Spatial and seasonal variability of PM2.5 acidity at two Chinese megacities: insights into the formation of secondary inorganic aerosols

摘要

Aerosol acidity is one of the most important parameters influencingatmospheric chemistry and physics. Based on continuous field observationsfrom January 2005 to May 2006 and thermodynamic modeling, we investigatedthe spatial and seasonal variations in PM acidity in two megacitiesin China, Beijing and Chongqing. Spatially, PM was generally moreacidic in Chongqing than in Beijing, but a reverse spatial pattern was foundwithin the two cities, with more acidic PM at the urban site inBeijing whereas the rural site in Chongqing. Ionic compositions ofPM revealed that it was the higher concentrations of NOat the urban site in Beijing and the lower concentrations of Cawithin the rural site in Chongqing that made their PM more acidic.Temporally, PM was more acidic in summer and fall than in winter,while in the spring of 2006, the acidity of PM was higher in Beijingbut lower in Chongqing than that in 2005. These were attributed to the moreefficient formation of nitrate relative to sulfate as a result of theinfluence of Asian desert dust in 2006 in Beijing and the greater wetdeposition of ammonium compared to sulfate and nitrate in 2005 in Chongqing.Furthermore, simultaneous increase of PM acidity was observed fromspring to early summer of 2005 in both cities. This synoptic-scale evolutionof PM acidity was accompanied by the changes in air masses origins,which were influenced by the movements of a subtropical high over thenorthwestern Pacific in early summer. Finally, the correlations between[NO]/[SO] and [NH]/[SO]suggests that under conditions of high aerosol acidity, heterogeneousreactions became one of the major pathways for the formation of nitrate atboth cities. These findings provided new insights in our understanding ofthe spatial and temporal variations in aerosol acidity in Beijing andChongqing, as well as those reported in other cities in China.