Effect of mass resolution and mass spectrometer on trace gas measurements in a boreal forest comparing three nitrate CIMS

Abstract. Accurate measurements of oxygenated organic molecules (OOMs) are essential to understand their relevance in secondary organic aerosol formation. Online measurements of OOMs are primarily performed with chemical ionization mass spectrometry (CIMS). These measurements often yield crowded mass spectra due to the molecular complexity of atmospheric OOMs, presenting a challenge for peak assignment and molecular identification. To investigate how mass resolution influences the interpretation of the mass spectrum, we compare three nitrate chemical ionization mass spectrometers: two atmospheric pressure interface time-of-flight (APi-ToF) instruments equipped with different ionization inlets (Eisele type and MION) and an Orbitrap with a MION-inlet. Their resolving powers are 2,500, 7,500 and 280,000 at 200 m/z, respectively. The simultaneous measurements in a Finnish boreal forest demonstrate the advantages of high resolving power for peak identification. In the mass range 260-550 m/z, the MION-Orbitrap identified 1333 non-isotopic peaks compared to the 483 non-isotopic peaks identified by the MION-ToF. By using the Orbitrap peak list, the MION-ToF could identify 1327 peaks. When measuring ions at concentrations higher than 5e5 molecules cm^-3, the instruments correlate very well, where the MION-Orbitrap and MION-ToF have an average correlation of ρ=0.77. At lower concentrations, the correlations are weak (ρ=0.42), likely due to peak overlap in the ToF mass spectra. Peak lists constructed using Orbitrap-derived molecular compositions showed clear improvements in the number of identified peaks over the conventional ToF peak list. These results demonstrate that high-resolution Orbitrap measurements can substantially enhance molecular identification in widely used ToF datasets, leading to more reliable quantification of atmospheric OOMs.