Modeling Dust and Dissolved Iron Deposition to the Southern Ocean: GEOS-Chem Application

Transcription

1 Modeling Dust and Dissolved Iron Deposition to the Southern Ocean: GEOS-Chem Application Matthew Johnson, Nicholas Meskhidze, Fabien Solmon, Duncan Fairlie, Santiago Gassó, Diego Gaiero, Daniel Jacob, Bob Yantosca, Philippe Le Sager Funding: National Science Foundation (NSF) Grant No and the NASA North Carolina Space Grant Graduate Research Fellowship Program MODIS-Aqua January 23 rd, Matthew Johnson North Carolina State University 4 th GEOS-Chem Users Meeting April 7, 2009

2 Importance of Dissolved Iron (DFe) Deposition to the Southern Ocean (Chisholm 2000, Nature; biological pump) Fe is supplied to the oceans by windblown dust from the continents Iron Hypothesis (Fe) [Martin et al., 1990]

3 Modeling Dust and DFe Deposition to the South Atlantic Ocean Using GEOS-Chem/DFeS Patagonia/South Atlantic Ocean (SAO). GEOS-Chem implemented with prognostic Fe dissolution scheme (GEOS-Chem/DFeS) dissolution rates dependant on solution ph, temperature, dust mineralogy and the specific surface area of minerals Acid Mobilization Yearlong model simulation between Oct Sep proved to be the first application of a prognostic dust laden-fe model to this region

4 Dust Emission Regions and Source Strength g m -2 yr -1 GEOS-Chem predicted annual emission rates overlaid on Google Earth images Past modeling studies predicted emission rates of g/m 2 /yr similar source locations

5 Mineral Dust Transport and Deposition Pathway µg m -2 day Wagner et al., Li et al., 2008 Percentage of annual dust deposited by wet deposition processes Dust deposition pathway and rates are in agreement with past models Wet deposition dominates total dust deposition over the SAO µg m -2 day -1

6 Mineral Dust Deposition Tg/yr Duce and Tindale (1991) Jickells and Spokes (2000) Gao et al., 2003 Gaiero et al., 2003 Li et al., 2008 Summer (DJF) Fall (MAM) Winter (JJA) Spring (SON) GEOS Chem Total mineral dust deposition (~18 Tg) and seasonality were well captured

7 Modeling DFe Deposition to the SAO Using GEOS-Chem/DFeS Annual DFe deposition rates Annual DIF values µg m -2 yr Gigagrams per year was deposited to the SAO. Patagonian dust encountered limited Fe dissolution over the SAO DIF = ~ % %

8 Sensitivity Study: Clay Dissolution Smectite and Illite dissolution proved to contribute a ~78% increase in DFe deposition. Large increases were simulated in major dust transport pathways.

9 Sensitivity Study: Effect of Doubled Dimethyl Sulfide (DMS) Emissions Gigagrams DFe / month Baseline Double DMS Double Natural SO2 Emissions Emissions 2 0 Dry DeposiRon Wet DeposiRon Total DeposiRon ~8.5 % increase in DFe deposition when DMS emissions were doubled.

10 Limited increases in Sensitivity DFe deposition during the volcanic eruption Study: Cerro Hudson Volcanic Eruption Volcanic SO2 plume transported above Patagonian dust transport Average SO2 concentrations (Dobson Units) over the SO during the volcanic eruption. Strong and explosive eruptions may lead to concentrations in the 10 DU range, even as high as 100 DU. μg m 2 day 1 (

11 Conclusions and Future Work First applicaron of GEOS Chem for dust and bioavaliable Fe deposiron to the SAO GEOS Chem/DFeS demonstrated the ability to capture dust outbreaks and transport pathways over the SAO Acid mobilizaron doesn t seem to be the major mechanism for the producron of bioavaliable Fe in Patagonian dust SensiRvity studies highlight the possible importance of clay minerals and natural sources of SO 2 More in situ measurements are needed to be^er constrain mineral dust and DFe fluxes to the SAO Future work: Continued model evaluation Further model developments including: 1. Anthropogenic/Combustion Fe emissions 2. Photo reduction of Fe-containing minerals 3. Photochemical cycling of Fe(II)/Fe(III) 4. Cloud processing of mineral dust Stop by my poster for the assessment of simulated DFe deposition on chlorophyll production and carbon sequestration in the SAO THANK YOU!

12 Implementation of Patagonian Dust Mineralogy Implemen=ng Patagonian dust mineralogy would allow mineral dust in GEOS Chem to be representa=ve to Patagonia. Only the key minerals that can affect the ph of the deliquesced aerosol solu=on were chosen. Clays > 50% of total mineral dust mass. Smec=te and Illite are the main clays found in Patagonia. Mineral Percent of Mineral Dust Aerosol (wt%) Percent Fe of Mineral (wt%) Hematite Albite 8 - Microcline 8 - Illite Smectite Calcite Quartz 29 - (Gaiero et al., 2004; Smith et al., 2003; Ramsperger et al., 1998)

13 Dust Sulfate Interac=ons SO 4 is produced in aqueous solution surrounding dust particles by: 1. Condensation of gas phase sulfuric acid (H 2 SO 4 ). 2. Direct absorption of SO 2. The fraction of H 2 SO 4 deposited on dust is determined by surface area ratio of dust (S d ) and anthropogenic mode surface area (S a ) (defines the surface area of sulfate, nitrate, and ammonia aerosol species). The heterogeneous (aqueous phase) absorption depends on: 1. Dust concentration 2. SO 2 concentration 3. Bulk uptake coefficient = 9e -8 m 3 µg -1 s -1 (determined by airplane measurements) [Herring et al., 1996]

14 Dust Nitrate Interac=on NO 3 is produced in aqueous solution surrounding dust particles by: 1. Gas phase Nitric Acid (HNO 3 ) condensation 2. Direct absorption of NO 2. The condensation and absorption are determined by the bulk uptake coeffiecient = 10e -7 m 3 µg -1 s -1 [Herring et al., 1996]. S eq represents either the rate of condensation of gas-phase HNO 3 to the dust mode deliquesced solution or the volatilization of dissolved NO 3 from solution. Calculated by the thermodynamic module ISORROPIA.

15 * Constants and equation used to determine the rate of mineral dissolution [Meskhidze et al., 2005]. R net i = Mineral rate of mineral dissolution dissolution determined (moles/gram(dust)/sec through ph and temperature K r = rate constant which is temperature dependant (shown above) (mol/m dependant dissolution kinetics. 2 /sec) a(h + ) = hydrogen ion activity (calculated binary activity coefficients for dissolved Once determined all cations are converted to equivalent molar species using the formulation of Kusik and Meissner [1978] and q parameters for each concentrations of sodium before entering aqueous-phase salt). equilibrium reactions in ISORROPIA to determine speciation and m = empirical parameter f(δgr) solution = function ph. of Gibbs free energy A i = specific surface area of the mineral W i = weight fraction of mineral in dust

16 B Maximum Dust Emission Regions Topography of maximum dust sources was analyzed through Google Earth topography maps. 20 km

17 Capturing Episodic Dust Events The major fraction of desert dust is deposited to the world s oceans in just a few large dust events [Loye-Pilot et al., 1986]. Between October 2006 September 2007 Patagonian station reports were available with visibility reports that denoted suspended dust particles or dust storms. GEOS-Chem proved to simulate 75% of dust events (dust column concentration >1.0 g/m 2 ) reported by stations in Patagonia during the austral summer (DJF), which is characterized by active dust advection. GEOS-Chem daily averaged column dust concentration compared with Patagonian visibility reports during the austral summer of 2006/2007 (DJF).

18 On December 27, 2006 a clear image of dust advection off the coast of Patagonia was captured by MODIS Terra real-time imagery. GEOS-Chem proved to capture this dust outbreak which is shown by elevated mineral dust fluxes for the same time period. MODIS Terra imagery of a dust outbreak off the coast of Patagonia on December 27, :25 UTC. GEOS-Chem predicted average daily dust fluxes (blue line) off the coast of Patagonia (39-52 S) for the month of December 2006.

19 Spatial distribution of percent increase show that DMS oxidation (left) had a much larger influence on DFe dissolution compared to volcanic emissions (right).

20 Decreased Initial DIF % A proportional change in DFe deposition compared to initially prescribed DIF was observed. This punctuates the necessity to constrain initial dust laden -Fe solubility. Presently an initial DIF range of 0.001% to 2% [Bonnet and Guieu, 2004] is assumed. In areas that have limited Fe dissolution during atmospheric transport, such as the SAO, initial DIF becomes the dominant factor in bioavaliable Fe supply. ppb Initial DIF is most important due to limited sulfate concentrations present over Patagonia and the SAO! GEOS-Chem/DFeS predicted total DFe deposition to the SAO during baseline simulation (blue) and the decreased initial DIF sensitivity study (red).

21 Dissolved Species Treated in GEOS Chem/DFeS (Solmon et al., 2009; Meskhidze et al., 2005)

22 Once the concentrations of species are determined they will be speciated by a modified version of ISORROPIA. The model invokes thermodynamic equilibrium between the gas phase and aerosol during each time step. The original model of ISORROPIA treats the sodium, ammonium, chloride, sulfate, nitrate, and water gas/aerosol system. For Fe dissolution calculations, additional reactions were implemented to include gas-phase CO 2, aqueous phase carbonates, and Fe-species. Most important reaction once in solution (aqueous phase reactions) is dissolution of CaCO 3 (calcium carbonate) to form CaSO 4 (gypsum). CaSO 4 is precipitated out of solution during long range transport due to it becoming oversaturated. This mechanism is central in calculating the mineral dust (calcium) buffering effect. Additional aqueous-phase reactions added to ISORROPIA [Meskhidze et al., 2005]. The base model of ISORROPIA is described in depth in Nenes et al [1998].

23 Fe Dissolu=on DFe is produced from the ini=al dissolved iron frac=on (DIF) (0.45%) and hema=te dissolu=on. Rates are determined by a 3 stage kine=c process depending on solu=on ph. *Six aqueous forms and one solid phase of Fe treated within the model. The model only considers the Fe(III) form due to the assumption the deliquesced solution never becomes over-saturated to Fe(III); thus formation and precipitation of Fe(OH) 3 will not occur.

24 Supplemental Material Model pathways to form sulfate (SO 4 ) 1. DMS+OH = SO 2 +MO 2 +CH 2 O 1.20E 11 exp( 260/T) 2. DMS+OH+O 2 = 0.75SO MS A+MO 2 K1 = 1.7E 42exp(7810/T) K2 = 5.5E 31exp(7460/T) K = K1/(1.0+K2*[O2]) 3. DMS+NO 3 = SO 2 +HNO 3 +MO 2 +CH 2 O 1.90E 13 exp(500/t) 4. SO 2 + H 2 O 2 or O 3 (aq) > SO 4 5. SO 2 +OH+M = SO 4 +HO 2 LPL: 3.00E 31(300/T)^3.3 HPL:1.50E 12 Fc: 0.6 Background tropospheric and stratospheric concentra=ons of SO 4 are present with dependence on RH (0, 50, 70, 80, 90, 95, 99%) and Temp. SO 4 is directly emiied by anthropogenics and volcanic ac=vity. Typical concentra=ons of: DMS = 0 10ppb SO 2 = 0 1ppm (rural), up to 50ppm (urban) SO 4 = 0 10 s of ppb NO 3 = 5e9 molecules cm 3