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### Unmonitored Area Analysis for Ozone in the San Joaq uin Valley

Overview

An unmonitored area analysis (UAA) is required by t he United States Environmental Protection Agency (USEPA). The UAA is intended to identify unmonitored areas where future year design values may be greater than the N AAQS. Since a simple spatial interpolation of observational data cannot identify unmonitored areas with concentrations higher than those measured at monito rs, the USEPA recommends a UAA that combines both observations and modeling. We followed USEPA’s methodology in this analysis, and found that the mo nitoring network within the San Joaquin Valley (SJV) adequately captures elevated o zone levels within a 15-km radius
of the monitors.

Methodology

Following USEPA guidance, this analysis used the Mo deled Attainment Test Software (MATS), developed by the USEPA, in the UAA. For th is 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 monito r 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 outpu t 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 fiel d for the respective modeled day. These data were then input into MATS to generate a single gradient- adjusted DV spatial field.2

3) Apply gridded model RRFs to the gradient adjusted s patial fields

. In addition to the DVs and base year simulated ozon e, MATS requires future year simulated ozone. The same methodology of calc ulating the daily maximum 8-hr ozone for the base year (Step 2) was a lso used in this step to calculate a total of ten daily spatial gradient dis tributions for the future year. MATS uses the base year and future year simulated o zone in each grid cell to calculate an average RRF for each grid cell. The f uture 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 are as

. Results from Step 3 were analyzed to determine if a ny unmonitored regions may have concentrations higher than those observed at existing monitoring areas based on the future year gradient adjusted co ncentration.

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 consist ent with the 15km radius of representativeness for ambient monitoring stations that is used in the modeled attainment test. These square ‘monitored area’ reg ions of grid cells are illustrated on the figures with a dot in the center cell to repres ent the grid cell containing each DV
station.

Analysis of the gradient adjusted future year conce ntrations (Figure 4) shows that the monitoring network in the SJV provides adequate cov erage 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 spati al field in Figure 4 shows elevated values occurring in one grid cell immediately to th e south and west of several ‘monitored areas’ in the southern SJV near Bakersfield. Secti on 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 whe ther they are likely to exist in the ambient air or whether they may be caused by an err or 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 o f the analysis in that grid cell. For example, the model concentration gradients used for this analysis include emissions for an overestimated emissions category, “Industrial Na tural Gas Combustion (Unspecified)” (EIC 5099501100000). Localized emis sion changes in this sub-region3

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 th e 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 t his analysis. The new location provides slightly better coverage to the west.4

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

Figure 1 . Interpolated baseline concentration (DV) from MAT S.5

2) Adjust the spatial fields using gridded model ou tput gradients (base year values).

Figure 2 . Interpolated gradient adjusted baseline concentra tion from MATS.6

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

Figure 3 . Relative Response Factor from MATS.7

4) Determine if any unmonitored areas are predicted to have higher concentrations than those at existing monitored are as.

Figure 4 . Interpolated gradient adjusted future year concen tration from MATS.