Experimental and numerical analysis of stomatal absorption of sulphur dioxide and transpiration by pine needles

We present the experimental results of flow chamber measurements on SO2 dry deposition, and of separate field measurements on transpiration of Scots pine (Pinus sylvestris L) twigs under typical northerly summertime conditions. These results are interpreted by a numerical model, which solves the steady-state diffusion equation for a single stoma approximated to have cylindrical symmetry. An analytical method to estimate the maximal effect of interference (merging concentration fields of adjacent stomata) between stomatal pores is introduced. As a result, a functional pore radius is found to be of order of 2–3 μm, which is significantly smaller than the maximal anatomical size of stomatal aperture. This indicates that stomata are capable to transfer vapours to different degrees. If the obtained estimates for needle resistances are divided by a factor of 4, the leaf area index of local pine canopy, the bulk stomatal resistance for SO2 is of the order of 200–300 s m−1.