[1] The scattering and absorption properties of black carbon (BC) particles internally mixed with secondary organic aerosol (SOA) were investigated experimentally at the large aerosol chamber facility AIDA. Diesel soot particles were coated with secondary organic compounds produced by the in situ ozonolysis of a-pinene. It was found that the organic coating strongly affects the optical and microphysical properties of the soot aggregates. Amplification factors of the internally mixed BC of 1.8 to 2.1 compared to the specific absorption cross section of externally mixed BC were measured. These amplification factors are well reproduced by a Mie model for concentrically coated spheres over a wide range of organic coating/BC mixing ratios. Other optical properties in particular of thinly coated soot particles, namely, the single scattering albedo, the Å ngstrøm exponent, and the hemispheric backscattering ratio, are less well reproduced by the model, most likely because of the restructuring and the incomplete enclosure of the porous soot aggregates.

[1] The low-temperature aerosol and cloud chamber AIDA (Aerosol Interactions and Dynamics in the Atmosphere) of Forschungszentrum Karlsruhe was used to investigate the effect of sulfuric acid coating on the ice nucleation efficiency of soot aerosol particles from a spark discharge generator. The uncoated (sulfuric acid-coated) soot aerosol showed a nearly lognormal size distribution with number concentrations of 300-5000 cm À3 (2500-56,000 cm À3 ), count median diameters of 70-140 nm (90-200 nm), and geometric standard deviation of 1.3-1.4 (1.5-1.6). The volume fraction of the sulfuric acid coating to the total aerosol volume concentration ranged from 21 to 81%. Ice activation was investigated in dynamic expansion experiments simulating cloud cooling rates between about À0.6 and À3.5 K min À1. At temperatures between 186 and $235 K, uncoated soot particles acted as deposition nuclei at very low ice saturation ratios between 1.1 and 1.3. Above 235 K, ice nucleation only occurred after approaching liquid saturation. Coating with sulfuric acid significantly increased the ice nucleation thresholds of soot aerosol to saturation ratios increasing from $1.3 at 230 K to $1.5 at 185 K. This immersion mode of freezing nucleates ice well below the thresholds for homogeneous freezing of pure sulfuric acid solution droplets measured in previous AIDA experiments. A case study indicated that in contrast to the homogeneous freezing the nucleation rate of the immersion freezing mechanism depends only weakly on relative humidity and thereby the solute concentration. These results show that it is important to know the mixing state of soot and sulfuric acid aerosol particles in order to properly assess their role in cirrus formation. Citation: Möhler, O., et al. (2005), Effect of sulfuric acid coating on heterogeneous ice nucleation by soot aerosol particles,

Abstract. The homogeneous freezing of supercooled H2SO4/H2O solution droplets was investigated in the aerosol chamber AIDA (Aerosol Interactions and Dynamics in the Atmosphere) of Forschungszentrum Karlsruhe. 24 freezing experiments were performed at temperatures between 189 and 235 K with aerosol particles in the diameter range 0.05 to 1 µm. Individual experiments started at homogeneous temperatures and ice saturation ratios between 0.9 and 0.95. Cloud cooling rates up to -2.8 K min-1 were simulated dynamically in the chamber by expansion cooling using a mechanical pump. Depending on the cooling rate and starting temperature, freezing threshold relative humidities were exceeded after expansion time periods between about 1 and 10 min. The onset of ice formation was measured with three independent methods showing good agreement among each other. Ice saturation ratios measured at the onset of ice formation increased from about 1.4 at 231 K  to about 1.75 at 189 K. The experimental data set including thermodynamic parameters as well as physical and chemical aerosol analysis provides a good basis for microphysical model applications.

Abstract. Secondary organic aerosol (SOA) formation has been investigated as a function of temperature and humidity for the ozone-initiated reaction of the two monoterpenes α-pinene (243-313 K) and limonene (253-313 K) using the 84.5 m 3 aerosol chamber AIDA. This paper gives an overview of the measurements done and presents parameters specifically useful for aerosol yield calculations. The ozonolysis reaction, selected oxidation products and subsequent aerosol formation were followed using several analytical techniques for both gas and condensed phase characterisation. The effective densities of the SOA were determined by comparing mass and volume size distributions to (1.25±0.10) g cm −3 for α-pinene and (1.3±0.2) g cm −3 for limonene. The detailed aerosol dynamics code COSIMA-SOA proved to be essential for a comprehensive evaluation of the experimental results and for providing parameterisations directly applicable within atmospheric models. The COSIMA-assisted analysis succeeded to reproduce the observed time evolutions of SOA total mass, number and size distributions by adjusting the following properties of two oxidation product proxies: individual yield parameters (α i ), partitioning coefficients (K i ), vapour pressures (p i ) and effective accommodation coefficients (γ i ). For these properties temperature dependences were derived and parameterised. Vapour pressures and partitioning coefficients followed classical Clausius -Clapeyron temperature dependences. From this relationship enthalpies of vaporisation were derived for the two more and less volatile product proxies of α-pinene: (59±8) kJ mol −1 and (24±9) kJ mol −1 , and limonene: (55±14) kJ mol −1 and (25±12) kJ mol −1 . The more volatile proxy components had a notably low enthalpy Correspondence to: H. Saathoff (harald.saathoff@imk.fzk.de) of vaporisation while the less volatile proxy components gave enthalpies of vaporisation comparable with those of typical products from α-pinene oxidation, e.g. pinonaldehyde and pinonic acid.

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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. Zusammenfassung An der Aerosolkammer AIDA (Aerosol-Interaktionen und Dynamik in der Atmosphäre) des Forschungszentrums Karlsruhe wurde die heterogene Eisnukleation auf Rußpartikeln mit unterschiedlichem Gehalt an organischem Kohlenstoff (OC) untersucht. In der Aerosolkammer wurden hierfür dynamische Wolkenprozesse durch eine kontrollierte Druckabsenkung des Kammervolumens simuliert. Ausgehend von nahezu eisgesättigten Bedingungen nahm die relative Eisfeuchte während der Eisnukleationsexperimente mit einer Rate von etwa 30 %/min zu. Durch Variation des Brennstoffgemisches (Propan zu synthetischer Luft) für den CAST (combustion aerosol standard)-Brenner konnten zwei verschiedene Rußproben mit einem OC-Gehalt von 16 % (CS16) und 40 % (CS40) erzeugt werden. Bei dem Expansionsexperiment mit CS16-Ruß, das bei einer Temperatur von etwa 207 K durchgeführt wurde, entstanden Eiskristalle bei einem Eissättigungsverhältnis von 1,45, nahe dem Wert für die Eisnukleation auf Rußpartikeln aus einem Graphit-Funkengenerator, die mit Schwefelsäure beschichtet worden waren. Das zweite Experiment mit CS40-Ruß wurde bei nahezu identischen thermodynamischen Bedingungen durchgeführt. Es bildeten sich nur sehr wenige Eispartikel bei einem Eissättigungsverhältnis zwischen 1,5 und 1,7. Bis zu einem maximalen Eissättigungsverhältnis von etwa 1,9 war die Bildungsrate von Eispartikeln auf CS40-Ruß mindestens um zwei Größenordnungen niedriger als auf CS16-Ruß. Die Eisbild...

Abstract. Hydroxyl radicals (OH) are the major oxidizing species in the troposphere. Because of their central importance, absolute measurements of their concentrations are needed to validate chemical mechanisms of atmospheric models. The extremely low and highly variable concentrations in the troposphere, however, make measurements of OH difficult. Three techniques are currently used worldwide for tropospheric observations of OH after about 30 years of technical developments: Differential Optical Laser Absorption Spectroscopy (DOAS), Laser-Induced Fluorescence Spectroscopy (LIF), and Chemical Ionisation Mass Spectrometry (CIMS). Even though many measurement campaigns with OH data were published, the question of accuracy and precision is still under discussion.Here, we report results of the first formal, blind intercomparison of these techniques. Six OH instruments (4 LIF, 1 CIMS, 1 DOAS) participated successfully in the ground-based, international HOxComp campaign carried out in Jülich, Germany, in summer 2005. Comparisons were performed for three days in ambient air (3 LIF, 1 CIMS) and for six days in the atmosphere simulation chamber SAPHIR (3 LIF, 1 DOAS). All instruments were found to measure tropospheric OH concentrations with high sensitivity and good time resolution. The pairwise correlations between different data sets were linear and yielded high correlation coefficients (r 2 =0.75−0.96). Excellent absolute agreement wasCorrespondence to: H.-P. Dorn (h.p.dorn@fz-juelich.de) observed for the instruments at the SAPHIR chamber, yielding slopes between 1.01 and 1.13 in the linear regressions. In ambient air, the slopes deviated from unity by factors of 1.06 to 1.69, which can partly be explained by the stated instrumental accuracies. In addition, sampling inhomogeneities and calibration problems have apparently contributed to the discrepancies. The absolute intercepts of the linear regressions did not exceed 0.6×10 6 cm −3 , mostly being insignificant and of minor importance for daytime observations of OH. No relevant interferences with respect to ozone, water vapour, NO x and peroxy radicals could be detected. The HOxComp campaign has demonstrated that OH can be measured reasonably well by current instruments, but also that there is still room for improvement of calibrations.

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