Sources: Code of Federal Regulations, Title 40, Subpart 52.21; Federal Register, Volume 75, page 64864.

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6 4.11.3 Update to Section 4.11.2.4 of the Draft Solar PEIS: Visibility Protection

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8• A discussion of existing visibility conditions resulting from fine soil and

25Dust sources vary greatly spatially and temporally but play a more important role in

26visibility degradation in the arid parts of the western United States than in the eastern United

27States due to the desert environment. Windblown dust, both local and regional, has been found to

1 be a significant contributor to visibility impairment in the six-state study area. An attribution

2study found that on the majority of these “worst dust days,” the dust event could largely be

3attributed to both local and regionally transported dust sources with some level of confidence

4 (dust from Asian dust events made up a much smaller contribution) (Kavouras et al. 2009). Over

5the life of a solar facility, combustion-related emissions from the engine exhaust from heavy

6equipment and vehicles would be sizable during the construction phase and minimal during the

7operation phase. Fugitive dust from wind erosion and anthropogenic activities, including

8 agriculture, construction, grazing, mining, and vehicle traffic on paved and unpaved roads would 9 be a major concern in the arid desert environment where major solar development would occur. 10

11Figure 4.11-6 based on aerosol measurements taken at IMPROVE and Chemical

12Speciation Network (CSN) sites shows the impact of fugitive dust on visibility. The IMPROVE

13sites, governed by a steering committee composed of representatives from federal and regional

14and state organizations, are mostly located in remote/rural settings, while CSN sites, operated by

15the EPA, are located in urban/suburban settings.

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17 Figure 4.11-6(a) presents annual mean fine soil (FS) extinction coefficient (bext)1 spatial 18 patterns for 2005–2008. These patterns are the same as the mass concentration patterns (not

19 shown here) (Hand et al. 2011). In general, the southwestern states (in particular, Arizona,

20 southeastern California, and southern Nevada) have higher FS bext, but their values are relatively 21 low. The highest bext of 4.41 Mm–1 (corresponding to an annual average concentration of

22 4.41 µg/m3) occurred in Douglas, Arizona, which is adjacent to the U.S.–Mexican border and 23 has a semi-arid climate with a history of mining. The largest percent contributions to PM2.5 24 aerosol bext from FS occurred in about half of the six-state study area, as shown in

25 Figure 4.11-6(b). Percent contributions of FS were highest at 18.4% in Douglas, Arizona, but FS 26 was not a major contributor to PM2.5 aerosol bext at urban CSN sites (less than 10%).

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28As shown in Figure 4.11-7(a), the highest bext of 12.67 Mm–1 (corresponding to an

29annual average concentration of 21.12 µg/m3) from CM occurred at Douglas, Arizona, which

30was most likely associated with mineral dust (Hand et al. 2011). CM bext values higher than

3110 Mm–1 occurred in southern Arizona and Fresno, California. As shown in Figure 4.11-7(b), the

32annual mean fractional contributions of bext of CM to total aerosol bext was higher (20% or

33higher) in about two-thirds of Arizona and south-central New Mexico, with a peak of about

3434.5% in Douglas, Arizona. The contributions of CM to total aerosol bext were typically more

35than 10% in most of six-state study area. (CM is not measured by the CSN network.)

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1The extinction coefficient (bext) represents the ability of the atmosphere to scatter and absorb light primarily by particles and, to some extent, by gases, and has unit of inverse length (inverse megameters, Mm-1). The bext is related to visual range and deciview (a haziness index designed to be linear with respect to human perception of

visibility, analogous to the decibel scale in acoustics). A higher bext corresponds to a lower visual range and higher deciview values.

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2 (a)

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