Figure 6-4 shows the combined impact of emission changes (Table 6-1) from ERCOT and the Eagle Ford Shale on the episode average daily maximum 8-hr ozone concentration for each grid cell in the air quality simulation. For the $1.89 per MMBTU case, the episode average daily maximum 8-hr ozone concentration decreased by a maximum of 1.3 ppb. For the $3.87 per MMBTU and $7.74 per MMBTU scenarios, the episode average daily maximum 8-hr ozone concentrations increased by 1.0 ppb and 1.9 ppb, respectively. The spatial locations of the maximum ozone impacts in all scenarios occur in similar spatial locations, which correspond to the locations of coal-fired power plants, predominantly in northeastern Texas (Figure 6-1). The results in Figure 6-4 indicate that the maximum regional changes in the episode average of the daily maximum 8-hr ozone concentration are driven by emission changes at coal-fired power plants, rather than by emission changes in the Eagle Ford Shale production region.
Figure 6-5 is a sensitivity analysis that shows the impact on the episode average of the daily maximum 8-hr ozone concentrations for each scenario considering only the emission changes in the Eagle Ford Shale production region while keeping emissions from ERCOT constant at the base case ($2.88 per MMBTU) level. These sensitivity analyses allow for an estimate of the ozone impacts due only to changes in ozone precursor emissions from the Eagle Ford Shale. When comparing to Figure 6-4, it is important to note the difference in maximum scale for the Figures (±2 ppb in Figure 6-4 versus ±0.5 ppb in Figure 6-5). For the $1.89 per MMBTU case, the episode average of the daily maximum 8-hr ozone concentrations increased by a maximum of 0.4 ppb. For the $3.87 per MMBTU and $7.74 per MMBTU scenarios, the episode average of the daily maximum 8-hr ozone concentrations decreased by 0.4 ppb and 0.5 ppb, respectively. For the oil and gas emission sensitivity scenarios, the largest ozone
concentrations occur in the areas of the Eagle Ford with the highest well density (Figure 6-1) and, thus, apportioned emissions based on study methodology.
Figure 6-4. Change in average daily maximum 8-hr ozone concentration over the 33-day episode compared to the $2.88 per MMBTU base case. Emissions changes for ERCOT and the Eagle Ford Shale are outlined for each scenario in Table 6-1. Increased ozone concentrations compared to the base case are yellow to red in color. Decreased ozone concentrations compared to the base case are blue in color.
Figure 6-5. Change in average daily maximum 8-hr ozone concentration over the 33-day episode compared to the $2.88 per MMBTU base case considering only changes in oil and gas emissions from the Eagle Ford Shale. For these simulations, the Eagle Ford Shale emissions outlined in Table 6-1 were used for each scenario, but ERCOT emissions were for the $2.88 per MMBTU scenario in all simulations. Increased ozone concentrations compared to the base case are yellow to red in color. Decreased ozone concentrations compared to the base case are blue in color.
Comparing the episode average 8-hr daily maximum ozone results from Figure 6- 4 (emission changes in both ERCOT and Eagle Ford Shale) to the results from Figure 6-5 (emission changes only from the Eagle Ford Shale) indicates that an ozone trade-off may exist due to increased natural gas use in the power sector if the additional natural gas production was sourced from the Eagle Ford Shale. Comparing the $1.89 per MMBTU
scenario to the base case, the episode average daily maximum 8-hr ozone concentration decreased 0.5 ppb - 1.8 ppb throughout large sections of northeastern Texas (Figure 6- 4a). The sensitivity analysis (Figure 6-5a), which included the 44% increase in emissions from the Eagle Ford Shale but kept emissions from ERCOT constant with base case values, indicated that the episode average daily maximum 8-hr ozone concentration could increase by 0.1 ppb to 0.4 ppb in several high well-density production areas of the Eagle Ford Shale. Detailed analysis (Figure 6-6) of a grid cell in Milam county [(25,41) in the 12-km eastern Texas domain] that showed the highest ozone sensitivity to changes in Eagle Ford Shale emissions (Figure 6-5) could be greater than 2 ppb in that grid cell during morning and evening hours (NG_1.89 and NG_1.89OG in Figure 6-6). Despite a constant emissions profile from the oil and gas sector, the ozone impacts during the mid- day hours for the grid cell were lower in magnitude, indicating either that other emissions sources largely drove ozone formation in the grid cell during mid-day hours or that dilution during the time of day with large mixing heights reduced the impacts of emissions. This is an important result since the periods associated with daily maximum 8-hr ozone concentration typically occurred during daytime hours, when the ozone impact of the Eagle Ford Shale emissions is lower.
Figure 6-6. Difference in episode average ozone concentration for each hour of the day in ground-level grid cell (25,41), which has the largest ozone sensitivity to oil and gas emissions from the Eagle Ford Shale, compared to the $2.88 per MMBTU base case. OG indicates a sensitivity case in which the Eagle Ford emissions were changed but ERCOT emissions were kept constant at base case levels.
Another important ozone metric to consider when examining the combined impacts of changing emissions in the natural gas production and power sectors is the difference in the episode maximum 8-hr ozone concentration for each grid cell. This metric can be important since Federal ozone standards are based on a three-year average of the 4th highest annual daily maximum 8-hr ozone concentration (EPA 2014c). Thus, reductions to the daily maximum 8-hr ozone concentrations during a photochemical modeling episode would be an important for regulatory consideration if one of those days included one of the top four highest daily maximum 8-hr ozone concentrations for the year.
Figure 6-7 shows the change in episode maximum 8-hr ozone concentration compared to the $2.88 per MMBTU base case and including emission changes from the
-4.00 -3.00 -2.00 -1.00 0.00 1.00 2.00 3.00 4.00 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 D iff e re n ce in E p isod e Hou rl y Av e rag e Ozo n e C o n ce n tr ation in M ilam Co u n ty Gr id C e ll Simulation Hour NG_1.89 NG_1.89OG NG_3.87 NG_3.87OG NG_7.74 NG_7.74OG
electricity generation and the Eagle Ford production sectors. Note that the values represented in the grid cells in Figures 6-7 and 6-8 are not necessarily paired in time with surrounding grid cells. Episode maximum 8-hr average ozone concentration results are similar between the Eagle Ford Shale and northeastern Texas, which is different than the results for the episode average daily maximum 8-hr ozone concentration metric. For example, despite additional upstream production emissions of 19.6 tons per day of NOx in the $1.89 case (Table 6-1), the episode maximum 8-hr ozone concentration decreased throughout much of the Eagle Ford shale (Figure 6-7a), which were areas with increased episode average daily maximum 8-hr ozone concentrations under the same emissions scenario (Figure 6-5). However, the same increase in production emissions from the Eagle Ford Shale without ERCOT emissions reductions drove an increase in the episode maximum 8-hr average ozone concentration (Figure 6-8a) compared to the base case. Thus, the episode maximum 8-hr average ozone concentration throughout much of the Eagle Ford was more impacted by changes in EGU emissions than local, upstream production emissions.
In addition, several hotspots resulted from changes in the power sector and natural gas emissions when using the episode maximum 8-hr average ozone concentration metric (Figure 6-7). One hotspot occurred on the Texas-Oklahoma border in a pattern consistent with the shape of a plume from a large coal-fired power plant (Figure 6-7a), despite the fact that NOx emissions decreased from that facility in the $1.89 per MMBTU scenario compared to the base case. The ozone results from this hotspot had two separate drivers. For the dark red dot in the same grid cell as the coal-fired power plant, the decreased NOx emissions from the coal-fired power plant led to lower titration of ozone in the fresh power plant plume, causing an increased ozone concentration values in the $1.89 per MMBTU case compared to the base case. However, the surrounding yellow color
(Figure 6-7a) also appears in the simulation with only oil and gas emissions (Figure 6- 8a), indicating that the ozone increase for the yellow region was driven by increased transport from the Eagle Ford Shale production area. The second hotspot of interest occurred in the Houston area (Figure 6-7a) and was likely driven by increased NOx emissions from area natural gas EGUs in the $1.89 per MMBTU scenario that increased ozone during the peak hours of the episode. This hotspot was not observed in the cases which considered only emissions from the oil and gas production sector in the Eagle Ford (Figure 6-8a).