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

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 (C-14) 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.