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

State	Monthly MDA8	Delta (Base-BG)
12-km Domain	Plot	Plot
Arizona	Plot	Plot
California	Plot	Plot
Colorado	Plot	Plot
Idaho	Plot	Plot
Montana	Plot	Plot
New Mexico	Plot	Plot
Nevada	Plot	Plot
Oregon	Plot	Plot
Utah	Plot	Plot
Washington	Plot	Plot
Wyoming	Plot	Plot

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

State	AQS	CASTNet
Colorado	Plot	Plot
Utah	Plot	Plot
Wyoming	Plot	Plot

Table 3.

Month	O3	NOx	CO	PM2.5
February	Base Diff	Base Diff	Base Diff	Base Diff

Table 3.

Month
FebruaryMarch	O3Base Diff	NOXBase Diff	COBase Diff	PM2.5Base Diff
April	Base Diff	Base Diff	Base Diff	Base Diff
May	Base Diff	Base Diff	Base Diff	Base Diff
June	Base Diff	Base Diff	Base Diff	Base Diff
July	Base Diff	Base Diff	Base Diff	Base Diff
August	Base Diff	Base Diff	Base Diff	Base Diff
September	Base Diff	Base Diff	Base Diff	Base Diff
October	Base Diff	Base Diff	Base Diff	Base Diff
November	Base Diff	Base Diff	Base Diff	Base Diff
December	Base Diff	Base Diff	Base Diff	Base Diff