Rapid formation of aerosol precursors from the autoxidation of aromatic carbonyls and the remarkable enhancing influence of NO addition

Aromatic carbonyl compounds are common constituents of urban atmospheres. Their rapid oxidation yields highly oxygenated organic molecules (HOMs) that are key direct secondary organic aerosol (SOA) precursors in ambient air. However, the formation of HOM and SOA is thought to be suppressed by high NOx (NO + NO2) concentrations, typical of polluted environments. Furthermore, the vast diversity of gaseous hydrocarbons and their reactivity leads to uncertainties in the net effect of NOx on oxidation processes. Here, we investigate the OH initiated oxidation of three aromatic carbonyls, benzaldehyde, phenylacetaldehyde, and acetophenone in variable reaction time experiments (0.9 s to 14 s) in a flow tube reactor coupled to a nitrate chemical ionization mass spectrometer (NO3– CIMS). We observe HOMs with 12 O atoms from phenylacetaldehyde within 0.9 s while benzaldehyde and acetophenone produce HOMs with 10 and 11 O atoms within 1.1 s and 2.7 s, respectively. Remarkably, our experiments with varying high NO concentrations (10 ppb to 1 ppm) at 14 s reaction time show that phenylacetaldehyde and acetophenone HOM yields are enhanced by a factor of up to 16.9 for O8 HOMs, starkly contrasting the traditional understanding of oxidation suppression by NOx. We use quantum chemical methods to characterize the relevant aromatic carbonyl autoxidation mechanisms, which suggest that OH addition is key to the observed rapid molecular functionalization. The fastest formation of HOM from phenylacetaldehyde is in agreement with our calculations and our kinetic simulations at atmospherically relevant conditions. We also determine theoretical saturation concentrations of the measured HOMs and note that production of low to extremely-low volatility compounds is considerably enhanced in presence of NO. This observation represents a significant advancement in our understanding of polluted air and will aid in accurate estimation of HOM and secondary aerosol budgets, especially in high NOx environments.