Methane mole fraction and δ$^{13}$C above and below the trade wind inversion at Ascension Island in air sampled by aerial robotics

Abstract

The Authors.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 m 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 CH$_{4}$, but no distinct shift in δ$^{13}$C was observed compared to the air below. Back trajectories indicate that lofted CH$_{4}$ emissions from Southern Hemisphere Africa have bulk δ$^{13}$C$_{CH4}$ signatures similar to background, suggesting mixed emissions from wetlands, agriculture, and biomass burning. The campaigns illustrate the usefulness of unmanned aerial system sampling and Ascension's value for atmospheric measurement in an understudied region.This work was part of the investigation of the southern methane anomaly: causes, implications, and relevance to past global events funded by the UK Natural Environment Research Council (NERC) (grant NE/K006045/1) and NERC studentship. Data will be deposited in the UK Centre for Environmental Data Analysis on completion of Rebecca Brownlow’s PhD thesis