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 January 10 to February 4, 2014 to study the mass concentrations, chemical processes, and sources of sub-micrometer particulate matter (PM1). The average PM1 concentration during this study was 57.3 µg m−3 (ranging from 2.1 to 229.7 µg m−3 for hourly averages) with organic aerosol (OA) accounting for 51.2 %, followed by nitrate (16.5 %), sulphate (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 µg m−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 compared to summer, largely due to more favoured partitioning to the particle phase at low air temperature. The mass fractions of both OA and nitrate increased by ~5 % with the increase of the total PM1 mass loading, while the average sulphate fraction decreased by 6 %, indicating the importance of OA and nitrate for the heavy air pollution events in Lanzhou. The size distributions of OA, nitrate, sulphate, ammonium, and chloride all peaked at ~500 nm with OA being slightly broader, suggesting that aerosol particles were externally mixed during winter, likely due to stagnant air conditions (average wind speed: 0.82 m s−1). All species showed evident diurnal variations reflecting the important local/regional sources. 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. 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 semi-volatile (SV-OOA) and the other one of low-volatility (LV-OOA). SV-OOA was the most abundant OA component (24 % of OA mass), followed by COA (20%), LV-OOA (19 %), CCOA (18 %), BBOA (11 %), and HOA (9 %). 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 haze in Lanzhou during winter. The primary OA sources were more complex during winter than during summer. 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. Combining with the PMF results, we also found that a large fraction (57 %) of the secondary OC was from non-fossil OC.