Evaluating the Effects of Projected Utility SO2 and NOx Emissions ...

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1. Evaluating the Effects of Projected Utility SO2 and NOx Emissions Reductions on Particulate Nitrate Concentrations in the Southeast Presented To VISTAS Data Analysis Work Group February 19, 2004

3. Background Methods Used observed data (circa 1999-2000) of sulfate, nitrate, ammonium, ammonia, nitric acid, and meteorological data from ARIES/SEARCH Varying number of days modeled but distribution representative of longer records at each site. The data were used in the thermodynamic equilibrium model SCAPE2 SCAPE2 was run to replicate the data SCAPE2 is then used to estimate changes in fine particulate nitrate resulting from changes in nitric acid, ammonia, and sulfate.

4. Table 1. Statistical summary of distributions of particulate nitrate for the modeled days and for the full three years of data at Jefferson Street (1998-2000).

5. Figure A-1. Comparisons between measured and model-predicted nitrate (top) and between measured and model-predicted ammonia (bottom). The comparisons were determined from 45 days in 1999 with data from ARIES and from the SEARCH site at Jefferson Street. The simulations for ARIES used measurements of ammonia whereas the simulations for SEARCH calculated the ammonia concentrations based upon the assumption of equilibrium between ammonium nitrate and nitric acid.

6. Figure A-2. Comparisons between measured and predicted nitric acid, fine particulate nitrate, and fine particulate ammonium concentrations at four SEARCH sites. Units are mg m-3.

7. Background Caveats Since the model does not deal with emissions, we assume a linear response between NH3, NOx, and SO2 emissions and ambient NH3, HNO3, and SO2 + SO4 concentrations, respectively. We also do not differentiate among NOx source types (i.e., mobile vs. power plant NOx). We believe these assumptions to be conservative and are assessed qualitatively.

8. Background Scenarios Expected reductions reflected the following SO2 scenarios: Completion of Title IV reductions (i.e., EGU reductions of ~ 25%) Phase 1 of CSI (e.g., EGU reductions of ~ 62% in GA) Phase 2 of CSI (e.g., EGU reductions of ~ 73% in GA) Additional NOx reductions of EGU NOx reflected the following scenarios: Mobile Source NOx in 2020 (~ 40%) Phase 1 of CSI (e.g., EGU reductions of ~ 64% in GA) Phase 2 of CSI (e.g., EGU reductions of ~ 73% in GA) Zero-out PP NOx (e.g., EGU reductions of 100% in GA)

9. Background Scenarios These reductions were then applied to the fraction of EGU emissions (based on EPA’s 1999 NEI Inventory) in each relevant state to get the reduction in Total SO2 and Total NOx. The effect of changes in ammonia emissions were also assessed. The overall effect on PM2.5 concentrations were also assessed.

10. Findings The mean concentrations of particulate nitrate resulting from the CSI 2 SO2 reductions increased from < 0.3 - 0.8 ug/m3 and were, thus, marginal to small. A change of less than 0.4 ug/m3 is below our ability to reliably measure or model and is, thus, considered marginal. The results for BHM show nitrate decreases after CSI-2 SO2 reductions, with further decreases in response to NOx reductions.

11. Table 2. Modeled mean fine particulate nitrate concentrations for SEARCH sites under eight scenarios. The first scenario used current conditions, and the second (S61N00) was done with sulfate concentrations reduced by 61 percent (56 % at Birmingham). Additional simulations were done with a reduced sulfate concentrations and nitric acid reductions of 10 to 55 percent. Units are ug m-3. The Atlanta simulations were carried out using measured ammonia concentrations from ARIES, while all other simulations used model-estimated ammonia levels (Appendix A).

12. Figure 1. Statistical distributions of modeled fine particulate nitrate concentrations at Jefferson Street under current conditions and for scenarios with sulfate concentrations reduced by 61 percent and nitric acid concentrations reduced by varying amounts. The first scenario used current conditions, and the second was done with sulfate concentrations reduced by 61 percent (S61N00). Additional simulations were done with a 61 percent sulfate reduction and nitric acid reductions of 10, 17, 26, 35, 45, and 55 percent. The distributions show the 10th, 25th, 50th, 75th, and 90th percentiles, and individual points above the 90th percentile or below the 10th percentile.

13. Table 4. Statistical summary of modeled fine particulate nitrate concentrations for 219 days at Yorkville under eight scenarios. The first scenario used current conditions, and the second was done with sulfate concentrations reduced by 61 percent (S61N00). Additional simulations were done with a 61 percent sulfate reduction and nitric acid reductions of 10, 17, 26, 35, 45, and 55 percent. Units are ug m-3.

14. Table 3. Statistical summary of modeled fine particulate nitrate concentrations for 164 days of Atlanta ARIES measurements under eight scenarios. The first scenario used current conditions, and the second was done with sulfate concentrations reduced by 61 percent (S61N00). Additional simulations were done with a 61 percent sulfate reduction and nitric acid reductions of 10, 17, 26, 35, 45, and 55 percent. Units are ug m-3.

15. Findings Under the CSI-2 SO2 reductions, particulate nitrate formation: changes from being largely ammonia limited to largely nitric acid limited at JST and YRK but stays largely ammonia limited at CTR, OAK, and OLF. The results for BHM show nitric acid limitation currently and after CSI-2 SO2 reductions, with subsequent responses to NOx reductions.

18. Findings Under the CSI-2 SO2 reductions, the amount of NOx emissions reductions needed to offset the marginal to small increases in particulate nitrate: range from 35 to 55% at JST and YRK to greater than 55% at CTR, OAK, and OLF, the latter cases being essentially non-responsive to NOx reductions.

19. Table 2. Modeled mean fine particulate nitrate concentrations for SEARCH sites under eight scenarios. The first scenario used current conditions, and the second (S61N00) was done with sulfate concentrations reduced by 61 percent (56 % at Birmingham). Additional simulations were done with a reduced sulfate concentrations and nitric acid reductions of 10 to 55 percent. Units are ug m-3. The Atlanta simulations were carried out using measured ammonia concentrations from ARIES, while all other simulations used model-estimated ammonia levels (Appendix A).

20. Findings The amount of NOx emissions reductions needed to offset the increase in particulate nitrate resulting from CSI-2 SO2 reductions increases as ammonia emissions increase. This is largely due to the fact that the amount of particulate nitrate increase is greater. With lesser SO2 reductions there is less of a sulfate decrease, less of a nitrate increase, less response of nitrate to NOx decreases, and more areas, days remaining ammonia limited. We should compare these results with eulerian air quality results.

22. Findings Two additional considerations: Expected Mobile NOx Reductions Overall Effect on PM2.5 Mobile Sources are expected to decline by about 40% by 2020 with CSI-2 resulting in an additional 17% for a total of about 57% CSI Phase 2 NOx reductions provide little, if any, additional benefit to PM2.5 concentrations.

24. Policy Inferences Large SO2 emissions reduction will result in marginal to small increases in nitrate concentrations in certain locations. CSI-2 NOx reductions or even elimination of power plant NOx emissions would moderate to a very small degree but cannot fully offset the small particulate nitrate increases projected to occur from CSI-2 SO2 reductions. Even this small benefit would be limited to those areas where the CSI-2 SO2 reductions resulted in nitrate formation becoming nitric acid limited.

25. Policy Inferences For those areas remaining largely ammonia limited after CSI-2 SO2 reductions, there would be an even smaller increase in particulate nitrate and essentially no reduction in particulate nitrate from power plant nor any NOx emissions reductions. While the relative effect of power plant versus mobile NOx was not assessed, it is likely that in Atlanta where mobile NOx dominates, power plant reductions would be even less effective in reducing particulate nitrate increases. Since increasing ammonia leads to the need for even more NOx emissions reductions to offset the particulate nitrate increases resulting from CSI-2 emissions reductions, capping and/or reducing ammonia emissions would be a far more effective strategy for solving this, albeit small, problem.

27. Table 5. Statistical summary of modeled fine particulate nitrate concentrations for 95 days at Pensacola OLF under eight scenarios. The first scenario used current conditions, and the second was done with sulfate concentrations reduced by 61 percent (S61N00). Additional simulations were done with a 61 percent sulfate reduction and nitric acid reductions of 10, 17, 26, 35, 45, and 55 percent. Units are ug m-3.

28. Table 6. Statistical summary of modeled fine particulate nitrate concentrations for 251 days at Oak Grove under eight scenarios. The first scenario used current conditions, and the second was done with sulfate concentrations reduced by 61 percent (S61N00). Additional simulations were done with a 61 percent sulfate reduction and nitric acid reductions of 10, 17, 26, 35, 45, and 55 percent. Units are ug m-3.

29. Table 7. Statistical summary of modeled fine particulate nitrate concentrations for 169 days at Centreville under eight scenarios. The first scenario used current conditions, and the second was done with sulfate concentrations reduced by 61 percent (S61N00). Additional simulations were done with a 61 percent sulfate reduction and nitric acid reductions of 10, 17, 26, 35, 45, and 55 percent. Units are ug m-3.

32. Figure 2. Statistical distributions of modeled fine particulate nitrate concentrations at suburban Pensacola (OLF) and Oak Grove under current conditions and for scenarios with sulfate concentrations reduced by 61 percent and nitric acid concentrations reduced by varying amounts. The first scenario used current conditions, and the second was done with sulfate concentrations reduced by 61 percent (S61N00). Additional simulations were done with a 61 percent sulfate reduction and nitric acid reductions of 10, 17, 26, 35, 45, and 55 percent. The distributions show the 10th, 25th, 50th, 75th, and 90th percentiles, and individual points above the 90th percentile or below the 10th percentile.

34. Figure 3. Distributions of excess ammonia at Jefferson Street, Yorkville, and Centreville under current conditions and for scenarios with sulfate concentrations reduced by 61 percent and nitric acid concentrations reduced by varying amounts. Particulate nitrate formation is limited by the availability of ammonia when excess ammonia is negative.

37. Figure 4. Distributions of excess ammonia at Pensacola (OLF) and Oak Grove under current conditions and for scenarios with sulfate concentrations reduced by 61 percent and nitric acid concentrations reduced by varying amounts. Particulate nitrate formation is limited by the availability of ammonia when excess ammonia is negative.

39. Table 8. Statistical summary of modeled fine particulate nitrate concentrations for 164 days of ARIES measurements under eight scenarios, with 20 percent increases in ammonia concentrations for the seven hypothetical cases. The first scenario used current conditions, and the second was done with sulfate concentrations reduced by 61 percent (S61N00). Additional simulations were done with a 61 percent sulfate reduction and nitric acid reductions of 10, 17, 26, 35, 45, and 55 percent. Units are ug m-3.

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