Investigation of a regional ozone reduction event over eastern India by integrating in situ and satellite measurements with WRF-Chem simulations

This study examines various physical and chemical processes contributing towards an anomalous low surface ozone (O₃) event at a tropical urban site Bhubaneswar (20.29° N, 85.83° E) located in eastern India. Surface O₃ generally peaks during winter (~ 40 ppbv) at Bhubaneswar. However, during 5–11 December 2010, surface O₃ decreased rapidly from 48°ppbv on December 4 to 14°ppbv on December 6. Average surface O₃ concentration before and after the event was 50.4 ± 5.8 ppbv and 47.8 ± 2.4 ppbv, respectively, with two-day running mean ranging from 15.5 to 30.5 ppbv during the event. This event is analyzed using in situ surface O₃, meteorological parameters, backward air trajectories, and simulations using the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem). Our analysis shows that this low O₃ event was driven by advection of cleaner low O₃ marine masses to Bhubaneswar associated with a rare low-pressure event over Bhubaneswar (occurred only once in winter during 2005–2014). Analysis of modeled spatial distribution of surface O₃ showed that this event was not confined to Bhubaneswar only and significantly reduced surface O₃ over a large area comprising most of the Indian land mass, the Bay of Bengal (BOB), and Burma as well. The study elucidates the importance of transport processes in controlling trace gas levels and highlights the advantage of Bhubaneswar as a strategic location to study atmospheric chemistry in contrasting air masses, i.e., polluted air masses from the Indo-Gangetic Plain (IGP) versus pristine air masses from the BOB.