A comparison of some aerodynamic resistance methods using measurements over cotton and grass from the 1991 California ozone deposition experiment

Measurements of dry, deposition velocities (V_d) of O₃ (using the eddy correlation technique) made over a cotton field and senescent grass near Fresno California during July and August 1991 were used to test some dry deposition velocity models. Over the cotton field, the observed maximum daytimeV_d was about 0.8 cm s^–1 and the average nighttime value was about 0.2 cm s^–1. Over the grass, daytime values averaged about 0.2 cm s^–1 and nighttime values about 0.05 cm s^–1. Application of a site-specific model known as ADOM (Acid Deposition and Oxidant Model) over the cotton field generally overestimated the observations except for a few hours in the afternoon when the observations were underestimated The overestimation was attributed to inadequacies in the surface resistance formulation and the underestimation to uncertainties in the aerodynamic formulation. Unlike previous studies which focused on the role of surface resistance, we perform additional tests using a large variety of aerodynamic resistance formulae, in addition to those in ADOM, to determine their influence on the modelledV_d of O₃ over, cotton. Over grass, ADOM considerably overestimated the observations but showed improvement when other surface resistance formulations were applied.