Spatial and seasonal variability of PM<sub>2.5</sub> acidity at two Chinese megacities: insights into the formation of secondary inorganic aerosols

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 PM_2.5 acidity was investigated at Beijing and Chongqing, two megacities in northern and southwestern China, respectively. PM_2.5 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 PM_2.5 at urban Beijing and rural Chongqing comparing to their rural and urban sites, respectively. PM_2.5 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 PM_2.5 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 PM_2.5 and rain water over the surrounding region. Meanwhile, PM_2.5 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

Variation of Urban Atmospheric Ammonia Pollution and its Relation with PM2.5 Chemical Property in Winter of Beijing, China

“…The ratio of NO 3 -to SO 4 2-exhibited an increasing trend in Beijing in recent years (He et al, 2012;, consistent with the effective control of SO 2 emissions and the rapid increase of the number of motor vehicles. As a major inorganic species in PM 2.5 , particulate NO 3 -can be primarily formed through the homogeneous reaction between gaseous HNO 3 and NH 3 and the heterogeneous reaction between NH 3 and the product from the hydrolysis of N 2 O 5 Shon et al, 2013).…”

“…As a major inorganic species in PM 2.5 , particulate NO 3 -can be primarily formed through the homogeneous reaction between gaseous HNO 3 and NH 3 and the heterogeneous reaction between NH 3 and the product from the hydrolysis of N 2 O 5 Shon et al, 2013). Generally, during daytime, the homogeneous formation is predominant, whereas during nighttime, heterogeneous formation occurring through the hydrolysis of N 2 O 5 on the surface of the pre-existing moist aerosols under relatively high humidity is predominant (Ianniello et al, 2011 (Huang et al, 2011;He et al, 2012). Fig.…”

Secondary Formation of Sulfate and Nitrate during a Haze Episode in Megacity Beijing, China

“…5, an obvious increase of nitrates was also observed during haze episodes, and nitrates dominated SNA during the measurement periods (discussed in section 3.1.2), which implied that nitrate had become a major concern during haze pollution. -was dominated by the homogenous reaction of HNO 3 (g) + NH 3 (g) ↔ NH 4 NO 3 (s, aq) (Pathak et al, 2009;He et al, 2012). To demonstrate the homogenous formation of nitrate, a scatter plot of the molar concentrations of nitrate and "excess ammonium" is shown in Figs.…”

“…To demonstrate the homogenous formation of nitrate, a scatter plot of the molar concentrations of nitrate and "excess ammonium" is shown in Figs. 10(c) with the axis, respectively on non-haze and haze day (He et al, 2012)). As illustrated in Figs.…”

“…Both laboratory experiments and in situ observations suggest that the aqueous oxidation of SO 2 is attributed to catalysis by metals, such as iron and manganese (Turšič et al, 2003;Li et al, 2011;Wang et al, 2012). The formation pathway of nitrates, however, is less well known compared to the formation of sulfate, and it is influenced by the emission load of the precursors, the concentrations of oxidants in both the gas and the liquid phase, the characteristics of pre-existing particles and meteorological conditions (Seinfeld and Pandis, 2006;Pathak et al, 2009;Guo et al, 2010;Ye et al, 2011;He et al, 2012). Generally, fine nitrate is more concentrated in homogeneous gas-phase reactions between ammonia and nitric acid in ammoniumrich conditions and is attributed to heterogeneous hydrolysis of N 2 O 5 on the surface of humid and acidic pre-existing particles under ammonium-poor conditions (Seinfeld and Pandis, 2006;Pathak et al, 2009).…”