Optical and morphological properties of light absorbing internally mixed aerosol particles
G.R. Lawson 1 , J.M. Langridge 2 , K. Szpek 2 , J. Bowles 2 , M.I. Cotterell 1 1 School of Chemistry, University of Bristol, UK, 2 Met Office, Exeter, UK
Interactions of aerosols with light, both directly and indirectly, affect the overall radiative forcing of the Earth’s climate. Despite this importance, they remain one of the largest uncertainties in climate models, limiting future improvements as illustrated by the IPCC report (2021) 1 .The optical properties of aerosols can be quantified by their extinction cross sections (σ ext ) which represent the sum of the scattering (σ sca ) and absorption (σ abs ) cross sections. The use of cavity ring down spectroscopy (CRDS) and photoacoustic spectroscopy (PAS) to quantify the extinction and absorption cross sections respectively to high accuracy is well established 2 . This project utilises a suite of CRDS and PAS spectrometers developed by the Met Office to systematically study how the complex refractive index of internally mixed two-component aerosol particles changes with composition, and subsequently tests the performance of commonly applied refractive index mixing rules. Previous work by Cotterell et al. 2 used a differential mobility analyser (DMA) to size select aerosols prior to their optical characterisation. This project instead uses an aerodynamic aerosol classifier (AAC) which has a greater transmission efficiency and eliminates the multiple charge artefacts present when using a DMA, which can add uncertainty to optical property calculations. The aerosols being studied are comprised of mixtures of ammonium sulphate, a non-absorbing atmospherically relevant compound, and nigrosin, a highly absorbing organic dye. These internally mixed systems, with varying mass fractions of light absorbing dye, produce test aerosol systems with tuneable absorption strengths. Test aerosols are generated through atomisation, they are dried and aerodynamically size selected using an AAC. These aerodynamically classified aerosol particles are then characterised using CRDS and PAS, and a scanning mobility particle sizer (SMPS) to measure the mobility size distribution. The coefficients measured by CRDS and PAS are converted to cross sections using the total number concentration as measured by a condensation particle counter (CPC). Measured optical cross sections are compared to predicted values calculated using Mie theory and a grid search algorithm applied to retrieve the complex refractive index. The results of these retrievals for mass fraction mixtures of ammonium sulphate and nigrosin measured at an optical wavelength of 405 nm are shown in figure 1, also shown are predictions for the same mixtures using different mixing rules. The refractive index results show closest agreement with the molar refraction mixing rule, a rule with a physical basis that takes into account the density of the mixture. These results demonstrate the importance of using a mixing rule with a physical basis that takes into account particle density, such as the molar refraction mixing rule, to predict complex refractive indices.
References 1. IPCC. 2021. Summary for policymakers, in: Climate change 2021, Masson-Delmotte, V., et al. eds., 3−32. Cambridge, UK and New York, NY, USA: Cambridge UniversityCotterell, M.I., Szpek, K., Haywood, J.M. and Langridge, J.M., (2020), Aerosol Science and Technology, 54:9, 1034-1057
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© The Author(s), 2023
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