Atmospheric aerosol particles, tiny solid or liquid particles suspended in air, play a crucial role in environmental processes by interacting with both radiation and clouds, including fog. These interactions contribute to a range of environmental and health-related issues, both on their own and through complex interactions. One of the most polluted regions in Europe is the Italian Po Valley, frequently covered in fog. With a more than 30-year long history in aerosol-fog measurements, the research station San Pietro Capofiume (SPC), located in a rural area near Bologna, is an ideal site for studying aerosol-fog interactions in a changing atmosphere. Despite the fact that aerosol-fog interactions have been extensively studied worldwide, especially in recent years, many uncertainties and challenges persist.

This thesis aims to investigate aerosol-fog interactions in the Po Valley, contributing to a deeper understanding of fog processes such as formation and dissipation, as well as shifts in fog occurrence in the context of a warming climate and cleaner air. These insights are relevant for various fields such as air quality or the transportation sector, but are e.g. also needed to validate fog model results and remote-sensing retrievals of aerosol particles and fog with detailed in-situ measurements. Therefore, during winter and spring 2021/22, we conducted the Fog and Aerosol InteRAction Research Italy (FAIRARI) campaign at the research station SPC. We measured processes on the scale of gas molecules up to droplets and captured not only the wintertime fog period but also the springtime new particle formation period and the transition, which was associated with an increase in the air temperature and global radiation, as well as a decrease in PM_2.5 concentration.

We found a large variability in the microphysical characteristics of the fog events, which lasted between a few minutes and 13 hours. While the dry aerosol mostly consisted of particles around 100 nm, the droplets grew to diameters around 20 μm. Organics and nitrate dominated the dry mass of the measured submicron chemical composition. In the fog residuals, nitrate and sulfate fractions were enhanced, whereas the organic aerosol fraction was reduced compared to the ambient aerosol during non-fog conditions. Notably, the organics present in the fog residuals showed a higher relative contribution of organic nitrogen, potentially resulting from aqueous-phase fog processing. Moreover, we provide new κ-values for several organic compounds and show that a value of 0.1 reliably estimates their hygroscopicity in both supersaturation-limited (Po Valley) and particle-limited (Arctic) environments. The high amount of nitrate found in the Po Valley led to an overall high hygroscopicity of the particles. The high number concentration of accumulation mode particles in combination with high hygroscopic growth factors make hygroscopically grown - but not activated - particles important for various reasons: These hydrated aerosol particles do not only reduce visibility through scattering but also influence the interpretation of aerosol-fog relationship measurements. For example, the effective diameter of the droplets increases by 81 % when excluding hydrated aerosol particles, while the cloud droplet number concentration decreases by 87 %. The contribution of hydrated aerosol particles to the liquid water content was shown to be dependent on the accumulation mode number concentration and the supersaturation. Moreover, not the total number concentration but the shape of the upper tail of the dry particle number size distribution was shown to be crucial when predicting fog microphysical parameters using Large Eddy Simulations. Additionally, hydrated aerosol particles impact the physical and chemical measurements of interstitial aerosol and fog residuals as those are defined by ambient diameter thresholds which are lower than the largest hydrated aerosol particles.

This thesis emphasizes the importance of aerosol chemical composition and particle size in determining aerosol-cloud and aerosol-fog behavior across contrasting environments. It highlights the role of fog and aqueous-phase processing in transforming aerosol properties and affecting visibility, climate-relevant fog parameters, and potentially human health.

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.