07 Jun 2023
 | 07 Jun 2023

Characterization of water-soluble brown carbon chromophores from wildfire plumes in the western US using size exclusion chromatography

Lisa Azzarello, Rebecca A. Washenfelder, Michael A. Robinson, Alessandro Franchin, Caroline C. Womack, Christopher D. Holmes, Steven S. Brown, Ann Middlebrook, Tim Newberger, Colm Sweeney, and Cora J. Young

Abstract. Wildfires are an important source of carbonaceous aerosol in the atmosphere. Organic aerosol that absorbs light in the ultraviolet to visible spectral range is referred to as “brown carbon” (BrC), and its impact on Earth’s radiative budget has not been well characterized. We collected water-soluble brown carbon using a particle into liquid sampler (PILS) onboard a Twin Otter aircraft during the Fire Influence on Regional to Global Environments and Air Quality (FIREX-AQ) campaign. Samples were collected downwind of wildfires in the western United States from August to September 2019. We applied size exclusion chromatography (SEC) with ultraviolet-visible spectroscopy to characterize the molecular size distribution of BrC chromophores. The wildfire plumes had transport ages of 0 to 5 h and the absorption was dominated by chromophores with molecular weights <500 Da. With BrC normalized to a conserved biomass burning tracer, carbon monoxide, a consistent decrease in BrC absorption with plume age was not observed during FIREX-AQ. These findings are consistent with the variable trends in BrC absorption with plume age reported in recent studies. While BrC absorption trends were broadly consistent between the offline SEC analysis and the online PILS measurements, the absolute values of absorption and their spectral dependence differed. We attribute this difference to the organic modifier used in the chromatographic separation and demonstrate how this affects the molecular structure of the compounds comprising BrC, with implications for interpretation of absorption measurement of BrC field samples.

Lisa Azzarello et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2023-1128', Anonymous Referee #1, 13 Jun 2023
  • RC2: 'Comment on egusphere-2023-1128', Anonymous Referee #2, 20 Jun 2023
  • RC3: 'Comment on egusphere-2023-1128', Anonymous Referee #3, 21 Jul 2023

Lisa Azzarello et al.


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Short summary
We present a molecular size-resolved offline analysis of water-soluble brown carbon collected on an aircraft during FIREX-AQ. The smoke plumes were aged 0 to 5 h where absorption was dominated by small molecular weight molecules, brown carbon absorption downwind did not consistently decrease, and the measurements differed from online absorption measurements of the same samples. We show how differences between online and offline absorption could be related to different measurement conditions.