Gulf Coast Mercury Research
April 25, 2007
The area surrounding ARL’s mercury site in the Grand Bay National Estuarine Research Reserve in Mississippi underwent a prescribed burn on March 12. The landscape is termed “Savanna Pine” and consists of grasses, low shrubs, and evergreen trees. The burn affected primarily the grasses, shrubs, and small trees (< 2m). The fire came within 50 m of the mercury measurements at its closest approach around noon. Prior to the burn, the decision was made to keep the mercury speciation system operating unless smoke was visible around the sensor inlet. The site operator, Mr. Jake Walker, never observed visible smoke at the sensor inlet; therefore, the sampling was never halted. The controlled burn started at 9:15 am and continued sporadically until roughly 8:00 pm. The wind was light and variable from the west and south putting the mercury site generally downwind. System glassware and filters were changed the following day.
Our Tekran mercury sensor suite pulls ambient air for 1 hour through a front-end denuder and a filter system. These collect, respectively, reactive gaseous mercury (RGM) and fine particulate mercury (FPM). The mercury remaining in the air stream is gaseous elemental mercury (GEM) which is analyzed in continuous 5-min blocks. During RGM/FPM analysis, the ambient sampling is interrupted for 1 hour while the system is first flushed with mercury-free air then sequentially heated and cooled. The system thus samples 1 hour every 2, providing twelve 5-min averages of GEM concentration over each sampled hour along with a 1-hr average each of RGM and FPM. During the burn, six 1-hr samples were collected providing a rare dataset during an actual burn event.
Recent studies have indicated that wildfires and prescribed burns in the U.S. emit roughly 50 tons of mercury annually, comparable to the total mercury emissions from all U.S. coal-fired power plants. The combustion of litter and green vegetation is thought to release essentially all stored mercury. Laboratory studies have suggested that the mercury is emitted predominately as GEM (> 95%) with trace amounts of FPM and no detectable RGM (Friedli et al. 2001, Science). At Grand Bay half the sampling periods reported GEM portions below 95% and readily detectable RGM in portions from 1.8% to 6.3%. The presence of a nontrivial portion of RGM, a species with a lifetime of only hours in near-surface air, suggests local “hotspots” of mercury deposition.
Steve B. Brooks
Phone: (304) 463-4739