A33C-01 INVITED
Investigations on tropospheric multiphase chemistry in field, laboratory experiments and modelling
An overview on recent developments in tropospheric multiphase chemistry studies is given. First, recent laser- based time-resolved studies of radical reaction kinetics in aqueous solution are described where reactions of the hydroxyl and nitrate radical with organic particle phase constituents have been investigated. Such measurements form the basis for the development of chemical mechanisms and the current state of the Chemical Aqueous Phase Radical Mechanism (CAPRAM) is being reviewed. Field experiments on aerosol-cloud-interaction have been performed at the Schmücke measurement site in the Thuringian Forest in Germany and results from these campaigns are being compared to model results. To conclude, currently open questions and possible further developments are discussed.
A33C-02 INVITED
Chemical and physical modification of aerosol particles by cloud processing
While it is well established that sulphate is formed in clouds, only recently it has been suggested that also secondary organic mass can be formed in clouds from a variety of anthropogenic and biogenic organic precursors. A multiphase mechanism will be presented that includes detailed gas and aqueous phase reactions and predicts the formation of various organic compounds that are exclusively formed in clouds (e.g. macromolecular compounds, oxalate). The amount of organic mass formed in clouds depends on a variety of parameters such as initial NOx/VOC ratios, and cloud properties (liquid water content). The importance of these parameters for organic aerosol mass yields will be discussed. Even though the resulting composition of the organic fraction can be very complex, it will be shown that, in general, the hygroscopic properties of the water-soluble organic fraction can be greatly simplified in models and robust predictions of cloud properties (droplet number concentrations) in subsequent cloud cycles are possible.
A33C-03
Variability of CCN Sizes
Cloud condensation nuclei (CCN) are characterized by their critical supersaturation (Sc), which is a function of particle size and chemistry. The relationship between particle size and Sc represents CCN solubility. Small variability of size-Sc measurements has been cited as evidence that CCN can be deduced from particle size measurements alone. Since particle size is easier to measure than particle chemistry or CCN this would have advantages for indirect aerosol effect investigations; e.g., remote sensing of CCN. However, we present size-Sc measurements with a greater range of variability, which appears to limit or cast doubt on the practicality of deducing CCN from particle size measurements. CCN size is determined by passing an aerosol through a differential mobility analyzer (DMA) and then to a CCN spectrometer (i.e., Hudson, 1989), which provides a mean value of Sc. Several different sizes provide a size-Sc relationship (Hudson and Da, 1996). Airborne measurements in three different environments have confirmed Hudson and Da (1996) that CCN are significantly smaller in cleaner air masses where they behave like NaCl or ammonium sulfate. CCN are two to four times larger in polluted air masses. Associated volatility measurements suggest that they are probably sulfate internally mixed with less soluble material. Dusek et al. (2006) found only very large CCN and a small size-Sc range. This led them to conclude that CCN can be determined solely based on particle size. The much larger range of CCN sizes that we measured in a greater variety of air masses indicates that this conclusion might only be valid in polluted air masses such as those that they measured. Dusek et al. (2006) also said that, although there might be different size-Sc ranges in different air masses, if there were limited variability in size-Sc within each air mass, then it still might be possible to deduce CCN from size measurements, as long as the size-Sc relationship is determined for each air mass. However, this would require not only measurements of such, but also an analysis similar to Dusek et al. (2006) for each air mass. Mixed air masses would present a problem. Dusek et al., (2006), Science, 312, 1375-1378. Hudson, (1989), J. Atmos. & Ocean. Techn., 6, 1055-1065. Hudson and Da, (1996), J. Geophys. Res., 101, 4435-4442.
A33C-04 INVITED
Heterogeneous oxidation and photochemistry of PAHs
Until recently, the atmospheric degradation of polycyclic aromatic hydrocarbons (PAHs) was thought to occur primarily via gas phase reaction with OH or NO3. We have measured the heterogeneous oxidation kinetics of naphthalene, anthracene, fluoranthene, phenanthrene, pyrene, and benzo[a]pyrene sorbed on an "urban grime" substrate with gas phase ozone over a wide range of ozone concentrations. Including these kinetics in mass- balance models shows that they may be very important in establishing the fate of PAHs in urban environments. The photolysis rates of anthracene and naphthalene at the air-ice interface were measured, and the kinetics were compared to those observed in water solution and at the air-water interface. Reaction at the ice surface is 4-6 X faster than at the water surface, probably due to an enhancement in the absorption cross sections and/or photolysis quantum yields.
A33C-05
Photochemistry of Model Organic Aerosol Systems
Up to 90 percent of urban aerosol particles have been shown to contain organic molecules. Reactions of these particles with atmospheric oxidants and/or sunlight result in large changes in their composition, toxicity, and ability to act as cloud condensation nuclei. For this reason, chemistry of model organic aerosol particles initiated by oxidation and direct photolysis is of great interest to atmospheric, climate, and health scientists. Most studies in this area have focused on identifying the products of oxidation of the organic aerosols, while the products of direct photolysis of the resulting molecules remaining in the aerosol particle have been left mostly unexplored. We have explored direct photolytic processes occurring in selected organic aerosol systems using infrared cavity ringdown spectroscopy to identify small gas phase products of photolysis, and mass-spectrometric and photometric techniques to study the condensed phase products. The first model system was secondary organic aerosol formed from the oxidation of several monoterpenes by ozone in the presence and absence of NOx, under different humidities. The second system modeled after oxidatively aged primary organic aerosol particles was a thin film of either alkanes or saturated fatty acids oxidized in several different ways, with the oxidation initiated by ozone, chlorine atom, or OH. In every case, the general conclusion was that the photochemical processing of model organic aerosols is significant. Such direct photolysis processes are believed to age organic aerosol particles on time scales that are short compared to the particles' atmospheric lifetimes.
A33C-06
Direct Observation of Extremely Rapid Oligomer Formation Via OH Radical Initiated Oxidation of Organic Aerosols
Ambient aerosols are known to play a significant role in a variety of atmospheric processes such as direct and indirect effects on radiative forcing. Chemical composition can be an important factor in determining the magnitude of these effects (optical density, hygroscopicity, etc.) (1). However, a major fraction(80 - 90%) of organic aerosols can not be resolved on a molecular level. Recent identification of high mass oligomeric species as a major component in laboratory and ambient organic aerosols has received much attention due to the possibility that these species may account for much of the unknown organic mass in ambient aerosols (2, 3). Although, a few mechanisms have been proposed, the origin and formation processes of these compounds remain largely unknown. Here we provide strong evidence for a previously unidentified mechanism of extremely rapid oligomer formation, via OH radical initiated oxidation of organic aerosols. This process appears capable of converting a sizable fraction of an organic particle to higher mass oligomers within only a few hours of exposure to OH radicals at typical atmospheric concentrations. Furthermore, we have found that rapid volatilization, followed by oligomerization, is also important for specific reaction systems, and can lead to the loss of a large fraction (> 60%) of a particle within 15 minutes of exposure to atmospheric OH. We propose that such a rapid processing (oligomerization and volatilization) is possible due to a radical chain reaction which quickly propagates throughout the entire particle and is only initiated by the surface OH reaction. References 1. J. H. Seinfeld, S. N. Pandis, Atmospheric Chemistry and Physics (Wiley, New York, 1998). 2. M. Kalberer et al., Science 303, 1659 (2004). 3. V. Samburova et al., J Geophys Res-Atmos 110, D23210 (2005).
A33C-07
Modeling and Computation of Thermodynamic Equilibrium for Mixtures of Inorganic and Organic Species
A series of modules has been developed in the atmospheric modeling community to predict the phase transition, crystallization and evaporation of inorganic aerosols. Modules for the computation of the thermodynamics of pure organic-containing aerosols have been developed more recently; however, the modeling of aerosols containing mixtures of inorganic and organic compounds has gathered less attention. We present here a model (UHAERO), that is flexible, efficient and rigorously computes the thermodynamic equilibrium of atmospheric particles containing inorganic and organic compounds. It is applied first to mixtures of inorganic electrolytes and dicarboxylic acids, and then to thermodynamic equilibria including crystallization and liquid-liquid phase separation. The model does not rely on any a priori specification of the phases present in certain atmospheric conditions. The multicomponent phase equilibrium for a closed organic aerosol system at constant temperature and pressure and for specified feeds is the solution to the equilibrium problem arising from the constrained minimization of the Gibbs free energy. For mixtures of inorganic electrolytes and dissociated organics, organic salts appear at equilibrium in the aqueous phase. In the general case, liquid-liquid phase separations happen and electrolytes dissociate in both aqueous and organic liquid phases. The Gibbs free energy is modeled by the UNIFAC model for the organic compounds, the PSC model for the inorganic constituents and a Pitzer model for interactions. The difficulty comes from the accurate estimation of interactions in the modeling of the activity coefficients. An accurate and efficient method for the computation of the minimum of energy is used to compute phase diagrams for mixtures of inorganic and organic species. Numerical results show the efficiency of the model for mixtures of inorganic electrolytes and organic acids, which make it suitable for insertion in global three-dimensional air quality models. Preliminary results for mixtures of inorganic and organic species are presented and exhibit the influence of liquid phase separation on the salt cristallyzation.