Aerosols are colloidal systems comprised of mesoscale solid or liquid particles (with diameters ranging from 10 nm to 100 µm) suspended in a gas. They are pervasive in our atmosphere and their sources include anthropogenic emissions from burning of fossil fuels, and natural sources from sea spray, desert dust, and biomass burning sources such as wildfires. These aerosols have significant impacts on our environment, affecting regional air quality as well as global climate through interacting with sunlight and cloud droplets. Indeed, the representation of aerosols in atmospheric models is one of the largest uncertainties in predicting future climate. Globally, the net aerosol cooling effect provided by aerosol particles scattering sunlight back to space partially offsets the warming impact of greenhouse gases. On a regional basis, light absorbing aerosols such as those from combustion can heat the atmosphere, driving changes in atmospheric dynamics and regional meteorology such as the formation of clouds. However, large uncertainties in quantifications of aerosol-light interactions degrade the confidence we have in models of regional meteorology and of future climate. Improvements to our understanding of aerosol-light interactions could lead to more effective risk mitigation strategies in managing climate change impacts.

Measurement Approaches

The interaction of light with a particle is governed by the particle extinction cross section (σ_ext) and its partitioning to scattering (σ_sca) and absorption (σ_abs) components (σ_ext = σ_sca + σ_abs). We use techniques to characterise the extinction and absorption cross sections for aerosol samples, from which particle physicochemical properties may be retrieved. The figure opposite summarises the key spectroscopy approaches exploited in our group. Cavity Ring-Down Spectroscopy (CRDS, Fig. (a)) is a cavity enhanced technique allowing direct (calibration free) measurements of the light extinction coefficient (i.e., the fractional amount of power removed from the probe beam by both scattering and absorption losses) with superior sensitivity. Photoacoustic Spectroscopy (PAS, Fig. (b)) is the technique-of-choice for measurements of aerosol light absorption. We apply these techniques in the laboratory to aerosol ensembles (100s – 1000s of particles per cubic centimetre). Alternatively, working with the Met Office, these same instruments can be installed on the UK FAAM atmospheric research aircraft for studying aerosols in the natural environment. In addition, we apply these spectroscopic techniques to study single particles. We use CRDS to interrogate the evolving optical properties of single particles levitated in either an optical (Bessel laser beam) trap, or a linear electrodynamic quadrupole trap (Fig (c)). Concurrently, we record the angularly varying elastic light scattering from a further laser beam, from which the size of micron-sized particles (with particle radii greater than ~300 nm) is retrieved by comparing these light intensity distributions with predictions from an optical model (Mie theory). The advantages to single particle studies include the ability to study the same particle on extended timescales, of up to several days if desired, that cannot be achieved in laboratory studies on ensembles (which are lost via wall impaction and sedimentation on characteristic timescales of less than a few hours in typical chambers), enabling studies of processes occurring on long timescales.

From cross section measurements, either on ensembles with a known (or controlled) particle size or single particles of measured size, the complex refractive index is derived from comparisons of measured data with electromagnetic interaction models. The complex refractive index, m = n + ik, quantifies the intensive optical properties of the material constituting an aerosol particle, with the real component n dependent on the mean molecular polarizability and density of the particle, and the imaginary component k characterizing the attenuation (absorption) of light. Therefore, the complex refractive index is connected to the particle composition. Our research uses these refractive indices to: (i) Infer changes in chemical composition for aerosol particles in controlled environments and undergoing physiochemical changes, such as those driven by the evaporation of volatile components, chemical changes driven by photobleaching or chemical reaction via in-droplet chemistry or heterogeneous reactions at the gas particle interface (which form the basis of Michael’s recent EPSRC Standard Grant award and PhD studentship funding); and (ii) Develop effective medium approximations that predict the intensive optical properties with changes in aerosol mixing state (e.g., for internally-mixed particles containing known mixing ratios of organic and inorganic species).