Hydroxyl radicals dominate daytime tropospheric chem., responsible for reactive removal of most trace gases (e.g., volatile org. compds.[VOC]), and is an active participant in the NO and NO2 cycle. Pump-and-probe LIDAR was used to examine the OH- chem. in the planetary boundary layer. This method consists of a high OH- concn. prodn. by a first laser beam (pump), including a flash photolysis of O3: O3 + hv (256 nm) -> O(1D) + O2, and the reaction, O(1D) + H2O -> 2OH. Its relaxation is followed by measuring the time evolution of the radical by laser-induced fluorescence (probe beam). A simulation of this expt. showed that in a short delay time, a direct est. of the total hydrocarbon reactivity defined as SVOC was obtained. The pump-and-probe technique allows measurements which can distinguish between locations where O3 formation mainly depends on NOx concns. and those where it mainly depends on VOC concns.