In:
Atmospheric Chemistry and Physics, Copernicus GmbH, Vol. 18, No. 23 ( 2018-12-05), p. 17207-17224
Abstract:
Abstract. Methane (CH4) is a powerful greenhouse gas. Its atmospheric mixing
ratios have been increasing since 2005. Therefore, quantification of
CH4 sources is essential for effective climate change mitigation.
Here we report observations of the CH4 mixing ratios measured at the
Zeppelin Observatory (Svalbard) in the Arctic and aboard the research
vessel (RV) Helmer Hanssen over the Arctic Ocean from June 2014 to
December 2016, as well as the long-term CH4 trend measured at the
Zeppelin Observatory from 2001 to 2017. We investigated areas over the
European Arctic Ocean to identify possible hotspot regions emitting
CH4 from the ocean to the atmosphere, and used state-of-the-art
modelling (FLEXPART) combined with updated emission inventories to identify
CH4 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 (δ13C in
CH4) as tracers for ocean-to-atmosphere CH4 emission. We
found that the average methane / 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 CH4 mixing
ratio in the Arctic increased by 5.9±0.38 parts per billion by
volume (ppb) per year (yr−1) from 2001 to 2017 and ∼8 pbb yr−1 since 2008, similar to the global trend of
∼ 7–8 ppb yr−1. Most large excursions from the baseline
CH4 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 CH4 emissions. However, we also
identify a potential hotspot region with ocean–atmosphere CH4 flux
north of Svalbard (80.4∘ N, 12.8∘ E) of up to
26 nmol m−2 s−1 from a large mixing ratio increase at the
location of 30 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.
Type of Medium:
Online Resource
ISSN:
1680-7324
DOI:
10.5194/acp-18-17207-2018
DOI:
10.5194/acp-18-17207-2018-supplement
Language:
English
Publisher:
Copernicus GmbH
Publication Date:
2018
detail.hit.zdb_id:
2092549-9
detail.hit.zdb_id:
2069847-1
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