Aerosol dry deposition

Measurement of total nutrient deposition requires studies integrating rainfall, through-fall, and stemflow in the wet component, as well as aerosol dry deposition. No studies in Amazonia have yet integrated all these components. Throughfall studies indicate a large enrichment of potassium, on the order of 2.7 to 10.2, leached from plant leaves (Holscher et al. 1998). This large variation in potassium enrichment could be caused by different floristic composition and precipitation patterns. 

Petroff, A., A. Mailliat, A. Muriel and A. Fabien (2008) Aerosol dry deposition on vegetative canopies. Part I. Review of present knowledge. Atmospheric Environment 42, 3625-3653 Petsch, S. T, T. I. Eglinton and K. J. Edwards (2001) C-dead living biomass Evidence for microbial assimilation of ancient organic carbon during scale weathering. Science 292, 1127-1131... 

An aerosol is a suspension of either a solid or a liquid in a gas. Fog, for example, is a suspension of small liquid water droplets in air, and smoke is a suspension of small solid particulates in combustion gases. In both cases the liquid or solid particulates must be small enough to remain suspended in the gas for an extended time. Solid aerosol particulates, which are the focus of this problem, usually have micrometer or submicrometer diameters. Over time, solid particulates settle out from the gas, falling to the Earth s surface as dry deposition. 

Existing methods for monitoring the transport of gases were inadequate for studying aerosols. To solve the problem, qualitative and quantitative information were needed to determine the sources of pollutants and their net contribution to the total dry deposition at a given location. Eventually the methods developed in this study could be used to evaluate models that estimate the contributions of point sources of pollution to the level of pollution at designated locations. 

Deposition. The products of the various chemical and physical reactions in the atmosphere are eventually returned to the earth s surface. Usually, a useful distinction is made here between wet and dry deposition. Wet deposition, ie, rainout and washout, includes the flux of all those components that are carried to the earth s surface by rain or snow, that is, those dissolved and particulate substances contained in rain or snow. Dry deposition is the flux of particles and gases, especially SO2, FINO, and NFl, to the receptor surface during the absence of rain or snow. Deposition can also occur through fog, aerosols and droplets which can be deposited on trees, plants, or the ground. With forests, approximately half of the deposition of SO(, NH+,andH+ occurs as dry deposition. 

Consider a lake with a smaU watershed in a forest ecosystem. The forest and vegetation can be considered as an acid concentrator. SO2, NO2, and acid aerosol are deposited on vegetation surfaces during dry periods and rainfalls they are washed to the soil floor by low-pH rainwater. Much of the acidity is neutralized by dissolving and mobilizing minerals in the soil. Aluminum, calcium, magnesium, sodium, and potassium are leached from the soil into surface waters. The ability of soils to tolerate acidic deposition is very dependent on the alkalinity of the soil. The soil structure in the... 

Sulfur oxides (SO,) are compounds of sulfur and oxygen molecules. Sulfur dioxide (SO2) is the predominant form found in the lower atmosphere. It is a colorless gas that can be detected by taste and smell in the range of 1, (X)0 to 3,000 uglm. At concentrations of 10,000 uglm , it has a pungent, unpleasant odor. Sulfur dioxide dissolves readily in water present in the atmosphere to form sulfurous acid (H SOj). About 30% of the sulfur dioxide in the atmosphere is converted to sulfate aerosol (acid aerosol), which is removed through wet or dry deposition processes. Sulfur trioxide (SO3), another oxide of sulfur, is either emitted directly into the atmosphere or produced from sulfur dioxide and is readily converted to sulfuric acid (H2SO4). 

Impurities travel from atmosphere to ice sheet surface either attached to snowflakes or as independent aerosols. These two modes are called wet and dry deposition, respectively. The simplest plausible model for impurity deposition describes the net flux of impurity to ice sheet (which is directly calculated from ice cores as the product of impurity concentration in the ice, Ci, and accumulation rate, a) as the sum of dry and wet deposition fluxes which are both linear functions of atmospheric impurity concentration Ca (Legrand, 1987) ... 

Apart from the economic significance of such loss there are potentially adverse effects on the environment arising from acidification of rain and soil. Ammonia may react with hydroxyl radicals in the atmosphere to produce NOx contributing to the acidification of rain (4). Wet and dry deposition of NH3/NH4+ inevitably contributes to soil acidification through their subsequent nitrification. This effect can be accentuated in woodland by absorption of aerosols containing NH4+ within the canopy followed by transport to the soil in stem flow (5). In more extreme cases, NH3 emission from feedlots, pig and poultry... 

PCDD/Fs are transported to the ocean primarily via effluent discharges, stormwater runoff and by wet and dry deposition of aerosols. Because of their high molecular weight and low volatiUty most PCDD/F contamination is local, with adsorption onto sediments being the primary sink, although transport to the Arctic is occurring. 

Calcium and/or magnesium sulphates can originate from dry deposition of aerosols, from deposition of dust or from sulphur-containing rocks in the catchment. Since carbonate seems not to be the only calcium and magnesium source, the alkalinity consumption calculated earlier must be considered as an overestimation. 

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. 

Slinn, "Dry Deposition and Suspension of Aerosol Particles -A New Look at Some Old Problems" ERDA Symposium, 1976, 1. 

Particles in the accumulation range tend to represent only a small portion of the total particle number (e.g., 5%) but a significant portion (e.g., 50%) of the aerosol mass. Because they are too small to settle out rapidly (see later), they are removed by incorporation into cloud droplets followed by rainout, or by washout during precipitation. Alternatively, they may be carried to surfaces by eddy diffusion and advection and undergo dry deposition. As a result, they have much longer lifetimes than coarse particles. This long lifetime, combined with their effects on visibility, cloud formation, and health, makes them of great importance in atmospheric chemistry. 

Main, H. H., and S. K. Friedlander, Dry Deposition of Atmospheric Aerosols by Dual Tracer Method—I. Area Source, Atmos. Environ., 24A, 102-108 (1990). 

A second factor in the deposition process is the physical size of the radioaerosol particles deposited during dry deposition. Particle size affects the efficiency of foliar collection of dry aerosol particles and may also be important in determining the rate of solution of soluble constituents within the aerosol particle. Lockhart et al. (7) indicated that nuclear debris in surface level air is generally associated with aerosol particle sizes about 0.5-1.0p in diameter. Martell (8) stated that high yield detonations causing condensation of "Sr at stratospheric altitudes leads to the formation of particles with diameters less than 1 fi. Hence,... 

Deposition of POPs may take place by (1) snow and rain scavenging of gases and aerosols (wet deposition), (2) dry particle deposition and (3) gas exchange with surfaces [61], The distributions of POPs between the gas and particle phases depend on their physical-chemical properties (Fig. 8) as well as the environmental conditions in the atmosphere, such as temperature, amounts and composition of particles [26, 35]. 

Porstenddrfer, J., Robig, G. Ahmed, A. (1980) Washout and dry deposition of atmospheric aerosols. In Radiation Protection, vol. I, pp. 583-7. Oxford Perga-mon. 

Shinn, J.H., Homan, D.N. Gay, D.D. (1983) Plutonium aerosol fluxes and pulmonary exposure rates during re-suspension from bare soils near a chemical separation facility. In Precipitation Scavenging, Dry Deposition and Re-suspension, ed. H.R. Pruppacher, R.G. Semonin W.G.N. Slinn, pp. 1131 43. Amsterdam Elsevier. 

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