Satellite observations reveal higher persistence of fog in polluted conditions in the Po valley, Italy

Fog and low stratus clouds (FLS) form as a result of complex interactions of atmospheric and land surface drivers and their analysis helps to improve traffic safety, solar power planning and to better understand inversion-topped boundary layer clouds in its interaction with the air quality of larger basin areas. A major factor impacting FLS occurrence and life cycle is aerosol loading, but its impact is challenging to disentangle from confounding meteorological factors and measuring both FLS occurrence and aerosol loading simultaneously is challenging.Here we use satellite observations of FLS persistence, ERA5 reanalysis data and aerosol robotic network (AERONET) observations to disentangle the effect of meteorology and aerosol loading on FLS persistence in the Po valley in northern Italy, one of the most polluted regions in central Europe. After selecting 430 FLS events in the winter (DJF) months from 2006-2015 using a regional FLS occurrence threshold, we apply k‐means clustering to latitudinal transects of relative humidity from 550-1000hPa to identify FLS events with similar FLS formation pathways. Analyzing the average synoptic conditions for the two clusters identified shows that FLS formation in the Po valley is either based on radiative processes or moisture advection from the Mediterranean sea. Radiatively formed FLS events are more persistent, likely due to a stable boundary layer combined with a temperature inversion, whereas advective FLS events form under more dynamic conditions and are on average 2-3 hours shorter. Analysis of AERONET observations at three locations reveals that FLS persistence is significantly higher under high aerosol loading, particularly for radiatively formed FLS events. Aerosol loading further shows a clear increasing trend ahead of persistent FLS events, suggesting an accumulation of pollutants and a subsequent increase in cloud condensation nuclei prolonging FLS events through aerosol-cloud interactions. Our results show the combined effect of meteorology, aerosol loading and geographic conditions on FLS persistence in the Po valley and underline the need for further observational studies on the effect of aerosols on the FLS life cycle over different geographic and synoptic backgrounds.