Estimating the San Joaquin Valley 1-hr O3 Standard Carrying Capacity

The photochemical modeling used in the 2013 San Joaquin Valley (SJV) SIP for the 1-hr O3

Standard showed attainment of the 124 ppb standard in 2017. The Edison monitoring site was 

the design site with a modeled future year design value 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 of 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 analysis 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 band-RRF analysis (adapted from Figure 23, 

Appendix G of the 2013 SJV SIP for the 1-hr O3 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 

possible to extrapolate to future design values for NOx emissions levels greater than the 2017 

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

increase above the 247.1 tons/day in the 2017 inventory (e.g., less than ~15%).

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From Figure 1, it can be seen that as NOx emissions are reduced from 2017 levels, the 

simulated change in ozone design value per ton of NOx emissions reductions increases (i.e., 

the ozone isopleths are closer together at lower NOx emissions levels). As a result, estimating 

a design value 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 conservative estimate of the design value.

Figure 2 shows the future year design value 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).

Figure 2. Future Year 1-hr ozone design value 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/day will result in a 

future year design value 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 value and NOx carrying capacity in the region. This estimate was verified with

additional modeling based on the original 1-hr O3 SIP modeling used to generate Figure 1, 

which showed that a 13% increase in NOx emissions above 2017 levels resulted in a design 

value of 123.4 ppb at Edison. Since the 123.4 ppb design value is less than the 124 ppb 

estimated from the linear extrapolation in Figure 2, the 13% increase in NOx emissions 

represents a conservative estimate.

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