Catalytic effect of (H2O)n (n = 1–3) clusters on the HO2 + SO2 → HOSO + 3O2 reaction under tropospheric conditions

Abstract

The HO₂ + SO₂ → HOSO + ³O₂ reaction, both without a catalyst and with (H₂O)_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 H₂O exerts the strongest catalytic role in the hydrogen atom transfer processes of HO₂ + SO₂ → HOSO + ³O₂ as compared with (H₂O)₂ and (H₂O)₃. In the atmosphere at 0 km altitude within the temperature range of 280.0–320.0 K, the reaction with H₂O 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 HO₂ + SO₂ → HOSO + ³O₂ reaction with H₂O depends heavily on its ability to compete with the primary loss mechanism of HO₂ radicals (such as the HO₂ + HO₂ and HO₂ + NO₃ reactions) and SO₂ (such as the SO₂ + HO reaction). The calculated results show that the HO₂ + SO₂ → HOSO + ³O₂ reaction with H₂O cannot be neglected in the primary loss mechanism of the HO₂ radical and SO₂. The calculated results also show that for the formation of HOSO and ³O₂, the contribution of H₂O 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 HO₂ + SO₂ → HOSO + ³O₂ reaction with H₂O cannot compete with the primary loss mechanism of HO₂ radicals. The present results provide new insight into (H₂O)_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.