Secondary Formation of Sulfate and Nitrate during a Haze Episode in Megacity Beijing, China

A heavy haze episode that occurred in Beijing from 20 September to 27 September, 2011 was observed to explore the secondary processes of the haze episode. During the haze episode, the relatively stable synoptic conditions and regional transport from polluted areas in the south and southwest of Beijing favored the formation of haze. Significant increases of PM2.5/PM10 ratio was observed during haze period, which implied that the haze was caused by fine particles. Additionally, the presence of secondary inorganic pollutants (SO42–, NO3– and NH4+) sharply increased during the haze episode, which indicated that secondary processes significantly strengthened the haze episode. The sulfur oxidation ratio (SOR) sharply increased from a non-haze episode with a highest value of 0.11 to a haze episode with a highest value of 0.62. Low correlations between SOR and O3 and the temperature were found, whereas a high correlation between SOR and RH was found during the haze episode, which implied that sulfate was mainly produced by the aqueous-phase oxidation of SO2 rather than the gas-phase conversion of SO2 to sulfate in haze episode in Beijing. Furthermore, a fine linear relationship between SOR and the surface area (dS) of particles smaller than 1 µm confirmed the heterogeneous processes of sulfate formation in haze episode. The nitrogen oxidation ratio (NOR) also sharply increased from a non-haze episode with a highest value of 0.03 to a haze episode with a highest value of 0.26, which indicated more intense secondary formation of nitrate in haze episode. Nitrate was found to be mainly produced by a homogenous reaction under ammonium-rich conditions. Higher RH in haze episode reduced the thermodynamic equilibrium constant Ke’, and favored the thermodynamic equilibrium reaction of HNO3(g) + NH3(g) ↔ NH4NO3(s, aq) to formed nitrate, which might help explain the enhanced homogenous production of nitrate in haze episode. In addition, a good empirical fit (R2 = 0.70) between NOR and dS was found, which indicated that the particle surface area significantly contributed to the intense homogeneous production of nitrate in haze episode.