资源环境科技发展态势分析平台

Abstract. To understand the chemical evolution of aerosols in the transport process, the chemistry of PM_2.5 and nitrogen isotope compositions on the mountainside of Mt. Hua (~1120 m a.s.l.) in inland China during the 2016 summertime were investigated and compared with parallel observations collected at surface sampling site (~400 m a.s.l.). PM_2.5 exhibited a high level at the surface (aver. 76.0±44.1 μg/m³) and could be transported aloft by anabatic valley winds, leading to the gradual accumulation of daytime PM_2.5 with a noon peak at the mountainside sampling site. As the predominant ion species, sulfate exhibited nearly identical mass concentrations in both sites, but its PM_2.5 mass fraction was moderately enhanced by ~4 % at the higher elevation. The ammonium variations were similar to the sulfate variations, the chemical forms of both of which mainly existed as ammonium bisulfate (NH₄HSO₄) and ammonium sulfate ((NH₄)2SO₄) at the lower and higher elevations, respectively. Unlike sulfate and ammonium, nitrate mainly existed as ammonium nitrate (NH₄NO₃) in fine particles and exhibited decreasing mass concentration and proportion trends with increasing elevation. This finding was ascribed to NH₄NO₃ volatilization, in which gaseous HNO₃ from semi-volatile NH₄NO₃ subsequently reacted with dust particles to form nonvolatile salts, resulting in significant nitrate shifts from fine particles into coarse particles. Such scavenging of fine-particle nitrate led to an enrichment in the daytime ¹⁵N of nitrate at the mountainside site compared with to the lower-elevation site. In contrast to nitrate, at the higher elevation, the ¹⁵N in ammonium depleted during the daytime. Considering the lack of any significant change in ammonia sources during the vertical transport process, this ¹⁵N depletion in ammonium was mainly the result of unidirectional reactions, indicating that additional ammonia would partition into particulate phases and further neutralize HSO₄- to form SO₄^2-. This process would reduce the aerosol acidity, with a higher pH (3.4±2.2) at MS site and lower ones (2.9±2.0) at MF site. Our work provides more insight into physicochemical behaviors of semi-volatile nitrate and ammonium, which will facilitate the improvement in model for a better simulation of aerosol composition and properties.