Abstract - Topic #5

Organic Speciation Effects on Regional and Global Scale Atmospheric Chemistry and Climate

Topic Leader: Mark Jacobson
Contributors: Martin Schnaiter & Song Gao

Talks in this session will touch on one or more of these topics. A discussion of numerical techniques for treating secondary organic aerosol formation in atmospheric models will be given by Mark Jacobson. A discussion of the optical properties of mixtures of organic carbon and black carbon will be given by Martin Schnaiter. A discussion of the reaction pathways forming and the composition of secondary organic aerosol will be given by Song Gao.

Numerical methods for treating size-resolved SOA formation and evolution among multiple size distributions in atmospheric models

Mark Z. Jacobson, Stanford University
Numerical techniques for treating formation and evolution of secondary organic and other aerosol types over multiple aerosol size distributions are described. The main processes discussed are homogeneous nucleation, condensation, dissolution, coagulation, and reversible chemistry. All numerical techniques developed are unconditionally stable, mass conserving, and positive definite. In the case of homogeneous nucleation and condensation, the processes are solved together rather than operator split. Some findings include the following: (a) in a competition for vapor between homogeneous nucleation and condensation, the relative importance of condensation increases with an increasing number of background particles; (b) in the absence of a continuous source of new particles, coagulation, condensation, dissolution, hydration, and chemical reaction may internally mix most particles within half a day under moderately polluted conditions; (c) condensation increases the fractional coating of small particles more than large particles;(d) Coagulation internally mixes particles of different original composition over the entire size distribution; (e) coagulation internally mixes a greater fraction of larger than smaller particles; (f) coagulation internally mixes larger particles with more other distributions than it does smaller particles; and (g) a solution real refractive index generally increases with decreasing particle size. Application of these techniques in global-model studies of the mixing state and climate response of aerosols will be discussed.

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Optical properties of black carbon affected by refractive organic coatings

Martin Schnaiter, Forschungzentrum Karlsruhe
Particulate organic components may have a significant influence on the absorption and scattering properties of the atmospheric aerosol. Especially in cases where the organic aerosol mass is mixed internally with other components, the microphysical and thus the optical properties of the aerosol is affected. For instance, when accounting for the internal aerosol mixing in global circulation models a doubling of the climate warming potential of black carbon (BC) was found compared to the external case (1).This study uses a relatively simple optical model for concentric core shell spherical particles to calculate the optics of the aerosol. Since the overall diversity and complexity of aged aerosol could not be accounted for in optical models, lab investigations which simulate the atmospheric aging processes under realistic conditions are desirable to get a more reliable assessment of the direct climate impact of atmospheric aerosols.

The influence of refractive organic coatings on the optical properties of BC was investigated experimentally. This was realized by the nucleation and condensation of low volatile organic compounds formed by the in situ ozonolysis of alpha-pinene in the large aerosol chamber AIDA of Forschungszentrum Karlsruhe. In a first set of experiments secondary organic aerosol (SOA) was nucleated homogeneously to characterize the optical properties of this non-absorbing aerosol. In subsequent experiments BC particles were coated by the condensation of SOA mass. A comprehensive set of optical and microphysical properties were measured for varying amounts of condensed organic matter.

It was found that the organic coating affects strongly the BC optical and microphysical properties. The most important result for the direct climate impact of BC emissions is an increase of the visible BC specific absorption cross section by factor of ~1.8. To judge the quality of the concentric core shell model for the optics of internally mixed aerosol the experimental results were compared with modeling results. It was found that the model represents the BC absorption quite well whereas significant discrepancies remain in case of the angular dependent scattering properties.

References

(1) M.Z. Jacobson, "Strong radiative heating due to the mixing state of black carbon in atmospheric aerosols," Nature 409, 695 (2001).

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Song Gao, California Institute of Technology
An accurate understanding of the molecular composition of secondary organic aerosols (SOA) is crucial for atmospheric chemistry, climate research, and human health studies. However, this has proven a difficult task due to the intrinsic complexity of SOA components and a lack of suitable analytical techniques, particularly for polar compounds as well as high-molecular-weight species. As a result of incomplete speciation
and sampling or analysis artifacts, the current knowledge of SOA composition may be misleading in a number of ways. In light of these uncertainties, a series of controlled laboratory experiments have been carried out in Caltech's 28 m3 dual teflon chambers. The ozonolysis of a series of homologous hydrocarbons (C5 - C8) and alpha-pinene were studied in the presence of seed particles under dark conditions. The SOAs
produced were collected on PTFE membrane filters several hours from the onset of ozonolysis and were analyzed by an LC-MS system for mainly low-molecular-weight (low-MW, MW < 250 Da) species and an LCQ classic ion trap MS for mainly high-molecular-weight (high-MW, MW > 250 Da) species. Both MS were operated with an electrospray ionization source to preserve molecular integrity. Four categories of compounds were found to be dominant low-MW components of SOA from cycloalkene ozonolysis, i.e., dicarboxylic acid, hydroxyl-diacid, carbonyl monoacid, and diacid alkyl ester. Assuming a particle density of 1.4 ug/m3, they together comprise 50 - 85% of the total SOA mass. More interestingly, high-MW (MW 250 - 700
Da) species were found present in most SOA from cycloalkene ozonolysis, with abundance often comparable to and sometimes exceeding that of low-MW species. High-MW species (MW 250 - 1600 Da) were found present in all SOA from alpha-pinene ozonolysis, at a variety of initial seed pH. MS/MS analyses revealed that these components are very likely oligomers, and
they are probably formed through acid-catalyzed heterogeneous reactions. Three such reactions are proposed. Even though oligomers appear to be ubiquitous in SOA regardless of the initial seed pH or state (dry/wet), higher acidity leads to faster formation of larger oligomers (MW > 450 Da), possibly as a result of faster catalysis. With the alpha-pinene ozonolysis system, oligomers in total have a much higher abundance than low-MW species in SOA. If the MS response factors are similar, oligomers are the predominant species in SOA from ozonolysis of alpha-pinene and some cycloalkenes. Reliable quantification of oligomers and accurate knowledge of aerosol density (with high-MW species accounted for) are required for carrying out a correct speciation closure of SOA. Furthermore, the current conceptual model for the atmospheric formation of SOA may need to be revised accordingly, and the presence of oligomers may affect a variety of SOA properties, such as hygroscopicity and radiation, to a yet unknown extent.

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