The measurement of tropospheric OH radicals by laser-induced fluorescence spectroscopy during the POPCORN field campaign

A highly sensitive OH measurement instrument has been developed. It is based on laser-induced fluorescence (LIF) detection of OH using the A(2) Sigma(+) v'=0 - X(2) Pi v ''=0 transition at 308.15 nm at low pressure. The LIF instrument detects OH directly and with high specificity, a fact that was demonstrated by recording laser excitation spectra (Q(1)(3), Q(21)(3) and P-1(1) lines) of ambient OH. For high time resolution (typ. 60-100 s), the laser wavelength was modulated on- / off- resonance with the P-1(1) line. Here, we report some of the OH measurements obtained by this technique during its first application in a tropospheric field campaign (''POPCORN''), which was conducted in August 1994 in a rural environment in the North-East of Germany. These include diurnal OH concentration profiles with maximum OH concentrations up to 1.4 x 10(7) cm(-3) at noon. Minimum OH concentrations were measured in the morning and evening down to the detection limit of (3-6) x 10(5) cm(-3) (SNR = 2, measurement time 1 min.). During the day, OK fluctuations were observed on a time scale of minutes and hours. These were highly correlated to the flux of the solar UV radiation which is responsible for the primary OH production by photolysis.