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Flux measurements of formaldehyde and OH reactivity in the atmosphere

Dr. Lisa Whalley (SOC), Prof. Dwayne Heard (SoC), Dr. Trevor Ingham (SoC), Dr. Ben Langford (CEH) and Dr. Eiko Nemitz (CEH)

Project partner(s): Centre for Ecology and Hydrology (CEH)

Contact email: l.k.whalley@leeds.ac.uk

The atmospheric concentration of OH largely determines the atmospheric oxidation and lifetime of volatile organic compounds (VOCs), an important group of compounds emitted from natural and manmade sources. The reactions of VOCs with OH leads to the formation of secondary pollutants such as ozone, oxygenated VOCs and secondary organic aerosols (SOA) which impact human and ecosystem health as well as the climate system. In areas of London up to 1 in 12 deaths are at least partly attributable to air pollution (Public Health England), yet big uncertainties relating to the emissions, transformation and removal rates of primary pollutants remain. Understanding the reactivity of VOCs and the contribution made by different VOC compounds in a particular environment is important to minimise these uncertainties and improve model predictions of air quality and climate change.

Figure 1: BT tower in London where flux measurements of OH reactivity and HCHO can be made to investigate VOC emissions.

Formaldehyde (HCHO) is formed during the oxidation of nearly every VOC and so is considered a good target species to evaluate the completeness of the oxidation schemes used in atmospheric models (e.g. Kaiser et al., 2015). HCHO, however, may also be directly emitted into the atmosphere and so primary sources of HCHO, for example from traffic or agriculture, need to be considered in parallel to these secondary sources. A direct measure of total OH reactivity, the inverse of the lifetime of OH, can provide further insight into VOC oxidation. Numerous field measurements of OH reactivity have failed to reconcile observations with the OH reactivity calculated from individually measured VOCs suggesting unidentified sinks for OH which can lead to under-predictions of secondary pollutants (Yang et al., 2016). Measurements have been made over a range of environments but the largest discrepancies between measured and predicted OH reactivity have so far been reported from forested environments (e.g Edwards et al., 2013). In urban centres this missing reactivity has the potential to significantly increase the calculated ozone production when included in models. For example, measurements of OH reactivity in Tokyo, and subsequent modelling studies, demonstrated that the unidentified OH reactivity had the potential to increase ozone production by up to 8 ppbv hr-1 (Yoshino et al., 2012). Similarly, from our own observations of OH reactivity in London and comparison to calculated OH reactivity determined from the coordinated measurements of individual VOCs, it was found that the calculated in situ ozone production was substantially lower (by 60%) if oxidation products of the measured VOCs were not considered or if the measured VOC suite under-represented the actual primary VOC emissions (Whalley et al., 2016). Ozone acts as a respiratory irritant and damages crops and ecosystems and so the ability to accurately model ozone is important. Even where the total VOC emission is well constrained, the transformation into different oxidised products is often uncertain and this greatly affects the total OH reactivity and O3 forming potential (Dunmore et al., 2015).

Combined, measurements of OH reactivity and HCHO, allow primary HCHO which has been directly emitted to be distinguished from HCHO which has been formed during the oxidation of VOCs. During field observations in the Po Valley in Italy, the discrepancy between modelled and measured OH reactivity was small, but HCHO concentrations were substantially under-predicted, demonstrating an unidentified non-photochemical ground-level source of HCHO (Kaiser et al., 2015). The atmospheric degradation of this unidentified source of HCHO (postulated to be directly emitted from agricultural land) was shown to increase ozone production by 12%.

With highly time resolved measurements (1 second or faster), it becomes possible to determine the net flux of a compound using micrometeorological methods such as eddy-covariance or disjunct eddy-covariance, which can provide further valuable insights into production and loss mechanisms of species occurring in the atmosphere. Unlike concentration measurements which reflect a complex mixture of local sources, long range transport and boundary layer dynamics, canopy-scale flux measurements provide robust estimates of emission or deposition of compounds occurring in the area immediately upwind of the measurements, or “flux footprint”. By focusing on OH reactivity and HCHO, on a flux, rather than concentration basis, the influence of advection or long-range transport of pollutants is removed enabling the impact of local emissions on the chemistry, local air quality and climate to be fully evaluated.

Previously we have developed techniques to measure VOC fluxes above both urban (Langford et al., 2010a) and forest environments (see Fig. 3) (Langford et al., 2010b). More recently, DiGangi et al. (2011) conducted the first set of HCHO flux measurements above a Pine forest in Colorado but to date flux measurements of OH reactivity have only been measured using branch enclosure techniques (Nölscher et al., 2013).

Figure 2: OH initiated oxidation of VOC in the presence of NOx leading to formation of ozone, oxygenated VOC (e.g. HCHO) and SOA Figure 3: Instrumentation measuring VOC flux above a forest (Bosco Fontana) in the Po Valley, Italy.

Objectives

This studentship will develop a system capable of measuring OH reactivity fluxes at the canopy-scale as well as a system to measure HCHO fluxes analogous to methodologies described by DiGangi et al. (2011). The project will involve a three stage process of technological development, testing and validation, and field deployments and analysis.  Specific research activities will involve:

1.   Development of methodologies to measure total OH reactivity  and HCHO fluxes

The initial stages of the studentship will involve making modifications to an existing laser flash photolysis pump-probe OH reactivity instrument (Whalley et al., 2016; Stone et al., 2016) to improve the time-resolution to 1 Hz to enable eddy covariance flux measurements. A new detector which has been shown to improve the sensitivity of other laser-induced fluorescence instruments by a factor of 3 will be fitted to the instrument and further inlet modifications will be carried out to achieve this resolution. The instrument will also be re-designed to make it more compact and able to run independently from other field instruments to permit OH reactivity flux measurements to be made from towers and buildings where space is limited, for example from the BT tower in London.  The Leeds HCHO instrument currently measures at 1 Hz, but, software modifications will be made to both the OH reactivity instrument and HCHO instrument to enable the measurements (from each instrument) to be synchronised with the three-dimensional wind-speeds from a sonic anemometer.

2.    Preliminary tests of OH reactivity and HCHO flux instruments at a managed grassland site in Edinburgh

The Easter Bush field site is a managed grassland site close to the Centre for Ecology and Hydrology in Edinburgh. VOC fluxes have been measured at this site in the past, for example, during slurry spreading (Twigg et al., 2011). It is a well-equipped measurement facility offering the necessary power and infrastructure required to fully test and validate the new eddy covariance HCHO and OH reactivity flux systems.

3.    OH reactivity flux intercomparison

To assess the reliability of the OH reactivity flux measurements further, the instrument will be run alongside a newly built Comparative Rate Method (CRM) OH reactivity instrument (Sinha et al., 2008) which utilises disjunct eddy covariance to determine fluxes and is based at the Max Planck Institute in Mainz, Germany.  This comparison will provide opportunity to compare instruments which use different methodologies to determine the OH reactivity (laser flash photolysis pump-probe and CRM) and its flux (eddy covariance and disjunct eddy covariance).

4.    Flux measurements in urban and forested environments

Where opportunities arise, the instruments will be deployed as part of other collaborative field campaigns. Long-term flux measurements of green housegases are made from the King’s College campus and the BT tower in central London; this has included campaign-based measurements of VOC fluxes (Valach et al., 2015). HCHO and OH reactivity flux would complement these existing measurements allowing missing VOC fluxes to be identified and direct emissions of HCHO from various sources (e.g. traffic) to be quantified. Similarly, VOC flux measurements have previously been measured at an Oak forest site in Southern England (Alice Holt, Forestry Commission) and so this site may offer further opportunities to investigate missing VOC fluxes in a forest setting, an environment, where typically the largest missing OH reactivity is reported. The studentship will assess whether better closure can be achieved for the OH reactivity based on local fluxes rather than for the concentrations and look to gain unique insights into the nature of the missing reactivity based on its biophysical behaviour.

5.      Modelling studies

The HCHO and OH reactivity flux measurements from the different environments studied will be compared to those predicted using a photochemical box model, containing a very detailed chemical oxidation scheme based on the detailed Master Chemical Mechanism (MCM). The model will be constrained to known emissions and used to evaluate missing sources or sinks. The impact of these sources (or sinks) in terms of secondary pollutant formation (e.g. ozone) will be investigated.

Potential for high impact outcome

The project will deliver improved mechanisms for the oxidation of a range of important VOCs which will lead to more accurate simulations of climate and air-quality related trace gases and aerosols. The work will be of particular interest to the Department of Energy and Climate Change, 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 to the scientific community through high quality publications in leading journals and at international conferences. We anticipate that the project would lead to several publications in high quality international journals.

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 project directly addresses several NERC strategic challenges listed in “Next Generation Science for Planet Earth 2007-2012”, including in the Climate System theme: “Determine the likely interactions between climate change and air quality”, in the Environment, Pollution and Human Health theme: “Develop better models of the behaviour and persistence of chemicals and radionuclides in the environment”, in the Earth System Science theme “Build knowledge of how atmospheric composition is controlled and feeds back to global change” and in the Technologies theme: “Develop a base of skilled people in crucial areas and monitor emerging technologies and new ideas.

There have been several high impact journal publications on topics related to OH radical oxidation of VOCs (for example in Nature and Science), and further advances to solve long-standing uncertainties will result in dissemination with high impact.

Training

During the studentship, you will develop transferable skills in making field measurements using state of the art instrumentation, along with numerical and data skills associated with data analysis and modelling. The student project will be co-supervised by Dr Lisa Whalley, Prof Dwayne Heard and Dr Trevor Ingham at the University of Leeds and Dr Ben Langford and Dr Eiko Nemitz at the Centre for Ecology and Hydrology (CEH) in Edinburgh. You will work in well-equipped laboratories and be part of an active, thriving and well-funded atmospheric chemistry community. The Leeds group 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 University of Leeds provides a comprehensive training programme for PhD students with a range of courses on both hard and soft skills. You will also have access to training provided by the National Centre for Atmospheric Science such as the Arran instrumental Summer School and the Earth System Science Summer School (ES4). You will also spend time working at the Centre for Ecology & Hydrology, Edinburgh, where you will work as part of the Reactive Gases and Aerosols research group. CEH has a vibrant PhD student community from a range of environmental science disciplines and supports a further training programme addressing both generic (presentation, paper-writing) and more specific skills (programming, statistics etc.).

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. Candidates should have an interest in instrumentation and field work as well as a solid mathematical foundation. Experience in programming would be an advantage.

References

DiGangi, J., et al.: Atmos. Chem. Phys., 11, 10565–10578, 2011

Dunmore, R., et al.: Atmos. Chem. Phys., 15, 9983-9996, 2015

Edwards, P.M. et al.: Atmospheric Chemistry and Physics, 13, 9497-9514, 2013

Kaiser, J. et al.: Atmospheric Chemistry and Physics, 15, 1289-1298, 2015

Langford, B. et al.: Atmospheric Chemistry and Physics, 10, 627-645, 2010a

Langford, B. et al.: Atmospheric Chemistry and Physics, 10, 8391-8412, 2010b

Nölscher, A.C. et al.: Biogeosciences, 10, 4241-4257, 2013

Sinha, V., et al.: Atmospheric Chemistry and Physics, 8, 2213-2227, 2008

Stone, D., et al.: Atmospheric Measurement Techniques, 9, 2827-2844, 2016

Twigg, M. et al.: Agricultural and Forest Meteorology, 151, 1488-1503, 2011

Valach, A., et al.: Atmospheric Chemistry and Physics, 15, 7777–7796, 2015

Whalley, L.K. et al.: Atmospheric Chemistry and Physics, 16, 2109-2122, 2016

Yang, Y., et al.: Atmospheric Environment, 134, 147-161, 2016

Yoshino, A. et al.: Atmospheric Environment, 49, 51-59, 2012

Related undergraduate subjects:

  • Chemistry
  • Environmental science
  • Mathematics
  • Physics