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 centr al role in the troposphere. However, recent investigations indicate an important influence of the halogen-oxide radicals BrO, IO, and possib ly ClO on tropospheric chemistry. Therefore, the knowledge of the conc entration of those species in the atmosphere is a key requirement for the investigation of atmospheric chemistry. Unfortunately the low conc entration of free radicals makes measurements particularly difficult. Among several techniques applied to the problem UV/visible differentia l 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 10(6) to 10(7) molec/cm3 have been reached, which are sufficiently low to resolve the diurnal variation profiles of the obse rved 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 wit h a scenario combining BrO catalyzed ozone loss and atmospheric disper sion to explain the observed rapid changes in ozone with the measured levels of BrO.