Determination of the sorption- and desorption-properties from PAH-coated ultrafine particles with the photoelectric aerosol sensor

Abstract

Polycyclic aromatic hydrocarbons (PAHs) are one of the most important and ubiquitous enviromnental pollutants. Many of them are identified as carcinogens or mutagens. Four different "trace catcher" aerosols, carbon, sodium chloride, aluminium oxide, and Aerosi1200 were used for the following studies. From the different polydisperse aerosols monodisperse fractions were obtained by electrostatic classification. The required particle size and the monodispersity of the particles were verified by diffusion battery measurements. The monodisperse aerosols became coated with PAHs through a known condensation technique [1]. A valuable method to detect the PAH-coated particles is the photoelectric aerosol sensor (PAS) [2, 3]. The PAHcoated particles, charged by irradiation with UV-light (6.7eV) were continuously detected and recorded by application of an aerosol electrometer. The photoelectric activity was measured as a function of the PAH vapour pressure, and the particle size. The desorption of the adsorbed PAHs were investigated with a combination of a thermodenuder, consisting of a heated quartztube, a subsequently fixed unheated charcoal-denuder, and a PAS as detector. the photoelectric signal is observed at low vapour pressures. The linear relationship between vapour pressure and photoelectric signal exists only as long as the diameter of the coated particles remained unchanged. These results were tested independently by fluorimetric determination of probe samples on glass fibre filter. It demonstrates that the PAS "sees" only PAH molecules of the outer layer of a microstructured particle. Comparing the vapour pressure required for one PAH but enriched on different particle systems, different vapour pressures were necessary to get the same photoelectric signal. The PAH vapour pressureincreases in the order: C < NaC1 < A1203 < Aerosil 200. Different factors can explain this behaviour. At first really smaller PAH coverages exists on the particle surfaces and in the second case it is possible that the adsorption of the PAH is followed by a chemical reaction or by a modification of the planarity of the PAH. This modification of the molecular structure could result a decrease of the photoelectric yield which was observed in case of the Aerosil 200 particles.