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

Abstract. To understand the chemical evolution of aerosols in transport process, chemistry PM2.5 and nitrogen isotope compositions on mountainside Mt. Hua (∼1120 m above sea level, a.s.l.) inland China during 2016 summertime were investigated compared with parallel observations collected at surface sampling site (∼400 a.s.l.). The exhibited a high level mountain foot (MF; average 76.0±44.1 µg m−3) could be transported aloft by anabatic valley winds, leading to gradual accumulation daytime noon peak (MS). As predominant ion species, sulfate nearly identical mass concentrations both sites, but its fraction was moderately enhanced ∼4 % MS site. ammonium variations similar variations, forms which mainly existed as bisulfate (NH4HSO4) ((NH4)2SO4) MF respectively. Unlike ammonium, nitrate (NH4NO3) fine particles decreasing concentration proportion trends increasing elevation. This finding ascribed NH4NO3 volatilization, gaseous HNO3 from semi-volatile subsequently reacted dust form nonvolatile salts, resulting significant shifts into coarse particles. Such scavenging fine-particle led an enrichment 15N In contrast nitrate, site, depleted daytime. Considering lack any change ammonia (NH3) sources vertical this depletion result unidirectional reactions, indicating that additional NH3 would partition particulate phases further neutralize HSO4- SO42-. process reduce aerosol acidity, higher pH (3.4±2.2) lower ones (2.9±2.0) Our work provides more insight physicochemical behaviors will facilitate improvement model for better simulation composition properties.