PASSIVE TRACER TRANSPORT RELEVANT TO THE TRACE A EXPERIMENT

This paper explores some of the mechanisms governing the accumulation of passive tracers over the tropical southern Atlantic Ocean during th e northern hemisphere fall season. There has been a pioneering observa tion regarding ozone maxima over the South Atlantic during austral spr ing. The understanding of the formation of this maxima has been the pr ime motivation for this study. Using a global model as a frame of refe rence, we have carried out three kinds of experiments during the perio d of the Transport and Atmospheric Chemistry Near the Equator-Atlantic (TRACE A) project of 1992. The first of these is a simple advection o f total ozone (a passive tracer) in time using the Florida State Unive rsity global spectral model. Integration over the period of roughly 1 week showed that the model quite closely replicates the behavior of th e observed total ozone from the total ozone mapping spectrometer (TOMS ). This includes many of the changes in the features of total ozone ov er the tropical and subtropical region of the southern Atlantic Ocean. These studies suggest a correlation of 0.8 between the observed ozone over this region and ozone modeled from ''dynamics alone,'' i.e., wit hout recourse to any photochemistry. The second series of experiments invoke sustained sources of a tracer over the biomass burn region of A frica and Brazil. Furthermore, sustained sources were also introduced in the active frontal ''descending air'' region of the southern hemisp here and over the Asian monsoon's east-west circulation. These experim ents strongly suggest that air motions help to accumulate tracer eleme nts over the tropical southern Atlantic Ocean. A third series of exper iments address what may be required to improve the deficiencies of the vertical stratification of ozone predicted by the model over the flig ht region of the tropical southern Atlantic during TRACE A. Here we us e the global model to optimally derive plausible accumulation of burn elements over the fire count regions of Brazil and Africa to provide p assive tracer advections to closely match what was observed from recon naissance aircraft-based measurements of ozone over the tropical south ern Atlantic Ocean.