Real-time organic aerosol characterization via Orbitrap mass spectrometry in urban and agricultural environments

Abstract. Mass spectrometry techniques traditionally deployed in the field often operate at low mass resolution, making it hard to unambiguously identify and attribute organic molecules. In this regard, in-situ, accurate and precise online mass-spectrometric measurements of organic molecules in atmospheric organic aerosol (OA) are essential for understanding its sources, formation and chemical composition. In this study, we demonstrate the field applicability of a high-resolution (Orbitrap) mass spectrometer with Atmospheric Pressure Chemical Ionization (APCI-Orbitrap-MS) for real-time ambient OA measurements, achieving online, molecular resolution at atmospherically relevant concentrations with a high temporal resolution of 1 s, mass resolution of R=120,000 at m/z 200, and mass accuracy of ±1.5 ppm. These features enable chemically reliable measurements in environments that are exhibiting chemically complex aerosol composition, through molecular-level detection and identification of anthropogenic pollutants, biogenic and biomass burning tracers. As proof of principle, we deployed the APCI-Orbitrap-MS for in-situ measurements in a mobile laboratory container at an urban background station at Campus Riedberg (CR, Frankfurt am Main, Germany) and an agricultural field site in Schivenoglia (SKI, Italy) in the heavily polluted Po Valley. The APCI-Orbitrap-MS showed good agreement with the organic aerosol mass of an aerosol chemical speciation monitor (ACSM), with Pearson's R values of 0.91 and 0.70 for the urban and agricultural sites, respectively. In SKI, we resolved distinct diurnal variations in compounds such as MBTCA (C₈H₁₂O₆), a biogenic marker of photochemical aging, and C₈H₁₃O₈N, an organic nitrate indicative of nighttime chemistry. Additionally, nighttime biomass burning events were detected frequently, with durations ranging from 10 to 40 minutes, emphasizing the importance of high temporal resolution. During these events we found up to 30 isobaric peaks per unit mass that are baseline-resolved. For the first time, the hydroxypinonyl ester of cis-pinic acid (C₁₉H₂₈O₇) could be measured and confirmed with MS² experiments in ambient aerosol by an in-situ method at CR. In addition, laboratory experiments were performed to confirm the broad applicability of the APCI-Orbitrap-MS for the real-time detection of biogenic and biomass burning tracers, as well as specific anthropogenic pollutants, such as pesticides, organophosphates or organic esters from aircraft lubrication oil.