Evaluation of a filter-based black carbon (BC) instrument using a brown carbon (BrC) surrogate as well as pure and coated BC surrogates

Abstract

We evaluate a filter-based Aethalometer and investigate a well characterized, commercially available colloid suspension as a potential material to mimic atmospheric black carbon (BC) for use in the evaluation of such filter-based BC instruments. We size selected BC surrogate particles of 300 nm in mobility diameter, determined the particle mass, m_p, and used a condensation particle counter (CPC) as a reference method for measurement of number density, Np. The number concentration was used to determine the mass concentration and to evaluate the Aethalometer as a function of particle loading, size, wavelength, and coating. For uncoated BC surrogate particles and mass concentrations less than 2.5 μg m⁻³ the CPC-derived BC and filter-based BC measurements could, after a correction was applied, be made to agree to within 5% at all Aethalometer wavelengths (370 to 950 nm). At higher concentrations shadowing occurs, necessitating a correction for loading on the filter. Pure brown carbon (BrC) particles generated with a humic acid (HA) surrogate produced only a minor signal at 880 nm (2% of an equivalent mass of BC), but a substantial signal at shorter wavelengths (up to 20% of an equivalent mass of BC at 370 nm). The Aethalometer indicated an Attenuation Angstrom Exponent, A_tAE, value near unity (0.71) for the pure BC surrogate, but much larger (3.7) for the BrC surrogate. Uncertainty in filter-based BC measurements increased substantially for BC particles coated with minimally absorbing ammonium sulfate or with the brown carbon surrogate, and varies with detection wavelength. We present information that may improve a widely used filter-based instrument for measurements of BC and absorption properties of atmospheric aerosols.

Copyright © 2021 American Association for Aerosol Research

Acknowledgments

We would like to acknowledge Chris Zangmeister, James Radney and Joseph Conny from NIST and Gary Norris at the EPA Office of Research and Development for use of the crucial instruments. CG was supported by NIST. There are no competing interests of the work in this article.

Additional information

Funding

This research was supported by the National Institute of Standards and Technology (grant no. 70NANB19H037).