Chamber Studies of the Oxidation of Key Atmospheric Intermediates

Product studies on the atmospheric oxidation of volatile organic compounds (VOCs) are vital in quantifying the production of tropospheric ozone (air quality and climate), toxic intermediates (air quality, health) and aerosol precursors (air quality and climate). Understanding these processes improves our predictive models of air quality (e.g. the Master Chemical Mechanism – MCM) or climate, influencing abatement strategies and environmental policy.

The Highly Instrumented Reactor for Atmospheric Chemistry (HIRAC) in the School of Chemistry is a unique facility in the UK to study the gas phase oxidation of VOCs. HIRAC can operate over the temperatures (230 – 330 K) and pressures (0.2 – 1 bar) relevant to the Earth’s troposphere and can detect both stable species and radical intermediates with a range of state-of-the-art instrumentation.

A range of VOCs will be studied but we illustrate the principles and ideas via two important VOCs: methyl formate (CH3OCHO) and hydroxyacetone (CH3C(O)CH2OH).

Methyl formate (MF) – MF is formed in the atmosphere from the oxidation of ethers and is also directly emitted as a solvent and potential biofuel. Reaction with OH is the main oxidation route and a key question is: ‘Where does the OH abstract? From the CH3 or –CHO?’. Abstraction at the CH3 group leads to formic acid HCOOH and there is currently considerable uncertainty as to the sources of acids in the atmosphere with measured quantities being much higher than model predictions. Abstraction at the CHO site is predicted to form formaldehyde (a known carcinogen). Determining the product distribution in MF oxidation should constrain the initial branching ratios and help quantify the HCOOH budget via modelling studies with the MCM.

Hydroxyacetone (HA) – HA is formed from isoprene oxidation; isoprene is by far the largest VOC emission, hence HA is a key intermediate. Once again the initial oxidation is the abstraction of an H by the OH radical and again, this can occur at multiple sites. It is predicted that the dominant (>90%) abstraction site will be the CH2 group. The resulting radical (CH3C(O)CHOH) reacts rapidly with O2 at room temperature to give methylglyoxal CH3C(O)C(O)H, a key precursor to aerosol formation. Prof Heard’s group has developed an instrument for monitoring glyoxal (HC(O)C(O)H) and this instrument can be adapted to monitor methylglyoxal. Interestingly, previous studies have shown that at lower temperatures the methylglyoxal yield decreases and acids start to be formed. As well as experimental studies, it would be possible to use computational chemistry to explore the mechanism of such processes.

The above systems are examples of the VOCs that could be studied and the work illustrates the kind of skills that you could develop. These skills include: instrument development, use of state-of-the-art laser and mass spectrometry systems, modelling and computational chemistry. We can focus the project around the skills that you most want to develop.

In addition to the work in HIRAC it should be possible to carry out complementary studies using laser flash photolysis and laser induced fluorescence and to spend some time working at another chamber in either Europe or the US to gain additional experience. The supervision team have a number of joint projects and working with a large group will give you exposure to a wide range of studies relevant to atmospheric chemistry.