Mercury Dry Deposition Almost Two Decades Later: Musings of an O.F.

Steve Lindberg
ERC (enjoying retirement completely), PO Box 1895, Graeagle, CA

Daniel Obrist
Desert Research Institute, Division of Atmospheric Sciences,
2215 Raggio Parkway, Reno, NV 89512


At the inaugural ICMGP (International Conference on Mercury as a Global Pollutant) around 1989, a paper was presented titled “The Dry Deposition of Mercury.” Unfortunately, the audience was small, but was polite enough to make little noise when they exited en masse mumbling “what the heck is he talking about?” Also unfortunately, that paper was mine (perhaps it was my graphics, see Fig. 1 below). Both we, and the science, have come a long way since then, but do we yet understand the role of dry deposition? I fondly recall the struggles involved with our original NADP Mercury Deposition Initiative back in 1994. And now, somewhat to my amazement and much to my delight, the MDN continues to set records for longevity and breadth as it enters its second decade. Last year the NADP proposed expansion of the original initiative to include dry deposition.

In the ensuing decade(s), sufficient progress has been made to at least suggest the importance of dry deposition, though precious few actual measurements exist. As might be expected, most all of the methodology developed during the acid rain era for S & N has been applied to Hg in some fashion. On one hand, dry deposition has been estimated from leaf-washing xperiments (with little assurance that dry deposited Hg can be effectively washed from leaves), from various gas exchange chambers (does only gaseous Hg matter?), from the ecosystem fluxes throughfall and litterfall (what about complications from soil uptake and foliar leaching?), and data exist for deposition to surrogate surfaces (of exactly what one wonders?). On the other hand, a few estimates have been derived from micrometeorological measurements (including concentration gradients, modified Bowen ratio, and relaxed eddy accumulation). Of course (and I say this fondly), there are any number of model estimates (“caution, you are entering a data-free zone”). In these musings I’ll attempt to explore what we do, what we don’t, and what we only think we know about Hg dry deposition, and why that matters.