Impacts of nonrefractory material on light absorption by aerosols emitted from biomass burning

• Published in: Journal of geophysical research. Atmospheres 2014-11, Vol.119 (21), p.12,272-12,286
• Main Authors: McMeeking, G. R., Fortner, E., Onasch, T. B., Taylor, J. W., Flynn, M., Coe, H., Kreidenweis, S. M.
• Format: Article
• Language: English
• Subjects: Absorption; Aerosols; Air sampling; Albedo; Atmospheric aerosols; Biomass; biomass burning; Black carbon; Carbon; Emittance; Exponents; Geophysics; Light absorption; Optical properties; Photometers; Soot; Wavelengths
• ISSN: 2169-897X; 2169-8996
• DOI: 10.1002/2014JD021750

We present laboratory measurements of biomass‐burning aerosol light‐scattering and light absorption coefficients at 405, 532, and 781 nm and investigate their relationship with aerosol composition and fuel type. Aerosol composition measurements included nonrefractory components measured by a high‐resolution aerosol mass spectrometer (AMS), composition of refractory black carbon‐containing particles by a soot particle aerosol mass spectrometer (SP‐AMS), and refractory black carbon measured by a single‐particle soot photometer (SP2). All measurements were performed downstream of a thermal denuder system to probe the effects of nonrefractory material on observed optical properties. The fires studied emitted aerosol with a wide range of optical properties with some producing more strongly light‐absorbing particles (single‐scattering albedo or SSA at 781 nm = 0.4) with a weak wavelength dependence of absorption (absorption Ångström exponent or AAE = 1–2) and others producing weakly light‐absorbing particles (SSA at 781 nm ~1) with strong wavelength dependence of absorption (AAE ~7). Removal of nonrefractory material from the particles by the thermal denuder system led to substantial (20–80%) decreases in light absorption coefficients, particularly at shorter wavelengths, reflecting the removal of light‐absorbing material that had enhanced black carbon absorption in internally mixed untreated samples. Observed enhancements of absorption by all mechanisms were at least factors of 1.2–1.5 at 532 nm and 781 nm as determined from the heated samples. A mass absorption cross‐section‐based approach indicated larger enhancements, particularly at shorter wavelengths. Key Points At 781 nm, SSA ~1 corresponded to the largest absorption Angstrom exponents, ~7Nonrefractory material enhanced visible light absorption at least 20–50%A portion of the absorption enhancement was attributed to brown carbon