The influence of lightning activity on NO and O in the Pearl River Delta region.

Extremely high-temperature lightning generates NO by electrolyzing nitrogen and oxygen molecules, regulating ozone concentration. The Pearl River Delta (PRD) is located in the world's high-value area of lightning density, and lightning-generated NO (LNO) cannot be ignored. Using the flash data from Guangdong-Hong Kong-Macao Lightning Location System and multi-site atmospheric composition data, we estimate the NO variations in lightning activity and its impact on O across the PRD region. The cloud-to-groud (CG) frequency from 2013 to 2021 shows a decreasing trend driven by urban regions. We observe that the lightning density is steadily decreasing from the south-central part of Guangzhou City to the surrounding area. A comparison of the different sites with lightning days and non-lightning days shows that a significant amount (13. 84-20. 47 %) of ground-level NO concentration at urban stations can be attributed to lightning NO emissions. A lower lightning frequency and low background concentration observed at suburban sites indicated a limited contribution of LNO. The average decrease in O concentration at urban stations (15.92-25.06 %) was significantly higher than that at suburban stations (5.34-8.95 %) due to the influence of titration and lower actinic radiation. There was a greater fluctuation in NO and O concentrations during the cases, and the surface NO concentration displayed the most significant responsiveness to LNO under direct lightning striking in the tall tower. This phenomenon has not been reported, however, it is consistent with the laboratory-based observations suggesting the amount of LNO increases with peak current. LNO significantly impacts air quality in the PRD during the high convective season. Further in situ and vertical distribution observations are necessary to explore the ground-level impact of LNO.