Development of a ground-based LIF instrument for measuring HOx radicals: Instrumentation and calibrations

An instrument for measuring tropospheric OH/HO2 radicals by laser-induced f luorescence developed in our laboratory is presented in detail. It is based on FAGE (fluorescence assay by gas expansion) technique and OH is both exc ited and detected at 308 nm corresponding to its A-X(0,0) band. The alignme nt of the laser beam, the design of the sample gas inlet, and the devices f or the fluorescence detection are optimized so as to reduce the background signal while keeping the OH sensitivity as high as possible. A thermalized position of the expanding gas beam is probed in our system and we did not o bserve a severe decrease of the HOx sensitivities under humid conditions. A n optical fiber is used for delivering the laser light to the fluorescence detection cell mounted outside at a high position. Thus the laser beam alig nment is by far simplified and is made highly reproducible, once settled pr operly. For the calibration, two methods are employed: a system with laser absorption measurements of OH and a system of simultaneous photolysis of H2 O and O-2. The calibration factors are compared well within the combined un certainty. Using the latter system, the conversion efficiency of HO2 to OH by NO addition is measured to be around 90%. The detection limits for OH an d HO2 (S/N = 2) are estimated to be 3.3 x 10(6) and 3.6 x 10(6) cm(-)3 at n oon, respectively, with an integration time of 1 min. The results of test o bservations at our institute are also presented.