Enhanced molecular characterization of atmospheric organic aerosols in the Western Mediterranean basin by Fourier transform mass spectrometry

Atmospheric aerosols (AAs) significantly influence the global radiative balance, air quality, biogeochemical cycles, and human health. While their climate and health impacts are well-studied, their biogeochemical role, including contributions of phosphorus (P), nitrogen (N), and organic matter (OM) to oligotrophic regions like the Mediterranean basin, is less explored. Recent studies suggest variable atmospheric deposition of trace metals and nutrients associated with both natural (i.e. recurring Saharan dust storms, biomass burning episodes) and anthropogenic origin (i.e. polluted air masses from northern and central Europe) with atmospheric OM inputs comparable to rivers. However, the detailed composition of atmospheric organic aerosols in the region remains poorly understood.

Ultrahigh-resolution mass spectrometry (UHRMS) offers unparalleled resolving power and enables detailed characterization of complex natural and anthropogenic organic matter (OM) mixtures. It also provides stoichiometric insights into organic nitrogen (N) or phosphorus (P) molecules that are often undetectable by methods like NMR spectroscopy or lower-resolution mass spectrometers. Here, we present advanced analysis of the chemical composition of aerosol particles collected in the Western Mediterranean basin. We combined atmospheric pressure photoionization (APPI) and electrospray ionization (ESI), two complementary techniques, to achieve comprehensive coverage of both polar and nonpolar molecular components through  UHRMS. Electrospray ionization (+ESI) was paired with a 21-Tesla (T) Fourier-transform ion cyclotron resonance mass spectrometer (FT-ICR MS), delivering exceptional resolving power, sensitivity, acquisition speed, mass accuracy, and dynamic range. Meanwhile, APPI (+ /-) was coupled to an Orbitrap Fusion Lumos 1M to target condensed, polyaromatic, nonpolar compounds that are challenging or impossible to detect by ESI alone.

Approximately 28,000 distinct C_cH_hN_nO_oP_pS_s molecular formulas were assigned across all 30 samples collected in the Western Mediterranean basin to ESI(+) 21-T FT-ICR MS spectra after a solid phase extraction to isolate and desalt the samples, revealing an astonishing molecular chemodiversity mainly driven by nitrogen-containing compounds (CHNO) and oxygenated compound (CHO) with minor contribution of sulphur-containing (CHOS) and phosphorus-containing (CHOP) compounds, despite their inherent poor ionisation efficiency in complex mixture. APPI(+/-) / Orbitrap Lumos 1M proved to be a powerful approach for characterizing the molecular composition of highly condensed hydrocarbons, especially the large molecular species that cannot be eluted from gas chromatography columns.

We will explore the key factors driving the molecular composition of atmospheric aerosols (AAs) and their influence on variations and potential formation pathways. Our findings aim to improve understanding of their composition and sources with a focus on biogeochemical processes in the nutrient-limited, stratified open waters of the Mediterranean Sea.