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 n ear Fresno California during July and August 1991 were used to test so me dry deposition velocity models. Over the cotton field, the observed maximum daytime V(d) was about 0.8 cm s-1 and the average nighttime v alue was about 0.2 cm s-1. Over the grass, daytime values averaged abo ut 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 underesti mated. The overestimation was attributed to inadequacies in the surfac e resistance formulation and the underestimation to uncertainties in t he aerodynamic formulation. Unlike previous studies which focused on t he role of surface resistance, we perform additional tests using a lar ge variety of aerodynamic resistance formulae, in addition to those in ADOM, to determine their influence on the modelled V(d) of O3 over co tton. Over grass, ADOM considerably overestimated the observations but showed improvement when other surface resistance formulations were ap plied.