Deposition of Acid Pollutants

In the sequence of emission-transport/con-version-deposition, the last phase has been dealt with in detail (Guderian 2000, 2001). Less emission leads to less deposition. Air pollutants exist as dust particles and in gaseous form, and for the effect of air pollutants their concentration in the air (immission) and their quantity deposited on receptors (deposition) is important. In ecosystems, both nutrients and harmful substances are deposited, and air pollutants may act as either acidic or alkaline, and as reductive or oxidative. The impact of immissions on plants may occur either directly (on plant leaves) or indirectly (through the soil), visible or invisible, latent or acute and chronic. Since they serve as sensitive receptors, plants and ecosystems (e.g., crops, forests, natural vegetation), soils and waters are of major interest. In order to characterize the atmospheric pollutant load of a special site in the landscape, it is first necessary to know the concentration situation and the deposited quantity per time and surface unit - the surface load. Concentrations may be measured over shorter or longer ( 24 h) periods. The concentration of gaseous air pollutants is measured in ppm, ppb or jg m.  

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The deposition of particles 1 pm is measured by means of dry-only samplers, which are shielded during precipitation. 

The accumulation of dry-deposited material occurs both on the canopy and within the leaves. The amount of dry deposition is determined by the air pollution load (air concentration), the aerodynamic character of the site, and the filtering capacity of the recipient (forest tree with or vdthout leaves) (Hicks and Matt 1988). Dry deposition variations are due to differences in forest structure and site. 

Deposition of SO2 and NO2 to plants mainly occurs through stomatal uptake. The uptake by the surfaces of the receptors mentioned takes place by solution in water or water films. For highly soluble SO2, transfer to moist surfaces is limited only by atmospheric transfer, as long as no equilibrium exists between gas phase and solution phase. During rainfall the accumulated material will be leached out (leaves, soil) or washed off (twigs, trunks). 

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Small particles ranging in size from a few hundredths of a micron to several tenths of a micron in diameter have a long residence time as a function of their size and can thereby be transported over long distances of hundreds and possibly thousands of kilometers prior to deposition. Basically two mechanisms are responsible for the deposition of acidic pollutants carried in the atmosphere. Final deposition of the acidic aerosol may occur either directly via sedimentation and impaction (dry deposition) or indirectly as an acidic solution (wet deposition) which is formed during condensation of ati spheric water vapor prior to, or collected during, rain events. Segregation of the relative contribution of wet and drg deposition processes is difficult and must be carefully assessed ... 

While the wet deposition of acidic pollutants may produce dramatic changes in the characteristics of the physical environment and suggest associated biological effects, it may be... 

Indirect mechanisms involving effects on trees (and flora and crops too) caused by alteration of the root environment, i.e. the soil, are due to both wet and dry deposition of acid pollution. Because this mechanism of damage is complex, and involves other factors, a whole spectrum of arguments have evolved in linking acid precipitation with forest damage. 

Forests in particular must endure the combined stresses imposed by climatic extremes/changes, invasion of insects and diseases, and forest management practices in addition to the added stress of acidic pollutants. All of these stresses modify forest health and productivity. Under this complex situation, it has not been possible to establish the exact role that acidification has had on forest decline nor to develop critical deposition levels at which damages are believed to become important 14), However, the geographical coincidence of forest decline and elevated levels of acidic pollutants offer strong evidence that a linkages exists. 

Experimental exposure of several agricultural crops to ambient levels of acidic pollutants has not established measurable yield responses although foliar damage has been observed. Complex interactions with other airborne pollutants, particularly ozone, makes it difficult to exactly establish the damages that may be attributed to acidic deposition 14), It has not been possible to establish any critical level of acidic deposition in relation to crop damages. 

Pollutants can enter through direct dumping, piped outflow, and channeled waste streams as localized point sources, or as diffuse nonpoint sources they can enter rivers, lakes, streams, and groundwater through runoff and soil percolation. Nonpoint sources are considered to be major contributors to air, water, and soil pollution which include runoff from paved streets and parking lots, agricultural lots, soil erosion from logging, atmospheric deposition of acidic or toxic air pollutants (Table 1). The source is particularly... 

Acid deposition, the deposition of acidic substances from the atmosphere onto Earth s surface, is another phenomenon associated with atmospheric pollution. Acid rain refers to the low pH of precipitation that has been observed in... 

The dry and wet deposition of acidic sulphur compounds arrives on the soil and on the vegetation. Most rain occurs over land surfaces and washes the pollutants absorbed on vegetation through the soil into freshwater systems. As the rain passes through the soil it is affected by many chemical reactions which can change its composition before it reaches a head-water stream or upland lake. These chemical reactions can be driven by the increased deposition of acidic sulphur compounds and can lead to the acidification of the soils and the freshwaters. Acidified soils are not as productive as weU-buffered soils and have depleted communities of flora and fauna. Acidified freshwaters have depleted fisheries and populations of insects, amphibians, mammals and birds. 

The main corrosive effect in this respect is caused by dry deposition of air pollutants. The influence of acid precipitation may differ for different materials and depends also on the pollution level. 

The characteristics of the bulk soil, measured on one field replicate at each site, are presented in Tables 1 and 2. The pH values of the bulk soil are more acidic at site 1, close to the smelter, and gradually increase toward site 3. The deposition of atmospheric pollutants, such as sulfur compounds, is most probably responsible for the soil acidification observed close to the smelter. The EC and CEC values of the three sampling locations do not follow any specific trend, although site 1 exhibits both the highest EC value and the lowest CEC. Soil organic C content increases with distance from the smelter, reflecting the increase in canopy density. The amounts of Fe and Al extracted by AAO and DC also increase gradually from site 1 to 3. The proportion of sand in the bulk soil decreases from site 1 to 3 as the silt and the clay contents increase. 

Nevertheless Section 9 of the report does examine the impact on buildings of acid pollutants/ since in many urban environments this damage can be quite widespread. Also the report does consider the importance of distance from sources as a factor in influencing the likely type of pollutant which may cause damage/ its deposition level and resultant impact. On the other hand respiratory health effects caused by high urban SO2 with particulate concentrations was considered not to be part of the acid rain phenomenon. 

This section deals with the chemical transformation of acid pollutants and their deposition or removal from the atmosphere. 

E. Joranger, J. Schaug A. Semb, Deposition of Air Pollutants in Norway International Conference on Ecological Impact of Acid Precipitation, Sardefjord, 1980. 

In polluted acidic environments the corrosion of zinc would be expected to be linear early in the exposure. The AIB term of Eq. 2.2 would be the intercept, the B term representing the slope with a constant corrosion rate. The intercept would be proportional to the steady thickness of the film when the rate of film dissolution is equal to the rate of its formation. Diffusivity (A) through the film can be affected by impurities in the film. The solution rate (B) is expected to be a function of the rate of delivery of acidic pollutants to the film The B term may be separated into environmental terms involving wet and dry deposition. The wet and dry delivery mechanisms provide means of describing the factors that control the rate of corrosion. 

Study, much cropland would be destroyed direcdy by fast-moving fires. Also of special concern are the heavy deposits of air pollutants from the atmosphere which would take place in the months during and following the war. If an appreciable fraction of the NOx formed in the nuclear explosions and in the resulting fires were to be deposited in rain, the rainwater would be highly acidic with an average pH of less than 4. 

Receptors. The receptor can be a person, animal, plant, material, or ecosystem. The criteria and hazardous air pollutants were so designated because, at sufficient concentrations, they can cause adverse health effects to human receptors. Some of the criteria pollutants also cause damage to plant receptors. An Air QuaUty Criteria Document (12) exists for each criteria pollutant and these documents summarize the most current Hterature concerning the effects of criteria pollutants on human health, animals, vegetation, and materials. The receptors which have generated much concern regarding acid deposition are certain aquatic and forest ecosystems, and there is also some concern that acid deposition adversely affects some materials. 

Selection of pollution control methods is generally based on the need to control ambient air quaUty in order to achieve compliance with standards for critetia pollutants, or, in the case of nonregulated contaminants, to protect human health and vegetation. There are three elements to a pollution problem a source, a receptor affected by the pollutants, and the transport of pollutants from source to receptor. Modification or elimination of any one of these elements can change the nature of a pollution problem. For instance, tall stacks which disperse effluent modify the transport of pollutants and can thus reduce nearby SO2 deposition from sulfur-containing fossil fuel combustion. Although better dispersion aloft can solve a local problem, if done from numerous sources it can unfortunately cause a regional one, such as the acid rain now evident in the northeastern United States and Canada (see Atmospheric models). References 3—15 discuss atmospheric dilution as a control measure. The better approach, however, is to control emissions at the source. 

Three different types of chemical mechanisms have evolved as attempts to simplify organic atmospheric chemistry surrogate (58,59), lumped (60—63), and carbon bond (64—66). These mechanisms were developed primarily to study the formation of and NO2 in photochemical smog, but can be extended to compute the concentrations of other pollutants, such as those leading to acid deposition (40,42). 

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