Harald Saathoff

Absorption of light by soot particles: determination of the absorption coefficient by means of aethalometers

Duringa soot aerosol measurement campaig n the response of two di*erent aethalometers (AE10 with white light and AE30 with multiwavelength capability) to several types of soot was investigated. Diesel soot, spark-generated carbon particles, and mixtures of these soot particles with ammonium sulfate and oxidation products of � -pinene were used in this evaluation. The determination of the particles light absorption coe%cient (babs) with the AE10 aethalometer is a di%cult task because of an ill-de7ned spectral sensitivity of this instrument. Provided that the proper numerical corrections are performed, the AE30 instrument allows for the measurement of babs over a wide spectral range (� = 450–950 nm). Duringall experiments it was found that with increasing7lter load the optical path in the aethalometer 7lter decreased. As a result, an increased underestimation of the measured aethalometer signals (babs or black carbon mass concentrations) occurs with increasing7lter loads. This e*ect, which is attributed to a “shadowing ” of the particles in the 7ber matrix, is very pronounced for “pure” soot particles while almost negligible for aged atmospheric aerosols. An empirical correction for this bias is presented and requires information on the light scattering behavior (i.e. light scattering coe%cient) of the sampled particles. Without this additional information, the applicability of the instruments is limited. Comparison with a reference method shows that multiple scatteringin the nearly unloaded 7ber 7lter is responsible for enhanced light absorption by a factor of about 2.14. ? 2003 Elsevier Ltd. All rights reserved.

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).

The effect of organic coating on the heterogeneous ice nucleation efficiency of mineral dust aerosols

The effect of organic coating on the heterogeneous ice nucleation (IN) efficiency of dust particles was investigated at simulated cirrus cloud conditions in the AIDA cloud chamber of Forschungszentrum Karlsruhe. Arizona test dust (ATD) and the clay mineral illite were used as surrogates for atmospheric dust aerosols. The dry dust samples were dispersed into a 3.7?m3 aerosol vessel and either directly transferred into the 84?m3 cloud simulation chamber or coated before with the semi-volatile products from the reaction of ?-pinene with ozone in order to mimic the coating of atmospheric dust particles with secondary organic aerosol (SOA) substances. The ice-active fraction was measured in AIDA expansion cooling experiments as a function of the relative humidity with respect to ice, RHi, in the temperature range from 205 to 210?K. Almost all uncoated dust particles with diameters between 0.1 and 1.0??m acted as efficient deposition mode ice nuclei at RHi between 105 and 120%. This high ice nucleation efficiency was markedly suppressed by coating with SOA. About 20% of the ATD particles coated with a SOA mass fraction of 17?wt% were ice-active at RHi between 115 and 130%, and only 10% of the illite particles coated with an SOA mass fraction of 41?wt% were ice-active at RHi between 160 and 170%. Only a minor fraction of pure SOA particles were ice-active at RHi between 150 and 190%. Strong IN activation of SOA particles was observed only at RHi above 200%, which is clearly above water saturation at the given temperature. The IN suppression and the shift of the heterogeneous IN onset to higher RHi seem to depend on the coating thickness or the fractional surface coverage of the mineral particles. The results indicate that the heterogeneous ice nucleation potential of atmospheric mineral particles may also be suppressed if they are coated with secondary organics.

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.

Resurgence in Ice Nuclei Measurement Research

Understanding cloud and precipitation responses to variations in atmospheric aerosols remains an important research topic for improving the prediction of climate. Knowledge is most uncertain, and the potential impact on climate is largest with regard to how aerosols impact ice formation in clouds. In this paper, we show that research on atmospheric ice nucleation, including the development of new measurement systems, is occurring at a renewed and historically unparalleled level. A historical perspective is provided on the methods and challenges of measuring ice nuclei, and the various factors that led to a lull in research efforts during a nearly 20-yr period centered about 30 yr ago. Workshops played a major role in defining critical needs for improving measurements at that time and helped to guide renewed efforts. Workshops were recently revived for evaluating present research progress. We argue that encouraging progress has been made in the consistency of measurements using the present generation of ic...

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.

Deactivation of ice nuclei due to atmospherically relevant surface coatings

The ice nucleation characteristics of Arizona test dust (ATD) and illite clay, surrogates for atmospheric ice nuclei, have been determined at the Aerosol Interactions and Dynamics in the Atmosphere (AIDA) chamber located at the Research Center Karlsruhe in Germany. The objective of this research was to determine the effect of sulfuric acid and ammonium sulfate coatings on the ability of these mineral dust surrogates to nucleate ice in an environment where particles realistically compete for water vapor. Coated ATD particles required higher saturations at all temperatures considered, from −20 to −45 ◦ C, than did identical uncoated particles. Freezing of coated particles often required saturations approaching those for the homogeneous freezing of aqueous solutions of the coating material alone. Less pronounced effects were found for illite, although the presence of a coating consistently increased the saturation or decreased the temperature required for ice formation. Analysis of ice residue at the single particle level suggests that the first coated particles to freeze had thinner or incomplete coatings when compared to particles that froze later in the expansion. This observation highlights a need to verify coating properties since an assumption of homogeneity of a group of coated aerosols may be incorrect. The increase in saturation ratio for freezing suggests that gas-phase uptake of sulfates, a large fraction of which are due to anthropogenic emissions, will reduce the ice and mixed-phase cloud formation potential of atmospheric ice nuclei.

Ice nucleation on flame soot aerosol of different organic carbon content

The aerosol chamber AIDA (Aerosol Interactions and Dynamics in the Atmosphere) was used as a moderate expansion cloud chamber to investigate the effect of the organic carbon (OC) content on the ice nucleation properties of soot aerosol particles. Two different soot samples with OC contents of 16 % (CS16) and 40 % (CS40) where produced with the CAST (Combustion Aerosol Standard) burner operated at different air/fuel (propane) ratios. In dynamic expansion experiments with about 30 %/min increase of relative humidity with respect to ice, the CS16 sample started to nucleate ice crystals at an ice saturation ratio S in of 1.45 (at a temperature of 207 K). This value is very close to the ice saturation ratio of ice nucleation onset on carbon spark generator soot particles coated with a significant amount of sulphuric acid investigated in previous AIDA expansion experiments. A second experiment with CS40 soot performed at almost identical thermodynamic conditions showed ice nucleation onset to occur at S in between 1.5 and 1.7. The formation rate of ice crystals was at least two orders of magnitude less than for CS16 soot, even at ice saturation ratios up to values of 1.9, which is very close to water saturation at a temperature of 207 K. Therefore, increasing the amount of OC seems to significantly suppress the ice nucleation on flame soot particles. In contrast, similar expansion experiments with dry and untreated mineral dust particles (Arizona test dust) in the temperature range 194 to 241 K showed ice nucleation to occur at much lower ice saturation ratios of only 1.05 to 1.15.

The AIDA soot aerosol characterisation campaign 1999

An intensive soot aerosol characterisation campaign was organised in October 1999 at the large aerosol chamber facility AIDA in Karlsruhe, with the participation of scientists from nine Austrian, German, Russian, and Swiss Research Centres and Universities who contributed special equipment and expertise. The main goal was a comprehensive physical and chemical characterisation of soot aerosol from a modern turbo Diesel passenger car equipped with an oxidation catalyst, in comparison with arti8cial soot aerosol (“Palas” soot) from a commercial spark discharge generator which is often used as a surrogate for combustion soot in laboratory studies. Included were experiments with pure ammonium sulphate aerosol as well as its external mixtures with soot aerosols, and their evolution to partially internal mixtures on time scales up to 45 h. E%ects of organic coatings on various aerosol properties, generated in situ by heterogeneous nucleation of products from the reaction of � -pinene with ozone were also investigated. The purpose of this paper is to present an overview of the whole campaign. This includes the description of technical and modelling tools, standard procedures, and the presentation of experimental parameters in tabular form, as a common background for a series of companion papers which focus on selected scienti8c issues. Included is a comparison between Diesel and spark generated soot in terms of their Raman and ESR spectra. The most remarkable di%erence is the large spin density in spark generated soot, which exceeds that of Diesel soot by an order of magnitude. However, the spin densities in both materials are too small to a%ect the surface properties of soot aerosols to a signi8cant extent. ? 2003 Elsevier Ltd. All rights reserved.