Control Of Acids

As discussed in Chapter 8, the major contributors to acid deposition are sulfuric and nitric acids, with a significant contribution being increasingly recognized from organic acids. The chemistry of formation of nitric and organic acids has been discussed in Chapters 7 and 8 and has been shown to be part of the complex VOC-NO, chemistry that also leads to ozone formation. Hence control strategies applied for ozone will also impact the formation of these acids as well, although not necessary proportionately, as can be seen from the detailed chemistry.

For example, Meng et al. (1997) applied an Eulerian grid model that included both gas and aerosol chemistry to the Los Angeles area for conditions representative of a smog episode on August 27 and 28, 1987. Model predictions showed that the changes in the gas-phase HN03 due to reductions in VOC and NO, were not proportionate, which is not surprising given the complex chemistry involved. For example, a 50% reduction in NO, emissions alone gave a predicted 17% reduction in the maximum 1-h average concentration of HN03 at Riverside, in the eastern end of the air basin; a concurrent reduction of 50% in VOC gave a predicted reduction in peak HN03 of 39%. interestingly, the peak HN03 was predicted to actually increase by 17% for VOC reductions from 35 to 50% without concurrent reductions in NO, emissions. This increase in HN03 was associated with predicted simultaneous decreases in PAN of 58-76%. The HN03 increase was attributed to lowered concentrations of RC03 radicals, which resulted in less N02 being tied up in PAN and hence being available to react with OH to form HN03.

In short, the development of control strategies for HN03 is intimately tied with that of 03. Although the control of organic acids has not been examined in detail, similar considerations are expected to apply there as well.

As discussed in Chapter 8, the formation of H2S04 from S02 occurs largely in the aqueous phase. The major oxidants in fogs and clouds are H202 and 03, so that control of photochemical oxidants is again expected to impact the rate of S02 oxidation and formation of H2S04.

Given that the source of oxidants for S02 in both the gas and liquid phases is the VOC-NOx chemistry 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).

One of the key issues in developing effective control strategies for acid deposition has been what is known as "linearity." This term has been subject to a variety of interpretations and meanings and applied on microscopic, i.e., molecular, to macroscopic scales. A detailed treatment and discussion of linearity encompassing these scales is given by Hales and Renne (1992).

In its simplest form of interest for policy and regulatory purposes, linearity is often treated in terms of source-receptor relationships. That is, if the emissions of the precursor S02 are lowered by 50%, will the deposition of sulfate also decrease by 50% at all receptor sites?

A major factor involved in determining the relationship between S02 emissions and sulfate deposition is the chemistry. As discussed above, the oxidation of S02 by OH in the gas phase generates H02 and hence OH in the presence of NO. The regeneration of OH means that the oxidation will not be oxidant limited in the gas phase, and hence a reduction in S02 might be expected to be accompanied by a corresponding decrease in the formation of H2S04.

However, the situation is not as clear-cut for the liquid-phase oxidation, which, we have seen, predominates in many (perhaps most) situations. In this case, a less than 1:1 relationship between the reduction in H2S04 formed and S02 emitted may result for a number of reasons operating on the microscopic scale. For example, less H202 is available in many clouds than is needed to oxidize all of the S(IV) that is present, and hence the oxidation can be limited by the availability of oxidant (e.g., see Dutkiewicz et al., f995). Another important factor that comes into play is the interplay between the acidity of the aqueous phase, the reaction kinetics, and the solubility of S(IV). Thus, as seen in Chapter 8, the solubility of S(IV) decreases as the aqueous phase becomes more acidic, limiting the total sulfur available for oxidation. In addition, all oxidations in the aqueous phase except that by H202 are pH dependent; as a result, the contribution of various oxidants can vary significantly with time in an air mass as the oxidations take place and removal by rainout and washout occurs.

Examples of these complications have been reported in a number of field and modeling studies. For example, the amount of dissolved S02 in rain that has passed through a power plant plume has been found in some cases to be much less than might be expected, due to the acidification of the rain by other plume components that decreased the solubility of S02 (e.g., Dana et al., 1975). Similarly, studies using the PLU-VIUS model developed for application to gas-phase species and their interaction with clouds and precipitation (Hales, 1989) suggest that the deposition rate of sulfate from an urban source mixed in with contributions from the "background" may initially be nonlinear (Hales, 1991). However, at longer times this deposition rate becomes linear, and since most of the removal occurs at these longer times, the overall integrated deposition does indeed appear to be linear.

In short, as is the case for acids formed in VOC-NOx chemistry, the chemical and physical processes associated with the formation and deposition of sulfuric acid are also quite complex.

There are a number of field measurements that have addressed this relationship between the mandated reductions in S02 emissions in the United States and the subsequent changes in sulfate deposition downwind. For example, one analysis of the trends in the atmospheric concentrations of sulfate in the northeastern United States suggests that from 1977 to 1989, the sulfate concentration decreased by about 22-28% during which the emissions of S02 were estimated to have decreased by 25% (Shreffler and Barnes, 1996).

Similarly, Husain et al. (1998) have reported trends in sulfate at two sites in New York state, from 1979 to 1996 at Whiteface Mountain in a remote area in the eastern part of the state and from 1983 to 1996 at Mayville, in the western part. The trends at both sites were highly correlated. Figure 16.41 shows the relationship between sulfate or total sulfur (defined as sulfate plus gas-phase S02) at Whiteface Mountain as a function of the estimated anthropogenic emissions of S02 upwind in the Midwest. The relationship is well described as linear, with a correlation coefficient of r2 = 0.8f. (However, a negative intercept suggests that this cannot be directly extrapolated down to very small S02 emissions.)

The results of a global 3-dimensional model simulation also suggest that in the boundary layer in the United States, a 50% reduction in anthropogenic S02 emissions in the United States will result in a similar (53%) annual reduction in the total (wet plus dry) deposition of sulfur (Chin and Jacob, 1996).

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