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- Title
Beyond the Ångström Exponent: Probing Additional Information in Spectral Curvature and Variability of In Situ Aerosol Hyperspectral (0.3–0.7 μm) Optical Properties.
- Authors
Jordan, Carolyn E.; Anderson, Bruce E.; Barrick, John D.; Blum, Dani; Brunke, Kathleen; Chai, Jiajue; Chen, Gao; Crosbie, Ewan C.; Dibb, Jack E.; Dillner, Ann M.; Gargulinski, Emily; Hudgins, Charles H.; Joyce, Emily; Kaspari, Jackson; Martin, Robert F.; Moore, Richard H.; O'Brien, Rachel; Robinson, Claire E.; Schuster, Gregory L.; Shingler, Taylor J.
- Abstract
Ångström exponents (α) allow reconstruction of aerosol optical spectra over a broad range of wavelengths from measurements at two or more wavelengths. Hyperspectral measurements of atmospheric aerosols provide opportunities to probe measured spectra for information inaccessible from only a few wavelengths. Four sets of hyperspectral in situ aerosol optical coefficients (aerosol‐phase total extinction, σext, and absorption, σabs; liquid‐phase soluble absorption from methanol, σMeOH‐abs, and water, σDI‐abs, extracts) were measured from biomass burning aerosols (BBAs). Hyperspectral single scattering albedo (ω), calculated from σext and σabs, provide spectral resolution over a wide spectral range rare for this optical parameter. Observed spectral shifts between σabs and σMeOH‐abs/σDI‐abs argue in favor of measuring σabs rather than reconstructing it from liquid extracts. Logarithmically transformed spectra exhibited curvature better fit by second‐order polynomials than linear α. Mapping second order fit coefficients (a1, a2) revealed samples from a given fire tended to cluster together, that is, aerosol spectra from a given fire were similar to each other and somewhat distinct from others. Separation in (a1, a2) space for spectra with the same α suggest additional information in second‐order parameterization absent from the linear fit. Spectral features found in the fit residuals indicate more information in the measured spectra than captured by the fits. Above‐detection σMeOH‐abs at 0.7 μm suggests assuming all absorption at long visible wavelengths is BC to partition absorption between BC and brown carbon (BrC) overestimates BC and underestimates BrC across the spectral range. Hyperspectral measurements may eventually discriminate BBA among fires in different ecosystems under variable conditions. Plain Language Summary: New measurement techniques for atmospheric aerosols are becoming available to obtain high spectral resolution optical information over a broad range of ultraviolet (UV)–visible–infrared wavelengths. Here, we measured UV‐visible spectra from aerosols emitted by five wildfires that exhibited curvature not adequately represented by a widely used single mathematical parameter known as the Ångström exponent. The observed spectral curvature differed across the five wildfires. We argue that the differences in absorption spectral curvature, in particular, were driven by different molecular composition of the smoke aerosols. We hypothesize that the observed curvature may eventually be used to optically differentiate wildfire aerosols and to better predict the optical and chemical evolution of wildfire smoke plumes downwind. We hope to encourage more routine measurements of this kind and to motivate development of improved measurement techniques and analytical approaches to maximize the information we can glean from such data. Key Points: Measured in situ atmospheric aerosol hyperspectral (<0.001 μm, resolution) optical properties in biomass burning plumes from 0.3 to 0.7 µmLog‐transformed spectra better fit by second‐order polynomials than Ångström exponents; measured spectra have details missed by fitsSpectral curvature in absorption driven by myriad organic chromophores over full wavelength range
- Subjects
ALBEDO; ATMOSPHERIC aerosol measurement; AEROSOLS; OPTICAL properties; ATMOSPHERIC aerosols; SMOKE plumes; CARBONACEOUS aerosols; CANALS
- Publication
Journal of Geophysical Research. Atmospheres, 2022, Vol 127, Issue 21, p1
- ISSN
2169-897X
- Publication type
Article
- DOI
10.1029/2022JD037201