NO3-initiated oxidation of biogenic hydrocarbons

Abstract

To test the recent hypothesis of a possible non-photochemical, NO₃-initiated production of peroxy radicals, differential optical absorption spectroscopy (DOAS) measurements of NO₃ and its precursor species were made at a mediterranean Eucalyptus forest site in the framework of the CEC FIELDVOC'94 project. Our data set is presented in combination with other FIELDVOC'94 results for various important reactants in night-time radical chemistry. Despite a high NO₃ production (as calculated from O₃ and NO₂ measurements), its concentration remained below the detection limit of 6 pptv. This is most likely due to the large abundance of olefins, which are very effective NO₃ scavengers. In fact, it is shown that among the known NO₃ sinks, the reaction with olefins was by far the most important NO₃ loss process during our measurements. It was suggested that this reaction could be a non-photochemical source of peroxy radicals (RO₂) and probably OH. Kinetic model calculations based on our observations lead to night-time RO₂ concentrations between 10⁸ and 10¹⁰ molecule $\text{cm}{}^{\mathrm{-3}}$. While during most nights observed RO₂ values were considerably lower, elevated RO₂ concentrations in the low ppt range were occasionally found in the late evening, demonstrating the presence of a night-time RO₂ source. The data set suggests that heterogeneous scavenging of peroxy radicals (which is not accounted for in our model) may be the dominant night-time sink for peroxy radicals.