Articles | Volume 10, issue 19
Atmos. Chem. Phys., 10, 9415–9429, 2010

Special issue: AMMA Tropospheric Chemistry and Aerosols

Atmos. Chem. Phys., 10, 9415–9429, 2010

Research article 06 Oct 2010

Research article | 06 Oct 2010

HOx observations over West Africa during AMMA: impact of isoprene and NOx

D. Stone1,2, M. J. Evans1, R. Commane2,*, T. Ingham2,3, C. F. A. Floquet2,**, J. B. McQuaid1, D. M. Brookes4, P. S. Monks4, R. Purvis5,6, J. F. Hamilton6, J. Hopkins6,7, J. Lee6,7, A. C. Lewis6,7, D. Stewart8, J. G. Murphy8,***, G. Mills8, D. Oram8, C. E. Reeves8, and D. E. Heard2,3 D. Stone et al.
  • 1School of Earth and Environment, University of Leeds, Leeds, UK
  • 2School of Chemistry, University of Leeds, Leeds, UK
  • 3National Centre for Atmospheric Science, University of Leeds, Leeds, UK
  • 4Department of Chemistry, University of Leicester, Leicester, UK
  • 5Facility for Airborne Atmospheric Measurement, Cranfield University, Cranfield, UK
  • 6Department of Chemistry, University of York, York, UK
  • 7National Centre for Atmospheric Science, University of York, York, UK
  • 8School of Environmental Sciences, University of East Anglia, Norwich, UK
  • *now at: School of Engineering & Applied Sciences, Harvard University, Cambridge, USA
  • **now at: National Oceanography Centre, University of Southampton, Southampton, UK
  • ***now at: Department of Chemistry, University of Toronto, Toronto, Canada

Abstract. Aircraft OH and HO2 measurements made over West Africa during the AMMA field campaign in summer 2006 have been investigated using a box model constrained to observations of long-lived species and physical parameters. "Good" agreement was found for HO2 (modelled to observed gradient of 1.23 ± 0.11). However, the model significantly overpredicts OH concentrations. The reasons for this are not clear, but may reflect instrumental instabilities affecting the OH measurements. Within the model, HOx concentrations in West Africa are controlled by relatively simple photochemistry, with production dominated by ozone photolysis and reaction of O(1D) with water vapour, and loss processes dominated by HO2 + HO2 and HO2 + RO2. Isoprene chemistry was found to influence forested regions. In contrast to several recent field studies in very low NOx and high isoprene environments, we do not observe any dependence of model success for HO2 on isoprene and attribute this to efficient recycling of HOx through RO2 + NO reactions under the moderate NOx concentrations (5–300 ppt NO in the boundary layer, median 76 ppt) encountered during AMMA. This suggests that some of the problems with understanding the impact of isoprene on atmospheric composition may be limited to the extreme low range of NOx concentrations.

Final-revised paper