Abstract
The dominant role played by the hydroxyl radical in tropospheric photochemistry makes essential a better definition of its atmospheric abundance. The measurement techniques employed to date are critically evaluated and the reported concentrations of HO in ambient air are reviewed. Photochemical models used to predict HO concentrations are discussed, and a comparison between measured and modelled concentrations made. Tropospheric concentrations in the ranges
(0.5−5) × 10
6
HO cm
−3
daytime mean (measurements) and
(0.3−3) × 10
6
HO cm
−3 24
h mean
(models) are suggested. A seasonal variation of about 3-fold is indicated by model studies.