Formation of droplet-mode secondary inorganic aerosol dominated the increased PM during both local and transport haze episodes in Zhengzhou, China.

The size distribution and formation of secondary inorganic aerosol play a key role in the increasing PM concentration. Size-segregated data including mass, number, and chemical component concentrations were obtained during a haze episode from January 12 to 23 in Zhengzhou to gain insight into the dominant factors for the growth of PM. PM levels during two local processes (LP1 and LP2) were mainly affected by the accumulation and secondary formation of local pollutants. The transport process (TP) was affected by the air mass transported from the northern area of Zhengzhou. Results show that the growth of particle mass concentration in LP1 mainly occurred in the size range of 400-640 nm and 640-1000 nm. With the aggravated particles increases (LP2), 640-1000 nm and 1-1.6 μm particles dominated the increasing PM concentration. The particles carried by northern air mass (TP) were concentrated in the size range of 1-1.6 μm. Variation trends of hourly PM chemical components and size distribution of water-soluble inorganic ions suggested that the formation and growth of droplet-mode nitrate, sulfate, and ammonium dominated the increase of PM, and the particle sizes of these components increased with the increasing PM. High concentrations of aerosol water content and large surface area in droplet-mode were beneficial for the heterogeneous reactions for droplet-mode nitrate formation. Moreover, large particle surface area in droplet-mode particles also provided adequate carriers for the adsorption and condensation of gaseous HNO onto these particles. Elevated aerosol water, surface area, and particle acidity enhanced the HO and transition metal (TMI) oxidation for aqueous-phase droplet-mode sulfate formation. The contribution of TMI-catalyzed oxidation significantly increased in LP2 because of the high TMI concentration and particle acidity. Relatively low aqueous-phase sulfate production rates in TP suggest that the observed high concentration of droplet-mode sulfate was mainly originated from the completely transformed SO carried by air masses. Moreover, droplet-mode particles exhibited moderate acidity, which enhanced the gas-particle partitioning of NH/NH.