Atmospheric Black Carbon: Measurements in the Los Angeles Atmosphere and Aging by Condensation of Organic Aerosol.

摘要

Aerosol particles in the atmosphere scatter and absorb solar radiation; an interaction that yields the largest uncertainty in models of future climate change. While most aerosols scatter light and, therefore, cool the environment, absorbing aerosol warms the environment. In particular, black carbon (BC) aerosol, the largest component of absorbing aerosol, may exhibit the second-largest forcing on climate behind greenhouse gases. In addition, the mixing state of BC aerosol, or the degree to which a BC core is coated with a scattering substance, may significantly increase the absorbing potential of BC. This thesis presents results from field and laboratory studies of BC aerosol, its mixing state in the atmosphere, and how it ages in the presence of condensing secondary organic aerosol.;A major field study, CalNex 2010, was conducted in Southern California to study air quality and climate change issues. Measurements of BC aerosol in and around the Los Angeles (LA) Basin reveal the evolution of BC aerosol from a thinly coated state near sources in the eastern LA Basin to a more thickly coated state in the outflow regions of the Basin. While the majority of BC aerosol emitted in the LA Basin remains near the surface, some BC aerosol is transported to the free troposphere through sea-breeze and mountain-flow coupling. BC aerosol above the inversion layer tends to be thickly coated, indicating that it is more aged than the BC measured near the surface.;To understand how the mixing state of BC evolves with secondary formation of species in the atmosphere, carefully controlled environmental chamber experiments were conducted. Two types of secondary organic aerosol (SOA) precursors, alpha-pinene and naphthalene, were reacted in the chamber to condense secondary products onto BC seed aerosol. The rate of growth and magnitude of absorption enhancement due to the secondary coating on BC was measured, revealing that growth of coatings is diffusion-limited. Particle composition measurements reveal that condensed SOA onto BC seed particles is nearly identical to nucleated SOA from the same parent hydrocarbon. Measurements of coating thickness and optical properties provide insight to single-particle SOA growth and volatility.