Secondary inorganic aerosol; Chemistry transport modelling; Particulate matter; Acidification and Eutrophication; Europe
500 Naturwissenschaften und Mathematik 550 Geowissenschaften 551 Geologie, Hydrologie, Meteorologie
Secondary inorganic aerosol (SIA), which are sulphate, nitrate and ammonium, is involved in the eutrophication and acidification of ecosystems, the formation of health relevant particulate matter and climate change by affecting the radiation balance of the earth. A thorough understanding of the SIA budget is of scientific interest and is required to be able to devise emission mitigation strategies that are effective for biodiversity, climate change and human health. SIA concentrations and the deposition of sulphur and nitrogen compounds show non-linear responses to emission changes of sulphur dioxide, nitrogen oxides and ammonia. At the start of this study chemistry transport models (CTMs) did not incorporate all processes leading to these non-linearities and previous studies highlighted that the performance of these models showed clear deficiencies in comparison to observations. Within the here presented study the process description of SIA formation and wet removal is improved within two state-of-the-art regional CTMs followed by a comprehensive operational and dynamic evaluation of the models against observations with the focus on the non-linearity aspects.
The chemical transport model REM-Calgrid has been improved by the implementation of pH dependent aqueous-phase sulphate formation and a new wet deposition scheme including cloud liquid water content dependent in-cloud scavenging and pH dependent droplet saturation. A model sensitivity study varying droplet pH within atmospheric ranges has revealed a significant impact on resultant SIA air concentrations and wet deposition fluxes. Furthermore, the comparison of model results to observations has shown that using a modelled droplet pH is preferable to using a constant pH leading to an increased model performance concerning air concentrations and wet deposition fluxes.
Using the improved RCG model system two PM episodes over Central Europe characterized by a high SIA contribution have been analysed. To investigate the response of modelled SIA concentrations to precursor emission changes several model runs for different emission scenarios varying emissions of sulphur dioxide, nitrogen dioxide and ammonia have been performed. It was shown that the incorporation of the pH dependent aqueous phase chemistry added non-linear responses to the system and significantly modified the models' response to precursor emission variations compared to when using a constant droplet pH.
Finally, a dynamic model evaluation over Europe from 1990 to 2009 has been performed using observations at European rural background sites. Therefore, the improved aqueous phase chemistry scheme was implemented in the LOTOS-EUROS model as the latter includes a source apportionment module that enables the analysis of changes in SIA formation efficiency over time. The model was able to capture the observed non-linear responses to the emission changes within 1990 and 2009. The source apportionment exercise revealed increases in formation efficiency for sulphate and nitrate showing that the model is able to reproduce that changes in the formation efficiency due to changes in the chemical regime from 1990 to 2009 are at the basis of the observed non-linearity in the emission-concentration relationship.
With this modelling study, added knowledge and an improved understanding has been obtained with respect to the non-linearity between emissions of sulphur and nitrogen compounds and the resulting concentrations and deposition fluxes.
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