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 United States (USA), 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 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 USA using extended ground-level measurements and regional chemical transport model simulations. We show that aerosols in China are significantly less acidic than in the USA, 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 USA, 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 USA - increasing the aerosol mass concentrations (by a factor of 8.4) but keeping the relative component contributions the same in the USA as the level in China increases the aerosol pH by ~1.0 units and further shifting the chemical composition from US conditions to China's that are richer in ammonia increases the aerosol pH by ~0.9 units. Therefore, China being both more polluted than the USA and richer in ammonia explains 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.
Highlights
As an intrinsic aerosol property, aerosol acidity plays an important role in a variety of aerosol physical and chemical processes (Pye et al, 2020)
The sensitivity test to adjust the Clean Air Status and Trends Network (CASTNET)-measured concentrations based on the reported average differences between weekly-duration and 24 h duration samples shows little difference between the unadjusted and adjusted pH values in the United States (USA) (2.69 ± 0.85 and 2.74 ± 0.83 on average for the unadjusted and adjusted pH, respectively), suggesting that the weekly duration of the CASTNET sampling has little impact on the calculated aerosol pH
The aerosol pH values calculated based on observational data show a significant difference between China and the USA
Summary
As an intrinsic aerosol property, aerosol acidity (usually characterized by aerosol pH) plays an important role in a variety of aerosol physical and chemical processes (Pye et al, 2020). A comprehensive, nationwide comparison of aerosol pH between China and the USA can provide a better understanding of the factors driving aerosol pH and its effect on aerosol formation mechanisms and properties (Pathak et al, 2009; Guo et al, 2017a; Wang et al, 2020). Such comparisons are still scarce (Guo et al, 2017b; Nenes et al, 2020; Zheng et al, 2020), primarily because of a lack of extensive simultaneous measurements of aerosol composition and semi-volatile gaseous compounds in China. We propose a new method to identify the factors driving the pH difference between these two countries and discuss the causes and implications of the pH difference
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