Impact of halogen monoxide chemistry upon boundary layer OH and HO[subscript 2] concentrations at a coastal site

The impact of iodine oxide chemistry upon OH and HO[subscript 2] concentrations in the coastal marine boundary layer has been evaluated using data from the NAMBLEX (North Atlantic Marine Boundary Layer Experiment) campaign, conducted at Mace Head, Ireland during the summer of 2002. Observationally constrained calculations show that under low NO[subscript x] conditions experienced during NAMBLEX (NO ≤ 50 pptv), the reaction IO + HO[subscript 2] → HOI + O[subscript 2] accounted for up to 40% of the total HO[subscript 2] radical sink, and the subsequent photolysis of HOI to form OH + I comprised up to 15% of the total midday OH production rate. The XO + HO[subscript 2] (X = Br, I) reactions may in part account for model overestimates of measured HO[subscript 2] concentrations in previous studies at Mace Head, and should be considered in model studies of HO[subscript x] chemistry at similar coastal locations.