Comparison of chemical composition and acidity of size-resolved inorganic aerosols at the top and foot of Mt. Hua, Northwest China: The role of the gas-particle distribution of ammonia.

Aerosol pH is not only a diagnostic indicator of secondary aerosol formation, but also a key factor in the specific chemical reaction routes that produce sulfate and nitrate. To understand the characteristics of aerosol acidity in the Mt. Hua, the chemical fractions of water-soluble inorganic ions in the atmospheric PM and size-resolved particle at the top and foot of Mt. Hua in summer 2020 were studied. The results showed the mass concentrations of PM and water-soluble ions at the foot were 2.0-2.6 times higher than those at the top. The secondary inorganic ions, i.e., SO, NO, and NH (SNA) were 56 %-61 % higher by day than by night. SO was mainly distributed in the fine particles (D < 2.1 μm). NO showed a unimodal size distribution (peaking at 0.7-1.1 μm) at the foot and a bimodal (0.7-1.1 μm and 4.7-5.8 μm) size distribution at the top. At the top site, the distribution of NO in coarse particles (> 2.1 μm) was mainly attributed to the gaseous HNO volatilized from fine particles reacting with cations in coarse particles to form non-volatile salts (such as Ca(NO)). The pH values of PM were 2.7 ± 1.3 and 3.3 ± 0.42 at the top and foot, respectively. NH/NH(g) plays a decisive role in stabilizing aerosol acidity. In addition, the increase of the liquid water content (LWC) at the foot facilitates the gas-particle conversion of NH, while the H concentration was diluted, resulting in a decrease in acidity at the foot. NH/NH had good linear correlations with SO, NO, and LWC during the daytime at both sites, indicating that SO, NO, and LWC together affect the gas-particle distribution of ammonia by day: however, the effect of LWC at night was not evident.