Exploring the cause of PM2.5 pollution episodes in a cold metropolis in China

Harbin, a cold metropolis of China, frequently endures fine particles (PM2.5) pollution during its long heating seasons, which has almost become the main environmental issue. To better understand pollution causes, multifaceted factors including the meteorology conditions, secondary transformation, source apportionment and regional transportation, were analyzed based on the definition of three pollution levels and eight pollution episodes. The results showed that chemical species varied dynamically and followed similar trends to PM2.5, and secondary organic carbon and nitrate played more significant driving roles in PM2.5 as the pollution level increased. Five sources were apportioned using the positive matrix factorization model, and the order of importance for the entire study period was as follows: secondary aerosol, traffic emissions, biomass burning, mineral dust, and coal combustion. The PM2.5 in Harbin is mainly affected by transportation in the Harbin-Changchun megalopolis and slightly affected by long-distance transmission from the northwestern region of Harbin. A pollution episode in the fall was quite significant, as it was characterized by long duration time and extremely high pollutant levels, and was most likely attributed to extensive emissions from biomass burning and enhanced secondary transformation. The pollution episodes in winter were characterized by enhanced heterogeneous reactions and weakened photochemical reactions, which were especially sensitive to synoptic variations, and most likely induced by high relative humidity (>60%), low wind speed (3 m/s), suppressed planetary boundary layer height (