The evolution and spatial extent of the smoke plume is studied through the behaviour of the AOT500. Figure 9 shows the modelled AOT500 and the horizontal flow at 1400 m above the surface, at selected days during the analyzed period. On 23 August the smoke plume showed a relative maximum close to the emission sources, centred near 10° S and 60° W, with values higher than 2. The smoke plume had its greater longitudinal extension between

the Equator and 10° S. This feature is related with the dominance of the easterlies in that region. An outflow zone from South America towards the west is observed between 5° S and 10° S. To the south, at higher latitudes, the smoke plume had an important branch oriented from the NW to the SE reaching the latitude 20° S, with AOT values higher than 0.5. These features are due to the transport patterns at low and middle atmospheric levels, which were dominated by the flow at the western branch of the high-pressure system and the channelling effect of the Andes barrier.

On 24 August, the region with higher AOT values near the sources increased. The smoke plume had a greater latitudinal extent over Argentina, reaching 35° S. The core of the low — level jet was associated with a relative minimum. Contrarily, a relative maximum over northern Argentina appears west of the jet core. On 25 August the southern edge of the plume continues to travel towards higher latitudes and presents a shape associated with the anticyclonic circulation and the barrier effect of the Andes. Optical depths ranging from 0.3 to 0.5 cover NE of Argentina and Uruguay. An outflow region from South America towards the west is observed between 5° S and 15° S. On 26 August, Buenos Aires had AOT values between 0.5 and 0.75. The smoke plume has N-S orientation from latitudes near 15° S to 30° S. The greater values are observed near the sources, over central Brazil and Bolivia. On its southernmost extreme the plume shows a curvature associated with the high-pressure system centred over the Atlantic Ocean, near 32° S and 35° W. Cordoba is affected by aerosol optical thicknesses ranging from 0.75 to 1, which are higher than those at Buenos Aires.

On 27 August the smoke plume reached latitudes higher than 40° S. The AOT over Buenos Aires ranged from 0.75 to 1. On 28 August, the cold front succeeded in crossing the Andes and reached Argentina and afterwards, the plume started to be displaced towards the east but was still over Buenos Aires due to its pre-frontal location. During the next day, the smoke plume displaced towards the northeast, owing to the fast movement of the cold front, and reached southern Brazil. On 30 August the plume had clearly the shape of the frontal zone and reached Sao Paulo. During the next day the surface cold front was stationary over Sao Paulo. There is a region associated to the postfrontal anticyclone with a low-level recirculation of the aerosols towards the west of the plume centre. This occurs at the north­western edge of the frontal region, where the forced convection is weaker.