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

This Project has been filled

Background

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.

Training

The project will be undertaken in the Department of Chemistry at the University of York (YDC) under the supervision of Dr. Katherine Manfred and Dr. David Carslaw. As part of the iDTP programme, the PhD student will have access to a broad range of training programmes. The University of York provides additional opportunities to help students complete their postgraduate degree and prepare for a successful career.

The PhD student will be based in the Wolfson Atmospheric Chemistry Laboratories, a world-leading group of approximately 60 researchers from the University of York and National Centre for Atmospheric Science (NCAS) with expertise in a wide range of atmospheric chemistry specialties including both experimental and modelling research. WACL has state-of-the-art laboratory facilities and provides access to data management and computing infrastructure. The student will also have opportunities to present research at national and international conferences, and can take part in a variety of outreach and public engagement activities.

Dr. Katherine Manfred is a NERC/DfT Independent Research Fellow with a strong background in developing innovative laser-based techniques. She will provide any necessary training in optics theory, safe laser use, instrument control and software design, data analysis, and aerosol sampling. Dr. David Carslaw will provide additional support, particularly for fieldwork experiment design and interpreting measurements in the context of atmospheric modelling. In addition to general training, the PhD student will have training from researchers at WACL for skills directly linked to the project. These are likely to include: R/Python, Matlab, LabVIEW, laser-based measurement techniques, and electron microscopy.

Candidate

A successful candidate will have a First or 2:1 degree in Physics, Chemistry, Engineering, or a related field. The candidate will be enthusiastic about hands-on lab work, and show interest in environmental and/or public health issues.  This project involves both chemistry and physics concepts, but the student will receive comprehensive training in all required mathematical and experimental techniques.

References

  1. AQEG. Fine particulate matter (PM2.5) in the UK. (2012).
  2. Dolgos, G. & Martins, J. V. Polarized Imaging Nephelometer for in situ airborne measurements of aerosol light scattering. Opt. Express 22, 21972–21990 (2014).
  3. Manfred, K. M. et al. Investigating biomass burning aerosol morphology using a laser imaging nephelometer. Atmos. Chem. Phys. 18, 1879–1894 (2018).
  4. Kahn, R. A. et al. Multiangle Imaging Spectroradiometer (MISR) global aerosol optical depth validation based on 2 years of coincident Aerosol Robotic Network (AERONET) observations. J. Geophys. Res. D Atmos. 110, 1–16 (2005).
  5. Kahnert, M., Nousiainen, T. & Veihelmann, B. Spherical and spheroidal model particles as an error source in aerosol climate forcing and radiance computations: A case study for feldspar aerosols. J. Geophys. Res. D Atmos. 110, 1–12 (2005).
  6. Broday, D. M. & Rosenzweig, R. Deposition of fractal-like soot aggregates in the human respiratory tract. J. Aerosol Sci. 42, 372–386 (2011).

 

Figure 1: (a) Scanning electron micrographs of particles emitted from wood combustion showing range of particle shapes that can be emitted from a single source. (b) Model of respiratory tract (RT) deposition of aerosol particles by shape (Dfm) and size (Np) for different age groups (adapted from Broday & Rosenzweig, 2011).6 (c) Measured (blue line) scattered light as a function of scattering angle (e.g. between incident radiation and viewing direction) compared to different theoretical models for different particle shapes.3 Coloured lines qualitatively indicate three types of shapes: open, branched structures (yellow), collapsed structures (black), and spheres (red).

(a) Scanning electron micrograph showing particles <2 micron collected from biomass burning including spherical, collapsed, and open fractal shapes (b) Plot of total RT deposition vs fractal dimension for small (N=10) and large (N=150) agglomerates. Deposition monotonically increases with fractal dimension for small particles, whereas for large particles trends differ for infant, toddler, and adult lung models. (c) Plot of light scattering (au) as a function of scattering angle showing data from biomass burning aerosol emissions compared to models assuming different particle morphology. The data lie between a model for fossil fuel (e.g. very open fractal) primary emissions and a model for biomass burning (e.g. collapsed fractal) primary emissions.