Quick cycling of quicksilver

Nature Geoscience (February 2012 issue) contains a “News and Views” column that I wrote about some exciting new airborne measurements of mercury. An article in the same issue, by Seth Lyman and Dan Jaffe, reports the first measurements of elemental and oxidized mercury at high altitudes. Previous modeling work by myself and others suggested that oxidation should be very fast near the tropopause and above. These observations confirm that model result and suggest that particle settling controls the fate of mercury in the upper atmosphere.

Atmospheric mercury cycle. Oxidation occurs throughout the atmosphere and new measurements confirm model predictions that it is fastest at high altitudes near the tropopause and above.

Assessment of climate impact from aviation (and its uncertainties) published in PNAS

Aircraft emit nitrogen oxides (NOx) that indirectly warm and cool the climate by increasing tropospheric ozone and decreasing methane, two major greenhouse gases. These opposing effects on climate (quantified as radiative forcing, RF) mostly cancel. Through a survey of prior modeling studies we find that the net steady-state RF from aviation NOx is +4.5 ± 4.5 mW/m2. The 100-year global warming potential (GWP100y) from aviation NOx is thus 52 ± 52. Despite the large relative uncertainty in NOx effects, the climate warming due to aviation CO2 emissions is about 14 times greater, over a 100 year timespan. This work diagnoses the sources of uncertainty in RF with two complementary methods. We find strong correlations among the O3 and CH4 responses to aviation NOx across widely differing models, which is likely caused by differing NOx abundances in each model. Measurements of NOx and other reactive species in the free troposphere could reduce the uncertainty in the climate impact from aviation NOx and other emission sources.

Steady-state radiative forcing (RF) from O3 and CH4 caused by aviation NOx emissions. RF components are strongly correlated across models, indicating an underlying factor that determines the net climate RF. The UCI CTM reproduces this variability through changes to background NOx.

Global mercury cycle analysis published in ACP

We show that atmospheric oxidation of elemental mercury solely by atomic bromine can explain observed global patterns of mercury abundance, gradients, seasonal cycle, and deposition. The model using this oxidation mechanism predicts greater mercury deposition to the high-latitude oceans, where many productive fisheries are located.

Mean seasonal cycle of total gaseous mercury (TGM) at 15 sites in the northern hemisphere mid-latitudes and 3 sites in the Arctic. See paper for details.