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

Abstract. Secondary Inorganic Aerosol (SIA) constitutes a considerable fraction of total particulate matter exposure, making it an important component of any atmospheric composition and air quality forecasting system. The subsequent loss of SIA to the surface, via both dry and wet deposition, determines the exposure time for humans and the extent of damage imposed on sensitive ecosystems due to increased surface acidity. This study provides a description and evaluation of recent updates to aerosol production, scavenging, and wet deposition processes in the global IFS-COMPO chemical forecasting system, used within the Copernicus Atmosphere Monitoring Service. The implementation of the EQSAM4Clim simplified thermodynamic module in IFS-COMPO cycle 49R1 alters the phase transfer efficiency of SIA precursor gases (sulphur dioxide, nitric acid, and ammonia), which significantly affects particulate SIA concentrations by modifying the fraction converted into aerosol form. Comparisons with surface observational data from Europe, the U.S., and Southeast Asia during 2018 indicate reductions in the global annual mean bias for both sulphates and nitrates. Updating the IFS-COMPO model to cycle 49R1 increases the burden and lifetime of sulphate and ammonium particles by one-third. Coupling EQSAM4Clim with IFS-COMPO improves the representation of ammonia-ammonium partitioning across regions, while the effect on sulphate is minimal. For nitric acid and nitrates, the phase partitioning is also significantly altered, with lower particulate concentrations leading to an excess of gas-phase nitric acid and an associated improvement in surface nitrate predictions. The impact on total regional wet deposition is generally positive, although sulphates in the U.S. and ammonium particles in Southeast Asia are strongly influenced by precursor emission estimates. Overall, these results provide confidence in the ability of IFS-COMPO cycle 49R1 to deliver accurate global-scale deposition fluxes of sulphur and nitrogen.