Distribution and Properties of Aerosol and Gas Phase Constituents within Biomass Burning Regional Haze in Brazil, 2012, during the Sambba (South American Biomass Burning Analysis) Field Campaign

Abstract:

Biomass Burning (BB) aerosols (BBA) impact upon weather, climate, ecosystems and human health at global and regional scales. Yet quantitative evaluation is impeded by a limited understanding of BB processes and a dearth of in-situ measurements. Thus large model uncertainties prevail, especially in data poor, intensive BB regions such as Brazil. Hence the timely nature of the SAMBBA campaign, utilizing aircraft (UK Facility for Airborne Atmospheric Measurement BAe-146) and ground based observations out of Porto Velho in Sept-Oct 2012. This work utilizes aircraft measurements to characterize BB regional haze - the inhomogeneous accumulation of aged BBA capped within the boundary layer, present across swathes of Brazil.

As context, aerosol optical depth (AOD) and meteorological climatologies are presented and compared to the synoptic conditions of 2012. Throughout the early flights an expansive area of elevated (>1) AOD persisted, although in transitioning toward the wet season, rain out and advection significantly reduced its spatial extent and magnitude in western regions of Brazil. Concurrent decreases in haze BBA concentrations (~50%) were observed from the aircraft measurements sampling in these deforested/forested areas. However, the relative vertical structure, composition, physical and optical properties remained similar. The lofted maxima in aerosol concentrations at ~1.5km, typically not captured in models, is potentially important for regional climate.

Significant differences were observed, however, during flights over the eastern savannah-like regions of Brazil, which remained drier throughout. Here, haze BBA concentrations resembled those in the west prior to wash out, with the exception of high loadings of refractive black carbon. This acted to lower the single scattering albedo and alter the number size distribution. The observed haze BBA west-east split is also present at source and remains similar throughout fresh plume evolution, thus we conclude haze BBA physiochemical properties are determined at source as a function of local burn conditions (combustion phase, fuel, etc.).

This work presents a synthesis of the aerosol, gas phase and thermodynamic state of the Brazilian atmosphere under the influence of biomass burning regional haze and assesses regional climate implications.