AAAR 35th Annual Conference October 17 - October 21, 2016 Oregon Convention Center Portland, Oregon, USA
Abstract View
Direct Measurements of Time-Dependent Optical Extinction Cross-Sections of Single Aerosol Particles
JONATHAN P. REID, Michael I. Cotterell, Rose Willoughby, Hongze Lin, Andrew J. Orr-Ewing, University of Bristol
Abstract Number: 94 Working Group: Single Aerosol Particle Studies - Techniques and Instrumentation
Abstract Aerosol particles scatter and absorb light, directly influencing the radiative balance of the atmosphere and climate. The optical cross-section depends on particle size, composition (refractive index) and morphology, and varies with the wavelength of incident light. In addition, the optical cross-section changes in response to environmental conditions (e.g. relative humidity), heterogeneous chemistry (e.g. the formation of light absorbing brown carbon components), and mixing state (e.g. internal and external mixtures). A refined model of optical extinction by atmospheric aerosol requires improved characterisation of the dependencies of refractive index on relative humidity and wavelength, and the ability to predict the refractive index for complex multicomponent particles. We will present a method for the accurate determination of the optical extinction cross-sections of an individual particle at multiple wavelengths and the dependence on environmental conditions. Such measurements provide accurate data for benchmarking and refining predictive models.
Cavity ringdown spectroscopy (CRDS) has become a widely used technique for interrogating the optical properties of ensembles of aerosol particles, both in field measurements and in laboratory studies. Uncertainties of +/-2 % are typical in the determination of the real part of the refractive index and larger for the imaginary part. We will discuss the advantages of coupling CRDS with a single particle trap formed from a Bessel beam for studying the optical properties of accumulation mode aerosol; for example, this approach allows us to directly resolve the change in optical cross-section of a single particle with change in relative humidity. The refractive index can be retrieved at three wavelengths simultaneously with an accuracy of better than +/-0.1 %. In addition, we will discuss the application of this approach to study the optical cross-sections of non-spherical, crystalline and multiphase particles, and to measurements of the complex refractive index of absorbing aerosol.