Air–sea fluxes of CO₂ and CH₄ from the Penlee Point Atmospheric Observatory on the south-west coast of the 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 27 m above mean sea level, a.m.s.l.), each from a different period during 2014–2015. At sampling heights  ≥  18 m a.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 ± 6 µmole m⁻² d⁻¹ at 15, 18, 27 m a.m.s.l.) than during falling tides (14 ± 2; 22 ± 2; 21 ± 5 µmole m⁻² d⁻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 20 µmole m⁻² d⁻¹ over hourly timescales (4 µmole m⁻² d⁻¹ over 24 h).