Gas and Aerosol Partitioning Over the Equatorial Pacific

Wenxian ZhangApril 21, 2009

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In the Marine Boundary Layer…

Sea salt aerosol - Main components: Na and Cl - Large water content - Diameter larger than 0.5um - Average lifetime: 6 days (Pham et al., 1995) - Emission rate around 1000 particles m-2 s-1

(Smith et al., 1993)

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In the Marine Boundary Layer…DMS (CH3-S-CH3)- Produced by marine phytoplankton- The dominant sulfur compound emitted from the oceans- Volatile in the ocean- Oxidized by OH and NO3 radicals- Exclusive source for methane sulfonic acid (MSA) and the

dominant source of SO2 in the marine atmosphere MSA (CH3(O)S(O)OH)- One of the product of DMS oxidation- Non-volatile and can exist in aqueous phase- MS/NSS ratio have been studies to understand the sulfur

cycle.

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Over the Marine Boundary Layer…

Buffer Layer- 650~1500m- Intermittent turbulence- Reduced concentration of sea salt

aerosols Free Troposphere- Less vertical mixing- Aerosol size is less than 0.5um

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Gas/Aerosol Partitioning

Equilibrium Condition

μ is the chemical potentialwhose difference is the cause for chemical reactionor for mass transfer fromone phase to another.

K is the equilibrium constant,which is highly depend on T

α is the activity of a species,which depends on the concentration of this component

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Deliquescent Relative Humidity The threshold RH for the solid particle to

absorb water spontaneously Depends on the particle composition

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ISORROPIA A Thermodynamic equilibrium model for

multiphase multicomponent inorganic aerosols

Components:

Sodium, ammonium, chloride, sulfate, nitrate, and water

Possible species:

Gas: NH3, HNO3, HCl, H2O Liquid: NH4,Na,H,Cl,NO3,SO4,HSO4,OH,H2O Solid:

(NH4)2SO4,NH4HSO4,(NH4)3H(SO4)2,NH4NO3, NH4Cl,NaCl,NaNO3,NaHSO4,Na2SO4

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Three box model

Assumptions

- Lifetime of DMS is 0.5day

- DMS is entirely converted to MS and NSS - tMS/tNSS=0.35 - Aerosol sulfate comes from only non – sea salt sulfate

T=285KRH=40%

T=290KRH=90%

T=298RH=80%

EDMS=2.3x109molec cm-2s-1

(Davis et al.,1999)

k1= 10 m2s-1

k2= 1 m2s-1

650m

1500m

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Total Concentrations

DMS MS NSS Cl Na NOx NH4

FT 0.002 0.0005 0.0015 0 0

0.00040.0008 - 0.004

BuL 0.007 0.0018 0.0052 0.022 0.022

BL 0.017 0.0044 0.0126 0.086 0.086

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Aerosol Composition in BL

20 30 40 50 60 70 80 90 1000

0.002

0.004

0.006

0.008

0.01

0.012

tA/ppt

Con

cent

ratio

n (u

mol

/m3

air)

SO4--

HSO4-Na2SO4

20 30 40 50 60 70 80 90 1000

0.01

0.02

0.03

0.04

0.05

0.06

tA (ppt)

Con

cent

ratio

n (u

mol

/m3

air)

Cl-

HCl

20 30 40 50 60 70 80 90 1000

100

200

300

400

500

600

700

Total ammonia (ppt)

Gas

con

cent

ratio

ns (

ppt)

NH3

HClHNO3

• Sulfate poor, sodium rich• Only solid is Na2SO4• HCl can exist as gas• Most nitrate in the aerosol

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Aerosol Composition in BuL

20 30 40 50 60 70 80 90 1000

1

2

3

4

5

6x 10

-3

tA/ppt

Con

cent

ratio

n (u

mol

/m3

air)

SO4

HSO4

20 30 40 50 60 70 80 90 1000.01

0.0105

0.011

0.0115

tA/ppt

Con

cent

ratio

n (u

mol

/m3

air)

Cl

HCl

20 30 40 50 60 70 80 90 1000

1

2

3

4

5

x 10-4

tA/ppt

Con

cent

ratio

n (u

mol

/m3

air)

HNO3

NO3

20 30 40 50 60 70 80 90 1000

0.5

1

1.5

2

2.5

3

3.5x 10

-3

tA/ppt

Con

cent

ratio

n (u

mol

/m3

air)

NH3

NH4

• Sulfate poor, sodium rich• No solid• Nitrate in aerosol

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Aerosol Composition in FT

20 30 40 50 60 70 80 90 1000

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

x 10-3

tA/ppt

Concentr

ation (

um

ol/m

3 a

ir)

SO4

HSO4

NH4HS4LC

NH42S4

tA/tS=1.25 tA/tS=2

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Aerosol Composition in FT

20 30 40 50 60 70 80 90 1000

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6x 10

-3

tA/ppt

Con

cent

ratio

n (u

mol

/m3

air)

NH3

NH4

20 30 40 50 60 70 80 90 1000

1

2

3

4

5

x 10-4

tA/ppt

Con

cent

ratio

n (u

mol

/m3

air)

HNO3

NO3

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pH and MSA gas

20 30 40 50 60 70 80 90 1001

2

3

4

5

6

7

8

tA/ppt

pH

BL

BuL

FT

20 30 40 50 60 70 80 90 100

4.85

4.9

4.95

5

5.05

5.1

5.15

5.2x 10

-4

tA/pptM

SA

(um

ol/m

3 ai

r)

MSA MS- + H+

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Summary

The marine atmosphere becomes more acidic with increase in altitude.

The results suggest that MSA can be released to gas phase in pronounced amount in the dry and acidic FT.

Ammonia does not significantly affect the aerosol/gas partitioning in BL and BuL. However, the aerosol component is sensitive to ammonia in FT.