COMPARISON OF MEASURED OH CONCENTRATIONS WITH MODEL-CALCULATIONS

The influence of chemical precursors and sunlight on the atmospheric O H abundance is investigated by a comparison of locally measured tropos pheric OH with model calculations. The latter are based on the gas pha se reaction mechanism of the regional acid deposition model (RADM2) wh ich incorporates an explicit inorganic and a comprehensive organic che mistry. The experimental data were obtained in the planetary boundary layer during two sets of campaigns. In Deuselbach (1983) and Schauinsl and (1984), rural conditions were encountered with NO(X) concentration s on the average of 2.2 and 0.9 ppb, respectively. This data set was a lready compared with model calculations based upon an older and less d etailed chemical reaction scheme (Perner et al., 1987). Since then the experimental data were reanalyzed leading to modified measured OH con centrations and also to modified precursor concentrations. For a consi stent comparison with the more recent campaigns in Julich (1987 and 19 88) we have redone the calculations. The modeled and measured OH conce ntrations of the campaigns in 1983 and 1984 correlate well with a coef ficient of correlation of r = 0.73. The model overpredicts OH by about 20%. Under more polluted conditions in Julich with average NO(x) conc entrations of 4 ppb the correlation coefficient between experimental a nd modeled data are significantly smaller (r = 0.61). Possible reasons are the influence of not measured precursors, for example isoprene, a nd the inapplicability of a quasi-steady state model under the spatial ly inhomogeneous conditions in Julich. Again the model overpredicts th e OH concentration by about 15%, which is somewhat smaller than for th e rural case. The precision of the comparison is limited by the uncert ainties of the chemical reaction rate constants.