Seasonal and diurnal variation in the deposition velocity of ozone over a spruce forest in Denmark

Abstract

The flux of O3 was measured by the eddy-correlation method over Norway spruce in periods when the trees had a very low activity, periods with optimum growth, and periods with water stress. The aerodynamic resistance (ra), viscous sub-layer resistance (rb) and surface resistance (rc) to O3 were calculated from meteorological parameters and the deposition velocity. The canopy stomatal resistance to O3 was calculated from measurements of the water vapour flux. The deposition velocities showed a diurnal pattern with night-time values of 3.5 mm s-1 and day-time values of 7 mm s-1, when the trees had optimal growth conditions. The surface resistance was highly dominating in day-time and the influence of meteorology low. In night-time the surface resistance to 03 was lower than the canopy stomatal resistance. A low surface resistance was also found in winter-time, when the activity of the trees was low. The surface resistance increased when the trees were subject to water stress. It is concluded that stomatal uptake is an important parameter for the deposition of O3. However, other processes such as destruction of O3 at surfaces, reaction with NO emitted from the soil, and reactions with radicals produced from VOC's emitted from the forest, should also be taken into consideration. © 1995 Kluwer Academic Publishers.