Atmospheric aerosol particles are tiny solid or liquid particles suspended in the air. Even though most of them are too small for us to see, they exert a significant influence on the atmosphere by playing crucial roles in both environmental and climatic processes. Aerosol particles contribute to air pollution, which represents the greatest environmental threat to human health, while their interactions with solar radiation and clouds affect Earth’s radiative balance and, consequently, the climate. However, the mechanisms underlying these aerosol–cloud interactions remain insufficiently understood, contributing to substantial uncertainties in future climate projections.

In order to improve our understanding on aerosol-cloud interactions and to constrain the related uncertainties, we must obtain more in-situ cloud measurements. This thesis investigates aerosol-cloud interactions in a polluted environment to increase our understanding on the processes involved as well as the importance of the aerosol chemical composition. Therefore, we conducted the Fog and Aerosol InteRAction Research Italy (FAIRARI) campaign during the winter and spring of 2021/2022 at the rural research station San Pietro Capofiume, 30 km from Bologna. The Italian Po Valley is one of the most polluted regions in Europe, where fog is a common occurrence throughout the winter months. The methods consisted of a combination of state-of-the-art measurement techniques quantifying the chemical composition of the ambient aerosol particles, fog residuals, fog water, interstitial (unactivated) particles, as well as the microphysical properties of the fog.

We were able to show that the fog residuals were more internally mixed, and consisted of larger particles. Furthermore, they contained a higher fraction of inorganic species, where nitrate (NO₃⁻), ammonium (NH₄⁺), and sulfate (SO₄²⁻) together contributed to 69% of the fog residual mass compared to 53% of the ambient aerosol mass. The organic aerosol (OA) and black carbon (BC) were comparatively less abundant in the fog than in the ambient aerosol, highlighting their lower hygroscopicity. BC occupied a larger mass fraction in the lower particle sizes. These particles were also scavenged by the fog, despite being smaller than the dry activation diameters, likely due to collision/coalescence processes. Furthermore, hydrated particles proved to have a large impact on the fog microphysical properties, and could grow hygroscopically without activating. These hydrated particles contributed to 87% of the measured fog droplet number concentration. Further investigation into the OA revealed a considerable enhancement of organic nitrogen (ON) compounds in the fog, compared to both the ambient and interstitial aerosol. A small fraction of this fog-enhanced ON included C_xH_yN₂⁺ ions originating from imidazoles. 1H-imidazole was found at high levels in all fog water samples, whereas only trace amounts were observed in the ambient PM₁ filter samples. This strongly suggests that imidazoles were formed in the Po Valley fog.

This doctoral thesis focuses on understanding the role of the aerosol physicochemical properties in their interactions with fog. The composition, solubility, and size of aerosol particles determine their ability to take up water, activate into fog droplets, and participate in chemical reactions within the aqueous phase. These processes, in turn, modify aerosol characteristics such as composition, hygroscopicity, size, and light-scattering properties, ultimately affecting visibility, air quality, and climate.