Insufficient mass spectrometric detection of synthesized peroxy acids from α -pinene ozonolysis

Abstract. Biogenic volatile organic compounds (BVOCs) are major precursors of secondary organic aerosol (SOA) and new particle formation (NPF), and therefore play an important role in the climate system, altering the abundance of cloud condensation nuclei (CCN). Ozonolysis of the most atmospherically abundant monoterpene, α-pinene, generates RO2 radicals which undergo autoxidation, resulting in the formation of oxygenated organic molecules (OOMs) with low volatility, an essential step in nucleation and early particle growth. However, quantitative interpretation of widely used mass spectrometric OOM measurements remains limited by reagent-ion selectivity and the lack of authentic monomeric standards, an issue that is particularly important for hydroperoxides and peroxy acids, which constitute a significant fraction of autoxidation products. Here, we synthesize two α-pinene-derived monomeric OOM standards, peroxy norpinonic acid (PNPA; C9H14O4) and peroxy pinonic acid (PPA; C10H16O4), and confirm their structures by 1H and 13C NMR. We then evaluate their detectability using a MION-Orbitrap operated with nitrate (NO3-) and uronium (CH5N2O+) chemical ionization and compare these gas-phase schemes to heated electrospray ionization (H-ESI). NMR shows that freshly prepared standards are dominated by peroxy acids and contain only a fraction of the corresponding carboxylic acids, whereas Orbitrap measurements consistently yield substantially lower peroxy-to-carboxylic-acid ratios. These ratios vary strongly across ionization modes, with greater apparent peroxy acid loss under harder (de)protonation and improved, though still incomplete, preservation under softer nitrate and uronium adduct formation. Together with the paired temporal profiles of the peroxy and carboxylic acid signals, the results are consistent with ionization-associated loss of peroxy acid functionality. Notably, uronium provides significantly higher sensitivity for these moderately oxygenated compounds, complementing nitrate's strong selectivity toward highly oxygenated organic molecules (HOMs). Together, our results suggest that peroxy acids formed via α-pinene autoxidation may be systematically under-quantified by commonly used mass spectrometric approaches, with possible implications for molecular assignments, oxygen-to-carbon (O : C) ratios, volatility-basis-set derivations, and inferred rates of nucleation and early particle growth. The broader atmospheric magnitude of these effects remains to be established.