Abstract. Although the anthropogenic emissions of SO2 have decreased
significantly in China, the decrease in SO42- in PM2.5 is
much smaller than that of SO2. This implies an enhanced formation rate
of SO42- in the ambient air, and the mechanism is still under
debate. This work investigated the formation mechanism of particulate
sulfate based on statistical analysis of long-term observations in
Shijiazhuang and Beijing supported with flow tube experiments. Our main
finding was that the sulfur oxidation ratio (SOR) was exponentially
correlated with ambient RH in Shijiazhuang (SOR = 0.15+0.0032×exp(RH/16.2)) and Beijing (SOR = -0.045+0.12×exp(RH/37.8)). In
Shijiazhuang, the SOR is linearly correlated with the ratio of aerosol water
content (AWC) in PM2.5 (SOR = 0.15+0.40×AWC/PM2.5).
Our results suggest that uptake of SO2 instead of oxidation of S(IV) in
the particle phase is the rate-determining step for sulfate formation.
NH4NO3 plays an important role in the AWC and the change of
particle state, which is a crucial factor determining the uptake kinetics of
SO2 and the enhanced SOR during haze days. Our results show that
NH3 significantly promoted the uptake of SO2 and subsequently the
SOR, while NO2 had little influence on SO2 uptake and SOR in the
presence of NH3.