Sulfate acid deposition

One could argue that if the emissions are reduced by 50%, and because Whatever goes up must eventually come down, that the sum of sulfate acid deposition (wet and dry) averaged over a year would also decrease by 50%. Such an answer implies that sulfate deposition varies linearly with changes in S02 emissions. We know enough about the processes and pathways connecting the S02 emissions with the sulfate deposition (Figure 20.12) to critically evaluate the validity of such an assumption. 

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... 

Black and colored plates can also be obtained from chromic acid baths. The plates are mostly oxides (177). Black chromium plating bath compositions are proprietary, but most do not contain sulfate. The deposit has been considered for use in solar panels because of its high absorptivity and low emissivity (175). 

Acid deposition refers to the transport of acid constituents from the atmosphere to the earth s surface. This process includes dry deposition of SO2, NO2, HNO3, and particulate sulfate matter and wet deposition ("acid rain") to surfaces. This process is widespread and alters distribution of plant and aquatic species, soil composition, pH of water, and nutrient content, depending on the circumstances. 

Acid deposition and the associated particulate nitrates and sulfates are implicated in the deterioration of certain sensitive ecosystems, decreased visibility, negative human health effects, and increased degradation of certain stone building materials and cultural resources, especially those made of limestone and marble. Fine particulate nitrate and sulfate particles... 

Fig. 16-4 pH sensitivity to SO4- and NH4. Model calculations of expected pH of cloud water or rainwater for cloud liquid water content of 0.5 g/m. 100 pptv SO2, 330 ppmv CO2, and NO3. The abscissa shows the assumed input of aerosol sulfate in fig/m and the ordinate shows the calculated equilibrium pH. Each line corresponds to the indicated amoimt of total NH3 + NH4 in imits of fig/m of cloudy air. Solid lines are at 278 K, dashed ones are at 298 K. The familiar shape of titration curves is evident, with a steep drop in pH as the anion concentration increases due to increased input of H2SO4. (From Charlson, R. J., C. H. Twohy and P. K. Quinn, Physical Influences of Altitude on the Chemical Properties of Clouds and of Water Deposited from the Atmosphere." NATO Advanced Research Workshop Acid Deposition Processes at High Elevation Sites, Sept. 1986. Edinburgh, Scotland.)... 

It is roughly estimated that there are more than 1,200,000 water bodies in eastern North America that are currently affected by acid deposition. A subset of these lakes has been sampled since early 1980s in order to monitor the changes in lake water chemistry induced by the declining sulfur dioxide emissions and wet sulfate deposition... 

The Eastern Canadian Acid Rain Program was highly successful at reducing SO2 emissions and sulfate wet deposition in eastern Canada (see Figure 10). Sulfur emissions actually declined more than the desired 50% by 1994, and have continued to decline modestly in the present. These SO2 emissions in the United States have also reduced dramatically, particularly since the implementation of the Canada-United States Air Quality Accord in 1991. This has been especially important to the aquatic and terrestrial ecosystems in eastern Canada, since US emissions are responsible for a large proportion of the acid deposition received in eastern Canada due to transboundary transport. 

Jeffries, D. S., Lam, D. C. L. (1993). Assessment of the effect of acidic deposition on Canadian lakes determination of critical loads for sulfate deposition. Water Science Technology, 28, 183-187. 

Reaction (13.2) is highly undesired because SO, reacts with water present in the flue gas in large excess and with ammonia to form sulfuric acid and ammonium sulfate salts. The ammonium sulfate salts deposit and accumulate on the catalyst if the temperature is not high enough, leading to catalyst deactivation, and on the cold equipment downstream of the catalytic reactor, causing corrosion and pressure drop problems. The catalyst deactivation by deposition of ammonium sulfate salts can be reversed upon heating. 

Efforts to reduce acid deposition have had mixed results thus far. For example, measurements at five locations in Nova Scotia, New Brunswick, Newfoundland, and Labrador by Canadian researchers found that sulfate deposition dropped between 27 and 50 percent between 1980 and 1995. During the same time, however, there was a significant reduction in acid deposition at only one of the five monitoring sites. 

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). 

Attention has been focused recently on sedimentary microbial reduction of sulfate. This process neutralizes atmospheric sulfuric acid deposited into soft-water lakes (8-10) through the production of two equivalents of alkalinity per mole of sulfate reduced (11). 

Recent concern for the role of sulfate in acidic deposition has intensified the need for a more accurate estimation of natural sulfur emissions in the United States. The magnitude and the spatial and temporal distribution of natural sulfur emissions must be quantified in order to be useful in efforts to predict the effectiveness of various control strategies for acid deposition. The inventoiy estimates described in this paper predict monthly sulfur emissions from each county in the contiguous United States. Improvements in the methodology used to calculate this natural emissions inventory may be a useful guide in the calculation of the national emissions inventories of other naturally emitted compounds which have the potential to make significant contributions to regional air quality. 

The level of emissions of SO2 and N0X, and their effects in the United States and in other industrialized countries have been the target of considerable debate. When SO2 and N0X are emitted into the atmosphere, a large fraction of these pollutants can be oxidized to sulfate and nitrate during atmospheric transport, and then deposited as acidic compounds (4-8). Acid rain is the popular term to describe this complex phenomenon. Acidic compounds can be deposited in both wet and dry forms, and this process is more properly referred to as acid deposition or acid precipitation. The wet forms of acid deposition include rain, snow, fog, and dew. Dry deposition occurs via absorption of SO2 and N0X on surfaces and via... 

After calcination, palladium sulfate was deposited by impregnation to incipient wetness with an acidic solution of PdS04 2H20 (Alfa products) in water. An additional amount of sulfuric acid was added to the solution to aid the palladium sulfate dissolution. The amount of sulfuric acid in the solution was adjusted to obtain a S042YPd2+ ratio of 10 in the catalyst. Catalysts were dried and stored in air at 350 K. 

As demonstrated by papers presented at this Symposium, the increase of our knowledge about acid deposition in recent years has been enormous. The SURE project (Hidy, This Volume) demonstrated that huge field projects can be conducted with good quality control of samples, analyses and data. Kok, Tanner (This Volume) and others have developed highly sophisticated systems for measuring concentrations of many species, including the very important H2O2, in clouds and clear air with aircraft. In the area of mechanisms, we know that oxidation by -OH radicals is the dominant gas-phase reaction in the conversion of S02 to and sulfate (5). Furthermore, we know that oxidation in solution by is rapid and that by 03 and O9 (the latter catalyzed by metal ions or carbon soot) canoe important under some conditions (6 Schwartz, This Volume). 

In August, 1983, members of the Research Staff of Ford Motor Company carried out a field experiment at two rural sites in southwestern Pennsylvania involving various aspects of the acid deposition phenomenon. This presentation will focus on the wet (rain) deposition during the experiment, as well as the relative importance of wet and dry deposition processes for nitrate and sulfate at the sites. Other aspects of the experiment have been discussed elsewhere the chemistry of dew and its role in acid deposition (1), the dry deposition of HNO3 and SO2 to surrogate surfaces (2), and the role of elemental carbon in light absorption and of the latter in visibility degradation (3). 

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