Projections of atmospheric mercury levels and their effect on air quality in the United States.

The individual and combined effects of global climate change and emissions changes from 2000 to 2050 on atmospheric mercury levels in the US are investigated by using the global climate-chemistry model, CAM-chem, coupled with a mercury chemistry-physics mechanism (CAM-Chem/Hg). Three future pathways from the Intergovernmental Panel on Climate Change (IPCC) Special Report on Emissions Scenarios (SRES) are considered, with the A1FI, A1B and B1 scenarios representing the upper, middle and lower bounds of potential climate warming, respectively. The anthropogenic and biomass burning emissions of mercury are projected from the energy use assumptions 10 in the IPCC SRES report. Natural emissions from both land and ocean sources are projected using dynamic schemes. The zonal mean surface total gaseous mercury (TGM) concentrations in the tropics and mid-latitudes of the Southern Hemisphere are projected to increase by 0.5-1.2 ngm^-3 in 2050. TGM concentration increases are greater in the low latitudes than they are in the high latitudes, indicative of a larger meridional 15 gradient than in the present day. In the A1FI scenario, TGM concentrations in 2050 are projected to increase by 2.1-4.0 ng m^-3 for the eastern US and 1.4-3.0 ngm^-3 for the western US. This pattern corresponds to potential increases in wet deposition of 10-14 µgm^-2 for the eastern US and 2-4 µgm^-2 for the western US. The increase in Hg(II) emissions tends to enhance wet deposition and hence increase the risk of 20 higher mercury entering the hydrological cycle and ecosystems. In the B1 scenario, mercury concentrations in 2050 are similar to present level concentrations; this indicates that the domestic reduction in mercury emissions is essentially counteracted by the effects of climate warming and emissions increases in other regions. The sensitivity analyses presented show that anthropogenic emissions changes contribute 32-53% 25 of projected mercury air concentration changes, while the independent contribution by climate change accounts for 47-68 %. In summary, global climate change could have a comparable effect on mercury pollution in the US to that caused by global emissions changes. [ABSTRACT FROM AUTHOR]