insights into the dynamics of surface O₃ and its implications for urban secondary pollutants.