Variation Characteristics And Sources Of Peroxyacetyl Nitrate In Nanling Background Atmosphere

Over the past decade,secondary pollution characterized by active photochemistry has sharply increased and become the major contributor to air pollution in China,despite the great reduction of primary emissions.PAN（Peroxyacetyl nitrate:PAN）is an important secondary photochemical oxidation product in the atmosphere.This study conducted two online measurements of atmospheric PAN in Nanling in the fall of 2018 and 2020.The results of this study are helpful to understand the concentration level and source of PAN in the background atmosphere of South China,and has an important scientific significance for exploring the problem of regional photochemical pollution.The main results of the study are as follows:（1）The average concentration of PAN at the foot and top of Nanling Tianjing Mountain in 2018 was 0.20±0.14ppb and 0.52±0.45 ppb,respectively.The PAN concentration at the top mountain in 2020（0.47±0.35 ppb）is lower than 2018.In general,the PAN level of background stations is higher than the observation results of other remote background areas.The higher background concentration of PAN may also be one of the factors for the higher PAN level in South China over the years;（2）The hourly mean values of PAN and O₃、NO₂ present a positive correlation.The ratio of PAN/O₃in Nanling is equivalent to that in the suburbs of the Pearl River Delta,indicating the photochemical reaction of Nanling forests is relatively active.Combining with the correlation analysis between PAN and O_x found that the photochemical reaction in 2018 had a greater impact on PAN than in 2020;From the distribution of active nitrogen（NO_y）,the PAN/NO_y（14.88%）in2018 was slightly lower than that in 2020（17.05%）;（3）Although the concentration of PAN at the foot of the mountain was lower in 2018,the concentration of PA radical was significantly higher than the mountain top area.Compared with some rural areas in southern China,the concentration of PA free radicals in the mountain top area was equivalent or.Higher,indicating the strong photochemical oxidation ability in the background area of Nanling.From the calculation of PAN reaction rate and photochemical contribution,both results found that the photochemical reaction at the foot of the mountain had a higher impact on PAN than the mountain top area,and the mountain top is more affected by regional transportation or other unknown sources.From the perspective of PAN atmospheric lifetime,PAN at the foot of the mountain has a shorter atmospheric lifetime（1.5 h）,while the temperature and NO/NO₂ ratio are much higher than the mountain top in 2018 and 2020.The average atmospheric lifetime of PAN in 2020（42.4 h）is more than twice in2018（19.9 h）,this result was related to the lower temperature and NO/NO₂ ratio;（4）The backward trajectory of regional air mass transport and the distribution of the potential source contribution of PAN indicate that the high concentration of PAN mainly comes from Central China（such as Jiangxi,Hubei,Henan and Hunan）in 2018.Except for the potential source contribution of Central China,there are also potential sources of high-concentration PAN in the southwest of the site in 2020;（5）Therefore,it was also found the frequency of downward transport events from September to November in 2018 and 2020 was 3.7%and 7.5%,respectively.