Shekhar Chandra Students’ Seminar CAOS@IISc

1. Can the State of Mixing of Black Carbon Aerosols Explain the Mystery of ‘Excess’ Atmospheric Absorption? Shekhar Chandra Students’ Seminar CAOS@IISc

2. Basic Associated Terms Aerosols: Tiny solid particles suspended in the Atmosphere. Mixing State: How Different type of Aerosols are mixed, means the approximated pattern they have followed;though highly variable both with space and time. External Mixing: All type of aerosol species are in complete isolation with another species optically, chemically and physically. Internal Mixing: Different type of aerosols species are arrayed in form of layers and optical properties of such mixture is volume averaged. Core-Shell Structure: One aerosol species is coated over another species to from core-shell structure.

3. Preclusion of the Work: (1) In order to know the response of the earth-atmospheric system to anthropogenic aerosol forcing, its very essential to know the relative impact of various aerosol species on the radiation budget, which in turn depends on various parameters viz., size, shape, chemical property, optical property etc. Most of the models while estimating radiative forcing have assumed that aerosol species are either externally mixed or internally mixed.However,its possible that one aerosol species may be coated over another species and resulting radiative impact may be significantly different than those of externally-mixed or internally-mixed aerosols. Our study suggests that changes in the state of mixing of black carbon (BC) may be on of the possible cause for excess atmospheric absorption as reported by many investigators.We show that past estimates of climate forcing due to anthropogenic black carbon aerosols represent aerosols represent the lower bound and actual values may be larger than current estimates.

4. Motivationfor this work: 1) Seasonal and time dependent similarity between observed and estimated radiative flux. [Kiehl et al.,1998] 2) Presence of an ‘unidentified’ absorber in the atmosphere having properties very similar to black carbon. [Halthore et al.,1998] 3) Overestimation of diffuse downward irradiance at the surface in a cloud-free atmosphere by 9 to 40% while correctly calculating direct normal solar irradiance. [Halthore et al.,1998]

5. Introduction: (1) Aerosols are capable of modifying incoming solar and outgoing long-wave radiation [Charlson et al.,1992;Kaufman et al.,2003]. The consequent changes in radiative fluxes caused by aerosols is known as ‘radiative forcing’ [Intergovernmental panel on Climate Change, 2001]. Its common practice to incorporate measured aerosol properties in radiative transfer models to estimate radiative forcing [Satheesh et al.,1999;Kiehl et al.,2000;Satheesh and Ramanathan, 2000]. (2) Recent studies have suggested that the clear-sky atmosphere absorb more short-wave radiation than predicted by radiative transfer models [Arking,1996;Halthore et al.,1998;Sato et al.,2003].These investigations have shown that radiative transfer models consistently overestimate diffuse downward irradiance at the surface in a cloud-free atmosphere by 9 to 40% while ……….

6. Correctly calculating direct solar normal irradiance [Halthore et al.,1998].These studies have led to a hypothesis that an ‘unidentified’ absorber may be present in the atmosphere, which has optical properties very much similar to black carbon [Halthore et al.,1998;Sato et al.,2003]. Several investigators on the other hand, have found agreement between models and observations [Cess et al.,1995; Kiehl et al.,1998; Satheesh et al.,1999].

7. Layout of our conjecture and there on: [4] Extensive aerosol measurements simultaneous with surface radiative fluxes were carried out at Kashidhoo Climate Observatory (KCO), established in 1998 as a part of the Indian Ocean Experiment (INDOEX) at Kashidhoo island of Republic of Maldives [Satheesh et al.,1999; Ramanathan et al.,2001]. Based on this measurements Satheesh et al., [1999] reported that during February and March (dry season), measured and estimated (assuming that aerosols are externally mixed) surface radiative fluxes agree well within instrumental uncertainties (3 Wm-2). [5] Things may significantly differ in case of core-shell structure than those of externally-mixed or internally-mixed aerosols [Jacobson 1997a, 1997b, 1999,2000,2001].

8. Methodology for Mixing of different type of aerosols and Results. [5]We know that aerosol size distribution is assumed to be following log normal distribution. Firstly, we plot the size number distribution for both type of aerosol species those are to be mixed then subsequently both are mixed such that resulting mixture forming core-shell structure also follows log normal distribution. Case is repeated for all possibilities of core-shell formation in case of each of the species and optical properties are calculated.

10. Cartoon of mixing of different species in core-shell structure

11. Percentage contribution of aerosol optical-depth in different mixing state

13. Discussion [1] The large values of aerosol radiative forcing obtained when there is a change in the mixing state of black carbon raise several issues. Our study implies that the warming due to black carbon aerosols may be much larger than the current estimates [Penner et al., 1998; Haywood et al., 1999; Kiehl et al., 2000; Takemura et al., 2002] based on external mixture as also acknowledged byJacobson [2000] and Sato et al., [2003]. In summary, the current estimates of radiative forcing represent the lower bound and the actual values may be much larger consistent with the few previous reports [Jacobson, 2000; Sato et al., 2003] [2] The results we have presented may have a possible linkage to the issue of ‘excess’ atmospheric absorption paradox as observed by [Arking,1996;Halthore et al.,1998;Sato et al.,2003].

14. [3] IPCC [2001] has focused primarily on anthropogenic aerosol forcing. Our study shows that when natural and anthropogenic aerosols co-exist in mixed state (in the form of core-shell), its very difficult to separate the individual forcing from the composite forcing. Our study also suggests that the presence of natural aerosols influences the anthropogenic aerosol forcing. Another issue is the reduction of cloud cover by BC induced atmospheric heating suggested by Ackerman et al., [2000].

15. Peroration: [1] Our study suggests that past estimates of climate forcing due to anthropogenic carbon aerosols represent the lower bound and actual values may be much larger than the current estimates consistent with the previous reports. [2] Changes in the mixing state of black carbon aerosols may be on of the possible causes of ‘excess’ atmospheric absorption reported by many investigators. [3] More detailed observations and simulations are needed to understand the climate impact of BC on other aerosol species.

16. Thank You