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The reduction of divalent gaseous mercury (Hg-II) to elemental gaseous mercury (Hg-0) in a commercial coal-fired power plant (CFPP) exhaust plume was investigated by simultaneous measurement in-stack and in-plume as part of a collaborative study among the U.S. EPA, EPRI, EERC, and Southern Company. In-stack continuous emission monitoring data were used to establish the CFPP's real-time mercury speciation and plume dilution tracer species (SO2, NOX) emission rates, and an airship was utilized as an airborne sampling platform to maintain static position with respect to the exhaust plume centerline for semicontinuous measurement of target species. Varying levels of Hg-II concentration (2.39-3.90 mu g m(-3)) and percent abundance (similar to 87-99%) in flue gas and in-plume reduction were observed. The existence and magnitude of Hg-II reduction to Hg-0 (0-55%) observed varied with respect to the types and relative amounts of coals combusted, suggesting that exhaust plume reduction occurring downwind of the CFPP is influenced by coal chemical composition and characteristics.