Different physicochemical behaviors of nitrate and ammonium during transport: a case study on Mt. Hua, China

To understand the chemical evolution of aerosols in the transport process, the chemistry of PM2.5 and nitrogen isotope compositions on the mountainside of Mt. Hua (1/41120m above sea level, a.s.l.) in inland China during the 2016 summertime were investigated and compared with parallel observations collected at surface sampling site (1/4400ma.s.l.). The PM2.5 exhibited a high level at the mountain foot site (MF; average 76.0±44.1µgm-3) and could be transported aloft by anabatic valley winds, leading to the gradual accumulation of daytime PM2.5 with a noon peak at the mountainside sampling site (MS). As the predominant ion species, sulfate exhibited nearly identical mass concentrations at both sites, but its PM2.5 mass fraction was moderately enhanced by 1/44% at the MS site. 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 MF and MS sites, 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 MS site compared with to the MF site. In contrast to nitrate, at the MS site, the 15N in ammonium depleted during the daytime. Considering the lack of any significant change in ammonia (NH3) sources during the vertical transport process, this 15N depletion in ammonium was mainly the result of unidirectional reactions, indicating that additional NH3 would partition into particulate phases and further neutralize HSO4- to form SO42-. This process would reduce the aerosol acidity, with a higher pH (3.4±2.2) at the MS site and lower ones (2.9±2.0) at the MF site. Our work provides more insight into physicochemical behaviors of semi-volatile nitrate and ammonium, which will facilitate the improvement in the model for a better simulation of aerosol composition and properties.