Ozone impacts of gas–aerosol uptake in global chemistry transport models

Abstract. The impact of six heterogeneous gas–aerosol uptake reactions on tropospheric ozone and nitrogen species was studied using two chemical transport models, the Meteorological Synthesizing Centre-West of the European Monitoring and Evaluation Programme (EMEP MSC-W) and the European Centre Hamburg general circulation model combined with versions of the Hamburg Aerosol Model and Model for Ozone and Related chemical Tracers (ECHAM-HAMMOZ). Species undergoing heterogeneous reactions in both models include N₂O₅, NO₃, NO₂, O₃, HNO₃, and HO₂. Since heterogeneous reactions take place at the aerosol surface area, the modelled surface area density (S_a) of both models was compared to a satellite product retrieving the surface area. This comparison shows a good agreement in global pattern and especially the capability of both models to capture the extreme aerosol loadings in east Asia.

The impact of the heterogeneous reactions was evaluated by the simulation of a reference run containing all heterogeneous reactions and several sensitivity runs. One reaction was turned off in each sensitivity run to compare it with the reference run. The analysis of the sensitivity runs confirms that the globally most important heterogeneous reaction is the one of N₂O₅. Nevertheless, NO₂, HNO₃, and HO₂ heterogeneous reactions gain relevance particularly in east Asia due to the presence of high NO_x concentrations and high S_a in the same region. The heterogeneous reaction of O₃ itself on dust is of minor relevance compared to the other heterogeneous reactions. The impacts of the N₂O₅ reactions show strong seasonal variations, with the biggest impacts on O₃ in springtime when photochemical reactions are active and N₂O₅ levels still high. Evaluation of the models with northern hemispheric ozone surface observations yields a better agreement of the models with observations in terms of concentration levels, variability, and temporal correlations at most sites when the heterogeneous reactions are incorporated. Our results are loosely consistent with results from earlier studies, although the magnitude of changes induced by N₂O₅ reaction is at the low end of estimates, which seems to fit a trend, whereby the more recent the study the lower the impacts of these reactions.