1. 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

2. Introduction of sulfate phase & composition Solids: AS (NH 4 ) 2 SO 4 LET (NH 4 ) 3 H(SO 4 ) 2 AHS (NH 4 ) HSO 4 Aqueous: SO4aq SO 4 2-, H +, NH 4 +, H 2 O Extent of neutralization X= [NH 4 ]/2[SO 4 ] X = 1 X = 0.75 X = 0.5 0  X  1 Phase transition deliquesce Crystallization

3. 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 humidityDeliquesce relative humidity

4. 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

5. Approach Emission (SO 2 and NH 3 ) Deposition (dry and wet) Lab data Martin et al. (2003) CRH(x) SO 4 2- NH 4 + (NH 4 ) 3 H(SO 4 ) 2 (NH 4 ) HSO 4 (NH 4 ) 2 SO 4 DRH LET DRH AHS DRH AS aqueous solids Sulfate-water system Optical properties Wang & Martin (2007) Surface reflectance Koelemeijer et al. (2003) RTM (Fu & Liou, 1998) Forcing calculations GEOS-Chem CTM Park et al. (2003)

6. Model results % solids 23% 30% 45% 38% Global annual (natural + anthropogenic) burden: 1.938 mg SO 4 2- m -2 % of solids: 34%

7. 13% 17% 29% 24% % solids Annual global: 0.017; solids: 21%. (natural + anthropogenic) sulfate aerosol optical thickness

8. optical thickness & full-sky forcing of anthropogenic aerosols Global & annual average of % Solids: 26% in , 31% in F clr, 37% in F fky. 16% 22% 36% 27% 26% 31% 47% 38% % solids

9. 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 sideUpper side All aqueous “lower side” and “upper side” difference

10. Regional difference can be ~20%

11. 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 O 3.

13. Sensitivity to other compounded factors compositionhysteresis  F (NH 4 ) 2 SO 4 yes-2%+4% (NH 4 ) 2 SO 4 Upper side5%9% (NH 4 ) 2 SO 4 Upper side Backscattered fraction of aqueous = that of solids 5%25.9%

14. Full-sky (anthropogenic) sulfate direct climate forcing F fky = F clr × cloud fraction Annual global: 0.17 Wm -2 Solids: 37% F fky_sd /F clry_sd = 0.5 F fky_aq /F clr_aq = 0.4