Volatile organic compound fluxes over a winter wheat field by PTR-Qi-TOF-MS and eddy covariance

Abstract

Abstract. Volatile organic compounds (VOC) contribute to air pollution through the formation of secondary aerosols and ozone and contribute to increasing the lifetime of methane in the atmosphere. Tropospheric VOC are 90 % originating from biogenic sources at the global scale. Forests are the main contributors to these emissions with isoprene and monoterpenes being the most emitted compounds. Crops are also a potentially large, yet poorly characterised, source of VOC. In particular, measurements of VOC fluxes for wheat at the ecosystem scale are scarce, although this is the most cultivated crop in Europe. Available evidence indicates that crops may contribute to 30% of the VOC emissions in Europe, especially oxygenated, low-molecular-weight VOC such as methanol, acetone and acetaldehyde. In this study, which is part of the COV3ER French national project, we investigated VOC fluxes over a wheat field in-situ by eddy covariance using a PTR-Qi-TOF-MS with an outmost sensitivity and mass resolution. We found 264 compounds to have a flux three times above the flux detection limit. Methanol was the most emitted compound, with an averaged flux of 63 µg m−2 h−1, representing around 60 % of summed VOC emissions on a molar basis (40 % on a mass basis). This finding is in line with previous measurements at canopy and plant scales. We also measured acetone, acetaldehyde and dimethyl sulphide among the five most emitted compounds. The second most emitted VOC corresponded to the ion m/z 93.037, tentatively identified as C6H4O. This compound was not reported previously as one of the most emitted compound by terrestrial ecosystems. Summed VOC emissions amounted around 150 µg m−2 h−1. Summed VOC deposition amounted to around −125 µg m−2 h−1, which represented about 70 % of the VOC emissions on a mass basis. The most depositing VOC were tentatively identified as hydroxyacetone and fragments of oxidised VOC with a flux of −16 µg m−2 h−1. Overall, our results reveal that wheat fields represent a non-negligible source and sink of VOC to be considered in regional VOC budgets, and underline the usefulness and limitations of eddy covariance measurements with PTR-Qi-TOF-MS.