Studies of Emissions from Anthropogenic and Natural Dust Sources in the Western United States

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Dust is a significant component of visibility degradation in the western United States. As implied by the national goal defined in the Regional Haze Rule (RHR),1 only visibility impairments from man-made pollution must be prevented and/or remedied. However, dust sources, particularly fugitive dust sources, can result from both man-made and natural conditions. Historically, particulate matter (PM) and related visibility issues have focused on anthropogenic sources. Unlike ozone modeling, where biogenic (i.e., natural) sources of hydrocarbons are estimated and included in the modeling inventories (but not the reporting inventories), most PM and visibility modeling does not explicitly include “natural” sources of PM. Nor do standard reporting PM inventories discriminate between natural and anthropogenic sources of PM. As planners and policy-makers formulate their plans to improve visibility and PM air quality, there is an increasing need to discriminate between uncontrollable and controllable sources in emissions inventories to better assess PM control and visibility improvement strategies.

The Western Regional Air Partnership’s (WRAP) Dust Emission Joint Forum (DEJF) has developed a draft “dust” definition with broad criteria for categorizing dust emissions as natural or anthropogenic. Conceptually, dust emissions can be categorized as (1) purely anthropogenic, resulting from direct human activities, and directly subject to human management or control; (2) purely natural, generally spontaneous, and not subject to human management or control; or (3) natural sources that may be influenced by human actions. The WRAP’s draft dust definition is summarized in Table 1.2 Category 3 sources, that is, natural sources that may be anthropogenically influenced, are the most difficult to characterize with respect to partitioning emissions between anthropogenic and natural. For example, partitioning windblown emissions from grazed land requires an analysis of emissions with and without the anthropogenic
disturbance of grazing to soil and vegetation. In addition, Category 3 sources represent a significant portion of the
overall dust emissions in the western United States.

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