Effect of the Alkoxy Radical Chemistry on the Ozone Formation from Anthropogenic Organic Compounds Investigated in Chamber Experiments

The photooxidation of five anthropogenic volatile organic compounds (propane, propene, isopentane, n-hexane, trans-2-hexene) at different levels of nitric oxide (NO) was investigated in the atmospheric simulation chamber SAPHIR, Forschungszentrum Jülich. Measured time series of trace gases and radical concentrations are compared to zero-dimensional box model calculations, based on the Master Chemical Mechanism (agreement within 30%) and complemented by state-of-the-art structure–activity relationships (SAR). Including RO₂ isomerization reactions from SAR, validated with theoretical calculations, improves particularly the model–measurement agreement by ∼20% for n-hexane. The photooxidation of the chosen compounds generates different types of peroxy radicals (RO₂) which produce HO₂ after one or multiple RO₂+NO reaction steps, depending on the formed alkoxy radical (RO). Measurements show that the HO₂/RO₂ ratio is up to ∼40% lower and the number of odd oxygen (O_x = O₃+NO₂) formed per OH+VOC reaction $\left({P\left(O_x\right)}_{VOC}\right)$ is up to ∼30% higher if RO regenerates RO₂ instead of forming HO₂ directly. Though, the formation of organic nitrates nearly completely compensates for the ozone production from the second NO reaction step for nitrate yields higher than 20%. Measured and modelled HO₂/RO₂ ratios agree well as does $P{\left(O_x\right)}_{VOC}$, derived from measured/modelled radical concentrations and calculated from measured O_x.

A large model−measurement discrepancy of ozone production rates was observed, especially in urban environments. This study highlights that VOCs forming HO₂ after several RO₂+NO reaction steps unlikely explain the model−measurement disagreement of the ozone production, observed in field campaigns.