Prepared for Capital Area Council of Governments (CAPCOG) P.O. Box Austin, TX and

Transcription

1 Analysis of the Impact of Reductions in Anthropogenic NO x and VOC Emissions on Ozone Concentrations in the Austin Area using the Rider 8 Photochemical Modeling Episode for May 31-July 2, 2006 Prepared for Capital Area Council of Governments (CAPCOG) P.O. Box Austin, TX and Texas Commission on Environmental Quality (TCEQ) P.O. Box Austin, TX Prepared by Gary McGaughey Ling Huang Yosuke Kimura Cyril Durrenberger Elena McDonald-Buller The University of Texas at Austin Center for Energy and Environmental Resources, MS R Burnet Road Austin, TX PREPARED IN COOPERATION WITH THE TEXAS COMMISSION ON ENVIRONMENTAL QUALITY The preparation of this report was financed through grants from the State of Texas through the Texas Commission on Environmental Quality. September 2012

2 Executive Summary The purpose of this study was to examine the relative sensitivity of predicted daily maximum 8-h ozone concentrations in Travis County and at the Audubon and Murchison monitors to reductions in NO x or VOC emissions in the Austin area. These ozone precursor response studies were conducted by reducing anthropogenic emissions in the May 31-July2, 2006 Rider 8 photochemical modeling episode (hereafter referred to as the June 2006 basecase ), which has been fully described elsewhere (McGaughey et al., 2012a). A summary of the meteorological and large-scale transport conditions during June 2006 has been provided in the conceptual model of ozone for the Austin area (McGaughey et al., 2012b). Throughout this report, the term 1-h ozone concentration(s) will be used for ozone concentration(s) averaged over 1 hour. Similarly, the term 8-h ozone concentration(s) will be used for ozone concentration(s) averaged over 8 hours. As requested by CAPCOG, the original basecase was modified to reduce facility-wide emissions at the Sandow Power Plant, located in southern Milam County, to represent conditions for 2010 instead of This modification was performed to quantify the impacts of the emissions reductions at Sandow, which was the point source with the largest change in NO x emissions in the local area, on predicted ozone concentrations in Travis County as well as to isolate these effects from other impacts associated with reductions in anthropogenic NO x emissions in the 5- county Austin area. A comparison of the results between the original basecase and the modified basecase (Section 3.1 of this report) found that maximum reductions in the predicted daily maximum 8-h ozone concentrations for Travis County, Audubon, and Murchison were 2.3 ppb, 3.3 ppb, and 1.2 ppb, respectively. Overall during June 2006, reductions >0.5 ppb occurred on seven days in Travis County, three days at Audubon, and five days at Murchison. Four additional modeling scenarios were performed to achieve reductions of: (1) 25% NO x, (2) 50% NO x, (3) 25% VOC, and (4) 50% VOC equally across all anthropogenic emissions sources in the 5-county Austin area (Bastrop, Caldwell, Hays, Travis and Williamson counties). Example results are shown in Figure ES-1 for Travis County. For days with observed 8-h ozone concentrations >=75 ppb, reductions of anthropogenic NO x emissions were predicted to be more effective than VOC reductions for reducing peak 8-h ozone concentrations. The largest predicted reduction in 8-h ozone associated with reductions in VOC emissions was on June 8 th at Murchison; the daily maximum 8-h ozone concentration was decreased by 0.88 ppb for the 50% VOC scenario compared to a decrease of 6.28 ppb for the corresponding 50% NO x scenario. At all locations, the largest reductions in 8-h ozone concentrations were predicted for the 50% NO x scenario, with maximum 8-h ozone concentration reductions for Travis County, Audubon, and Murchison of 7.10 ppb (June 13 th ), 8.08 ppb (June 28 th ) and ppb (June 13 th ), respectively. For most days and locations, the amount of predicted ozone reduction for the 25% NO x scenarios was 44-48% of that predicted for the 50% NO x scenario. 1

3 Reduction in Daily Max 8-h Ozone (ppb) The University of Texas at Austin August 2012 Figure ES-1. Detailed precursor response results on days with observed concentrations >= 75 ppb. Results are presented for the absolute change in the maximum daily average 8-h ozone concentration (ppb) for Travis County for each of the four emissions reduction scenarios. (a) Travis County >= 75 ppb Days 25% VOC Reduction 50% VOC Reduction 25% NOx Reduction 50% NOx Reduction /03 06/08 06/09 06/10 06/13 06/14 06/28 06/29 06/30-1 As shown in Figure ES-1, the absolute reductions in 8-h ozone concentrations varied widely between individual days; for example, the reduction in predicted daily maximum 8-h ozone concentrations for the 50% NO x scenario ranged from 1.76 ppb on June 10 th to 7.10 ppb on June 13 th. A comparison of the spatial patterns of ozone reductions between these two days (Figure ES-2) demonstrated that light northeasterly near-surface winds on June 13 th were associated with maximum ozone reductions over portions of Travis and Hays counties while moderate southerly winds on June 10 th transported most of the ozone reductions northward into Williamson, Bell, and Coryell counties. This comparison demonstrated that the relative effectiveness of NO x emissions reductions in the 5-county Austin area on reducing 8-h ozone concentrations in Travis County was directly affected by the daily transport conditions for locally-formed ozone. 2

4 Figure ES-2. Differences in predicted daily maximum 8-h ozone concentrations (ground-level) on June 10 th (left) and June 13 th (right) between the basecase and the 50% anthropogenic NO x emissions reduction scenario at 1100 CST. Figure ES-3 shows the absolute reductions in predicted maximum 8-h ozone concentrations at Murchison for days with maximum observed 8-h ozone concentrations (1) >= 75 ppb, (2) ppb, and (3) ppb for the 50% NO x reduction scenario. There is wide variability between individual days within each category, but days with little change in predicted 8-h ozone concentrations are more common on lower ozone days; for example, the average reduction for the five ppb ozone days is 2.8 ppb compared to 4.8 ppb for the eight >=75 ppb days. The amount of daily ozone reduction had a strongly linear relationship with the contribution of ozone formed from local emissions sources in the 5-county Austin area (as quantified in McGaughey et al., 2012c), indicating that days dominated by high levels of background ozone transported into the Austin area had relatively small changes in ozone concentrations in response to reductions in local emissions. 3

5 6/03 6/08 6/09 6/13 6/14 6/28 6/29 6/30 6/04 6/07 6/10 6/26 6/27 6/02 6/06 6/12 6/15 6/25 8-h Ozone Reduction (ppb) The University of Texas at Austin August 2012 Figure ES-3. The absolute reduction in the daily average maximum 8-h ozone concentrations at Murchison for days with observed concentrations (1) >= 75 ppb, (2) ppb, (3) ppb for the 50% NOx reduction scenario >= 75 ppb Days (AVG = 4.8 ppb) Murchison ppb Days (AVG = 2.8 ppb) ppb Days (AVG = 1.8 ppb) Date To summarize the overall results of these studies, precursor response graphs were generated that show the average predicted daily maximum 8-h ozone concentrations for each reduction scenario for (1) >=75 ppb, (2) ppb, and (3) ppb ozone days. As shown in Figure ES-4 for Audubon, the predicted average 8-h ozone concentrations for the 25% and 50% VOC scenarios changed little compared to the basecase. For the NO x reduction scenarios on >=75 ppb days, the average basecase, 25% NO x, and 50% NO x daily maximum 8-h ozone concentrations were 80.2 ppb, 77.9 ppb, and 75.2 ppb, respectively. Figure ES-4 also demonstrates that the relative effectiveness of NO x reductions in lowering 8-h ozone concentrations decreases, on average, for the ppb days compared to >=75 ppb days. Similar results were seen for Travis County and Murchison. 4

6 Daily Max 8-h Ozone (ppb) The University of Texas at Austin August 2012 Figure ES-4. Average maximum predicted 8-h ozone concentrations for days with observed concentrations (1) >= 75 ppb, (2) ppb, (3) ppb as a function of anthropogenic NO x or VOC emissions reductions in the 5-county Austin area. Results are shown for Audubon. (b) 82 Audubon >=75 ppb Days (VOC) ppb Days (VOC) ppb Days (VOC) >=75 ppb Days (NOx) ppb Days (NOx) ppb Days (NOx) % 10% 20% 30% 40% 50% 60% Precursor Reduction 5

7 CONTENTS Executive Summary Introduction Methodology Basecase Simulation Anthropogenic VOC and NO x Reduction Scenarios Geographic Regions for Analysis of Results Results Differences in Predicted 8-h Ozone Concentrations between the June 2006 Basecase and Basecase with Sandow at 2010 Average Emissions Results for the Precursor Response Scenarios Summary References Acknowledgements...32 Appendix A: Additional results describing the reductions in daily maximum 8-h averaged ozone concentrations between the original and revised basecases Appendix B: Daily results for the precursor response scenarios Appendix C: Precursor Response Scenarios for the September 13-20, 1999 CAMx Episode with 2007 Projected Emissions 6

8 1.0 Introduction The objective of the photochemical modeling studies provided in this report are to examine the relative sensitivity of maximum predicted daily ozone concentrations in Travis County and at the Audubon and Murchison monitoring locations to reductions in anthropogenic NO x or VOC emissions in the local area. These ozone precursor response studies were conducted by reducing all anthropogenic emissions of NO x or VOCs in the daily emission inventory files for the May31-July2, 2006 Rider 8 episode within the 5-county Austin area. Because all anthropogenic emissions are targeted and not specific source categories, the results provide a quantitative indication of whether air quality in the area is predicted to be more responsive to reductions in NO x emissions or VOC emissions. These results become the foundation for modeling studies of specific emissions control programs that may be investigated in the future. 2.0 Methodology The analyses presented in this report utilize TCEQ s Rider 8 photochemical modeling episode for May31-July 2, 2006, hereafter referred to as the June 2006 basecase or basecase. The description of this episode, including results for the performance evaluation for ozone in the Austin area, has been previously documented by McGaughey et al. (2012a). The predicted ozone contributions from boundary conditions, initial conditions, and geographic emissions source regions within and outside of Texas (including the 5-county Austin area) on maximum ozone concentrations in Travis County and at the Audubon and Murchison monitors has been quantified using the Anthropogenic Precursor Culpability Analysis (APCA) tool described in detail elsewhere (McGaughey et al., 2012c). 2.1 Basecase Simulation As requested in an from Andrew Hoekzema (CAPCOG) to Gary McGaughey (UT) on June 11, 2012, the basecase used in support of the precursor response studies was modified from the original June 2006 basecase described in McGaughey et al. (2012a). The modification to the basecase was to reduce facility-wide emissions of NO x, VOC, and CO at the Sandow Power Plant, which is located in southern Milam County, to represent emissions conditions for 2010 instead of Since 2006, portions of the Sandow facility have been owned and operated by Alcoa, Inc. and Luminant (formerly TXU). The emissions files provided by TCEQ in support of the June 2006 basecase are available in two formats that have similar information but were created for different purposes: (1) CAMx-ready binary files that contain the processed emissions data for direct input to CAMx, and (2) AIRS Facility Subsystem (AFS) data that provide the emissions information for point sources in a detailed raw format that cannot be directly used by CAMx. The AFS file reports compliance and enforcement data for stationary sources of air pollution, including geographic location, source identification (e.g. plant and owner name, address, Standard Industrial Classification (SIC) Code), emissions release characterization (e.g. stack height), pollutant type, and emission rates 7

9 (e.g., tons of NO x emissions for each hour during June 2006). The data originate from two sources: (1) Continuous Emission Monitoring (CEM) for EPA s Acid Rain Program (ARP) and (2) daily emissions reported to State of Texas Air Reporting System (STARS). For the purposes of chemical transport modeling with CAMx, AFS files are processed using the Emission Processing System (EPS) program into two distinct binary emission file formats for (1) elevated point sources, and (2) surface-layer (gridded) sources. The data provided in the CAMxready binary emissions file for elevated point sources have been optimized for the application: Emissions, including NO x and VOCs, are speciated for the model chemical mechanism. Hourly emissions are estimated if provided for a daily (or longer) period. Nearby emission points with similar stack parameters may be aggregated into a single emissions point for modeling. Emissions points modeled with the Plume-in-Grid (PiG) approach are flagged. (The PiG approach is used in CAMx to disperse mass associated with newly-emitted plumes gradually into the CAMx grid cells.) Information not directly related to the photochemistry, such as the identification information for individual facilities, is dropped. For surface (ground-level) emissions sources, all emissions for a particular species within a model grid cell are summed by EPS for each hour and formatted for input to the CAMx gridded emission file. Emissions from point sources are primarily processed using an elevated point source format, but low-level point sources are sometimes processed into the gridded surface emissions file if the plume rise is not expected to be significant. For this application, the preferable approach would be to modify the facility-wide Sandow emissions through modification of the emission rates for individual point sources in the raw AFS file and re-running the EPS program to create a new CAMx-ready emissions input file. However, replicating TCEQ s EPS methodology and generating an updated CAMx binary file can be a time-consuming and complex effort because the EPS program requires auxiliary data files (e.g. chemical speciation information, temporal emissions profiles by SCC code, master stack list file for PiG processing) and multiple processing steps. Therefore, it was decided to alter the emissions contained in the CAMx binary files for the Sandow facility directly through the application of a multiplicative factor to achieve the desired change to facility-wide emissions. Figure 1(top) presents a map showing the locations of the 10 Luminant and 276 Alcoa point sources contained in the AFS data files provided by TCEQ. Elevated point sources were extracted from the CAMx binary file using the AFS locations to define the geographic region of interest. Figure 1(bottom) shows the locations of the 70 unique CAMx sources. Since the Sandow facility is geographically isolated, there is a high level of certainty that the extracted CAMx sources correspond to the Sandow sources represented in the original AFS dataset. 8

10 As shown in Figure 1(bottom), the locations of the elevated point sources in the AFS and CAMx files do not match. One reason for the locational offset is that the EPS program will aggregate nearby stacks with similar release parameters into a single modeled point source. This action is performed to limit the number of explicitly simulated stacks since the impact on modeled concentrations is expected to be negligible. This is particularly beneficial for nearby tall stacks (which often have the greatest emissions rates) that use the PiG option because the PiG algorithm requires relatively large computational resources. Of the 70 unique point sources identified in the CAMx binary elevated emissions file, only one emissions point (model stack ID 15 shown in Figure 1(bottom)) had hourly emissions that varied throughout the episode period. Figure 2 shows the hourly time series of NO x emissions from model source ID 15, which mostly is at 0.51 tons/hr (24% of the episode) or tons/hr (69% of the episode). Table 1 compares the daily average emission rates of CO, NO x, and VOC at the Sandow facility (i.e., combined Luminant and Alcoa sources) between the AFS and CAMx datasets. The NO x emissions are essentially identical. The facility-wide CAMx emissions of CO and VOC are ~12% and 33% lower, respectively, compared to the AFS values. Table 1 shows emissions associated with elevated point sources only; adding surface-based emissions (as a quality assurance action) corrected the differences in total NO x and CO emissions; however, the differences remained for VOC. The most likely cause of the discrepancy is the chemical speciation applied by the EPS program to convert total VOC to CAMx model species, but the exact cause of the discrepancy was not identified. As a practical matter, the absolute emissions of VOC are quite low and would not be expected to have a significant impact on predicted ozone concentrations in the Austin area. In addition, the APCA results for this modeling episode (McGaughey et al., 2012c) demonstrate that ozone formation conditions in Central Texas are primarily NO x -limited; as such, NO x is the species of importance from the Sandow facility. Operational changes at the Sandow facility since 2006 have resulted in substantially lower emissions for more recent years. CAPCOG requested that the facility-wide emissions be reduced to the values shown in Table 2 that represent emissions conditions for After an average emissions rate was applied to the previously discussed model stack ID 15 that had variable hourly emissions, reduction factors (Table 3) were calculated by taking the ratio of the facilitywide Sandow emissions for 2010 (Table 2) by the CAMx emissions shown in Table 1. These factors were applied equally to the 70 elevated emissions point sources in the CAMx binary files. Figure 3 compares the daily NO x emissions for the Sandow facility between the original basecase (2006 emissions) and the revised basecase (2010 emissions). In summary, the only difference between the original June 2006 basecase and the revised basecase used in this report in support of the precursor response sensitivity runs is that facilitywide emissions at the Sandow facility were substantially reduced from the original 2006 values to values representative of emissions conditions for This modification was performed to 9

11 quantify the impacts of the emissions reductions at Sandow, which was the point source with the largest change in NO x emissions in the local area, on predicted ozone concentrations in Travis County as well as to isolate these effects from other impacts associated with reductions in anthropogenic NO x emissions in the 5-county Austin area. 10

12 Figure 1. Maps showing the locations of: (top) AFS point source NO x emissions at the Sandow facility for Alcoa (red) and Luminant (yellow), and (bottom) overlain CAMx elevated emissions sources (green) including the identification of model stack 15. The location symbols are sized by the NO x emissions rate in tons per day. 11

13 Figure 2. Hourly NO x emissions (tons) from model stack ID 15. Table 1. Average daily emissions (tons per day) from the Sandow facility (i.e., combined Luminant and Alcoa emissions sources) as contained in TCEQ s AFS and CAMx-ready emissions files for the June 2006 basecase. CO (tons/day) NO x (tons/day) VOC (tons/day) AFS CAMx Difference -11.5% -0.5% -33.4% Table 2. Sandow facility (combined Alcoa, Inc. and Luminant sources) emissions for Annual Emissions Daily Emission Species (tons/year) (tons/day) NO x CO VOC

14 Table 3. Reduction factors applied to the 70 elevated emissions points in the CAMx elevated emissions file. CO NO x VOC Original Sandow Emissions (2006) Revised Sandow Emissions (2010) Reduction Factor Figure 3. Comparison of daily NO x emissions (tons per day) for the Sandow facility between the original basecase (2006) and revised basecase (Sandow with 2010 emissions). 13

15 2.2 Anthropogenic NO x and VOC Reduction Scenarios The precursor response sensitivity runs used the revised basecase described in Section 2.1. Four additional modeling runs were then performed that reduced either NO x or VOC emissions in the 5-county Austin area (i.e., Bastrop, Caldwell, Hays, Travis and Williamson Counties). The emissions reductions were applied to the 4-km horizontal grid cells shown in Figure 4 to achieve: (1) 25% NO x, (2) 50% NO x, (3) 25% VOC, and (4) 50% VOC reductions from the combined (surface and elevated) anthropogenic emissions sources. Table 4 summarizes the changes in total anthropogenic and biogenic 5-county NO x and VOC emissions for each reduction scenario. Figure 4. The 4-km grid cells in the 5-county Austin area where anthropogenic NO x or VOC emissions were reduced. The grid cells containing the Audubon and Murchison monitors are shown in green. 14

16 Table 4. Summary of total (biogenic and anthropogenic) basecase NO X and VOC emissions and changes in anthropogenic NO X and VOC emissions for each reduction scenario in tons per day (tpd). Basecase (tpd) 5-County 25% NO x Reduction Scenario (tpd) 5-County 50% NO x Reduction Scenario (tpd) 5-County 25% VOC Reduction Scenario (tpd) 5-County 50% VOC Reduction Scenario (tpd) NO x VOC NO x VOC NO x VOC NO x VOC NO x VOC Travis Williamson Hays Bastrop Caldwell County Total Geographic Regions for Analysis of Results For consistency with the APCA analyses reported elsewhere (McGaughey et al., 2012c), the precursor response results were analyzed for three geographic regions. The primary region was Travis County. For each episode day and hour, the predicted 8-h ozone concentrations at the km grid cells that comprise Travis County were averaged. The daily results were then analyzed for the hour with the maximum 8-h ozone concentration. Daily results were also analyzed for the 4-km grid cells that contain the Audubon and Murchison monitoring stations using the hour with the maximum predicted 8-h ozone concentration. The analysis of modeling results quantifies the change in maximum predicted daily ozone concentrations in Travis County and at the Audubon and Murchison monitors between each modeling scenario and the basecase. For each grid cell for each day, the differences in predicted maximum 8-h ozone concentrations are calculated as the basecase values minus the scenario (i.e., reduction run) values; thus, positive differences indicate that the predicted 8-h ozone concentrations for the scenario were less than the 8-h ozone concentrations predicted in the basecase. 15

17 3.0 Results 3.1 Differences in Predicted 8-h Ozone Concentrations between the June 2006 Basecase and Basecase with Sandow at 2010 Average Emissions Figure 5 shows the daily differences in maximum 8-h average ozone concentrations in Travis County between the original June 2006 basecase and the revised basecase that reduced Sandow emissions to values representative of The 8-h ozone concentration differences shown in Figure 5 represent the predicted impact on 8-h ozone concentrations due to reductions in Sandow emissions between 2006 and 2010; note, however, that the revised basecase used average hourly emissions whereas the original basecase used actual emissions for 2006 that has substantial dayto-day differences in NO x emissions associated with the varying emissions rate from the previously discussed model stack ID 15 (e.g., refer to Figure 3). The reductions in the daily maximum 8-h ozone concentrations for Travis County (Figure 5a) ranged from no change on several days during the episode to 2.3 ppb on June 3 rd. Differences greater than 0.5 ppb occurred on seven days (June 1 st, 2 nd, 3 rd, 12 th, 14 th, 19 th, and 27 th ). Results for Audubon (Figure 5b) showed maximum 8-h ozone concentration reductions of 3.3 ppb (June 12 th ) and 2.5 ppb (June 14 th ) with reductions greater than 0.5 ppb on one other day (June 28 th ). At Murchison (Figure 5c), the maximum 8-h ozone concentration reduction was 1.2 ppb on two days (June 3 rd, 14 th ) with >0.5 ppb reduction on five days (June 1 st, 2 nd, 3 rd, 14 th, 27 th ). As requested by CAPCOG, Appendix A includes a table of the results shown graphically in Figure 5 as well as tile plots that show the spatial extent of reductions in the maximum 8-h ozone concentrations. The tile plots show, for each grid cell, the differences in the daily maximum 8-h ozone concentrations between the original basecase and revised basecase. 16

18 5/31 6/1 6/2 6/3 6/4 6/5 6/6 6/7 6/8 6/9 6/10 6/11 6/12 6/13 6/14 6/15 6/16 6/17 6/18 6/19 6/20 6/21 6/22 6/23 6/24 6/25 6/26 6/27 6/28 6/29 6/30 7/1 7/2 Reduction in Daily Maximum 8-hr O3 (ppb) The University of Texas at Austin August 2012 Figure 5. Reduction in daily maximum 8-h averaged ozone concentrations for (a) Travis County, (b) Audubon, and (c) Murchison between the original June 2006 basecase and the revised basecase that has Sandow at 2010 average emissions. (a) 3.5 Travis County

19 5/31 6/1 6/2 6/3 6/4 6/5 6/6 6/7 6/8 6/9 6/10 6/11 6/12 6/13 6/14 6/15 6/16 6/17 6/18 6/19 6/20 6/21 6/22 6/23 6/24 6/25 6/26 6/27 6/28 6/29 6/30 7/1 7/2 Reduction in Daily Maximum 8-hr O3 (ppb) 5/31 6/1 6/2 6/3 6/4 6/5 6/6 6/7 6/8 6/9 6/10 6/11 6/12 6/13 6/14 6/15 6/16 6/17 6/18 6/19 6/20 6/21 6/22 6/23 6/24 6/25 6/26 6/27 6/28 6/29 6/30 7/1 7/2 Reduction in Daily Maximum 8-hr O3 (ppb) The University of Texas at Austin August 2012 Figure 5. (continued) (b) 3.5 Audubon Figure 5. (continued) (c) 3.5 Murchison

20 3.2 Results for the Precursor Response Scenarios Table 5 shows the results for Travis County, Audubon, and Murchison for days with observed daily maximum 8-h ozone concentrations >= 75 ppb. Ten days met this criterion, including June 3 rd, 8 th, 9 th, 10 th, 13 th, 14 th, 18 th, 28 th, 29 th, and 30 th. However, McGaughey et al. (2012a) noted that model performance on one of these days, June 18 th, was poor and suggested that this day be considered independently from others in future analyses. This day was not considered here. The reductions in predicted daily maximum 8-h concentrations are provided as absolute values as well as the percentage change relative to the basecase. The full daily results for Travis County, Audubon, and Murchison have been provided in Appendix B. Figure 6 displays the absolute reductions in 8-h ozone concentrations (ppb) presented in Table 5 as bar charts. Reductions of anthropogenic NO x emissions were predicted to be more effective than VOC reductions for reducing peak 8-h ozone concentrations on all days except June 13 th at Audubon (discussed in more detail shortly). The largest predicted reduction in 8-h ozone concentration associated with reductions in VOC emissions was on June 8 th at Murchison; the daily maximum 8-h ozone concentration was decreased by 0.88 ppb for the 50% VOC reduction scenario compared to a decrease of 6.28 ppb for the corresponding 50% NO x reduction scenario. At all locations, the largest reduction in 8-h ozone concentrations was predicted for the 50% NO x emissions scenario, with maximum reductions in 8-h ozone concentrations for Travis County, Audubon, and Murchison of 7.10 ppb (June 13 th ), 8.08 ppb (June 28 th ) and ppb (June 13 th ), respectively. For most days and locations, the reductions in 8-h ozone concentrations with 25% NO x reductions were 44-48% of the corresponding reductions in 8-h ozone concentrations with 50% NO x reductions. 19

21 Table 5. Predicted daily maximum 8-hour ozone concentrations on days with observed 8-h ozone concentrations >= 75 ppb and the absolute change (ppb) and percentage change (%) in the daily maximum 8-hour ozone concentration relative to the basecase. Travis County Predicted Basecase Daily Maximum 8-h Ozone Concentration (ppb) 25% VOC (a) (ppb) 50% VOC (a) (ppb) 25% NO x (b) (ppb) 50% NO x (b) (ppb) 25% VOC (a) (%) 50% VOC (a) (%) 25% NO x (b) (%) 50% NO x (b) (%) Audubon Predicted Basecase Daily Maximum 8-h Ozone Concentration (ppb) 25% VOC (a) (ppb) 50% VOC (a) (ppb) 25% NO x (b) (ppb) 50% NO x (b) (ppb) 25% VOC (a) (%) 50% VOC (a) (%) 25% NO x (b) (%) 50% NO x (b) (%) High Ozone Days During June rd 8 th 9 th 10 th 13 th 14 th 28 th 29 th 30 th AVG

22 Table 5. (Continued) Murchison Predicted Daily Maximum Basecase 8-h Ozone Concentration (ppb) % VOC (a) (ppb) 50% VOC (a) (ppb) 25% NO x (b) (ppb) 50% NO x (b) (ppb) 25% VOC (a) (%) 50% VOC (a) (%) 25% NO x (b) (%) 50% NO x (b) (%) (a) Reduction in predicted daily maximum 8-h ozone concentration corresponding to this reduction in VOC emissions. (b) Reduction in predicted daily maximum 8-h ozone concentration corresponding to this reduction in NO x emissions. 21

23 Reduction in Daily Max 8-h Ozone (ppb) The University of Texas at Austin August 2012 Figure 6. Detailed precursor response results on days with observed concentrations >= 75 ppb. Results are presented for the absolute change in the maximum daily average 8-h ozone concentration (ppb) for (a) Travis County, (b) Audubon, and (c) Murchison. (a) Travis County >= 75 ppb Days 25% VOC Reduction 50% VOC Reduction 25% NOx Reduction 50% NOx Reduction /03 06/08 06/09 06/10 06/13 06/14 06/28 06/29 06/

24 Reduction in Daily Max 8-h Ozone (ppb) Reduction in Daily Max 8-h Ozone (ppb) The University of Texas at Austin August 2012 Figure 6. (continued) (b) Audubon >= 75 ppb Days % VOC Reduction 50% VOC Reduction 25% NOx Reduction 50% NOx Reduction /03 06/08 06/09 06/10 06/13 06/14 06/28 06/29 06/30-1 Figure 6. (continued) (c) Murchison >= 75 ppb Days 25% VOC Reduction 50% VOC Reduction 25% NOx Reduction 50% NOx Reduction /03 06/08 06/09 06/10 06/13 06/14 06/28 06/29 06/30 23

25 Across the individual high ozone days, the absolute reductions in 8-h ozone concentrations varied widely. For example, the reduction in the maximum 8-h ozone concentration for the 50% NO x emissions reduction scenario in Travis County ranged from 1.76 ppb on June 10 th to 7.10 ppb on June 13 th, respectively. To investigate the differences in the effectiveness of reductions of NO x emissions on predicted 8-h ozone concentrations between these two days, tile plots were generated that show the spatial extent of reductions in the maximum 8-h ozone concentrations. Figure 7 shows, for each grid cell, the differences in the daily maximum 8-h ozone concentrations between the basecase and the 50% NO x scenario for June 10 th (left) and June 13 th (right). On June 13 th, a weak cold front had moved through Central Texas and winds in Austin were light and from the northeast. The spatial pattern of ozone reductions is consistent with this wind flow pattern, and maximum reductions in ozone concentrations were located over portions of Travis and Hays counties. The relatively light wind speeds were favorable to conditions of poor horizontal dispersion, and ozone formed from local emissions primarily remained within and to the southwest of Austin. In contrast, the near-surface winds on June 10 th were from the south at moderate wind speeds and the Austin urban plume was transported rapidly northward. The geographic areas with relatively large reductions in ozone concentrations on June 10 th extended north from Travis County into Williamson, Bell, and Coryell counties. The ozone reduction maps for June 10 th and 13 th demonstrate that the relative effectiveness of NO x emissions reductions in the 5-county Austin area on reducing 8-h ozone concentrations in Travis County is directly related to the associated transport conditions for locally-formed ozone. On June 10 th, the relatively strong winds transported most of the ozone reductions to geographic areas located outside of Travis County; expanding the analysis area from Travis County to the 5- county Austin area for both days would likely result in more similar reductions in area-averaged ozone concentrations between June 10 th and 13 th. 24

26 Figure 7. Differences in predicted daily maximum 8-h ozone concentrations (ground-level) on June 10 th (left) and June 13 th (right) between the basecase and the 50% anthropogenic NO x emissions reduction scenario at 1100 CST. Example Case Study: NOx Disbenefit on June 13th at Audubon As shown in Figure 6 for >= 75 ppb ozone days, the only location/scenario that showed an increase in the predicted 8-h ozone concentrations with a reduction in NO x emissions occurred on June 13 th at Audubon; that is, a reduction in emissions increased 8-h ozone concentrations in the local area. Figure 8 shows the predicted 1-h ozone concentrations at Audubon for the basecase and 50% NO x reduction scenario for the 0000 CST June 13 th CST June 14 th period. The maximum 1-h ozone concentrations occurred at 1800 CST for both the basecase (86.72 ppb) and 50% NO x reduction scenario (84.51 ppb). Relative to the basecase, reducing anthropogenic NO x emissions in the Austin area by 50% produced a decrease in 1-h ozone concentrations during the morning and afternoon period; however, 1-h ozone concentrations during the evening hours were increased. Ozone photochemistry would be expected to decline rapidly as available sunlight diminishes. A hypothesis to explain the relative increase in 1-h ozone concentrations for the reduction scenario during the evening hours is that freshly emitted NO is reacting with ozone in the vicinity of the Audubon monitor to decrease ozone concentrations; the reduced amount of NO in the 50% NO x reduction scenario results in relatively less destruction of ozone compared to the basecase. The different hourly ozone diurnal profiles between the two cases shifted the start-hour for the daily maximum 8-h ozone concentrations from 1200 CST for the basecase (76.56 ppb) to 1700 CST for the 50% NO x reduction scenario (77.02 ppb). 25

27 Ozone (ppb) The University of Texas at Austin August 2012 Figure 8. Time series of predicted 1-h ozone concentrations for the (1) basecase and (2) 50% NO x reduction scenario at the 4-km grid cell containing the Audubon monitor CST June CST June 14 Start Hour for Basecase Max 8-h Ozone = ppb Start Hour for 50% NOx Reduction Max 8-h Ozone = ppb Hour (CST) Basecase 50% NOx Reduction Results for >=60 ppb days Figure 9 shows the absolute reduction in predicted maximum 8-h ozone concentrations for Travis County for days with maximum observed 8-h ozone concentrations (1) >= 75 ppb, (2) ppb, and (3) ppb for the 50% NO x emissions reduction scenario. There is wide variability between individual days within each category, but days with little change in predicted 8-h ozone concentrations are more common on lower ozone days; for example, the average reduction in 8-h ozone concentrations for the seven ppb ozone days is 1.9 ppb compared to 3.8 ppb for the nine >=75 ppb days. The results shown previously suggest that the relative effectiveness of NO x reductions for reducing ozone concentrations is often impacted by the local transport patterns that determine the amount of ozone in Travis County formed from local emissions; that is, days with very high levels of background ozone transported into the 5-county Austin area would be expected to show relatively small changes in 8-h ozone concentrations in response to reductions in local emissions. 26

28 6/3 6/8 6/9 6/10 6/13 6/14 6/28 6/29 6/30 6/7 6/12 6/15 6/25 6/26 6/27 6/2 6/6 6/11 6/1 6/17 8-h Ozone Reduction (ppb) The University of Texas at Austin August 2012 Figure 9. The absolute reduction in the daily average maximum 8-h ozone concentrations in Travis County for days with observed concentrations (1) >= 75 ppb, (2) ppb, (3) ppb for the 50% NO x reduction scenario. 8 7 >= 75 ppb Days (AVG = 3.8 ppb) ppb Days (AVG = 1.9 ppb) ppb Days (AVG = 1.7 ppb) Date The APCA analyses quantified the amount of ozone in Travis County attributed to emissions sources in the 5-county Austin area as well as from all other sources except the 5-county Austin area. To demonstrate the relationship between the amount of locally-formed ozone and the effectiveness of NO x emissions controls, Figure 10 shows the contribution of ozone from local emissions sources in the 5-county Austin area (y-axis) versus the amount of reduction of the 8-h ozone concentration in Travis County for the 25% NO x (blue symbols) and 50% NO x (red symbols) reduction scenarios. To demonstrate the full response across days with widely varying conditions conducive to the formation and accumulation of ozone, Figure 10 includes results from all June 2006 days (e.g., both high and low ozone days). Visually, each NO x reduction scenario has a generally linear relationship between the amount of locally-formed ozone in Travis County (as quantified in the APCA analysis) and the amount of ozone reduction. A best-fit linear curve applied to the two datasets found that the coefficients of correlation were 0.97 and 0.96 for the 25% and 50% NO x reductions scenarios, respectively. This result indicates that >95% of the variation in the amount of day-to-day ozone reductions is predicted by the total amount of locally-formed ozone. This result clearly demonstrates that days with relatively lower amounts of locally-formed ozone have a relatively lower change in 8-h ozone concentrations in response to NO x reductions in the local area. 27

29 Ozone Contribtuion from Austin Sources (ppb) The University of Texas at Austin August 2012 Figure 10. For each day in June 2006, the contribution from 5-county Austin emissions sources to the average maximum 8-h ozone concentrations in Travis County (y-axis) versus the amount of daily ozone reduction for the 25% (blue symbols) and 50% NO x (red symbols) reduction scenarios % NOx Reduction 50% NOx Reduction Ozone Reduction (ppb)) Precursor Response Curves The precursor response studies discussed in this report provide a quantitative indication of whether 8-h ozone concentrations in the Austin area may be more responsive to reductions in NO x emissions or VOC emissions. The average results for (1) >= 75 ppb, (2) ppb, and (3) ppb ozone days are summarized for Travis County, Audubon, and Murchison in Figure 11(a)-(c). The graphs demonstrate that the average results are similar for each of the three receptor regions. Little change in the maximum 8-h ozone concentrations is predicted in the 25% and 50% VOC reduction scenarios compared to the basecase. For the 25% NO x scenario, the range of reductions of 8-h ozone concentrations compared to the basecase varies from 0.84 ppb for ppb days in Travis County to 2.10 ppb for >=75 ppb days at Audubon. For the 50% NO x scenario, 8-h ozone concentration reductions ranges from 1.74 ppb for ppb days in Travis County to 4.55 ppb for >=75 ppb days at Murchison. For comparison purposes, Appendix C describes the results of the precursor response analysis for the September 13-20, 1999 CAMx episode with 2007 projected emissions, conducted in support of Austin s Early Action Compact. 28

30 Daily Max 8-h Ozone (ppb) The University of Texas at Austin August 2012 Figure 11. Average maximum predicted 8-h ozone concentrations for days with observed concentrations (1) >= 75 ppb, (2) ppb, (3) ppb as function of anthropogenic NO x or VOC emissions reductions in the 5-county Austin area. Results are shown for (a) Travis County, (b) Audubon, and (c) Murchison. (a) 82 Travis County >=75 ppb Days (VOC) ppb Days (VOC) ppb Days (VOC) >=75 ppb Days (NOx) ppb Days (NOx) ppb Days (NOx) % 10% 20% 30% 40% 50% 60% Precursor Reduction 29

31 Daily Max 8-h Ozone (ppb) Daily Max 8-h Ozone (ppb) The University of Texas at Austin August 2012 Figure 11. (continued) (b) 82 Audubon >=75 ppb Days (VOC) ppb Days (VOC) ppb Days (VOC) >=75 ppb Days (NOx) ppb Days (NOx) ppb Days (NOx) % 10% 20% 30% 40% 50% 60% Precursor Reduction Figure 11. (continued) (c) 82 Murchison >=75 ppb Days (VOC) ppb Days (VOC) ppb Days (VOC) >=75 ppb Days (NOx) ppb Days (NOx) ppb Days (NOx) % 10% 20% 30% 40% 50% 60% Precursor Reduction 30

32 4.0 Summary The primary objective of this study was to quantify the impacts of reductions in anthropogenic NO x or VOC emissions in the 5-county Austin area on maximum predicted 8-h ozone concentrations in Travis County and at the Audubon and Murchison monitors. The results were generally similar across the three receptor regions. For days with observed 8-h ozone concentrations >=75 ppb, reductions of anthropogenic NO x emissions were predicted to be more effective than VOC reductions for reducing peak 8-h ozone concentrations. The largest predicted reduction in 8-h ozone concentrations associated with reductions in VOC emissions was on June 8th at Murchison; the 8-h daily maximum ozone concentration was decreased by 0.88 ppb for the 50% VOC scenario compared to a decrease of 6.28 ppb for the corresponding 50% NO x scenario. At all locations, the largest reductions in 8-h ozone concentrations were predicted for the 50% NO x scenario, with maximum 8-h ozone concentration reductions for Travis County, Audubon, and Murchison of 7.10 ppb, 8.08 ppb, and ppb, respectively. For most days and locations, the amount of predicted reduction in 8-h ozone concentration for the 25% NO x scenarios was 44-48% of the 8-h ozone concentration predicted for the 50% NO x scenario. The absolute reductions in 8-h ozone concentrations varied widely between individual days; for example, the reduction in predicted daily maximum 8-h ozone concentration in Travis County for the 50% NO x scenario ranged from 1.76 ppb on June 10th to 7.10 ppb on June 13th. Limited case-study analysis showed that the relative effectiveness of NO x emissions reductions was directly affected by the daily transport conditions for locally-formed ozone over Travis County. The amount of daily reduction in 8-h ozone concentrations had a strongly linear relationship with the contribution of ozone formed from local emissions sources in the 5-county Austin area, indicating that days dominated by high levels of background ozone transported into Austin had relatively small changes in 8-h ozone concentrations in response to reductions in local emissions. To summarize the overall results of these studies, precursor response graphs were generated that showed the average predicted daily maximum 8-h ozone concentrations for each reduction scenario for (1) >=75 ppb, (2) ppb, and (3) ppb ozone days. The predicted average 8-h ozone concentrations for the 25% and 50% VOC scenarios changed little compared to the basecase. For the NO x reduction scenarios on >=75 ppb days at Murchison, the average basecase, 25% NO x, and 50% NO x daily maximum 8-h ozone concentrations were 80.4 ppb, 78.3 ppb, and 75.6 ppb, respectively, respectively. The relative effectiveness of NO x reductions in lowering 8-h ozone concentrations decreased, on average, for the ppb days compared to >=75 ppb days; for example, the average basecase, 25% NO x, and 50% NO x daily maximum 8-h ozone concentrations were 64.9 ppb, 64.1 ppb, and 63.0 ppb, respectively. Similar trends were also seen at Audubon and Travis County. 31

33 5.0 References McGaughey, G., L. Huang, Y. Kimura, C. Durrenberger, E. McDonald-Buller, Description of the Rider 8 Photochemical Modeling Episode for May 31-July 2, 2006 including a Performance Evaluation for Ozone in the Austin Area, prepared for the Capital Area Council of Governments (CAPCOG) and the Texas Commission on Environmental Quality, July 2012a. McGaughey, G., C. Durrenberger, E. McDonald-Buller, Conceptual Model for Ozone for the Austin Area, prepared for the Capital Area Council of Governments (CAPCOG) and the Texas Commission on Environmental Quality, July 2012b. McGaughey, G., L. Huang, Y. Kimura, C. Durrenberger, E. McDonald-Buller, Analysis of the Impact of Regional Transport on Ozone Concentrations in the Austin Area using the Anthropogenic Precursor Culpability Assessment (APCA) Tool with the Rider 8 Photochemical Modeling Episode for May 31-July 2, 2006, prepared for the Capital Area Council of Governments (CAPCOG) and the Texas Commission on Environmental Quality, July 2012c. 6.0 Acknowledgements This report was prepared in cooperation with the Texas Commission on Environmental Quality. The preparation of this report was financed through grants from the State of Texas through the Texas Commission on Environmental Quality. The authors acknowledge the Texas Advanced Computing Center (TACC) at The University of Texas at Austin for providing high performance computational resources that have contributed to the work described within this report. URL: 32

34 Appendix A: Additional results describing the reductions in daily maximum 8-h averaged ozone concentrations between the original and revised basecases. 33

35 Table A-1. Reduction in daily maximum 8-h averaged ozone concentrations for (a) Travis County, (b) Audubon, and (c) Murchison between the original June 2006 basecase and the revised basecase that has Sandow at 2010 average emissions. Results are shown for days with observed Austin maximum 8-h ozone concentrations >= 75 ppb. Travis County Murchison Monitor Audubon Monitor Modeled Modeled Observed Observed O3 O3 O3 O3 Reduction Reduction Observed O3 (ppb) (Max/Min) Modeled O3 Reduction 06/ / / / / / / / / / / / / / / / / / The remainder of Appendix A shows the differences in predicted daily maximum 8-h ozone concentrations (ground-level) for each day during May 31 July 2, 2006 between the original and modified basecases. 34

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45 Appendix B: Daily Results for the Precursor Response Scenarios (Predicted maximum daily 8-h ozone concentrations and percentage change relative to the basecase for Travis County, Audubon, and Murchison) 44

46 Table B-1. Predicted daily maximum 8-h average ozone concentrations in Travis County for the original Rider 8 basecase, the basecase after the Sandow modification, and the precursor sensitivity response scenarios. Date Obs (min/max) ppb Base Case Rider 8 Model ppb Base Case After Sandow Modification ppb 25% VOC Reduction Ppb 50% VOC Reduction ppb 25% NO x Reduction ppb 50% NO x Reduction ppb 05/ / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / /