Remote Sensing of Sulfates in a Dry Power Plant Plume: a Feasibility Study.

It is proposed that the relative abundances of acid sulfates and neutral sulfates in a power plant plume can be remotely determined by infrared backscattering spectroscopy. Assessing the feasibility of this proposed technique requires an understanding of the processes leading to sulfate particle formation in plumes, in some instances sulfuric acid droplets, a precursor to acid rain, and in others, neutral sulfates such as ammonium sulfate. In addition, the problem of predicting the infrared backscattering properties of plume particles must be formulated in terms of particle shape, composition and size distribution. Small, irregularly shaped particles of ammonium sulfate show strong backscattering cross section resonances in the infrared. A method is developed whereby the backscattering cross sections of irregularly shaped particles are approximated by averaging over a range of electromagnetic microstates, in this instance, randomly oriented particles that are continuously distributed in ellipsoidal shape (CDE). Other particles, such as sulfuric acid droplets, show no cross section resonances in the infrared. The absence of backscattering resonances for acid sulfate particles and their occurrence for neutral sulfate particles suggest the possibility that characteristic infrared backscattering spectra can be used to infer particle composition. A clear understanding of the chemical and physical process that determine the nature of sulfate formed in a plume--acid or neutral--has yet to be formulated. In this work, a one-dimensional model of the physical and chemical processes in dry power plant plume is developed, providing information on the composition and volume of plume sulfates formed by gas-to-particle conversion. It is found that in the presence of substantial atmospheric ammonia, the favored sulfate product is neutral ammonium sulfate. As ammonia is depleted from the plume, the formation of sulfuric acid solution dominates the gas-to-particles process. The plume model simulates sulfate formation on background particles for several sets of initial conditions. Subsequent calculations of backscatter spectra show that significant spectral features are discernible in the frequency range of a tunable CO(,2) lidar. These spectral features are found to provide strong indications of the presence of dry neutral sulfate particles, or of acid sulfate droplets, in an evolving plume.