Measurements of ozone deposition to vegetation quantifying the flux, the stomatal and non-stomatal components

Ozone deposition to vegetation represents the major sink for boundary layer ozone and yet the underlying mechanism of reaction and uptake at the surface is poorly understood. While overall rates Of O-3 deposition are known, the fractions of the flux absorbed by stomata and deposited to non-stomatal surfaces in the field have been poorly quantified. This paper reports 4 years of continuous fluxes by micrometeorological methods to moorland vegetation in southern Scotland. The flux has been partitioned between stomatal and non-stomatal fluxes and shows over a seasonal scale that the non-stomatal deposition (50 kg O-3 ha(-1) y(-1)) dominates the overall flux (77 kg O-3 ha(-1) y(-1)) and represents 70% of the total deposition. The surface resistance for non-stomatal O-3 deposition (r(ns)) decreases with temperature from 400 s m(-1) at 0 degreesC to 200 s m(-1) at 15 degreesC in dry conditions and is consistent with thermal decomposition of ozone at the surface with an apparent activation energy of about 36 kJ mole(-1). The 4 years of continuous flux measurements show that stomatal conductance, when O-3 concentrations are 80 mug m(-3), is substantially smaller than for smaller O-3 concentrations, although whether this is a response to VPD or O-3 remains unclear.