Micrometeorological measurement of the dry deposition flux of sulphate and nitrate aerosols to coniferous forest

Dry deposition fluxes of sulphate and nitrate have been determined over a coniferous canopy using the aerodynamic gradient technique. Vertical concentration gradients of sulphate and nitrate were measured with filters; the gradient of ammonium bisulphate was measured with thermodenuders. Filter measurements of sulphate yielded a deposition velocity ranging from near-zero under stable atmospheric conditions to 0.04 m s−1 under unstable conditions. The average deposition velocity over 23 measurements was 0.007 ms−1. Values of similar magnitude were derived from concentration gradients of ammonium bisulphate. This deposition 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 and 1 ms−1 with an overall average of 0.011 ms−1 This Vd exceeds values observed in previous micrometeorological studies over forests and is twice as high as the Vd predicted from a parametrization which is currently used to estimate aerosol deposition in The Netherlands. The annual flux of sulphate to this site is estimated at 190 mol S04 ha−1 a−1, implying that sulphate aerosol contributes approximately 30% of the dry deposition of SOx. For nitrate a number of measurements showed deposition velocities larger than the maximum theoretically possible when the temperature was above 20°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 of equilibrium processes between gas-phase NH3 and HNO3 and aerosol-phase 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 overestimation of the deposition velocity. It is therefore concluded that the observed gradients should be used with caution to estimate the dry deposition velocity of nitrate. As the size distributions of nitrate and sulphate in The Netherlands are similar (bimodal with mass median diameters of 0.6 and 4.5 μm for the fine and coarse modes, respectively) 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.