Abstract
This intercomparison has taken thirteen chemical mechanisms and compared how they treat VOC oxidation and degradation and its relationship to the photochemical formation of ozone and hydroxyl radicals. Here, we have looked in some detail at the incremental responses of hydroxyl radicals to incremental additions of a range of organic compounds under conditions appropriate to the background atmosphere. Most of the time, with most organic compounds and most chemical mechanisms, incremental additions of an organic compound led to depletion of hydroxyl radical concentrations. The chemical mechanisms studied demonstrated increasingly negative incremental hydroxyl radical reactivities with increasing carbon numbers for the alkanes ethane, propane and n-butane. Hydroxyl radical incremental reactivities for the simple alkenes, ethylene and propylene, were reasonably consistent across the chemical mechanisms studied. However, this consistent representation did not extend to trans but-2-ene, where reactivity estimates spanned a range of a factor of five. Incremental reactivities were reasonably well-defined for isoprene which was encouraging in view of its importance to background tropospheric chemistry. The most serious discrepancies emerging from this study were found with the aromatics toluene and o-xylene, and with the Master Chemical Mechanism and these are discussed in some detail.
Highlights
Air quality models are employed by policy-makers to formulate emission control strategies with a view to combatting photochemical air pollution, particulate matter and acid rain in Europe, Asia and North America
This second major impact of organic compounds on Changes to the sources and sinks of the hydroxyl radical have an important impact on tropospheric chemistry because of the vast numbers of trace gases whose sinks are controlled by ozone and hydroxyl (OH) radical driven oxidation and degradation
The methodology chosen to address the impact of organic compounds on the major free radical species involved adding increments of each organic compound and following the changes in hydroxyl radical number densities ([OH]) and the rates of oxidation of organic compounds (ROC)
Summary
Air quality models are employed by policy-makers to formulate emission control strategies with a view to combatting photochemical air pollution, particulate matter and acid rain in Europe, Asia and North America. Chemical mechanisms are essential components of air quality models because ozone (O3 ) is a secondary pollutant that is not emitted into the atmosphere. Attention is focussed on the representation of the oxidation and degradation of the organic compounds and its relationship to the photochemical formation of ozone and hydroxyl (OH) radicals. The representation of these processes presents a formidable challenge to mechanism developers because of their complexity, because of the myriad of organic compounds that need to be considered and because of the limited nature of current understanding. Attention is given to the photochemical ozone production rates and the OH number densities in the base case and their responses to incremental additions of organic compounds
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