Improving Molecular-Level Understanding of Atmospheric Oxygenated Organic Molecules Using Online High-Resolution...

Atmospheric oxygenated organic molecules (OOMs) are key intermediates driving secondary organic aerosol (SOA) formation and new particle growth, yet their molecular-level characterization is constrained by the insufficient mass resolution of current online mass spectrometers (MS). Here, we developed a high-resolution nitrate chemical-ionization Orbitrap MS (NO₃^--Orbitrap) and conducted parallel field and laboratory intercomparison with a widely used nitrate chemical-ionization time-of-flight MS (NO₃^--TOF). The results show that the NO₃^--Orbitrap unambiguously separates multiple neighboring high-m/z OOMs with peak distances smaller than 0.01 Da. During field observations, the NO₃^--Orbitrap detected 1363 OOM peaks, of which 404 overlapped with those identified by the NO₃^--TOF, which detected 930 OOM peaks, indicating that a large proportion of TOF-assigned peaks were misidentified. Leveraging the Orbitrap-derived peak list, we introduce a new framework for refitting NO₃^--TOF spectra (NO₃^--TOF_Modified), which substantially reduces peak misidentification and increases the number of OOMs with strong cross-instrument agreement (r ≥ 0.7) from 45 to 221 OOMs. Specifically, the average OOM concentration measured by NO₃^--TOF_Modified was 1.9 times that measured by NO₃^--Orbitrap. Severe interference from adjacent peaks due to the low mass resolution of the NO₃^--TOF_Modified, together with the lower sensitivity of the NO₃^--Orbitrap, largely explains the weak correlations in the time series of the remaining OOMs and the concentration differences among instruments. Our study highlights the essential role of ultrahigh-resolution MS for resolving atmospheric OOM complexity and provides a new framework for enhancing both current and historical OOM identifications.