Aging of Soot Aerosol from OH-Initiated Oxidation of Toluene

Ubiquitous in the atmosphere, soot particles interact with solar radiation and alter the cloud formation process. Aging of soot particles through interactions with other atmospheric pollutants can significantly change particle properties and hence modify impacts of soot on the climate and environment. We have investigated the role of secondary organic aerosol formation from oxidation of anthropogenic volatile organic compounds (VOCs) on the properties of soot particles. In an environmental chamber, monodisperse soot particles were exposed to products of the toluene-OH oxidation reaction. An integrated aerosol analytical system was used to simultaneously monitor the changes in the particle size, mass, organic mass faction, morphology, effective density, hygroscopicity, and optical properties. We show that properties of the aged soot particles are largely governed by the thickness of the organic coating. With the development of the organic coating, the soot core is changed from a highly fractal to compact form. The organic coating makes soot aerosol hygroscopic, causing further compaction on particles when exposed to elevated relative humidity. The scattering and absorption cross-sections and single scattering albedo are also enhanced by the organic coating. Our results suggest that oxidation products of anthropogenic aromatic VOCs can significantly alter the composition and properties of soot particles, considerably enhancing the atmospheric impacts of soot aerosol.