Particulate mercury emissions in regional wildfire plumes observed at the Mount Bachelor Observatory

Atmospheric mercury is composed primarily of Hg0 (>95%), but Hg+2 and particle bound mercury are also found in some environments. The three forms of mercury were measured at the Mount Bachelor Observatory beginning in 2005. Using data gathered from 2005 to 2007, 15 periods were identified during which PHg was above the instrument detection limit of 3 pg m−3 for nine or more consecutive hours. Peak PHg concentrations ranged from 6.0 to 44.3 pg m−3. During these events, PHg is strongly correlated with CO and sub-micron aerosol scatter coefficient (typically R2 > 0.6). Our data suggest that the 15 PHg events were likely due to regional wildfires in California and Oregon. Wildfires were identified as the primary PHg source using a combination of air-mass back-trajectories, MODIS satellite data, and chemical and physical tracers of combustion. Slopes of the PHg/σsp and PHg/CO relationships ranged from 0.20 to 1.57 pg (Mm−1)−1 and 0.11 to 0.61 pg m−3 ppb−1, respectively. The range of slopes may indicate different types of burning (e.g. flaming vs. smoldering), differing amounts of chemical processing, different fuel sources, or different physical parameters such as the plume injection height. The slopes provide constraints for the relationship between PHg, CO, and aerosol scatter from wildfires. Asian long-range transport was not a source of PHg but we cannot rule out the possibility of local U.S. industrial sources of PHg for some of the events. Assuming our observations are representative of global fire emissions, we estimate that PHg represents 15% of the total mercury released from wildfires and is a source of PHg comparable to anthropogenic sources.