Pollution characteristics of peroxyacetyl nitrate in karst areas in Southwest China

In recent years, the issue of photochemical pollution in the Guangxi region of China has escalated considerably. However, there remains a notable dearth of related research in this area. Peroxyacetyl nitrate (PAN), recognized as a reliable indicator of photochemical pollution, was the focus of our study. This study marks the inaugural observation of PAN levels in Guilin, a renowned world tourist destination situated in a typical karst region, during 1-31 October 2021. Throughout this observation period, the average volume concentration of PAN ranged from 0.087 to 2.559 ppb, which was 3.61 times higher than the South China background site of the Nanling. Combined with meteorological factors and potential source analysis, the causes of a typical high-value PAN process were explored during 24-29 October. The results showed that, during this high-value PAN event, pollution primarily originated from the horizontal transport of polluted air masses and the descent of high-altitude air masses from Hunan Province in the northeast direction. Additionally, the meteorological conditions, including high temperatures, intense radiation, and low humidity, fostered local PAN formation. Notably, traffic emissions emerged as the primary source of PAN's locally generated precursor volatile organic compounds. Furthermore, we estimated the background concentration of O3 to be approximately 20.347 ppb based on PAN monitoring data, constituting 44.4% of the total O3 levels in Guilin City. This study offers valuable insights for addressing and mitigating photochemical pollution in southern Chinese cities, while providing a theoretical foundation for regional pollution control efforts.