Analysis of secondary inorganic aerosols over the Greater Area of Athens using the EPISODE-CityChem source dispersion and photochemistry model

Abstract. Secondary inorganic aerosols (SIA) are major components of fine particulate matter (PM_2.5), having substantial implications for climate and air quality in an urban environment. In this study, a state-of-the-art thermodynamic model has been coupled to the source dispersion and photochemistry city-scale chemistry transport model EPISODE-CityChem, able to simulate pollutants on a horizontal resolution of 100 x 100 m², to determine the equilibrium between the inorganic gas and aerosol phases over the Greater Area of Athens, Greece, for the year 2019. In agreement with in-situ observations, sulfate (SO₄^2-) is calculated to have the highest annual mean surface concentration (2.15 ± 0.88 μg m^-3) among SIA in the model domain, followed by ammonium (NH₄⁺; 0.58 ± 0.14 μg m^-3) and fine nitrate (NO₃^-; 0.24 ± 0.22 μg m^-3). Simulations denote that NO₃^- formation strongly depends on the local nitrogen oxide emissions, along with the ambient temperature, the relative humidity, and the photochemical activity. Additionally, we show that anthropogenic combustion sources may have an important impact on the NO₃^- formation in an urban area. During the cold period, the combined effect of decreased temperature in the presence of non-sea salt potassium favors the partitioning of HNO₃ in the aerosol phase in the model, raising the NO₃^- formation in the area. Overall, this work highlights the significance of atmospheric composition and the local meteorological conditions for the equilibrium distribution of nitrogen-containing semivolatile compounds and the acidity of inorganic aerosols, especially in urban areas where atmospheric trace elements from natural and anthropogenic sources coexist.