Size-resolved aerosol chemical composition, acidity, and gas-aerosol partitioning of nitrate in the Netherlands

In the ongoing CAINA project (Cloud-Aerosol Interactions in a Nitrogen-dominated Atmosphere) we investigate multiple aspects of aerosol-cloud interactions under the high concentrations of reactive nitrogen present in the Netherlands. Here, we present results of year-long side-by-side deployment of two aerosol composition instruments with differing size cut inlets (PM_2.5 and PM₁₀), to investigate size-dependent composition, acidity, and nitrate speciation at the Cabauw tower, in the central Netherlands. Aerosol and gaseous composition were measured by two Monitors for AeRosols and Gasses in Ambient air (MARGA 2060IC), run at adjacent locations for over 1 year of measurements. We supplement and interpret these in-situ observations using thermodynamic equilibrium models such as ISORROPIA2 and interpret potential sources aided by back-trajectory modeling using HYSPLIT. We observe strong seasonal variations, with the highest monthly average gas-phase NH₃ concentration of 15 μg m^-3 observed in April 2025, accompanied by large NH₄NO₃ aerosol concentrations (as high as the wintertime maximum) and resulting in the highest pH period of ~ 5. We interpret this low aerosol acidity in terms of its impact on deposition pathways. Mineral dust contributions appear episodically in spring and winter, dominantly in the PM₁₀ fraction, but they occasionally constitute the majority of aerosol mass (both PM₁₀ and PM_2.5) for up to a few days.