Observations of trace gases above & below a forest canopy during ACROSS

The ACROSS campaign (Atmospheric chemistry of the suburban forest) is a collaborative effort to better understand how the mixing between urban and biogenic air masses impacts atmospheric composition and reactivity. A main objective is to provide a detailed description of physico-chemical processes involved in atmospheric transformations of biogenic Volatile Organic Compounds (VOCs), and the myriad of intermediate species formed as a consequence, which have yet to be characterized. This multi-platform campaign took place during summer 2022 for a duration of 6 weeks and included a supersite in the Rambouillet forest located on the path of pollution plumes from Paris. An unprecedented suite of analytical instruments was deployed at this site to probe air masses at the ground level and above the forest canopy using a 40-m tower.
This presentation will focus on trace gas observations, including VOCs, RO_x (OH+HO₂+RO₂) and selected inorganic species (O₃, NO_x – NO+NO₂), both above and below the forest canopy. A descriptive analysis will be presented to highlight biogenic VOC oxidation regimes taking place under unpolluted (oceanic air masses) and polluted (Paris plumes) conditions, with a focus on primary/secondary species. Chemical compositions observed above and below-canopy will also be contrasted. On the basis of these observations, this presentation will highlight scientific questions related to RO_x, VOC and NO_x budgets that will be further investigated to fill important gaps in our understanding of biogenic VOC transformations and secondary organic aerosol formation.

Acknowledgments. This work is supported by the French national research agency (ANR-20-CE01-0010) & LABEX-CaPPA (ANR-11-LABX-005-01) and the French national program LEFE/CHAT INSU.