Vehicle exhaust emissions and ammonia slip are major sources of atmospheric ammonia and ammonium

Haze pollution that occurs frequently in winter in cities in northern China has caused widespread concern. Since the implementation of the Air Pollution Prevention and Control Action Plan in 2013, the atmospheric concentration of acid gases such as sulfur dioxide and nitrogen oxides has decreased significantly in this region. However, the concentration of ammonia gas (NH3) and inorganic ammonium (NH4+) salts such as ammonium sulfate and ammonium nitrate in atmospheric aerosol particles continued to increase.

Ammonia gas (NH3), combined with acid gases in the atmosphere, can form PM precursors2.5 (small atmospheric aerosolized particulate matter in air 2.5 micrometers or less in diameter) and increase the contribution of ammonium deposition to atmospheric nitrogen deposition. Measurement of the natural abundance of stable nitrogen isotopes (δ15N) is an efficient and rapid approach for the detection of the source of NH4+ in PM2.5and it is vital for the formulation of reasonable air pollution control programs.

Researchers Abubakari Said Mgelwa and FANG Yunting from the Institute of Applied Ecology of the Chinese Academy of Sciences, together with the team of Professor ZHANG Yanlin from Nanjing University of Information Science and Technology, collected PM2.5 samples in Beijing, Tianjin, Baoding, Shijiazhuang, Tangshan and Gucheng, and measured the concentrations of ammonium and 15N ammonium abundance (δ15N-NH4+) in PM2.5 particles.

The researchers found that polluted days (defined as days with PM2.5 concentration less than 35 μg m-3), the average atmospheric concentration of NH4+ was almost three times higher on unpolluted days (PM2.5 > 35 μg m-3) in the six cities.

They found that vehicle exhaust emissions and urban ammonia leakage, two non-agricultural sources of NH4+ in the atmosphere, together accounted for 56% to 86% and 72% to 94% of atmospheric NH4+ concentration on polluted and unpolluted days respectively, according to the degree of urbanization of cities.

In particular, NH related to combustion4+ sources, vehicle exhaust emissions in particular, were positively correlated with population density, proportion of urban area, and number of vehicles on the road in the six cities.

These results highlight the critical impacts of local emission sources on the formation of atmospheric haze pollution and the importance of δ15N analysis for the detection of ammonia and ammonium sources in the atmosphere.

This study, funded by the National Key R&D Program of the Ministry of Science and Technology of China, was published in Environmental Pollution under the title “Isotopic imprints of aerosol ammonium over the north China plain”.

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