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.
Continue with Facebook
