Sulfate formation in point source plumes: A review of recent field studies

Data in twelve power plant and smelter plumes reported by eight different organizations have been reviewed in an attempt to establish the existence of recognizable patterns concerning the extent and rates of sulfate formation. The data reviewed here were collected in Australia, Canada and the U.S., during warm as well as cold seasons, and during day and night. The primary variables examined and compared are S_p/S_T, the paniculate fraction of excess sulfur sampled in the plume, and the corresponding plume age. The variation of the extent of sulfate formation (S_p/S_T) is also examined in terms of plume exposure to solar radiation dose during transport.In spite of wide geographical, seasonal, background and source variations, a distinct difference is observed in day and night sulfate formation. During the daytime, there exists a substantial variability in sulfate formation rates (typically 0–5% h⁻¹) and extent (typically up to 25%) for any given physical plume age. This variability is significantly less for plumes with similar exposure to sunlight dose, showing that environmental factors linked to solar radiation strongly influence the rate and extent of sulfate formation in point-source plumes. The diurnal and seasonal co-variance of all insolation-related factors makes it difficult to distinguish the roles of specific environmental factors or conversion mechanisms. Night-time data show that S_p/S_T seldom exceeds 3% even after 5–10 h of plume transport.A surprising result is observed related to geographical variability of the conversion rate. Except in the case of the data of the Navajo coal-fired power plant plume collected as part of the VISTTA study, all other data fall within an identifiable envelope ranging over 15–30% sulfate formation (S_p/S_T) corresponding to solar dose equivalent to one July day in St. Louis. The Navajo data yield about an order of magnitude lower conversion. Evidently, factors other than those related to insolation are also important.