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Significant Contrasts in Aerosol Acidity between China and the United States
BINGQING ZHANG, Huizhong Shen, Pengfei Liu, Hongyu Guo, Yongtao Hu, Yilin Chen, Shaodong Xie, Ziyan Xi, T. Nash Skipper, Armistead G. Russell, Georgia Institute of Technology
Abstract Number: 435
Working Group: Aerosol Chemistry
Abstract
Aerosol acidity governs several key processes in aerosol physics and chemistry, thus affecting aerosol mass and composition, and ultimately climate and human health. Previous studies have reported aerosol pH values separately in China and the US, implying different aerosol acidity between these two countries. A full picture of the pH difference and the underlying mechanisms responsible 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 in China and the US using extended ground-level measurements and regional chemical transport model simulations. We show that 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 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 the differences in mass concentrations and chemical composition play equally important roles in driving the aerosol pH difference between China and the US — increasing the aerosol mass concentrations, but keeping the relative component contributions the same, in the US to the level in China (by a factor of 8.4) increases the aerosol pH by ~1.0 unit, and further shifting the chemical composition from US conditions to China’s that is richer in ammonia increases the aerosol pH by ~0.9 units. Therefore, both China being more polluted than the US and richer in ammonia together explain the aerosol pH difference. The difference in aerosol acidity highlighted implies potential differences in formation mechanisms, physicochemical properties, and toxicity of aerosol particles.