The fractal-like structure of atmospheric soot (e.g., elemental carbon) provides a large surface area available for heterogeneous chemistry in the upper troposphere and lower stratosphere <Blake and Kato, 1995>. One potentially important reaction is ozone decomposition on soot. Although extensively studied in the laboratory, a wide range of reaction probabilities have been observed (&ggr;~10-3 to &ggr;~10-7) which have been attributed to differences in reactivity between fresh (i.e., nonoxidized) versus aged (i.e., oxidized) soot <Schurath and Naumann, 1998>. The importance in understanding soot-ozone chemistry is particularly important in light of recent nighttime field measurements <Berkowitz et al., 2000> made over Portland, Oregon. The data revealed episodes of an anticorrelation between ozone mixing ratio and aerosol surface area density. During these episodes a single scattering albedo in the range 0.8--0.9 was measured, indicating an increased absorptive component of the aerosol, perhaps due to elemental carbon. In addition, an increase in the concentration of aerosols contained in the small size range of the fine mode (<0.1--0.15 μm) was observed, suggestive of new aerosol formation. In this article we attempt to explain these field observations. One explanation of the field observations is ozone loss occurring on atmospheric soot aerosol. Here we present laboratory results obtained using a static aerosol reactor that indicate that direct ozone loss on soot aerosol is unlikely under ambient conditions in the troposphere. An alternative and more likely explanation of the field data is based on ozone-mediated organic aerosol production. This could occur by either nighttime nitrate radical oxidation or direct ozone oxidation of hydrocarbons as suggested previously <Starn et al., 1998; Griffin et al., 1999; Kamens et al., 1999; Yu et al., 1999; De Gouw and Lovejoy, 1998>.