MODELING VISIBILITY FOR ASSESSMENT

The visibility assessment scoping model (VASM), a Monte Carlo techniqu e for simulating and assessing regional-scale impairment of visibility , is described. The input requirements of the method for each receptor are modest: geometric means and standard deviations describing the wi thin-season distributions of daily concentration of each of the six pr incipal particle species; matrices of interspecies partial correlation s of daily concentrations; relative humidity (RH) climatology; and sea sonal mean concentrations for each particle species. The lognormal dis tribution parameters and the correlation matrices are abstracted from past observations. For those particle species with emission precursors that are varied in the policy scenarios, the concentration means are obtained from a regional atmospheric transport model, the means of the other particle species are assumed to be statistically similar to pre vious observations. Seasonal distributions of haze intensity are produ ced from sets of short-term Monte Carlo realizations of concentrations and RH and appropriate formulae for light extinction. VASM simulation s for rural sites in both the relatively hazy eastern United States an d the more pristine western United States compare favorably with trans missometer observations. Application of VASM, in concert with utility SO, emission projections and atmospheric transport modeling, indicates that the median seasonal reduction of haziness expected at Shenandoah by the year 2010, relative to 1990 conditions, will range from 1.3 de civiews in winter to 2.2 deciviews (dv) in summer. If calculated as vi sual range, the corresponding improvements are 14 and 24%, respectivel y. (C) 1997 Elsevier Science Ltd.