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

Biogenic volatile organic compounds (BVOCs) 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 quadrupole ion guide time-of-flight mass spectrometer (PTR-Qi-TOF-MS). In total, 42 BVOCs were significantly emitted or deposited. Methanol dominated emissions during fruit development (about 90 % of total molar flux), followed by monoterpenes, acetone, and isoprene. During senescence, formaldehyde and methanethiol emerged as additional contributors and emission fluxes of oxygenated compounds like methanol, acetone, acetaldehyde, formaldehyde, formic acid and acetic acid were larger than during the fruit development period. Deposition fluxes over both periods were mainly attributed to formic acid (about 50 %), with smaller contributions from other oxygenated BVOCs. Several BVOCs, including formaldehyde and acetic acid, exhibited bidirectional fluxes. Besides, our findings show that the oxygenated BVOC SEFs, that were calculated in the present study with a slightly modified version of the MEGAN2.1 equations, were generally smaller than what is estimated in MEGAN2.1, while terpenoid BVOCs (isoprene, monoterpenes) SEFs may be underestimated in that model. Finally, our study points out that the contribution to total OH reactivity is mostly due to terpenoids (about 40 %), which suggests a larger role of croplands in secondary organic aerosol formation than currently represented in models.