ECN publication
Micrometeorological measurement of the dry deposition flux ofsulphate and nitrate aerosols to coniferous forest
Published by: Publication date:
ECN 1995
ECN report number: Document type:
ECN-RX--95-065 Other
Number of pages: Full text:
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Dry deposition fluxes of sulphate and nitrate have been determined over aconiferous canopy using the aerodynamic gradient technique. Vertical concentration gradients of sulphate and nitrate were measured with filters; the gradient of ammonium(bi)sulphate 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 m s"-"1. Values of similar magnitude were derived from concentration gradients of ammonium(bi)sulphate. The annual flux of sulphate to this site is estimated at 190 mol SO4 ha"-"1 a"-"1, implying that sulphate aerosol contributes approximately 30% of the dry deposition of SOx. For nitrate a number of measurements in the first series showed deposition velocities larger than the maximum theoretically possible when the temperature was above 20C. 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 ammoniumnitrate aerosol and nitric acid and ammonia. The influence of equilibrium processes between gas phase NH3 and HNO3 and aerosol phase NH4NO3 was investigated. 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 mum 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. 6 figs., 2 tabs., 35 refs.

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