OH-Initiated Oxidation of m-Xylene on Black Carbon Aging

Laboratory experiments are conducted to investigate aging of size-classified black carbon (BC) particles from OH-initiated oxidation of m-xylene. The variations in the particle size, mass, effective density, morphology, optical properties, hygroscopicity, and activation as cloud condensation nuclei (CCN) are simultaneously measured by a suite of aerosol instruments, when BC particles produced from incomplete combustion of propane are exposed to the oxidation products of the OH-m-xylene reactions. The BC aging process is governed by the coating thickness (Δrve), which is correlated to the reaction time and initial concentrations of m-xylene and NOx. For an initial diameter of 100 nm and Δrve = 44 nm, the particle size and mass increase by a factor of 1.5 and 10.4, respectively, and the effective density increases from 0.43 to 1.45 g cm^-3 due to the organic coating and shrinking of the BC core. The BC particles are fully converted from a highly fractal to nearly spherical form for a Δrve value of 30 nm, as reflected by a decreased BC core size and the dynamic shape factor. The scattering, absorption cross-section, and single scattering albedo are enhanced accordingly. The critical supersaturation for CCN activation is reduced to 0.1%, with an initial particle size of 100 nm and Δrve = 44 nm. Our results imply that the oxidation of m-xylene exhibits larger impacts in modifying the BC particle properties than those for the OH-initiated oxidation of isoprene and toluene.