Impacts of mechanistic changes on HO_x formation and recycling in the oxidation of isoprene

Abstract

Recently reported model-measurement discrepancies for the concentrations of the HOx radical species (OH and HO2) in locations characterized by high emission rates of isoprene have indicated possible deficiencies in the representation of OH recycling and formation in isoprene mechanisms currently employed in numerical models; particularly at low levels of NOx. Using version 3.1 of the Master Chemical Mechanism (MCM v3.1) as a base mechanism, the sensitivity of the system to a number of detailed mechanistic changes is examined for a wide range of NOx levels, using a simple box model. The studies consider sensitivity tests in relation to three general areas for which experimental and/or theoretical evidence has been reported in the peer-reviewed literature, as follows: (1) implementation of propagating channels for the reactions of HO2 with acyl and β-oxo peroxy radicals with HO2, with support from a number of studies; (2) implementation of the OH-catalysed conversion of isoprene-derived hydroperoxides to isomeric epoxydiols, as characterised by Paulot et al.~(2009a); and (3) implementation of a mechanism involving respective 1,5 and 1,6 H atom shift isomerisation reactions of the β-hydroxyalkenyl and cis-δ-hydroxyalkenyl peroxy radical isomers, formed from the sequential addition of OH and O2 to isoprene, based on the theoretical study of Peeters et al. (2009). All the considered mechanistic changes lead to simulated increases in the concentrations of OH, with (1) and (2) resulting in respective increases of up to about 7% and 16%, depending on the level of NOx. (3) is found to have potentially much greater impacts, with enhancements in OH concentrations of up to a factor of about 3.3, depending on the level of NOx, provided the (crucial) rapid photolysis of the hydroperoxy-methyl-butenal products of the cis-δ-hydroxyalkenyl peroxy radical isomerisation reactions is represented, as also postulated by Peeters et al.~(2009). Additional tests suggest that the mechanism with the reported parameters cannot be fully reconciled with atmospheric observations and existing laboratory data without some degree of parameter refinement and optimisation which would probably include a reduction in the peroxy radical isomerisation rates and a consequent reduction in the OH enhancement propensity. However, an order of magntitude reduction in the isomerisation rates is still found to yield notable enhancements in OH concentrations of up to a factor of about 2, with the maximum impact at the low end of the considered NOx range. A parameterized representation of the mechanistic changes is optimized and implemented into a reduced variant of the Common Representative Intermediates mechanism (CRI v2-R5), for use in the STOCHEM global chemistry-transport model. The impacts of the modified chemistry in the global model are shown to be consistent with those observed in the box model sensitivity studies, and the results are illustrated and discussed with a particular focus on the tropical forested regions of the Amazon and Borneo where unexpectedly elevated concentrations of OH have recently been reported.