Nocturnal Atmospheric Oxidative Processes in the Indo‐Gangetic Plain and Their Variation During the COVID‐19 Lockdowns

This study investigates selected secondary atmospheric responses to the widely reported emission change attributed to COVID‐19 lockdowns in the highly polluted Indo‐Gangetic Plain (IGP) using ground‐based measurements of trace gases and particulate matter. We used a chemical box‐model to show that production of nighttime oxidant, NO3, was affected mainly by emission decrease (average nighttime production rates 1.2, 0.8 and 1.5 ppbv hr−1 before, during and relaxation of lockdown restrictions, respectively), while NO3 sinks were sensitive to both emission reduction and seasonal variations. We have also shown that the maximum potential mixing ratio of nitryl chloride, a photolytic chlorine radical source which has not been previously considered in the IGP, is as high as 5.5 ppbv at this inland site, resulting from strong nitrate radical production and a potentially large particulate chloride mass. This analysis suggests that air quality measurement campaigns and modeling explicitly consider heterogeneous nitrogen oxide and halogen chemistry. Plain Language Summary The Indo‐Gangetic Plain (IGP) is one of the most polluted regions on earth, with poor air quality affecting the majority of the Indian population. The atmospheric chemistry that transforms major regional emissions into harmful secondary pollutants is complex. Here, we quantify, for the first time, several important oxidative processes and show the potential for substantial oxidation of biogenic volatile organic compounds and the production of chlorine through unconventional chemistry in the IGP. We further show how these chemical cycles varied due to the emission reductions as a result of COVID‐19 lockdown, findings that will serve to define their sensitivity to future emission changes in the region. Key Points Atmospheric response in the Indo‐Gangetic Plain varied according to seasonal changes and emissions reductions due to COVID‐19 lockdown NO3 production was mainly affected by emission changes, while NO3 sinks were sensitive to both emissions and seasonal changes Nitryl chloride, a photolytic chlorine radical source not previously considered in the inland Indo‐Gangetic Plain, may be up to 5.5 ppbv