Measurement of hydroxyl radical in the upper atmosphere using lidar from the space shuttle

The hydroxyl radical (OH) is a key species in the chemistry of the atmosphere participating in such processes as catalytic destruction of ozone and aerosol formation. Despite its importance it has been measured only a handful of times at distinct geographical locations and local times. The measurement of the latitudinal and diurnal variation of OH would provide some excellent tests for the current picture of stratospheric and mesospheric chemistry. Such a measurement is possible using a lidar system on board the Space Shuttle. Two lidar systems are described. The first using excitation via the X(2)Pi?A(2)Sigma (0,1) transition at 282 nm and broadband detection gives the best signal to noise ratio for nighttime measurements at altitudes above 40 km. Its performance is degraded at lower altitudes by ozone absorption at 282 nm and during daytime operation by its sensitivity to backscattered solar UV flux. The second system uses (0,0) excitation at 308 nm with detection of a single line at 309 nm of the (0,0) fluorescence band. This system gives better performance during the day and at lower altitudes.