Gas-to-particle conversion of sulfur in power plant plumes—III. Parameterization of plumecloud interactions

Abstract

Long-range plume transport-transformation models generally require bulk rates of chemical transformations averaged over plume segments whose spatial-temporal scales exceed those of most clouds. There arises, therefore, the need to average out over the entire plume segment the effect of processes occurring quite rapidly in random and discrete clouds which occupy only parts of the segment. An attempt is made in this paper to quantify the relationship between the fast in-cloud sulfate formation rate ( K t L ) and the slower bulk sulfate formation rate by liquid phase mechanisms in an entire plume segment ( K t L ). The ratio K t L K t L is interpreted statistically in terms of the probability and extent of plume-cloud interactions within any plume segment. A quantitative expression is derived relating this plume-cloud interaction function to plume dimensions and to specific cloud-related quantities. A short-term and a long-term application of this plume-cloud interaction parameterization are attempted, respectively, for a case study of plume measurements on a summer day in the U.S. Midwest, and for statistically averaged summer and winter days in the same region. It is shown that K t L K t L is likely to be less than 0.1 during much of the long-range transport of a plume. On an average summer day, plume-cloud interaction is most likely in the midday hours when low-level cumulus formation is quite common. For the case study day, which is shown to be quite representative of the average summer day in the region, it is estimated that the in-cloud sulfate formation rate ( K t L ), averaged for the hours 1000–1700, was about (12 ± 6)% h −1. For the statistically average seasonal days, the regional cloud data needed to evaluate the interaction parameter are identified. The adequacy and reliability of the cloud data base of the NWS Surface Weather Observation Network are assessed in this context, and the probability and extent of plume-cloud interactions are evaluated to the extent permitted by the available data base.