Comparison of the Tropospheric Concentration of OH with Model Calculations

The influence of chemical precursors and sunlight on the atmospheric OH abundance is investigated by a comparison of measured tropospheric OH with model calculations. The latter are based on the gas phase reaction mechanism of the Regional Acid Deposition Model (RADM2) which incorporates an explicit inorganic and a comprehensive organic chemistry. For rural environments in Deuselbach and Schauinsland with a low burden of hydrocarbons and nitrogen oxides the calculated and measured OH-concentrations correlate well. The model overpredicts OH by about 10%. Under more polluted conditions in Julich the correlation coefficient between experimental and modeled data are significantly smaller. On the average the model overpredicts OH concentrations by a factor of 1.8. Possible reasons are the influence of not measured precursors, for example isoprene, and the inapplicability of a quasi steady state model under the spatially inhomogeneous conditions in Julich.