The Effect of Nucleating Particulates an Photochemical Aerosol Formation

Journal/Book: Reprinted from APCA Journal Vol. 16 No. S March 1966. 1966;

Abstract: ALEXANDER GOETZ Ph.D. Assoc. Professor of Physics and RUDOLF F. PUESCHEL M.S. Assoc. Research Physicist California Institute of Technology Pasadena California Summary Systematic tests of the photochemical aerosol formation processes in a steady flow channel System with NO2 1-octene and monodisperse latex nuclei as photochemical reactants in ultra-filtered air can be summarized as follows: The photochemical reaction produces fractional oxidation products of relatively low volatility and with the tendency to agglomerate (or polymerize) and thus to convert from the gaseous (molecular disperse) into the aero-colloidal state by a process of auto-nucleation. In the presence of particulates already present at the time of photo-activation these act as centers for this accumulant formation whereby they grow substantially in size. The tests show that e.g. a 10-fold higher nuclei concentration will produce under the same conditions ten times the mass of total accumulant while autonucleation is suppressed. The growth process appears thus principally different from that of fog formation by H2O-condensation where for the same degree of super-saturation the growth of the nuclei is inversely proportional to their concentration. In the absence of such nucleating particulates the rate of auto-nucleation depends (at the same irradiation level) largely upon the reactant concentration. It follows that if the latter is sufficiently large and the nuclear level low the formation of auto-nucleates will compete with that of accumulants an the nuclei-also that this reaction will favor accumulant formation the more the lower the reactant concentration or the larger that of the nucleating particulates. Due to the fact that in natural or polluted air masses particulates abound in comparison with the relatively low reactant levels photochemical aerosol generation is more likely due to accumulant growth than to autonucleation. This indicates that-aside from the reactant levels-the locally prevailing or induced particulate matter is a very significant factor in the control of photochemical aerosol formation. ___MH

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