Modeling and measurements of Modeling and measurements of oxygen isotope tracers of sulfate oxygen isotope tracers of sulfate

formation: Implications for the sulfur formation: Implications for the sulfur budget in the marine boundary layerbudget in the marine boundary layer

Becky Alexander, Rokjin Park, Daniel J. Jacob, Robert M. Yantosca

Harvard University, Department of Earth and Planetary Sciences

Joël Savarino, Charles. C.W. Lee, Mark H. Thiemens

University of California, San Diego, Department of Chemistry and Biochemsitry

Mian Chin

NASA Goddard Space Flight Center

AGU, Fall 2003

Mass-Dependent FractionationMass-Dependent Fractionation

-60

-40

-20

0

20

40

60

-100 -80 -60 -40 -20 0 20 40 60 80 100

18O

17O

SMOWRain and Cloud Water

Basaltic and Sedimentary Rocks

-80

-60

-40

-20

0

20

40

60

80

-100 -80 -60 -40 -20 0 20 40 60 80 100

18O

17O

Starting O2

Experimental:

O + O2 O3*Thiemens and Heidenreich 1983

Residual O2

Product O3

Measured: Tropospheric O3

Mass-Independent Fractionation:

17O/18O1

Mass-Independent FractionationMass-Independent Fractionation

Mass-Dependent Fractionation:

17O/18O0.5

17O 30‰

Mass-Independent Fractionation:

17O=17O-0.5*18O

Source ofSource of 1717OO SulfateSulfate

SO2 in isotopic equilibrium with H2O :

17O of SO2 = 0 ‰

1) SO2 + O3 (17O=30-35‰) 17O ~ 8-9 ‰

17O of SO42- a function relative amounts of OH, H2O2, and O3 oxidation

Savarino et al., 2000

3) SO2 + OH (17O=0‰) 17O = 0 ‰

2) SO2+ H2O2 (17O=1-2‰) 17O ~ 0.5-1 ‰ Aqueous

Gas

Aqueous versus Gas Phase OxidationAqueous versus Gas Phase Oxidation

Biological regulation of the climate?

(Charlson et al., 1987)

DMSOH

NO3 SO2 H2SO4OH

New particle formation

CCN

H2O2

Light scattering

Gas-phaseAqueous-phase

Aqueous-phase

O3

pH dependency of OpH dependency of O33 oxidation and its oxidation and its

effect on effect on 1717O of SOO of SO442-2-

1.0E-15

1.0E-14

1.0E-13

1.0E-12

1.0E-11

1.0E-10

1.0E-09

1.0E-08

1.0E-07

1.0E-06

1.0E-05

1.0E-04

1.0E-03

1.0E-02

1.0E-01

1.0E+00

1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0

pH

Oxi

dat

ion

rat

e (M

/sec

)

H2O2

O3

1.0E-151.0E-141.0E-13

1.0E-121.0E-111.0E-101.0E-091.0E-08

1.0E-071.0E-061.0E-051.0E-041.0E-03

1.0E-021.0E-011.0E+00

1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0

pH

Oxi

dat

ion

rat

e (M

/sec

)

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

17

O (

‰)

H2O2

O3

Lee et al., 2001

Pre-INDOEX Jan. 1997 INDOEX March 1998

INDOEX cruises – INDOEX cruises – 1717O sulfateO sulfate

•Sulfate collected on front deck with a High Volume Air Sampler

•Collection time of ~48 hours per sample

•Laboratory conversion: SO42- O2

•Triple isotope mass-spectrometer (17O, 18O)

GEOS-CHEMGEOS-CHEM

• Global 3-D model of atmospheric chemistry and transport

•4ºx5º horizontal resolution, 26-30 layers in vertical

• Driven by assimilated meteorology

•Off-line sulfur chemistry (uses monthly mean OH and O3 fields from a coupled chemistry/aerosol simulation)

•Includes aqueous and gas phase chemistry:

S(IV) + OH (gas-phase)

S(IV) + O3/H2O2 (in-cloud, pH=4.5)

S(IV) + O3 (sea-salt aerosols, function of sea-salt

alkalinity flux to the atmosphere)Modeled after results from Chamedies and Stelson (1992)

http://www-as.harvard.edu/chemistry/trop/geos/index.html

GEOS-CHEM GEOS-CHEM 1717O Sulfate SimulationO Sulfate Simulation

SO2 + OH (gas phase) 17O=0‰

S(IV) + H2O2 (in cloud) 17O=0.85‰

S(IV) + O3 (in cloud, sea-salt) 17O=8.75‰

Assume constant, global 17O value for oxidants

17O ‰ method reference

O3 35 Photochemical model

Lyons 2001

H2O2 1.3-2.2 (1.7)

Rainwater measurements

Savarino and Thiemens 1999

OH 0 Experimental Dubey et al., 1997

Pre-INDOEX Sagar Kanya Cruise #120Pre-INDOEX Sagar Kanya Cruise #120

January 1997

0

0.5

1

1.5

2

2.5

3

3.5

-15.0 -10.0 -5.0 0.0 5.0 10.0 15.0

Latitude (degrees)

SO

42-

nss

17

O (

‰)

0

0.5

1

1.5

2

2.5

3

3.5

-15.0 -10.0 -5.0 0.0 5.0 10.0 15.0

Latitude (degrees)

SO

42-

nss

17

O (

‰)

0

0.5

1

1.5

2

2.5

3

3.5

-15.0 -10.0 -5.0 0.0 5.0 10.0 15.0

Latitude (degrees)

SO

42-

nss

17

O (

‰)

ITCZ

0.00

0.50

1.00

1.50

2.00

2.50

-15 -10 -5 0 5 10 15Latitude (degrees)

SO

42

- ns

s 1

7 O (

‰)

0.00

0.50

1.00

1.50

2.00

2.50

-15 -10 -5 0 5 10 15Latitude (degrees)

SO

42

- ns

s 1

7 O (

‰)

0.00

0.50

1.00

1.50

2.00

2.50

-15 -10 -5 0 5 10 15Latitude (degrees)

SO

42

- ns

s 1

7 O (

‰)

INDOEX Sagar Kanya Cruise #133INDOEX Sagar Kanya Cruise #133

March 1998

ITCZ

0% 50% 100%

Percent (%) change in concentrations (yearly average)

Case A: SO2/SO42- concentration without sea-salt chemistry

Case B: With sea-salt chemistry

SO2 (decrease) SO42- (small increase)

|100|

CaseA

CaseBCaseA

Effect of sea-salt chemistry on SOEffect of sea-salt chemistry on SO22 and and

SOSO442-2- concentrations concentrations

50%0% 100%

Effect of sea-salt chemistry on gas-phase Effect of sea-salt chemistry on gas-phase sulfate production ratessulfate production rates

|100|

CaseA

CaseBCaseA

Mar/Apr/May Jun/Jul/Aug

Sep/Oct/Nov Dec/Jan/Feb

Percent (%) decrease (seasonal average):

Aqueous versus Gas Phase OxidationAqueous versus Gas Phase Oxidation

Biological regulation of the climate?

(Charlson et al., 1987)

DMSOH

NO3 SO2 H2SO4OH

New particle formation

CCN

H2O2

Light scattering

Gas-phaseAqueous-phase

Aqueous-phase

O3

ConclusionsConclusions

•Sulfate17O provides information on relative oxidation pathways (gas OH versus aqueous O3,H2O2) in the atmosphere

•Measurements from INDOEX show that O3 oxidation is an important mechanism for sulfate formation over the ocean

•The magnitude and trend of 17O sulfate can be represented in the GEOS-CHEM model only with the addition of chemical formation in sea-salt aerosols

•Sulfate17O measurements provide an additional constraint for chemical transport models improve our understanding of sulfur chemistry and the sulfur budget

AcknowledgementsAcknowledgements

Funding:

Measurements: NSF

Modeling: NASA

NOAA CGC Postdoctoral Fellowship (B. Alexander)