The relative role of Amazonian and non-Amazonian fires in building up the aerosol optical depth in South America: a five year study (2005-2009)

In South America (SA) biomass burning is the major source of atmospheric aerosols. Fires are mostly registered in the dry season (July-November) and are mainly concentrated in the Amazonia and Cerrado regions. Nonetheless, the growing systematic employment of fires for land clearing and pasture maintenance across the SA continent is introducing other, potentially significant, sources of BB aerosols. This study investigates the relative contributions of different SA biomass burning regions in building up the continental aerosol load. To this purpose, the SA continent is divided into four biomass burning sources regions and their impact on the aerosol optical depth (AOD) is evaluated in eight different SA target domains. The dataset used includes multi-year (2005-2009) satellite observations of both aerosol and fires and model-based atmospheric trajectories. The methodology followed couples fire counts and atmospheric transport through the definition of a specific quantity, referred to as 'fire weighted residence time' (FWRT), which is used to assess the contribution of the four identified fire source regions to the continental aerosol load. Results show that local fires play an important role in building up the regional aerosols load all over SA. Nevertheless, in some regions, contribution of BB aerosols transported from outside their boundaries is comparable to the local one. The major 'smoke exporter' regions are found to be the eastern Brazil and the Amazonia-Cerrado regions. In the dry season, due to the typical continental circulation pattern, the first is estimated to contribute to half of the AOD in Northern Amazonia, Southern Amazonia and Cerrado regions while up to 40% of the AOD in Paraguay and North Argentina derives from the Amazonia-Cerrado fires. Due to the presence of the inter tropical convergence zone, which decouples wind circulation of the two hemispheres, regions north of the Equator (Venezuela, Guyana, Suriname) are found to receive almost no contribution to the local AOD from fires occurring in the nearby active regions of Amazonia and Caatinga. Similarly, Venezuela fires are shown not to impact the northern Amazonia AOD. Finally, in excluding the continental fires driver of some AOD enhancements observed in the wet season, this study indirectly points to an important role of aerosol transoceanic transport from Africa.