Dry deposition of SO2 on agricultural crops.

A micrometeorological method has been used to estimate dry deposition rates of sulphur dioxide on agricultural crops from vertical gradients of SO2 concentration, windspeed and air temperature above the crop surface. Field measurements in a wide range of atmospheric and surface conditions enabled analysis of the results to separate the various atmospheric and surface processes controlling the flux. For wheat deposition velocity (Vg) 1 m above the surface varied between 0.1 and 1.5cm s−1 and was controlled primarily by surface processes, surface resistance generally contributing 70% of total resistance (rt). Values of surface resistance are determined essentially by deposition at two sinks, the sub-stomatal cavity and leaf cuticle acting in parallel. With stomata open 1/3 of the total SO2 flux was to the leaf surface and 2/3 to the sub-stomatal cavity. When foliage was wet with rain or dew, provided the pH of the liquid was >3.5 surface resistance is negligible and Vg then controlled by atmospheric resistance may exceed 1 cm s−1. For agricultural areas of Britain (14 × 106ha) dry deposition has been estimated at 72 kg SO2 ha−1 annually, 60% of which is deposited during the winter (October–March, inclusive), equivalent to a deposition velocity of 0.6cm s−1 for the area considered