Observations of OH and HO2 radicals in coastal Antarctica

OH and HO2 radical concentrations have been measured in the boundary layer of coastal Antarctica for a six-week period during the austral summer of 2005. The measurements were performed at the British Antarctic Survey's Halley Research Station ( 75 degrees 35' S, 26 degrees 19' W), using the technique of on-resonance laser-induced fluorescence to detect OH, with HO2 measured following chemical conversion through addition of NO. The mean radical levels were 3.9 x 10(5) molecule cm(-3) for OH, and 0.76 ppt for HO2 ( ppt denotes parts per trillion, by volume). Typical maximum ( local noontime) levels were 7.9 x 10(5) molecule cm(-3) and 1.50 ppt for OH and HO2 respectively. The main sources of HOx were photolysis of O-3 and HCHO, with potentially important but uncertain contributions from HONO and higher aldehydes. Of the measured OH sinks, reaction with CO and CH4 dominated, however comparison of the observed OH concentrations with those calculated via the steady state approximation indicated that additional co-reactants were likely to have been present. Elevated levels of NOx resulting from snowpack photochemistry contributed to HOx cycling and enhanced levels of OH, however the halogen oxides IO and BrO dominated the CH3O2 - HO2 - OH conversion in this environment, with associated ozone destruction.