Enrichment of organic nitrogen in fog residuals observed in the Italian Po Valley

Abstract. While aerosol-cloud interactions have been extensively investigated, large knowledge gaps still exist. Atmospheric organic nitrogen (ON) species and their formation in the aqueous phase are potentially important due to their influence on aerosol optical and hygroscopic properties, and their adverse effects on human health. This study aimed to characterize the wintertime aerosol and fog chemical composition, with focus on the formation of ON, at a rural site in the Italian Po Valley. Online chemical characterization of interstitial aerosol (non-activated particles) and fog residuals (dried fog droplets) were performed in parallel. Fog residuals were sampled using a Ground-based Counterflow Virtual Impactor (GCVI) inlet and analyzed by a Soot-Particle Aerosol Mass Spectrometer (SP-AMS), while the interstitial aerosol was characterized by a High-Resolution Time of Flight AMS (HR-TOF-AMS). Our results revealed an enhancement of nitrate (NO₃^-; 43.3 % vs. 34.6 %), ammonium (NH₄⁺; 15.2 % vs. 11.7 %), and sulfate (SO₄^2-; 10.5 % vs. 6.6 %) in the fog residuals compared to the ambient non-fog aerosol, while the organic aerosol (OA; 27.6 % vs. 39.4 %) and refractory black carbon (rBC; 2.3 % vs. 6.3 %) were less abundant. An enrichment of ON was observed in the fog, mainly consisting of C_xH_yN₁⁺ ions, partly originating from amines. C_xH_yN₂⁺ ions, fragments linked to imidazoles were over proportionally present in the fog, suggesting aqueous-phase formation, which was verified by proton nuclear magnetic resonance (1H-NMR) spectroscopy. This study demonstrates that fogs and clouds are potentially important sinks for gaseous nitrogen species and medium for aqueous production of nitrogen-containing organic aerosol in the atmosphere.