Meteorological conditions associated with vertical distributions of aerosols off the west coast of Africa

Abstract

Vertical distributions of aerosol backscattering were obtained on Transport and Atmospheric Chemistry Near the Equator‐Atlantic (TRACE A) flights parallel to the west coast of Africa using the airborne differential absorption lidar (DIAL) instrument. Aerosol distributions on the flight of October 15, 1992 (from 22°S to 5.5°S) exhibit strong horizontal and vertical gradients. The top of the aerosol layer ranges from 3.5 to 5.7 km above sea level, while its thickness ranges from 1.4 to 4.5 km. The greatest aerosol loading generally occurs near 4.0‐ to 4.5‐km altitude between 8° and 12°S. Meteorological conditions are found to exert a major influence on the aerosol distributions. Dropsonde data along the flight track indicate numerous temperature inversions and stable layers in each sounding. The top of the aerosol region is associated with strong inversions due to subsidence. Five‐day backward trajectories are calculated along the flight track at vertical intervals of 1 km using global meteorological analyses. Trajectories arriving at most locations of large aerosol loading originate over southern Africa, where biomass burning is occurring and deep surface‐based mixed layers are common. Conversely, the air with less aerosol loading originates over the Atlantic Ocean. The exception is the northernmost segment of the flight above 3.5 km. Although this segment receives flow off Africa at these altitudes, lower level stable layers inhibit transport to higher levels. In addition, trajectories arriving at this part of the flight pass over a portion of Africa with reduced biomass burning and extensive deep convection that penetrates the stable layers.