AAAR 31st Annual Conference
October 8-12, 2012
Hyatt Regency Minneapolis
Minneapolis, Minnesota, USA
Abstract View
Black Carbon Optical Properties Measured in Pasadena, Los Angeles During CalNex
Jonathan Taylor, JAMES ALLAN, Michael Flynn, Patrick Hayes, Jose-Luis Jimenez, Barry Lefer, Hugh Coe, University of Manchester
Abstract Number: 114 Working Group: Carbonaceous Aerosols in the Atmosphere
Abstract In climate models, black carbon absorption is commonly modelled using Mie theory, assuming a concentric sphere core-shell configuration. The resultant modelled absorption is sensitive to the physical size and complex refractive index of the particles. As nonrefractory species condense to form a thicker coating, the amount of light absorbed per unit mass of black carbon is increased, analogous to a lensing effect focusing light onto the core.
This effect has been studied in detail theoretically and observed in numerous laboratory studies. Many ambient measurements of mass absorption cross-section (MAC) are available, though most used filter-based methods, which are subject to artefacts when nonrefractory material is present. Core-shell measurements are available using the single-particle soot photometer (SP2), but the SP2’s scattering measurements are sensitive to assumed refractive indices. We discuss the ambiguity in determination of the core refractive index, the sensitivity to calibration material, and the effect this has on calculated optical properties.
We present results from the CalNex study, which took place in Pasadena, Los Angeles in summer 2010, to provide a real-world assessment of the validity of models and parameters used to represent black carbon optical properties. An SP2 was deployed alongside a 3-wavelength photoacoustic soot spectrometer (PASS) measuring absorption. Combining both measurements yields the MAC in high time-resolution, free from filter-based artefacts. We compare measured MAC to that calculated from SP2 core-shell measurements using Mie theory under a range of different atmospheric conditions, to quantitatively constrain the applicability of the core-shell model in an urban environment.