Quantum chemical study of potential energy surface in the formation of atmospheric sulfuric acid

Abstract

A new potential energy surface (PES) for the atmospheric formation of sulfuric acid from OH+SO 2 is investigated using density functional theory and high-level ab initio molecular orbital theory. A pathway focused on the new PES assumes the reaction to take place between the radical complex SO 3 ·HO 2 and H 2 O. The unusual stability of SO 3 ·HO 2 is the principal basis of the new pathway, which has the same final outcome as the current reaction mechanism in the literature but it avoids the production and complete release of SO 3 . The entire reaction pathway is composed of three consecutive elementary steps: (1) HOSO 2 +O 2 →SO 3 ·HO 2 , (2) SO 3 ·HO 2 +H 2 O→SO 3 ·H 2 O·HO 2 , (3) SO 3 ·H 2 O·HO 2 →H 2 SO 4 +HO 2 . All three steps have small energy barriers, under 10 kcal/mol, and are exothermic, and the new pathway is therefore favorable both kinetically and thermodynamically. As a key step of the reactions, step (3), HO 2 serves as a bridge molecule for low-barrier hydrogen transfer in the hydrolysis of SO 3 . Two significant atmospheric implications are expected from the present study. First, SO 3 is not released from the oxidation of SO 2 by OH radical in the atmosphere. Second, the conversion of SO 2 into sulfuric acid is weakly dependent on the humidity of air.

Influence of Ammonia and Water on the Fate of Sulfur Trioxide in the Troposphere: Theoretical Investigation of Sulfamic Acid and Sulfuric Acid Formation Pathways

“…SO 3 is considered to be one of the major pollutants − present in Earth’s atmosphere because of its role in the formation of atmospheric aerosols and its contributions toward acid rain via formation of sulfuric acid (SA), that is, H 2 SO 4 . − SO 3 is mainly formed in atmosphere by gas-phase oxidation of sulfur dioxide (SO 2 ). ,− In atmosphere, SO 3 rapidly reacts with water to form SA. SO 3 has drawn the attention of atmospheric chemists for a long time because of its pivotal role in the atmospheric acid rain and various nucleation processes. − Formation of SA through gas-phase hydrolysis of SO 3 has been extensively studied by various research groups, both experimentally ,− as well as theoretically. − Various investigations suggested that SA formation through the reaction between SO 3 and water monomer (WM) is unfavorable under atmospheric conditions because of high energy barrier (>28 kcal mol –1 ). − As a result, the effect of different catalysts, namely, water, ,− ,− ,− hydroperoxy radical, formic acid, , SA (as an auto catalyst), nitric acid, and ammonia (AM), on the hydrolysis of SO 3 , was studied by various research groups. Considering the combined effect of rate constants and catalysts’ concentrations, WM and AM were proposed to have significant contribution in total SA formation in the troposphere.…”