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| WRAP Dust Assessment
Assessment of the Principal Causes of Dust-Resultant Haze at IMPROVE Sites
in the Western United States
Background
Suspended dust is one of the principal causes of haze at Class I areas in the Western United States. The magnitude of the impact of dust on haze varies by region as well as by season. Dust is the principal component of haze on the 20% worst haze days of the year ("worst days" hereafter) most frequently in the southwestern United States. However, every state in the Western Regional Air Partnership (WRAP) contains an IMPROVE monitor that registers dust as the principal component of haze on a worst day. On a seasonal basis, dust has the greatest impact on the largest number of monitoring sites in the spring, followed by summer and fall. Sources of fugitive dust can vary greatly in spatial scale, time, location, and causes of emission. For example, paved and unpaved road dust emissions tend to follow the diurnal trends associated with motor vehicle traffic, with some additional dependence on seasonal occurrences such as snow and agricultural activities. Windblown dust emissions generally occur over larger spatial scales and the magnitude of dust emissions during these events can eclipse the comparatively smaller, but more regular road dust emissions. On a transcontinental scale, enormous, regional dust storms can be transported across continents and impact the entire WRAP region.
Methods
The study, presented here, utilized several methods to specifically identify the primary causes of dust measured in the WRAP region. The methodology focused on periods when dust was the principal contributor to haze during worst days (worst dust days, hereafter). That is, we only considered days that were among the 20% worst for the given calendar year - with regard to reconstructed extinction - when dust was the largest component of Bext. These tools were combined using a semi-quantitative approach to preliminarily determine the likely source of dust on a worst dust day at a given site. Due to limitation of the information and capabilities of the tools, the causes of some worst dust days were not determined with any confidence.
Using 2001-2003 data from IMRPOVE (and some protocol) monitors in the WRAP regions, each worst dust day was associated with one of these events:
 | Transcontinental transport of large scale events from Asia |
| Windblown dust events |
| Transport of windblown dust from sources upwind (i.e. not from immediate vicinity of site) |
| Undetermined Events. |
Further specification if windblown and upwind transport events appear to be regional in nature based on scale of meteorological phenomenon causing dust and number of sites affected.
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Page updated 20 Feb 2006
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