Impact of pH computation from EQSAM4Clim on inorganic aerosols in the CAMS system

The Integrated Forecasting System (IFS) of ECMWF is used within the Copernicus Atmosphere Monitoring Service (CAMS) to provide global analyses and forecasts of atmospheric composition, including aerosols as well as reactive trace gases and greenhouse gases. Inorganic gas/aerosol equilibrium involving the major sulphate and nitrate anions, i.e., H₂SO₄/HSO₄^-/SO₄^2- and HNO₃/NO₃^-, largely determines the aerosol acidity, while the gas/liquid/solid phase partitioning of semi-volatile cations, NH₃/NH₄⁺, and the liquid/solid partitioning of non-volatile mineral cations, particularly Ca²⁺, Mg²⁺, and K⁺, overall control the gas-liquid-solid aerosol equilibrium partitioning of reactive nitrogen compounds. For the NO₃^- and NH₄⁺ equilibrium, our recent developments have focused on EQSAM4Clim, which has been recently integrated into the IFS as a computationally efficient means of describing aerosol pH in a global modeling system.

EQSAM4Clim is used in the IFS to estimate gas-liquid-solid partitioning and the aerosol associated liquid water content, which is subsequently used to estimate the associated aerosol, cloud and rain acidity. The aerosol, cloud and rain pH is computed by considering the liquid water (H₂O) content in the respective liquid water phase using either the aerosol associated water computed by EQSAM4clim, and if present, the cloud and/or rain water of the IFS. The pH is coupled to the aqueous phase chemistry in IFS(CB05) and in the wet deposition of SO₂ and NH₃, which in-turn affects the aerosol composition through changes in the SO₂/SO₄^2-, NH₃/NH₄⁺and HNO₃/NO₃^- partitioning, the aerosol associated liquid water content and solution pH.

Here we present first results of the of the impact of improved aerosol acidity in the solution (aerosol/cloud/rain water) on PM2.5 forecasts simulated in the IFS which are subject to gas-particle partitioning. In particular, the NH₄⁺, NO₃^- and SO₄^2- concentrations have been compared against observational datasets at the surface, showing promising improvements as a direct result of the new pH computations. When coupling the EQSAMClim pH into the aqueous phase chemistry routine, the surface concentrations of SO₄^2- and the SO₂ + SO₄^2- wet deposition fluxes are improved over most of Europe, but degraded over parts of US. The relative impact of the improved pH appears generally small as compared to other related changes such as updates in aqueous chemistry rates. In the pH coupling, the aqueous chemistry component dominates the impact on the wet deposition of SO₂/NH₃. All of these results are highly dependent on the emissions input (SO₂/NO_x/NH₃).