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Unmonitored Area Analysis for Ozone in the San Joaquin Valley 

Overview

An unmonitored area analysis (UAA) is required by the United States Environmental 

Protection Agency (USEPA). The UAA is intended to identify unmonitored areas where 

future year design values may be greater than the NAAQS. Since a simple spatial 

interpolation of observational data cannot identify unmonitored areas with 

concentrations higher than those measured at monitors, the USEPA recommends a 

UAA that combines both observations and modeling. We followed USEPA’s 

methodology in this analysis, and found that the monitoring network within the San 

Joaquin Valley (SJV) adequately captures elevated ozone levels within a 15-km radius 

of the monitors. 

Methodology 

Following USEPA guidance, this analysis used the Modeled Attainment Test Software 

(MATS), developed by the USEPA, in the UAA. For this analysis, MATS was used with 

the default configuration. Using MATS, the UAA is comprised of four primary steps: 

1) Interpolate base year ambient data to create a set of spatial fields. 

8-hour Ozone Design Value (DV) concentrations from within the modeling 

domain are input into MATS, which creates a gridded set of DV spatial fields. 

The same design values that were used in the monitor based model 

attainment test are used for the UAA (DVs for years 2000-2002, 2001-2003, 

and 2002-2004). 

2) Adjust the spatial fields using gridded model output gradients (base year 

values). 

For the 2007 SJV 8-hr ozone SIP, two five-day ozone episodes were 

simulated (July 9 – July 13, 1999 and July 29 – August 2, 2000). The daily 

maximum 8-hr ozone concentration was calculated for each day and each 

model grid cell, resulting in a total of ten daily spatial gradient distributions, 

each representing the daily maximum 8-hr ozone field for the respective 

modeled day. These data were then input into MATS to generate a single 

gradient- adjusted DV spatial field. 

 
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3) Apply gridded model RRFs to the gradient adjusted spatial fields. 

In addition to the DVs and base year simulated ozone, MATS requires future 

year simulated ozone. The same methodology of calculating the daily 

maximum 8-hr ozone for the base year (Step 2) was also used in this step to 

calculate a total of ten daily spatial gradient distributions for the future year. 

MATS uses the base year and future year simulated ozone in each grid cell to 

calculate an average RRF for each grid cell. The future year gradient adjusted 

concentration is then calculated as the product of the RRF and base year 

gradient adjusted concentration. 

4) Determine if any unmonitored areas are predicted to have higher 

concentrations than those at existing monitored areas. 

Results from Step 3 were analyzed to determine if any unmonitored regions 

may have concentrations higher than those observed at existing monitoring 

areas based on the future year gradient adjusted concentration. 

Results 

Results of the UAA are presented below (Figures 1-4). By default in the MATS 

analysis, a ‘monitored area’ is defined as a 7-by-7 grid of 4km grid cells surrounding a 

monitoring station. This is intended to be consistent with the 15km radius of 

representativeness for ambient monitoring stations that is used in the modeled 

attainment test. These square ‘monitored area’ regions of grid cells are illustrated on 

the figures with a dot in the center cell to represent the grid cell containing each DV 

station. 

Analysis of the gradient adjusted future year concentrations (Figure 4) shows that the 

monitoring network in the SJV provides adequate coverage over the regions which 

experience elevated ozone levels within the valley. In particular, the network 

adequately captures elevated ozone levels within a 15-km radius of the monitors. 

It should be noted that the gradient adjusted spatial field in Figure 4 shows elevated 

values occurring in one grid cell immediately to the south and west of several ‘monitored 

areas’ in the southern SJV near Bakersfield. Section 3.4.3 of USEPA’s Modeling 

Guidance states, “It is recommended that predicted violations of the unmonitored area 

analysis are carefully scrutinized to determine whether they are likely to exist in the 

ambient air or whether they may be caused by an error or uncertainty in the modeling 

system.” As recommended in USEPA’s guidance, staff looked carefully at factors such 

as emissions that might affect the interpretation of the analysis in that grid cell. For 

example, the model concentration gradients used for this analysis include emissions for 

an overestimated emissions category, “Industrial Natural Gas Combustion 

(Unspecified)” (EIC 5099501100000). Localized emission changes in this sub-region 
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would affect the model concentration field that is used by MATS to interpolate the ozone 

concentration spatial fields. As noted in Section 3.4.3 of USEPA’s modeling guidance, 

the spatial field in this particular sub-area of the domain is likely not certain enough to 

draw conclusions. Additionally, the Arvin monitor has already been relocated slightly 

north and west from the location that was used in this analysis. The new location 

provides slightly better coverage to the west. 

 
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1) Interpolate base year ambient data to create a set of spatial fields. 

Figure 1. Interpolated baseline concentration (DV) from MATS. 

 
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2) Adjust the spatial fields using gridded model output gradients (base year 

values). 

Figure 2. Interpolated gradient adjusted baseline concentration from MATS. 

 
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3) Apply gridded model RRFs to the gradient adjusted spatial fields. 

Figure 3. Relative Response Factor from MATS. 

 
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4) Determine if any unmonitored areas are predicted to have higher 

concentrations than those at existing monitored areas. 

Figure 4. Interpolated gradient adjusted future year concentration from MATS.