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Discussion papers
https://doi.org/10.5194/acp-2018-1200
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2018-1200
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 21 Nov 2018

Research article | 21 Nov 2018

Review status
This discussion paper is a preprint. A revision of this manuscript was accepted for the journal Atmospheric Chemistry and Physics (ACP) and is expected to appear here in due course.

Measuring Light Absorption by Organic Aerosols: Correction Factors for Solvent Extraction-Based Photometry Techniques

Nishit Shetty1, Apoorva Pandey1, Stephen Baker2, Wei Min Hao2, and Rajan K. Chakrabarty1,3 Nishit Shetty et al.
  • 1Center for Aerosol Science and Engineering, Department of Energy, Environmental and Chemical Engineering, Washington University in St. Louis, St. Louis, MO 63130, USA
  • 2USDA Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory, Missoula, Montana, USA
  • 3McDonnell Center for the Space Sciences, Washington University in St. Louis, St. Louis, MO 63130, USA

Abstract. Recent studies have shown that organic aerosol (OA) could have a non-trivial role in atmospheric light absorption at shorter visible wavelengths. Good estimates of OA absorption are therefore necessary to accurately calculate radiative forcing due to these aerosols in climate models. One of the common techniques used to measure OA light absorption is the solvent extraction technique from filter samples which involves the use of a spectrophotometer to measure bulk absorbance of the solvent-soluble organic fraction of particulate matter. Measured bulk absorbance is subsequently converted to particle-phase absorption coefficient using correction factors. The appropriate correction factors to use for performing this conversion under varying scenarios of organic carbon (OC) to total carbon (TC) mass ratios has been an unexplored area of research. The conventional view is to apply a correction factor of 2 for water-extracted OA based on Mie calculations.

Here, we performed a comprehensive laboratory study involving three solvents (water, methanol, and acetone) to investigate the corrections factors for converting from bulk-to-particle phase absorption coefficients (babs,OA/babs,bulk) for primary OA emitted from biomass burning. We parametrized these correction factors as a function of OC / TC mass ratio and single scattering albedo (SSA). We observed these correction factors to be a function of the OC / TC ratio of the aerosol, and that the conventionally used correction factor of 2 for water-extracted OA could severely underpredict OA absorption at high EC mass fractions. We recommend using babs,OA/babs,bulk values between 2 and 11 for water extracts and values between 1 and 4 for methanol extracts based on OC / TC ratios, for EC mass fractions less than 0.25. Furthermore, a linear correlation between SSA and OC / TC ratio was also established. Finally, from the spectroscopic data, we analyzed the differences in Absorption Ångström Exponents (AÅE) obtained from bulk- and particulate-phase measurements. We noted that AÅE from bulk measurements deviate significantly from their OA counterparts.

Nishit Shetty et al.
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Interactive discussion
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Status: closed
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Nishit Shetty et al.
Nishit Shetty et al.
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Short summary
We determined correction factors to estimate particle-phase absorption coefficients in the visible spectrum for organic aerosol from bulk-phase absorbance measurements of solvent extracts. These empirical correction factors were systematically studied as a function of organic-to-total carbon mass ratios and aerosol single scattering albedo. Differences in the absorption angstrom exponents from bulk- and particle-phase measurements were also investigated.
We determined correction factors to estimate particle-phase absorption coefficients in the...
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