MICROMETEOROLOGICAL MEASUREMENT OF THE DRY DEPOSITION FLUX OF SULFATEAND NITRATE AEROSOLS TO CONIFEROUS FOREST

Dry deposition fluxes of sulphate and nitrate have been determined ove r a coniferous canopy using the aerodynamic gradient technique. Vertic al concentration gradients of sulphate and nitrate were measured with filters; the gradient of ammonium bisulphate was measured with thermod enuders. Filter measurements of sulphate yielded a deposition velocity ranging from near-zero under stable atmospheric conditions to 0.04 ms (-1) under unstable conditions. The average deposition velocity over 2 3 measurements was 0.007 ms(-1). Values of similar magnitude were deri ved from concentration gradients of ammonium bisulphate. This depositi on velocity has a near-linear dependency on u(), and ranges from < 0. 0008 ms(-1) at u() < 0.1 ms(-1) to 0.034 ms(-1) at u(*) between 0.7 a nd 1 ms(-1) with an overall average of 0.011 ms(-1). This V-d exceeds values observed in previous micrometeorological studies over forests a nd is twice as high as the V-d predicted from a parametrization which is currently used to estimate aerosol deposition in The Netherlands. T he annual flux of sulphate to this site is estimated at 190 mol SO4 ha (-1) a(-1), implying that sulphate aerosol contributes approximately 3 0% of the dry deposition of SOx. For nitrate a number of measurements showed deposition velocities larger than the maximum theoretically pos sible when the temperature was above 20 degrees C. These observed high deposition velocities for nitrate are not caused by artifacts related to simple measurement errors. There are however strong indications in the data that there is an effect of shifts in the equilibrium between ammonium nitrate aerosol and nitric acid and ammonia. The influence o f equilibrium processes between gas-phase NH3 and HNO3 and aerosol-pha se NH4NO3 was investigated and it appeared that these processes could very well explain the extraordinary large gradients of nitrate aerosol observed in some of the experiments. This effect would lead to overes timation of the deposition velocity. It is therefore concluded that th e observed gradients should be used with caution to estimate the dry d eposition velocity of nitrate. As the size distributions of nitrate an d sulphate in The Netherlands are similar (bimodal with mass median di ameters of 0.6 and 4.5 mu m for the fine and coarse modes, respectivel y) it can be assumed that their deposition velocities will be similar. Based on this assumption an annual nitrate flux to the forest of 270 mol ha(-1) a(-1) is estimated. Copyright (C) 1996 Elsevier Science Ltd