The MIT/NCAR Three-Dimensional Interactive Aerosol-Climate Model

1. An Interactive Aerosol-Climate Model based on CAM/CCSM: Progress and challenging issuesChien Wang and Dongchul Kim (MIT) Annica Ekman (U. Stockholm)Mary Barth and Phil Rasch (NCAR)Acknowledgments: NSF, NASA, Ford-MIT alliance, and MIT Global Change Joint Program

2. Concentrations of Aerosols Atmospheric Aerosol Model 6 Aerosol modes Advection, convection, mixing, as well as wet and dry deposition AGCM NCAR CCM3/CAM + CLM Circulation and State of Atmosphere Clouds and Precipitation Radiation Winds, T, H2O, Precipitation & Vertical Fluxes MIT EPPA + Emission Processor NCAR DMS emissions OGCM or SSTData Emissions A-O Exchanges The MIT/NCAR Three-Dimensional Interactive Aerosol-Climate Model References: Kim et al., 2006; Wang 2004; Ekman et al., 2005, 2006; Wilson et al., 2001; Barth et al., 2000; Mayer et al., 2000; Wang et al., 1998; Kiehl et al., 1998; Boville and Gent, 1998

3. SO4 acc BC/SO4 mixed SO4 ait nucleation coagulation condensation dry deposition wet deposition H2SO4(g) SO4 nuc OC pure BC emission growth A Size-Resolving Aerosol Model Prognostic variables: Q and N for each mode + Qbc in mixed mode

4. Size distributions of various modes WANG 12.806 05S

5. Modeling the Mixed Aerosol: The “black carbon core” model; diffusive growth and coagulation are allowed; radiative properties are calculated based on particle size and BC/acid volume ratio (Toon and Ackerman, 1981; revised by W. Wiscombe) An Example for the BC-Core Model: TEM images of various soot-containing aerosols • Air-rich fresh methane soot • Air-rich fresh propane soot • Fuel-rich propane soot after exposure to H2SO4 vapor • Kerosene soot after exposure to H2SO4 vapor. (Courtesy by R. Zhang, Texas A&M; unpublished; 2006)

6. Modeled vs. Observed Surface Sulfate Concentration Seasonal means; Const. emissions; observations are from EPA monitoring stations; Model results = accumulation sulfate + sulfate in mixed mode

7. Model vs. Observations: Vertical Sulfate Profile Obs: ACE Asia, south Japan flights, late April; see Bahreini et al., 2003 Model: April-May mean; Kim et al., 2006 Model vs. Observations: Aerosol Optical Depth (From selected AERONET stations; all are annual means)

8. Critical SSA Calculated based on a simple reflection model (Seinfeld and Pandis, 1998) For:  = 0.15; ocean = 0.06; 20-yr mean surface albedo 0.05 TOA Forcing of Mixed Aerosols Clear sky, no-feedback (W/m2) Internal mixing scheme, size- and BC/acid volume ratio dependent, BC core model

9. Atmospheric Forcing of Black Carbon (W/m2) Mass based, external mixing scheme BC mass = external BC + mixed mode BC Atmospheric Forcing of Mixed Aerosols (W/m2) Internal mixing scheme, size- and BC/acid volume ratio dependent, BC core model

10. Accumulate Sulfate Aitken Sulfate Challenges in connecting aerosol processes with global models Redistribution of Various Aerosols in CRM Simulations (Ekman et al., 2006) BC BC3D: 3h

11. Nucleation scavenging of aerosols • One of the major sinks of aerosols and the connecting point to the indirect forcing • Required information: supersaturation, aerosol size distribution • Current assumption: assuming a given “typical supersaturation” to calculate the minimum size of droplet activation • Recycling of aqueous S(VI) • Determines a significant supply of sulfuric acids in the air for aerosol nucleation and diffusive growth • Required information: stored aqueous concentration of S(VI) and the model (not net) evaporation measure of liquid particles • Current assumption: assuming an arbitrary evaporation ratio of total aqueous S(VI) based on CRM results • Conversion of cloud droplet to rain • Critical process in simulating aerosol’s role in hydrological cycle • Required information: precipitation efficiency in various clouds • Current assumption: ?

12. Summary • Model is “functioning”; aerosol-forced integrations are undergoing • Including size distribution and chemical composition in calculation brings some interesting effects and would provide useful information for aerosol-rainfall and indirect forcing projects • Atmospheric forcing of BC/acid mixed mode aerosol is ~ 2x of that of BC estimated using external mixing scheme; Note their TOA forcings could differ significantly (SSA and surface albedo). • Interesting Issues • The role of aerosol in precipitation and more generally in hydrological cycle • How to use CRM and aerosol process models to derive parameterization of aerosol processes for the global models • Coupling with tropospheric chemistry model (oxidation; heterogeneous reactions, etc.)