Molecular Characterization of Cloud Water Samples Collected at the Puy de Dôme (France) by Fourier Transform Ion Cyclotron Resonance Mass Spectrometry

Cloud
droplets contain dynamic and complex pools of highly heterogeneous
organic matter, resulting from the dissolution of both water-soluble
organic carbon in atmospheric aerosol particles and gas-phase soluble
species, and are constantly impacted by chemical, photochemical, and
biological transformations. Cloud samples from two summer events,
characterized by different air masses and physicochemical properties,
were collected at the Puy de Dôme station in France, concentrated
on a strata-X solid-phase extraction cartridge and directly infused
using electrospray ionization in the negative mode coupled with ultrahigh-resolution
mass spectrometry. A significantly higher number (n = 5258) of monoisotopic
molecular formulas, assigned to CHO, CHNO, CHSO, and CHNSO, were identified
in the cloud sample whose air mass had passed over the highly urbanized
Paris region (J1) compared to the cloud sample whose air mass had
passed over remote areas (n = 2896; J2). Van Krevelen diagrams revealed
that lignins/CRAM-like, aliphatics/proteins-like, and lipids-like
compounds were the most abundant classes in both samples. Comparison
of our results with previously published data sets on atmospheric
aqueous media indicated that the average O/C ratios reported in this
work (0.37) are similar to those reported for fog water and for biogenic
aerosols but are lower than the values measured for aerosols sampled
in the atmosphere and for aerosols produced artificially in environmental
chambers.