Abstract
We measured gaseous elemental mercury (GEM), particulate mercury (PHg), and reactive gaseous mercury (RGM), along with CO, ozone, and aerosol scatter at the Mount Bachelor Observatory (2.7 km above sea level), Oregon, from May to August 2005. The mean mercury concentrations (at standard conditions) were 1.54 ng/m3 (GEM), 5.2 pg/m3 (PHg), and 43 pg/m3 (RGM). RGM enhancements, up to 600 pg/m3, occurred at night and were linked to a diurnal pattern of upslope and downslope flows that mixed in boundary layer air during the day and free tropospheric air at night. During the night, RGM was inversely correlated (P < 0.0001) with CO (r = −0.36), GEM (r = −0.73), and H2O (r = −0.44), was positively correlated with ozone (r = 0.38), and could not be linked to recent anthropogenic emissions from local sources or long‐range transport. Principal component analysis and a composite of change in RGM versus change in GEM during RGM enhancements indicate that a nearly quantitative shift in speciation is associated with increases in ozone and decreases in water vapor and CO. This argues that high concentrations of RGM are present in the free troposphere because of in situ oxidation of GEM to RGM. A global chemical transport model reproduces the RGM mean and diurnal pattern but underestimates the magnitude of the largest observed enhancements. Since the only modeled, in situ RGM production mechanisms are oxidation of GEM by ozone and OH, this implies that there are faster reaction rates or additional RGM production mechanisms in the free troposphere.
Highlights
[2] The extent to which oxidized forms of mercury are present in the atmosphere remains a key uncertainty in our understanding of the global cycling of mercury [Schroeder and Munthe, 1998; Stratton et al, 2001]
Reactive gaseous mercury (RGM) is an operationally defined fraction of atmospheric mercury based on its collection by a KCl denuder [Landis et al, 2002]
Anthropogenic emissions to the atmosphere are dominated by gaseous elemental mercury (GEM) (60 – 70%), with the balance being RGM and PHg [Carpi, 1997; Pacyna et al, 2006; Streets et al, 2005]
Summary
[2] The extent to which oxidized forms of mercury are present in the atmosphere remains a key uncertainty in our understanding of the global cycling of mercury [Schroeder and Munthe, 1998; Stratton et al, 2001]. Most studies [e.g., Sheu and Mason, 2001; Landis et al, 2002; Malcolm et al, 2003; Weiss-Penzias et al, 2003; Poissant et al, 2005] report that the reactive and particulate fractions are less than 5% and often less than 2% of the total airborne mercury (THg = GEM + RGM + PHg). Several groups have conducted aircraft studies of mercury above the planetary boundary layer, but have generally measured only THg or have assumed that all of the mercury which was detected was present in the atmosphere as GEM They found either little or no change in mercury mixing ratios with height [Ebinghaus and Slemr, 2000; Banic et al, 2003] while several others report a slight decrease with height [Brosset, 1987; Kvietkus, 1995; Friedli et al, 2004]. In the spring of 2004, they observed well correlated enhancements of THg and CO which were linked to Asian anthropogenic
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