Concentrations of OH and HO 2 radicals during NAMBLEX: measurements and steady state analysis

OH and HO2 concentrations were measured si- multaneously at the Mace Head Atmospheric Research Sta- tion in the summer of 2002 during the NAMBLEX (North Atlantic Marine Boundary Layer EXperiment) field cam- paign. OH was measured by laser-induced fluorescence employing the FAGE (Fluorescence Assay by Gas Expan- sion) technique, with a mean daytime detection limit of 2.7◊10 5 molecule cm 3 (5 min acquisition period; signal-to- noise ratio = 1). HO2 was detected as OH following its chem- ical conversion through addition of NO, with a mean detec- tion limit of 4.4◊10 6 molecule cm 3 . The diurnal variation of OH was measured on 24 days, and that of HO2 on 17 days. The local solar noon OH concentrations ranged between (3- 8)◊10 6 molecule cm 3 , with a 24 h mean concentration of 9.1◊10 5 molecule cm 3 . The local solar noon HO2 concen- trations were (0.9-2.1)◊10 8 molecule cm 3 (3.5-8.2 pptv), with a 24 h mean concentration of 4.2◊10 7 molecule cm 3 (1.6 pptv). HO2 radicals in the range (2-3)◊10 7 molecule cm 3 were observed at night. During NAMBLEX, a com- prehensive suite of supporting measurements enabled a de- tailed study of the behaviour of HOx radicals under primar- ily clean marine conditions. Steady state expressions are used to calculate OH and HO2 concentrations and to eval- uate the effect of different free-radical sources and sinks. The diurnally averaged calculated to measured OH ratio was 1.04±0.36, but the ratio displays a distinct diurnal variation, being less than 1 during the early morning and late after- noon/evening, and greater than 1 in the middle of the day. For HO2 there was an overprediction, with the agreement be- tween calculated and measured concentrations improved by including reaction with measured IO and BrO radicals and uptake to aerosols. Increasing the concentration of IO radi- cals included in the calculations to above that measured by a DOAS instrument with an absorption path located mainly over the ocean, reflecting the domination of the inter-tidal region as an iodine source at Mace Head, led to further im- provement. The results are compared with previous measure- ments at Mace Head, and elsewhere in the remote marine boundary layer.