Tropospheric Aqueous‐Phase Free‐Radical Chemistry: Radical Sources, Spectra, Reaction Kinetics and Prediction Tools

Abstract

The most important radicals which need to be considered for the description of chemical conversion processes in tropospheric aqueous systems are the hydroxyl radical (OH), the nitrate radical (NO(3)) and sulphur-containing radicals such as the sulphate radical (SO(4)(-)). For each of the three radicals their generation and their properties are discussed first in the corresponding sections. The main focus herein is to summarize newly published aqueous-phase kinetic data on OH, NO(3) and SO(4)(-) radical reactions relevant for the description of multiphase tropospheric chemistry. The data compilation builds up on earlier datasets published in the literature. Since the last review in 2003 (H. Herrmann, Chem. Rev. 2003, 103, 4691-4716) more than hundred new rate constants are available from literature. In case of larger discrepancies between novel and already published rate constants the available kinetic data for these reactions are discussed and recommendations are provided when possible. As many OH kinetic data are obtained by means of the thiocyanate (SCN(-)) system in competition kinetic measurements of OH radical reactions this system is reviewed in a subchapter of this review. Available rate constants for the reaction sequence following the reaction of OH+SCN(-) are summarized. Newly published data since 2003 have been considered and averaged rate constants are calculated. Applying competition kinetics measurements usually the formation of the radical anion (SCN)(2)(-) is monitored directly by absorption measurements. Within this subchapter available absorption spectra of the (SCN)(2)(-) radical anion from the last five decades are presented. Based on these spectra an averaged (SCN)(2)(-) spectrum was calculated. In the last years different estimation methods for aqueous phase kinetic data of radical reactions have been developed and published. Such methods are often essential to estimate kinetic data which are not accessible from the literature. Approaches for rate constant prediction include empirical correlations as well as structure activity relationships (SAR) either with or without the usage of quantum chemical descriptors. Recently published estimation methods for OH, NO(3) and SO(4)(-) radical reactions in aqueous solution are finally summarized, compared and discussed.

Secondary organic aerosol formation in cloud droplets and aqueous particles (aqSOA): a review of laboratory, field and model studies

“…Kinetic data for aqueous phase reactions with OH are available or can be estimated based on structurereactivity-relationships (e.g. Ervens et al, 2003b;Herrmann, 2003;Monod et al, 2005;Monod and Doussin, 2008;Minakata et al, 2009;Herrmann et al, 2010). Most OH reactions with organic compounds are near the diffusion limit in the aqueous phase (k = 10 9 -10 10 M −1 s −1 ).…”

Structure–activity relationship for the estimation of OH-oxidation rate constants of carbonyl compounds in the aqueous phase

“…This latter choice makes this SAR simpler to implement but it lowers its prediction performance. This may explain why, in their thorough comparison of the robustness of the two SARs for alkanes, mono-alcohols, poly-alcohols, and carboxylic acids, Herrmann et al (2010) found that the SAR proposed by Monod and Doussin (2008) performed better and was therefore eventually implemented as an extension of the CAPRAM 3.0i mechanism whenever possible (Herrmann, 2012).…”

“…In these media, organic compounds are known to undergo oxidation by a number of radicals, among which OH radicals are the most reactive oxidants (Herrmann et al, 2010). This reactivity initiates chain reactions that are related to atmospherically important issues such as the oxidizing capacity of the atmosphere (Monod and Carlier, 1999;Monod et al, 2007;Poulain et al, 2010;Ervens et al, 2013), the fate of organic compounds (Blando and Turpin, 2000;Monod et al, 2005) and the formation of secondary organic aerosol (SOA) (Altieri et al, 2006;Carlton et al, 2007;Volkamer et al, 2009;Ervens and Volkamer, 2010;Tan et al, 2010Tan et al, , 2012Lim et al, 2010;Ervens et al, 2011).…”

Kinetics of the autoxidation of sulfur(iv) co-catalyzed by peroxodisulfate and silver(i) ions

“…Therefore, laser flash photolysis was used to study the kinetics of this process. Sulfate ion radicals were generated through the photolysis of peroxodisulfate ions initiated by a laser pulse at 266 nm: [53][54][55][56][57] …”