Vertical and spatial distribution of Chloromethane and Bromomethane from boundary layer to upper troposphere over the Amazon rainforest
Notice bibliographique
Résumé
Chloromethane (CH3Cl, average ambient mixing ratio 546 ppt (1)) and Bromomethane (CH3Br, average ambient mixing ratio 6.52 ppt (1)) are the most abundant chlorine-, respective bromine-containing atmospheric trace gases from natural origin. Due to their relatively long tropospheric lifetimes (0.9 y; 0.8 y respectively (2)), they are significant carriers of chlorine and bromine into the stratosphere, where they are photolyzed and contribute to the catalytic destruction of ozone. Chloromethane is mainly emitted by tropical vegetation and soils, biomass burning and from oceans. Vegetation and soils have been shown to act as both, source and sink for Chloromethane. Bromomethane has anthropogenic and biogenic sources, including oceans, fumigation, biomass and fossil fuel burning, crops and vegetation. For both methyl halides the known tropospheric sinks, such as reaction with hydroxyl radicals, loss to soils and oceans, and loss to the stratosphere, do not balance the currently known sources. In particular the strengths of the tropical sources still have substantial uncertainties. Due to the Montreal Protocol, anthropogenic emissions of chlorine and bromine compounds are declining. Therefore, biogenic contributions will become an even more important fraction of the global budget in the following decades. A better understanding of the mechanisms behind emission and tropospheric distribution of Chloro- and Bromomethane is essential to improve predictions of future stratospheric ozone levels.Here, we present the first airborne measurements of Chloromethane and Bromomethane over a tropical forest covering altitudes from the planetary boundary layer to the upper troposphere (300 – 14000 m). The measurements were conducted in Dec 2022 and Jan 2023 over the Amazon Rainforest with the German research aircraft HALO (High Altitude Long Range) in the scope of the CAFE-Brazil campaign. Chloromethane, Bromomethane and other Halocarbons and VOC were measured with Fast GC-MS with a time resolution of 3 minutes. On average elevated levels of both species were found in the boundary layer. Interestingly, the vertical distribution of both compounds also showed a layer with elevated mixing ratios in the upper troposphere, generating a C-shaped profile. The mean Chloromethane mixing ratio reached a maximum of 590 ± 30 ppt between 11 - 12 km altitude, whereas for Bromomethane the maximum appears to be higher than 14 km (where 11.8 ± 1.9 ppt were observed) and thus outside the covered altitude range. The high local convective activity and that of the more distant ITCZ may explain these observations. Surprisingly, high variability of mixing ratios in the boundary layer (300 – 1000 m) with seasonal and regional trends for both methyl halides was observed. Chloromethane mixing ratios between 499 and 686 ppt were observed, in agreement with earlier works which report that the rainforest (including vegetation and soil) can act as source and sink of Chloromethane. Bromomethane mixing ratios in the boundary layer varied between 6.0 to 20.9 ppt, indicating the rainforest to be a source of Bromomethane. (1) World Meteorological Organization (WMO). Scientific Assessment of Ozone Depletion: 2022. Geneva; 2022. Report No.: 278.(2) World Meteorological Organization (WMO). Scientific Assessment of Ozone Depletion: 2018. Geneva; 2018. Report No.: 58.
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Comment cette classification a été obtenuedéplier
Prédiction machine sur la base complète
Imitation des enseignantsNi prévalence calibrée, ni vérité terrain. Validation humaine à venir. Le volet Gemma est une étiquette directe du modèle pour chaque travail de la base, lue sur la notice réduite au titre. Le volet Codex est un classifieur appris des 10 348 étiquettes directes de Codex et calibré sur les taux pondérés de l'échantillon; les champs sans appui suffisant ne portent aucun appel Codex. Le mode candidate est l'union des deux volets; le consensus est leur intersection. Ces sorties portent le statut machine_predicted_unvalidated et ne sont pas des étiquettes humaines.
Scores du classifieur distillé par catégorie (deux têtes)
| Catégorie | Codex | Gemma |
|---|---|---|
| Métarecherche | 0,000 | 0,000 |
| Méta-épidémiologie (sens strict) | 0,000 | 0,000 |
| Méta-épidémiologie (sens large) | 0,000 | 0,000 |
| Bibliométrie | 0,001 | 0,001 |
| Études des sciences et des technologies | 0,000 | 0,000 |
| Communication savante | 0,000 | 0,000 |
| Science ouverte | 0,000 | 0,000 |
| Intégrité de la recherche | 0,000 | 0,000 |
| Charge utile insuffisante (le modèle a refusé de juger) | 0,001 | 0,000 |
Scores machine (provisoires)
Les deux têtes enseignantes du modèle étudiant, lues sur ce travail. Un score ordonne la base pour la relecture; il n'affirme jamais une catégorie, et le statut de validation accompagne chaque rangée tel quel.
Scores de référence d'un modèle non mature (critères de maturité non atteints, 7 itérations). Un score ordonne; il n'affirme jamais une catégorie.
score_only:v0-immature-baseline · tel quel depuis la passe de notation : score_only signifie que le nombre peut ordonner les travaux, et qu'aucune étiquette de catégorie n'en découleClassification
machine, non validéePrédiction automatique; un appel candidat d’une seule source (Gemma direct ou Codex distillé), pas un consensus.
Le détail, modèle par modèle et score par score, se trouve en fin de page sous « Comment cette classification a été obtenue ».