Wintertime organic and inorganic aerosols in Lanzhou, China: sources, processes, and comparison with the results during summer

Xu, Jianzhong; Shi, Jinsen; Zhang, Qi; Ge, Xinlei; Canonaco, Francesco; Prévôt, André S. H.; Vonwiller, Matthias; Szidat, Sönke; Ge, Jinming; Ma, Jianmin; An, Yanqing; Kang, Shichang; Qin, Dahe (2016). Wintertime organic and inorganic aerosols in Lanzhou, China: sources, processes, and comparison with the results during summer. Atmospheric chemistry and physics, 16(23), pp. 14937-14957. European Geosciences Union 10.5194/acp-16-14937-2016

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Lanzhou, which is located in a steep alpine valley in western China, is one of the most polluted cities in China during the wintertime. In this study, an Aerodyne high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS), a seven-wavelength aethalometer, and a scanning mobility particle sizer (SMPS) were deployed during 10 January to 4 February 2014 to study the mass concentrations, chemical processes, and sources of submicrometer particulate matter (PM1/. The average PM1 concentration during this study was 57.3 μgm-3 (ranging from 2.1 to 229.7 μgm-3 for hourly averages), with organic aerosol (OA) accounting for 51.2 %, followed by nitrate (16.5 %), sulfate (12.5 %), ammonium (10.3 %), black carbon (BC, 6.4 %), and chloride (3.0 %). The mass concentration of PM1 during winter was more than twice the average value observed at the same site in summer 2012 (24.5 μgm-3/, but the mass fraction of OA was similar in the two seasons. Nitrate contributed a significantly higher fraction to the PM1 mass in winter than summer (16.5% vs. 10 %), largely due to more favored partitioning to the particle phase at low air temperature. The mass fractions of both OA and nitrate increased by ~5% (47 to 52 for OA and 13 to 18% for nitrate) with the increase of the total PM1 mass loading, while the average sulfate fraction decreased by 6% (17 to 11 %), indicating the importance of OA and nitrate for the heavy air pollution events in Lanzhou. The size distributions of OA, nitrate, sulfate, ammonium, and chloride all peaked at ~500 nm, with OA being slightly broader, suggesting that aerosol particles were internally mixed during winter, likely due to frequently calm and stagnant air conditions during wintertime in Lanzhou (average wind speed: 0.82ms-1).
The average mass spectrum of OA showed a medium oxidation degree (average O/C ratio of 0.28), which was lower than that during summer 2012 (O/C=0.33). This is consistent with weaker photochemical processing during winter. Positive matrix factorization (PMF) with the multi-linear engine (ME-2) solver identified six OA sources, i.e., a hydrocarbon-like OA (HOA), a biomass burning OA (BBOA), a cooking-emitted OA (COA), a coal combustion OA (CCOA), and two oxygenated OA (OOA) factors.

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