(1) University of California Santa Cruz, CA (2) Laboratoire d’aérologie, Toulouse, France

1. Modelling of Soluble Iron Formation Transport and Deposition to the North Pacific. Anthropogenic impacts. F. Solmon(1,2), P. Chuang(1) , N.Meskhidze(3) (1) University of California Santa Cruz, CA (2) Laboratoire d’aérologie, Toulouse, France (3) North Carolina State University, Raleigh, NC

2. Iron is an important nutrient for marine ecosystems. Atmospheric deposition of dust has been considered as the main source of iron for open ocean. In High Chlorophyll Low Nurient regions, iron input stimulates biological productivity and CO2 pumping => Climate / Paleoclimate studies. At the source, dust iron (~ 3.5 % in mass) is mostly unsoluble i.e not bioavailable At the source, the Dissolved Iron Fraction : DIF~ 0 - 1 % Measurments at remote sites show that DIF increases during atmospheric transport : DIF ~ 1-30 % with a large variability. Modeling effort to better to characterize DIF (e.g Fan et al., 2006; Luo et al., 2007) Possible anthropogenic influences on soluble iron: Atmospheric processing of dust Released by combustion activities North Pacific Ocean Why study atmospheric iron cycle ?

3. Iron dissolution modelling GEOS-CHEM O 3 Anthro. aerosols 2: Scavenging of soluble gaz species by dust H2O Acidic attack H2SO4, HNO3, HCl, carbonates Uptake coeff Fe Therm. Equilibrium (HNO3, NH3), ISORROPIA 3 : Mineral dissolution (production of dissolved iron) Alkaline compounds, NH3 Kinetics processes (calcite, …, hematite) pH sensitive 4 : Chemical speciation, Thermo. Equilibrium, pH Specific mechansims : eg Ca2+ / CaSO4 ISORROPIA Meskhidze et al., 2005 1: Assume an initial mineral composition for the dust Dust alkaline

4. Dissolved iron modelling One mode representative of dust (aggregation of bins 1 and 2) Tracers are transported and removed (wet and dry dep) as dust in GC At the source : FETOT = 3.7 % * DUST ; DIF = FEDI / FETOT = 0.45 % 11 new tracers in GC representing mineral species in the dust mode : (Fe, Ca, Al, Na, Sil, K, Mg, SO42-, NO3-, NH4)aq, (CaCO3)s Dissolved iron FEDI (oxydation III)

5. Test Case Simulation (2 x 2.5, Full chemistry) : MARCH-APRIL 2001 Week 2 Week 3 Week 4 DUST (< 1µm) vert av. (µg.m-3) Anthropogenic SO4 vert av. (ppb) Validation of GC aerosol fields Heald et al., 2006 « LODU » « HIDU » High dust regime Low dust regime

6. Dust mode / Anthro mode aerosol partition Dust mode 30 % 80 % SO4DI NO3DI Results in line with e.g Jordan et al. 2003, Song and Carmichael 2001 Gas scavenging by dust « LODU » SO2 HNO3 vert av. (ppb) GC-ref GC-Fe

7. Dust (bin 1) Dust mode pH µg.m-3 8 « LODU » vert av. 4 1 Dust mode pH evolution OD Lagrangian Box model >

8. Large dust event are not necessarily the most FEDI productive (consistency with the 0D scheme, Meskhidze et al., 2005) Soluble iron formation vert av. HIDU LODU DUST µg.m-3 FEDI (ppt) 3.5 % 3.5 % 3.5 % DIF %

9. Comparison with surface concentration observations 400 ng.m-3 FETOT 6 ng.m-3 FEDI Cor = 0.4 Consistency of model results with obs. 3.5 % Underestimation of FEDI and DIF DIF Transport issues Chen, 2005; remote pacific site 9-26 April 2001 : Soluble Iron highly correlated with dust Missing processes ? photoreduction of FeIII promoted by organic acids

10. Comparison with surface concentration obs. : Kosan Data analysis ; FEDI shows : No significant correlation with total iron carried by dust Good correlation (R=0.67) with BC particles (anthro. combustion) Cor = -0.1 ! Chuang et al., 2005 (ACE-Asia) Model FEDI vs measurments ? April 1-30, 2001

11. Contribution of combustion soluble iron Simple approach : FEbc = BC x 0.02 Slope obtained by Chuang et al., 2005 FEbc (vert. av.ppt) FEDI (vert. av.ppt)

12. Wet deposition of soluble iron FEbc ng.m-2.d-1 70 FEDI 70 « HIDU » « LODU »

13. Conclusions Impact of anthropogenic pollution on soluble iron carried by dusts in the East Asian outflow. Longer term simulations, further validations (global) and sensitivity studies are required. Importance of chemical buffering effects : low intensity events (more frequent) are more efficient to produce soluble iron compared to big storms. =>Validation and further development of dust/anthro heterogeneous chemistry and aerosol µ-physic in GEOS-CHEM is an important issue for iron modelling. Other mechanisms for dust iron processing and DIF increase (iron III photoreduction / dissolution promoted by organic acids). Mechanisms not fully understood yet. Potential importance of continuous anthropogenic emission of soluble iron (Luo et al., 2007). Experimental characterisation of combustion iron and processing is an issue. How will soluble iron deposition and ecosystem response evolve in the future ?

14. Thank you

15. FeDI Vertical distribution 6 km 4 km 2 km Transpacific transport and simulated DIF Week 2 (high dust) Week 3 Week 4 (low dust) DIF ~ 0.15 – 0.6 % DIF ~ 0.6 – 1.1 % DIF ~ 2– 2.7 %

16. Importance of iron in High Nutrients Low Chlorophyll regions Annual average Nitrate concentration in surface water (levitus ocean atlas) e.g / SOIREE expriment Regional Iron Fertilisation experiment in different HNLC: Bloom of biological activity and carbon sequestation A 30 to 90 atm drawdown in surface pCO2 Paleoclimate : ‘The Iron hypothesis’ (Martin) Kohfeld et al., 2005; Archer et al., 2000; Mahowald et al.,1999 … Climate change mitigation (!)