Catalytic effect of (H 2 O) n ( n = 1-3) clusters on the HO 2 + SO 2 → HOSO + 3 O 2 reaction under tropospheric conditions.

The HO 2 + SO 2 → HOSO + 3 O 2 reaction, both without a catalyst and with (H 2 O) n ( n = 1-3) as a catalyst, has been investigated using CCSD(T)/CBS//M06-2X/aug-cc-pVTZ methods, and canonical variational transition state theory with small curvature tunneling (CVT/SCT). The calculated results show that H 2 O exerts the strongest catalytic role in the hydrogen atom transfer processes of HO 2 + SO 2 → HOSO + 3 O 2 as compared with (H 2 O) 2 and (H 2 O) 3 . In the atmosphere at 0 km altitude within the temperature range of 280.0-320.0 K, the reaction with H 2 O is dominant, compared with the reaction without a catalyst, with an effective rate constant 2-3 orders of magnitude larger. In addition, at 0 km, it is worth mentioning that the relevance of the HO 2 + SO 2 → HOSO + 3 O 2 reaction with H 2 O depends heavily on its ability to compete with the primary loss mechanism of HO 2 radicals (such as the HO 2 + HO 2 and HO 2 + NO 3 reactions) and SO 2 (such as the SO 2 + HO reaction). The calculated results show that the HO 2 + SO 2 → HOSO + 3 O 2 reaction with H 2 O cannot be neglected in the primary loss mechanism of the HO 2 radical and SO 2 . The calculated results also show that for the formation of HOSO and 3 O 2 , the contribution of H 2 O decreases from 99.98% to 27.27% with an increase in altitude from 0 km to 15 km, due to the lower relative concentration of water. With the altitude increase, the HO 2 + SO 2 → HOSO + 3 O 2 reaction with H 2 O cannot compete with the primary loss mechanism of HO 2 radicals. The present results provide new insight into (H 2 O) n ( n = 1-3) catalysts, showing that they not only affect energy barriers, but also have an influence on loss mechanisms. The present findings should have broad implications in computational chemistry and atmospheric chemistry.