Instantaneous nitric oxide effect on secondary organic aerosol formation from m-xylene photooxidation

- Enhanced SOA formation found at sub-ppb NO levels during m-xylene photooxidation.
- Multiple reaction scenarios used to simulate continuous NOx and NO2/NO ratios typical of urban atmospheres.
- Gas-phase chemical modeling of experiments applied to identify major SOA routes.
- Comprehensive mechanism showing competition between VOC-Radicals -NOx proposed.
- Details of the interplay of VOC-NOx and HO/HO2/RO2 radical levels on SOA formation included.

Secondary organic aerosol (SOA) formation from aromatic hydrocarbon photooxidation is highly sensitive to NO concentration. The instantaneous effect of NO on SOA formation from m-xylene photooxidation is investigated in this work by data mining 10 years of aromatic hydrocarbon chamber experiments conducted in the UC Riverside/CE-CERT chamber. First, the effect of sub-ppb NO concentrations on SOA formation is explored. The relationship of SOA growth rate to 1) NO2/NO ratio; 2) instantaneous HC/NO; 3) absolute NO concentration; 4) peroxy radical reaction branching ratio and 5) hydroxyl radical concentration are illustrated. Second, continuous and stepwise NO, NO2 and HONO injection are applied to m-xylene photooxidation experiments to simulate continuous NO sources in an urban area. The influence of these reaction scenarios on radical concentrations and SOA formation is explored. [HO2]/[RO2] shows a strong correlation with SOA yields in addition to [OH]/[HO2], [OH], [HO2] and [RO2]. Enhanced SOA formation is observed when low NO levels (<1 ppb) are artificially maintained by continuous or step-wise injection; consistent with earlier research, SOA formation is observed to be suppressed by large initial NO injections. It is proposed that NO at sub-ppb level enhances OH formation increasing HO2 and RO2 and therefore promoting SOA formation. Further, two NO pathways (one promoting and one suppressing SOA formation) and one extremely low NO phase (NO “free”) are used to demonstrate the evolution of NO impact on SOA formation during photooxidation. This study implies that SOA yields from aromatic hydrocarbon and low NOx photooxidation is previously underestimated due to differences between traditional environmental chamber experiments and atmospheric reactivity.