Aircraft observations of biomass burning pollutants in the equatorial lower stratosphere over the tropical western Pacific during boreal winter

Abstract. Recent studies hypothesize that emissions from fires reaching the stratosphere can provide aerosols and aerosol precursors that initiate stratospheric ozone loss and lead to radiative heating of the stratosphere and cooling of the surface. Air from the troposphere enters the stratosphere primarily over the tropical western Pacific (TWP) during boreal winter. We report observations in the TWP of persistent, ubiquitous pollution in the tropical tropopause layer (TTL) and lower stratosphere (LS) during the Airborne Tropical TRopopause EXperiment (ATTREX) campaign in February–March 2014. We found carbon monoxide (CO) concentrations increased by up to 65 % above background levels in the deep tropics (5° S–15° N; 16–17 km). Correlations of CO with hydrocarbon and halocarbon species indicated a biomass burning source, with the largest CO increases in cloud-free air. Satellite observations did not detect the thin layers of CO observed by the aircraft and revealed Africa, Indonesia, and the western and central Pacific as geographical hot spots for CO in the TTL. Backward trajectories also identified convective encounters in these areas, highlighting delivery of biomass burning pollutants to the TWP by both nearby and remote convection. Contributions from Africa and Indonesia were nearly 60 %, with transport timescales of 10 d to several weeks. Our study confirms that air in the TTL over the TWP is affected by emissions from distant fires that can rapidly reach the tropical LS, the principal source region for air entering the global stratosphere, supporting the view that fires in tropical regions can impact stratospheric composition and temperature.