K.-H. Naumann

UV-VIS-NIR spectral optical properties of soot and soot-containing aerosols

Abstract The UV-VIS-NIR spectral optical properties of soot and soot containing aerosols were investigated in detail during the AIDA Soot Aerosol Campaign 1999. One aim of the campaign was a comprehensive comparison of the microphysical properties of Diesel and spark generator soot. The mass specific extinction cross section at λ=450 nm of Diesel soot is 10.6±0.5 m 2 g −1 which is almost a factor of two larger than the corresponding value of 5.7±0.3 m 2 g −1 measured for spark generator soot. Coagulation-induced particle growth does not affect the soot extinction cross section and has a weak influence on the scattering properties of the soot aggregates. Atmospheric processing of freshly emitted soot was simulated in mixing experiments. The formation of mixed Diesel soot and dry ammonium sulfate particles by coagulation has only a minor effect on the soot absorption cross section. The coating of spark generated soot with organic material results in a strong increase of the single scattering albedo. A significant increase of the absorption coefficient at λ=473 nm during the coating process can be attributed to an enhancement of the specific soot absorption cross section by more than 30%.

Transmission electron microscopical and aerosol dynamical characterization of soot aerosols

Abstract Size, morphology and microstructure of Palas soot, Diesel soot and of Diesel soot/ammonium sulfate mixtures were studied by transmission electron microscopy (TEM). The diameter of the primary particles derived from TEM is 6.6±1.7 nm for Palas soot and 22.6±6.0 nm for Diesel soot. Palas soot predominantly consists of amorphous carbon. In a few cases, nanocrystalline graphite with domain sizes on the order of 1 nm were observed. Primary particles of Diesel soot always show an onion-shell structure of nanocrystalline graphite with domain sizes between 2– 3 nm . Fractal properties of 37 Diesel soot agglomerates were determined from TEM images by two different techniques. The average fractal dimension of Diesel soot derived from TEM is 1.70±0.13. TEM further showed that the initially external mixture of Diesel soot and ammonium sulfate developed with time in a significant degree of internal mixing. A second independent approach to determine the fractal properties of soot is based on computer simulations of the aerosol dynamics. A good reproduction of the time evolution of mass and number concentrations and of the mobility size distribution was achieved. The primary particle diameters obtained from the computer simulations ( 7.3±0.8 nm for Palas soot, 25±3 nm for Diesel soot) are in excellent agreement with the TEM results. The fractal dimension of Diesel soot received from the COSIMA algorithm of 1.9±0.2 (overlap of primary particles was taken into consideration) is consistent with the value obtained from TEM image analysis. For Palas soot, the computer simulation yielded a fractal dimension of 2.0±0.1 (overlap was not corrected, as the overlap coefficient is not known).

Aging of biogenic secondary organic aerosol via gas-phase OH radical reactions

The Multiple Chamber Aerosol Chemical Aging Study (MUCHACHAS) tested the hypothesis that hydroxyl radical (OH) aging significantly increases the concentration of first-generation biogenic secondary organic aerosol (SOA). OH is the dominant atmospheric oxidant, and MUCHACHAS employed environmental chambers of very different designs, using multiple OH sources to explore a range of chemical conditions and potential sources of systematic error. We isolated the effect of OH aging, confirming our hypothesis while observing corresponding changes in SOA properties. The mass increases are consistent with an existing gap between global SOA sources and those predicted in models, and can be described by a mechanism suitable for implementation in those models.

Coating of soot and (NH4)2SO4 particles by ozonolysis products of α-pinene

The ozonolysis of � -pinene in a large aerosol chamber was usedto generate second ary organic aerosol (SOA) mass by homogeneous nucleation, or by heterogeneous nucleation, either on soot, or on (NH4)2SO4 seedaerosols. The rate of the � -pinene + ozone reaction andthe aerosol yieldof ∼19% are in goodagreement with literature data. The organic coating of soot particles leads to a compaction of the fractal agglomerates expressedby an increase in fractal d imension from 1.9 to 2.1 for Diesel soot, andfrom 2.0 to 2.3 for spark generated“Palas” soot. The d ielectric coating of the soot particles with SOA layers between 2 to 11 nm gives rise to a substantial enhancement of their single scattering albedo, from about 0.2 to 0.5, and increases the e+ective absorption coeCcients of both soot types by ca. 30%. The coating of both soot types increases the hygroscopic growth factors (HGF) to values close below the HGF measuredfor pure SOA material d=d0∼1:12 at 90% RH. ? 2003 Elsevier Ltd. All rights reserved.

COSIMA—a computer program simulating the dynamics of fractal aerosols

Abstract The sectional aerosol behavior code COSIMA simulates the time evolution of the structural, dynamical, and optical properties of airborne agglomerate particles as well as their heterogeneous chemical interactions with reactive trace gases utilizing a formalism based on fractal scaling laws. The modeled processes include diffusion to the walls and sedimentational deposition, Brownian and gravitational coagulation, molecular transport from the gas phase to the accessible particle surface, surface adsorption and reactions, gas phase reactions, and dilution effects due to sampling (e.g. during aerosol chamber experiments). The effect of hydrodynamic interactions and shielding on particle mobility is considered within the framework of the Kirkwood–Riseman theory. Rayleigh–Debye–Gans theory is used to deal with light absorption and scattering. The code is validated against new experimental data on the dynamics of Diesel and graphite spark soot as well as recent theoretical and simulation results. Applying the Kirkwood–Riseman formalism to compute the mobility of fractal like agglomerates significantly enhances coagulation rates as well as wall and depositional loss but does not affect the form of the self preserving size distributions attained in the long time regime if Brownian coagulation dominates the aerosol dynamics.

Carbon mass determinations during the AIDA soot aerosol campaign 1999

During the soot aerosol campaign particle carbon mass concentrations of Diesel soot, spark generated “Palas” soot, external and internal mixtures of Diesel soot with (NH4)2SO4, and particles coated with secondary organic aerosol material were determined by several di&erent methods. Two methods were based on thermochemical @lter analysis with coulometric and NDIR detection of evolved CO 2 (total carbon, TC and elemental carbon, EC) and four methods employed optical techniques: aethalometry (black carbon, BC), photoacoustic soot detection (BC), photoelectron emission, and extinction measurement at 473 nm. Furthermore, � -attenuation (total particulate mass), FTIR spectroscopy (sulphate), and COSIMA model calculations were used to determine particle mass concentrations. The general agreement between most methods was good although some methods did not reach their usual performance. TC determined by coulometric @lter analysis showed good correlations with optical extinction, photoacoustic BC signal, and photoelectron emission data. However, the evolution of the photoelectron emission signal correlated with changes in accessible surface area rather than mass concentration and was very sensitive to surface conditions. The BC content as measured by the aethalometers approximately equal to less than 70% of the EC content for Diesel soot and amounts to less than 25% of the EC content of “Palas” soot. ? 2003 Elsevier Ltd. All rights reserved.

Conformal solution methods based on the hard convex body expansion theory

Abstract Naumann, K.-H. and Leland, T.W., 1984. Conformal solution methods based on the hard convex body expansion theory. Fluid Phase Equilibria, 18: 1–45. This paper develops an improvement in the hard convex body equation of state of Boublik by introducing a new parameter which forces the equation to predict the computer calculated virial coefficients for hard sphero-cylinders. The technique is analogous to that used in developing the Carnahan-Starling equation for hard spheres. The new hard convex body equation is applicable to both pure components and mixtures. It agrees with computer simulation data for sphero-cylinders which have a length-width ratio as large as three. As the constituents of a mixture approach spherical shape, the new hard convex body equation approaches the hard sphere mixture equation of Mansoori, Carnahan, Starling and Leland. Analytical relationships have been derived between the dimensionless size parameters in the new equation and the Pitzer acentric factor. The convex body surface, mean radius, and volume parameters required in this equation for each component of the hard convex body mixture can then be determined from the acentric factor and an equation of state for the pure component. New pseudo-parameters have been derived which predict the molecular attraction contribution to properties of a mixture of convex molecules from this attraction contribution in a zero acentric factor reference fluid and in a second reference with a larger acentric factor. A theoretical basis is derived for combining the molecular attraction portion of these two references in the Lee-Kesler form to predict attraction properties in a mixture of convex molecules. A new average acentric factor is derived for this purpose. It is shown that molecular repulsion effects in either pure convex molecule components or in mixtures containing them cannot be predicted by the Lee-Kesler technique.

HETEROGENEOUS PROCESSES INVOLVING ATMOSPHERIC PARTICULATE MATTER

Atmospheric aerosol particles differ widely by size, surface area, and chemical composition. The particles are either dry solid or deliquescent, depending on relative humidity. According to their surface properties, aerosol particles are suspected, and in a number of cases have been shown, to interact with gaseous environmental chemicals, radicals, and other reactive intermediates. However, for many potential surface reactions the reaction probabilities are still unknown. Reaction probabilities which were determined under typical laboratory conditions may differ from reaction probabilities under real atmospheric conditions. The following classes of heterogeneous reactions, which directly or indirectly affect the degradation rates of airborne environmental chemicals and/or modify their atmospheric residence times, will be reviewed: hydrolysis of N205 on atmospheric aerosols; reactions of soot and other oxidising compounds on soot particles which exhibit pronounced surface ageing effects; reactions of NO, and water vapour on soot and other particulate matter which generate HONO as a photochemical OH source; reactions of

Computer simulations on the dynamics of fractal aerosols

Abstract An extended version of the aerosol code PARDISEKO is used to study the dynamics of solid fractal particles. Compared to compact ones, they show slower sedimentation and much enhanced coagulation rates. Most important, however, is the dramatically increased influence of the gravitational coagulation process. Recalculations of coagulation experiments with UO2-aerosols based on the fractal formalism result in excellent agreement between measured and computed data.

Aerodynamic properties of fractal aerosol particles

Abstract Theoretical relations between the mass equivalent and the mobility equivalent radii of fractalparticles are derived for step as well as for exponential cutoff conditions. These are used to determine the fractal dimension and the average radius of the primary particles from form factor measurements. A computer simulation study demonstrates the enormous effect of low fractal dimensions on the sedimentation and coagulation behaviour of aerosol systems.