An evaluation of the regional distribution and wet deposition of secondary inorganic aerosols and their gaseous precursors in IFS-COMPO preparatory to cycle 49R1

Abstract

Abstract. Secondary Inorganic Aerosol (SIA) makes up a considerable fraction of the total particulate matter exposure and, thus, is an important product from any forecasting system of atmospheric composition and air quality. The subsequent loss to the surface of SIA via dry and wet deposition determines the duration of the exposure time for humans and the extent of acidification imposed on sensitive ecosystems. Here we provide a description and evaluation of the most recent updates made towards aerosol production, aerosol scavenging and wet deposition components of the global Integrated Forecast System-COMPOsition (IFS-COMPO) chemical forecasting system, which is used as part of the Copernicus Atmosphere Monitoring Service. The implementation of the EQSAM4Clim simplified thermodynamic module in IFS-COMPO, for use in cycle 49R1, changes the efficacy of phase transfer of SIA precursor gases (sulphur dioxide, nitric acid and ammonia) which significantly impacts the respective SIA particulate concentrations by changing the fraction converted into SIA. Comparisons made against observational composites at the surface for Europe, the US, and Southeast Asia during 2018 show reductions in the global yearly mean bias statistics for both sulphates and nitrates. Updating the IFS-COMPO model towards cycle 49r1 increases both the burden and lifetime of sulphate and ammonium particles by one third. Coupling EQSAM4Clim into IFS-COMPO provides a better description of the partitioning between state phases involving ammonia and ammonium across regions, whereas changes for sulphate are minimal. For nitric acid and nitrates, the partitioning changes significantly, leading to lower particulate concentrations and a corresponding increase in gas-phase nitric acid with an associated improvement in surface nitrate. There is also a shift in the particle size distribution, with less nitrate production in the coarse mode and more in the fine mode. The impact on the total regional wet deposition values is generally positive, except for sulphates in the US and ammonium particles in Southeast Asia which are strongly influenced by the precursor emission estimates. This provides confidence that this update to IFS-COMPO has the ability to provide accurate deposition fluxes of S and N at global scale.

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