Michael Cotterell

My research concerns laser-based spectroscopy of aerosol particles (mesoscale solid or liquid particles suspended in a gas, with particle diameters ranging from 10 nm to 100 µm) to improve our understanding of their optical, physical, and chemical properties. Applications of our research include: atmospheric aerosols and their impact on Earth’s climate; the potential for accelerated reactions in aerosols; the morphological properties and non-ideal mixing for internally mixed aerosol particles. 

Novel Spectroscopic Approaches 

We develop sensitive laser-based spectroscopy techniques for the non-contact interrogation of aerosol particles to provide new insights into their physicochemical properties. The interaction of light with a particle is governed by the particle extinction cross section and how it partitions into scattering and absorption components. We develop Cavity Ring-Down Spectroscopy (CRDS) instruments for measurements of aerosol light extinction coefficients, while Photo-Acoustic Spectroscopy (PAS) is our technique-of-choice for accurate measurements of light absorption coefficients. 
 
Excitingly, our group applies these spectroscopic techniques to study single, levitated aerosol particles. We use CRDS to follow the evolving optical properties of single particles levitated in either optical (Bessel-shaped or hollow laser beam) traps, or a linear electrodynamic quadrupole trap. Concurrently, we record the angularly or spectrally resolved elastic light scattering from a further light source, from which the sizes of micron-sized particles are quantified. We also apply these techniques in the laboratory to aerosol ensembles (with ~10²-10⁶ particles per cubic centimetre within the spectroscopic probe beam at any instance) with the ensemble having undergone prior particle size selection (using techniques such as electrical mobility or aerodynamic classification). We collaborate closely with the Met Office on projects developing the next generation of instrumentation for in-situ aerosol observations; installed on the FAAM atmospheric research aircraft, these instruments provide accurate measurements of optical properties for aerosols aloft in Earth’s atmosphere. 

Photobleaching Kinetics 

When light is incident on an aerosol particle, complicated electromagnetic fields develop that may be greatly enhanced in specific regions inside the particle. The role of light in driving chemistry in aerosol droplets is uncertain and has been studied poorly. We are applying our spectroscopy and single particle levitation toolkit to follow the changes in aerosol optical properties for aerosol droplets containing dyes that photobleach when irradiated with UV-visible light. We are unravelling the dependence of photobleaching rates on the particle size, viscosity, and pH for chemical systems important to atmospheric and physical chemistry. 

Reaction Acceleration in Microdroplets 

Very recent observations have reported accelerated reactions in aerosol droplets, at rates up to a million-fold faster than in macroscopic (beaker-scale) vessels. We are developing a new measurement platform to quantify the rates of formation of chromophores in single, levitated aerosol particles. 

Light Absorption and Interfacial Dynamics for Volatile Aerosol Particles 

Our research aims to improve our understanding of how the water content of aerosol particles changes their light absorption properties. We are developing new photoacoustic spectroscopy approaches to study aerosol particles that contain volatile species, such as water. From these same photoacoustic measurements, we are studying the mass accommodation coefficient of water (an important quantity that controls the formation of clouds) and the dynamics of molecular adsorption at the gas-particle interface. 

Morphology of Dried Particles 

We are assessing the impacts of systematic changes in chemical composition for organic-inorganic mixtures on the effective densities and internal structures of dried particles, unravelling the effects of particle viscosity and drying rates on particle shape, structure, and optical properties. These efforts include CRDS and PAS measurements of plumes of dried aerosol particles, characterising their mobility and aerodynamic size distributions, and the collection the particles on filter substrates for electron microscopy assessments of particle shapes and compositional distributions. 

Associated Research Themes

Advanced Functional Materials and Interfaces

Energy and Sustainable Chemistry

Innovative Measurement and Photon Science

Kinetics, Dynamics and Mechanism