Impacts of α-pinene ozonolysis products on the morphology and optical properties of black carbon

Black carbon (BC) is known to absorb solar radiation and thus have a warming impact on climate. The atmospheric aging of fresh BC has been suggested to enhance BC's light absorption, but the extent of variation observed varies largely in field and laboratory measurements. Biogenic VOCs with much higher abundance than anthropogenic emissions are reactive towards atmospheric oxidants, and act as major sources of secondary organic aerosol (SOA) in the atmosphere. In this study, we performed experiments of BC aging in the presence of condensable organic vapors generated from the ozonolysis of α-pinene. Significant changes in the particle size, mass, effective density, morphology as well as optical properties are observed upon coating of BC by α-pinene ozonolysis products. While the BC aging rate depends on the initial concentrations of precursors forming condensable materials and the particle size, the extent of transformation of BC at a given size is mainly controlled by the volume equivalent coating thickness. The highly fractal BC core is converted to a more compact morphology for a coating thickness of ∼25–30 nm depending on the initial particle size. The absorption and scattering of light by BC also increase by more than 1.29- and 5.30- fold during the coating process. The enhancement of optical properties can be attributed to both formation of coatings and restructuring of the BC core, with the latter accounting for a large fraction of optical enhancement. Our results indicate that the ozonolysis of α-pinene has profound effects on the morphology and optical properties of BC, with a magnitude comparable to sulfuric acid and ammonium nitrate, but much higher than other reported anthropogenic hydrocarbons. Although present in relatively low concentrations, α-pinene oxidation reactions can considerably enhance the impacts of BC on air quality and climate forcing.