Atmos. Chem. Phys. Discuss., 11, 25557-25603, 2011
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Spatial and seasonal variability of PM2.5 acidity at two Chinese megacities: insights into the formation of secondary inorganic aerosols
K. He1, Q. Zhao1, Y. Ma1, F. Duan1, and F. Yang2
1State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing 100084, China
2School of Earth Science, Graduate University of Chinese Academy of Sciences, Beijing 100049, China

Abstract. Aerosol acidity is one of the most important parameters influencing atmospheric chemistry and physics. Based on continuous field observation from January 2005 to May 2006, both spatial and seasonal variation in PM2.5 acidity was investigated at Beijing and Chongqing, two megacities in northern and southwestern China, respectively. PM2.5 was generally more acidic at urban and rural sites in Chongqing than Beijing, but a reverse spatial pattern was found within the two cities, with more acidic PM2.5 at urban Beijing and rural Chongqing comparing to their rural and urban sites, respectively. PM2.5 was more acidic in summer and fall than in winter, while large inter-annual variations were evident during the springs of 2005 and 2006, with Beijing and Chongqing exhibiting opposite trends, however. The higher acidity of PM2.5 in the spring of 2006 in Beijing was attributed to the influence of Asian desert dust which significantly enhanced the formation of nitrate relative to sulfate; in contrast, the more acidic aerosols found during the spring of 2005 in Chongqing was mainly due to the greater wet deposition of ammonium, and precipitation was probably one of the key factors that controlled the partition of ammonium in PM2.5 and rain water over the surrounding region. Meanwhile, PM2.5 became more acidic at both cities from spring into early summer of 2005. The synoptic-scale evolution of its acidity was found to be closely associated with the greater contribution of air masses from between the Northern China Plain to the south of Beijing, and from central China to the east of Chongqing, and was ended by the northward movement of a subtropical high over the northwestern Pacific, a major element of the Asian summer monsoon. Under conditions of high aerosol acidity, heterogeneous reactions may become one of the major pathways for the formation of nitrate at both cities. These findings may also explain the significant variation in inter-annual aerosol acidity observed during other seasons at Beijing, Chongqing and many other cities in China, as reported in previous studies.

Citation: He, K., Zhao, Q., Ma, Y., Duan, F., and Yang, F.: Spatial and seasonal variability of PM2.5 acidity at two Chinese megacities: insights into the formation of secondary inorganic aerosols, Atmos. Chem. Phys. Discuss., 11, 25557-25603, doi:10.5194/acpd-11-25557-2011, 2011.
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