Comparison of ISC2 Dry Deposition Estimates Based on Current and Proposed Deposition Algorithms

摘要

The ability to accurately estimate deposition of particulate matter is of special concern in assessing environmental impacts from a variety of sources including Superfund sites, municipal waste incinerators, and surface coal mines. The current deposition algorithm in ISC2 simulates deposition as the movement of particles toward the surface by the combined processes of atmospheric turbulence and gravitational settling. The method was developed for applications to large particles dominated by gravitational settling; these are typically particles greater than 20 micrometers diameter. The current algorithm was not intended for use with particles smaller than about 20 micrometers which are often of concern in air toxics assessments. In light of this limitation, a new deposition algorithm is being considered for use in ISC2. The proposed algorithm is intended to simulate processes important over the entire range of significant particle sizes (0.1 to 100 micrometers). The proposed algorithm employs a deposition velocity based on a resistance model. In this approach, deposition flux is calculated as the product of the near-surface air concentration and the deposition velocity. The latter is computed as the inverse sum of the aerodynamic layer and deposition layer resistances, plus gravitational settling. The new deposition algorithm has been tested within the framework of the ISC2 model and comparisons of deposition estimates using the old (current) and new deposition algorithms have been made for a range of source types and particulate emission scenarios. Similar comparisons have been made of particulate concentration estimates as affected by the old and new deposition algorithms.