Reducing uncertainties in Ozone formation via Chamber Studies of VOC Oxidation

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Chambers are a vital tool in understanding the chemistry of our atmosphere. They are an ideal vehicle to test proposed mechanisms under simplified, controlled but realistic conditions. The HIRAC (Highly Instrumented Reactor for Atmospheric Chemistry) chamber was constructed during grant NE/C513493/1 and contains several significant features: 1) Uniquely, for an indoor chamber, we can monitor OH and HO2 radicals directly using the FAGE technique. 2) As the chamber is of a stainless steel construction, it is possible to vary pressures over tropospherically relevant conditions. 3) Many important species can be measured by a variety of techniques. This allows us to investigate the possibility of systematic measurement errors. Concentrations of tropospheric ozone are predicted to rise with implications for climate change and air quality. The current proposal seeks to study a number of reactions that are important in determining ozone levels. The mechanims are complex and can only be unravelled via experimental measurements. HIRAC, with a range of complementary detection systems, is an ideal tool to tackle these systems. The reactions to be studied are: 1) Ozone/alkene reactions have been shown to be an important source of OH radicals especially at night. We propose to measure OH and co-product yields as a function of alkene structure in order to construct temperature and pressure dependent structure activity relationships. 2) OH + aromatics - The peroxy radical formed from the attack of OH on the aromatic ring and addition of O2 can react via two channels, either generating a phenol + HO2 (radical regeneration) or ring opening to give dicarbonyl species which are important radical and aerosol precursors. The branching ratio for peroxy radicals formed from reaction of OH with benzene and toluene will be investigated, supported by ab initio calculations. 3) Recent work in our laboratory has identified significant fragmentation of the CH3CO radical formed from OH abstraction of ethanal. If confirmed, this would have important implications for PAN formation. Modelling simulations shown that monitoring CO and CO2 yields should confirm the presence or absence of chemically activated acetyl decomposition under atmospheric conditions. 4) OH + alkynes. The reaction with OH is the major atmospheric removal process for acetylene, an abundant anthropogenic and biomass pollutant. In the prescence of O2, there are two product channels generating glyoxal/OH and HCOOH/HCO. Glyoxal can be photolysed at relatively long wavelengths to give 2HCO radicals and is hence an important HOx source but, being relatively involatile, is also known to be a good aerosol precursor. The atmospheric budget of formic acid is poorly constrained. 5) OH + alkenes. Ethene oxidation is the single largest contributor to European O3 formation. The major uncertainties in this reaction is the rate of reaction of the intially formed hydroxyperoxy radical with NO and the fate of the product alkoxy radical. Both issues can be addressed by product studies in HIRAC As part of the proposal temperature control will be provided for HIRAC to simulate all conditions of the troposphere. One element of the study will be a systematic study (through a tied studentship) of a range of OH, Cl and NO3 reactions via relative and absolute techniques to construct temperature dependent structure activity relationships (SAR). Such SAR are vital in the development of the MCM for temperature dependent modelling of O3 formation and other issues.