Ecosystem-scale biogenic volatile organic compound fluxes over rapeseed by eddy-covariance

Abstract. Biogenic volatile organic compounds (BVOC) are key precursors of ozone and secondary organic aerosols, yet their emissions from croplands remain poorly characterized. Here, we report what is, to our knowledge, the first ecosystem-scale quantification of BVOC fluxes over a rapeseed crop during fruit development and senescence. Measurements were conducted at the FR-Gri ICOS site near Paris (France) in May–June 2017 using eddy covariance with a proton-transfer-reaction quadripole ion guide time-of-flight mass spectrometer (PTR-Qi-TOF-MS). Complementary flux estimates were obtained via an aerodynamic resistance approach using a five-level vertical concentration profile. In total, 42 BVOC were significantly emitted or deposited. Methanol dominated emissions during fruit development (about 90 % of total molar flux), followed by acetone, monoterpenes and isoprene. During senescence, formaldehyde and methanethiol emerged as additional contributors. Deposition fluxes were mainly attributed to formic acid (about 50 %), with smaller contributions from other oxygenates. Several BVOC, including formaldehyde and acetic acid, exhibited bidirectional fluxes. Agreement between aerodynamic resistance and eddy-covariance fluxes was generally acceptable for non-reactive BVOC (R² ranging 5–31 %), but diverged for reactive compounds due to longer residence times in profile tubing. This effect helped reveal episodic, atypical deposition of monoterpenes, isoprene, and siloxanes, likely linked to herbicide-related advection from neighbouring fields. Our findings eventually demonstrate that BVOC contribution to total OH reactivity is mostly due to terpenoids (about 40 %), suggesting that MEGAN2.1 would substantially underestimate terpenoid emissions from oilseed rape, implying a larger role of croplands in secondary organic aerosol formation than currently represented.