Dry deposition measurement

Both local and long-range emission sources contribute to atmospheric deposition. Total atmospheric deposition is determined using both wet and dry deposition measurements. Although the term acid rain is widely recognized, the dry deposition portion ranges from 20 to 60 percent of total deposition. 

A review of dry deposition measurement techniques is presented, focusing on surrogate surfaces. Detailed field data from the literature are available for three types of collectors filter paper with rainshields positioned overhead, Petri dishes, and flat Teflon or polyethylene plates. The data suggest that deposition velocities of submicron particles increase in the order flat plates < Petri dishes < filter paper. For supermicron particles, the order is filter paper < Petri dishes < flat plates. These results are interpreted in terms of the geometry of the collector, surface roughness, and peripheral shielding of the surface. 

A complicating factor in dry deposition measurements is the presence of sources of the depositing substance in the footprint of the measurement. Whereas the flux of S02 is nearly always unidirectional (downward) and the surface is a sink for S02, gases such as H2S, NH3, and NO may have surface sources. It may be possible in some cases to specify a surface emission rate. For N02, the situation appears to be even more complex than just adding a surface emission rate to the resistance model. As much as 50% of the N02 initially removed at the surface can reappear as NO as a result of surface emissions (Meyers and Baldocchi 1988). 

Dry deposition measurements can be divided into two general categories direct and indirect. In the direct methods, an explicit determination is made of the flux of material to the surface, either by collecting material deposited on the surface itself or by measuring the vertical flux in the air near the surface. Indirect methods derive flux values by measurements of secondary quantities, such as the mean concentration or vertical gradients of the mean concentration of the depositing material, and relating these quantities to the flux. 

Direct methods require that fewer assumptions be made in analyzing the data but generally demand considerably more effort and relatively sophisticated instrumentation. A detailed treatment of micrometeorological measurement techniques is provided by Businger (1986). Erisman (1993a,b) and Baldocchi et al. (1988) di.scuss dry deposition measurement methods. 

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... 

Dry Deposition. Dry deposition occurs in two steps the transport of pollutants to the earth s surface, and the physical and chemical interaction between the surface and the pollutant. The first is a fluid mechanical process (see Fluid mechanics), the second is primarily a chemical process, and neither is completely characterized at the present time. The problem is confounded by the interaction between the pollutants and biogenic surfaces where pollutant uptake is enhanced or retarded by plant activity that varies with time (47,48). It is very difficult to measure the depositional flux of pollutants from the atmosphere, though significant advances were made during the 1980s and early 1990s (49,50). 

Figure 13-5 is the box model of the remote marine sulfur cycle that results from these assumptions. Many different data sets are displayed (and compared) as follows. Each box shows a measured concentration and an estimated residence time for a particular species. Fluxes adjoining a box are calculated from these two pieces of information using the simple formula, S-M/x. The flux of DMS out of the ocean surface and of nss-SOl back to the ocean surface are also quantities estimated from measurements. These are converted from surface to volume fluxes (i.e., from /ig S/(m h) to ng S/(m h)) by assuming the effective scale height of the atmosphere is 2.5 km (which corresponds to a reasonable thickness of the marine planetary boundary layer, within which most precipitation and sulfur cycling should take place). Finally, other data are used to estimate the factors for partitioning oxidized DMS between the MSA and SO2 boxes, for SO2 between dry deposition and oxidation to sulfate, and for nss-SO4 between wet and dry deposition. 

For the hybridization, samples with deposited DNA were placed in plastic envelopes containing 2 mL of the hybridization buffer (10 mM Tris-HCl, pH 7.6,1 mM EDTA, 0.5% SDS). Twenty mL of the boiled probe was added to the same envelope. The envelopes were then sealed and placed into the water thermostat at 60°C, with stirring overnight. After the hybridization, the samples were strongly washed with distilled water for 10 min, dried, and measured. Cold hybridization was performed at room temperature. 

Elicks BB, Hosker Jr RP, Meyers TP, Womack JD. 1991. Dry deposition inferential measurement techniques. I. Design and tests of a prototype meteorological and chemical system for determining dry deposition. Atmos Environ 25A 2345-2359. 

Meyers TP, Finkelstein P, Clarke J, Ellestad T, Sims PF. 1998. A multilayer model for inferring dry deposition using standard meteorological measurements. J Geophys Res 103 22645-22661. 

Two observationally constrained box-models, based on the Master Chemical Mechanism and with different levels of chemical complexity, have been used to study the HOx radical chemistry during the SOAPEX-2 campaign, which took place during the austral summer of 1999 (January-February) at the Cape Grim Baseline Air Pollution Station in northwestern Tasmania, Australia. The box-models were constrained to the measured values of long lived species and photolysis rates and physical parameters (NO, NO2, O3, HCHO, j(01D), j(N02), H2O and temperature). In addition the detailed model was constrained to the measured concentration of CO, CH4 and 17 NMHCs, while the simple model was additionally constrained only to CO and CH4. The models were updated to the latest available kinetic data and completed with a simple description of the heterogeneous uptake and dry deposition processes. 

Rangarajan C, Eapen CD, Gopalakrishnan SS. 1986. Measured values of the dry deposition velocities of atmospheric aerosols carrying natural and fallout radionuclides using artificial collectors. Water Air Soil Pollut 27 305-314. 

Pattenden NJ, Branson JR, Fisher EMR. 1982. Trace element measurement in wet and dry deposition and airborne particulate at an urban site. Deposition Atmos Pollut Proc Colloq, 173-184. 

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

Droppo, J. G Jr., Concurrent Measurements of Ozone Dry Deposition Using Eddy Correlation and Profile Flux Methods, J. Geo-phys. Res., 90, 2111-2118 (1985). 

U. Shahin, S.-M. Yi, R. D. Paode, and T. M. Holsen, Long-Term Elemental Dry Deposition Fluxes Measured Around Lake Michigan, Environ. Sci. 

Substances can be removed from the atmosphere by dry deposition to surfaces. A method for obtaining the parameters of dry deposition uses eddy correlation flux measurements that require chemical sensors with very fast... 

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