A new approach to detection of highly oxidized products from ethylbenzene oxidation using matrix assisted ionization in vacuum-mass spectrometry

Abstract

Aromatic hydrocarbon oxidation by hydroxyl radicals (OH) efficiently forms secondary organic aerosol (SOA) particles, likely through formation of low volatility highly oxidized products which form and grow these particles. However, the nature of these oxidation products and their mechanisms of formation are not sufficiently well characterized to be included in models with confidence, leaving gaps in predicting SOA particle formation and growth. Challenges for highly oxidized product characterization include the small concentrations, low volatility, propensity for loss on surfaces and instability upon heating which is often part of the analytical detection techniques. A new approach is reported here in which oxidation products generated from the OH oxidation of ethylbenzene (C₈H₁₀) are condensed on glutaric acid particles. These seed particles provide a large condensation surface area and also act as the matrix for detection of surface-bound products via matrix assisted ionization in vacuum-mass spectrometry (MAIV-MS). Series of oxidation products consistent with the formation of peroxides (ROOH and ROOR) from several formation pathways were detected, indicating that they were scavenged from the gas phase. For comparison, gas-phase measurements using nitrate chemical ionization mass spectrometry (NO₃⁻ CIMS) showed highly oxidized C₈ products with up to 8 oxygen atoms, and weaker signals from highly oxidized C₁₆ products with up to 12 oxygen atoms. MAIV-MS of the condensable products showed relatively higher signal intensities for larger mass ROOR products compared to NO₃⁻ CIMS, supporting the importance of ROOR in the formation and growth of particles. Two product series with unexpectedly low numbers of hydrogens that have not been reported previously, C₁₆H₁₆O_5–8 and C₁₆H₁₈O_5–8, were detected with MAIV. Prominent C₁₆ accretion products were observed from a range of formation pathways, including multiple OH attacks to the aromatic ring and contributions from RO₂ isomerization. The use of MAIV-MS for condensable product detection provides direct insights that complement other gas phase measurements and leads to a more comprehensive picture of SOA particle formation and growth.