Reaction of SO with HONO and Implications for Sulfur Partitioning in the Atmosphere.

Sulfur trioxide is a critical intermediate for the sulfur cycle and the formation of sulfuric acid in the atmosphere. The traditional view is that sulfur trioxide is removed by water vapor in the troposphere. However, the concentration of water vapor decreases significantly with increasing altitude, leading to longer atmospheric lifetimes of sulfur trioxide. Here, we utilize a dual-level strategy that combines transition state theory calculated at the W2X//DF-CCSD(T)-F12b/jun'-cc-pVDZ level, with variational transition state theory with small-curvature tunneling from direct dynamics calculations at the M08-HX/MG3S level. We also report the pressure-dependent rate constants calculated using the system-specific quantum Rice-Ramsperger-Kassel (SS-QRRK) theory. The present findings show that falloff effects in the SO + HONO reaction are pronounced below 1 bar. The SO + HONO reaction can be a potential removal reaction for SO in the stratosphere and for HONO in the troposphere, because the reaction can potentially compete well with the SO + 2HO reaction between 25 and 35 km, as well as the OH + HONO reaction. The present findings also suggest an unexpected new product from the SO + HONO reaction, which, although very short-lived, would have broad implications for understanding the partitioning of sulfur in the stratosphere and the potential for the SO reaction with organic acids to generate organosulfates without the need for heterogeneous chemistry.