Application of a New Land-Surface, Dry Deposition, and PBL Model in the Models-3 Community Multi-Scale Air Quality (CMAQ) Model System

Abstract

The U.S. EPA has developed a new comprehensive air quality modeling system, known as the Models-3 Community Multi-scale Air Quality (CMAQ) model (Byun and Ching, 1999). The CMAQ system includes a comprehensive emissions processor, a Chemical Transport Model (CTM), and a meteorology model. The community modeling approach aims to provide a focal point for diverse model development research which will lead to many alternative algorithms and model components that can be inter-compared and evaluated. An early example of this process is the development of a more advanced surface exchange, planetary boundary layer (PBL), and dry deposition model and its incorporation into the CMAQ system. Accurate simulation of air quality depends on realistic modeling of land surface and PBL processes. Quantities that exert first-order control on trace chemical concentrations and photochemistry include PBL height, temperature, wind speed, and dry deposition velocity. These quantities are all directly related to air-surface exchange processes of heat, moisture, momentum, and trace chemical species. In addition, cloud cover, which is important for photolysis rates, is greatly influenced by surface flux and PBL processes. For these reasons it is especially critical to apply realistic techniques for land-surface and PBL modeling within an air quality system. Like most air quality modeling systems, CMAQ divides the treatment of meteorological and chemical/transport processes into separate models run sequentially. A potential drawback to this approach is that it creates the illusion that these processes are minimally interdependent and that any meteorology model with a good reputation is adequate for air quality work. However, most mesoscale meteorology models are developed for operational weather forecasting and meteorological research. These foci do not emphasize all the same critical capabilities as air quality applications. Conversely, CTMs are often developed to accept basic meteorological inputs from a variety of sources