Regional measurements and modeling of windblown agricultural dust:: The Columbia Plateau PM10 Program

The Columbia Plateau PM,, Program [CP3] is a multi-investigator study of windblown dust in the Pacific Northwest with an emphasis upon the role of agricultural lands in regional dust storms. Ambient measurements of PM,, within the source areas of the central basin of Washington during several autumn dust periods show that typical background concentrations [nonwind-event periods] decrease from an average of 34 mu g m(-3) in early fall to 10 mu g m(-3) in late fall. During wind events, ambient concentrations at downwind urban receptors can exceed 500 mu g m(3) on an hourly basis, with 24 hour averaged values as high as 300 mu g m(-3). Particle counts during wind events are enhanced by as much as a factor of 5 for particle sizes greater than 5 mu m, and also for sizes between 1 and 5 mu m compared to nonwindy periods. Analysis of the synoptic conditions which exist during these dust storms showed a common situation where a surface low is moving rapidly across British Columbia while a surface high is positioned in the Great Basin of Nevada. A regional windblown dust air quality model, developed for the CP3 study, predicts large dust plumes stretching across eastern Washington with maximum concentrations in the source regions exceeding 10,000 mu g m(-3). Total mass emissions during a storm are estimated to equal 100 Gg dy(-1), which represents about 1% of recent estimates of the global annual dust emission rate. In the initial applications of the model, available PM10 observations are used to calibrate the dust emission algorithm. Changes in the dust constant for two modeled events are consistent with changes in soil cover and accumulated precipitation between an early fall event and a late fall event. The estimated fluxes are in a range similar to those in the literature but appear to be much less than estimated from global modeling of recently disturbed soils.