Significant contrasts of aerosol acidity between China and the United States
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Zhang, Bingqing
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Abstract
Aerosol acidity governs several key processes in aerosol physics and chemistry, thus affecting aerosol mass and composition, and ultimately the climate and human health. Previous studies have reported aerosol pH values separately in China and the United States (US), implying different aerosol acidity between these two countries. However, there is debate about whether mass concentration or chemical composition is the more important driver of differences in aerosol acidity. A full picture of the pH difference and the underlying mechanisms responsible for it is hindered by the scarcity of simultaneous measurements of particle composition and gaseous species, especially in China. Here we conduct a comprehensive assessment of aerosol acidity based on annual mean data in China and the US using extended ground-level measurements and regional chemical transport model simulations. We show that annually aerosol in China is significantly less acidic than in the US, with pH values 1–2 units higher. Based on a proposed multivariable Taylor Series method and a series of sensitivity tests, we identify major factors that potentially leading to the pH difference. Compared to the US, China has much higher aerosol mass concentrations (gas + particle, by a factor of 8.4 on average) and a higher fraction of total ammonia (gas + particle) in the aerosol composition. Our assessment shows that such differences in mass concentrations and chemical composition play equally important roles in driving the aerosol pH difference between China and the US. Therefore, both the facts that China is more polluted than the US and is rich in ammonia together explain the aerosol pH difference. The difference in aerosol acidity highlighted in the present study implies potential differences in formation mechanisms, physicochemical properties, and toxicity of aerosol particles in these two countries.
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2021-04-22
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