An evaluation of the performance of chemistry transport models - Part 2: Detailed comparison with two selected campaigns

Abstract

This is the second part of a rigorous model evaluation study involvingfive global Chemistry-Transport and two Chemistry-Climate Modelsoperated by different groups in Europe. Simulated trace gas fields wereinterpolated to the exact times and positions of the observations toaccount for the actual weather conditions and hence for the specifichistories of the sampled air masses. In this part of the study we focuson a detailed comparison with two selected campaigns, PEM-Tropics A andSONEX, contrasting the clean environment of the tropical Pacific withthe more polluted North Atlantic region. The study highlights thedifferent strengths and weaknesses of the models in accuratelysimulating key processes in the UT/LS region includingstratosphere-troposphere-exchange, rapid convective transport, lightningemissions, radical chemistry and ozone production. Model simulatedRadon, which was used as an idealized tracer for continental influence,was occasionally much better correlated with measured CO than simulatedCO pointing towards deficiencies in the used biomass burning emissionfields. The abundance and variability of HO(x) radicals is in generalwell represented in the models as inferred directly from the comparisonwith measured OH and HO(2) and indirectly from the comparison withhydrogen peroxide concentrations. Components of the NO(y) family such asPAN, HNO(3) and NO were found to compare less favorably. Interestingly,models showing good agreement with observations in the case ofPEM-Tropics A often failed in the case of SONEX and vice versa. A betterdescription of NO(x) and NO(y) emissions, chemistry and sinks is thoughtto be key to future model improvements with respect to therepresentation of chemistry in the UT/LS region.