The core theme of my research is the measurement of atmospheric aerosols. Particulate matter is produced from both natural and anthropogenic sources and is known to have significant effects on both health and climate. However, the processes governing their lifecycle and effects are poorly understood, which hampers our predictive capability of them and their impacts. These are some examples of areas of research in which I am actively involved.

Urban aerosols. As part of major collaborative projects, I have studied particulates (principally PM₁) in urban environments and worked to identify and quantify specific sources, such as traffic emissions, cooking, domestic burning, secondary aerosol formation and regional transport, and investigate the factors governing their concentrations.

Secondary organic aerosols (SOA). Secondary aerosols from both biogenically and anthropogenically emitted organic gases can form a very large fraction of the submicron aerosol mass but it still relatively poorly understood and represented by models. I have conducted measurements in a variety of locations including urban, rural, boreal forests and tropical rainforests and found a range of different processes and behaviours at play. These measurements are used in conjunction with laboratory work to develop process and large-scale models.

Black carbon (BC). As well as having potential impacts on human health, BC (soot) is known to be a strong absorber of radiation, having a short-lived warming effect on climate, particularly on local scales. However, the addition of secondary material can significantly alter its optical properties, lifetime and thus impacts. Quantifying these processes requires a combination of measurements to quantify an aerosol’s BC content and properties on a variety of different spatial scales since emission, which can then be used to develop models used to represent the key properties.

A large aspect of my work is the application of specialist aerosol instrumentation, mainly with those that utilise in situ analysis during intensive measurements.

Aerosol Mass Spectrometry. Much of my work has focused on the development and application of the Aerodyne Research Inc (ARI) Aerosol Mass spectrometer (AMS). The Centre for Atmospheric Science currently has three instruments based around Tofwerk Time-Of-Flight (TOF) mass spectrometers; two Compact (C-TOF) and one High Resolution (HR-TOF) versions. One of the C-TOF instruments is mainly used for airborne measurements on the Facility for Airborne Atmospheric Measurements (FAAM). This instrument is designed for the online analysis of aerosol composition through thermal desorption and analysis with Electron Ionisation (EI). This is able to study matter from particles smaller than 1 µm in size and produces data on ammonium, nitrate, sulphate, chloride and organic matter, which in most environments represents the majority of particulate matter in this size range.

Positive Matrix Factorisation (PMF). This is a multivariate technique frequently applied to environment data, designed to separate complex datasets into a number of discrete ‘factors’. When applied to AMS data, this can be used to quantitatively estimate the sources of particulate organic matter present. Examples include traffic, domestic burning, cooking and secondary processes. This technique continues to offer new insights into the sources and processes of aerosols in the atmosphere and is particularly powerful when used in conjunction with other gas and particle phase measurements.

Single Particle Soot Photometry. The Single Particle Soot Photometer (SP2) by Droplet Measurement Technologies (DMT) is an instrument designed to study BC on a particle-by-particle basis. The aerosol is passed through the active cavity of a Nd:YAG laser, where particles that contain BC absorb the laser energy and vaporise. The BC is quantified by measuring the intensity of the incandescent light emitted when a particle vaporises and the overall size is indicated by the amount of light scattered by the particle (note that particles that do not contain BC will still scatter light). This information is highly useful when studying the complex BC processes in the atmosphere.

Other techniques. Others include the DMT  Photoacoustic Soot Spectrometer (PASS), which measures bulk light absorption by converting a modulated laser beam to sound in an acoustic. The ARI Cavity Attenuated Phase Shift (CAPS) is a new technique used to absolutely quantify the total aerosol extinction (light scattering plus absorption) by measuring the phase shift of modulated LED light through an optical cavity. The ARI/DMT SP-AMS is a new variant of the AMS, which uses an active cavity laser identical to that used in the SP2 to vaporise particles, thereby studying the composition of BC and its coatings.