Radiation fogs characterize the wintertime climate of many mid-latitude environments, especially in orographic depressions, and exert a profound impact on visibility, surface temperatures and soil-air water vapor fluxes. Trend analysis of fog occurrence based on visibility data supports the potential impact of air pollution decline on reduced fog frequency in several areas of the world, net of the effects of climate warming (van Oldenborgh et al., 2010; Gray et al., 2019). Microphysical models indicate that aerosol-fog interactions manifest on a range of fog physical properties for which, however, we miss relevant observations over decadal time scales. Here, we present a 30-year long timeline of radiation fog liquid water content (LWC) from the Po Valley, Italy, and analyse long-term trends, concomitantly with the trends in potential meteorological and atmospheric composition drivers. In particular, we reconstructed the entire time series of cloud condensation nuclei (CCN) concentrations using a machine learning approach trained on an extended (ca. 15-year long) observation record of differential mobility particle sizer (DMPS) measurements available from the same site (Leinonen et al., 2022). Our results show a consistent decrease of fog LWC in the 90s’ and early 2000’s at a time when aerosols and CCN concentrations underwent a steep decline. By contrast, in the last ten years, aerosol concentrations stabilized while fog LWC has recovered, probably because of an increased daily temperature excursion as a feature of current climate change in the Po Valley. Although the time evolution of Po Valley fog microphysics could not be followed over the three decades, the comparison of the detailed observations performed during recent intensive field campaigns in 2021 and 2022 (Neuberger et al., 2025) with those carried out in the early studies of 1989 and 1994 with state-of-the-art instrumentation (Fuzzi et al, 1992; Wendisch et al., 1998) support the hypothesized effects of CCN on fog LWC be mediated by changes in fog droplet concentration, size distribution and deposition rates. This research provides new insights on the effects of anthropogenic activities on fog occurrence and characteristics, as well as on the mechanisms of aerosol-cloud interactions in regime of high CCN and low supersaturations.
This work was funded by the EU Horizon 2020 project FORCeS (grant no. 821205) and the Horizon Europe project CleanCloud (Grant No. 101137639).
References
Fuzzi et al., The Po Valley fog experiment 1989, Tellus, 44B, 448–468, 1992.
Gray et al., J. Geophys. Res., 124, 10.1029/2018JD029419, 2019.
Leinonen et al., Comparison of particle number size distribution trends in ground measurements and climate models, Atmos. Chem. Phys., 22., 10.5194/acp-22-12873-2022, 2022.
Neuberger et al. From Molecules to Droplets: The Fog and Aerosol Interaction Research Italy (FAIRARI) 2021/22 Campaign. Bull. Amer. Meteor. Soc., 106, E23–E50, https://doi.org/10.1175/BAMS-D-23-0166.1, 2025.
van Oldenborgh et al., On the roles of circulation and aerosols in the decline of mist and dense fog in Europe over the last 30 years, Atmos. Chem. Phys., 10, 10.5194/acp-10-4597-2010, 2010.
Wendisch et al., Drop size distribution and LWC in Po Valley fog, Contrib. Atmos. Phys., 71, 87–100, 1998.