Expanding the horizons of the Visibility Assessment Scoping Model (VASM)

Abstract

INTRODUCTIONThe Visibility Assessment Scoping Model (VASM) has successfully simulated seasonal distributions of hourly visual impairment, or haziness, that compare favorably with observations for both relatively polluted eastern sites and relatively pristine western sites in the United States (Shannon et a/., 1997). The VASM approach combines regional modeling of seasonal mean concentrations of key anthropogenic particle species with Monte Carlo statistical techniques using past monitoring data and relative humidity (RH) climatology. The Monte Carlo method requires as input the seasonal geometric mean and standard deviation of daily concentrations of six particle species: sulfate, nitrate, organic carbon, elemental carbon, and dust in the fine particle mode (aerodynamic diameter less than 2.5 pm), and dust in the coarse particle mode (diameter between 2.5 and 10 pm). (NOTE: the coarse-particle mode is treated as dust regardless of actual speciation.) In addition, the Monte Carlo method uses inter-species seasonal correlations of daily concentrations, RH climatology (seasonal means, standard deviations of daily averages, and typical diurnal patterns), and predicted future mean seasonal concentrations for one or more particle species. The VASM technique thus far has been applied in integrated assessment only where fine-particle monitoring and speciation measurements have taken place for a time sufficient for reliable statistical input into the Monte Carlo process (Henrion et al., 1997;Shannon, 1998). Thus, haziness changes resulting from emission changes have been quantitatively evaluated only at those monitoring locations. For use in comprehensive cost-benefit analyses consistent with estimation of the total costs of emissions controls, it is desirable to extend the estimation of changes in haziness to the entire region of interest. APPROACHThe simplest approach to extending the results of VASM analyses to benefits analysis for large regions (the eastern United States, for example) would be to assume that the mean or median decrease in haziness at the selected sites examined can be applied to the entire region. The focus is on a single emission species (SO*). The emissions policies examined are quite broad in their effects on emission patterns (and thus on the regional sulfate pattern). The simple averaging approach becomes more problematic when multiple pollutant species are being more tightly controlled or when the emissions changes are more focused spatially.In any case, to demonstrate that such a simplified approach is adequate, one would need to compare it with a more spatially complete assessment of improvements in visibility.Although the monitoring of haziness and fine-particle concentrations is likely to increase because of new and proposed regulations about PM2.5 concentrations and regional haze, considerable time must pass before suitable quality controlled data become available for use in assessment. Thus, some form of interpolation or extrapolation of the data from monitoring sites is likely to remain...