Abstract
Abstract. The GABRIEL airborne field measurement campaign, conducted over the Guyanas in October 2005, produced measurements of hydroxyl radical (OH) concentration which are significantly higher than can be simulated using current generation models of atmospheric chemistry. Based on the hypothesis that this "missing OH" is due to an as-yet undiscovered mechanism for recycling OH during the oxidation chain of isoprene, we determine that an OH recycling of about 40–50% (compared with 5–10% in current generation isoprene oxidation mechanisms) is necessary in order for our modelled OH to approach the lower error bounds of the OH observed during GABRIEL. Such a large amount of OH in our model leads to unrealistically low mixing ratios of isoprene. In order for our modelled isoprene mixing ratios to match those observed during the campaign, we also require that the effective rate constant for the reaction of isoprene with OH be reduced by about 50% compared with the lower bound of the range recommended by IUPAC. We show that a reasonable explanation for this lower effective rate constant could be the segregation of isoprene and OH in the mixed layer. Our modelling results are consistent with a global, annual isoprene source of about 500 Tg(C) yr−1, allowing experimentally derived and established isoprene flux rates to be reconciled with global models.
Highlights
During the GABRIEL airborne field campaign, conducted over the Guyanas in October 2005, concentrations of OH were measured in excess of those which can be reproduced by models of atmospheric chemistry based on current understanding (Lelieveld et al, 2008, and other papers in this issue)
In this study we present detailed comparisons of the measurements taken during GABRIEL with the ECHAM5/MESSy AC-GCM (Atmospheric Chemistry General Circulation Model), and attempt to reconcile our simulations with the measurements
We present the results of simulations performed with a global three dimensional AC-GCM using a number of different isoprene oxidation mechanisms, including several with differing degrees of imposed OH regeneration, and compare these with measurements taken during the GABRIEL campaign in order to estimate the degree of OH recycling required for our model to agree with the GABRIEL measurements
Summary
During the GABRIEL airborne field campaign, conducted over the Guyanas in October 2005, concentrations of OH were measured in excess of those which can be reproduced by models of atmospheric chemistry based on current understanding (Lelieveld et al, 2008, and other papers in this issue). In this study we present detailed comparisons of the measurements taken during GABRIEL with the ECHAM5/MESSy AC-GCM (Atmospheric Chemistry General Circulation Model), and attempt to reconcile our simulations with the measurements. OH plays an important role in the chemistry of the troposphere, being the oxidising species primarily responsible for the removal of reactive pollutants Spivakovsky et al, 1990, 2000) or compounds in the atmosphere which are removed by reaction with OH, most notably methyl chloroform Direct measurements of OH in the troposphere are possible using a variety of techniques (e.g. Heard and Pilling, 2003, and references therein)
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