Comparison of measured OH concentrations with model calculations

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Abstract
The influence of chemical precursors and sunlight on the atmospheric OH abundance is investigated by a comparison of locally 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. The experimental data were obtained in the planetary boundary layer during two sets of campaigns. In Deuselbach (1983) and Schauinsland (1984), rural conditions were encountered with NOxconcentrations on the average of 2.2 and 0.9 ppb, respectively. This data set was already compared with model calculations based upon an older and less detailed chemical reaction scheme (Perner et al., 1987). Since then the experimental data were reanalyzed leading to modified measured OH concentrations and also to modified precursor concentrations. For a consistent comparison with the more recent campaigns in Jülich (1987 and 1988) we have redone the calculations. The modeled and measured OH concentrations of the campaigns in 1983 and 1984 correlate well with a coefficient of correlation ofr= 0.73. The model overpredicts OH by about 20%. Under more polluted conditions in Jülich with average NOxconcentrations of 4 ppb the correlation coefficient between experimental and modeled data are significantly smaller (r= 0.61). 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 Jülich. Again the model overpredicts the OH concentration by about 15%, which is somewhat smaller than for the rural case. The precision of the comparison is limited by the uncertainties of the chemical reaction rate constants.

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