The Measurement of OH and HO2 in the Atmosphere

Measurements of the OH and HO2 radicals form stringent tests of our knowledge of atmospheric photochemistry. Owing to the extremely low concentrations of these species, their determination has posed a considerable experimental challenge; but now, after many years of development, it appears that reliable measurements are possible. The major experimental methods for OH are laser absorption, laser-induced fluorescence, and chemical conversion. HO2 is determined through electron spin resonance, chemical conversion to OH, or chemical amplification producing NO2. Recent field determinations of both compounds have provided some significant surprises, suggesting that our understanding of atmospheric photochemistry is not yet complete. This overview article discusses features of these methods, needs for OH and HO2 measurement campaigns including instrument intercomparisons, and some questions posed by the field measurements.