A33C-0163
Isotope and methane dynamics above and below the Trade Wind Inversion at Ascension Island using UAVs

Wednesday, 16 December 2015
Poster Hall (Moscone South)
Rebecca Brownlow1, David Lowry1, Euan G Nisbet1, Rebecca Elizabeth Fisher2, James France3, Mathias Lanoisellé1, Rick Thomas4, Tom Richardson5, Colin Greatwood5, Jim E Freer5 and A Rob MacKenzie6, (1)Royal Holloway University of London, Egham, United Kingdom, (2)Royal Holloway, Univ London, Egham, United Kingdom, (3)University of East Anglia, Norwich, NR4, United Kingdom, (4)University of Birmigham, Birmingham, United Kingdom, (5)University of Bristol, Bristol, United Kingdom, (6)School of Geography, Earth and Enviromental Sciences, University of Birmingham, Birmingham, United Kingdom
Abstract:
Ascension Island (8oS, 14 oW) is a South Atlantic background site for atmospheric measurement. Royal Holloway, in collaboration with the UK Met Office, installed a Picarro 1301 CRDS in 2010 for continuous methane monitoring. This has high precision and accuracy, with a 6-gas calibration and target suite, to measure long term methane mole fraction. Regular flask sampling is also carried out for NOAA and RHUL (co-located), to measure δ13CCH4 isotopic trends.

Ascension Island experiences near-constant SE Trade winds below the Trade Wind Inversion (TWI), with air from the remote S. Atlantic. In flask samples and in continuous monitoring at the Airhead location, atmospheric methane mole fraction has been increasing since 2007 whilst the δ13CCH4 isotope record has shifted to more depleted values. Above the normally well-defined TWI (1200 - 1800m altitude), variable tropical air masses pass over Ascension. This air last mixed with the boundary layer over Africa or South America.

Field work undertaken in September 2014 and July 2015, in collaboration with U. Bristol and U. Birmingham, using UAVs (octocopters) collected samples with Tedlar bags or aluminium flasks from different heights above and below the TWI. The maximum altitude reached was 2700masl. Sample bags were immediately analysed on Ascension for CH4 mole fraction using the Picarro CRDS and subsequently analysed at RHUL for δ13CCH4 using continuous-flow gas chromatography/isotope-ratio mass spectrometry (CF-GC/IRMS). The TWI was clearly identified by an increase in CH4 mole fraction above the TWI. Back trajectory analysis was used to distinguish the origins of the air masses, with air above showing inputs from the land surfaces of equatorial and southern Africa, and from southern S. America.

The campaigns have extended the envelope of altitudes accessed by micro-UAVs for atmospheric science, demonstrating their utility for probing the remote free troposphere and for penetrating the TWI. Sampling at Ascension is able to measure both the deep S. Atlantic air and also the air that has been mixed by convective systems in the equatorial and southern savannah tropics. Biomass burning plumes in southern hemisphere winter may also be accessible. Ascension is thus potentially a measurement site of global significance.

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