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

Abstract

Abstract. Volatile organic compounds (VOCs) contribute to air pollution through the formation of secondary aerosols and ozone and extend the lifetime of methane in the atmosphere. Tropospheric VOCs originate to 90 % from biogenic sources on a global scale, mainly from forests. Crops are also a potentially large yet poorly characterized source of VOCs (30 % of the VOC emissions in Europe, mostly oxygenated). In this study, we investigated VOC fluxes over a winter wheat field by eddy covariance using a PTR-Qi-TOF-MS with high sensitivity and mass resolution. The study took place near Paris over a 5-week period and included flowering, crop maturity and senescence. We found a total of 123 VOCs with fluxes 3 times above the detection limit. Methanol was the most emitted compound with an average flux of 63 µg m−2 h−1, representing about 52 % of summed VOC emissions on a molar basis (36 % on a mass basis). We also identified ethanol, acetone, acetaldehyde and dimethyl sulfide among the six most emitted compounds. The third most emitted VOC corresponded to the ion m/z 93.033. It was tentatively identified as furan (C6H4O), a compound not previously reported to be strongly emitted by crops. The average summed VOC emissions were about 173 ± 6 µg m2 h−1, while the average VOC depositions were about 109 ± 2 µg m−2 h−1 and hence 63 % of the VOC emissions on a mass basis. The net ecosystem flux of VOCs was an emission of 64 ± 6 µg m−2 h−1 (0.5 ± 0.05 nmol m−2 s−1). The most deposited VOCs were identified as hydroxyacetone, acetic acid and fragments of oxidized VOCs. Overall, our results reveal that wheat fields represent a non-negligible source and sink of VOCs to be considered in regional VOC budgets and underline the usefulness and limitations of eddy covariance measurements with a PTR-Qi-TOF-MS.

Closing the Reactive Carbon Flux Budget: Observations From Dual Mass Spectrometers Over a Coniferous Forest

“…This is partly because the upward fluxes were dominated by a few major species (all of which have explicit CTM representation), and partly because of offsetting model errors between those major species. This work has provided a chemically detailed analysis of VOC surface-atmosphere exchange for one pine forest ecosystem, and builds upon a small number of similar studies that relied solely upon PTRMS (Loubet et al, 2022;Millet et al, 2018;Park et al, 2013). Further measurements employing multiple high-resolution mass spectrometers in different ecosystems are required to better understand surface-atmosphere VOC fluxes across the full suite of relevant compounds, diagnose the underlying environmental drivers, and advance the ability of current CTMs to capture the ensuing impact on atmospheric chemistry.…”

“…Park et al (2013) performed detailed flux measurements over an orange orchard and detected 555 species contributing to the net VOC flux budget-with 10 commonly-known compounds making up 68% of the total flux. A recent study over a winter wheat field measured fluxes for 264 VOCs, with only four ubiquitous oxygenated VOCs accounting for 85% of total emissions (Loubet et al, 2022). A third study over a mixed temperate forest observed 377 VOCs with detectable surface-atmosphere exchange (Millet et al, 2018).…”

Measurement report: Ammonia in Paris derived from ground-based open-path and satellite observations

“…The measurements made by the miniDOAS over the January 2020-June 2022 period (N = 16 888) show an average ammonia concentration of 2.23 µg m −3 in Paris over this period, with a standard deviation of 2.02 µg m −3 , indicating a high NH 3 variability. In comparison, the average concentration measured by the miniDOAS in an agricultural site at Grignon (Loubet et al, 2022) in September-October 2021 (France) is 6.52 ± 8.44 µg m −3 (Caville et al, 2023), almost 3 times higher than in Paris. The relatively low concentrations observed in Paris are explained by the distance to the major emission sources which are related to agricultural activities.…”

Online field measurements of Chlorothalonil show that volatilization is key to pesticide exposure

“…Ion counts per second (cps) were computed by combining each ion peak. The cps were used to calculate the "uncalibrated" Chlorothalonil concentration following Loubet et al 74 using Toluene as a reference.…”