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.