Abstract. On April 15 and 19, 1998, two intense dust storms were generated over the Gobi desert by springtime low-pressure systems descending from the northwest. The windblown dust was detected and its evolution followed by its yellow color on SeaWiFS satellite images, routine surface-based monitoring, and through serendipitous observations. The April 15 dust cloud was recirculating, and it was removed by a precipitating weather system over east Asia

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Aerosol data from the Interagency Monitoring of Protected Visual Environments (IMPROVE) network, air mass backward trajectories, land use maps, soil characteristics maps, diagnostic ratios of elemental composition, and multivariate linear regression were utilized as part of a semiquantitative analysis. The purpose of the analysis was to determine the types of dust‐causing events that contribute to low visibility at a given site when the sum of extinction from coarse mass (CM) and fine soil (FS) was larger than any other aerosol component and the reconstructed aerosol extinction coefficient was among the 20% highest (calculated on a calendar year basis) for that site. For these “worst dust days,” the above tools were used to ascribe the cause of low visibility to one of the following types of events: (1) transcontinental transport of dust originating from Asia; (2) windblown dust events from sources located nearby the site and; (3) transport of windblown dust from sources upwind of the site. Depending on the weight of evidence, a low or high level of confidence was associated with the assignment of one of these three events. Absence of convincing evidence resulted in ascribing the worst dust day to “undetermined events.” Of the 610 worst dust days over the 2001–2003 period, 51% were associated with one of the three event types with high confidence and an additional 30% were accounted for with low confidence. Of the 496 worst dust days associated with an event (either low or high confidence), Asian dust was the assigned event on 55 days (for 2001–2002), locally generated windblown dust on 201 days, and transport from upwind source areas susceptible to wind erosion on 240 days. Events associated with windblown episodes from source areas in the United States and Mexico exhibited the highest dust concentrations. Asian dust events were associated with lower dust concentrations and a larger FS‐to‐CM ratio. Some variations between Asian dust and continental North American dust were observed in organic matter (OMC), black carbon (LAC), and nitrate (NO3−) content. None of the tools used in this study was adequate for identifying events associated with mechanically released dust by anthropogenic activities including, agriculture, construction and motor vehicle travel on paved and unpaved roads. Some of the worst dust days may have been caused by these types of activities, especially in central Arizona and northern and Southern California, where the fraction of undetermined events was higher than in other regions within the western United States. All in all, the methods and results of this study can help improve the performance of large‐scale dust emission models and provide insight into the distribution of the types of events that cause dust resultant haze in relatively remote areas of the western United States.

This paper reports the first empirical estimate of particle emissions from unpaved shoulders along paved roads. 1Its objectives are to develop and demonstrate an emission rate measurement methodology that can be applied in different areas; identify the mechanisms that suspend dust from unpaved shoulders and the observables related to this suspension process; and quantify PM 10 mass emissions in the form of an emission rate. To achieve these objectives, fast-response observations from nephelometers and a sonic anemometer were used to characterize shortlived dust plumes generated by passing vehicles. In addition, detailed soil surface measurements determined the mechanical properties of the shoulder surfaces.Large traffic-induced turbulence events that led to significant dust entrainment were almost exclusively caused by "large" vehicles such as trucks, semis, and vehicles pulling trailers, all traveling 50-65 mph. PM 10 emission rates for these large, fast-traveling vehicles were determined to be 8 ± 4 grams per vehicle kilometer traveled under dry conditions. Emissions due to smaller vehicles such as cars, vans, and sport utility vehicles were negligible IMPLICATIONS Particle emissions from unpaved shoulders along paved roads may be the source of a significant fraction of ambient PM 10 in some non-attainment areas. However, relatively little is known about entrainment mechanisms and emission rates needed to calculate source strengths. This knowledge is essential to determine necessity and costeffectiveness of particulate suppression measures for unpaved shoulders. This work identifies entrainment mechanisms, develops a new emission rate measurement methodology, and uses it to quantify particle emission rates from unpaved shoulders along paved roads. These results will be useful for future State Implementation Plans to attain particulate standards.for normal on-road driving. These results indicate that the majority of PM 10 emissions from unpaved shoulders is caused by relatively few vehicles.