Preliminary Results on the Positive Radiative Forcing of Climate by Organic Aerosol Components

Carynelisa Erlick and Natalia LitvakDepartment of Atmospheric Sciences, The Hebrew University of Jerusalem

Yinon RudichDepartment of Environmental Sciences, Weizmann Institute of Science

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”

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]

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.

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.”

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

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).

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]

What Controls Which Organic Molecules Might Absorb in the VisibleLine Broadening

line broadening (pressure broadening, different conformations of an organic system, vibrational excitation) of ultraviolet peaks (excitations of valence electrons) into the visible

What Controls Which Organic Molecules Might Absorb in the VisibleConjugation

additional double and triple bonds longer chain molecules p electrons are more delocalized (less strongly bound)

polyene

What Controls Which Organic Molecules Might Absorb in the VisibleOxidation and Bonding With Inorganic Species

example: nitrated aromatics[Jacobson, 1999, Figure 2]

Influence of Absorbing Organics on the Properties of Fossil Fuels

nominal BC, nonabsorbing OC

nominal BC, nominal OC

Influence of Absorbing Organics on the Properties of Fossil Fuels

nominal BC, nonabsorbing OC

nominal BC, strongly absorbing OC

Influence of Absorbing Organics on the Properties of Dust

nominal dust, nonabsorbing OC

nominal dust, nominal OC

Influence of Absorbing Organics on the Properties of Dust

nominal dust, nonabsorbing OC

nominal dust, strongly absorbing OC

Influence of Absorbing Organics on the Properties of Dust

nonabsorbing dust, nominal OC

strongly absorbing dust, nominal OC