Atmospheric Mercury:From emission to deposition

Shannon CappsApril 22, 2008

Mercury cycling

From http://people.uwec.edu/piercech/Hg/mercury_water/cycling.htm

Model Components

125 x 125 km domain

Losses

Dry deposition schemes

Wet deposition estimations

ChemistryGaseous and aqueous

chemical mechanisms

Chemical kinetic constants

Production

Anthropogenic Emissions

Natural (Re)emissions

Chemical SpeciesGaseous elemental mercury (Hg(0)) (GEM)

Atmospheric lifetime, τ, of 0.5 to 2 years due to dry deposition and chemical reactions (Lin et al., 2006)

Reactive gaseous mercury (Hg(II)) (RGM)Produced by oxidation of GEM, this species’

atmospheric lifetime is governed by wet and dry deposition

Particulate mercury (PHg)Nonreactive species whose atmospheric

lifetime is governed by wet and dry deposition

Chemical ReactionsGaseous reduction of Hg(0)Aqueous reactions of Hg(II)

OxidationReduction

Gaseous Hg(0) OxidationContinental troposphere

O3

OH

Marine boundary layer and upper troposphereCl

Br

Alternate oxidantsH2O2, Cl2, Br2, BrO, I, I2

0 20 3 1 1(g) 3(g) (s, g) 2(g)Hg + O HgO + O , k = 3 490 10  cm molec s

0(g) (g)

14 3 1 1Hg + OH Hg(II) Products, k = 8.7 9.0 10  cm molec s

0(g) (g)

13 3 1 1Hg + Cl Hg(II) Products, k = 7.6 100 10  cm molec s

0 12 3 1 1(g) (g)Hg + Br Hg(II) Products, k = 3.2 10  cm molec s

0 2 6 1 1(aq) (aq) (aq)Hg + HOCl Hg + HO + Cl , k = 2.09 10  M s

0 - 2 6 1 1(aq) (aq) (aq)Hg + OCl Hg + HO + Cl , k = 1.99 10  M s

Aqueous Hg(II) ReactionsOxidation

OH

O3

Cl

Br

0 2 7 1 1(aq) 3 (aq) (aq)Hg + O Hg + Products, k = 4.7 10  M s

0 2 9 1 1(aq) (aq) (aq)Hg + OH Hg + Products, k = 2.0 10  M s

0 - 2 1 1(aq) (aq) (aq)Hg + OBr Hg + HO + Br , k = 0.273 M s

0 2 1 1(aq) (aq) (aq)Hg + HOBr Hg + HO + Br , k = 0.279 M s

0 2 1 1(aq) 2(aq) (aq)Hg + Br Hg + 2Br , k = 0.196 M s

Aqueous Hg(II) ReactionsReduction

SO3

HO2•

Light

2- 0 -4 -13 2(aq) (aq)Hg(SO ) Hg +S(VI), k = 10 s

31.971T 12595

0 13(aq) (aq)HgSO Hg +S(VI), k = T*  s

Te

• 0 4 -1 -1(aq) 2(aq) (aq)Hg(II) +HO Hg +Products, k =1.7×10 M s

0(aq) (aq)

-7 -1

Hg(II) +h ( <420 nm) Hg +Products,

j =3×10 s (midday 60 N)

Sources and SinksAs with any

other chemical we have studied, a budget is as simple or complex as a flow diagram.

from Selin et al., 2008

Compared to 1970 in preindustrial

times.

Inventories are in 106 g and rates are in 106 g y-1

Natural (Re)emissionsEvapotranspiration

The combination of transpiration from vegetation and evaporation from land surfaces

VolatilizationReduction-oxidation

reactions occur within the soil and due to heat, the Hg(II) is released from the soil pool into the atmosphere

Immediate recyclingFast return into atmosphere

Freshly deposited mercury has a reemission lifetime of days to months after deposition

After a few months, the reemission lifetime is identical to the mercury strongly bound to the soil already

~20% of wet deposition of Hg(II) results in quick release of Hg(0) into the atmosphere

Fossil fuel combustionPoints

represent gas turbine power generation sites

(rking’s public Google map)

Yorkville MDN

site

Anthropogenic Emissions

680

160

80

40

20 20 9.01

Mercury Emissions in Atlanta

Bartow CoCoweta CoCobb CoFulton CoDekalb CoGwinnett CoClayton Co

Values in pounds emitted per year as estimated in the EPA’s 1999 NEI Total: 1040 lbs/year

Atlanta in April ScenarioSources (kg/day)

Anthropogenic Emissions1

1.29

Evapotranspiration2

0.099

Volatilization2 0.090

Prompt Recycling

f(deposition)

1. EPA NEI 1999, http://www.epa.gov/air/data/reports.html2. Selin et al., 2008, submitted, http://www-as.harvard.edu/chemistry/trop/3. Lin et al., 20064. National Weather Service, http://www.weather.gov/climate/index.php?wfo=ffc

Sinks (cm/day)

Dry Deposition, Vd3 864

Wet Deposition, P 4 0.432

Flux = [Hgi]* Vd

Flux = [Hg2+]* P

Checking ResultsSimilar studies have been carried out (Selin

et al., 2008, Lin et al., 2006, etc.)Data from these will be useful for verifying that

the programming has been done correctlyMeasurements of wet deposition (Mercury

Deposition Network) has extensive dataChecking against actual measurements will

provide verification for the model inputs and resulting values

Modeled estimates

(Selin et al., 2008), submitted

MeasurementsMercury

Deposition Network

Weekly measurements of wet deposition of mercuryYorkville, GA in

Paulding County

April 2006, 0.75 kg/day

ConclusionsExpected challenges

Difficulty finding stable initial conditions for the Runge-Kutta fourth order solver

Comparisons of the wet depositions ratesAnticipated findings

Similar results to the Selin et al. findings due to simplicity

Gain range of the impact of the thermodynamics (gas-liquid-solid partitioning)

Natural Occurrences of MercurySources

Natural Anthropogenic

Relevance Human health impacts Ecosystem impact

Cycling Air Surface/water Transfer

Wet/dry deposition Etc.

Chemistry General – species of Hg Atmospheric reactions – photochemistry/oxidation