Isoprene oxidation mechanisms: measurements and modelling of OH and HO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; over a South-East Asian tropical rainforest during the OP3 field campaign

D Stone,D E Heard,D Oram,R Commane,A C Lewis,D Stewart,M J Evans,C E Reeves,J Hopkins,D M Brookes,R J Leigh,P S Monks,A R Rickard,P M Edwards,T Ingham

doi:10.5194/acp-11-6749-2011

Abstract

Abstract. Forests are the dominant source of volatile organic compounds into the atmosphere, with isoprene being the most significant species. The oxidation chemistry of these compounds is a significant driver of local, regional and global atmospheric composition. Observations made over Borneo during the OP3 project in 2008, together with an observationally constrained box model are used to assess our understanding of this oxidation chemistry. In line with previous work in tropical forests, we find that the standard model based on MCM chemistry significantly underestimates the observed OH concentrations. Geometric mean observed to modelled ratios of OH and HO2 in airmasses impacted with isoprene are 5.32−4.43+3.68 and 1.18−0.30+0.30 respectively, with 68 % of the observations being within the specified variation. We implement a variety of mechanistic changes into the model, including epoxide formation and unimolecular decomposition of isoprene peroxy radicals, and assess their impact on the model success. We conclude that none of the current suggestions can simultaneously remove the bias from both OH and HO2 simulations and believe that detailed laboratory studies are now needed to resolve this issue.

Highlights

Atmospheric oxidation, predominantly initiated by the hydroxyl radical (OH), removes biogenically and anthropogenically emitted volatile organic compounds (VOCs) from the atmosphere
Isoprene (C5H8) is produced by the biosphere and is the dominant biogenic VOC emitted into the atmosphere
In our previous study as part of the African Monsoon Multidisciplinary Analysis (AMMA) campaign (Commane et al, 2010; Stone et al, 2010) we found that modelled HO2 concentrations were consistent with observed HO2 even in high isoprene airmasses over the rainforest but were unable to investigate our understanding of OH concentrations owing to instrumental issues

Summary

Introduction

Atmospheric oxidation, predominantly initiated by the hydroxyl radical (OH), removes biogenically and anthropogenically emitted volatile organic compounds (VOCs) from the atmosphere. The reaction of OH with VOCs leads to a complex cascade of reactions and species. Understanding this cascade is a significant challenge for atmospheric chemistry and climate. Isoprene (C5H8) is produced by the biosphere and is the dominant biogenic VOC emitted into the atmosphere. Chamber studies and theoretical chemical studies have all recently investigated the oxidation of isoprene under low NOx conditions

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