Representing acidity in the IFS using a coupled IFS-EQSAM4Clim approach
Authors: Metzger Swen , Remy Samuel , Williams Jason E. , Huijnen Vincent , Meziane Mehdi , Kipling Zak , Flemming Johannes , Engelen Richard 
Published: March 2022

&lt;p&gt;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. Here we provide a first description and assessment of cloud and aerosol pH as computed in EQSAM4CLIM when implemented in the IFS. The more flexible pH computation is furthermore coupled to the aqueous phase chemistry governing the SO&lt;sub&gt;2&lt;/sub&gt; in-cloud oxidation, as well as to the wet deposition routine for gases.&lt;span&gt;&amp;#160;&lt;/span&gt;&lt;/p&gt;&lt;p&gt;Currently the IFS describes acidity via the scavenging strong inorganic acids (HNO&lt;sub&gt;3&lt;/sub&gt;, H&lt;sub&gt;2&lt;/sub&gt;SO&lt;sub&gt;4&lt;/sub&gt;) and the contribution from SO&lt;sub&gt;2&lt;/sub&gt; oxidation and buffering via NH&lt;sub&gt;3&lt;/sub&gt; using a simplistic description, which results in a pH ranging between 3-5. To provide a more diverse range in pH we have coupled the CB05 chemistry scheme and AER-aerosol components of IFS exploiting EQSAM4Clim-based [H&lt;sup&gt;+&lt;/sup&gt;] calculations by introducing the resulting pH into the aqueous phase chemistry and wet deposition processes. The subsequent representation of aerosol acidity in the solution (aerosol/cloud/rain water) has been evaluated against observations and previous modelling studies using the GEOS-Chem global CTM. The use of aerosol induced acidity in the computation of aqueous phase chemical reaction rates was found to be most important for inorganic soluble gas and aerosol phase species, e.g. SO&lt;sub&gt;2&lt;/sub&gt;, and their subsequent oxidative products, e.g. HNO&lt;sub&gt;3&lt;/sub&gt;. The impact on simulated SO&lt;sub&gt;2&lt;/sub&gt; concentrations at the surface is significant, through changes to the aqueous phase chemistry and subsequent wet deposition. The impact on PM2.5 is generally small but regionally positive, over Europe, US and China. There is a positive impact on surface O&lt;sub&gt;3&lt;/sub&gt; with a reduction in the annual mean bias at the surface for Europe, the US and China when compared against observational networks for 2019. Surface SO&lt;sub&gt;4&lt;/sub&gt;&lt;sup&gt;2-&lt;/sup&gt;concentrations are generally closer to observations, especially during wintertime over Europe and U.S., while surface NH&lt;sub&gt;3&lt;/sub&gt; shows only moderate changes. NH&lt;sub&gt;3&lt;/sub&gt; shows no significant improvement in observed biases against observations, but the impact on simulated surface concentration of NH&lt;sub&gt;4&lt;/sub&gt;&lt;sup&gt;+&lt;/sup&gt; aerosol is generally positive, particularly in winter. Further improvements here can be expected by improving the coupling with mineral cations (Na&lt;sup&gt;+&lt;/sup&gt;, K&lt;sup&gt;+&lt;/sup&gt;, Ca&lt;sup&gt;2+&lt;/sup&gt;, Mg&lt;sup&gt;2+&lt;/sup&gt;), and by including major organic acids in the aerosol neutralisation reactions. For EQSAM4Clim, the neutralisation reactions and the total liquid water content (of aerosols, fog/cloud, rain) are key for the pH computation and the associated gas/aerosol partitioning, and most critical for NH&lt;sub&gt;3&lt;/sub&gt;. Ammonia forms here the only volatile cation (NH&lt;sub&gt;4&lt;/sub&gt;&lt;sup&gt;+&lt;/sup&gt;), those presence in the aerosol phase critically depends on water and mineral cations.&lt;/p&gt;&lt;p&gt;In summary, the production efficacy of sulphate and ammonium aerosol is critically dependent on an accurate representation of acidity in aqueous droplets and aerosol species at global scale. This development is not only expected to bring a much improved constraint on the modelling of surface concentrations of sulfur and nitrogen, together with its deposition. Also the cloud and rain water pH itself are within reach as new products of the CAMS global service.&lt;/p&gt;