Impacts of mechanistic changes on HOx formation and recycling in the oxidation of isoprene
1School of Chemistry , University of Bristol, Bristol, BS8 1TS, UK
2rdscientific, Newbury, Berkshire, RG14 6LH, UK
3Atmospheric Chemistry Services, Okehampton, Devon, EX20 1FB, UK
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. These studies place emphasis on processes for which experimental or theoretical evidence has been reported in the peer-reviewed literature, in addition to examining the impact of an intrinsic simplification in the MCM v3.1 chemistry. Although all the considered mechanistic changes lead to simulated increases in the concentrations of OH at low NOx levels, the greatest impact is achieved by implementation of a recently postulated mechanism involving isomerisation of the δ-hydroxyalkenyl peroxy radical isomers, formed from the sequential addition of OH and O2 to isoprene. In conjunction with necessary rapid photolysis of the resultant hydroperoxyaldehyde products, this mechanism yields approximately a factor of three increase in the simulated OH concentration at low NOx, and is the only considered mechanism which achieves enhancements which approach those necessary to explain the reported model-measurement discrepancies. Combination of all the considered mechanistic changes has an effect which is approximately additive, yielding an overall enhancement of about a factor of 3.2 in the simulated OH concentration at the lowest NOx input rate considered, with the simulated mean NOx mixing ratios at this input rate being 42 ppt and 29 ppt with the base case and modified mechanisms respectively.
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.