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Long-Range Transport of Black Carbon in the Tropical Tropopause Layer

J. R. Spackman (1,2), R. S. Gao (2), J. P. Schwarz (1,2), L. A. Watts (1,2), D. W. Fahey (1,2), L. Pfister (3), T. P. Bui (3), N. J. Livesey (4)

(1) CIRES (2) NOAA ESRL Chemical Sciences Division (3) NASA Ames Research Center (4) Jet Propulsion Laboratory, California Institute of Technology

The role of black carbon (BC) in climate change through direct and indirect radiative forcing is still emerging. The chemistry and dynamics of the stratosphere is sensitive to radiative changes in the tropical tropopause layer (TTL) possibly induced by perturbations to BC in this region. Also, the role of BC in altering ice cloud properties is particularly uncertain. While most BC particles are removed in the lower troposphere, a small fraction are lofted to higher altitudes via convection. Measurements of BC mass loadings have now been made in the TTL aboard the NASA WB-57F research aircraft during both summer (wet) and winter (dry) in Costa Rica. Below the TTL, BC mass mixing ratios declined sharply with altitude from the ground to approximately 5 km. In the TTL, a factor of 6 times more BC mass was measured in the wet season than the dry season. These BC mass loadings were examined using back trajectories to determine the most recent influence from convection. Depending on the BC mass loadings in the region of entrainment, convection can either dilute or enhance the background BC mass in the TTL. Results from the convective influence back trajectories suggest seasonal variability in the (1) long-range transport of clean air from convection in the Southern Hemisphere and (2) local and nonlocal convective lofting of BC from various emission sources followed by long-range transport contribute to the differences in the vertical profiles between the 2 seasons. We use satellite measurements of short-lived tracer species to examine the relative contributions of local anthropogenic, African biomass burning, and Asian anthropogenic emissions to the observed enhancement of BC in the TTL during the summer in Costa Rica. This analysis highlights the coupled processes of convection and long-range transport that control the global concentrations of BC in the TTL.