A study of peroxyacetyl nitrate at a rural site in Beijing based on continuous observations from 2015 to 2019 and the WRFChem model

Peroxyacetyl nitrate(PA N)is one of the most important photochemical pollutants and has aroused much concern in China in recent decades.However,few studies described the long-term variations in PAN in China.In this study,we continuously monitored the PAN,03 and NOx concentrations at a regional background site near Beijing from August 2015 to February 2019.Based on the observed concentrations and climate data,w e analyzed the seasonal PAN variations.The results revealed that the monthly mean PAN concentration ranged from 0.33-2.41 ppb,with an average value of 0.94 ppb.The PAN concentration exhibited a distinct seasonal variation,with high values in spring and low values in winter.After analyzing the corresponding meteorological data,we found that stronger ultraviolet(UV)radiation,a relatively longer lifetime and a higher background PAN concentration contributed to the high PAN concentrations in spring.In addition,with the utilization of the WRF-Chem(Weather Research and Forecasting with Chemistry)model,the cause of the extremely high PAN concentration in spring 2018 wa s determined.The model results demonstrated that an anomalously low pressure and the southwesterly winds in northern China might be the main causes of the increased PAN concentration in Beijing and its surrounding area in spring 2018.