Intercomparison of Chemical Mechanisms for European Air Quality Policy Formulation and Assessment

Abstract

An intercomparison and evaluation of nine chemical mechanisms has been made for their suitability for European air quality policy formulation and assessment. Box modelling techniques were employed using a range of background environmental conditions across Europe. Although the chemical mechanisms gave strikingly similar base case ozone production rates, their responses to 30% NO_x and VOC reductions showed significant dispersion. These 30% reductions in NO_x and VOCs also produced changes in the hydroxyl radical number densities which were again chemical mechanism dependent.

Questions and Answers

Questioner: Emmanouil Oikonomakis

Question: Why do the ozone production rates and OH radical number densities diverge in low NO_x conditions for the different chemical mechanisms but in high NO_x conditions they do not?

Answer: Under high NO_x situations, the reactions of OH with VOCs generate HO₂ and RO₂ radicals which are efficiently recycled back to OH to continue the fast photochemistry. However, as NO_x levels fall, HO₂ and RO₂ radicals increasingly react amongst themselves to form organic hydroperoxides and terminate the free radical chain reactions and reduce photochemical ozone production. These reactions have not been well studied in smog chamber systems so there are large divergences between the chemical mechanism under these conditions.

Questioner: Johannes Bieser

Question: Based on your analysis, can you give a ranking of the different chemical mechanism?

Answer: Which is the best mechanism to use depends on the implementation and application. If the application describes ozone production on the regional scale from largely man-made VOC precursors, then mechanism choice is not an important issue and all the tested mechanisms will work satisfactorily. However, if the application deals with individual VOCs, then the choice of mechanism will be crucial. Such applications include situations with large emissions of biogenic VOCs such as isoprene, the fate and behaviour of air toxics, studies of local ozone formation from highly reactive VOCs and the generation of VOC reactivity scales. For these applications, explicit mechanism such as the Master Chemical Mechanism are recommended.