Measurements of the optical properties of light absorbing aerosol particles using cavity ring-down and photoacoustic spectroscopy

Abstract

The interaction of atmospheric aerosols with radiation represents one of the largest uncertainties in climate models. In particular, the role of aerosol absorption and the change in optical properties under humidified conditions are not well constrained. Limitations surrounding the current measurement techniques for absorption of aerosols contribute to this large uncertainty. Measurements of the extinction cross-section and how these partitions between the scattering and absorption cross-sections are important for understanding the direct interactions of aerosols with radiation. This thesis combines measurements of aerosol extinction cross-sections made using cavity ring-down spectroscopy (CRDS) with measurements of aerosol absorption cross-sections made using photoacoustic spectroscopy (PAS) using an instrument developed by the Met Office for field measurements. The complex refractive index of aerosol particles size selected based on their aerodynamic diameter, using an aerodynamic aerosol classifier (AAC), were retrieved. The measured cross-sections were compared to model cross-sections calculated using Lorenz-Mie theory. The change in refractive index with mixing of an inorganic/organic system was compared to that for an organic/organic system. It was observed that physically based mixing rules are needed to accurately predict the complex refractive index for an inorganic/organic system where the effective densities of the mixed particles deviated from ideal predictions. The representation of aerosol optical properties in the Met Office’s Unified Model was then considered. A correction factor was developed to correct the imaginary refractive index calculated using a simple mixing rule, to one calculated from a more physically based rule. This correction factor allowed aerosol absorption to be represented in a more accurate way, while making computational cost savings compared to the full mixing rule calculations. Further cost savings were made using a waveband averaging approach. Finally, the optical properties of laboratory generated aerosols under elevated relative humidity conditions were investigated. The extinction cross-sections as a function of relative humidity measured using CRDS were used to quantify the hygroscopicity. A low bias in absorption cross-sections measured using PAS at high relative humidity was observed and the origin explored further.

Date of Award	20 Jan 2026
Original language	English
Awarding Institution	University of Bristol