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 impor- tant 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 par- ticle growth. However, quantitative interpretation of widely used mass spectrometric OOM measurements remains lim- ited by reagent-ion selectivity and the lack of authentic monomeric standards, an issue that is particularly impor- tant for hydroperoxides and peroxy acids, which consti- tute a significant fraction of autoxidation products. Here, we synthesize two α-pinene-derived monomeric OOM stan- dards, peroxy norpinonic acid (PNPA; C9H14O4) and per- oxy pinonic acid (PPA; C10H16O4), and confirm their struc- tures by 1H and 13C NMR. We then evaluate their detectabil- ity using a MION-Orbitrap operated with nitrate (NO− 3 ) 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 mea- surements consistently yield substantially lower peroxy-to- carboxylic-acid ratios. These ratios vary strongly across ion- ization modes, with greater apparent peroxy acid loss un- der harder (de)protonation and improved, though still incom- plete, preservation under softer nitrate and uronium adduct formation. Together with the paired temporal profiles of the peroxy and carboxylic acid signals, the results are consis- tent with ionization-associated loss of peroxy acid function- ality. Notably, uronium provides significantly higher sensi- tivity for these moderately oxygenated compounds, comple- menting nitrate’s strong selectivity toward highly oxygenated organic molecules (HOMs). Together, our results suggest that peroxy acids formed via α-pinene autoxidation may be sys- tematically under-quantified by commonly used mass spec- trometric approaches, with possible implications for molecu- lar 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.