Estimating the San Joaquin Valley 1

- hr O 3 Standard Carrying Capacity

The photochemical modeling used in the 2013 San Joaquin Valley ( SJV ) SIP for the 1

- hr O 3 Standard showed attainment of the 124 ppb standard in 2017. The Edison moni toring site was

the d esign site with a modeled future year d esi gn v alue of 119.3 ppb

, based on a 2017 emissions inventory for the SJV of 247.1 tons/day of NOx and 366.3 tons/day o f ROG. Because this design value was below the standard, staff evaluated the modeling results to determine the emissions levels required for the future design value to equal the standard (i.e., the carrying capacity).

As part of the Weight of Evidence an alysis for the SIP, ozone isopleths were generated for Edison, which show the model response to incremental (20%) emission reductions of NOx and
ROG starting from the 2017 baseline inventory (Figure 1)

Figure 1. Ozone isopleth for Edison based on b and

- R RF analysis (adapted from Figure 23, Appendix G of the 2013 SJV SIP for the 1

- hr O 3 Standard).

The linear (and horizontal) nature of the ozone isopleths in Figure 1 suggests that the photochemical regime for ozone formation at Edison is NOx limited and is relatively insensitive to changes in ROG emissions (also true for all other sites in the SJV) . Consequently, it is possib le to extrapolate to future design v alues for NOx emission s levels greater than the 2017

baseline inventory, provided the chang e in NOx emissions does not represent a significant

increase above the 247.1 tons/day in the 2017 inventory (e.g., less than

~ 15 %)

.

1From Figure 1, it can be seen that as NOx emissions are reduced from 2017 l evels, the simulated change in ozone design v alu e per ton of NOx emi ssions reductions increases (i.e.

, the ozone isopleths are closer together at lower NOx emissions levels). As a result, estimating a design v alue for NOx emissions levels above the 2017 baseline inventory through a linear regression of the top two points along the far right vertical axis in Figure 1 ([100% ROG, 100% NOx] and [100% ROG, 80% NOx]) provides a conservati ve estimate of the design v alue.

Figure 2 shows the future year design v alue as a function of NOx emissions for the case where ROG emissions are held constant at 366.3 tons/day and NOx emission are reduced in 20% increments from the baseline of 247.1 tons/day (the points along the far right vertical axis in Figure 1 ).

Fi gure 2. Future Year 1

- hr ozone design v alue at Edison as a function of NOx emissions when anthropogenic ROG emissions are held constant at 2017 levels

.

Based on the linear regression in Figure 2, NOx emissions of 278.9 tons/da y will result in a future year design v alue of 124 ppb at Edison . This amounts to an increase of just under 13% ( or 31.8 tons/day) in the NOx inventory, which is a small enough fraction of the 2017 inventory

that the linear regression provides a reasonable (and conservative) approximation of the design v alue and NOx c arrying c apacity in the region. This estimate was verified with additional modeling based on the original 1

- hr O 3 SIP modeling used to generate Figure 1

, which showed that a 13% increase in NOx emissi ons above 2017 level s resulted in a design

v alue of 123 .4 ppb a t Edison. Since the 123.4 ppb design v alue is less than the 124 ppb estimated from the linear extrapolation in F igure 2 , the 13% increase in NOx emissions represents a conservative estimate.

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