Modeling secondary organic aerosol over urban areas of Central Europe: uncertainties linked to different mechanisms and parameterizations

The most significant environmental health risk for the European population is air pollution, especially for people living in urban areas. Fine particulate matter (PM), a mixture of aerosols with an aerodynamic diameter less than or equal to 2.5 μm, is among the pollutants with the most critical threat to human health in European urban areas. Despite the substantial spatial and temporal variability of the chemical composition of fine PM in Central Europe, it is generally dominated by organic matter and secondary inorganic aerosols, with organic matter being the main contributor to total submicron PM. Modeling organic aerosol using chemical transport models (CTMs) has been problematic for decades, with CTMs often underestimating its concentrations. Potential sources of this deficiency in CTMs include:(1) simplifying assumptions applied in the model description of organic aerosol,(2) uncertainties in the model mechanisms describing gas phase chemistry since these mechanisms determine the concentrations of gaseous precursors of secondary organic aerosol,(3) missing emissions of intermediate-volatility and semivolatile organic compounds (IVOCs and SVOCs) in emission inventories used in CTM simulations. In this work, we focused on investigating the impacts of these sources of uncertainty on the concentrations of organic aerosol in six large cities of the Central European region (Prague, Vienna, Budapest, Berlin, Munich, and Warsaw). For this purpose, we performed a series of model simulations employing an offline coupled modeling framework consisting of the Weather Research and Forecast (WRF) Model and the Comprehensive Air quality Model with Extensions (CAMx) on the Central European domain with a horizontal resolution of 9 km for the period covering the years 2018 and 2019. More specifically, we focused on assessing the influence of two mechanisms for organic aerosol (SOAP, 1.5-D VBS), three mechanisms of gas-phase chemistry (CB6r2, CB6r5, and SAPRC07TC), and several different source-specific parameterizations of IVOCs and SVOCs.