Accurate spectroscopic quantification of the optical properties of nitroaromatic compounds in single aerosol particles Jamie Knight , Andrew Orr-Ewing, Michael Cotterell University of Bristol, UK The interaction of light with spherical, homogeneous aerosol particles is dependent on particle size and complex refractive index, m = n +i k . The real refractive index ( n ) is related to the mean molecular polarizability of the particle, and the imaginary refractive index ( k ) is related to the concentration and absorption strength of constituent chromophores. Light-absorbing organic compounds known as brown carbon (BrC) because of their relatively strong absorption in the ultraviolet and near-visible wavelengths are prevalent in the atmosphere, yet their contribution to the radiative balance of Earth is poorly understood. Novel ultra-sensitive spectroscopic techniques are required to improve knowledge of the optical properties for BrC aerosol particles and how these depend on ambient humidity, pH, and how these dependencies evolve in response to photo-oxidation processes. Single particle cavity ring-down spectroscopy (SP‑CRDS) is a technique that directly measures the total power removed from an incident beam of light by an aerosol particle, known as the extinction cross-section ( σ ext ) (Cotterell et al. 2022). Comparison of measurements of size-dependent σ ext to predictions from optical models such as Mie theory can yield both components of the complex refractive index. Further to our recent studies that represent the first SP-CRDS measurements for particles that absorb at the spectroscopic wavelength (Knight et al. 2022), our current research aims to characterise the optical properties of nitroaromatic compounds that are thought to contribute significantly to total atmospheric absorption by BrC. Figure 1 depicts the measured σ ext (black dots) for an evaporating, aqueous aerosol particle of 4-nitrocatechol, a commonly identified constituent of BrC. The complex refractive index for the aerosol particle was retrieved for the aqueous droplet. These measurements were then used in combination with physically based effective medium models to estimate the complex refractive index for pure 4-nitrocatechol, which was determined to be m = 1.418 + i0.108. Figure 1. Size-dependent variations in σ ext for an aqueous aerosol particle of 4-nitrocatechol (black dots) and the Mie theory prediction (red line).
The accuracy of the retrieved complex refractive indices for our SP-CRDS instrument has been assessed theoretically through the generation of synthetic SP-CRDS measurements of size- dependent σ ext and their subsequent inversion, via comparison to Mie theory predictions, to
retrieve the input complex refractive indices. The real and imaginary refractive index components were retrieved to an accuracy better than 0.005 and 0.002, respectively for almost all absorption strengths studied (Knight et al. 2023). We acknowledge funding from the EPSRC Centre for Doctoral Training in Aerosol Science (EP/S023593/1), and NERC (NE/S014314/1). References
1. Cotterell, M., Knight, J., Reid J. and Orr-Ewing A. (2022) J. Phys. Chem. A. 126 , 2619-2631. 2. Knight. J., Egan. J., Orr-Ewing. A., Cotterell., M. (2022) J. Phys. Chem. A. 126 , 1571-1577. 3. Knight, J., Orr-Ewing. A., Cotterell., M. (2023) Aerosol Sci. Technol. (submitted).
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