Chasing quicksilver northward: mercury chemistry in the Arctic troposphere

Abstract

Environmental Context. ‘Mercurial storms rage over the Arctic’ wrote Fred Pearce in New Scientist in June of 1997: he was referring to the recent discovery by Bill Schroeder and his colleagues (Nature, Vol. 394, 1998) of periods soon after Arctic dawn when the concentration of mercury in the atmosphere literally plummets to levels so low that they can be undetectable, even by the most sensitive of modern instruments. A decade and many measurement campaigns later, we think we understand how these so-called depletion events occur, if not all the mechanisms that go towards providing the conditions for them to happen. Nor do we really know what happens to the mercury removed from the atmosphere; the fear is that it is deposited and enters the Arctic ecosystem, where it is potentially extremely harmful. The present study questions whether that fear is grounded. Abstract. The tropospheric boundary layer chemistry of Hg has been simulated using a two-phase photochemical box model to see if our current (experimental and theoretical) understanding of Hg(g)0 reaction rates can account for the depletion events seen during Arctic spring, when the so-called ‘bromine explosion’ in the model is constrained by the measured ozone depletion rate. The simulations reveal that the observed rate of Hg(g)0 depletion can be accounted for; however, the measured concentrations of gas-phase oxidised Hg and HgP (Hg associated with particulate matter) cannot. Simulating the emission of Hg(g)0 from the snow pack to mimic the observed concentration recovery after a depletion event suggests the net Hg deposition from a depletion event is all but irrelevant.