Decadal changes in aerosol absorption across Brazil resulting from changes in biomass burning practices

Thursday, 17 December 2015: 09:15
3008 (Moscone West)
William Morgan1, Hugh Coe2, Eoghan Darbyshire1, James D Allan2, Michael Flynn2, Dantong Liu2, Justin Langridge3, Ben Thomas Johnson4, Jim Haywood4, Karla Longo5, Paulo Artaxo6, Eleanor Highwood7 and James Mollard7, (1)University of Manchester, Manchester, United Kingdom, (2)University of Manchester, School of Earth, Atmospheric and Environmental Sciences, Manchester, United Kingdom, (3)Met Office, Exeter, United Kingdom, (4)Met Office Hadley center for Climate Change, Exeter, United Kingdom, (5)CPTEC Center for Weather Forecasts and Climate Research, Grupo de Modelagem da Atmosfera e Interfaces - GMAI, Cachoeira Paulista, Brazil, (6)USP University of Sao Paulo, São Paulo, Brazil, (7)University of Reading, Reading, United Kingdom
Open biomass burning makes a substantial contribution to the global budget of black carbon, yet models significantly underestimate absorption aerosol optical depth compared to observations by approximately a factor of two over South America. These large differences need to be addressed. Recent work has shown that the number of deforestation fires has decreased across Amazonia over the last decade, giving rise to a decrease in the abundance of biomass burning aerosol across the region. At the same time there has been an increase in the frequency of agricultural burning across regions that have previously been deforested, as well as increased burning in the east of Brazil in the Cerrado regions.

We sampled both of these types of open burning extensively during a recent aircraft experiment. Significant concentrations of organic carbon as well as black carbon were observed, with this ratio providing the main control on the single scattering albedo (SSA).Deforestation fires and wild forest fires are prevalent across the south west of the Amazon Basin, where smouldering burning dominates. In the east of Brazil, agricultural burning proceeds via a much more efficient form of combustion and as a result, black carbon is a much larger fraction of the aerosol mass and SSAs are much lower than in the west.

We have analysed MISR data across the region to show that whilst aerosol optical depths have decreased during the dry season over the last decade, with greater rates of reduction occurring over the south western margins of Amazonia, absorption aerosol optical depths have significantly increased over the Cerrado and remained constant over south western Amazonia. This has led to a decline in SSA across the whole of the region with greater reductions occurring over the eastern states. This finding is consistent with our aircraft measurements. We will discuss the implications of these changes for air quality and climate across the region.