The Observation And Understanding Unusually Wintertime High Levels Of Peroxyacetyl Nitrate(PAN) In Urban Jinan,China

Peroxyacetyl nitrate(chemical formula CH3COO2NO2,shortened form PAN),is a typical secondary photochemical product.PAN has no natural sources in atmospheric environment,but photochemical reaction of VOCs and NOx under light conditions is the only way of PAN generation.Due to its high stability at low temperature,PAN is also considered as a short-term reservoir of NOX,the remote transport of which can affect the ambient concentrations of NOX in remote area and consequently the concentration of O3.This PAN study was the first time that PAN concentration was continuously detected at urban site of Shandong Province(2015.11-2016.7).During that period,ambient concentration of PAN was observed in real time by an automatic GC-ECD analyzer.During PAN observation,various atmospheric pollutants and the local meteorological data were detected at the same site for assisted analysis,such as concentrations of PM2.5,NO,NO2 and 03,HONO,NO/NO2 ratio,NO2 photolysis rate constant(JNO2),O3 photolysis rate constant(JO1D),wind speed,direction and temperature.Because of the significantly influence of temperature on PAN,PAN data obtained in this experiment were roughly divided into two periods:2015.11,4-2016.1.14(winter section)and 2016.4.1-2016.7.19(summer section).In this research,the maximum values of PAN in winter and summer were 9.61 ppbv and 13.47 ppbv,respectively,and the mean values were 1.89± 1.42 ppbv and.2.54± 1.44 ppbv,respectively.Compared with previous studies,the ambient concentrations of PAN in Jinan were significantly higher than those of most large cities in the world,indicating the serious photochemical pollution of Jinan.In this study,the PAN level in summer was much higher than that in winter,according with the significant seasonal difference of PAN found in previous researches.The wintertime level of PAN was unusually high compared with previous researches at same season,and the PAN behavior was highly consistent with haze events.Therefore,the main target of this paper is to explain the formation mechanism of unusually high levels of PAN in winter(especially during haze events).In order to explore the formation mechanism of high-level PAN in winter,a series of analyses were made as follows.72h Mass trajectory lines in 1000 m high air calculated by Hybrid Single Particle Lagrangian Integrated Trajectory Model and the wintertime wind speed and direction analysis found that the influence of the long-way transport of PAN was extremely low,and the high level o PAN mainly formed by local photochemical reaction or local accumulation.According to the chemical reaction mechanism of PAN formation and degradation,the lifetime of PAN in different situations can be calculated.Long lifetime of PAN in winter proved that winter environment benefits the local accumulation and transport of PAN,but significantly lower lifetime of PAN(5.5 days)during haze event with high concentration of PAN means PAN formation rate during haze episodes may be higher than common winter.The formation rate of PAN is mainly affected by the intensity of light radiation and the concentration of precursors,such as VOCs and OH radicals.Previous studies has repeatedly confirmed that VOCs concentrations increased significantly during haze episodes.The change of photolysis rate constant showed that the light radiation intensity during haze episodes might be enough to produce fast photochemical reaction.In addition,high concentration of HONO during haze episodes might be one of the contributing factors for accelerated PAN generation rate.Ozone was hard to accumulate in winter like PAN due to short lifetime,and the high concentration of NO during winter will hide part concentration of ozone,which might explain the irrelevance between ozone and PAN during the winter.This experiment showed that even in low-light condition,such as the haze episodes,it is possible to form serious photochemical pollution.