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 57.3µgm−3 (ranging from 2.1 to 229.7µgm−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 (24.5µgm−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  ∼ 500nm, 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. One of the OOAs was less oxidized (LO-OOA), and the other one more oxidized (MO-OOA). LO-OOA was the most abundant OA component (22.3% of OA mass), followed by CCOA (22.0%), COA (20.2%), MO-OOA (14.9%), BBOA (10.8%), and HOA (9.8%). The mass fraction of primary OA ( = HOA+BBOA+COA+CCOA) increased during high PM pollution periods, indicating that local primary emissions were a main reason for the formation of air pollution events in Lanzhou during winter. Radiocarbon (14C) measurement was conducted on four PM2.5 filter samples from this study, which allowed for a quantitative source apportionment of organic carbon (OC). The non-fossil sources on average accounted for 55±3% of OC, which could be mainly from biomass burning and cooking activities, suggesting the importance of non-fossil sources for the PM pollution in Lanzhou. Together with the PMF results, we also found that a large fraction (66±10%) of the secondary OC was from non-fossil OC.