Reaction of SO₃ with HONO₂ and Implications for Sulfur Partitioning in the Atmosphere

Abstract

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 3 + HONO 2 reaction are pronounced below 1 bar. The SO 3 + HONO 2 reaction can be a potential removal reaction for SO 3 in the stratosphere and for HONO 2 in the troposphere, because the reaction can potentially compete well with the SO 3 + 2H 2 O reaction between 25 and 35 km, as well as the OH + HONO 2 reaction. The present findings also suggest an unexpected new product from the SO 3 + HONO 2 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 3 reaction with organic acids to generate organosulfates without the need for heterogeneous chemistry.

Important Routes for Methanediol Formation by Formaldehyde Hydrolysis Catalyzed by Iodic Acid and for the Contribution to an Iodic Acid Sink by the Reaction of Formaldehyde with Iodic Acid Catalyzed by Atmospheric Water

“…Regarding the reaction mechanism of HCHO + HIO 3 , the hydrogen atom of the iodic acid is transferred to the oxygen atom of the carbonyl group in formaldehyde, while the oxygen atom in iodic acid is simultaneously added to the central carbon atom of formaldehyde, resulting in the formation of the product CH 2 (OH)­IO 3 . This process is similar to the HCHO + HCOOH/H 2 SO 4 , and SO 3 + HONO 2 reactions . As shown in Table S7 and Figure S7, the NBO analysis revealed a stronger stabilization (17.23 kcal/mol) of the lone pair donor orbital interactions in the lone pair electron O 1 and O 5 –H 6 antibonding orbitals [LP(2)­O 1 → σ*­(O 5 –H 6 )].…”

“…The binding enthalpy of CR2 was calculated to be −10.75 kcal/mol at the CCSD­(T)-F12a/def2-TZVPP//M06-2X/aug-cc-pVTZ-PP level of theory. The enthalpy of activation at 0 K of TS2 (2.96 kcal/mol) is 0.53 kcal/mol lower than that of the HCHO + H 2 SO 4 reaction at the W2X//QCISD/cc-pV­(T+d)­Z level of theory and 5.69 kcal/mol lower than that of the HCHO + HCOOH reaction at the CCSD­(T)-F12a/jun-cc-pV­(T+d)­Z//M06-2X/MG3S level of theory . This indicates that formaldehyde reacts more readily with iodic acid.…”

“…This process is similar to the HCHO + HCOOH/ H 2 SO 4 84,85 and SO 3 + HONO 2 reactions. 86 As shown in Table S7 and Figure S7, the NBO analysis revealed a stronger stabilization (17.23 S8.…”

“…85 This indicates that formaldehyde reacts more readily with iodic acid. Moreover, the reaction enthalpy (−14.34 kcal/mol at 0 K) for the reaction of formaldehyde with iodic acid is lower than that (−13.37 kcal/mol) of the HCHO + H 2 SO 4 reaction at the W2X//QCISD/cc-pV(T+d)Z level of theory 86 and that (−12.31 kcal/mol) of the HCHO + HCOOH reaction at the CCSD(T)-F12a/jun-cc-pV(T+d)Z//M06-2X/MG3S level of theory. 85…”

Quantitative Kinetics of the Reaction between CH2OO and H2O2 in the Atmosphere

“…The optimized geometry and frequency of the CH 2 OO + H 2 O 2 reaction were calculated using the CCSD­(T)-F12a , method combined with the cc-pVTZ-F12 and cc-pVDZ-F12 basis sets and DF-CCSD­(T)-F12b method with jun′-cc-pVDZ and jun-cc-pVDZ basis sets. , The W3X-L and W2X composite methods were used to do single-point energy calculations. W3X-L represents W2X along with the post-CCSD­(T) calculations .…”