Investigation of the ozone formation of anthropogenic VOCs in the atmospheric simulation chamber SAPHIR

The hydroxyl radical (OH) is the most important daytime oxidant in the troposphere, initiating chemical degradation of volatile organic compounds (VOCs) and hence contributing to the formation of secondary pollutants such as ozone (O₃). In the oxidation process of VOCs, peroxy radicals (RO₂) and hydroperoxy radicals (HO₂) are formed. In polluted areas, characterised by the presence of nitric oxide (NO), the OH radical is regenerated by the reaction of HO₂ with NO, enhancing atmospheric oxidation. Ozone is mainly produced from the photolysis of nitrogen dioxide (NO₂) which is formed in the reaction of HO₂ and RO₂ with NO, where the latter reaction also leads to the formation of an alkoxy radical (RO). Depending on the fate of the RO radical, additional O₃ may be produced.

Large discrepancies between measured and modelled HO₂ and RO₂ radical concentrations have been observed during daytime for several urban environments, both for low (< 1ppbv) and high (> 3ppbv) NO. As measured and modelled radical concentrations are commonly used to determine the instantaneous ozone production rate (P(O_x)), a large model-measurement discrepancy was also found for P(O_x) at high NO.

A systematic study of the photo-oxidation of different anthropogenic VOCs (propane, propene, iso-pentane, n-hexane, trans-2-hexene), associated with traffic emissions and involving different alkoxy chemistry, was conducted at the atmospheric simulation chamber SAPHIR at Forschungszentrum Jülich, Germany, for NO mixing ratios below 1ppbv and between 3 and 5ppbv.

Measured radicals as well as precursors and oxidation products are compared with results from a zero-dimensional box model using the Master Chemical Mechanism (MCM v3.3.1) which is complemented by structure-activity relationships (SAR). When including SAR, an improved model-measurement agreement of HO₂ and RO₂ radical concentrations was specifically found for n-hexane and trans-2-hexene. In addition, the O_x (= NO₂ + O₃) formation per oxidised VOC (P(O_x)_VOC) could be derived from modelled radical concentrations and measured O_x concentrations. Overall, a good agreement between the different P(O_x)_VOC was found.