Ozone chemistry and dynamics at a tropical coastal site impacted by the COVID-19 lockdown

The nationwide lockdown in India to curb the spread of Coronavirus disease 2019 (COVID-19) led to colossal reduction in anthropogenic emissions. Here, we investigated the impact of lockdown on surface ozone (O(3)) and nitrogen dioxide (NO(2)) over a tropical coastal station – Thumba, Thiruvananthapuram (8.5°N, 76.9°E). Daytime as well as night-time NO(2) showed reduction by 0.8 (40%) and 2.3 (35%) ppbv, respectively during the lockdown period of 25–30 March 2020 as compared with the same period of previous 3 years. Unlike many urban locations, daytime surface O(3) is found to be dramatically reduced by 15 ppbv (36%) with O(3) production rate being lower by a factor of 3 during the lockdown. Interestingly, a feature of O(3)-hump during the onset of land breeze typically observed during 1997–1998 has reappeared with magnitude of 5–10 ppbv. A photochemical box model, capturing this feature, revealed that significant O(3) sustained till onset of land breeze over the land due to weaker titration with NO(x) during lockdown. It is suggested that the transport of this O(3) rich air with onset of land breeze led to the observed hump. Our measurements unravel a remarkable impact of the COVID-19 lockdown on the chemistry and dynamics of O(3) over this tropical coastal environment. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s12040-021-01666-3.