References

ACP

Atmospheric Chemistry and Physics

ACP

1680-7324

Copernicus GmbH

Göttingen, Germany

10.5194/acp-10-8783-2010

Observations of OH and HO2 radicals over West Africa

Commane

¹ ⁴ Floquet

C. F. A.

¹ ⁵ Ingham

¹ ² Stone

¹ ³ Evans

M. J.

³ Heard

D. E.

¹ ²

School of Chemistry, University of Leeds, LS2 9JT, UK

National Centre for Atmospheric Science, University of Leeds, LS2 9JT, UK

Institute for Climate & Atmospheric Science, School of Earth & Environment, University of Leeds, LS2 9JT, UK

now at: School of Engineering & Applied Sciences, Harvard University, Cambridge, USA

now at: National Oceanography Centre, University of Southampton, Southampton, UK

17 09 2010

10 18 8783 8801

This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

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The full text article is available as a PDF file from http://www.atmos-chem-phys.net/10/8783/2010/acp-10-8783-2010.pdf

The hydroxyl radical (OH) plays a key role in the oxidation of trace gases in the troposphere. However, observations of OH and the closely related hydroperoxy radical (HO2) have been sparse, especially in the tropics. Based on a low-pressure laser-induced fluorescence technique (FAGE – Fluorescence Assay by Gas Expansion), an instrument has been developed to measure OH and HO2 aboard the Facility for Airborne Atmospheric Measurement (FAAM) BAe-146 research aircraft. During the African Monsoon Multidisciplinary Analyses (AMMA) campaign, observations of OH and HO2 (HOx) were made in the boundary layer and free troposphere over West Africa on 13 flights during July and August 2006. Mixing ratios of both OH and HO2 were found to be highly variable, but followed a diurnal cycle: OH varied from 1.3 pptv to below the instrumental limit of detection, with a median mixing ratio of 0.17 pptv. HO2 varied from 42.7 pptv to below the limit of detection, with a median mixing ratio of 8.0 pptv. A median HO2/OH ratio of 95 was observed. Daytime OH observations were compared with the primary production rate of OH from ozone photolysis in the presence of water vapour. Daytime HO2 observations were generally reproduced by a simple steady-state HOx calculation, where HOx was assumed to be formed from the primary production of OH and lost through HO2 self-reaction. Deviations between the observations and this simple model were found to be grouped into a number of specific cases: (a) within cloud, (b) in the presence of high levels of isoprene in the boundary layer and (c) within a biomass burning plume. HO2 was sampled in and around cloud, with significant short-lived reductions of HO2 observed. Up to 9 pptv of HO2 was observed at night, with HO2 above 6 pptv observed at altitudes above 6 km. In the forested boundary layer, HO2 was underestimated by a steady state calculation at altitudes below 500 m but overestimated between 500 m and 2 km. In a biomass burning plume, observed HO2 concentrations were significantly below those calculated.

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