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Mixing State of Aerosols: Excess Atmospheric Absorption Paradox

Term Paper Presentation for EAS-6410. Mixing State of Aerosols: Excess Atmospheric Absorption Paradox. Shekhar Chandra Graduate Student, EAS. Introduction:. Excess Atmospheric Absorption Paradox? Mixing State of Aerosols? Externally Mixed Internally Mixed Layered (Core- Shell) Structure.

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Mixing State of Aerosols: Excess Atmospheric Absorption Paradox

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  1. Term Paper Presentation for EAS-6410 Mixing State of Aerosols:Excess Atmospheric Absorption Paradox Shekhar Chandra Graduate Student, EAS

  2. Introduction: • Excess Atmospheric Absorption Paradox? • Mixing State of Aerosols? • Externally Mixed • Internally Mixed • Layered (Core- Shell) Structure

  3. Mixing State of Aerosols Different Cases of Mixing State: Case-1; Case-2 Case-3; Case-4 Internally Mixed

  4. Optical Properties of Aerosols • Absorption Coefficient • Scattering Coefficient • Extinction Coefficient (km-1) • Optical Depth (unit less) • Radiative Forcing (W/m-2) • TOA Forcing (W/m-2) • Surface Forcing (W/m-2) TOA Atmospheric Forcing (W/m-2) Surface

  5. Radiative Impact of Aerosol on Climate Direct Impact Indirect Impact Scattering and Absorption of Radiation Alter Cloud Properties

  6. Motivation • Recent studies suggest that clear sky absorbs more short wave model than predicted my radiative transfer models • (Arking et al., 1996; Halthore et al., 1998; Sato et al., 2003). • Overestimation of diffuse downward irradiance by 9-40% • at the surface in a cloud-free atmosphere while correctly calculating direct normal solar irradiance • (Halthore et al., 1998). • Unidentified absorber in the atmosphere • (Halthore et al., 1998; Sato et al., 2003)

  7. Motivation • Some observers reported an agreement between models and • observations within instrumental uncertainties • (Cess et al., 1995; Kiehl et al., 1998; Satheesh et al., 1999). • In précis some investigators report excellent agreement while • other report discrepancy between models and observations. • Optical properties of aerosols may significantly differ in case • of internally mixed aerosols (Jacobson, M.Z.,2001).

  8. Outline for Current Case Study • In this study, data set is size-segregated aerosol composition • along with observations from ground based radiometers over • Arabian Sea. ARMEX-2003

  9. Observed Facts from ARMEX • Over tropical Indian Ocean, during Moist season (August) estimated surface diffuse radiative fluxes exceed observations by ~92 Wm-2 . Same results showing excess absorption were reported by Charlock et al., (2003) • Investigators have reported that during the moist season (July-August) average optical depth was as high as ~0.7. If we consider case-I (BC shell and sulfate core), corresponding reduction in diffuse radiation will be around 200 Wm-2 • It was proposed by Sato et al., (2003) that estimations and observations do agree well if we increase the BC by a factor ~2-3

  10. Other Facts are: • Recent experiments over southern Arabian Sea have shown that the amount of BC mass fraction reduces from 11% during Jan-March to 0.5% in June (Babu, S.S., et al., 2004). Thus the possibility of increasing BC by a factor ~2-3 to let observed and modeled fluxes agree is an impossibility.In general: • Optical Depth is going up • SSA is going down (Absorption is more) • Forcing is more

  11. Different cases of mixing of BC (black carbon) with other aerosol species Case-I: BC in shell with Sulfate while all other species are externally mixed Case-II: BC in core with Sulfate while all other species are externally mixed Case-III: BC in shell with Sea-Salt while all other species externally mixed Case-IV: BC in core with Sea-Salt while all other species are externally mixed Case-V: All species are externally mixed

  12. Methodology of mixing and simulations • Ratio of core to shell size is constant Shell thickness is constant

  13. Tools Used • Mie treatment of coated sphere for estimating optical properties • SBDART for estimating radiative forcing

  14. SSA for different mixing cases of aerosols

  15. Optical depth contribution of different aerosol types at 550 nm

  16. Over Arabian Sea in August-2003 28% BC aerosols are assumed to be forming core-shell with sea-salt

  17. Précis • Mixing state of aerosols may have possible linkage with ‘excess’ • atmospheric absorption issue and mismatch between models and • observations • Past estimates of climate forcing due to anthropogenic aerosols • represent the lower bound and actual values may be higher than • the current estimates • IPCC has primarily focused on anthropogenic forcing but current • study suggests that when natural and anthropogenic aerosols • co-exist in core-shell form, one must talk about composite forcing

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