The main goal of this session was to address three sets of questions about organic speciation effects on regional and global chemistry and climate. Below, each question is given, followed by a discussion of the talks associated with the question.

5a. How does organic carbon, particularly its individual components, affect atmospheric chemistry, aerosol scattering and absorption, ultraviolet radiation, and climate?     

Introduction

Organic compounds affect the absorption and scattering properties of atmospheric aerosol particles. In particular, when organic aerosol components are mixed internally with other aerosol components, such as black carbon (BC), the microphysical and optical properties of the aerosol are affected. Several zero-dimensional modeling studies have examined the effect of internal mixing on BC absorption (1-6). In three dimensions, a global modeling study of the evolution of the mixing state and radiative effects of BC, which treated BC as a core component within aerosol particles, found that BC internally mixed as it aged, enhancing its direct radiative forcing by up to a factor of two over that of an external mixture (7). On the regional scale, one 3-D modeling study compared time-series model predictions of absorption coefficients and BC concentration with field data at several locations when BC was treated as internally-mixed as a core component in a core-shell model (8). However, few studies have compared theory with direct laboratory measurements (3,9). Results from the more recent of these studies (9) and subsequent work by the same group were reported here. Major findings are listed below.

Findings

• The measured optical properties of BC are altered significantly due to the coating by OC.

• The increase in particle size due to the coating results in a decrease in the hemispheric backscattering ratio.

• The coating of BC by OC also causes a structural rearrangement of BC aggregates to form more compact particles.

• The measured absorption coefficient of BC may increase by a factor of two due to a coating. The amplification is wavelength dependent, with a factor of 1.8 at 450 nm and 2.1 at 700 nm.

• The BC absorption enhancement due to OC coating is represented well by a core-shell model. The model has greater problems reproducing the single scattering albedo, backscatter ratio, and Angstrom exponent, most likely due to the structural rearrangement of the soot aggregate, which affects scattering more than absorption.

Recommendations

Further laboratory experiments are needed to test the effect on absorption and scattering of black carbon coatings by additional organic compounds, by inorganic compounds, such as sulfate, nitrate, ammonium, and sea spray, and by mixtures of inorganic and organic compounds. Experiments are also needed to examine the effect of particle attachments (resulting from coagulation) in addition to coatings, on absorption and scattering coefficients. Finally, additional model comparisons with experimental data are needed to test different mixing rules and model types (e.g., core-shell, random inclusion).

5b. What organic species participate in heterogeneous chemical reactions and secondary organic aerosol formation?             

Introduction

An accurate understanding of the molecular composition of secondary organic aerosols (SOA) is crucial for atmospheric chemistry, climate research, and human health studies. Although much work on the topic has been done (e.g., 1-6), obtaining a complete understanding has proven to be difficult 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 is incomplete. Three studies were presented that examined the issue of secondary organic aerosol formation. One of these talks examined reaction pathways resulting in the formation of oligomeric and low-molecular-weight components in secondary organic aerosols. Another talk examined water soluble organic carbon in Hong Kong, and a third talk discussed preliminary measurements of carboxylic and dicarboxylic acids with capillary electrophoresis.

Findings

• High molecular weight (250-700 Da) species were found in SOA from cycloalkene ozonolysis in abundance often comparable with and sometimes exceeding that of low-molecular-weight species.
• High molecular weight (250-1600 Da) species were found in all SOA from alpha-pinene ozonolysis at a variety of initial seed pHs. 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, possibly as a result of faster catalysis.
• With the alpha-pinene ozonolysis system, oligomers in total have a much higher abundance than low-molecular-weight 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.
• Water soluble organic carbon (WSOC) in Hong Kong exhibits a bimodal size distribution. The fine mode accounts for the major proportion of WSOC.
• Preliminary results of measurements of low molecular-weight carboxylic and dicarboxylic acids with a prototype analytical method were presented.

Recommendations

Further laboratory experiments are needed to verify the reaction pathways hypothesized and explore other reaction pathways involved. Reliable quantification of oligomers and accurate knowledge of aerosol density (with high-molecular-weight species accounted for) are needed to carry out a correct speciation closure of SOA. The presence of high-MW species in ambient aerosols needs to be explored.

Introduction

Numerical models of the atmosphere are now treating aerosol processes in more detail than in the past. Major processes that affect aerosol evolution include emission, nucleation, condensation, dissolution, coagulation, transport, dry deposition, rainout, and washout. A major fraction of aerosol composition in many regions of the atmosphere is secondary organic matter (SOM). SOM enters aerosol particles primarily by condensation and dissolution. Soluble organic gases dissolve in existing particles containing liquid solutions; high-molecular-weight organics with low saturation vapor pressures condense onto existing particle surfaces. The size distribution of SOM evolves further by coagulation, chemistry, and removal. Relatively few three-dimensional models have simulated speciated SOM formation within aerosol particles to date (e.g., 1-4). A talk in this session examined the treatment of aerosol formation and evolution in numerical models. The talk discussed a sectional approach of examining the issue. It looked at numerical methods of treating aerosol particles in general and specific methods of treating condensation and dissolutional growth (5,6). Most such methods apply to formation of SOM. Findings discussed in the talk are discussed below.

Findings

• A stable numerical solution to the problem of nonequilibrium growth/evaporation at long time step of multiple dissociating acids and a base was discussed.
• The solution eliminates nearly all oscillatory behavior observed otherwise observed at long time step.
• The solution is applicable across the entire relative humidity range, both in the presence and absence of solids.
• Analysis with the scheme suggests that, under some conditions of high relative humidity and concentration, some coarse-model particles <6 micrometers in diameter may reach equilibrium on a time scale of less than an hour.
• In a competition for vapor between homogeneous nucleation and condensation, the relative importance of condensation increases with an increasing number of background particles.
• 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.
• Condensation increases the fractional coating of small particles more than large particles.
• Coagulation internally mixes particles of different original composition over the entire size distribution.
• Coagulation internally mixes a greater fraction of larger than smaller particles.
• Coagulation internally mixes larger particles with more other distributions than it does smaller particles.

Comprehensive field campaign data are needed to validate aerosol modules. Data necessary for useful model comparisons and inputs include highly-resolved (e.g., 5 kmx5 km) emission data, meteorological soundings throughout the measurement domain, surface and elevated gas and aerosol measurements, and radiative measurements. Aerosol measurements should include size and composition, where composition should include organic and inorganic species. Large-scale computational resources are also needed for global-scale aerosol simulations. Finally, additional measurements of organic aerosol composition (e.g., single-particle measurements) and the properties of organic gases and aerosol particles are needed. Some such properties include vapor pressures, imaginary refractive indices as a function of wavelength, and deliquescence properties.

(1)     Jacobson, M. Z., Isolating nitrated and aromatic aerosols and nitrated aromatic gases as sources of ultraviolet light absorption, J. Geophys. Res., 104, 3527-3542, 1999.

(2)     Chung, S.H., and J.H. Seinfeld, Global distribution and climate forcing of carbonaceous aerosols. J. Geophys. Res., 107 (D19), 4407, doi:10.1029/2001JD001397, 2002.

(3)     Griffin, R. J., D. Dabdub, M. J. Kleeman, M. P. Fraser, G. R. Cass, and G. H. Seinfeld, Secondary organic aerosol 3. Urban/regional scale model of size and composition-resolved aerosols, J. Geophys. Res., 107 (D17), 4334, doe:10.1029/2001JD000544, 2002.

(4)     Zhang, Y., B. Pun, K. Vijayaraghavan, S.-Y. Wu, C. Seigneur, S.N. Pandis, M.Z. Jacobson, A. Nenes, and J.H. Seinfeld, Development and application of the Model of Aerosol Dynamics, Reaction, Ionization, and Dissolution (MADRID), J. Geophys. Res., 109, D01202, doi:10.1029/2003JD003501.

(5)     Jacobson, M.Z., Analysis of aerosol interactions with numerical techniques for solving coagulation, nucleation, condensation, dissolution, and reversible chemistry among multiple size distributions, J. Geophys. Res. 107 (D19), 4366, doi:10.1029/ 2001JD002044, 2002.

(6)     Jacobson, M.Z., A solution to the problem of nonequilibrium acid/base gas-particle transfer at long time step. Aerosol Sci. Technol., in review, 2004.