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Title:
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Micrometeorological measurement of the dry deposition flux ofsulphate and nitrate aerosols to coniferous forest
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Author(s):
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Published by:
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Publication date:
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ECN
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1995
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ECN report number:
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Document type:
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ECN-RX--95-065
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Other
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Number of pages:
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Full text:
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17
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Download PDF
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Abstract:
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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