Impact of COVID-19 lockdown on surface ozone build-up at an urban site in western India based on photochemical box modelling

Elevated ozone (O<inf>3</inf>) near the earth’s surface causes adverse impacts on human health and vegetation, besides impacting air chemistry and climate. Intense lockdown to contain the spread of Coronavirus disease 2019 (COVID-19) offered a rare opportunity to delineate the anthropogenic impact on urban O<inf>3</inf>build-up. In this regard, we incorporated observations of chemical species and environmental conditions into a photochemical box model (NCAR Master Mechanism) to study the O<inf>3</inf>changes at a semi-arid urban site in western India (Ahmedabad; 23°N, 72.6°E). In contrast with primary pollutants, daytime O<inf>3</inf>build-up is observed to be enhanced during the lockdown by ~39%. Model, driven by lower nitrogen oxides (NO<inf>x</inf>) during the lockdown, also simulated enhanced O<inf>3</inf>(by ~41%) showing the role of nonlinear dependence of O<inf>3</inf>on NO<inf>x</inf>. Further, a sensitivity simulation unravelled an important role of the meteorological changes in the O<inf>3</inf>enhancement (by ~16%) during the lockdown. The results highlight that the lockdown impacts can be modulated profoundly by the complex chemistry plus meteorological changes, offsetting the benefits of lower precursor levels in the context of O<inf>3</inf>pollution.