THE DEVELOPMENT OF A MODEL TO EXAMINE SOURCE-RECEPTOR RELATIONSHIPS FOR VISIBILITY ON THE COLORADO PLATEAU

This paper describes the development and application of the Visibility and Haze in the Western Atmosphere (VISHWA) model to understand the s ource-receptor relationships that govern chemical species relevant to visibility degradation in the western United States. The model was dev eloped as part of a project referred to as Visibility Assessment for R egional Emission Distributions (VARED), the objective of which is to e stimate the contributions of various geographical regions, compounds, and emission sources to light scattering and absorption by particles o n the Colorado Plateau. The VISHWA model is a modified version of a co mprehensive Eulerian model, known as the Acid Deposition and Oxidant M odel.(1) The modifications were designed to obtain the computational e fficiency required to simulate a one-year period at about 1/25th of re al time, and at the same time incorporate mechanistic features relevan t to realistic modeling of the fate and transport of visibility degrad ing species. The modifications included use of a condensed chemical me chanism; incorporation of reactions to simulate the formation of secon dary organic particles; and use of a semi-Lagrangian advection scheme to preserve concentration peaks during advection. The model was evalua ted with 1992 air quality data from Project MOHAVE (Measurements of Ha ze and Visual Effects) intensive experiments. An important conclusion of this evaluation is that aqueous-phase oxidation of SO2 to sulfate i n nonprecipitating clouds makes a significant contribution to observed sulfate levels during winter as well as summer. Model estimates of am bient sulfate for the winter intensive were within a factor of 2 of th e observations for 75% of the values. The corresponding statistic for the summer intensive was 90%. Model estimates of carbon were within a factor of 2 of the limited set of observations.