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Heterogeneous chemistry of reactive species on aerosol surfaces

Prof Dwayne Heard (SoC), Dr Lisa Whalley (SoC), Dr Steve Arnold (SEE)

Contact email: d.e.heard@leeds.ac.uk

Aerosols are solid, liquid or multiphase microscopic particles which are suspended in air, and impact both air quality and climate. The processing of aerosols by the exchange of reactive molecules alters the chemical composition and physical properties of the aerosols leading to changes in their toxicity, and their ability to scatter light and form clouds. A report by Public Health England showed that air pollutant exposure leads to 29,000 deaths a year in the UK, with life expectancy shortened by 6 months on average. The cost to society is estimated at up to £20 billion per year, and worldwide there are more than 5.5 million premature deaths annually as a result of air pollution exposure, with more than half occurring in India and China. However, large uncertainties still remain relating to the chemistry, transformation and removal rate of primary emissions in urban areas. This chemistry is largely controlled by short-lived free radical species, for example the hydroxyl radical, OH, the hydroperoxy radical, HO2, and organic peroxy radicals, RO2. Figure 1 shows the impact of the uptake of HO2 onto clouds, using a parameterisation which was validated during fieldwork performed during the Hill Cap Cloud Thuringia (HCCT) campaign in Germany (Whalley et al., 2015).

Figure 1. Annually averaged fraction change in surface HO2 (left) and O3 (right) with the inclusion of HO2 uptake onto clouds assuming a pH of 5. Taken from Whalley et al., 2015.

The goal of this project is to improve our understanding of the role aerosols play in modifying gas-phase composition through the processing and exchange of reactive gases, both in terms of uptake, and production at the surface in the presence of sunlight. There are considerable uncertainties associated with the uptake and production of reactive gas-phase species through heterogeneous interactions on aerosols, with our understanding being less developed than for gas phase chemistry. Figure 2 shows a significant change in surface O3 concentrations owing to halving the rate of just one heterogeneous reaction on aerosols.

Figure 2. The modelled change in surface ozone mixing ratio for July 2007 in response to a 50% decrease in the rate of the heterogeneous reaction N2O5 + H2O ® 2HNO3 taking place on aerosols.

In urban regions, HONO has been identified as the dominant precursor to the main daytime oxidant, the hydroxyl radical, OH, which controls the atmospheric lifetime of most trace gases and the formation of secondary pollutants such as ozone and secondary organic aerosols (Lee et al., 2016). As well as being harmful to health, ozone is an important greenhouse gas and is responsible for significant losses of crop yields globally. The role of aerosols in the atmosphere is currently the largest and most persistent source of uncertainty in estimates of anthropogenic impacts on climate, and so understanding HONO chemistry is important in reducing these uncertainties and improving climate predictions. We will measure the production of HONO from aerosols generated in the laboratory and collected on filters in urban areas (e.g. Beijing) when they are illuminated using a lamp to simulate solar radiation.

The project will use a combined laboratory and chemical modelling approach, developing new experimental techniques and modelling methods on a range of spatial scales. The project combines expertise in the School of Chemistry and the School of Earth and Environment. We have experience with the ultra-sensitive detection of radicals using laser-induced fluorescence spectroscopy (Stone et al., 2012; Heard and Pilling, 2003) and the heterogeneous uptake of HO2 using an aerosol flow-tube equipped with a sliding injector (George et al., 2013), as well as chemical modelling using both local “box” (zero-dimensional model) and large-scale global chemistry-transport models (Nicely et al, 2017; Arnold et al., 2009). The research will lead to an improved representation of chemical oxidation mechanisms in models that are used for the prediction of future changes in climate and air quality.

The new understanding will provide important input into models on a range of spatial scales leading to a better predictive capability of atmospheric composition linking to pollution and climate. More specifically this project will lead to an improved representation of chemical oxidation processes within models for calculating health and climate related trace gases and particles, which is controlled by OH and other oxidants, leading to improved predictive capability of atmospheric composition locally, regionally and globally.

Objectives

Specific areas of focus for the project will include:

  1. Measurement of HO2 and RO2 uptake coefficients onto a variety of atmospherically relevant sub-micron aerosols using our existing aerosol flow tube (George et al., 2013; Lakey et al., 2016), particularly as a function of temperature and relative humidity. New, sensitive methods developed at Leeds will be used to detect individual RO2 species, for example by chemical conversion to the corresponding alkoxy radical, RO, with subsequent detection by laser-induced fluorescence spectroscopy (Onel et al., 2017).
  2. To measure directly the production rate of nitrous acid (HONO) and radicals (HO2 and OH) from a range of aerosol surfaces when illuminated at actinic tropospheric wavelengths and under atmospheric conditions.
  3. To quantify the atmospheric impacts of the uptake of HO2 and RO2 on aerosols and the production of radicals and HONO on a range of spatial scales, for example using a box model incorporating the detailed Master Chemical Mechanism, the regional WRF-Chem (with an emphasis on polluted regions) and global UM-UKCA (UK community atmospheric chemistry-aerosol) models. A particular focus is the impact on the oxidative capacity and composition of remote and urban environments.

Potential for high impact outcome

The project will deliver an improved understanding of the role of aerosols in the capture/release of reactive gas-phase species, leading to a better predictive capability of atmospheric composition linking to pollution and climate. Specifically, this project will improve the accuracy of chemical oxidation processes and production of secondary pollutants within air quality and climate models, leading to improved predictive capability of atmospheric composition and climate both regionally and globally. The work will be of particular interest to the Department of Business, Energy and Industrial Strategy, and the Department for Environment, Food and Rural Affairs, and will also provide an ideal vehicle for Science in Society activities, for example presentations in Schools. The results from the project will be disseminated widely to the scientific community through high quality publications in leading international journals and at international conferences.

Training

The student will work under the supervision of Professor Dwayne Heard (School of Chemistry, Leeds, member of the Atmospheric and Planetary Chemistry Group), Dr Lisa Whalley (School of Chemistry, Leeds, member of the Atmospheric and Planetary Chemistry Group) and Dr Steve Arnold (School of Earth and Environment, Leeds, member of the Atmospheric Chemistry and Aerosols group of the Institute for Climate and Atmospheric Science (ICAS) and Director of the Centre of Excellence for Modelling Atmosphere and Climate (CEMAC). The supervisors lead active and vibrant research groups exploring the role of atmospheric chemistry and aerosol in air quality, using experimental and modelling approaches. They are also members of the Leeds Ecosystem, Atmosphere and Forest (LEAF) Centre. There will also be opportunities to work with international project partners and visit their laboratories.

You will work in well-equipped laboratories and be part of an active, thriving and well-funded atmospheric chemistry community. The Leeds groups receive funding from the National Centre for Atmospheric Science (NCAS) and are part of the Atmospheric Measurement Facility, and have an internationally leading reputation in atmospheric chemistry for field measurements of atmospheric composition, laboratory studies of chemical kinetics and photochemistry, and the development of numerical models and chemical mechanisms, including the Master Chemical Mechanism (MCM). Activities in these three areas are intimately linked and interdependent, providing a significant advantage. The PhD will provide a broad spectrum of experience in the use of high power lasers, vacuum systems, optics, electronics, computer controlled data acquisition systems and methods in numerical calculations.). By working with expert investigators the student will receive advanced technical training and enhance their skills base considerably.

The project will also include specialist training in (1) use and development of advanced numerical models, (2) big data analysis, visualisation and interpretation, (3) statistics and uncertainty and (4) scientific writing. We strongly support students to write publications during their PhD (see examples from previous students on the Web pages of the supervisors given above). You will be supported to attend both national and international conferences, and will receive a wide range of training, for example in communication skills, project management, and with other technical aspects (for example Labview and computing). Students have access to local high performance computing facilities as well as world-class support through the Centre for Expertise on Modelling the Atmosphere and Climate (CEMAC) – a dedicated team providing training for advanced models and data.

You will also have access to training provided by the National Centre for Atmospheric Science such as the Arran Instrumental Summer School and other courses. The successful PhD student will have access to a broad spectrum of training workshops that include an extensive range of training workshops in numerical modelling, through to managing your degree, to preparing for your viva (http://www.emeskillstraining.leeds.ac.uk/).

Eligibility

Standard NERC eligibility rules apply. Applicants should have a First or 2:1 degree in Chemistry, Physics, Environmental Sciences or a related discipline, or have a 2:2 degree and also a Masters qualification. Applicants should be a UK Citizen, or an EU Citizen who has lived in the UK for the last 3 years undertaking undergraduate or masters education. Candidates should have an interest in instrumentation as well as possessing a solid mathematical foundation. Experience in programming would be an advantage.

References

Arnold, S. R., Spracklen, D. V., Williams, J., Yassaa, N., Sciare, J., Bonsang, B., Gros, V., Peeken, I., Lewis, A. C., Alvain, S., and Moulin, C.: Evaluation of the global oceanic isoprene source and its impacts on marine organic carbon aerosol, Atmos. Chem. Phys., 9, 1253-1262, 2009

George, I.J.; Matthews, P S J; Whalley, L. K.; Brooks, B.; Goddard, A.; Baeza-Romero, M.T.; Heard, D.E. Measurements of uptake coefficients for heterogeneous loss of HO2 onto submicron inorganic salt aerosols, Physical Chemistry Chemical Physics, 2013, 15, 12829-12845

Heard D.E.; Pilling M.J. Measurement of OH and HO2 in the troposphere, Chemical Reviews, 103, 5163-5198, 2003

Lakey, P. S. J., Berkemeier, T., Krapf, M., Dommen, J., Steimer, S. S., Whalley, L. K., Ingham, T., Baeza-Romero, M. T., Pöschl, U., Shiraiwa, M., Ammann, M., and Heard, D. E.: The effect of viscosity and diffusion on the HO2 uptake by sucrose and secondary organic aerosol particles, Atmos. Chem. Phys., 16, 13035-13047, 2016

Lee JD, Whalley LK, Heard DE, Stone D, Dunmore RE, Hamilton JF, Young DE, Allan JD, Laufs S, Kleffmann J; Detailed budget analysis of HONO in central London reveals a missing daytime source Atmospheric Chemistry and Physics 16 2747-2764, 2016

Nicely, J. M., et al. (2017), Quantifying the causes of differences in tropospheric OH within global models, J. Geophys. Res. Atmos., 122, 1983–2007

Onel, L., Brennan, A., Seakins, P. W., Whalley, L., and Heard, D. E.: A new method for atmospheric detection of the CH3O2 radical, Atmos. Meas. Tech., accepted, 2017

Stone, D.; Whalley, L.K.; Heard, D.E. Tropospheric OH and HO2 radicals: Field measurements and model comparisons, Chemical Society Reviews, 2012, 41, 6348-6404

Whalley L.K.; Stone D.; George I.J.; Mertes S.; Van Pinxteren D.; Tilgner A.; Herrmann H.; Evans M.J.; Heard D.E.; The influence of clouds on radical concentrations: Observations and modelling studies of HOx during the Hill Cap Cloud Thuringia (HCCT) campaign in 2010. Atmospheric Chemistry and Physics 15 3289-3301, 2015

Related undergraduate subjects:

  • Atmospheric science
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