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Recently reported model-measurement discrepancies for the concentrations ofthe HOx radical species (OH and HO2) in locations characterized byhigh emission rates of isoprene have indicated possible deficiencies in therepresentation of OH recycling and formation in isoprene mechanismscurrently employed in numerical models; particularly at low levels ofNOx. Using version 3.1 of the Master Chemical Mechanism (MCM v3.1) as abase mechanism, the sensitivity of the system to a number of detailedmechanistic changes is examined for a wide range of NOx levels, using asimple box model. The studies consider sensitivity tests in relation tothree general areas for which experimental and/or theoretical evidence hasbeen reported in the peer-reviewed literature, as follows: (1)implementation of propagating channels for the reactions of HO2 withacyl and β-oxo peroxy radicals with HO2, with supportfrom a number of studies; (2) implementation of the OH-catalysed conversionof isoprene-derived hydroperoxides to isomeric epoxydiols, as characterisedby Paulot et al.~(2009a); and (3) implementation of a mechanism involvingrespective 1,5 and 1,6 H atom shift isomerisation reactions of the β-hydroxyalkenyland cis-δ-hydroxyalkenyl peroxy radical isomers,formed from the sequential addition of OH and O2 to isoprene, based onthe theoretical study of Peeters et al. (2009). All the consideredmechanistic changes lead to simulated increases in the concentrations of OH,with (1) and (2) resulting in respective increases of up to about 7% and16%, depending on the level of NOx. (3) is found to have potentiallymuch greater impacts, with enhancements in OH concentrations of up to afactor of about 3.3, depending on the level of NOx, provided the(crucial) rapid photolysis of the hydroperoxy-methyl-butenal products of thecis-δ-hydroxyalkenyl peroxy radical isomerisation reactions isrepresented, as also postulated by Peeters et al.~(2009). Additional testssuggest that the mechanism with the reported parameters cannot be fullyreconciled with atmospheric observations and existing laboratory datawithout some degree of parameter refinement and optimisation which wouldprobably include a reduction in the peroxy radical isomerisation rates and aconsequent reduction in the OH enhancement propensity. However, an order ofmagntitude reduction in the isomerisation rates is still found to yieldnotable enhancements in OH concentrations of up to a factor of about 2, withthe maximum impact at the low end of the considered NOx range. A parameterized representation of the mechanistic changes is optimized andimplemented into a reduced variant of the Common RepresentativeIntermediates mechanism (CRI v2-R5), for use in the STOCHEM globalchemistry-transport model. The impacts of the modified chemistry in theglobal model are shown to be consistent with those observed in the box modelsensitivity studies, and the results are illustrated and discussed with aparticular focus on the tropical forested regions of the Amazon and Borneowhere unexpectedly elevated concentrations of OH have recently beenreported

Impacts of mechanistic changes on HO&lt;sub&gt;x&lt;/sub&gt; formation and recycling in the oxidation of isoprene

Impacts of mechanistic changes on HO<sub>x</sub> formation and recycling in the oxidation of isoprene