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 deposition velocities (V _d), using (a) a module in the air quality model known as ADOM (Acid Deposition and Oxidant Model), which was modified to apply over a specific site, (b) a version of the ADOM module that employs 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 locally since it was tuned to the site's ozone data. Here it is used as a benchmark for model comparisons. The observed V _dhad an average value of about 0.5 cm s^?1 during the day and about 0.2 cm s^?1 at night. Compared 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 larger than the observations, the Massman values were slightly smaller, and the modified ADOM showed a better agreement. We speculate that the underestimation of the Massman V _dvalues during the day may have been caused by the ADOM aerodynamic resistance rather than the Massman canopy resistance. It is hypothesized that the original ADOM module and its modified version may need a revised aerodynamic or an additional canopy resistance at night in order to bring the estimates closer to the observations.