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Volume 16, issue 9 | Copyright
Atmos. Chem. Phys., 16, 5745-5761, 2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 11 May 2016

Research article | 11 May 2016

Air–sea fluxes of CO2 and CH4 from the Penlee Point Atmospheric Observatory on the south-west coast of the UK

Mingxi Yang1, Thomas G. Bell1, Frances E. Hopkins1, Vassilis Kitidis1, Pierre W. Cazenave1, Philip D. Nightingale1, Margaret J. Yelland2, Robin W. Pascal2, John Prytherch3, Ian M. Brooks3, and Timothy J. Smyth1 Mingxi Yang et al.
  • 1Plymouth Marine Laboratory, Prospect Place, Plymouth, PL1 3DH, UK
  • 2National Oceanography Centre, European Way, Southampton, SO14 3ZH, UK
  • 3Institute for Climate and Atmospheric Science, School of Earth and Environment, University of Leeds, Leeds, UK

Abstract. We present air–sea fluxes of carbon dioxide (CO2), methane (CH4), momentum, and sensible heat measured by the eddy covariance method from the recently established Penlee Point Atmospheric Observatory (PPAO) on the south-west coast of the United Kingdom. Measurements from the south-westerly direction (open water sector) were made at three different sampling heights (approximately 15, 18, and 27m above mean sea level, a.m.s.l.), each from a different period during 2014–2015. At sampling heights  ≥ 18ma.m.s.l., measured fluxes of momentum and sensible heat demonstrate reasonable ( ≤ ±20% in the mean) agreement with transfer rates over the open ocean. This confirms the suitability of PPAO for air–sea exchange measurements in shelf regions. Covariance air–sea CO2 fluxes demonstrate high temporal variability. Air-to-sea transport of CO2 declined from spring to summer in both years, coinciding with the breakdown of the spring phytoplankton bloom. We report, to the best of our knowledge, the first successful eddy covariance measurements of CH4 emissions from a marine environment. Higher sea-to-air CH4 fluxes were observed during rising tides (20±3; 38±3; 29±6µmolem−2d−1 at 15, 18, 27ma.m.s.l.) than during falling tides (14±2; 22±2; 21±5µmolem−2d−1), consistent with an elevated CH4 source from an estuarine outflow driven by local tidal circulation. These fluxes are a few times higher than the predicted CH4 emissions over the open ocean and are significantly lower than estimates from other aquatic CH4 hotspots (e.g. polar regions, freshwater). Finally, we found the detection limit of the air–sea CH4 flux by eddy covariance to be 20µmolem−2d−1 over hourly timescales (4µmolem−2d−1 over 24h).

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Coastal seas are sources of methane in the atmosphere and can fluctuate from emitting to absorbing carbon dioxide. Direct air–sea transport measurements of these two greenhouse gases in near shore regions remain scarce. From a recently established coastal atmospheric station on the south-west coast of the UK, we observed that the oceanic absorption of carbon dioxide peaked during the phytoplankton bloom, while methane emission varied with the tidal cycle, likely due to an estuary influence.
Coastal seas are sources of methane in the atmosphere and can fluctuate from emitting to...