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

Why study atmospheric iron cycle ?

Atmospheric deposition of dust has been considered as the main source of iron for open ocean.

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

Iron is an important nutrient for marine ecosystems.

In High Chlorophyll Low Nurient regions, iron input stimulates biological productivity and CO2 pumping => Climate / Paleoclimate studies.

Acidic attack

H2SO4, HNO3, HCl, carbonates

H2O

Alkaline compounds, NH3

2: Scavenging of soluble gaz species by dust

Uptake coeff

Therm. Equilibrium (HNO3, NH3), ISORROPIA

Iron dissolution modelling

Dustalkaline

Meskhidze et al., 2005

Fe

3 : Mineral dissolution (production of dissolved iron)

Kinetics processes (calcite, …, hematite)

pH sensitive

4 : Chemical speciation, Thermo. Equilibrium, pH

Specific mechansims : eg Ca2+ / CaSO4

ISORROPIA

GEOS-CHEM

Anthro.aerosols

O 31: Assume an initial mineral composition for the dust

Dissolved iron modelling

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)

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 %

Test Case Simulation (2 x 2.5, Full chemistry) : MARCH-APRIL 2001

Week 2 Week 3 Week 4

« HIDU » « LODU »

High dust regime Low dust regime

DUST (< 1µm) vert av. (µg.m-3)

Anthropogenic SO4 vert av. (ppb)

Validation of GC aerosol fields

Heald et al., 2006

Gas scavenging by dust

SO2 HNO3

GC-ref

GC-Fe

« LODU »vert av. (ppb)

SO4DI NO3DIDust mode

Results in line with e.g Jordan et al. 2003, Song and Carmichael 2001

Dust mode / Anthro mode aerosol partition

30 % 80 %

CaCO3

dissolution

NH3

solubilisation.

NO3

volatilisation

From N. Meskhidze

SO4

Self neutralisation

Low dust case

High dust case

Increasing interaction with acidic compounds

CaSO4 formationCarbonate volatilisation

Dust mode pH evolution

>

OD Lagrangian Box model

Dust mode pH Dust (bin 1)

8

4

1

µg.m-3

« LODU »vert av.

Soluble iron formationHIDU LODU

DUST

µg.m-3

FEDI

DIF % 3.5 % 3.5 % 3.5 %

Large dust event are not necessarily the most FEDI productive (consistency with the 0D scheme, Meskhidze et al., 2005)

(ppt)

vert av.

DIF

FEDI

FETOT

Cor = 0.4

3.5 %

6 ng.m-3

400 ng.m-3

Chen, 2005; remote pacific site

9-26 April 2001 : Soluble Iron highly correlated with dust

Comparison with surface concentration observations

Consistency of model results with obs.

Underestimation of FEDI and DIF

Transport issues

Missing processes ?

photoreduction of FeIII promoted by organic acids

Chuang et al., 2005 (ACE-Asia)

Comparison with surface concentration obs. : Kosan

April 1-30, 2001Model FEDI vs measurments ?

No significant correlation with total iron carried by dust

Data analysis ; FEDI shows :

Good correlation (R=0.67) with BC particles (anthro. combustion)Cor = -0.1 !

FEDI (vert. av.ppt)

FEbc (vert. av.ppt)

Contribution of combustion soluble iron

Simple approach : FEbc = BC x 0.02Slope obtained by

Chuang et al., 2005

Wet deposition of soluble iron

ng.m-2.d-1FEbc

FEDI

70

70

« HIDU » « LODU »

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.

Potential importance of continuous anthropogenic emission of soluble iron (Luo et al., 2007). Experimental characterisation of combustion iron and processing is an issue.

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.

How will soluble iron deposition and ecosystem response evolve in the future ?

Other mechanisms for dust iron processing and DIF increase (iron III photoreduction / dissolution promoted by organic acids). Mechanisms not fully understood yet.

Conclusions

Thank you

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 %

FeDI Vertical distribution

4 km

6 km

2 km

Importance of iron in High Nutrients Low Chlorophyll regions

Annual average Nitrate concentration in surface water (levitus ocean atlas)

Regional Iron Fertilisation experiment in different HNLC:

e.g / SOIREE expriment

Bloom of biological activity and carbon sequestation

A 30 to 90 atm drawdown in surface pCO2

Kohfeld et al., 2005; Archer et al., 2000; Mahowald et al.,1999 …

Paleoclimate : ‘The Iron hypothesis’ (Martin)

Climate change mitigation (!)