Trends and modeling of the total gaseous mercury flux and deposition in the leaf litter fall in a Northeastern red maple canopy

Transcription

1 Trends and modeling of the total gaseous mercury flux and deposition in the leaf litter fall in a Northeastern red maple canopy Jesse Bash EPA/NERL/AMD

2 Outline Bi-directional framework in CMAQ Site and measurement description Results TGM canopy fluxes Atmospheric canopy compensation points Under canopy fluxes Wet and litter fall deposition Soil and vegetation concentrations Conclusions

3 Background Natural mercury emissions/re-emissions Estimated to be a large fraction of the total mercury emissions Believed to contribute to long range transport of mercury though re-emissions Residence time in terrestrial media can be on the order of decades Once in the terrestrial system mercury is available for methylation Largest pools of mercury are in the terrestrial system Emission/re-emission processes are a means of transport through the atmosphere

4 χ a CMAQ Dry Deposition Model R a R w R b χ c R m R ac R g R st Atmosphere canopy soil resistance model to calculate deposition velocities

5 Modeling bi-directional surface exchange Bi-directional surface exchange capability is being developed in CMAQ Adaptation of NH 3 bi-directional algorithms for mercury Modification of dry deposition routines Adds canopy, soil and vegetation concentrations to parameterize a concentration gradient Uses a resistance analogy to model exchange coefficients Requires knowledge of surface properties and incanopy air movement Where mercury is deposited will determine the mechanisms of its re-emission

6 χ a Bi-directional Model R b R w R a χ w χc R b χ s R ac R bg χ g R st Modification of the two layer canopy compensation (χ c ) point model of Nemitz et al., 21

7 Total gaseous mercury flux Located in rural Coventry, CT Employed relaxed eddy accumulation technique Fluxes taken at 1.2 canopy heights in a 21 meter closed red maple stand Wetland to west and southwest Oak stand to the north east on a slightly elevated hill measurements

8 1/13 1/6 TGM flux Under canopy air-soil flux Atmosphere-canopy flux Measured using the dynamic flux chamber technique Measured using the REA micrometeorological technique Not continuously sampled y = x R 2 = ng m -3 ng m -3 h /29 9/22 9/15 9/8 9/1 8/25 8/18 8/11 8/4 7/28 7/21 7/14 7/7 6/3 6/23 6/16 1/1 1/6 1/2 9/28 9/24 9/2 9/16 9/12 9/8 9/4 8/31 8/27 8/23 8/19 8/15 8/11 8/7 8/3 7/3 7/26 7/22 7/18 7/14 7/1 7/6 7/2 6/28 6/24 6/2 6/16 Under canopy flux 24 flux 25 flux 24 concentrations 25 Concentrations Linear (24 flux) ng m -2 h -1

9 25 average daily flux Aug 18 th through Sept 12 th 25 W m o C and mv Morning peak in flux around the time that dew evaporates from canopy Afternoon peak in flux around peak in ambient temperature ng m -2 h Solar radiation (W m-2) Ambient temp (C) LWS (mv) ng m -3 Net 24 growing season evasive flux of µg m TGM Flux (ng m-2 h-1) TGM Concentration (ng m-3)

10 Compensation Points Wet canopy compensation point of 1.76 ng m 3 (2.71 ng m 3 for stable conditions) Dry canopy compensation point of 1.43 ng m 3 (2.12 ng m 3 for stable conditions) ( conditions Mean TGM concentration of 1.54 ng m3 (1.59 ng m 3 under stable Compensation point increase through the growing season

11 Under Canopy Soil Flux mean daily under canopy flux Measured using a Teflon dynamic flux chamber Under canopy flux measurements were taken on the drier elevated and transitional areas Soils under canopy were a consistent emissions source in 24 ng m -2 h hour 12 1 Flux was not correlated with soil moisture ng m -3 h Less than 1% variation in soil moisture from May through October 2-2 6/16 6/23 6/3 7/7 7/14 7/21 7/28 8/4 8/11 8/18 8/25 9/1 9/8 9/15 9/22 9/29 1/6 1/13 Under canopy flux

12 Vegetation Concentrations Acer Rubrum leaf mercury concentrations from shade leaves (z m ~ 4 m) were consistently higher ( m than from sun leaves (z m ~ Sun leaves are exposed to more solar radiation, higher temperatures, and higher wind speeds ng/g 3. Soil was consistent emission source In-canopy concentration gradient is unknown Annual fall leaf litter deposition of 12.1 µg m -2 5/23/6 6/6/6 6/2/6 7/4/6 7/18/6 8/1/6 Sun leaves 9/12/6 8/29/6 8/15/6 Shade leaves 9/26/6 1/1/6 1/24/6

13 26 soil concentrations.7.6 ug g Leaf Litter 5 cm 2 cm 5 cm.2.1. Elevated (n=5) Transition (n=5) Wetland (n=4) Mercury concentrations highest in organic layer Soils in the wetland area had the lowest mercury concentration but the highest amount of organic matter TGM concentrations from the surface to 5 cm depth were best correlated ( 26 Lee, with soil mercury flux (Sigler and

14 Wet Deposition Event based samples Highest concentrations and deposition in June and July Annual wet deposition from 24 through mid 26 was 6.57 µg m -2 Monthly deposition totals is often driven by several large events Volume Weighted Concentration ng/l Wet deposition ug m Jan Feb Mar April May June July Aug Sept Oct Nov Dec Jan Feb Mar April May June July Aug Sept Oct Nov Dec

15 Conclusions Seasonality in the bi-directional TGM flux was documented over a red maple canopy Under canopy soils were a constant emissions source and the largest pool of mercury Under story leaves had a higher mercury concentration than more exposed leaves The atmospheric-canopy compensation point was lowest during dry unstable conditions Mercury concentrations in the soils were lower in the wetlands Soil canopy atmosphere mercury exchange needs further investigation

16 Modeling Needs Speciated flux measurements Over a variety of land cover types Are fluxes over other forest canopies this large? Atmosphere-canopy-soil source/sink relationship In canopy concentrations, vegetative and soil concentrations Where mercury deposits in the canopy/soil system will determine how it is re-emitted Leaf level parameters Mesophyll and cuticular concentrations of various species Leaf washing experiments Identifying the mechanisms of emissions and deposition under wet canopy conditions Does the presence of water mobilize mercury bound to leaves and soils? Speciated measurements critical

17 χ a Future modeling needs R b R w R a χ w χc R b χ s R ac R bg χ g Speciation In canopy gradients Canopy storages R st Partitioning on above ground biomass and biomass concentrations

18 Disclaimer The research presented here was performed under the Memorandum and Understanding between the U.S. Environmental Protection Agency (EPA) and the U.S. Department of Commerce s National Oceanic and Atmospheric Administration (NOAA) and under agreement number DW This work constitutes a contribution of the NOAA Air Quality Program. Although it has been reviewed by EPA and NOAA and approved for publication, it does not necessarily reflect their policies or views.

19 Thank you ( UCONN ) David R. Miller ( UCONN ) Patricia Bresnahan ( UCONN ) Kate Knight ( NOAA/ASMD ) Ellen Cooter ( NOAA/ASMD ) John Pleim ( UNH ) Jeff Sigler ( Yale ) Xuhui Lee ( EPA/NRMRL ) John Walker Generous funding by the Connecticut River Airshed- Watershed Consortium (CRAWC) and the University of Connecticut Experiment Station