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. An Aerodyne high-resolution time-of-flight aerosol mass spectrometer(HR-ToF-AMS) was deployed along with other online instruments to study thehighly time resolved chemistry and sources of submicron aerosols (PM 1 )at Waliguan (WLG) Baseline Observatory, a high-altitude (3816 m a.s.l.)background station located at the northeast edge of the Qinghai–Tibet Plateau(QTP), during 1–31 July 2017. The average PM 1 mass concentrationduring this study was 9.1  µ g m −3 (ranging from 0.3 to 28.1  µ g m −3 ), which was distinctly higher than those(2.0–5.7  µ g m −3 ) measured with the Aerodyne AMS at other high-elevation sitesin the southern or central QTP. Sulfate showed a dominant contribution(38.1 %) to PM 1 at WLG followed 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 thisstudy, mainly related to the enhanced sulfate contribution. All chemicalspecies peaked at the accumulation mode, indicating the well-mixed andhighly aged aerosol particles at WLG from long-range transport. Positivematrix factorization (PMF) on the high-resolution organic mass spectraresolved four distinct organic aerosol (OA) components, including atraffic-related hydrocarbon-like OA (HOA), a relatively fresh biomassburning OA (BBOA), an aged biomass burning OA (agBBOA) and a more-oxidizedoxygenated OA (OOA). On average, the two relatively oxidized OAs, OOA andagBBOA, contributed 34.4 % and 40.4 % of organics, respectively, whilethe rest were 18.4 % for BBOA and 6.8 % for HOA. Source analysis for airmasses showed that higher mass concentrations of PM 1 and enhancedcontributions of sulfate and biomass-burning-related OA components (agBBOA + BBOA) were from the northeast of the WLG with shorter transport distance,whereas lower PM 1 mass concentrations with enhanced OOA contributionwere from the west after long-range transport, suggesting their distinct aerosolsources and significant impacts of regional transport on aerosol massloadings and chemistry at WLG.

Chemical characterization and sources of submicron aerosols in the northeastern Qinghai–Tibet Plateau: insights from high-resolution mass spectrometry