Please use this identifier to cite or link to this item: http://hdl.handle.net/10397/6640
Title: Summertime PM₂.₅ ionic species in four major cities of China : nitrate formation in an ammonia-deficient atmosphere
Authors: Pathak, RK
Wu, WS
Wang, T 
Keywords: Acidity
Aerosol composition
Aerosol formation
Ammonium
Gas phase reaction
Hydrolysis
Ionic composition
Nitrate
pH
Sulfate
Issue Date: 2009
Publisher: Copernicus GmbH
Source: Atmospheric chemistry and physics, 2009, v. 9, no. 19, p. 1711–1722 How to cite?
Journal: Atmospheric chemistry and physics 
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 PM₂.₅ 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 PM₂.₅ mass concentrations were high (with a mean value of 55–68 gμgm⁻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 PM₂.₅ mass) in the ammonium-poor samples. The Beijing and Shanghai aerosols were characterized by high levels of aerosol acidity (~220–390 nmolm⁻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 N2O5 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 nmolm⁻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.
URI: http://hdl.handle.net/10397/6640
ISSN: 1680-7316
EISSN: 1680-7324
DOI: 10.5194/acp-9-1711-2009
Rights: © Author(s) 2009. This work is distributed under the Creative Commons Attribution 3.0 License(https://creativecommons.org/licenses/by/3.0/).
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