Enhanced Aromatic HOM Production at Low Temperatures Accelerates Particle Growth

Aromatic volatile organic compounds are major anthropogenic precursors driving new particle growth in urban atmospheres. However, the influence of temperature on aromatic oxidation pathways and product condensation remains poorly understood. While colder winter temperatures reduce product volatility, they are also associated with lower OH levels, potentially suppressing multi-generation oxidation that forms low-volatility species. Moreover, temperature-dependent changes in following reaction branching ratios remain largely unknown. Here, we present a series of controlled chamber experiments at the CERN CLOUD facility examining the temperature dependence of aromatic oxidation and its contribution to particle growth. A representative aromatic mixture—toluene, 1,2,4-trimethylbenzene (TMB), and naphthalene—was oxidized at 5 and 20 °C, under varying NO and OH levels to simulate urban boundary-layer conditions. We find that lower temperatures enhance multi-generation OH oxidation, increasing the yield of highly oxygenated organic molecules (HOM) by ~60%. At the same time, the branching ratio of organonitrate formation from aromatic RO₂ + NO reactions rise at low temperatures, leading to a ~20% greater fraction of organonitrates among HOM. Despite organonitrate typically higher volatility, the overall particle growth rates increased due to enhanced HOM production and a decrease in products volatility. These results reconcile the elevated wintertime organonitrate fractions observed in urban and highlight the pivotal role of temperature in controlling multi-generation aromatic oxidation and particle growth under anthropogenic environments.