Acid Deposition Atmospheric Processes in Eastern North America (1983) / Chapter Skim
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Appendix A: The Chemistry of Acid Formation
Appendix A: The Chemistry of Acid Formation
Pages 155-201
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From page 155... ...
Reactions can occur in the gas phase; in the solution phase in cloud water and rainwater, for example; and in reactions on surfaces of solid particles in the atmosphere. Thus we are concerned with the rates of exchange of gaseous reactants and their reaction products between liquids and the surfaces of solids as well as the rates of interactions among gaseous molecules, aqueous phase molecules and ions, and species adsorbed on solid 155
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From page 156... ...
is so slow under catalyst-free conditions in the gas phase that it can be neglected as a source of sulfuric acid in the atmosphere. The thermal oxidation of NO and NO2 in the gas phase is also slow.
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From page 157... ...
A number of the more complex pathways involve photochemistry. In one, sulfur dioxide absorbs light in the ultraviolet region of the solar radiation incident in the troposphere, and, in principle, excited states of SO2 generated in this fashion could lead to SO2 oxidation in the troposphere.
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From page 158... ...
The reactions of this species with various atmospheric gases and many atmospheric impurities have been studied extensively (Calvert et al.
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From page 159... ...
of NO and NO2 is unimportant. Reactive Transient Species in the Troposphere Most of the gas-phase tropospheric chemistry of SO2, NO, NO2, and other impurity molecules involves reactions with a variety of reactive excited molecules, atoms, and free radicals (neutral fragments of stable molecules)
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From page 160... ...
Its reactions with carbon monoxide and the hydrocarbons (RH) lead to another important class of reactive transients, the peroxy radicals: HO + CO ~ H + CO2, (21)
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From page 161... ...
(29) The most common fate of the smaller alkoxy radicals in the lower atmosphere is reaction with oxygen, leading to HO2 radicals and a carbonyl compound.
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From page 162... ...
(35) For current purposes the complex array of interactions that occur among the reactive species outlined here and with the various atmospheric impurities need not be considered.
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From page 163... ...
ultimately leads to the generation of sulfuric acid aerosol. However, HOSO2 is not a stable molecule; it is a free radical that is probably highly reactive with several atmospheric compounds.
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From page 164... ...
. ~Rate constants are expressed as second-order reactions for I atm of air at 25°C; see Calvert and Stockwell (198,3)
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From page 165... ...
3' new , MU _4 ~ J /^ / / / I 1 1 1 1 1 11 1 1 1 1 1 1 1 1 1 1 1 1.0 1n 100 10 FIGURE A.2 Comparison of the experimental data for the effective second-order rate constants for the reaction (56) with M = N2.
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From page 166... ...
166 10 \\/ \\~ _ _ ~I_ _ ~ - 1 1 1 1 1 0 10 20 30 40 50 Altitude (km) / Pressure \\ \ \ - All' '\ \ \ \ \ \ 0.01 300 250 a)
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From page 168... ...
It is difficult to distinguish the homogeneous component of this reaction in laboratory experiments, since reaction at a moist cell wall can be more important than the homogeneous reaction for most experimental systems. Morris and Niki (1973)
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From page 169... ...
169 10 5 2 0.5 x a) a, o E mp 0.05 cat 0.2 0.1 0.02 0.01 \ \ \ /k \ my\ / \ \ Pressure \ ~ 1 1 1 1 1 1 1 1 1 1 1 1 0 5 10 15 20 25 30 1 000 100 Al 10 ' in 1 o 35 40 45 50 Altitude (km)
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From page 170... ...
(1976) are all reasonably consistent both among themselves and with theoretical estimates based on computer models (Calvert and McQuigg of the complex atmospheric chemistry 1975, C hang et al.
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From page 171... ...
. and NO2 to provide a significant quantity of sulfuric and nitric acids in the troposphere.
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From page 172... ...
The conversion rates for NO2 observed by recent worker s are also consistent with the estimates presented here (Spicer 1982)
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From page 173... ...
Aerosol from gas-phase processes dominated the plume chemistry for low humidities and in the absence of clouds. The clearest evidence of the major input from precipitating clouds was derived in the Acid Precipitation Experiment (APEX)
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From page 174... ...
. A more direct and quantitative measure of seasonal variations in the atmospheric chemistry involved in acid deposition has been presented by Shaw and Paur (1983)
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From page 175... ...
it' 1::~ I V) 1::~ 1~ 1::~ 1L AUG FIGURE A.6 Average monthly concentrations of airborne pollutants at three sites in the Ohio River Valley in 1980-1981.
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From page 176... ...
An abundance of data based largely on laboratory measurements points to the probable involvement of the oxidizing agents hydrogen peroxide and/or ozone in the formation of acids in cloud water. Also suggested in theory as having a possible influence in these transformations are free radicals such as HO and HO2 either formed in the gas phase and transported to the liquid phase or formed in the liquid phase.
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From page 177... ...
0 10 20 FIGURE A.7 Comparison of isotopic ratios in sulfates formed by seven laboratory methods and in precipitation water.
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From page 178... ...
have shown that the times required to establish gas-liquid equilibrium between SO2 and aqueous aerosol droplets are usually much shorter than the chemical reaction times for reactant concentrations typical of the ambient atmosphere. The expected 1S(IV)
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From page 179... ...
Of the various oxidizing agents that can oxidize S(IV) in solution, two species, H2O2 and O3, appear to be especially significant for ambient levels of impurities.
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From page 180... ...
. For aqueous solutions with pa in the range 1-3, the rate of oxidation via ozone follows = 1.9 x 104[H+(aq)
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From page 181... ...
. There is a strong link between the gas-phase and liquid-phase atmospheric chemistry related to acid precipitation.
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From page 182... ...
. However, for conditions more typical of a nonurban air mass, where it is not uncommon to have very low NOx levels (~1 ppb)
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From page 183... ...
ensure that the aqueous droplets in the atmosphere will contain ozone concentrations of about 3-6 x 0~10 M These limited observations suggest that conditions of low NOX and high hydrocarbon and aldehyde impurity levels favor H2O2 formation, whereas those of relatively high NOy, RH, and RCHO favor high O3 generation rates. If homogeneous air masses containing preformed H2O2 and O3 encounter cloud water, rainwater, high aqueous acid aerosol levels, etc., then solution phase pathways for H2SO4 formation are favored as well.
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From page 184... ...
in the troposphere. The truly uncatalyzed oxidation of sulfite solutions by oxygen is very slow, and there is some question whether small impurities of metal ions such as iron may not account entirely for observed ~uncatalyzed" rates (Martin 1983)
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From page 185... ...
In the high pH range (pH > 4) , a condition uncommon for most natural atmospheric situations, ferric ion concentrations fall below 10-8 M, and the inconsistency of the rate laws observed in the laboratory for this system in this pH range may reflect the unrecognized problem of iron ion removal as a precipitate.
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From page 186... ...
This and other estimates of the rate constants for this reaction are shown as a function of pH in Figure A.12. The activation energy for the reaction is 27.3 kcal/mole (Hoather and Goodeve 1934a,b)
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From page 187... ...
For the combined Fe-Mn system the rate of sulfite oxidation was typically 3 to 10 times faster than that anticipated from the sum of the independent rates in the individual systems. 104 c' ~ 103 o E ._ of 1 o2 o 10 1 ~ _ / ,,,,/' / ,~W Hoather and Goodeve (1934a,b)
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From page 188... ...
. Subsequent catalytic decomposition of the intermediate hydrogen peroxide may produce HO and additional HO2 free radicals.
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From page 189... ...
The coupling of photolytic and metal-catalyzed processes is also consistent with the observed difference between daytime and nighttime SO2 conversion rates. Dissolved organic molecules can act as competitive complexing agents for metals.
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From page 190... ...
assumed that there are no limitations due to mass transport rates. The rate constants presented here have been applied to a hypothetical cloud containing 1 ml of liquid water per cubic meter of air at 25°C.
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From page 191... ...
The trend of rising conversion rates with increased pH results from either the rising equilibrium concentration of sulfur (IV) or the sensitivity of the rate constants to pH, or both.
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From page 192... ...
The result of such interactions could increase the solubility of SO2 in the droplet and conceivably retard sulfite oxidation by the oxidants. Although there is significant theoretical evidence that H2O2 may be the most important oxidizing agent for acid generation in cloud water and rain, unambiguous
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From page 193... ...
If these species are absorbed i non In aerosols, cloud water, or rainwater, for example, as is probable for H2O2 in view of its very high Henry's law constant, then oxidation by H2O2, and possibly other peroxides, can be most significant. The possible roles for peroxyacetylnitrate, peroxynitr~c acid, CH3C2H, and other peroxides in solution-phase sulfite oxidation remain to be evaluated.
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From page 194... ...
Both HONO2 and H2SO4 produced in the gas phase can be scavenged effectively by cloud water and precipitation. NO2 may be oxidized to HONO2 if sufficient O3 and NO2 are present.
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From page 195... ...
1976. An experimental investigation of the absorption of sulfur dioxide by water drops containing heavy metal ions.
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From page 196... ...
1975. The photolysis of gaseous nitrous acid -- a technique for obtaining kinetic data on atmospheric photooxidation reactions.
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From page 197... ...
1981. Conversion rates in power plant plumes based on filter pack data: the oil fired Northpor~ plume.
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From page 198... ...
1983. Kinetics and mechanisms of catalytic oxidation of dissolved sulfur dioxide in aqueous solution: an application to nighttime fog water chemistry.
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From page 199... ...
1983. Kinetic studies of sulfite oxidation in aqueous solutions.
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From page 200... ...
I Growth laws for secondary aerosols in power plant plumes: implications for chemical conversion mechanisms.
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From page 201... ...
1981. Studies of aerosol formation in power plant plumes.
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