The influence of aerosol particles on fog microphysics during the Fog and Aerosol InteRAction Research Italy (FAIRARI) campaign 2021/22

The Po Valley in northern Italy is an ideal laboratory to study fog-pollution interactions. The peculiar orography of the region (enclosed between the Alps and the Apennines) promotes stable meteorological conditions and radiation fog formation in wintertime. At the same time, high population density and the several agricultural and industrial activities are responsible for high levels of pollutants, among the highest in Europe. The interaction between those factors has been studied since the 1980s, however, the detailed microphysical processes behind the aerosol-fog interactions are still to be elucidated. Therefore, in winter 2021/22, the Fog and Aerosol InteRAction Research Italy (FAIRARI) campaign took place at the research station San Pietro Capofiume, in a rural area close to Bologna. Microphysical as well as chemical aerosol and fog processes from the molecular to the droplet scale were captured.

Stockholm University’s mobile atmospheric laboratory simultaneously measured the total dried aerosol and the dried fog droplets (fog residuals), which then both were analyzed with respect to their size and (re-)activation behavior. Moreover, the chemical composition of the dried aerosol particles was determined. Meteorological parameters such as horizontal wind, updraft, and visibility were measured as well as the size distribution of the fog droplets.

We will present and discuss the influence of aerosol particles on fog microphysics during FAIRARI. For example, hydrated but not activated aerosol particles contributed to more than 50% of the visibility reduction, having implications for the definition of the beginning of fog. This in turn impacted the fog describing parameters such as the effective diameter or liquid water content (LWC), which are crucial when comparing in-situ measurements to data retrieved from satellite observations and modelling predictions. During FAIRARI, if fog is defined as LWC > 0.01 g m^-3, the in-fog median LWC increases by 28% (from 0.18 g m^-3 to 0.23 g m^-3), compared to if fog is defined by visibility < 1km. The hygroscopicity parameter κ was calculated to be around 0.36 in the ambient aerosol out of fog and about 0.47 in the fog residuals. Moreover, sensitivity tests with the large-eddy simulation model MIMICA showed that with the same amount of aerosol particles in the air, changes in the size distributions lead to significant modifications of fog microphysical properties. This work will contribute to constrain the role of aerosol parameters on fog properties and facilitate model improvements.

Financial support from the European Union’s Horizon 2020 research and innovation program (project FORCeS No 821205 and H2020-INFRAIA-2020-1 under grant agreement No 101008004) and the European Research Council (Consolidator grant INTEGRATE No 865799) is gratefully acknowledged.