Unveiling the Impact of Lightning-induced NOx on Air Quality Over India

Nitrogen oxides (NOx = NO + NO₂) are significant air pollutants that pose direct health risks and drive the formation of secondary pollutants such as ozone (O₃) and fine particulate matter (PM₂.₅). While NOx emissions have declined in regions like Europe and the United States (Schneider et al., 2015), the South Asia has experienced a sharp increases, exacerbating air quality challenges. Despite regulatory efforts to curb anthropogenic NOx emissions, the contribution of natural NOx sources, particularly lightning, remains poorly understood. This study explores the role of lightning-induced NOx (LNOx) in shaping the NOx budget and its implications for air quality across the Indian subcontinent.

Focusing on the pre-monsoon (March-May) season, when deep convective activity is at its peak, we analyzed high-resolution lightning data from the Lightning Imaging Sensor (LIS) to identify periods of intense activity. Using the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem), we simulated LNOx emissions by varying NO emissions per lightning flash (200–800 mol) to quantify its contribution accurately. Sensitivity experiments were conducted to isolate the impact of LNOx on atmospheric composition. Our results reveal that lightning significantly alters surface concentration of NO₂ and O₃. At lightning-strike locations, surface NO₂ levels increased by up to 0.5 ppb/day, though reductions of ~0.25 ppb were observed in certain areas due to complex chemical interactions. O₃concentrations showed enhancements of up to 1.5 ppb/day, driven by NO_x- fueled ozone production. Although, these numbers are small compared to anthropogenic contributions, it may have significant impacts on human exposure. Additionally, we observed a notable increase in hydroxyl radical (OH) concentrations across the atmosphere, highlighting the role of LNOx in modulating oxidative capacity. Stratosphere-troposphere exchange processes further influenced surface levels of NO₂, O₃, and OH.

As climate change intensifies deep convective activity, the contribution of LNOx to air quality is expected to grow. This study underscores the need to incorporate LNOx as a significant natural source in air quality models and policymaking efforts to develop effective strategies for mitigating air pollution in the region.