Total OH Reactivity

Total OH reactivity is defined as the total loss rate of reaction of the hydroxyl radical OH with atmospheric chemical compounds. It represents the inverse of the OH radical lifetime. Since OH is the main atmospheric oxidant during daytime, measuring the total OH reactivity represents an estimate of the total loading of reactive molecules in the atmosphere. This chapter presents the measurement principle of total OH reactivity and then results from measurements in the Mediterranean basin, which are placed in the general context of worldwide measurements. The major applications of such measurements are highlighted: first, the total OH reactivity is compared to the sum of OH reactivities of individual VOCs, which allows one deriving a missing non-measured part. Then instantaneous photochemical ozone formation rates, and the chemical regime of such ozone formation, are presented.