Hydration increases the lifetime of HSO&amp;lt;sub&amp;gt;5&amp;lt;/sub&amp;gt; and enhances its ability to act as a nucleation precursor – a computational study

T Kurtén,T Berndt,F Stratmann

doi:10.5194/acp-9-3357-2009

Abstract

Abstract. Recent experimental findings indicate that HSO5 radicals may play a key role in the nucleation of atmospheric SO2 oxidation products. HSO5 radicals are metastable intermediates formed in the SO2 oxidation process, and their stability and lifetime are, at present, highly uncertain. Previous high-level computational studies have predicted rather low stabilities for HSO5 with respect to dissociation into SO3+HO2, and have predicted the net reaction HSO3+OH→SO3+HO2 to be slightly exothermal. However, these studies have not accounted for hydration of HSO5 or its precursor HSO3. In this study, we have estimated the effect of hydration on the stability and lifetime of HSO5 using the advanced quantum chemical methods CCSD(T) and G3B3. We have computed formation energies and free energies for mono- and dihydrates of OH, HSO3, HSO5, SO3 and HO2, and also reanalyzed the individual steps of the HSO3+O2→HSO5→SO3+HO2 reaction at a higher level of theory than previously published. Our results indicate that hydration is likely to significantly prolong the lifetime of the HSO5 intermediate in atmospheric conditions, thus increasing the probability of reactions that form products with more than one sulfur atom. Kinetic modeling indicates that these results may help explain the experimental observations that a mixture of sulfur-containing products formed from SO2 oxidation by OH radicals nucleates much more effectively than sulfuric acid taken from a liquid reservoir.

Highlights

The formation of atmospheric aerosol particles by nucleation from gas-phase molecules has recently received growing experimental and theoretical interest due to the climate and health – related effects of fine particles
Kinetic modeling indicates that these results may help explain the experimental observations that a mixture of sulfur-containing products formed from SO2 oxidation by OH radicals nucleates much more effectively than sulfuric acid taken from a liquid reservoir
We have investigated the effect of hydration on the HSO3+O2→ SO3+HO2 reaction, and especially on the stability of HSO5 intermediate radicals, using advanced quantum chemical methods

Summary

Introduction

The formation of atmospheric aerosol particles by nucleation from gas-phase molecules has recently received growing experimental and theoretical interest due to the climate and health – related effects of fine particles (see e.g. Kulmala, 2003; Kulmala et al, 2004). The nucleation rate typically depends on the 7th or 8th power of the sulfuric acid concentration in the laboratory experiments (Ball et al, 1999), but only on the 1st or 2nd power in the field experiments (Weber et al, 1995; Kulmala et al, 2006; Riipinen et al, 2007). Very recently, Berndt et al (2008) and Laaksonen et al (2008) have discussed the implications of the experimental results in terms of these mechanisms, and concluded that a nucleation mechanism involving HSO5 or its reaction products is likely to explain nucleation starting from OH+SO2 observed in atmosphere as well as in the laboratory. We have assessed the effect of mechanisms 1 and 2 above using advanced quantum chemical methods, and found them both to be significant

Computational details

Results and discussion

Effect of hydration

Kinetic modeling of flow-tube experiments

Conclusions