1 Reconciling Fugitive Dust Emissions with Ambient Measurements: Along the Unpaved Road

The discrepancy between PM10 emissions inventories for fugitive dust and the fraction of crustal material found on ambient filters remains a concern for the air quality community. This paper describes recently completed work that sheds light on some issues related to the transportable fraction vs. the deposited fraction of PM10 dust from an unpaved road. There are at least three different dispersion and deposition regimes downwind of an unpaved road located in a rural area. In the first regime that extends a few meters in the downwind direction, the “impact zone”, the dust plume is introduced to the surrounding landscape. Depending on the ground cover, the plume may be thinned when particles collect on available surfaces. This process more closely resembles filtration than dry deposition in the classic sense. In the second regime stretching hundreds of meters downwind, the “near-source ” region, the plume is rapidly expanding in the vertical direction. The concentration of dust particles is reduced due to both dispersion and deposition. In the third regime, the “far downwind ” region, the concentration profile is nearly invariant with height, except at the ground where particles continue to be removed by dry deposition. These three regions are examined in the context of a recently completed field study, a Gaussian dispersion and deposition model, and a proposed Box Model. Field data indicate that the fraction of PM10 dust particles deposited in the “near-source ” region may be small for conditions of the arid southwestern United States. Results are 2 compared with other studies where measurable removal of PM10 was observed within a few hundred meters of the source