1. Preliminary Results on the Positive Radiative Forcing of Climate by Organic Aerosol Components Carynelisa Erlick and Natalia Litvak Department of Atmospheric Sciences, The Hebrew University of Jerusalem Yinon Rudich Department of Environmental Sciences, Weizmann Institute of Science

2. Radiative Forcing of Climate [IPCC, Climate Change 2001] Black carbon: positive forcing  warming Organic carbon: negative forcing  cooling –fossil fuel:  0.1±0.3 W m -2, Scientific Understanding “Very Low” –biomass burning:  0.2±0.6 W m -2, Scientific Understanding “Very Low”

3. Negative Forcing Implicit assumption: organic aerosols do not absorb in the solar (visible) part of the spectrum, e.g., –only the relative fraction of black carbon dictates absorbing capacity [Ross et al., 1998; Haywood and Ramaswamy, 1998; Hansen et al., 1998; Penner et al., 1998; Grant et al., 1999; Iacobellis et al., 1999; Myhre et al., 2001] –absorption of visible light indicates the presence of black carbon only [Liousse et al., 1993; Chow et al., 1993; Huffman, 1995]

4. Some Absorption by Organic Aerosols? [Hansen et al., 1980, 1999, 2000] “The most notable feature of the aerosols is that they absorb substantially, with single- scattering albedo typically between 0.9 and 0.95. Much of this absorption comes from the black carbon aerosols, a significant amount comes from soil dust, and a very small amount comes from organic aerosols.” Fig. 2: Optical thickness and single scattering albedo of aerosols in GISS GCM for estimated aerosols of 1990.

5. Other Statements About Organic Absorption [Heintzenberg et al., 1997] : “Organic and mineral aerosols, though much less absorbing than BC, absorb radiation, especially in the near UV.” [Hansen et al., 1998] : “organics” (including black carbon) single scattering albedo = 0.92 (some absorption), but forcing =  0.22 W m -2 (still cooling) [Cooke et al., 1999] : fossil fuel organic carbon visible single scattering albedo ~0.97 (some absorption), but forcing =  0.16 W m -2 (still cooling) [Jacobson, 1991, 2001] : organic aerosol components as sources of ultraviolet light absorption [Bond, 2001] : A “fraction of coal-burning emissions consists of weakly absorbing, yellowish material.”

6. What Are Organic Aerosols? fossil fuel aerosols –primary: incomplete combustion of coal and oil, meat cooking debris, automobile emissions, tire wear, break lining and asphalt dust, natural gas emissions –secondary: oxidation of aromatic compounds biomass burning aerosols –primary: incomplete combustion of forests, savanna, agricultural waste, wood, dung, peat, cigarette smoke –secondary: oxidation of aromatic compounds biogenic –primary: plant debris (leaf wax, leaf fragments), humic matter (refractory macromolecules, such as humic and fulvic acid, produced by microbial degradation of plant remainders), microbial particles (bacteria, fungi, viruses, algae, pollen, spores) –secondary: oxidation of plant-emitted hydrocarbons, such as monoterpenes and pinenes marine aerosols: emissions from phytoplankton and algae, humic matter in sea surface, continental pollution, ship emissions

7. Organic Material on Dust Aerosols [Falkovich et al., 2004] pesticides biomass burning markers polycyclic aromatic hydrocarbons Figure 7: Inner part of particle is a Mg-based mineral; outer coating contains carbon with some oxygen (organic).

8. Which Organic Molecules Might Absorb? UV absorbers: polycyclic aromatic hydrocarbons (strongly), nitrated aromatics (strongly), benzaldehydes and other aldehydes (weakly to moderately), benzoic acids (weakly), aromatic polycarboxylic acids (weakly), phenols (weakly), certain organic bases [nitrobenzene, Foster, 1991, Figure 3] humic material [Havers et al., 1998, Figure 2]

9. What Controls Which Organic Molecules Might Absorb in the Visible Line Broadening line broadening (pressure broadening, different conformations of an organic system, vibrational excitation) of ultraviolet peaks (excitations of valence electrons) into the visible

10. What Controls Which Organic Molecules Might Absorb in the Visible Conjugation additional double and triple bonds  longer chain molecules  p electrons are more delocalized (less strongly bound) polyene

11. What Controls Which Organic Molecules Might Absorb in the Visible Oxidation and Bonding With Inorganic Species example: nitrated aromatics [Jacobson, 1999, Figure 2]

12. Influence of Absorbing Organics on the Properties of Fossil Fuels nominal BC, nonabsorbing OC nominal BC, nominal OC

13. Influence of Absorbing Organics on the Properties of Fossil Fuels nominal BC, nonabsorbing OC nominal BC, strongly absorbing OC

14. Influence of Absorbing Organics on the Properties of Dust nominal dust, nonabsorbing OC nominal dust, nominal OC

15. Influence of Absorbing Organics on the Properties of Dust nominal dust, nonabsorbing OC nominal dust, strongly absorbing OC

16. Influence of Absorbing Organics on the Properties of Dust nonabsorbing dust, nominal OC strongly absorbing dust, nominal OC