Modeling visibility for assessment

Thevisibility assessment scoping model (VASM), a Monte Carlo technique for simulatinga 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 within-season distributions of daily concentration of each of the six principal particle species; matrices of interspecies partial correlations of daily concentrations; relative humidity (RH) climatology; and seasonal mean concentrations for each particle species. The lognormal distribution 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 previous observations. Seasonal distributions of haze intensity are produced from sets of short-term Monte Carlo realizations of concentrations and RH and appropriate formulae for light extinction. VASM simulations for rural sites in both the relatively hazy eastern United States and the more pristine western United States compare favorably with transmissometer observations. Application of VASM, in concert with utility SO2 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 deciviews in winter to 2.2 deciviews (dv) in summer. If calculated as visual range, the corresponding improvements are 14 and 24%, respectively.