A detailed look at monoterpene oxidation reactions: results from the CERVOLAND field campaign and MCM modelling

Atmospheric oxidation reactions can be studied in the field, in the lab and by modelling, with each methodic approach having advantages and issues. The main drawback for field experiments is that both chemical and non-chemical processes (emission, advection, vertical dilution, etc.) can simultaneously impact the chemical composition of ambient air, making it difficult to assess their respective contributions. For this purpose, a mobile atmospheric chamber (DouAir) has been developed to trap ambient air at a measurement site and to investigate the chemistry taking place in this isolated air mass. Since the environment within the chamber is controllable, oxidation processes can be measured and modelled with relative ease, so that the underlying chemistry can be better understood.

During July 2018 the DouAir chamber was brought to the Landes Forest in the southwest of France for the CERVOLAND field campaign (Characterisation of Emissions and Reactivity of Volatile Organic compounds in the LANDes forest). The reactor was used to trap real air masses coming from the surrounding forest - consisting mainly of Pinus pinaster trees - and the captured air was subsequently oxidised within the chamber. Different oxidation regimes were studied: dark oxidation, light oxidation by natural sunlight and light oxidation by artificial UV light with a known spectrum. Oxidation processes within the chamber were monitored by a variety of online instruments, including PTR-ToF-MS (for VOCs), PERCA (for peroxy radicals), O3 and NOx analysers, and CPC (for particles).

Here, we present the experimental results from the CERVOLAND field campaign under different oxidation conditions and the results from the 0-D modelling of these experiments using MCM. The focus is on measured and modelled monoterpene oxidation products and possible explanations for measurement-model discrepancies.