Chemical characterization and sources of submicron aerosols in the northeastern Qinghai-Tibet Plateau: insights from high-resolution mass spectrometry
Abstract. An Aerodyne high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) was deployed along with other online instruments to study the highly time-resolved chemistry and sources of submicron aerosols (PM1) at Waliguan (WLG) Baseline Observatory, a high-altitude (3816 m a.s.l.) background station located at the northeastern edge of Qinghai-Tibet Plateau (QTP), during 1–31 July 2017. The average PM1 mass concentration during this study was 9.1 μg m−3 (ranging from 0.3 to 28.1 μg m−3), which was distinct higher than those (2.0–5.7 μg m−3) measured with Aerodyne AMS at other high-elevation sites in the southern or central QTP. Sulfate showed dominant contribution (38.1 %) to PM1 at WLG following by organics (34.5 %), ammonium (15.2 %), nitrate (8.1 %), BC (3.0 %) and chloride (1.1 %). Accordingly, bulk aerosols appeared to be slightly acidic throughout this study mainly related to the enhanced sulfate contribution. All chemical species peaked at the accumulation mode, indicating the well mixed and highly aged aerosol particles at WLG from long-range transport. Positive matrix factorization (PMF) on the high-resolution organic mass spectra resolved four distinct organic aerosol (OA) components, including a traffic-related hydrocarbon-like OA (HOA), a relatively fresh biomass burning OA (BBOA), an aged biomass burning OA (agBBOA) and a more-oxidized oxygenated OA (OOA). On average, the two relatively oxidized OAs, OOA and agBBOA, contributed 34.4 % and 40.4 % of organics, respectively, while the rest were 18.4 % for BBOA and 6.8 % for HOA. Source analysis for air masses displayed higher mass concentrations of PM1 and enhanced contributions of sulfate and biomass burning related OA components (agBBOA + BBOA) were from northeast of the WLG with shorter transport distance, whereas lower PM1 mass concentrations with enhanced OOA contribution were from west after long-range transport, suggesting their distinct aerosol sources and significant impacts of regional transport to aerosol mass loadings and chemistry at WLG.