Decreasing seasonal cycle amplitude of methane in the northern high latitudes being driven by lower-latitude changes in emissions and transport
Decreasing seasonal cycle amplitude of methane in the northern high latitudes being driven by lower-latitude changes in emissions and transport
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DOI：
10.5194/acp-23-7363-2023
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发表时间：
2023-07
影响因子：
6.3
通讯作者：
Emily Dowd;C. Wilson;M. Chipperfield;E. Gloor;A. Manning;R. Doherty
中科院分区：
文献类型：
--
作者：
Emily Dowd;C. Wilson;M. Chipperfield;E. Gloor;A. Manning;R. Doherty
抽象的。大气甲烷(CH4)浓度正在上升，预计这将导致全球季节循环幅度(SCA)相应增加--即其季节最大值和最小值之间的差值。CH4与其主汇OH之间的反应取决于大气中CH4和OH的量。OH的浓度随季节变化，由于大气中CH4的负荷增加，预计CH4的SCA将增加，这是因为通过与大气中的OH反应增加了CH4的去除。SCA在空间上的变化可能表明季节性源和汇的长期持续变化，但这种SCA变化尚未被调查。在这里，我们使用地表烧瓶测量和三维化学传输模型(TOMCAT)来诊断1995-2020年期间大气CH4的SCA的变化，并将这些变化归因于不同部门的贡献。我们发现，在这段时间内，在北部高纬度(NHL；60-90∘N)观测到的SCA减少了4 ppb(7.6%)，而全球SCA增加了2.5 ppb(6.5%)。Tomcat在全球各地的观测点再现了SCA的变化。因此，我们使用它来归类导致NHL SCA变化的地区，这表明SCA发生了不同于世界其他地区的意外变化。我们发现，混合良好的背景CH4对NHL中SCA下降的影响最大(占NHL总贡献的56.5%)。CH4可能来自更南纬的排放。除背景CH4外，来自加拿大、中东和欧洲的近期排放分别占NHL中SCA总变化的16.9%、12.1%和8.4%。其余的贡献是由于其他地区的排放和交通运输的变化。三个最大的区域贡献来自加拿大北方平原夏季排放量的增加，整个欧洲冬季排放量的减少，以及中东阿拉伯半岛和里海夏季排放量的增加和冬季排放量的减少。这些结果突出表明，观测到的季节周期的变化可能是当地和非当地地区排放制度变化的一个指标，特别是在非霍奇金淋巴瘤地区，那里的变化是违反直觉的。
Abstract. Atmospheric methane (CH4) concentrations are rising, which are expected to lead to a corresponding increase in the global seasonal cycle amplitude (SCA) – the difference between its seasonal maximum and minimum values. The reaction between CH4 and its main sink, OH, is dependent on the amount of CH4 and OH in the atmosphere. The concentration of OH varies seasonally, and due to the increasing burden of CH4 in the atmosphere, it is expected that the SCA of CH4 will increase due to the increased removal of CH4 through a reaction with OH in the atmosphere. Spatially varying changes in the SCA could indicate long-term persistent variations in the seasonal sources and sinks, but such SCA changes have not been investigated. Here we use surface flask measurements and a 3D chemical transport model (TOMCAT) to diagnose changes in the SCA of atmospheric CH4 between 1995–2020 and attribute the changes regionally to contributions from different sectors. We find that the observed SCA decreased by 4 ppb (7.6 %) in the northern high latitudes (NHLs; 60–90∘ N), while the SCA increased globally by 2.5 ppb (6.5 %) during this time period. TOMCAT reproduces the change in the SCA at observation sites across the globe. Therefore, we use it to attribute regions which are contributing to the changes in the NHL SCA, which shows an unexpected change in the SCA that differs from the rest of the world. We find that well-mixed background CH4, likely from emissions originating in, and transported from, more southerly latitudes has the largest impact on the decreasing SCA in the NHLs (56.5 % of total contribution to NHLs). In addition to the background CH4, recent emissions from Canada, the Middle East, and Europe contribute 16.9 %, 12.1 %, and 8.4 %, respectively, to the total change in the SCA in the NHLs. The remaining contributions are due to changes in emissions and transport from other regions. The three largest regional contributions are driven by increases in summer emissions from the Boreal Plains in Canada, decreases in winter emissions across Europe, and a combination of increases in summer emissions and decreases in winter emissions over the Arabian Peninsula and Caspian Sea in the Middle East. These results highlight that changes in the observed seasonal cycle can be an indicator of changing emission regimes in local and non-local regions, particularly in the NHL, where the change is counterintuitive.