Atmospheric Mercury Deposition Modeling

Transcription

1 Atmospheric Mercury Deposition Modeling Brief review and comments on remaining uncertainties Frank J. Marsik University of Michigan NADP Total Deposition Science Meeting October 28 th, 2011

2 Gaseous Dry Deposition V d [ R + R + R ] 1 = a b c R a R a is the aerodynamic resistance Function of atmospheric turbulence Often parameterized in terms of wind speed and solar radiation Parameterizations are fairly well-established, though measurements now available to assist evaluation R b R c R b is the quasi-laminar boundary layer resistance Function of diffusivity of depositing species Paramaterizations are fairly well-established

3 Florida Everglades Dry Deposition Study June 2000 Many modeling studies incorporate the use of bulk meteorological approaches to calculate turbulence quantities such as the friction velocity used in determination of aerodynamic resistance. Friction velocity (Ustar) values calculated using bulk approach [Hicks et al. (Water, Air, and Soil Pollution 36 (1987) )] did not perform well in the aquatic environment of the Everglades compared with measurements. The reason: despite significant solar insolation, the surface layer above this aquatic ecosystem never became unstable, as assumed by bulk parameterization for the level of solar insolation observed.

4 Florida Everglades Dry Deposition Study June 2000 When friction velocity (Ustar) values calculated using bulk approach of Hicks et al. (Water, Air, and Soil Pollution 36 (1987) ) assuming neutral to stable conditions at all time, the modeled and measured friction velocity values showed greater agreement. With the reduction in the cost of sonic anemometry, direct measurement of friction velocity (via momentum flux) is now possible and should be considered for routine monitoring applications.

5 Gaseous Dry Deposition V d [ R + R + R ] 1 = a b c R a R c is the canopy resistance Function of: solubility/reactivity of depositing species plant canopy characteristics ground surface characteristics Scales with the Leaf Area Index of plant canopies Should one use single- or two-sided LAIs? Parameterization of this resistance is handled very differently across different dry deposition models. R b R c

6 RADM Dry Deposition Scheme Wesely (Atmospheric Environment, 23(6), , 1989)

7 RADM Parameterizations Wesely (Atmospheric Environment, 23(6), , 1989) Various resistances: Are functions of Henry s Law Coefficient and reactivity factors which may not be known for a given species like mercury and must be assumed Are scaled with resistances of SO 2 and O 3

8 Zhang et al. (2003) (Atmos. Chem. Phys. Discuss., 3, , 2003) W st = fraction of stomatal blocking under wet conditions Again, some resistances are scaled with more highly characterized values of SO 2 and O 3. α and β are scaling factors and are a function of the chemical species Presents table of values for species modeled.

9 Potential areas of improvement Determination of resistances to uptake of GEM and RGM by various natural surfaces. Rather than estimating values using scaling relative to uptake of SO 2 and O 3 Rather than assuming RGM deposits similarly to HNO 3 based upon high solubility and reactivity for both species. Would be helped by better understanding of what species actually constitute RGM! Lin et al (Atmospheric Environment 40 (2006) ) found a two-fold difference in RGM Vd based upon assumption of RGM as HgCl 2 or HgO

10 Particle Dry Deposition Velocity d [ ] 1 Ra + Rb + RaRbV g Vg V = + Estimation of the dry deposition for particles includes terms for gravitational settling velocity V g dependent upon particle diameter and density Particle size distributions for particulate mercury are not wellcharacterized

11 Potential areas of improvement Improved network-based measurements of particulate Hg size distributions would be helpful Commonly used Tekran, Inc. Mercury Speciation Units quantify PM 2.5 Data collected by USEPA in Detroit, MI and Steubenville, OH showed both uni- and bi-model distributions of particulate Hg (Lynam, personal communication). When bi-model distributions were observed, coarse mode mass median diameter was >> 2.5 μm This could result in underestimation of particulate Hg dry deposition.

12 Compartmentalized Approach (Bash et al, 2007) New approaches attempt to address more complex interactions, such as: bidirectional fluxes using storage and near surface gradients in species concentrations Compensation points uptake of mercury by foliage and translocation of mercury across soil-root interfaces Various Gustin et al. studies

13 Compartmentalized Approach However, the approach requires knowledge or estimation of new quantities such as: concentration of mercury within different compartments Example: within leaf mesophyll various partitioning coefficients (K X )

14 Summary of some research needs Improved characterization of atmospheric mercury What is the predominant form(s) of RGM? What are representative particulate Hg size distributions? Seth Lyman has looked at impact on O 3 on characterization of RGM on denuders How much has this potential effect impacted past measurements of RGM?

15 Summary of some research needs Additional field/laboratory measurements Bulk approaches to modeling friction velocity may no longer be necessary with lower cost of sonic anemometry measurements. Therefore, why not implement sonic anemometry in current/future networks for determination of R a? Additional measurements needed to obtain a better characterization of mercury-specific resistances to uptake: Example: soil, leaf cuticular wax

16 Summary of some research needs Additional field/laboratory measurements Additional measurements needed to obtain a better characterization of mercury chemical partitioning parameters With water With cuticular waxes Additional measurements needed to obtain a better characterization of mercury air-surface exchange rates for use in model evaluation RGM GEM (Since bi-directional, can we achieve separation of emissions and deposition measurements through careful study design?)

17 For regional-scale modeling efforts More vertical profile measurements of speciated mercury concentrations are needed to evaluate model descriptions of vertical transport processes This impacts vertical distribution of speciated mercury and thus availability of elevated mercury for incorporation into clouds (wet deposition). This impacts amount of mercury remaining in the boundary layer for dry deposition and below cloud scavenging.