Density changes of aerosol particles as a result of chemical reaction

[摘要] This paper introduces the capability to study simultaneously changes in thedensity, the chemical composition, the mobility diameter, the aerodynamicdiameter, and the layer thickness of multi-layered aerosol particles as theyare being altered by heterogeneous chemical reactions. Avaporization-condensation method is used to generate aerosol particlescomposed of oleic acid outer layers of 2 to 30nm on 101-nm polystyrenelatex cores. The layer density is modified by reaction of oleic acid withozone for variable exposure times. For increasing ozone exposure, themobility diameter decreases while the vacuum aerodynamic diameter increases,which, for spherical particles, implies that particle density increases. Theaerosol particles are confirmed as spherical based upon the small divergenceof the particle beam in the aerosol mass spectrometer. The particle andlayer densities are calculated by two independent methods, namely one basedon the measured aerodynamic and mobility diameters and the other based onthe measured mobility diameter and particle mass. The uncertainty estimatesfor density calculated by the second method are two to three times greaterthan those of the first method. Both methods indicate that the layer densityincreases from 0.89 to 1.12g·cm^-3 with increasing ozoneexposure. Aerosol mass spectrometry shows that, concomitant with theincrease in the layer density, the oxygen content of the reacted layerincreases. Even after all of the oleic acid has reacted, the layer densityand the oxygen content continue to increase slowly with prolonged ozoneexposure, a finding which indicates continued chemical reactions of theorganic products either with ozone or with themselves. The results of thispaper provide new insights into the complex changes occurring foratmospheric particles during the aging processes caused by gas-phaseoxidants.

[学科分类] 大气科学