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.
Spatial and Temporal Variation in PM
2.5
Chemical Composition in the United States for Health Effects Studies
