Formation of nighttime sulfuric acid from the ozonolysis of alkenes in Beijing

Gaseous sulfuric acid (SA) has received a lot of attention for its crucial role in atmospheric new particle formation (NPF). And for this reason, studies until now have mainly focused on daytime SA when most NPF events occur. While daytime SA production is driven by SO2 oxidation of OH radicals of photochemical origin, the formation of SA during nighttime and its potential influence on particle formation remains poorly understood. Here we present evidence for significant nighttime SA production in urban Beijing during winter, yielding concentrations between 1.0 and 3.0 x 10(6) cm(-3). We found a high frequency (similar to 30 %) of nighttime SA events, which are defined by the appearance of a distinct SA peak observed between 20:00 and 04:00 local time, with the maximum concentration exceeding 1.0 x 10(6) cm(-3). These events mostly occurred during unpolluted nights with a low vapor condensation sink. Furthermore, we found that under very clean conditions (visibility > 16.0 km) with abundant ozone (concentration > 2.0 x 10(11) cm(-3), similar to 7 ppb), the overall sink of SA was strongly correlated with the products of O-3, alkenes and SO2 concentrations, suggesting that the ozonolysis of alkenes played a major role in nighttime SA formation under such conditions. This is in light of previous studies showing that the ozonolysis of alkenes can form OH radicals and stabilized Criegee intermediates (SCIs), both of which are able to oxidize SO2 and thus lead to SA formation. However, we also need to point out that there exist additional sources of SA under more polluted conditions, which are not investigated in this study. Moreover, there was a strong correlation between SA concentration and the number concentration of sub-3 nm particles on both clean and polluted nights. Different from forest environments, where oxidized biogenic vapors are the main driver of nighttime clustering, our study demonstrates that the formation of nighttime cluster mode particles in urban environments is mainly driven by nighttime SA production.

Automated summary

Significant nighttime sulfuric acid production was observed in urban Beijing during winter, with concentrations ranging between 1.0 and 3.0 x 10^6 cm(-3). Nighttime sulfuric acid events occurred most frequently on unpolluted nights with low vapor condensation sink, and were strongly correlated with O3, alkenes, and SO2 concentrations under very clean conditions. The concentration of sulfuric acid showed a strong correlation with the number concentration of sub-3 nm particles on both clean and polluted nights.