K. Siegmann

PHYSICAL AND CHEMICAL PROPERTIES OF AIRBORNE NANOSCALE PARTICLES AND HOW TO MEASURE THE IMPACT ON HUMAN HEALTH

Abstract Suspended particles with a diameter below 1 μm act as vehicles transporting toxic chemicals into the human respiratory system. It is therefore of interest to record the intensity of these particles and to determine the source from which they were emitted. It is shown that this can be done by simultaneously measuring the light scattering (LS), the photoelectric charging (PC), and the diffusion charging (DC). Particles carrying polycyclic aromatic hydrocarbons (PPAH) are detected by their large PC and are generated in combustion of organic materials, whereas particles from other sources only exhibit LS and DC. With combustion-generated particles, the ratio of PC/DC allows to determine the type of combustion from which the particles were emitted. In particular, particles from diesel emission can be distinguished from cigarette smoke. It is further demonstrated how changes of chemical composition or of particle size that might occur in the atmosphere are detectable. As an example of an application, we studied the nanoscale particles found on motorways in the industrialized world. The sources of the majority of these particles are diesel motors. The fraction of particle mass due to PPAH is independent of location and weather conditions. However, the particle concentration is much larger in Tokyo and in Paris than in Zurich due to the higher density of diesel vehicles.

Molecular Precursor of Soot and Quantification of the Associated Health Risk

Particles generated in the combustion of organic materials are intrinsically toxic. Our work focuses on finding a way to quantify the health risk despite the complexity of the particles. First the current experimental research on the primary combustion aerosols is summarized. We take samples from various locations inside a laminar methane diffusion flame and freeze their physical and chemical state by rapid dilution with cold inert gas. Formation and growth of large molecules, mostly polycyclic aromatic hydrocarbons (PAH) is detected by mass spectroscopy and laser induced ionization. Fullerenes are also found. Particle size distribution are measured by standard aerosol techniques. To test the sampling, the flame is optionally seeded with palladium aerosol of known size distribution. We believe that the experiments on the model flame reveal some general principles of soot formation, in particular the fact that soot particles do not nucleate by agglomeration of large PAH. Photoemission is applied to study surface properties of soot particles from the flame. It is shown that the surface of the particles is covered with PAH. By heating the PAH can be removed and the properties of the carbon core are revealed. One can thereby distinguish a soot growth- from a soot burnout region in the flame. Time resolved desorption experiments of perylene (a PAH) from model aerosol particles are presented. It is shown that they follow a first order rate law. The photoelectric PAH sensor is introduced as a personal air quality monitor. The danger from inhaling combustion aerosol can be expressed in units of standard cigarettes.

Large natural circular dichroism in photoionization

Abstract Circular dichroism in the photoionization of nanometer-sized aerosol particles from the chiral amino acid tyrosine is investigated. The Kuhn anisotropy factor in photoionization reaches values of 10%. This value is twenty times larger than previous results. We show that it strongly depends on properties of the particles. The largest anisotropies are obtained for highly ordered particles. The enhancement can be explained by an exiton-coupling mechanism between neighbouring molecules geometrically fixed in the crystal lattice. Implications of this large effect in the context of the origin of biological homochirality and for the construction of a bioaerosol sensor are discussed.

Photoelectron spectroscopy without vacuum: nanoparticles in gas suspension

Abstract The photoelectric yield Y of nanoparticles in gas suspension delivers a fingerprint of the particle surface as it interacts with the surrounding gas, but bulk properties of the particle can also be probed. We show in three new experiments how this can be applied to acquire remarkable insight into processes of considerable general interest. In the first example, soot reduction by heterogeneous catalysis is examined. We show that fuel additives lead to the formation of transition metal oxide nuclei in the combustion zone. The carbonaceous matter preferentially condenses on the surface of the nuclei where it is burnt in the last stage of the combustion. In the second example, we present a method for the investigation of the desorption dynamics from particle surfaces using excimer laser pulses for photoemission. The kinetics of thermal perylene desorption from various types of particles is measured with a time resolution of a few milliseconds. The desorption follows a first order rate law and can be described by the Arrhenius model. In the third example, we show that there exists circular dichroism (CD) in photoemission from particles built with chiral molecules. The dependence of CD on the size of the nanoparticles shows that the crystalline order in the particles is important.

THE EFFECT OF FERROCENE ADDITION ON PARTICLE FORMATION AND BURNOUT IN COMBUSTION PROCESSES

THE EFFECT OF FERROCENE ADDITION ON PARTICLE FORMATION AND BURNOUT IN COMBUSTION PROCESSES M. KASPER’, K. SATTLER2, K. SIEGMANN’ and U.MATTER’ ’ ETH-Zurich, Laboratory for Combustion Aerosols and Suspended Particles, 8093 Zurich, Switzerland; 2 University of Hawaii, Department of Physics and Astronomy, Watanabe Hall / 2505 Correa Road, Honolulu Hawaii 96822, USA KEYWORDS Combustion Aerosol; Fuel Additive: Ferrocene; Aerosol Photoemission; Size Distributions The soot suppressing properties of ferrocene (Fe(CsH5)2) are well-known from different combustion systems (Howard and Kausch 1980). Various studies provide evidence that ferrocene reduces carbonaceous matter in combustion by more efficient burnout rather than by inhibition of soot formation (e.g. Zhang and Megaridis 1996). In this work we show that in a ferrocene doped diffusion flame, iron oxide indeed nucleates before soot inception and subsequently serves as soot formation nuclei. This process has been postulated earlier (Ritrievi et al. 1987) but is now directly observed. With the technique of photoelectric charging of the particles in their natural gaseous environment (Burtscher 1992), we obtain a characteristic signal of the chemical state of the particle surface as it depends on the time spent in the combustion zone. Furthermore, we show that the transition metal (iron) reappears in the form of very fine respiratory oxide nuclei in the exhaust. Fig. 1 shows particle size distributions from laminar methane diffusion flames with and without ferrocene vapor added to the fuel gas. In the flame with ferrocene vapor, the first particles appear earlier than in the unseeded flame. These particles are not carbonaceous soot particles, because their photoelectric activity is much lower than that of soot particles sampled slightly above the soot inception point. Most likely, these are iron oxide particles formed from the Fe-ions liberated when the ferrocene molecules are cracked. These iron oxide particles increase their photoelectric activity as combustion proceeds approaching the photoelectric activity of the carbonaceous soot particles after the soot inception point. This shows that the iron oxide particles act as condensation nuclei for the carbonaceous particles as their surface cannot be distinguished any more from the one of the genuine soot particles. After the soot burnout at the tip of the flame, the iron oxide particles reappear with their original low photoelectric activity. The fast drop of the photoelectric activity at the burnout of the soot particles suggests that the carbonaceous matter that condensed on the iron oxide nuclei surface was burnt. Results obtained with an acetylene diffusion flame show that iron oxide incorporated in the soot particles acts as catalyst to promote soot burnout at the tip of the flame. Above the flame the iron oxide particles reappear in the exhaust. The phenomena observed in the model experiment are compared to the more complicated and less transparent case of the real diesel engine.

Carbon Formation in Combustion

Laser ionization mass spectroscopy studies and aerosol analyses of combustion byproducts from inside of a laminar, atmospheric pressure diffusion flame burning with argon diluted methane are presented. Large molecules and small carbonaceous particles are found. The molecules are Polycyclic Aromatic Hydrocarbons (PAHs), with molecular masses up to 800 amu. The diameters of the particles range between 2 and 50 nm. Formation and destruction of particles and PAHs can be monitored as a function of the height in the flame. It is found that particles are formed before large PAHs appear. The relation between PAHs and particles is discussed. It is proposed that PAHs are formed on the surface of the particles and evaporate into the gas-phase when synthesis is completed. A mechanism for soot particle formation in diffusion flames is presented.