Eastern Atlantic Spring Experiment 1997 (EASE97) 1.: Measurements of OH and HO2 concentrations at Mace Head, Ireland -: art. no. 4091

[1] We report measurements of the hydroxyl (OH) and hydroperoxy (HO2) radicals, taken over 20 days, in the remote marine boundary layer at Mace Head, Ireland, during April and May 1997. OH was monitored directly by laser-induced fluorescence (LIF) spectroscopy at 308 nm, and HO2 was measured by chemical conversion to OH upon the addition of NO, with subsequent detection by LIF. The detection limit of the instrument at midday for OH was 6.0 x 10(5) molecule cm(-3) (0.0024 parts per trillion by volume (pptv)) and for HO2 was 3.0 x 10(6) molecule cm(-3) (0.12 pptv), as defined for a signal integration period of 2.5 min and a signal-to-noise ratio of 1. Midday OH and HO2 concentrations were between 2.0-6.0 10(6) molecule cm(-3) (0.08-0.24 pptv) and 0.5-3.5 10(8) molecule cm(-3) (2.5-14 pptv), respectively. OH concentrations correlated well with the rate of OH production from ozone photolysis for clean air from the Arctic containing low concentrations of both NOx and nonmethane hydrocarbons. A lower correlation was observed in more polluted air that originated from the United Kingdom and continental Europe. Measurements of OH and HO2 were made throughout two nights, and although no evidence was seen for OH above the detection limit, up to 2 pptv of HO2 was observed. The measured HO2/OH ratio was in good agreement with the predictions of a steady state expression for NO in the range 75-400 pptv.