Air–sea fluxes of CO<sub>2</sub> and CH<sub>4</sub> from the Penlee Point Atmospheric Observatory on the south-west coast of the UK

Abstract

Abstract. We present air-sea fluxes of carbon dioxide (CO 2 ), methane (CH 4 ), 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 CO 2 fluxes demonstrate high temporal variability. Air-to-sea transport of CO 2 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 CH 4 emissions from a marine environment. Higher sea-to-air CH 4 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 CH 4 source from an estuarine outflow driven by local tidal circulation. These fluxes are a few times higher than the predicted CH 4 emissions over the open ocean and are significantly lower than estimates from other aquatic CH 4 hotspots (e.g. polar regions, freshwater). Finally, we found the detection limit of the air-sea CH 4 flux by eddy covariance to be 20 µmole m −2 d −1 over hourly timescales (4 µmole m −2 d −1 over 24 h).

Sea Spray Fluxes from the Southwest Coast of the United Kingdom – Dependence on Wind Speed and Wave Height

“…Because of the narrower size range of this second unit and thus more undetected film drop aerosols, data from the 2016-80 2017 period were less representative of the total number fluxes and only used to contrast between open water and fetch-limited conditions. Previous EC observations of momentum, sensible heat, CO 2 , and CH 4 fluxes at PPAO show that two wind sectors are representative of air-water transfer: the southwest sector for which airflow is from the open ocean, and the northeast sector with airflow from the fetch-limited Plymouth Sound (Yang et al 2016a;2016b;2019). A flux footprint model for spatially 85 homogeneous conditions (Kljun et al 2004) predicts that under typical southwesterly winds, the majority of the turbulent flux at a sensor height of 18 m above mean sea level comes from waters several hundred meters upwind of the site with a mean water https://doi.org/10.5194/acp-2019-771 Preprint.…”

“…The Penlee Point Atmospheric Observatory (PPAO, http://www.westernchannelobservatory.org.uk/penlee/) on the 70 southwest coastal of the United Kingdom has proven to be a suitable site for eddy covariance measurements of air-sea transfer (Yang et al 2016a(Yang et al , 2016b(Yang et al , 2019. PPAO sits about 11 m above mean sea level and a few tens of m away from the water's edge.…”

Detection of O<sub>4</sub> absorption around 328 and 419 nm in measured atmospheric absorption spectra

“…Yang et al, 2016). Similar to the instrument used in Lampel et al (2015a) during the MAD-CAT campaign, the EnviMeS 2 MAX-DOAS instrument is based on an Avantes ultra-low straylight AvaSpec-ULS2048x64 spectrometer (f = 75 mm) using a back-thinned Hamamatsu S11071-1106 detector.…”

Evaluation of a lower-powered analyzer and sampling system for eddy-covariance measurements of nitrous oxide fluxes

“…φ C n was calculated as the mean of the variance spectra from 1 to 5 Hz. Values of σ 2 C a were calculated as the second point of the autocovariance of the concentration signal (Yang et al, 2016). Blomquist et al (2010) estimated τ wc using the peak frequency of the variance cospectrum.…”

“…Flux detection limits (δF ) were calculated using the method described in Blomquist et al (2010) and Yang et al (2016) where the detection limit is calculated using the instrument noise and the variability of the concentration signal:…”