H. Burtscher

Hygroscopic properties of carbon and diesel soot particles

Laboratory experiments were performed in order to study the hygroscopic properties of freshly produced carbon and diesel soot particles at subsaturations (i.e. at relative humidity < 100%). Wetted carbon aggregates collapsed to a more compact structure. In contrast, the diesel combustion particles exhibited a much smaller restructuring combined with condensational growth. The hygroscopicity of the diesel particles was enhanced when the sulfur content of the fuel was increased or when the particles were subjected to an ozone and UV pre-treatment. By extrapolating the data with a Kohler model critical supersaturations were calculated. Freshly emitted combustion particles are unlikely to act as cloud condensation nuclei whereas an aging of the particles will enhance their nucleation ability.

Separation of volatile and non-volatile aerosol fractions by thermodesorption : instrumental development and applications

Abstract An instrument to remove volatile material from aerosol particles by thermal desorption is presented. The thermodesorber consists of a heated tube, where volatile material is desorbed from the particles, and a water- or air-cooled tube, consisting of activated charcoal. This last tube removes desorbed material and thus prevents it from re-adsorbing onto particles. Although designed for measuring particulate emissions from combustion processes it can also be applied to atmospheric aerosols. After theoretical and experimental determination of thermodesorber operating characteristics (temperature profile, losses, removal of desorbed material), examples of applications in several fields are given. Examples of atmospheric measurements at several remote and urban sites are presented. In combustion technology, the thermodesorber is applied to remove all volatile materials, allowing separation of volatile species and the non-volatile core (mainly elemental carbon) of combustion particles. Finally, the thermodesorber is used to study adsorption and desorption processes of polycyclic aromatic hydrocarbons on particles.

Aerosol emission in a road tunnel

Abstract Continuous measurements of aerosol emissions were performed within the scope of emission measurements in the Gubrist tunnel, a 3250 m long freeway tunnel near Zurich, Switzerland, from 20 September to 26 September 1993. The particles in the respirable size range (d

Real-time characterization of ultrafine and accumulation mode particles in ambient combustion aerosols

Abstract The diffusion charging sensor (DC), photoelectric aerosol sensor (PAS) and condensation particle counter (CPC) are real-time particle instruments that have time resolutions s and are suitable for field use. This paper shows how the relative fraction of nuclei mode particles (D⩽50 nm ) in ambient combustion aerosols can be determined, along with the coverage degree of the respective accumulation mode particles with a modal diameter of ∼100 nm . Main tools for interpretation are the diameter of average surface D Ave, S (obtained from CPC and DC measurements) and PAS/DC versus D Ave, S scatter plots. Compared to the scanning mobility particle sizer (SMPS), which is a standard instrument for aerosol particle size distribution measurements, the presented method has a limited accuracy, but is substantially faster. Additionally, it is experimentally less demanding than SMPS measurements, especially for field applications.

An accurate, continuously adjustable dilution system (1:10 to 1:104) for submicron aerosols

A new type of dilution system designed for submicron aerosols with high and medium particle concentrations is presented. The corresponding device was built on a small and portable scale. Dilution ratios can be selected continuously in the range between 1:10 and 1:104. Calibration measurements showed a linear relation between the dilution ratio and the control parameters within ± 2%. The dilution ratio was found to be independent of particle diameter in the size range between 0.01 and 0.7 μm. The dilution system was tested by sampling and measurement of aerosol particles from the stacks of different emission sources.

Interaction between carbon or iron aerosol particles and ozone

Abstract Ozone destruction by submicron particles has been experimentally investigated by measuring the ozone volume mixing ratio as a function of the particle concentration and by measuring the particle size with monolayer resolution. NOx-free O3 production in an air-like gas mixture is achieved by means of excimer UV radiation. Submicron carbon or iron aerosol particles are found to destroy ozone efficiently. For fresh carbon particles suspended in argon, the sticking coefficient of 03 to the particles is of the order of 10−4. Ozonised carbon or iron aerosol particles grow about 0.3 nm in diameter, which is attributed to adsorption of atomic oxygen and/or ozone molecules to the surface. If the ozonised carbon particles are separated from the ozone-containing gas stream, they shrink in size by more than 0.1 nm, indicating that carbon surface! atoms are transformed into CO or CO2. The results suggest that particles present in the stratosphere may represent a significant sink for O3.

Characterization of particles in combustion engine exhaust

A number of aerosol analysis tools, including differential mobility analysis, total- and black-carbon mass measurement, photoelectron emission, and low-pressure impaction are applied to characterize particles in the exhaust of technical combustion systems burning liquid fuel. These are a spark-ignition engine, two diesel engines and an oil stove. Black-carbon fraction, volatile species and structure of agglomerates are investigated at different operating conditions. A very high mainly constant volatile fraction is found for the spark ignition engine. For the diesel engine, the volatile fraction is much lower and decreases significantly with increasing load. On the other hand, the black-carbon concentration increases with increasing load.

Photoelectric quantum yield of free silver particles near threshold

The absolute photoelectric quantum yieldY(hv), 4.6 eV≦hv≦5.6 eV, of clean free silver particles with radii 2.7 nm, 3.8 nm and 5.4 nm is measured for the first time. The particles are produced and transported in ultrapure Helium. The photoemission constantc in the Fowler-Nordheim law is enhanced by a factor of ≈200 for particles compared to a flat surface.Y(hv) shows the same structure for different radii and flat surfaces. The dependence of the work function Φ on the radius follows the classical electrostatic model.

Multiple wavelength aerosol photoemission by excimer lamps

Abstract Submicron aerosol particles of carbon, different metals and exhaust particles from a diesel engine are characterized via the charge p they obtain by photoelectron emission when irradiated by specially designed excimer lamps. The high intensity of these monochromatic lamps makes it possible to charge all particles to the limit defined by Coulomb recapture of the electron. This maximum charge pmax(R, (Φ) depends only on the particle radius R and photothreshold Φ∞ and can therefore be used for identification and separation of different materials as well as detection of small changes in the state of the surface in terms of the outermost atomic layer. The latter effect is demonstrated on C, Cu and diesel particle coated with Zn or surface oxidized with O3. The function pmax(R, Φ) for spherical particles holds well for agglomerated structures, if particle size is defined by the electric mobility of the particles. As long as less than 10% of pmax are reached at short radiation time or small intensity, the photoemission probability μ linearly depends on a photoemission parameter c related to the absorption behaviour of the particles. The charge distribution under these conditions is explained by Poisson statistics. The parameters Φ∞, c and R can all be determined using excimer lamp photoelectric sensors containing lamps of different wavelengths. Excimer lamps represent an interesting alternative to conventional charging principles in electrostatic precipitators for gas cleaning.

Characterization of ultrafine aerosol particles in Mt. Etna emissions

Particulate emissions from Mt. Etna in the fine-size range below 100 nm were studied in June and September 1989. The aerosol particles were characterized by size, concentration and photoelectric activity. These quantities are sensitive to the physical and chemical properties of the magma. Concentrations varied from 104 to 107 cm-3. The size distributions peak below 20 nm (radius) and are very narrow. The particles are generated mainly by nucleation and condensation of magmatic volatiles in a strong temperature gradient. The photoelectric activity of these particles can indicate high magma levels and increased exsolution of volatiles. It is therefore related to the observed activity of the respective crater and may be helpful as a prediction tool when used in conjunction with other volcano-monitoring techniques.