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

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Abstract
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.