AAAR 36th Annual Conference October 16 - October 20, 2017 Raleigh Convention Center Raleigh, North Carolina, USA
Abstract View
Sensitivity of Radiative Forcing to Uncertainties in Real and Imaginary Refractive Indices as Determined from Single Trapped Particle Measurements
ANTONIO VALENZUELA, Jonathan P. Reid, Rose Willoughby, Allen E. Haddrell, Bryan R. Bzdek, Andrew J. Orr-Ewing, University of Bristol
Abstract Number: 306 Working Group: Aerosols, Clouds, and Climate
Abstract Radiative forcing (RF) caused by anthropogenic aerosols is thought to be the major contributor to the changing radiative balance of the Earth-atmosphere system. In clear-sky conditions scatter solar radiation back to space, reducing solar irradiance at the ground. This effect is usually known as ‘’the aerosol direct effect’’and is one of the largest uncertainties in quantifying the RF through the scattering and absorption of radiation. The parameter which governs how the light interacts with a particle is the extinction cross-section which depends on the complex refractive index, the radius of the particle and the wavelength of the radiation. The complex refractive index is defined as the sum of two terms, the real refractive index, which governs scattering, and the imaginary refractive index, which determines the extent of light absorption. However, determination of the complex refractive index is challenging because of the mixed composition of the aerosol in the atmosphere, the hygroscopic response of this parameter to ambient relative humidity, and the imprecision of current techniques. We will present a novel method for the accurate determination of the optical extinction cross-sections of individual particles and the dependence on environmental conditions, with continuous monitoring of the optical properties of the same particle over an indefinite timeframe. A combination of a quadrupole electrodynamic trap and cavity ring-down spectroscopy (CRDS) is used to trap a single particle and characterize the optical cross-section, leading to a determination of the complex refractive index. We will demonstrate that such a single particle technique leads to a significant reduction in the uncertainty in the complex refractive index reducing, for example, the uncertainty in the real part to ±0.3 %. We will report retrievals of optical properties of aerosol spanning from simple mixtures of organic components through to samples of laboratory surrogates of secondary organic aerosol. Finally, a sensitivity study of RF will be reported establishing the limits on the accuracy and precision of RF estimates from the refractive indices retrieved using our setup.