A highly sensitive OH measurement instrument has been developed. It is based on laser-induced fluorescence (LIF) detection of OH using the A²Σ+v′ = 0-X²Π 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 (Q1(3), Q21(3) and P1(1) lines) of ambient OH. For high time resolution (typ. 60–100 s), the laser wavelength was modulated on-/off- resonance with the P1(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 1.4×107cm−3at noon. Minimum OH concentrations were measured in the morning and evening down to the detection limit of (3–6) × 105cm−3(SNR=2, measurement time 1 min.). During the day, OH 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.
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