Acid Deposition and Oxidant Model

Karamachandani, P., and Venkatram, A. (1992) The role of non-precipitating clouds in producing ambient sulfate during summer—results from simulation with the Acid Deposition and Oxidant Model (ADOM), Atmos. Environ. 26, 1041-1052. 

Because of the expanded scale and need to describe additional physical and chemical processes, the development of acid deposition and regional oxidant models has lagged behind that of urban-scale photochemical models. An additional step up in scale and complexity, the development of analytical models of pollutant dynamics in the stratosphere is also behind that of ground-level oxidant models, in part because of the central role of heterogeneous chemistry in the stratospheric ozone depletion problem. In general, atmospheric Hquid-phase chemistry and especially heterogeneous chemistry are less well understood than gas-phase reactions such as those that dorninate the formation of ozone in urban areas. Development of three-dimensional models that treat both the dynamics and chemistry of the stratosphere in detail is an ongoing research problem. 

Given that the source of oxidants for S02 in both the gas and liquid phases is the VOC-NO chemistiy discussed earlier and that a major contributor to acid deposition is nitric acid, it is clear that one cannot treat acid deposition and photochemical oxidant formation as separate phenomena. Rather, they are very closely intertwined and should be considered as a whole in developing cost-effective control strategies for both. For a representative description of this interaction, see the modeling study of Gao et al. (1996). 

Carmichael, G. R L. K. Peters, and R. D. Saylor, The STEM-II Regional Scale Acid Deposition and Photochemical Oxidant Model. I. An Overview of Model Development and Applications, Atmos. Enciron., 25, 2077-2090 (1991). 

McHenry, J. N., and R. L. Dennis, The Relative Importance of Oxidation Pathways and Clouds to Atmospheric Ambient Sulfate Production As Predicted by the Regional Acid Deposition Model, J. Appl. Meteorol., 33, 890-905 (1994). 

Other important inputs to AQM are the transformation and deposition processes of air pollutants. Examples of air quality modeling where transformation and deposition processes need to be considered are acid deposition, regional haze, and photo-oxidants. 

The family of photo-oxidants includes tropospheric ozone, O3 (the bad ozone), ketones, aldehydes and nitrated oxidants, such as peroxy-acetylnitrate (PAN) and peroxybenzoylnitrate (PBN). The modeling of photo-oxidants is more complicated than that of acid deposition (NRC 1991). Here, the primary precursor is NOx, which as mentioned before, is emitted as a result of fossil fuel combustion. A part of NOx is the N02 molecule, which splits (photodissociates)... 

As the model suggests, the dietary need for amino acids is determined by the rates of depletion of the free amino acid pool by oxidation or synthesis of protein. During steady state conditions, the contribution to the free pool from dietary intake and protein breakdown should be exactly balanced by the flux out of the pool to synthesis and oxidation. Any condition that increases deposition of protein in the body or the rate of amino acid oxidation should produce an increased need for protein. For example, muscle hypertrophy is dependent on a positive balance of the protein turnover process. If synthesis of protein exceeds the catabolism of protein, then muscle mass will increase and the free amino acid pool would be depleted. Thus, a net increase in protein requires an increase in intake or a decrease in oxidation. Likewise, the same arguments hold for an increase in oxidation of amino acids. 

Secondly I think one has to look very carefully at transport phenomena. Several speakers in this Study Week have referred to the effect of the introduction of tall stacks which permit an increased dilution of emissions from power plants. The inclusion of a tall stack at a power plant does not cut the deposition in the vicinity of that stack — and you can use the term vicinity in any way you like — to zero and the deposition at a distance of 500 kilometers to 100%. A very substantial fraction of the deposition associated with emission from a particular source, even with the tall stack, occurs relatively near to that source and again, the question of how near is one, that is extremely difficult to get solid answers for — one simply does not have that kind of information. If you want to take an applied mathematician and send him into shock, you ask him to model the flow from a tall smokestack over a distance of about ten or twenty kilometers — that is just something that is not done. The overall transport phenomenon in acid rain is an extraordinarily complex multi-scale phenomenon. So far as the chemistry is concerned, I think that, too, varies dramatically with the climate, with the season, with the presence of oxidants of various types in the atmosphere, and I fear that there can be no single generalization concerning acid rain and the mitigation of acid deposition worldwide. This is something that has to be handled on a scale which in fact I think will be much smaller. 

FIGURE 4-28 An example of output from ACID, a regional-scale model that simulates transport of S02 and sulfate, oxidation of S02 to sulfate, and sulfate deposition. Each contour represents the average airborne sulfate concentration in micrograms per cubic meter that would result in the Adirondacks region of New York per 1014 g of sulfur emitted annually anywhere along the contour line. For example, if a 1014 g/year source of S02 were sited in Tennessee, the resultant average addition to the airborne sulfate concentration in the Adirondacks would be 20 /cg/m3. 

Atmospheric reactions modify the physical and chemical properties of emitted materials, changing removal rates and exerting a major influence on acid deposition rates. Sulfur dioxide can be converted to sulfate by reactions in gas, aerosol, and aqueous phases. As we noted in Chapter 17, the aqueous-phase pathway is estimated to be responsible for more than half of the ambient atmospheric sulfate concentrations, with the remainder produced by the gas-phase oxidation of S02 by OH (Walcek et al. 1990 Karamachandani and Venkatram 1992 Dennis et al. 1993 McHenry and Dennis 1994). These results are in agreement with box model calculations suggesting that gas-phase daytime S02 oxidation rates are l-5% per hour, while a representative in-cloud oxidation rate is 10% per minute for 1 ppb of H202. 

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