Methane mole fraction and δ¹³C above and below the trade wind inversion at Ascension Island in air sampled by aerial robotics

Ascension Island is a remote South Atlantic equatorial site, ideal for monitoring tropical background CH₄. In September 2014 and July 2015, octocopters were used to collect air samples in Tedlar bags from different heights above and below the well-defined Trade Wind Inversion (TWI), sampling a maximum altitude of 2700 meters above mean sea-level. Sampling captured both remote air in the marine boundary layer below the TWI, and also air masses above the TWI that had been lofted by convective systems in the African tropics. Air above the TWI was characterized by higher CH₄ but no distinct shift in δ¹³C was observed compared to the air below. Back-trajectories indicate that lofted methane emissions from southern hemisphere Africa have bulk δ¹³C_CH4 signatures similar to background, suggesting mixed emissions from wetlands, agriculture and biomass burning. The campaigns illustrate the usefulness of UAS sampling, and Ascension’s value for atmospheric measurement in an understudied region.