The role of chemistry in fog formation in the Italian Po Valley

The Po Valley in northern Italy is a densely populated region, with a variety of anthropogenic emissions of primary aerosol particles and precursor gases from industrial, agricultural, and urban activities. Especially in the winter, the orographic and meteorological conditions in this region are favorable to result in pollution levels among the highest in Europe (Daellenbach et al., 2020). The combination of these high aerosol loadings with prevalent cold temperatures in winter cause the formation of fog. Efforts over the last decades led to an increased characterization and understanding of this fog (Fuzzi et al., 1992); however, from an aerosol perspective, this applies mainly to the physical characteristics of the fog while the role of the chemical composition of aerosols remains much less determined. To address this lack in understanding the chemical characteristics, state-of-the-art chemical mass spectrometers measuring aerosol particles as well as trace gases were deployed during the Fog and Aerosol InteRAction Research Italy (FAIRARI) field campaign. The FAIRARI campaign covered the winter and spring 2022 in San Pietro Capofiume in the Po Valley in northern Italy with the overall aim to comprehensively investigate aerosol-fog interactions in this polluted environment by covering the entire size range, from gas molecules up to hydrometeors. First results from the FAIRARI campaign on the bulk chemical composition measurements indicate no clear pattern in the difference in composition when comparing periods of fog with periods where no fog was formed. The bulk aerosol composition was dominated by nitrate and organics during both conditions. In contrast to the ambient aerosol particles, the aerosol particles left when drying the fog droplets show presence of organonitrates. In this presentation, we will dive into the molecular-level chemical composition of organic molecules present in the particulate and gas phase in fog episodes, as well as these organonitrates, potentially formed in the fog droplets. Understanding the different phases and their composition will help to better determine potential sources driving fog formation in this region, which is relevant for both climate and health mitigations.   References Daellenbach, K. R., Uzu, G., Jiang, J., Cassagnes, L.-E., Leni, Z., Vlachou, A., Stefenelli, G., Canonaco, F., Weber, S., Segers, A., Kuenen, J. J. P., Schaap, M., Favez, O., Albinet, A., Aksoyoglu, S., Dommen, J., Baltensperger, U., Geiser, M., El Haddad, I., Jaffrezo, J.-L., and Prévôt, A. S. H.: Sources of particulate-matter air pollution and its oxidative potential in Europe, Nature, 587, 414–419, https://doi.org/10.1038/s41586-020-2902-8, 2020. Fuzzi, S., Facchini, M. C., Orsi, G., Lind, J. A., Wobrock, W., Kessel, M., Maser, R., Jaeschke, W., Enderle, K. H., Arends, B. G., Berner, A., Solly, I., Kruisz, C., Reischl, G., Pahl, S., Kaminski, U., Winkler, P., Ogren, J. A., Noone, K. J., Hallberg, A., Fierlinger-Oberlinninger, H., Puxbaum, H., Marzorati, A., Hansson, H.-C., Wiedensohler, A., Svenningsson, I. B., Martinsson, B. G., Schell, D., and Georgii, H. W.: The Po Valley Fog Experiment 1989., Tellus B, 44, 448–468, https://doi.org/10.1034/j.1600-0889.1992.t01-4-00002.x, 1992.