The occurrence of SALLJ in the 2002 was then determined and the pattern found was in agreement with previous studies for other years. Figure 1 shows the percent relative frequencies of SALLJ obtained for each month.

The low level flow was present all through the 2002 year, though presenting variability in its strength, frequency and location mainly related to the different synoptic conditions, and the greater scale climatological scenario. The higher frequencies of occurrence of SALLJ are observed in October and the lower in July. As previously mentioned, the aim of the present study is to relate the low-level jet east of the Andes with the dispersion of biomass burning products in South America. Figure 2 presents the number of fire spots in South America for each month in 2002. The important increase from August to October — namely the biomass burning season — is clearly evident. In consequence, we will restrict the further analysis to the events in those months.

Fig. 2. Number of fires in South America per month during 2002.

The main characteristics of the mean low-level flow are depicted in the composite fields, obtained averaging the days that comprise the SALLJ events for the burning season months (Figure 3). August and October were characterized by a northerly oriented flow, when mainly the northeast of Argentina was under its influence. In September the mean pattern was more north-westerly oriented with an outflow towards the Atlantic Ocean, over passing the southern region of Brazil. August shows the southernmost penetration, greatest horizontal wind speed gradient and vertical wind speed shear. During this month, the events are less frequent but much stronger. In the opposite, in October, there is a higher recurrence of generally weaker events. The mean low level north-westerly flow organizes at about 15° S and extends southward reaching 30-35° S.

The associated circulation patterns in conjunction with the occurrence of biomass burning caused the transport of aerosols and gases towards different regions with diverse impacts. Figure 4 shows the composites of the modelled vertically integrated aerosol optical thickness at 500 nm (AOT500) and the flow pattern for the SALLJ events. The mean plume

and flow are very well reproduced and the higher aerosol concentrations are directly related to the greater emissions during September and October.

The temporal behaviour of the AOT at the AERONET site in Buenos Aires is depicted in Figure 5 for the sub-samples SALLJ and NO-SALLJ along with the comparison with the CATT-BRAMS predicted values. The model is able to capture the evolution of the aerosol concentration. The underestimation of the values is linked to the comparison of point measurements and the model results resolution.

The relationship between the Angstrom coefficient and the AOT is frequently used to get more information about the aerosol characteristics. The greater aerosol load observed during the SALLJ events is clearly associated to higher Angstrom coefficients in agreement with the literature (Figure 6).

Fig. 6. Variation of the Angstrom coefficient (440-870nm) with the AOT 500nm obtained from the measurements at the Buenos Aires AERONET site (dots, diamond: SALLJ, square: NO-SALLJ) from August to October 2002.