To study the characteristics and causes of atmospheric heavy pollution episodes in winter of Tianjin, the concentrations of air pollutants, chemical characteristics of particulate matter and the meteorological parameters were analyzed in this study. The major conclusions were obtained as follows. During the heavy pollution episodes, the wind speeds kept below 4.0m/s the relative humidity were larger than 80%, and the mixing heights were only one third to one second of that in the clean days. The NO2/SO2 ratios were lower in heavy pollution days than that in clean days, and the NO3-/SO42- ratios were larger than 1during pollution days, indicating that both vehicle exhaust and stationary sources were dominant in Tianjin. Compared to clean days, the PM2.5/PM10 ratios during the heavy pollution episodes were larger while the PM1/PM2.5 ratios were relatively lower, probably because of the hygroscopic behavior of fine particles and the coarse particles from coal combustion. At the beginning of pollution processes, the NOR values were larger than SOR, however, as the development of pollution, the SOR values exceeded NOR due to the limit of ammonia's concentration, indicating that more attention should be focused on gaseous precursors especially sulfur dioxides. OC and EC were weakly correlated when both of them have high concentrations. SOC contributed 20% to 54% of OC, indicating that the residential bulk coal burning and secondary organic reactions greatly affected heavy pollution episodes.
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