Organic semiconductors that can be fabricated by simple processing techniques and possess excellent electrical performance, are key requirements in the progress of organic electronics. Both high semiconductor charge-carrier mobility, optimized through understanding and control of the semiconductor microstructure, and stability of the semiconductor to ambient electrochemical oxidative processes are required. We report on new semiconducting liquid-crystalline thieno[3,2-b ]thiophene polymers, the enhancement in charge-carrier mobility achieved through highly organized morphology from processing in the mesophase, and the effects of exposure to both ambient and low-humidity air on the performance of transistor devices. Relatively large crystalline domain sizes on the length scale of lithographically accessible channel lengths ( approximately 200 nm) were exhibited in thin films, thus offering the potential for fabrication of single-crystal polymer transistors. Good transistor stability under static storage and operation in a low-humidity air environment was demonstrated, with charge-carrier field-effect mobilities of 0.2-0.6 cm(2) V(-1) s(-1) achieved under nitrogen.

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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 m 3 aerosol vessel and either directly transferred into the 84 m 3 cloud simulation chamber or coated before with the semi-volatile products from the reaction of α-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 μm 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.

Based on results of 11 yr of heterogeneous ice nucleation experiments at the Aerosol Interaction and Dynamics in the Atmosphere (AIDA) chamber in Karlsruhe, Germany, a new empirical parameterization framework for heterogeneous ice nucleation was developed. The framework currently includes desert dust and soot aerosol and quantifies the ice nucleation efficiency in terms of the ice nucleation active surface site (INAS) approach. The immersion freezing INAS densities nS of all desert dust experiments follow an exponential fit as a function of temperature, well in agreement with an earlier analysis of AIDA experiments. The deposition nucleation nS isolines for desert dust follow u-shaped curves in the ice saturation ratio–temperature (Si–T) diagram at temperatures below about 240 K. The negative slope of these isolines toward lower temperatures may be explained by classical nucleation theory (CNT), whereas the behavior toward higher temperatures may be caused by a pore condensation and freezing mechanism. The deposition nucleation measured for soot at temperatures below about 240 K also follows u-shaped isolines with a shift toward higher Si for soot with higher organic carbon content. For immersion freezing of soot aerosol, only upper limits for nS were determined and used to rescale an existing parameterization line. The new parameterization framework is compared to a CNT-based parameterization and an empirical framework as used in models. The comparison shows large differences in shape and magnitude of the nS isolines especially for deposition nucleation. For the application in models, implementation of this new framework is simple compared to that of other expressions.

Abstract. The deposition mode ice nucleation efficiency of various dust aerosols was investigated at cirrus cloud temperatures between 196 and 223 K using the aerosol and cloud chamber facility AIDA (Aerosol Interaction and Dynamics in the Atmosphere). Arizona test dust (ATD) as a reference material and two dust samples from the Takla Makan desert in Asia (AD1) and the Sahara (SD2) were used for the experiments at simulated cloud conditions. The dust particle sizes were almost lognormally distributed with mode diameters between 0.3 and 0.5 µm and geometric standard deviations between 1.6 and 1.9. Deposition ice nucleation was most efficient on ATD particles with ice-active particle fractions of about 0.6 and 0.8 at an ice saturation ratio S i <1.15 and temperatures of 223 and 209 K, respectively. No significant change of the ice nucleation efficiency was found in up to three subsequent cycles of ice activation and evaporation with the same ATD aerosol. This indicates that the phenomenon of preactivation does not apply to ATD particles. The desert dust samples SD2 and AD1 showed a significantly lower fraction of active deposition nuclei, about 0.25 at 223 K and S i <1.35. For all samples the ice activated aerosol fraction could be approximated by an exponential equation as function of S i . This indicates that deposition ice nucleation on mineral particles may not be treated in the same stochastic sense as homogeneous freezing. The suggested formulation of ice activation spectra may be used to calculate the formation rate of ice crystals in models, if the number concentration of dust particles is known. More experimental work is needed to quantify the variability of the Correspondence to: O. Möhler (ottmar.moehler@imk.fzk. de) ice activation spectra as function of the temperature and dust particle properties.

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Abstract.We have retrieved the wavelength-dependent imaginary parts of the complex refractive index for five different Saharan dust aerosol particles of variable mineralogical composition at wavelengths between 305 and 955 nm. The dust particles were generated by dispersing soil samples into a laboratory aerosol chamber, typically yielding particle sizes with mean diameters ranging from 0.3 to 0.4 µm and maximum diameters from 2 to 4 µm. The extinction and absorption coefficients as well as the number size distribution of the dust particles were simultaneously measured by various established techniques. An inversion scheme based on a spheroidal dust model was employed to deduce the refractive indices. The retrieved imaginary parts of the complex refractive index were in the range from 0.003 to 0.005, 0.005 to 0.011, and 0.016 to 0.050 at the wavelengths 955, 505, and 305 nm. The hematite content of the dust particles was determined by electron-microscopical single particle analysis. Hematite volume fractions in the range from 1.1 to 2.7 % were found for the different dusts, a range typical for atmospheric mineral dust. We have performed a sensitivity study to assess how accurately the retrieved imaginary refractive indices could be reproduced by calculations with mixing rule approximations using the experimentally determined hematite contents as input.

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...