Characterizations and Potential Formation Pathways of Atmospheric Inorganic Ions at a National Background Site in the Northeastern Qinghai‐Tibet Plateau During Autumn Season

Abstract

AbstractAtmospheric particulate matter (PM) imposes uncertain impacts on both radiative forcing and human health. Ambient PM has been comprehensively studied in China's megacities, while its compositions, sources, and characterizations in the Qinghai‐Tibet Plateau (QTP) are not fully understood. An autumn observation campaign was conducted during 1–14 October 2013 at a national background monitoring station (3,295 m above sea level, a.s.l.) in the QTP. Real time concentrations of inorganic water‐soluble ions (WSIs) associated with PM2.5 (PM with aerodynamic diameter equal to or less than 2.5 μm) were measured in addition to PM2.5 concentrations, gaseous pollutants, and meteorological parameters. SO42− was the most abundant WSI (10.0 μg/m3) followed by NH4+ (2.0 μg/m3), and NO3− (1.7 μg/m3). Observed WSI concentrations were higher as compared to other QTP monitoring sites and some remote sites. The levels of SO42− were comparable to concentrations of some urban sites. The correlation analysis disclosed the relationship among WSIs and other gaseous pollutants and discussed the possibilities of biomass burning as the potential sources. Extended aerosol inorganic model (E‐AIM) model results revealed that particulate sulfate, nitrate, and ammonium (SNA) existed in both liquid and solid phases. High sulfate and nitrate oxidation ratios indicated strong secondary formation of both SO42− and NO3−. By analyzing the relationship among SOR/NOR, RH, and O3, we found that both photochemical and heterogeneous reactions contributed to the formation of SO42−, while the conversion of NO2 to NO3− was likely to occur via photochemical reactions in the presence of high O3 concentrations and strong sunlight.