Abstract

Despite the importance of methane for climate change mitigation, uncertainties regarding the temporal and spatial variability of the emissions remain. Measurements of CH₄ isotopic composition are used to partition the relative contributions of different emission sources. We report continuous isotopic measurements during 5 months at the Lutjewad tower (north of the Netherlands). Time-series of χ(CH₄), δ¹³C-CH₄, and δD-CH₄ in ambient air were analysed using the Keeling plot method. Resulting source signatures ranged from −67.4 to −52.4‰ vs V-PDB and from −372 to −211‰ vs V-SMOW, for δ¹³C and δD respectively, indicating a prevalence of biogenic sources. Analysis of isotope and wind data indicated that (i) emissions from off-shore oil and gas platforms in the North Sea were not detected during this period, (ii) CH₄ from fossil fuel related sources was usually advected from the east, pointing towards the Groningen gas field or regions further east in Germany. The results from two atmospheric transport models, CHIMERE and FLEXPART-COSMO, using the EDGAR v4.3.2 and TNO-MACC III emission inventories, reproduce χ(CH₄) variations relatively well, but the isotope signatures were over-estimated by the model compared to the observations. Accounting for geographical variations of the δ¹³C signatures from fossil fuel emissions improved the model results significantly. The difference between model and measured isotopic signatures was larger when using TNO-MACC III compared to EDGAR v4.3.2 inventory. Uncertainties in the isotope signatures of the sources could explain a significant fraction of the discrepancy, thus a better source characterisation could further strengthen the use of isotopes in constraining emissions.

摘要

尽管甲烷对于缓解气候变化具有重要意义，但有关排放的时间和空间可变性的不确定性仍然存在。CH ₄同位素组成的测量用于划分不同排放源的相对贡献。我们报告了Lutjewad塔（荷兰北部）在5个月内的连续同位素测量结果。时间序列χ（CH的₄），δ ¹³ C-CH ₄，和δD-CH ₄使用Keeling曲线方法在环境空气中进行了分析。所得源签名范围从-67.4至-52.4‰VS V-PDB和从-372到-211‰VS V-SMOW，对于δ ¹³C和δD分别表示生物源的普遍性。对同位素和风数据的分析表明，（i）在此期间未检测到北海近海油气平台的排放，（ii）来自化石燃料相关源的CH ₄通常从东部平流，指向格罗宁根气田或德国更东部的地区。来自两个大气传输模型CHIMERE和FLEXPART-COSMO的结果，使用EDGAR v4.3.2和TNO-MACC III排放清单，相对较好地再现了χ（CH ₄）变化，但相比之下，同位素签名被该模型高估了观察。占δ的地域差异¹³化石燃料排放的C签名显着改善了模型结果。与EDGAR v4.3.2清单相比，使用TNO-MACC III时模型和实测同位素特征之间的差异更大。源的同位素特征的不确定性可以解释差异的很大一部分，因此更好的源表征可以进一步加强同位素在约束排放中的使用。