Spectroscopic measurement of the free radicals NO3, BRO,IO, and OH in the troposphere

Free radicals are the driving force for most chemical processes in the atmosphere. In particular OH- and nitrate radicals (NO₃) play a central role in the troposphere. However, recent investigations indicate an important influence of the halogen-oxide radicals BrO, IO, and possibly ClO on tropospheric chemistry. Therefore, the knowledge of the concentration of those species in the atmosphere is a key requirement for the investigation of atmospheric chemistry. Unfortunately the low concentration of free radicals makes measurements particularly difficult. Among several techniques applied to the problem UV/visible differential absorption spectroscopy (also known as DOAS) appears to be the most successful for the observation of the above species. Detection limits of the order of 10⁶ to 10⁷ molec/cm³ have been reached, which are sufficiently low to resolve the diurnal variation profiles of the observed radicals. As examples tropospheric DOAS measurements of NO₃- and OH radicals at mid latitudes are presented, as well as observations of BrO in the Canadian Arctic. The latter measurements are discussed with a scenario combining BrO catalyzed ozone loss and atmospheric dispersion to explain the observed rapid changes in ozone with the measured levels of BrO.