Evaluation and Intercomparison of Five North American Dry Deposition Algorithms at a Mixed Forest Site

To quantify differences between dry deposition algorithms commonly used in North America, five models were selected to calculate dry deposition velocity (V _d) for O₃ and SO₂ over a temperate mixed forest in southern Ontario, Canada, where a 5-year flux database had previously been developed. The models performed better in summer than in winter with correlation coefficients for hourly V _d between models and measurements being approximately 0.6 and 0.3, respectively. Differences in mean V _d values between models were on the order of a factor of 2 in both summer and winter. All models produced lower V _d values than the measurements of O₃ in summer and SO₂ in summer and winter, although the measured V _d may be biased. There was not a consistent tendency in the models to overpredict or underpredict for O₃ in winter. Several models produced magnitudes of the diel variation of V _d (O₃) comparable to the measurements, while all models produced slightly smaller diel variations than the measurements of V _d (SO₂) in summer. A few models produced larger diel variations than the measurements of V _d for O₃ and SO₂ in winter. Model differences were mainly due to different surface resistance parameterizations for stomatal and nonstomatal uptake pathways, while differences in aerodynamic and quasi-laminar resistances played only a minor role. It is recommended to use ensemble modeling results for ecosystem impact assessment studies, which provides mean values of all the used models and thus can avoid too much overestimations or underestimations.