Identification of missing organic reactivity in the urban troposphere

The international societal response to deteriorating air quality and the changing climate is guided by the predictions of numerical models. These models contain estimates of future emissions of trace gases and aerosols from natural and human activity, their dispersal throughout the atmosphere, and their chemical transformations into a wide range of secondary products. Photo-oxidation in the troposphere is highly complex, and is initiated by short lived radical species, in the daytime dominated by the hydroxyl radical, OH, and at night by either NO3 radicals or ozone. Chemical oxidation cycles remove primary emitted trace species which are directly harmful to humans (e.g. CO) or to the wider environment (greenhouse gases e.g. CH4,). However, many of the secondary products produced by atmospheric photo-oxidation are also directly harmful, for example O3, NO2, acidic and multifunctional species, many of which are of low volatility and are able to partition effectively to the condensed phase, creating secondary organic aerosol (SOA).

In order to calculate the abundance of OH, and hence the lifetimes of other trace gases in the atmosphere, it is necessary to have fundamental knowledge of the rates of reaction of the processes that generate and remove OH. OH reacts with both organic and inorganic species, with the former generating significant uncertainty in any OH calculation, since many thousands of reactive volatile organic compounds (VOCs) exist in air. Practically, it has been impossible to identify all VOCs present in air and even where this has been attempted in a comprehensive manner, kinetic data on reaction rates are often missing. This complexity of VOCs, and limited associated data make it intrinsically difficult to reconcile observed OH concentrations and OH reactivity with model calculations. When attempted, significant mismatches are observed, highlighting some basic flaws in our ability to simulate the chemistry of the troposphere. Recent measurements of OH reactivity, combined with measurements of VOCs, have enabled the magnitude of missing OH sinks to be quantified, but not their chemical identity. Other measurements have shown that many unidentified organic components exist in ambient air when comprehensive two dimensional gas chromatography is used as the measurement technique.

This proposal combines for the first time ultra high resolution VOC measurements developed by Lewis and Hamilton in York with the FAGE free-radical measurement and MCM modelling techniques developed by Heard, Whalley and Rickard in Leeds. We will determine the identity of missing organic material that contributes towards the removal of OH, and assess the formation of degradation products from their oxidation. This will be achieved by coupling comprehensive two-dimensional gas chromatography with a time-of-flight mass spectrometer and flame ionization detector with an OH chemical reactor. By exposing ambient air samples to a controlled environment containing enhanced OH radicals, and by observing the relative change of chromatographic peak intensity for unidentified species relative to the change in intensity for known VOC species (and for which the reactivity with OH is known) the OH reactivity of the unidentified species will be determined. The use a mass spectral detector will allow us to positively identify those species which we observe as contributing significantly as OH sinks, and, provide an assessment of the formation rates of secondary products formed. Using a functional group classification of the major species contributing to losses, we will create surrogate parameterized mechanisms for use in the MCM to allow a more accurate description of processes controlling urban OH and O3 Understanding the functionality of the missing reactivity will enable the atmospheric effects upon air quality and climate due to policy changes regarding complex emissions (such as solvent and petrochemical evaporation) to be better assessed.

One of the main outcomes of this work will be the identification of classes of compounds that are key missing sinks of OH in the urban polluted boundary layer, which consequently will most likely play an important role in local ozone and secondary aerosol formation, and hence have important health implications. The outcomes of this project can be used to focus laboratory kinetic and mechanistic studies on key atmospheric species, the outputs of which can be included into air quality models and help improve emission estimates (e.g. via the National Atmospheric Emissions Inventory) of important primary species. The outcomes of this project will feed directly into the work being carried out by the Air Quality Expert Group (AQEG) to address air quality abatement strategies. The general public will benefit from subsequent policy changes and improvements in air quality.

The results from this project will be communicated to the wider scientific community via publication in high quality journals and presentations at international conferences. Novel state-of-the-art instrumentation will be developed in the project which will help to maintain the impact and international leadership of the UK atmospheric composition community. The investigators are active within the professional societies, for example the Royal Society of Chemistry, and the activities within this project will be publicised via our engagements with them.

A workshop will be held in Leeds towards the end of the project to provide a forum for discussion of (i) the instrumentation developed, (ii) the results obtained from the field campaign including comparison with model calculations, (iii) improved representation of chemical processing of organic material in the MCM, and (iv) an evaluation of the impact on air quality policy and future needs of the community. Representatives from the laboratory, field instrumentation and atmospheric modelling communities will be invited together with stakeholders from the Air Quality Expert Group, National Atmospheric Emissions Inventory, DEFRA and health communities. In this way knowledge exchange will be maximised between the scientific and other stakeholder communities.