Seasonal variability of black carbon mass in the tropical tropopause layer

While most black carbon (BC)‐containing particles are removed in the lower troposphere in the tropics, some are lofted to higher altitudes via convection where they may be distributed globally throughout the tropical tropopause layer (TTL). Single‐particle measurements of BC aerosol were made from the NASA WB‐57F aircraft during both the dry (February 2006) and wet (August 2007) seasons in Central America. BC mass loadings declined sharply with increasing altitude from the ground to 5 km. In the TTL, they were up to six times higher in the wet relative to the dry season. The variability in BC mass was examined using convective‐influence back trajectories to determine the source regions. The seasonal differences in the vertical profiles are explained by long‐range transport of (1) low‐ BC air from the southern hemisphere in the dry season and (2) high‐BC air from biomass‐burning or pollution sources in Africa and Asia advected by the Asian monsoon circulation in the wet season.