Measurement of Dry Deposition

When it is raining, the S02 and ozone upper-canopy resistance is calculated from 

The upper-canopy resistance for other species is calculated from 

All the formulas described above are for unstressed vegetation, which is the default vegetation status. Optionally, for vegetation stress due to lack of water, the stomatal resistance is increased by a factor of 10 and for inactive vegetation (winter deciduous), a stomatal resistance of 10,000sm 1 should be used, indicating a complete shutdown of this pathway. 

A broad range of techniques have been used to measure dry deposition (Businger 1986). Applicability to different spatial and temporal scales, chemical species, and complexities of terrain varies among the different techniques. 

Wesely (1989) recommends an alternate surface resistance equation when the ground surface is wet from rain or dew. For surfaces covered with dew, the upper-canopy resistances for SO2 and O3 are calculated from 

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Acid Deposition. Acid deposition, the deposition of acids from the atmosphere to the surface of the earth, can be dry or wet. Dry deposition involves acid gases or their precursors or acid particles coming in contact with the earth s surface and thence being retained. The principal species associated with dry acid deposition are S02(g), acid sulfate particles, ie, H2SO4 and NH HSO, and HN02(g). Measurements of dry deposition are quite sparse, however, and usually only speciated as total and total NO3. In general, dry acid deposition is estimated to be a small fraction of the total... 

Dasch, J. M Direct Measurement of Dry Deposition to a Polyethylene Bucket and Various Surrogate Surfaces, Environ. Sci. Tech-nol., 19, 721-725 (1985). 

There is no simple device which enables the measurement of dry deposition in a manner as convenient as for wet deposition. Instead, comparatively less direct methods must be used, none of which is fully proven as yet. For particle exchange, leaf-washing and through-fall techniques (1) can provide measurements of the accumulated deposit on natural surfaces. Likewise, accumulation on snow surfaces can be sampled, and subjected to subsequent chemical analysis. It is evident, however, that such methods only apply in certain circumstances. Budget techniques are sometimes advocated, such as in the case of calibrated watersheds, but these have rarely delivered unequivocal results. The difficulty that arises is that the dry deposition must necessarily be computed as the difference between poorly determined in-flow and out-flow measurements. These, and. a wide variety of other experimental methods, have been reviewed elsewhere (2). 

Input is measured in bulk precipitation, while output is measured in streamflow from the watershed. Positive net output means the watershed has other sources of the ion besides precipitation or is being depleted of its storage of the element. Ca2+, Mg2+, K+, and Na+ are being depleted by mineral weathering Na+ and Cl- are believed to have a significant road salt source. Both NHJ and NOy are retained as nutrients. Sulfate is not believed to have major sources on the watershed its excess of output over input is believed to be a measure of dry deposition. Units are equivalents per hectare per year. 

Measurement of Dry Deposition onto Surrogate Surfaces A Review... 

Dasch, J.M., "Measurement of Dry Deposition to a Deciduous Canopy," GMR 5019, General Motors Corp., Warren Ml, 1985. 

Dasch, J. M. "Measurement of Dry Deposition to a Deciduous Canopy," General Motors Research Publication GMR-5019, 1985. Dasch, J. M. In "Precipitation Scavenging, Dry Deposition, and Resuspension Pruppacher, H. R. Semonin, R. G. SIinn,... 

Measurement of dry deposition is more complicated, not only because dry deposition varies with the type of land cover (e.g., bare soil, grass, deciduous forest, coniferous forest) but also because deposited chemicals are more difficult to collect from these complex surfaces for analysis. The dry bucket of Fig. 4.37, originally intended to collect dry deposition, is not a very close approximation to natural surfaces, and therefore yields results of imcertain meaning. 

Odabasi, M., et al.. Measurement of dry deposition and air-water exchange of polycyclic aromatic hydrocarbons with the water surface sampler. Environmental Science and Technology, 1999. 33(3) 426 34. 

In contrast to relatively widespread monitoring of wet deposition, we have neither extensive nor reliable measurements of dry deposition. Minimum estimates can be derived by assuming that dry NH deposition is equal to at least 30% of the wet flux see Warneck, P. 1988. Chemistry of the Natural Atmosphere. London Academic Press, p. 440. 

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