* Corresponding author. Present address: Centre for Atmo- spheric Sciences, Chemistry Department, Cambridge Univer- sity, Lens"eld Road, Cambridge, CB2 IEW, UK. E-mail address: nick.savage@atm.ch.cam.ac.uk (N.H. Savage). Atmospheric Environment 35 (2001) 515}524 Steady-state modelling of hydroxyl radical concentrations at Mace Head during the EASE '97 campaign, May 1997 Nicholas H. Savage *, Roy M. Harrison, Paul S. Monks, Gavin Salisbury Division of Environmental Health and Risk Management, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK Department of Chemistry, University of Leicester, LE1 7RH, UK Received 11 January 2000; received in revised form 27 May 2000; accepted 8 June 2000 Abstract Two di!erent steady-state methods are applied to calculate OH radical concentrations based on the rates of known source and sink processes. The "rst method, which calculates only OH radical concentrations from measured data including HO  gives good correlation with measured OH concentrations but overpredicts by 30%. The second method applied calculates OH, HO  and RO  radical concentrations simultaneously. This second method overestimates the measured concentrations of OH by almost 3 times. This apparent overprediction may be a result of calculated concentrations of HO  which appear too high and may be indicative of a gap in our understanding of the relevant peroxy radical chemistry or a result of the limited peroxy radical chemistry assumed by the method.  2000 Elsevier Science Ltd. All rights reserved. Keywords: Hydroxyl radicals; Troposphere; Modelling; Steady state; Peroxy radicals 1. Introduction The hydroxyl radical plays a central role in the oxida- tion of a great many compounds of atmospheric import- ance (e.g. Crutzen, 1973; Finlayson-Pitts and Pitts, 1997) and the peroxy radicals produced by the oxidation of hydrocarbons by OH play a central role in the control of tropospheric ozone (e.g. PORG, 1993; Finlayson-Pitts and Pitts, 1997). Direct measurements of OH and the hydroperoxy radical (HO  ) were made during the ACSOE EASE '97 campaign in April and May 1997 at Mace Head. Under relatively clean conditions the primary source of OH is from the reaction of the O(D) radical, produced by the photolysis of ozone at wavelengths below & 324 nm, with water vapour, to give two hydroxyl rad- icals. Hydroxyl radicals react with various chemical spe- cies in the atmosphere to a variety of given peroxy radicals, which may in turn react to regenerate OH. However, several other reactions can act to terminate this radical recycling. A simpli"ed representation of the cycling of radicals is shown in Fig. 1. The Mace Head Atmospheric Research Station (53320N, 9354W) is ideally located for studying both Northern Hemisphere background conditions and polluted air from Europe under conditions of continental out#ow (Simmonds and Derwent, 1991). On average, over 60% of air masses arrive at Mace Head from a clean marine sector giving ideal conditions to make measurements under Northern hemisphere background conditions. However, during the campaign polluted air masses were also observed giving a wide range of conditions under which to study OH radical formation and removal. The EASE '97 campaign (part of the ACSOE programme) included measurements of a wide range of species including OH, HO  , hydrocar- bons, NO  and other compounds required to model OH concentrations along with measurements of the photoly- sis frequency of ozone to give OD radicals referred to as j(OD) hereinafter. AEA=3054=Durai=Venkatachala=BG 1352-2310/00/$ - see front matter  2000 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 0 0 ) 0 0 3 1 5 - 0