AIRCRAFT MEASUREMENTS AND MATHEMATICAL MODELING OF THE REMOVAL OF GASEOUS AIR POLLUTANTS AT NATURAL AIR-LAND AND AIR-WATER INTERFACES.

摘要

Natural removal mechanisms play an important role in determining the ultimate fate of gaseous air pollutants emitted into the earth's atmosphere. One of the more important natural removal mechanisms is absorption at the earth's surface. In this work, absorption at the earth's surface has been investigated by field measurements, theoretical developments, and comparative interpretation of the measurements and mathematical models.;The field program consisted of aircraft measurements of the concentration profiles of SO(,2) in a plume from a coal-fired power plant on the southern shore of Lake Erie. The measurements were restricted to 20 kilometers from the source. These measurements were conducted in an attempt to differentiate removal at air-land and air-water interfaces. There were no apparent differences is measured concentration profiles obtained over land when compared with profiles obtained over water. Therefore, nothing quantitative could be deduced about the surface flux from the aircraft measurement alone.;The experimental program was then coupled with a theoretical study of surface removal. Models were derived for surface removal based on the concept of the mass transfer coefficient. The removal process was divided into subprocesses, each characterized by an individual mass transfer coefficient. By making appropriate assumptions, the individual mass transfer coefficients could be combined into an overall mass transfer coefficient for the overall removal process. Calculations based on several types of surface boundaries under typical meteorological conditions showed that the overall mass transfer coefficient for soil, water, and short vegetation was approximately the same and ranged from .003 to .008 m/s, while that for a forest was three to four times greater. The large forest coefficient was primarily attributed to its large surface area.;The surface removal models were used as boundary conditions in a numerical solution of a two-dimensional form of the atmospheric transport equation. The calculations were based on an elevated source with an effective stack height of 150 m. Under neutral conditions, a forest can remove about 8.5 percent of the material from a plume at 50 kilometers downwind of a source as compared to 4.8 percent for the other surfaces studied. The 50 kilometer position represents a relatively short travel time, so that on a regional or global scale, the impact of surface removal can be quite important. A sensitivity study of the numerical model indicated that at least within 50 kilometers of the source surface removal was most sensitive to the wind velocity and atmospheric stability. At much further distances from the source, surface removal is equally sensitive to factors affecting the gas-phase and to factors affecting the absorbing media.;The two-dimensional model was extended to three dimensions in order to simulate the field measurements. For all of the experiments, the results indicated that surface removal was less than one percent within the measurement area. Simulations were also carried out to compare near ground releases with elevated releases. These results indicated that surface removal could be as much as 40 percent within 20 kilometers of the source for a near ground release. This was in general agreement with results reported in the literature.;Within the limitations of the available data, the results have shown that removal at air-water and air-vegetation interfaces can be characterized quantitatively by the models discussed in this work. However, for most of the available data concerning surface removal, concentrations in the absorbing medium are not measured and details of the absorbing medium required for calculation of the overall mass transfer coefficient are not reported. Further validation and refinement of the models will require experiments in which these data are measured and reported.