a Model of the Formation of Acid in Coal-Fired Power Plant Plumes.

A one-dimensional model was constructed to predict the growth and acidity of droplets in the stack plumes of coal-fired power plants. Standard cloud physics droplet growth equations were applied to the droplet size section of the model and a non-catalytic oxidation of SO(,2) in an aqueous solution was used to predict the hydrogen ion concentration. Sensitivity tests were performed on the model and it was found that any factor which increased droplet growth (relative humidity, increase of solute in the droplet, for example) increased the pH of the droplet. The amount of gaseous ammonia in the atmosphere also had an effect on the pH. A large ammonia concentration increased the pH, while a small amount (or absence) of ammonia caused a decrease. The model was checked to see if a dispersing plume would change the predicted values. It was found that under the constraints of the model dispersion had small effect. Dilution of the plume by the outside environment was another aspect that was applied to the model. Again, it was found that the model adjusted quickly (within .2 pH in 25 seconds) to the outside atmosphere. The model was applied to conditions simulating stratus and cumulus clouds. It was found that the pH decreased early in the model run (near cloud base) and then increased. This is consistent with predictions of pH in the stack plume. Two "special cases" were modeled. A low pH situation was computed where conditions were specified which kept droplet growth at a minimum in the presence of low ammonia amounts. As expected, this produced a very low pH (in the neighborhood of 1.1). Another situation was modeled where the stack plume merged with the cooling tower plume about 500 m downwind from the source. Here droplet growth and pH increased sharply when the moist conditions of the cooling tower plume were applied.