Characterizations and source analysis of atmospheric inorganic ions at a national background site in the northeastern Qinghai-Tibet Plateau: insights into the influence of anthropogenic emissions on a high-altitude area of China

Abstract. Atmospheric particulate matter (PM) imposes highly uncertain impacts on both radiative forcing and human health. While ambient PM has been comprehensively characterized in China’s megacities; its composition, source, and characteristics in the Qinghai-Tibet Plateau (QTP) are not yet fully understood. An autumn observational campaign was conducted during the 1^st–15^th October 2013 at a national background monitoring station (3295 m a.s.l.) in the QTP. Real time concentrations of inorganic water-soluble ions (WSIs) associated with PM_2.5 were measured in addition to PM_2.5 concentrations, gaseous pollutants, and meteorological parameters. SO₄²⁻ was the most abundant WSI (10.00 ± 4.39 μg/m³) followed by NH₄⁺ (2.02 ± 0.93 μg/m³), and NO₃⁻ (1.65 ± 0.71 μg/m³). Observed WSI concentrations were lower as compared to urban sites in eastern China; however, they were higher as compared to other QTP monitoring sites. High sulfate and nitrate oxidation ratios indicated strong secondary formation of both SO₄²⁻ and NO₃⁻. Both photochemical and heterogeneous reactions contributed to the formation of particulate SO₄²⁻, while the conversion of NO₂ to NO₃⁻ only occurred via photochemical reactions in the presence of high O₃ concentrations and strong sunlight. Correlation analysis between WSIs revealed that NH₄NO₃, (NH₄)₂SO₄, Na₂SO₄, and K₂SO₄ were the major atmospheric aerosol components. To better understand the potential sources of WSIs in the QTP, a Positive Matrix Factorization receptor model was used. Results showed that salt lake emissions, mixed factor emissions (livestock feces emission, occasional biomass burning, and crustal material), traffic emissions, secondary inorganic aerosols, and residential burning were the major emission sources at the study site.