Sensitivity of sulfate direct climate forcing to the hysteresis of particle phase transitions

Jun Wang, Andrew Hoffman, Scot Martin, Daniel Jacob

Present at 3rd GEOS–Chem Users' Meeting, April. 11, 2007

Introduction of sulfate phase & composition

Solids:AS (NH4)2SO4 LET (NH4)3H(SO4)2

AHS (NH4) HSO4

Aqueous:SO4aq SO4

2-, H+, NH4+, H2O

Extent of neutralization X= [NH4]/2[SO4]

X = 1 X = 0.75 X = 0.5

0 X 1

Phase transitiondeliquesce

Crystallization

To predict the phase transition requires: (a) Current phase (RH back-trajectory) which curve ?(b) RH in next time step which

direction?(c) CRH(X) & DRH(X) Phase changes?

The hysteresis of sulfate phase transition

aqueous

solids

Aerosol phase transition Aerosol direct forcing on climate?

Crystalline relative humidity Deliquesce relative humidity

CRH

DRH

Treatment of hysteresis effect in previous studies

All previous forcing estimates diagnose the sulfate phase based on local RH only.

A full consideration of the hysteresis loop has not been made in the past estimate of sulfate climate forcing.

Chung et al., 2003

F: 18%, Haywood et al., 1997

24%, Martin et al., 2004

Limiting case studies

Approach

Emission(SO2 and NH3)

Deposition(dry and wet)

Lab dataMartin et al. (2003)

CRH(x)

SO42-

NH4+

(NH4)3H(SO4)2

(NH4) HSO4

(NH4)2SO4

DRHLET

DRHAHS

DRHAS

aqueous solids

Sulfate-water system

Optical propertiesWang & Martin (2007)

Surface reflectanceKoelemeijer et al. (2003)

RTM (Fu & Liou, 1998)

Forcing calculations

GEOS-Chem CTMPark et al. (2003)

Model results

% solids

23%

30%

45%

38%

Global annual (natural + anthropogenic) burden: 1.938 mg SO42- m-2

% of solids: 34%

13%

17%

29%

24%

% solids

Annual global: 0.017; solids: 21%.

(natural + anthropogenic) sulfate aerosol optical thickness

optical thickness & full-sky forcing of anthropogenic aerosols

Global & annual average of % Solids: 26% in , 31% in Fclr, 37% in Ffky.

16%

22%

36%

27%

26%

31%

47%

38%

% solids % solids

Sensitivity analysis to the hysteresis effect

: -14%F: -7%

: +10%F: +8%

: +5%F: +5%

: 19%F: 12%

(compared to base case; anthropogenic component only)

Lower side Upper side

All aqueous “lower side” and “upper side” difference

Regional difference can be ~20%

Summary & Outlook

• Phase transition of sulfate aerosols is now developed in GEOS-Chem.

• For anthropogenic component only, solids contribute 26% to sulfate burden, 31% of clear-sky sulfate climate forcing, 37% of full-sky sulfate climate forcing, reflecting the correlation between solids and clear-sky conditions

• Hystereisis can result in the uncertainty in the forcing calculations by 12%.

• Using upper-side hysteresis loop overestimate forcing by +5%. Regional differences can be up to 20%.

• Future research is to look at the implication of modeled results for the study of cirrus cloud formation and modeling of O3.

Sensitivity to other compounded factors

composition hysteresis F

(NH4)2SO4 yes -2% +4%

(NH4)2SO4 Upper side 5% 9%

(NH4)2SO4 Upper side

Backscattered fraction of aqueous = that of solids

5% 25.9%

Full-sky (anthropogenic) sulfate direct climate forcing

Ffky = Fclr × cloud fraction

Annual global: 0.17 Wm-2

Solids: 37%

Ffky_sd/Fclry_sd = 0.5Ffky_aq/Fclr_aq = 0.4