Implementation and evaluation of pH-dependent cloud chemistry and wet deposition in the chemical transport model REM-Calgrid

The Chemistry Transport Model REM-Calgrid (RCG) has been improved by implementing an enhanced description of aqueous-phase chemistry and wet deposition processes including droplet pH. A sensitivity study on cloud and rain droplet pH has been performed to investigate its impact on model sulphate production and gas wet scavenging. Air concentrations and wet deposition fluxes of the model sensitivity runs have been analysed and compared to observations. It was found that droplet pH variation within atmospheric ranges affects modelled air concentrations and wet deposition fluxes significantly. Applying a droplet pH of 5.5 for July 2005, mean sulphate air concentrations increased by up to 10% compared to using a droplet pH of 5 while SO2 domain wet deposition sum increased by 110%. Moreover, model results using modelled droplet pH for January and July 2005 have been compared to model results applying a constant pH of 5 and to observations. The comparison to observations has shown that using a variable droplet pH improves the model performance concerning air concentrations and wet deposition fluxes of the investigated sulphur and nitrogen compounds. For SOx wet deposition fluxes the Root Mean Square Error (RMSE) decreased by 16% for July 2005 when using a variable droplet pH instead of a constant pH of 5. Concerning sulphate and SO2 air concentrations the RMSE was reduced by 8% and 16% for July 2005, respectively. The results have revealed that applying a variable droplet pH is preferable to using a constant pH leading to better consistency concerning air concentrations and wet deposition fluxes.