MEASUREMENT OF TROPOSPHERIC OH AND HO2 BY LASER-INDUCED FLUORESCENCE AT LOW-PRESSURE

The hydroxyl radical (OH) is the primary oxidant in the atmosphere, re sponsible for many photochemical reactions that affect both regional a ir quality and global climate change. Because of its high reactivity, abundances of OH in the troposphere are less than 1 part per trillion by volume (pptv) and thus difficult to measure accurately. This paper describes an instrument for the sensitive detection of OH in the tropo sphere using low-pressure laser-induced fluorescence. Ambient air is e xpanded into a low-pressure detection chamber, and OH is both excited and detected using the A2SIGMA+(v' = 0) --> X2PI(v'' = 0) transition n ear 308 nm. An injector upstream of the detection axis allows for the addition of reagent NO to convert ambient HO2 to OH using the fast rea ction HO2 + NO --> OH + NO2. Using recent advances in laser and detect or technologies, this prototype instrument is able to detect less than 1 X 10(5) molecules cm-3 (0.004 pptv) of OH with an integration time of 30 s with negligible interferences.