DRY DEPOSITION VELOCITY OF O3 OVER A VINEYARD OBTAINED FROM MODELS AND OBSERVATIONS - THE 1991 CALIFORNIA OZONE DEPOSITION EXPERIMENT

Measurements were made of concentrations and flux densities (using the eddy correlation technique) for O3 over a vineyard for 20 days during July and August 1991. These were compared with modelled dry depositio n velocities (V(d)), using (a) a module in.the air quality model known as ADOM (Acid Deposition.and Oxidant Model), which was modified to ap ply over a specific site, (b) a version of the ADOM module that employ s a modified canopy resistance and (c) versions of ADOM which use two new canopy resistance formulations that are referred to as Wesely and Massman, respectively. The Massman parameterization is valid only loca lly since it was tuned to the site's ozone data. Here it is used as a benchmark for model comparisons. The observed V(d) had an average valu e of about 0.5 cm s-1 during the day and about 0.2 cm s-1 at night. Co mpared to the modified ADOM, the new parameterizations yielded results that were in better agreement with the observations at night. During the daytime, the original ADOM and the Wesely estimates were much larg er than the observations, the Massman values were slightly smaller, an d the modified ADOM showed a better agreement. We speculate that the u nderestimation of the Massman V(d) values during the day may have been caused by the ADOM aerodynamic resistance rather than the Massman can opy resistance. It is hypothesized that the original ADOM module and i ts modified version may need a revised aerodynamic or an additional ca nopy resistance at night in order to bring the estimates closer to the observations.