Developing new laser-based instruments to characterise optical properties of aerosol particles from road traffic

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Road traffic remains among the most significant sources of particulate matter (PM) pollution in the UK. With the decline in tailpipe emissions, non-exhaust sources - including resuspended road dust and tyre and brake friction - of PM are becoming increasingly important. Non-exhaust PM are now thought to dominate over exhaust sources.1 Factors that influence non-exhaust emissions are poorly understood, making it difficult to develop solutions for PM reductions and predict future changes as electric and hybrid vehicles become more popular. In addition, there is a great deal of uncertainty in the physical characteristics of these particles that determine climate and public health effects. Optical instruments have become popular for in situ measurements of atmospheric aerosol with high (~1 Hz) time resolution. In order to retrieve quantitative information on the particle size and refractive index, mathematical models are often required to invert raw data. These models rely on assumptions that the particles are spherical (or ellipsoids) with a homogeneous or simple core-shell composition. This project will focus on developing innovative laser-based instruments to more accurately characterise aerosol particle properties, specifically focusing on improving accuracy in measurements of non-spherical particles. The PhD candidate will develop techniques to measure particle morphology in the field based on the angular distribution of scattered light (commonly referred to as the scattering phase function). Direct measurements of aerosol phase function in the field have only recently been demonstrated (see Figure 1), and this project will focus on leveraging these new capabilities to produce vital new data sets on traffic related PM emissions.2,3 These data sets will reduce uncertainty in the effects of traffic emissions on cardiopulmonary health, tropospheric chemistry, and radiative forcing. In addition to improving ground-based measurement capabilities, this research project will also enable improved validation of remote sensing measurements. Aerosol data products from ground- and satellite-based radiometers rely on algorithms that relate the viewing angle of the instrument relative to the sun to determine the size distribution and radiative effects of the particles. Inaccurate assumptions of the scattering phase function may contribute to poor agreement between climate models, satellites, and ground-based radiometers, particularly in areas characterised by non-spherical particles (e.g. deserts and urban areas).4,5 Objectives The objectives of this project include: 1. Develop novel laser-based instrumentation to characterise aerosol particle size and shape in situ; 2. Deploy instrument to roadside air quality monitoring site to measure traffic emissions; 3. Compare measured light scattering with algorithms implemented in models and remote sensing retrievals; 4. Investigate the role of vehicle type and atmospheric processes on traffic particle characteristics.