Methane at Svalbard and over the European Arctic Ocean

Stephen M. Platt, Sabine Eckhardt, Benedicte Ferré, Rebecca E. Fisher, Ove Hermansen, Pär Jansson, David Lowry, Euan G. Nisbet, Ignacio Pisso, Norbert Schmidbauer, Anna Silyakova, Andreas Stohl (+4 others)
2018 Atmospheric Chemistry and Physics  
<p><strong>Abstract.</strong> Methane (<span class="inline-formula">CH<sub>4</sub></span>) is a powerful greenhouse gas. Its atmospheric mixing ratios have been increasing since 2005. Therefore, quantification of <span class="inline-formula">CH<sub>4</sub></span> sources is essential for effective climate change mitigation. Here we report observations of the <span class="inline-formula">CH<sub>4</sub></span> mixing ratios measured at the Zeppelin Observatory (Svalbard) in the Arctic and aboard
more » ... he research vessel (RV) <i>Helmer Hanssen</i> over the Arctic Ocean from June 2014 to December 2016, as well as the long-term <span class="inline-formula">CH<sub>4</sub></span> trend measured at the Zeppelin Observatory from 2001 to 2017. We investigated areas over the European Arctic Ocean to identify possible hotspot regions emitting <span class="inline-formula">CH<sub>4</sub></span> from the ocean to the atmosphere, and used state-of-the-art modelling (FLEXPART) combined with updated emission inventories to identify <span class="inline-formula">CH<sub>4</sub></span> sources. Furthermore, we collected air samples in the region as well as samples of gas hydrates, obtained from the sea floor, which we analysed using a new technique whereby hydrate gases are sampled directly into evacuated canisters. Using this new methodology, we evaluated the suitability of ethane and isotopic signatures (<span class="inline-formula"><i>δ</i><sup>13</sup>C</span> in <span class="inline-formula">CH<sub>4</sub></span>) as tracers for ocean-to-atmosphere <span class="inline-formula">CH<sub>4</sub></span> emission. We found that the average methane<span class="thinspace"></span>/<span class="thinspace"></span>light hydrocarbon (ethane and propane) ratio is an order of magnitude higher for the same sediment samples using our new methodology compared to previously reported values, 2379.95 vs. 460.06, respectively. Meanwhile, we show that the mean atmospheric <span class="inline-formula">CH<sub>4</sub></span> mixing ratio in the Arctic increased by <span class="inline-formula">5.9±0.38</span> parts per billion by volume (ppb) per year (yr<span class="inline-formula"><sup>−1</sup></span>) from 2001 to 2017 and <span class="inline-formula">∼8</span><span class="thinspace"></span>pbb<span class="thinspace"></span>yr<span class="inline-formula"><sup>−1</sup></span> since 2008, similar to the global trend of <span class="inline-formula">∼</span><span class="thinspace"></span>7–8<span class="thinspace"></span>ppb<span class="thinspace"></span>yr<span class="inline-formula"><sup>−1</sup></span>. Most large excursions from the baseline <span class="inline-formula">CH<sub>4</sub></span> mixing ratio over the European Arctic Ocean are due to long-range transport from land-based sources, lending confidence to the present inventories for high-latitude <span class="inline-formula">CH<sub>4</sub></span> emissions. However, we also identify a potential hotspot region with ocean–atmosphere <span class="inline-formula">CH<sub>4</sub></span> flux north of Svalbard (80.4<span class="inline-formula"><sup>∘</sup></span><span class="thinspace"></span>N, 12.8<span class="inline-formula"><sup>∘</sup></span><span class="thinspace"></span>E) of up to 26<span class="thinspace"></span>nmol<span class="thinspace"></span>m<span class="inline-formula"><sup>−2</sup></span><span class="thinspace"></span>s<span class="inline-formula"><sup>−1</sup></span> from a large mixing ratio increase at the location of 30<span class="thinspace"></span>ppb. Since this flux is consistent with previous constraints (both spatially and temporally), there is no evidence that the area of interest north of Svalbard is unique in the context of the wider Arctic. Rather, because the meteorology at the time of the observation was unique in the context of the measurement time series, we obtained over the short course of the episode measurements highly sensitive to emissions over an active seep site, without sensitivity to land-based emissions.</p>
doi:10.5194/acp-18-17207-2018 fatcat:ord5cdc5yberpatfqsfaxu3deq