Aircraft Observations of Continental Pollution 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 continental 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 concentrations of carbon monoxide (CO) enhanced up to 65 % over 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 enhancements found in warmer, clear air. Satellite observations of CO did not detect the thin pollution layers observed by the aircraft, but did indicate Africa, Indonesia, and the western/central Pacific as geographical hot spots for CO in the TTL. Backward trajectories identified convective encounters in these areas as the dominant sources of polluted air in the TWP. Africa and Indonesia contributed about 60 % of the excess CO, transported to the TWP in two to four weeks. Our study confirms that air in the TTL over the TWP is affected by emissions from distant fires that can rapidly reach the LS in the principal source region for air entering the stratosphere, supporting the view that fires in tropical regions could impact stratospheric ozone and temperatures.