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.