Summertime PM<sub>2.5</sub> ionic species in four major cities of China: nitrate formation in an ammonia-deficient atmosphere

Abstract

Abstract. Strong atmospheric photochemistry in summer can produce a significant amount of secondary aerosols, which may have a large impact on regional air quality and visibility. In the study reported herein, we analyzed sulfate, nitrate, and ammonium in PM 2.5 samples collected using a 24-h filter system at suburban and rural sites near four major cities in China (Beijing, Shanghai, Guangzhou, and Lanzhou). Overall, the PM 2.5 mass concentrations were high (with a mean value of 55-68 g µgm −3 ), which reflects the long-known particulate pollution in China's large urban centers. We observed very high concentrations of sulfate and nitrate at the Beijing and Shanghai sites, and, in particular, abnormally high levels of nitrate (24-h average concentration up to 42gµgm −3 and contributing up to 25% of the PM 2.5 mass) in the ammonium-poor samples. The Beijing and Shanghai aerosols were characterized by high levels of aerosol acidity (∼220-390 nmol m −3 ) and low levels of insitu pH (−0.77 to −0.52). In these samples, the formation of the observed high concentrations of particulate nitrate cannot be explained by homogeneous gas-phase reaction between ammonia and nitric acid. Examination of the relation of nitrate to relative humidity and aerosol loading suggests that the nitrate was most probably formed via the heterogeneous hydrolysis of N 2 O 5 on the surface of the moist and acidic aerosols in Beijing and Shanghai. In comparison, the samples collected in Lanzhou and Guangzhou were ammoniumrich with low levels of aerosol acidity (∼65-70 nmol m −3 ), and the formation of ammonium nitrate via the homogeneous gas-phase reaction was favored, which is similar to many previous studies. An empirical fit has been derived to relate fine nitrate to aerosol acidity, aerosol water content, aerosol surface area, and the precursor of nitrate for the data from Beijing and Shanghai.

Carbonaceous and Secondary Inorganic Aerosols during Wintertime Fog and Haze over Urban Sites in the Indo-Gangetic Plain

“…In addition; nighttime secondary NO 3 − formation involves the hydrolysis of dinitrogen-pentaoxide (N 2 O 5 ) through the heterogeneous reaction on aerosol surfaces and higher relative humidity enhances the heterogeneous hydrolysis of N 2 O 5 (Ravishankara et al, 1997;Jacob, 2000;Pathak et al, 2009). Brown et al (2004) have suggested that the conversion efficiency of NO x to HNO 3 during nighttime can be equivalent to the daytime photochemical conversion.…”

Aerosol Acidity Measurement Using Colorimetry Coupled With a Reflectance UV-Visible Spectrometer

“…The F SA values for the ambient aerosols are diverse, reaching to more than 0.60 under ammonium-poor condition and to less than 0.40 under ammonium-rich condition (Pathak et al 2009). In this study, the C-RUV calibration curve obtained from ammonium-poor aerosols was extrapolated to ammonium-rich aerosol such as NH 4 HSO 4 aerosol (F SA = 0.50) and (NH 4 ) 3 H(SO 4 ) 2 aerosol (F SA = 0.40) under various RHs.…”

“…When the ammonia gas is not sufficient to completely neutralize sulfuric acid aerosol, the atmospheric aerosol tends to be acidic. The pH of ambient aerosols collected in US and some of the European and Asian countries ranges from −1 to 3, indicating that atmospheric aerosol is quite acidic (Pathak et al 2009). It has been known that both dry and wet deposition of acidic aerosol cause damage to building materials and endanger ecological integrity (Likens et al 1996).…”

Production of N<sub>2</sub>O<sub>5</sub> and ClNO<sub>2</sub> in summer in urban Beijing, China

“…While the formation mechanisms of nitrate are relatively well known, the relative contributions of different mechanisms can have large variability and uncertainties. Pathak et al (2009) found that the heterogeneous hydrolysis of N 2 O 5 contributed 50 %-100 % of the nighttime enhancement of nitrate concentration in Beijing. WRF-Chem model simulations showed only 21 % enhancement of nitrate during highly polluted days (Su et al, 2016).…”