Mercury Chemistry in the Global Atmosphere: Constraints from Mercury Speciation Measurements

1. Mercury Chemistry in the Global Atmosphere: Constraints from Mercury Speciation Measurements Noelle Eckley Selin EPS Grad Student Seminar Series 14 February 2006

2. Why study Mercury (Hg)? • Mercury is a global environmental pollutant • Current levels in atmosphere are 3x pre-industrial levels • Accumulates in food webs as methyl mercury; risk to humans & environment (neurotoxin) • National, regional, and international policy interest U.S. EPA recommended limit for mercury in hair: 1 ug/g Noelle’s hair: 1.1 ug/g EPA benchmark dose (10% of births show neurological defects): 11 ug/g www.greenpeace.org/usa/mercury

3. THE MERCURY CYCLE: CURRENT ATMOSPHERE 5000 Anthropogenic Emissions 2400 Wet & Dry Deposition 2600 Land emissions 1600 Net Wet & Dry Deposition 1900 Net Oceanic Evasion 1500 (1680-3120) (1800-3600) (700-3500) (1300-2600) (700-3500) SURFACE SOILS 1,000,000 OCEAN 289,000 Extraction from deep reservoirs 2400 River 200 (1680-3120) Quantities in Mg/year Uncertainty ranges in parentheses Adapted from Mason & Sheu, 2002 Net burial 200

4. k=8.7(+/-2.8) x 10-14 cm3 s-1 (Sommar et al. 2001) k=9.0(+/-1.3) x 10-14 cm3 s-1(Pal & Ariya 2004) Too high? (Calvert and Lindberg 2005) OH Hg0 1.7 ng/m3 Gaseous Phase Hg2+ 10-200 pg/m3 Oxidation O3 k=3(+/-2) x 10-20 cm3 s-1 (Hall 1995) Reported rate constants up to k=1.7 x 10-18cm3 s-1 Henry’s Constant 0.11 M/atm Henry’s Constant 1.4x106 M/atm Oxidation Hg0 Aqueous Phase ? Hg2+ Oxalate? SO3 HO2 Reduction • k=1.1-1.7 x 104 M-1 s-1 (Pehkonen & Lin 1998) • Shouldn’t occur (Gårdfeldt & Jonsson 2003) • k=0.0106 (+/- 0.0009) s-1 (vanLoon et al. 2000) • Occurs only where high sulfur, low chlorine Particulate Phase HgP 1-100 pg/m3

5. Approach • Use observations from latitudinal gradient, seasonal cycles, and short-term variability to constrain uncertainties in Hg chemistry and deposition, using GEOS-Chem mercury simulation and sensitivity simulations

6. Mercury Budget in GEOS-Chem Hg(0) 4500 (trop: 3900) OH:8400 Hg(II) 760 (trop:240) Hg(P) 1.9 (trop:1.9) k=6.9 x 10-14 cm3 s-1 O3:2500 k=3 x 10-20 cm3 s-1 hv (cloud):5932 200 720 1300 1500 500 2000 Anthropogenic Emission 4400 Land Re-emission 190 11 Ocean Emission 1500 Land (Natural) Emission Dry Deposition Wet Deposition Wet Deposition Inventories in Mg Rates in Mg/yr Dry Deposition

7. Constraints from annual mean TGM Average concentration at 22 land-based sites Measured: 1.60 ng/m3 Modeled: 1.60 ng/m3 High Atlantic cruise data? + Oxidation rate constant (OH) adjusted to correspond to mean concentrations. Shown above: oxidation rate k=6.5 x 10-14 cm3 s-1 k=8.7(+/-2.8) x 10-14 cm3 s-1 (Sommar et al. 2001)

8. Constraints from Interhemispheric Gradient Measurement-based estimates of interhemispheric gradient: Lamborg et al. (2002): 1.2-1.8 Temme et al. (2003): 1.49 (+/- 0.12) Consistent with TGM lifetime of 1 year Interhemispheric gradient Constrains TGM lifetime GEOS-Chem interhemispheric gradient: 1.21 GEOS-Chem TGM lifetime: 0.92 yr *=land-based stations; +=Temme, 2003 (Atlantic); Δ=Fitzgerald, 1995 (Pacific); ◊=Laurier, 2004 (Atlantic); red line=GEOS-Chem global average

9. Constraints from Seasonal Variations 12 sites Measurements Model (OH, O3, reduction) OH only O3 only Measurements Model

10. Constraints from Time Series at Okinawa [Jaffe et al. 2005] Diurnal variation of RGM: daytime production plus rapid sink (uptake onto sea-salt?) measurements, standard model, O3 only, without sea salt morning increase a constraint on OH oxidation  One grid box upwind

11. RGM model-measurement comparison at OkinawaA sea-salt sink for RGM? • PreviousGEOS-Chem vs. measurements at Okinawa by Jaffe et al. (2005): model overestimates measurements by a factor of 3 (note difference in scale), but captures some day-to-day variation in observations • RevisedModel and measured RGM including an implied sink for RGM (sea salt uptake?) are consistent with order of magnitude of Okinawa observations (same scale)

12. Okinawa Data: Hg(0) vs CO and Asian Emissionsmodel (red), measured [Jaffe et al 2005] (black) Hg(0)/CO ratio: check on Asian emissions Slope 0.0053 in measurements 0.0036 in model Pacyna et al 2003: 770 Mg/year Jaffe: 1460 Mg/year (based on data) GEOS-Chem Asia: (GEIA 2000 inventory) Hg(0): 586 Mg Hg(II): 365 Mg land reemission: 342 Mg total Hg(0)-Asia: 928 Mg Consistent with Jaffe underestimate of Asian emissions – but land reemission is a substantial portion!

13. Constraints from Annual Average RGM Variable measurements; 2 cruises average of all measurements 17.4 pg/m3, GEOS-CHEM 8.3 pg/m3 however, skewed by a few high measurements Limitations from RGM – HgP partitioning

14. Constraints from Time Series at Mt Bachelor [Swartzendruber et al. 2005] RGM concentrations higher in the free troposphere Negative correlation between Hg(0) and RGM at night @ Mt Bachelor (r=-0.67 for meas, r=-0.71 for GEOS-Chem). Negative correlation between relative humidity and RGM, reproduced in model (downwelling?) 

15. Constraints from Wet Deposition Comparison with measurements % deposition from U.S. Sources 2 patterns: latitudinal variation (OH oxidation) and regional enhancement (sources) Moderate correlation (r2=0.52 for 2003, 0.66 for 2004) GEOS-Chem underestimates wet deposition over U.S. by c. 25% Data from U.S. Mercury Deposition Network (2006)

16. Conclusions and Future Work • GEOS-Chem model suggests that • OH, O3 reactions, coupled with reduction, provide best explanation for Hg observations • Rapid RGM uptake onto sea-salt aerosol • Elevated RGM in free troposphere & stratosphere • Future work: land emissions parameterization • Acknowledgments: Prof. Daniel Jacob (advisor); Bob Yantosca (Harvard); Rokjin Park (Harvard); Sarah Strode (U.Wa); Lyatt Jaegle (U.Wa)