Stable discontinuities as determinants of the vertical distribution of aerosols and trace gases in the atmosphere

While sufficient to control the vertical transport of aerosols and trace ga ses in the troposphere, inversions of temperature are not necessary for ver tical mixing to be inhibited. In this paper the notion that absolutely stab le layers are both necessary and sufficient to determine the nature of the mixing process is explored. Layers of absolutely stable air in which the la pse rate of temperature is less than the saturated adiabatic lapse rate occ ur frequently and at preferred heights in the vicinity of similar to 700, s imilar to 500 and similar to 300 hPa over the surface of the plateau in sou thern Africa. The similar to 500 hPa layer is particularly persistent and i s the major determinant of the ubiquitous haze layer occurring over the sub continent, particularly in the winter hen of the year. TRACE-A and SAFARI t ropospheric aerosol and trace gas profiles measured over southern Africa an d elsewhere in the subtropics of the southern hemisphere are examined in th e light of the known stability structure of the regional atmosphere. Peaks in concentration, points of inflection in profiles and discontinuities in c oncentration loadings are closely related to the stable layers or the trapp ing that occurs between them. Estimates are given of percentage transfers o f air between layers for different airstreams within the laminated structur e. During SAFARI only 20 % of air parcels in the vertically integrated 70-5 00 hPa layer penetrated through the similar to 500 hPa stable layer. None f rom the 900-800 hPa layer did so. Not only are the stable layers shown to a ccount for the vertical structure of aerosol and trace gas distributions in the vertical in the southern African region, but also over Brazil, the cen tral South Atlantic Ocean and western Indian Ocean areas.