Significant contrasts in aerosol acidity between China and the United States

Abstract

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.

Detecting the pH-dependent liquid-liquid phase separation of single levitated aerosol microdroplets via laser tweezers-Raman spectroscopy

“…However, even those works on real-world aerosols used the diluted extracts of filtercollected samples for aerosol generation where the constituent concentration differed from pristine atmosphere aerosols. Recent out-field studies have reported the high acidity found in many ambient aerosol particles [20][21][22][23][24][25]. Thus, it is imperative to detect the LLPS in microdroplets at their pristine state, that is, in acidic environment.…”

Simulations of aerosol pH in China using WRF-Chem (v4.0): sensitivities of aerosol pH and its temporal variations during haze episodes

“…These results are in line with previous studies (Ding et al, 2019;Shi et al, 2017) that have demonstrated the role of NVCs in aerosol acidity. However, other studies found that NVCs have limited impacts on aerosol pH, which may be due to the relatively minor contribution of crustal ions on aerosol mass in their cases (Liu et al, 2017;Zheng et al, 2020;Zhang et al, 2021). Furthermore, the mass fraction of sulfate declines from clean periods (0.16) to light and moderate pollution periods (0.08), then slightly increases during heavy pollution periods (0.10).…”

“…In addition to Ca 2+ and Na + (i.e., the NVCs) abundances, NH 3 is also an important alkaline component and plays an essential role in aerosol pH by neutralizing acidic components (H 2 SO 4 and HNO 3 ) to form particulate sulfate and nitrate and thus driving NH 3 towards to the particle phase (S. Zheng et al, 2020;Zhang et al, 2021). After doubling NH 3 emissions, the response in PM 2.5 pH is not as large as that to NVCs.…”

Evaluation of Extinction Effect of PM2.5 and Its Chemical Components during Heating Period in an Urban Area in Beijing–Tianjin–Hebei Region

“…However, even in the presence of a large amount of NH 3 , particulate matter in China remains acidic, which will promote the catalytic effect of transition metals. Zhang et al [26] conducted a comprehensive assessment of aerosol acidity in China and America, using extended ground-level measurements and regional-scale air quality model simulations. They found that the pH values of aerosols in China are 1-2 units higher than those in America, which caused by the higher fraction of total ammonia (gas + particle) in the aerosols.…”