Atmospheric Modelling:Challenges in Linking Emissions to Deposition Robert Bloxam January 19, 2006 Collaborative Meeting on Modeling Mercury in Freshwater Environments and Lake Ontario
Critical Elements of a Regional Mercury Model • Emission inventory • Speciation • Natural / Re-volatilization emissions • Background / boundary concentrations • Chemical scheme • Deposition of mercury
Main Emitted Mercury Species Temporal Scale Elementary Mercury: Global Lifetime: - Hgo about a year Divalent Mercury: Local/Regional Lifetime: - HgCl2 hours to a day -HgO Particulate Mercury Regional Lifetime: 1-3 days
Global Anthropogenic Mercury Emissions (tonnes/year) Total=1881 Total=2269 Based on Pacyna J. Munthe J. Presentation at Workshop on Mercury, Brussels, March 29-30, 2004
Natural (including re-emission) Mercury Fluxes • Current estimates are about 4000 tonnes/year • Uncertainties • Fraction of deposited mercury that is re-emitted • Fraction of mercury emissions from oceans that are re-deposited after aqueous oxidation by Cl2 • Spatial variability over land estimated to be an order of magnitude or more
Speciation of Mercury Emissions • Pacyna et. al. estimated that the split between global anthropogenic elemental, divalent and particulate bound emissions was about 53%, 37% and 10% respectively • The speciation of mercury emissions for U.S., Canada and Europe are similar with a larger fraction of elemental emissions (~ 60%) than estimated globally • What portion of HgCl2 emissions are absorbed on particles in the plume or further downwind? • Measurements of HgCl2 and Hg(P) near source regions in Europe give similar concentrations for these species while emissions of HgCl2 are estimated to be 3 times higher
Uncertainties in Mercury Modelling • Chemical Scheme • -How accurate are the reaction rates? • -Is adsorption of HgCl2 on particulates in the air a significant process? • -To calculate adsorption of mercury on carbon particles in aqueous solution requires concentration of those particles. How accurate are these concentrations? • -A number of mercury models include reactions with Cl2. How good are the Cl2 concentrations used in these transformations?
Uncertainties in Mercury Modelling cont’d • Mercury removal rates • -Model such as CMAQ-Hg assume no dry deposition of elemental mercury. Is that reasonable for all land use categories? Even a small dry deposition rate could be significant since elemental mercury is >95% of the air concentrations. • -How well can models predict wet scavenging? The results are very sensitive to how well precipitation is modelled.
Comparing Model Results to Monitoring DataAvailable Data • Wet deposition: Mercury Deposition Network • Operational since ~1998 • Total mercury concentrations/deposition with some methyl mercury data • Air concentrations • Total gaseous mercury measurements are available at a number of Canadian and U.S. sites • Data at Canadian and U.S. sites on reactive mercury (HgCl2) is more limited
Comparing Model Results to Monitoring DataLimitations • Wet deposition depends on precipitation rates. Modelled precipitation can be significantly different from station data • Total mercury wet deposition doesn’t tell which mercury species were scavenged. A model’s aqueous scavenging / chemistry mechanism can only be evaluated for overall deposition • Comparisons with total gaseous mercury air concentrations is useful information. However a good comparison doesn’t necessarily show that modelled dry deposition rates are reasonable
How do Model Results Compare with Data? • Model runs for the Clean Air Mercury Rule gave wet deposition values biased about 25% lower than observations. • Although the modelled deposition was biased low, the results correlated quite well with monitored values. This indicates that the spatial variations in deposition are being captured by the model. • Comparisons of modelled total gaseous mercury with observed data shows that average values agree well but the measured values have more day to day variability (Gbor et. al., 2006)
Long Term Trends in Mercury Deposition • There have been significant reductions in emissions of mercury in the U.S., Canada and Europe since the early 1990s. • Data for Europe indicates that mercury wet deposition has decreased by about 30% over the period when European emissions decreased by about 60% (Pacyna and Munthe, 2004). • Is there any trend in U.S. and Canadian wet deposition rates? The time period over which the Mercury Deposition Network was operational might be too short to detect trends.
Summary • Estimates of anthropogenic and natural/re-emissions of mercury as well as mercury speciation have improved over the last 10 – 15 years. However, there are still some significant uncertainties in these emission estimates. • Advance in the modelling of mercury have also been made over that time, however gaps in our knowledge of mercury modelling remain. • Comparisons of measured mercury wet deposition and air concentrations with model results provides some level of confidence that mercury models are performing adequately, but these comparisons don’t provide enough information to judge whether the chemistry and deposition algorithms have any deficiencies. • Mercury models are the main tool to assess which sources are contributing to deposition and how deposition rates might change with the changes in emissions.