Environ. Sci. Technol. 1992, 26, 715-725
Sensitivity of Acid Production/Deposition to Emission Reductions Woo-Chul Shln and Gregory R. Carmlchael" Department of Chemical and Biochemical Engineering, University of Iowa, Iowa City, Iowa 52240
The sensitivity of sulfate and nitrate deposition during a spring storm in the eastern United States to changes in SO, and NO, emissions is evaluated by performing simulations with the STEM-I1 acid deposition model. The proportionality between emission reductions and deposition is found to vary from the dry and wet processes. Dry deposition of sulfate and nitrate is proportional to SO, and NO, emissions, respectively. Liquid-phase production of sulfate is nonproportional to SO, and NO, emissions. Total domain-integrated liquid-phase production of sulfate decreases by 35% when SO, emissions are reduced by 50% and increases by 15% with a 50% lowering of NO, emissions. The smallest changes in sulfate wet deposition occur in those regions with large SO2 emissions, and those experiencing heavy precipitation. The results are found to be sensitive to how the boundary and initial conditions are treated in the emission reduction simulations, with larger nonlinearities occurring when the boundary conditions are held fixed.
of oxidants, sulfate wet deposition can depend strongly on changes in NO, and/or reactive hydrocarbon levels. The conditions under which such relationships hold must be identified, anticipated, and assessed when acid deposition and oxidant abatement strategies are being considered. This requires the use of comprehensive science-based computer models. The STEM-I1 model is one example of a contemporary comprehensive acid deposition and photochemical oxidant model (I, 2). This model has recently been applied to the study of acid deposition in the eastern United States as part of the Department of Energy PRECP (Processing Emissions by Cloud and Precipitation) program. The results of a detailed analysis of a spring storm have been presented by Shin and Carmichael (3). In this paper, the sensitivity of the calculated sulfate and nitrate fields to reductions in the primary emissions is reported. The sensitivity of the results to the methodology of the modeled emission reductions is also discussed.
1. Introductian The development of successful acid rain control policies requires an in-depth understanding of processes causing acid deposition. However, the relationships between the emissions of primary pollutants and acid deposition are inherently difficult to determine because of the number and nature of the processes that occur. Sulfur- and nitrogen-containing species, along with reactive hydrocarbons, are emitted from a variety of anthropogenic arid natural sources. These compounds are mixed, transported, reacted, and finally removed from the air back to the earth's surface. Sulfur dioxide may react immediately with hydroxyl radicals in the atmosphere to produce SO,, which in turn reacts quickly with water vapor to produce sulfuric acid, or, depending on the meteorological conditions and the local availability of oxidizing substances, the SOz may be transported hundreds of kilometers before it reacts. Some SO2may also be deposited in gaseous form directly to the earth's surface. SOz may also be absorbed into cloud droplets, where it may react with Hz02,Os, or other oxidants. These reactions produce sulfuric acid in the liquid phase. This acid may be removed from the atmosphere through the formation of precipitation, or it may be injected into the gas phase through evaporation processes. In a somewhat similar manner, NO and NO2 can be transported, dry deposited, or reacted to form nitric acid. Gaseous nitric acid is usually absorbed immediately into available cloudwater and is eventually returned to the earth as nitrate ion in precipitation. Organic acids may also be formed from emitted reactive hydrocarbons and end up in precipitation. The acid deposition processes are thus dependent on a wide spectrum of emissions: the emission of NO, and SO,, which control the acid deposition and formation processes, and the emission of hydrocarbons, which drive the photochemical processes with NO,. The sensitivity of sulfate and nitrate wet deposition to changes in anthropogenic emissions depends critically on whether the system is oxidant or primary pollutant limited. For example, under situations when the system is limited by the availability
2. Description of the Base Simulation 2.1. Domain and Inputs. The STEM-I1 model is a comprehensive three-dimensional, Eulerian model which describes the transport, chemical, and removal processes affecting acid deposition of sulfate and nitrate. The model is described in detail by Carmichael et al. (1,2). To investigate the sensitivity of acid deposition to reductions in emissions, model simulations were performed for a spring storm that occurred from May 1 to May 5, 1985. This storm was studied as part of the DOE PRECP program, and the STEM-I1 model predictions for the base emissions have been presented in ref 3. The domain modeled is shown in Figure 1. There are 20 X 13 horizontal grids with a grid spacing of 80 km and 11 vertical grids equally spaced between the earth's surface and 5 km. The height of the domain was selected to encompass the entire depth of the cloud system for the simulation period. The meterological fields (i.e., winds, temperature, and precipitation) were prepared using objective analysis of surface and upper air observations. The event total surface precipitation and samples of the grided wind fields are shown in Figure 1. The meteorological conditions are typical of those for spring frontal storms in this region, with the front moving from west to east over a period of a few days. The primary precipitation occurred in a west to east band along the Ohio River Valley, southern Pennsylvania, Connecticut, Delaware, and southern Massachusetts. Rain showers covered large areas in Illinois, Indiana, and western Ohio on May 1. By May 2 the primary precipitation had shifted eastward and covered southern Pennsylvania, West Virginia, and eastern Kentucky, and on May 3 the precipitation was centered along the east coast between Baltimore and Boston. The front passed out of the domain by May 4,and only light precipitation occurred in southern Canada. Complete details are presented elsewhere (2, 3). The emission fields were prepared from NAPAP emissions inventory version 5.2. The NAPAP emissions inventory was speciated according to the Atkinson-LloydWinges chemical mechanism ( 4 ) . Biogenic hydrocarbon emissions were also included in the analyses (3).
0013-930X/92/0926-0715$03.00/0
0 1992 Amerlcan Chemlcal Society
Environ. Sci. Technol., Vol. 26, No. 4, 1992
715
MAY 2, 1985
MAY 1, 1985
a
1200 GMT
a t 1000 m
1200 GMT
at 1000 m
10
MAY 4,1985 1200 GMT
ti o m/sec
at 1000 m
MAY 3,1985 1200 GMT
a t 1000 m
b
* e * * * * * * * *
UNIT
: mm
e 0 . 0 *40.o *
*
*
*
e
e e
30.0 20.0
Figure 1. (a) Grided wind fields. (b) Total grided precipitation.
2.2. Predicted Sulfate and Nitrate Wet Deposition. The predicted sulfate and nitrate wet deposition fields calculated with the base emissions are presented in Figure 2. The deposition patterns of both sulfate and nitrate are qualitatively similar, showing maximum deposition around the high-source regions and those regions receiving heavy precipitation. The total wet deposition of nitrate is 30% 718
Environ. Scl. Technol., Vol. 26,
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larger than that for sulfate. Sixty percent of the sulfate wet deposited was due to liquid-phase oxidation of S(IV), with the reaction with H202 the dominant pathway. Comparison of the model predictions with observations has been presented by Shin and Carmichael (3). The model was shown to capture the major features of the observed wet deposition and liquid-phase concentrations.
Table I. Mass Inventories for the Sensitivity Calculations‘ case
final
dry dep wet dep
gas rxn
0.96(33) 0.96(33) 0.67(33) 0.56(33)
0.61(33) 0.61(33) 0.45(33) 0.36(33)
0.23(34) 0.17(34) 0.11(34) 0.11(34) 0.23/34) 0.23i34j 0.17(34) 0.11(34)
0.50(33) 0.50(33) 0.29(33) 0.25(33) 0.29(33) 0.25(33)
4.35(33) 4.31(33) 0.35(32) 4.29(33) 0.35(32) 4,.29(33) 0.21(32) 4.22(33) 0.16(32) 4.17(33) 0.21(32) 4.20(33) 0.15(32) 4.14(33)
base
0.33(33) 0.25(33) 0.17(33) 0.17(33) 0.33(33) SO, 2 0.33(33) both 1 0.25(33) both 2 0.17(33)
NO, 1 NO, 2 NO, 3 SO, 1
L! 230.3
base i
2
NO, 1 NO, 2 NO, 3 SO, 1 SO. 2 both 1 both 2
base
1
.
-
’.
Figure 2. PIedicted event total sulfate (top)and nbate (bottom) wet deposition. Results are for the base emission conditions.
3. Emission Reduction Studies 3.1. Methodology of Emission Reduction Simulations. Before discussing the details of the response of the predicted acid deposition to changes in emissions, we will first discuss the sensitivity of such results to the method by which the simulations were conducted. In an episodic storm simulation, the initial conditions and boundary conditions can significantly influence the model predictions and the model‘s response to emission reductions. These influences must be carefully characterized, particularly since such models are being applied to the development of acid rain control policies. There are no general rules for how to treat initial conditions and boundary conditions under emission reduction simulations. Boundary and initial conditions can either be held constant during emission reductions or be changed in some manner. For example, the initial conditions could be prepared by making a multiday simulation with a reduced emission field, while keeping the boundary conditions fixed at the base values, or initial conditions and boundary conditions could he uniformly reduced from values used in the base case simulation. A variety of simulations were performed to investigate the sensitivity of predicted deposition to the method used. Each run started with initial concentrations representative of rural conditions. Then a 48-h “seasoning” simulation was performed utilizing the meteorology of April 30, and with emissions reduced according to the scenario. The resulting concentration fields were used as initial conditions to the 5-day event (May 1through May 5 ) . Additional simulations were conducted in which the boundary conditions and the initial loadings were further modified. The domain-integrated results are presented in Table I. Looking a t nitrate wet deposition first, reducing the emissions of NO, by 50% while keeping the boundary conditions fixed resulted in a 26% decrease in total nitrate wet deposited. Decreasing both the initial conditions and boundary conditions of NO, by 50% (in addition to the NO, emissions) resulted in a 41% decrease in wet deposition. Reducing also the initial loading and the boundary
0.53(33) 0.54(33) 0.54(33) 0.54(33) 0.33(33) 0.26(33) 0.33(33) 0.26(33)
0.20(33) NO, 1 0.18(33) NO, 2 0.18(33) NO, 3 0.18(33) SO, 1 0.15(33) SO, 2 O.lO(33) both 1 0.14(33) both 2 0.95(32)
(3) Sulfate 0.88(32) 0.41(33)
0.35(33) 0.31(33) 0.29(33) 0.29(33) 0.22(33) 0.17(33) 0.20(33) 0.14(33)
liq r m small small small small small small small small 4.32(33) 4.35(33) 4.37(33) 4.37(33) 4.25(33) 4.21(33) 4.27(33) 4.25(33) 0.32(33) 0.35(33) 0.37(33) 0.37(33) 0.25(33) 0.21(33) 0.27(33) 0.25(33)
‘Units: molecules. NO, 1, SO, 1, both 1: emissions only are reduced by 50%. NO. 2, SO, 2, both 2: emissions, initial conditions, and boundary conditions of NO,, SO,, and sulfate are reduced by 50%. NO, 3 same as NO, 2 except HNO, initial conditions are also reduced by 50%. Both 1, both 2: NO, and SO, are both reduced. Values in parentheses represent X10”. Emission information: for the base case Conditions the domain total emissions of NO, during the event were 0.23(34), the emissions of HNO, were zero, the emissions of SO2 were 0.17(34), and the emissions of sulfate were 0.32(32). Initial mass: for the base case conditions the mass at the start of the 5-day simulation was for “OB, 0.47(33); for SO2, 0.62(33); and for sulfate, 0.19(33).
conditions of nitric acid by 50% brought the total wet depasition reduction to 50%. These results indicate that most of the nitric acid deposited is produced inside the model domain. This is apparent also from Table I where nitric acid production is nearly 5 times the initial nitric acid loading. The largest changes in nitrate deposition occurred in eastern Illinois and western Indiana. The response in this region reflects the fact that the precipitation first occurred in these grid cells and before the nitric acid concentrations were able to completely adjust to the changes in NO,. The fact that the nitric acid deposition was further reduced hy lowering the boundary conditions of NO, indicates that NO, transported into the domain was a significant precursor for nitrate production. The effects of different initial SO, loadings on sulfate wet deposition are also summarized in Table I. Reducing the emissions of SO, by 50% resulted in a reduction in total domain sulfate wet deposition of 22%, while reducing emissions, and the initial loading and boundary conditions of SO, by 50%, resulted in a reduction of 42%. The effects of reducing the initial loading of sulfate were more uniformly distributed throughout the domain that those for nitrate wet deposition (5). This difference can be accounted for by the importance of liquid-phase production of sulfate. As indicated in Table I, sulfate production in the storm is 3 times larger than the initial loading of sulfate. The sensitivity of the results to changes in the boundary conditions indicates that the influx of sulfur outside the domain is an important precursor of sulfate. Envlron. Sci. Technol.. VoI. 26, No. 4, 1992 717
Figure 3. Ratio 01 su late wet deposition predicted by reducing SO, emissions by 50% wlm oounoary condnons held fixed to those where lhe OoJndary cond 1on5 are also reouced oy 50%
Additional simulations were conducted to isolate the influence of the boundary conditions. The modeling domain is surrounded hy relatively clean environments except in the southwestern region, which includes emission sources in Illinois, Kentucky, and Tennessee. To evaluate the influence of the boundary conditions on the model predictions, sensitivity simulations were conducted. In case 1, initial conditions and emissions for SO, were reduced by 50% while the boundary conditions were held constant. In case 2. the boundary conditions were also reduced by 50%. Total sulfate wet deposition for the two cases is compared in Figwe 3. Plotted is the ratio of the quantity of wet deposition under case 1 to that under case 2. If the boundary conditions play no role, then the ratio values wound be unity. Deviation from unity shows the sensitivity to the boundary conditions. Significant differences are noted in upper Michigan, Maine, and most of the southeastern part of the domain where the inflow conditions occurred during the simulation period. The central region of the domain, where most of the precipitation and deposition occurred during the storm period, and which includes most of the major source areas, was not greatly affected by the boundary conditions (less than 5 % ) . The above results indicate that the predicted acid deposition fields can be highly sensitive to the method by which the emission simulations are conducted. Each method has its own merits. The situation where only the emissions are reduced represents the case where only the domain of interest controls emissions, while those areas outside take no action. A reduction in the emissions, initial loading conditions. and boundary conditions represents the maximum possible response since all areas inside and outside the domain take the same actions. Throughout the rest of this paper results will focus on those obtained when the emissions, initial conditions, and boundary conditions are uniformly reduced. 32. Results for the NO, Emission Reduction Cases. 3.2.1. S i t r a t e Deposition. The dominant pathway for nitrate wet deposition for this storm was found to be the wet scavenging of nitric acid (3). Therefore, the wet deposition of nitrate is determined by how the gaeous nitric acid changes with KOxemissions. The response of gaseous nitric acid to NO, emission reductions can be nonlinear because the gas.phase NO, chemistry is strongly coupled with the hydrocarbon, radical chemistry. (In this paper we use the term nonlinear to indicate any situation where the response of the species is not in direct proportion to the changes in the precursor.) The sensitivity results can be understood in the context of the photochemical oxidant cycle, the key components being the reactions controlling the HO?, RO?,and OH 718
Envlron. Scl. Technol.. Vol. 26, No. 4. 1992
radical concentrations. Useful expressions quantifying the relationships between these species and the NO,, anthropogenic hydrocarbon (AHC), and 0, levels have been presented by Sillman et al. (6) and McKeen et al. (17). Qualitatively, in those regions with NO, levels greater than -6-8 ppb, the reaction of NO with HOzand ROz, and the reaction of NOz with OH, play a major role in controlling the destructiodn of HOPand the OH concentration. Decreasing NO, levels under these conditions result in an increase in HO,, an increase in O,, and an increase in OH. When NO, levels are less than -2 ppb (as occurs throughout most of the domain), the peroxy radicals are controlled by the radical/radical reactions (HOz+ HO,, HO, ROz, etc), the reactions of free radicals with NO, play a less important role, and OH decreases with NO, reductions (6). The net effect on nitric acid production is that, a t low NO, levels, nitric acid production decreases with decreasing NO, levels, but at NO, levels of >-8 ppb, it is rather insensitive (9). Cloud systems may influence these relationships between NO, levels and the OH distribution by reducing the ambient OH levels. Figure 4 shows the effects of the precipitating cloud system on ambient OH predicted by the model. In this figure, results of the base simulation are compared to those where the cloud system was removed from the analysis. The areas of reduced OH correspond to the movement of the cloud system. Regions of intense cloud cover and heavy precipitation show dramatic decreases in OH (more than 1order of magnitude). Under these conditions where the photochemical production of OH is slowed down, the radical reactions with NOz play a more significant role. The integrated effects of the NO, reduction and the presence of the cloud system are illustrated in Figure 5. Plotted are the reductions in nitrate wet deposition due to a 50% reduction in emissions, initial conditions, and boundary conditions. The smallest reductions (25-30% reductions in nitrate deposition) occurred in the high-NO, areas of eastern Pennsylvania, New Jersey, and Long Island, the urban/industrial centers of Pittsburgh, Detroit, Chicago, and along the Ohio River Valley. At these locations the NO, levels exceeded 8 ppb, and gaseous nitric acid production was less sensitive to decreases in NO,. In other regions, reduction percentages ranged from 40 to 50%. The nonlinear response in the western part of the domain (Central Illinois and Indiana) is due to the initial loading for nitrate. In this region, precipitation occurred in the earlier stages of the simulation period and nitrate wet deposition was strongly influenced by the initial loading. In the remainder of the domain, a large fraction of initially loaded nitrate was scavenged by dry deposition and the wet deposition was controlled by emitted NO,. When the effects of NO, reductions are integrated over the entire domain, nitrate dry deposition is reduced by 42% and wet deposition is reduced by 41% (Table I). 3.2.2. Sulfate Deposition. Reductions in NO, emissions may also influence sulfate deposition. The most important pathway leading to the production of aqueous-phase sulfate under this spring storm is the reaction between SO2 and Hz02(3). Reductions in NO, emissions influence sulfate production by affecting gas-phase HzOz production. The effects of NO, concentrations on H,Oz formation have been investigated by several researchers (8,9). Hydrogen peroxide is produced in the gas phase via the reaction HOz + H 0 2 + M HzOz+ 0, + M (A)
+
-
In high-NO, regions the HOz levels are controlled by their
85050113 EST
85050213 EST
1.05 0.95 0.80 0.65 0.50 0.35
85050313EST
85050413 EST
Flgure 4. Ratio of column-integratedOH levels for the base emissions conditions whh rain to those when the cloud system is removed from the analysis. 0-2.5 km: May 1 (top left), May 2 (top right). May 3 (bonom left), and May 4 (bonom right),
'EPCEIT
~ 5 4 . 0 0
-b
15.00 i3.00
55.00 30.00
Flgure 5. Percent reduction in nitrate wet deposition when NO, emissions and the inniil and boundary wndiions of nhrate are reduced
Flgure 6. Percent changes in event-averaged gas-phase H,O, concentrations at the surfacedue to a 50% reduction in NO, emissions.
by 50%.
reaction with NOz, and reducing NO, emissions increases the concentration of HOz radicals and the production of H20, (8). In addition, NO, reductions enhance the gasphase oxidation of SOzin high-NO, regions hy increasing OH, hut reduce the gas-phase sulfate production in lowNO, regions. A general increase in calculated amhient HzOzlevels due to NO, reduction is shown in Figure 6. Average gas-phase H,Oz concentrations were increased throughout most of the domain, and the increase in the Ohio Valley region reached 60%. The increased gas-phase HzOzunder NO, reductions also resulted in the increased total wet deposition of HzOZ(4). From gas-phase photochemistry considerations (under clear-sky conditions), H202levels are found to increase with decreases in NO,, and the magnitude increases with increasing NO, levels (6,17). Thus it is expected that the largest increases in H2O2should
occur along the east coast and the urhan/industrial centers in the Midwest. Instead, however, the patterns in Figure 6 show maximum sensitivity in the regions receiving the lowest precipitation (see Figure 1). Under precipitation conditions, the amhient H2O2concentrationsare controlled by wet scavenging and the liquid-phase reaction with S(IV), and not by the gas-phase reactions controlling HO, (i.e., the HOz + NO and the HOz HOBreactions). The effects of increased HzOzproduction on aqueousphase sulfate oxidation depends on the H,O,/S(IV) ratio in the aqueous phase. If aqueous-phase sulfate production is oxidant limited, then NO, reductions will increase the aqueous sulfate and decrease the S(IV). If the oxidation is S(IV) limited, then NO, reductions will have no significant effect on sulfur oxidation. During the storm event, responses to NO, reductions are complicated due to the fact that the H,Oz/S(IV) ratios change with time. Base emission simulations revealed that H,O,/S(IV) ratios
+
Environ. Sci. Technol., Vol. 26, NO. 4,
1992
719
Figure 7. Percent change in S(IV) wet deposition due to a 5 0 % reduction in NO, emissions. I
Figure 8. Percent change in sulfate wet deposition due to a 50% reduction in NO, emissions.
gradually decrease with continued precipitation (3). This means that the effects of NO, reductions on sulfate production will be significant in regions where SO, emissions are high and the precipitation occurs for extended periods. Changes in S(IV)and sulfate wet deposition due to NO, emission reductions are shown in Figures 7 and 8, respectively. S(IV) wet deposition decreased at most locations experiencing prolonged precipitation. In the regions of northern Ohio and Pennsylvania, S(IV) wet deposition increased by 25%. The sulfate wet deposition increased by 20% along the eastern border of Kentucky and the New York metropolitan area. These regions experienced relatively low precipitation (Figure l),and large increases in gaseous H,O, Concentrations with decreasing NO, emissions (Figure 6). The regions of high sulfate wet deposition were not, however, significantly impacted by NO, emission reductions. When integrated over the entire domain, NO, reductions were found to increase sulfate wet deposition by -5%, increase the liquid-phase production of sulfate by 15%, decrease the gas-phase production of sulfate by 17%. and decrease sulfate dry deposition by 6% (see Table I). 3.3. 50% SO, Emission Reduction Cases. The most important gas-phase production pathway for the formation of sulfate is the reaction between SO, and the OH radical. The reaction between SO, and OH produces the HO, radical, which eventually regenerates the OH radical, for example, by reacting with NO
SO, + OH + 0, NO + HO,
-.
-
SO3
NO,
+ HO,
+ OH
(B)
(C)
Since reaction B exerts little influence on ambient OH fields ( I @ , HNO, production should be insensitive to the reduction in SO, emission. The results in Table I show indeed that reductions in SO, emissions have no impact on nitrate deposition. 720
Environ. Sci. Technoi.. Voi. 26, NO. 4,
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With uniform reductions in SO, emissions, boundary and initial conditions and ambient SOz and sulfate levels respond linearly throughout most of the domain. Event total dry deposition and total chemical destruction/production for SO, and sulfate, shown in Table I, reflect these relationships. A higher degree of nonlinearity appears when the boundary conditions are fixed (as shown in Section 3.1). Wet deposition of sulfate can however experience nonproportional responses to changes in SO, emissions. If SO, emission rates exceed the oxidant production, then the system will behave more nonlinearly to emission reduction. Because the degree of nonlinearity of the predided change in sulfate concentration, caused by a change in SO, emissions, depends on the H,O,/S(IV) ratio in cloud/rain droplets, the nonlinearity in event total sulfate wet deposition depends on those factorsaffecting the H,O,/S(IV) ratio. These include the emission intensity and the storm duration, both of which have strong influences on the wet scavenging of SO, and H,02 In general, high SO, emissions and long-lasting precipitation enhance the nonlinearity in sulfate production and wet deposition. With heavy precipitation, ambient H,O, concentrations are greatly reduced by the wet-scavenging process and the subsequent destruction by reaction with S(IV), and H,Oz production in the gas phase is significantly reduced due to the thick cloud cover and the decrease in gas-phase precursors. In addition to changes in H202 and S(N)concentrations in the aqueous phase, sulfate produced in the gas phase affects the response of sulfate wet deposition to SO, emission changes. If a large fraction of aqueous-phase sulfate comes from gas-phase processes, a more linear response in the sulfate wet deposition will occur (since production of sulfate by gas-phase chemistry is linear in SO,). Model results under the base emission conditions found that 46% of the wet-scavenged sulfate comes from the gas phase through nucleation (3). Therefore, the overall response of sulfate wet deposition will be more linear than the sensitivity of the aqueous-phase sulfate production itself. Percent changes in predicted sulfate wet deposition are shown in Figure 9a and b. In the 50% emission only reduction case (Figure 9a), the percent changes are smaller. In the central region of the domain, which is not affected by boundary conditions, higher reductions (more linear response) appear in northern Pennsylvania, where only light precipitation occurred. In the high-emission regions along the Ohio River Valley, the sulfate deposition decreased by 20-30%. These nonlinear effects near strong source regions also appear in Figure 9b, where percent changes in sulfate deposition with uniform reductions in emissions, boundary conditions, and initial conditions are shown. Responses in the Ohio River Valley region and the Long Island area are more nonlinear than those at other locations. However, the minimum reductions in sulfate are 35%, and most of the domain experiences changes greater than 40%. 3.4. 50% NO, and SO, Emission Reduction Case. Simulations were also conducted for cases where both NO, and SO, emissions were reduced simultaneously by 50%. As shown in Table I, simultaneous reductions in NO, and SO, emissions gave almost the same nitrate production and deposition responses as the NO,-only reduction simulation. Sulfate production and deposition was affected by NO, emission changes. Therefore, control over NO, and SO, should effect the sulfate deposition differently. Event total sulfate dry deposition with reduced NO, and SO, emissions
>
Table 11. Total Amount of Acids Deposited on the Ground during the Simulation Period (108 hy
ease PERCENT
base NO, 1 NO, 2 so. 1 so; 2 both 1 both 2
wet deposition total acids by wet dep reductn, % 2.38(9) 2.18(9) 2.03(9) 2.08(9) 1.81(9) 1.84(9) 1.46(9)
8.4 14.7 12.6 24.0 22.7
38.7
total deposition total acids by dry + wet dep reductn, %
~
4.26(9) 3.57(9) :1.23(9) :1.89(9) 3.56(9) 3.17(9) 3.54(9)
"Unit: equivalence. Total acids, ZIH,SO,I
+ [HNOJ.
16.2 24.2 8.7 16.4 25.6 40.4 N0,.1,
SOz 1: only emissions are reduced by 50%. NO. 2, SO, 2: emis-
RCLNT
45.0" 40.00 35.0" 30.00 25.00
1
.I
'
U
~
Figure 9. Percent changes in sullate wet deposition due to 50% reduction in SOx emissions: boundary conditions unchanged (top); boundary and initial conditions also reduced by 50% (bonom).
L
Figure I O . Percent change in sulfate wet deposition when both NO, and SO, emissions are reduced by 50% (relative to the NO, only
r e d w t b case).
was slightly smaller (
