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Discussion papers
https://doi.org/10.5194/acp-2019-157
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2019-157
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 28 Feb 2019

Research article | 28 Feb 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Transformation and aging of biomass burning carbonaceous aerosol over tropical South America from aircraft in-situ measurements during SAMBBA

William T. Morgan1, James D. Allan1,2, Stéphane Bauguitte3, Eoghan Darbyshire1, Michael J. Flynn1, James Lee4, Dantong Liu1, Ben Johnson5, Jim Haywood5,6, Karla M. Longo7,a, Paulo E. Artaxo8, and Hugh Coe1 William T. Morgan et al.
  • 1School of Earth & Environmental Sciences, University of Manchester, Manchester, UK
  • 2National Centre for Atmospheric Science, University of Manchester, Manchester, UK
  • 3Facility for Airborne Atmospheric Measurements, Cranfield University, UK
  • 4Department of Chemistry, University of York, York, UK
  • 5Met Office, Exeter, UK
  • 6College of Engineering, Mathematics and Physical Sciences, University of Exeter, UK
  • 7National Institute for Space Research (INPE), Sao Jose dos Campos, Brazil
  • 8Physics Institute, University of Sao Paulo, Sao Paulo, Brazil
  • anow at: NASA Goddard Space Flight Center and USRA/GESTAR, Greenbelt, MD, USA

Abstract. We present a range of airborne in-situ observations of biomass burning carbonaceous aerosol over tropical South America, including a case study of a large tropical forest wildfire and a series of regional survey flights across the Brazilian Amazon and Cerrado. The study forms part of the South American Biomass Burning Analysis (SAMBBA) Project, which was conducted during September and October 2012. We find limited evidence for net increases in aerosol mass through atmospheric aging combined with substantial changes in the chemical properties of organic aerosol (OA). Oxidation of the OA increases significantly and rapidly on the scale of 2.5–3 hours based on our case study analysis and is consistent with secondary organic aerosol production. The observations of limited net enhancement in OA coupled with such changes in chemical composition, imply that evaporation of OA is also occurring to balance these changes. We observe significant coatings on black carbon particles at source, but with limited changes with aging in both particle core size and coating thickness.

We quantify variability in the ratio of OA to carbon monoxide across our study as a key parameter representing both initial fire conditions and an indicator of net aerosol production with atmospheric aging. We observe ratios of 0.075–0.13 μg sm−3 ppbv−1 in the west of our study region over the Amazon tropical forest in air masses less influenced by precipitation and a value of 0.095 μg sm−3 ppbv−1 over the Cerrado environment in the east. Such values are consistent with emission factors used by numerical models to represent biomass burning OA emissions. Black carbon particle core sizes typically range from 250–290 nm, while coating thicknesses range from 40–110 nm in air masses less influenced by precipitation. The primary driver of the variability we observe appears to be related to changes at the initial fire source. A key lesson from our study is that the complex nature of the regional aerosol and its drivers precludes aggregating our observations as a function of atmospheric aging due to the many conflating and competing factors present.

Our study explores and quantifies key uncertainties in the evolution of biomass burning aerosol at both nearfield and regional scales. Our results suggest that the initial conditions of the fire are the primary driver of carbonaceous aerosol physical and chemical properties over tropical South America, aside from significant oxidation of OA during atmospheric aging. Such findings imply that uncertainties in the magnitude of the aerosol burden and its impact on weather, climate, health and natural ecosystems most likely lie in quantifying emission sources, alongside atmospheric dispersion, transport and removal rather than chemical enhancements in mass.

William T. Morgan et al.
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Short summary
We flew a large atmospheric research aircraft across a number of different environments in the Amazon Basin during the 2012 biomass burning season. Smoke from fires builds-up and has a significant impact on weather, climate, health and natural ecosystems. Our goal was to quantify changes in the properties of the smoke emitted by fires as they are transported through the atmosphere. We found that the major control on the properties of the smoke was due to differences in the fires themselves.
We flew a large atmospheric research aircraft across a number of different environments in the...
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