Ulrich Schurath

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.

Measurement of Wavelength-Resolved Light Absorption by Aerosols Utilizing a UV-VIS Extinction Cell

The principle, technical details, and performance of the long path extinction spectrometer (LOPES), a new folded–path extinction cell with a spectral range from the mid–UV (200 nm) to the near infrared (1015 nm), is presented. Using nonabsorbing glass beads the measured extinction spectrum of LOPES was validated by Mie calculations and was compared with scattering coefficients in the visible measured by a three–color integrating nephelometer (TSI, mod. 3563). For absorbing aerosols (here soot and biomass burning aerosol with single–scattering albedos of about 0.2 and 0.74 at 550 nm), LOPES was combined with a TSI 3563 nephelometer to determine the wavelength–dependent absorption coefficients from the difference of the extinction and scattering coefficients. These absorption coefficients were found to agree with the measurements by two state–of–the–art absorption techniques, the multiangle absorption photometry (MAAP) and photoacoustic spectrometry (PAS), which measure the absorption coefficient at the single wavelength of 532 nm and 670 nm, respectively. Finally, based on a comprehensive analysis of the TSI 3563 nephelometer errors and their propagation into the determination of the absorption coefficients from the LOPES and nephelometer data, we discuss implications for improving in situ measurements of the optical properties of atmospheric aerosols.

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.

Experimental investigation of ice nucleation by different types of aerosols in the aerosol chamber AIDA : implications to microphysics of cirrus clouds

The aerosol chamber AIDA was used as a moderate expansion cloud chamber with cooling rates at the onset of ice nucleation between -1.3 and -3.0 K min -1 to investigate the nucleation and growth of ice crystals in sulphuric acid, ammonium sulphate, and mineral dust aerosols at temperatures between 196 and 224 K. Supercooled sulphuric acid droplets with mean diameters of about 0.2 to 0.3 μm nucleated ice by homogeneous freezing at RH ice increasing from 144 to 166 % with temperatures from 220 and 196 K. This is in good agreement both with previous results of AIDA experiments and literature data. In contrast, ammonium sulphate particles of similar size nucleated ice at the significantly lower RH ice of 120 to 127 % in the same temperature range. Fourier-Transform infrared (FTIR) extinction spectra of the aerosol revealed that the ammonium sulphate particles, mainly consisted of the liquid phase. The number concentration of ice crystals formed during the homogeneous freezing experiments agree well with model results from the literature. Higher ice crystal number concentrations formed during the ammonium sulphate, compared to the sulphuric acid experiments, can be explained by the also somewhat higher cooling rates at ice nucleation. Deposition ice nucleation on mineral dust particles turned out to be the most efficient ice nucleation mechanism both with respect to RH ice at the onset of ice nucleation (102 to 105 % in the temperature range 209 to 224 K) and the ice crystal number concentration. Almost all mineral dust particles nucleated ice at the lower temperatures.

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.

Fasergekoppeltes In-situ-Laserspektrometer für den selektiven Nachweis von Wasserdampfspuren bis in den ppb-Bereich (Fibre-Coupled In-situ Laser Absorption Spectrometer for the Selective Detection of Water Vapour Traces down to the ppb-Level)

Abstract Ein neues, glasfasergekoppeltes In-situ-Absorptionsspektrometer auf Basis von Nah-Infrarot(NIR)-Diodenlasern eröffnet im Hinblick auf den in Industrie und Forschung bedeutenden In-situ-H2O-Nachweis neue Möglichkeiten, da es erstmals auch in mehrphasigen Gasströmungen die schnelle, selektive, probennahme- und kalibrationsfreie Feuchtemessung gestattet. Aufwändige Tests in der cryogenen Aerosolkammer AIDA ermöglichten mit einer gefalteten Absorptionstrecke von 82 m selbst in dichten Eiswolken eine Nachweisgrenze von 15 ppb (1σ, Δt = 2 s) und demonstrierten gleichzeitig die erste präzise Bestimmung der dynamischen Übersättigung der Gasphase während der Wolkenbildung.

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

Ozone destruction on ice

The destruction of ozone on ice surfaces was studied with an ice-coated fused silica column used as a very long flow tube. Ozone destruction was found to be very slow. At 223 K and 258 K the measured reaction probabilities fall in the range 2 × 10−8 - 4 × 10−10, decreasing with increasing ozone concentration (20–1000 nbar). The observed kinetics are explained by an at least two step mechanism. The low reaction rates observed are insignificant for ozone destruction in cirrus clouds.

Particle Habit Imaging Using Incoherent Light: A First Step toward a Novel Instrument for Cloud Microphysics

The imaging unit of the novel cloud particle instrument Particle Habit Imaging and Polar Scattering (PHIPS) probe has been developed to image individual ice particles produced inside a large cloud chamber. The PHIPS produces images of single airborne ice crystals, illuminated with white light of an ultrafast flashlamp, which are captured at a maximum frequency of ;5 Hz by a charge-coupled device (CCD) camera with microscope optics. The imaging properties of the instrument were characterized by means of crystalline sodium hexafluorosilicate ice analogs, which are stable at room temperature. The optical resolving power of the system is ;2 mm. By using dedicated algorithms for image processing and analysis, the ice crystal images can be analyzed automatically in terms of size and selected shape parameters. PHIPS has been operated at the cloud simulation chamber facility Aerosol Interaction and Dynamics in the Atmosphere (AIDA) of the Karlsruhe Institute of Technology at different temperatures between 2178 and 248C in order to study the influence of the ambient conditions, that is, temperature and ice saturation ratio, on ice crystal habits. The area-equivalent size distributions deduced from the PHIPS images are compared with the retrieval results from Fourier transform infrared (FTIR) extinction spectroscopy in case of small (,20 mm) and with single particle data from the cloud particle imager in case of larger (.20 mm) ice particles. Good agreement is found for both particle size regimes.

Kinetics and mechanism of the heterogeneous interaction of ozone with soot aerosol

INTRODUCTION Atmospheric soot concentrations typically range from 0.002 μg/m3 in the stratosphere up to 40 μg/m3 in the polluted urban troposphere. Due to its irregular agglomerate structure soot aerosol offers a large surface for interactions with reactive trace gases like ozone. Recent model results indicated a possible effect of the soot ozone interaction on tropospheric (Bekki et al., 1997) and stratospheric ozone concentrations (Lary et al., 1997). However, the model predictions are uncertain because they rely on soot surface reaction probabilities mainly measured on short time scales (seconds to minutes) using bulk soot samples and high surface to trace gas concentration ratios, thus being not directly comparable to atmospheric conditions.