Atmospheric organic matter is a complex mixture of thousands of individual organic compounds originating from a combination of primary emissions and secondary processes. To study the influence of regional biomass burning emissions and secondary processes, ambient samples of fog and aerosol were collected in the Po Valley (Italy) during the 2013 Supersito field campaign. After the extent of fresh vs. aged biomass burning influence was estimated from proton nuclear magnetic resonance ( 1 H-NMR) and high resolution time of flight aerosol mass spectrometry (HR-TOF-AMS) observations, two samples of fog water and two samples of PM1 aerosol were selected for ultrahigh resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) analysis. Over 4300 distinct molecular formulas were assigned to electrospray ionization FT-ICR MS anions and were sorted into four elemental groups (CHO, CHNO, CHOS and CHNOS) and 64 subclasses. Molecular weight distributions indicated that the water-soluble organic matter was largely non-polymeric without clearly repeating units, although some evidence of dimerization was observed for C 10 compounds and especially for C 8–9 CHNO species in the aged aerosol. The selected samples had an atypically large frequency of molecular formulas containing nitrogen and sulfur (not evident in the NMR composition) attributed to multifunctional organonitrates and organosulfates. While higher numbers of organonitrates were observed in aerosol (dry or deliquesced particles), higher numbers of organosulfates were mostly found in fog water and so chemical reactions promoted by liquid water must be postulated for their formation. Consistent with the observation of an enhanced aromatic proton signature in the 1 H-NMR analysis, the average molecular formula double bond equivalents and carbon numbers were higher in the fresh biomass burning influenced samples, whereas the average O : C and H : C values from FT-ICR MS were higher in the samples with an aged influence (O : C > 0.6 and H : C > 1.2). The aged fog had a large set of unique highly oxygenated CHO fragments in HR-TOF-AMS mass spectra, which reflects an enrichment of carboxylic acids and other compounds carrying acyl groups as highlighted by the NMR analysis. Fog compositions were more SOA-like than aerosols as indicated by the observed similarity between the aged aerosol and fresh fog, implying that fog nuclei must be somewhat aged. Overall, functionalization with nitrate and sulfate moieties in addition to aqueous oxidation trigger an increase in the molecular complexity in this environment, which is apparent in the FT-ICR MS results. This study demonstrates the significance of the aqueous phase to transform the molecular chemistry of atmospheric organic matter and contribute to secondary organic aerosol.