10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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A Combination of a Cavity Ring Down Spectrometer and an Electrodynamic Quadrupole to Retrieve Physical and Optical Constants from Single Trapped Particles
ANTONIO VALENZUELA, Jonathan P. Reid, Allen E. Haddrell, Bryan R. Bzdek, Rose Willoughby, Andrew J. Orr-Ewing, University of Bristol
Abstract Number: 736 Working Group: Aerosol Chemistry
Abstract The rapid changes in climate observed during recent decades have important social and economic impacts on both global and regional scales that have brought climate change into the spotlight worldwide. According to the Fifth Assessment Report (AR5) of the last Intergovernmental Panel on Climate Change (IPCC) in 2013, atmospheric aerosol produces a net cooling effect of the Earth’s climate. However, an accurate estimate of this cooling is extremely difficult because of the large uncertainty on the estimation of the aerosol radiative forcing (RF), where the largest uncertainty in RF is due to uncertainties in understanding aerosol properties. Reduction of these uncertainties requires improved methods of laboratory determination of the optical properties of aerosol particles and their change with ambient relative humidity. Cavity ring-down spectroscopy (CRDS) has been proven to be a sensitive tool for measuring the absolute extinction cross-sections for single aerosol particles with high precision by combining optical confinement and levitation of a particle with CRDS (1). Studies on single particles, confined within optical or electrodynamic traps, provide precise measurements of aerosol refractive indices, hygroscopic growth, evaporation of semivolatile components, and other properties of importance for their atmospheric behavior. However, overall, these studies have been focused on characterizing optical and physical properties of non-absorbing aerosols. In this sense, we have designed and built an Electrodynamic Quadrupole (EQ) trap to confine and levitate a wide range of different single aerosol particles even absorbing particles. We present a new experimental approach which uses a combination of CRDS and an EQ trap which will allow the measurements of absolute light extinction cross-sections and optical constants and their dependence on environmental conditions over an extended time frame for single aerosol particles. We expect to extend previous analysis centered on non-absorbing aerosols to the more challenging case of absorbing aerosols allowing direct determination of the change in physical and optical properties of micrometer-sized aerosol particles over time. 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 % and retrieving a more precise imaginary part for absorbing aerosols. 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.
References [1] Cotterell, M. I.; Preston, T. C.; Orr-Ewing, A. J.; Reid, J. P. Assessing the Accuracy of Complex Refractive Index Retrievals from Single Aerosol Particle Cavity Ring-Down Spectroscopy. Aerosol Sci. Technol. 2016, 50 (10), 1077−1095.