Formation and evolution of secondary particulate matter during heavy haze pollution episodes in winter in a severe cold climate region of Northeast China.

The formation and evolution of sulfate (SO) and nitrate (NO) secondary contaminants under different stages of pollution episodes and different meteorological and emission conditions were compared, based on the simultaneous observation of fine particulate matter (PM) and its chemical components in four heavy haze pollution episodes at 14 sampling sites in a severe cold climate region of Northeast China in winter from 2017 to 2019. The results yielded two main findings. (1) Nitrate formation during the day was mainly due to the combination of high emissions and high relative humidity (RH, 50-90%), high temperature (T, 0 to 5 °C), high atmospheric oxidizability (ozone (O) and nitrous acid (HONO) concentrations), and high ammonia (NH) concentrations. Nitrate was formed by a gas-phase homogeneous reaction of the hydroxyl radical (OH·) with nitrogen dioxide (NO), sulfur dioxide (SO), and ammonia (NH). (2) The main differences in SO formation between Northeast China and other regions were that the gas-phase oxidation process played an important role. This was mainly a result of the promotion of the gas-phase oxidation of SO due to the high oxidizing ability and the suppression of the aqueous reaction due to the low Ts in winter and low-sulfur coal emissions. Sulfate formation mostly occurred through an aqueous phase reaction in winter, but the highest yield and the fastest production capacity were produced by the gas-phase reaction.