The role of sulfate and its corresponding S(IV)+NO2 formation pathway during the evolution of haze in Beijing.

To better understand the role of stationary sources during the evolution of haze, we investigated sulfate formation characteristics at different stages of four haze events in Beijing, China. The mass fraction of sulfate in PM2.5 increased while that of nitrate declined slightly during the worsening process of most haze events, consistent with higher ratios of SO42-/NO3- on haze days (0.50 on average) than those on clean days (0.32 on average). Further calculations indicated that sulfate had a higher mass growth rate than nitrate during the haze-worsening process, probably due to regional transport of sulfate from heavy industrial areas accompanied by increased sulfate secondary transformation during polluted periods. We quantitatively evaluated the contribution of the S(IV) + NO2 reaction (pH-dependent) in sulfate formation during the haze evolution. The production rate (PS(IV)+NO2) of the S(IV) + NO2 pathway ranged from 1.97 × 10-4 to 5.91 (mean: 0.39) μg·m-3·h-1. Its proportion to sulfate total heterogeneous production rate (PS(IV)+NO2/Phet) was generally correlated positively with PM2.5 concentrations, indicating the relative importance of this pathway on haze days. Due to the mutual restriction between aerosol pH and aerosol liquid water content (ALWC) during haze evolution, the relative contribution of the S(IV) + NO2 pathway to sulfate heterogeneous formation was generally limited to 40%.