Hydrodynamic Mesoscale Modeling Of Atmospheric Transport And Pollutant Deposition In The Vicinity Of A Lake (michigan).

Abstract

Pollutants emitted into the atmosphere may be transported a few tens of kilometers or thousands of kilometers before being removed from the atmosphere. While mathematical models have been developed to describe pollutant transport and deposition over distances up to 50 km from their sources (Gaussian plume models), and distances over hundreds of kilometers (Long-Range Transport Models), there are few available tools for predicting their impact in between. This study identifies two meteorological situations common to lakeshore environments, where pollutants can be vertically redistributed resulting in concentration and deposition fields, different from these expected for homogeneous surfaces. The first situation studied was characteristic of summertime conditions, driven by a synoptic weather system with air moving from land, over a cooler lake surface. This flow is shown to become highly modified by a mesoscale meteorological phenomenon known as lake-breeze. The second situation was for wintertime flow driven by a similar synoptic pressure gradient with air moving from land over a relatively warmer lake. In order to study the effects of lakes on pollutant deposition, a hydrodynamic model was developed which included terms for pollutant transport, dispersion and deposition. The model has a variable grid, and is capable in operating in two (x,z) or three dimensions (x,y,z). The modeling domain was centered around a lake (Lake Michigan) to investigate the potential impact of nearby industrial sources. The model is capable of forecasting the local wind, temperature, and pressure patterns as well as evolution of the atmospheric planetary boundary layer parameters. These parameters are characteristic of the local atmospheric stability conditions which, along with the model-generated wind fields, play an important role in the determination of the spatial and temporal distribution of the atmospheric pollutants. It was found that the presence of a lake in the summertime limits pollutant deposition over its length, due to stabilization and plume entrainment in the lake-breeze. Also the mean height of the resultant plume was predicted to be higher with the influence of a lake-breeze than without. On the other hand, deposition was enhanced over the upwind land due to the lake-breeze effect, and the downwind shore due to enhanced mixing. In the wintertime, the presence of a warm lake caused deposition near the upwind lakeshore greater than predicted without the lake. These limited applications illustrate that dry deposition around a source located by a large lake may be significantly different from that expected around a source located in simple terrain.