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Background Air Quality Modeling Results

CAMx Base 2011 US Background Simulation

We ran a CAMx sensitivity simulation derived from the 3SAQS 2011 Base version A CAMx (CAMx_3SAQS_Base11a) configuration to estimate background air quality in the U.S. The CAMx 2011 U.S. Background simulation (CAMx_3SAQS_Base11a_USBG) is an emissions sensitivity in which all anthropogenic emissions sources in the U.S. are removed. The only emissions included in the USBG simulation include:

  • Canadian and Mexican anthropogenic sources
  • Domain-wide biogenic sources
  • Domain-wide wildfires (prescribed and ag fires are excluded)
  • Domain-wide windblown dust
  • Seasalt
  • Lightning NOx

Table 1 presents summary results comparing monthly averaged daily maximum 8-hour average O3 (MDA8) from simulations Base11a and Base11a_USGB. Bar plots compare MDA8 averaged for each month and annually across all of the AQS monitors in the 12-km domain and within each of the Western states. The delta plots in Figure 1 show the monthly and annual differences in MDA8 between the Base11a and Base11a_USBG simulations.

Table 1. AQS Base and Background Monthly Mean MDA8

StateMonthly MDA8Delta (Base-BG)
12-km DomainPlotPlot
New MexicoPlotPlot
Table 2 shows box and whisker plots of hourly O3 at monitors in the states of Colorado, Utah, and Wyoming. The box and whisker plots show the monthly median, quartiles, 5th percentile, 95th percentile, and data outliers. These plots compare the monthly data distributions between the measurements, the Base11a, and Base11a_USBG simulations at all of the network sites within each state. An interesting feature of these plots with respect to the USBG simulation is the upper tail (75th and 95th percentile) of the data distribution. These tails illustrate that although the median background O3 values are estimated are around 40-50 ppb, there are days in which the background O3 may be as high as 60-70 ppb.

Table 2. AQS and CASTNet Hourly O3 Distributions by Month

Table 3.

FebruaryBase DiffBase DiffBase DiffBase Diff
Table 3.

FebruaryMarchO3Base DiffNOXBase DiffCOBase DiffPM2.5Base Diff
AprilBase DiffBase DiffBase DiffBase Diff
MayBase DiffBase DiffBase DiffBase Diff
JuneBase DiffBase DiffBase DiffBase Diff
JulyBase DiffBase DiffBase DiffBase Diff
AugustBase DiffBase DiffBase DiffBase Diff
SeptemberBase DiffBase DiffBase DiffBase Diff
OctoberBase DiffBase DiffBase DiffBase Diff
NovemberBase DiffBase DiffBase DiffBase Diff
DecemberBase DiffBase DiffBase DiffBase Diff