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The troposphere is such a complex system that the prediction of air pollution scenarios often requires the use of sophisticated model calculations. Such models contain emissions, chemistry, transport (meteorology), and deposition processes. A large fraction of the information on the detailed processes which are put into the model, in particular where the chemistry is concerned, comes from laboratory measurements of homogeneous and inhomogeneous elementary reactions. The model is then tested against in situ measurements in the real troposphere. One of the key species in tropospheric chemistry is the OH radical which is an aggressive oxidizing agent. Due to its high reactivity, its concentration in the troposphere is low, generally below 5 X 106 radicals per cm3. This remains difficult to measure in general, although under optimal conditions long path optical absorption, as well as laser induced fluorescence (LIF) techniques, have now been successfully applied. If one wishes to investigate the reaction of, for example, OH radicals with molecule X, one approach called flash photolysis is often used in laboratory studies. A very high and easily observable concentration of OH radicals is created by a photochemical pathway using a short flash of light. The decay of the OH concentration to its equilibrium value in the presence of a large excess of X is studied in the time domain. This observation permits the determination of the rate constant k for the reaction OH + M yields products. We would like to test if our understanding of OH reactions in the atmosphere is sufficiently complete under many types of atmospheric conditions. The comparison of model calculations of OH concentrations with measurements is one solution to this problem, which, however, can not always be applied. Hence, we have proposed an alternative solution in which essentially a flash photolysis experiment is done in situ in the troposphere. Using a short laser pulse, for example a high concentration of OH radicals is created by photodisassociating O3 in the presence of water molecules: O3 + hv yieldsk O(1D) + O2. O(1D) + H2O yields 2OH. We then follow the decay of the OH concentration by different spatially resolved techniques like laser-induced fluorescence (LIF) or DIAL (differential absorption lidar). This is then done under different conditions (clean troposphere, polluted troposphere) where the significant chemical and meteorological parameters have been separately measured, so that model calculations can be compared with the time dependent OH concentration. In case of disagreement, the model is presumed to be incomplete and other terms must be added like additional homogeneous or inhomogeneous reactions of OH. We have termed this novel approach to in situ measurement of tropospheric chemistry pump-and-probe lidar. Clearly the technique is general and is not limited to the generation and/or detection of high concentrations of OH radicals, as is shown. In the present description we present a computer simulation of some chemical scenarios, in order to obtain some preliminary information on what can be learned from pump-and-probe lidar experiments. The emphasis is on the chemical kinetics. Transport is taken into account in a future paper. Hence, the scenarios, within the limits of the simplified chemical model, are only realistic for a static atmosphere without turbulence or diffusion, i.e., at short delays after the laser flash perturbation.
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