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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union
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Preprints
https://doi.org/10.5194/amt-2020-317
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/amt-2020-317
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  19 Aug 2020

19 Aug 2020

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A revised version of this preprint is currently under review for the journal AMT.

New In Situ Aerosol Hyperspectral Optical Measurements over 300–700 nm, Part 1: Spectral Aerosol Extinction (SpEx) Instrument Field Validation during the KORUS-OC cruise

Carolyn E. Jordan1,2, Ryan M. Stauffer3, Brian T. Lamb4, Charles H. Hudgins2, Kenneth L. Thornhill2,5, Gregory L. Schuster2, Richard H. Moore2, Ewan C. Crosbie2,5, Edward L. Winstead2,5, Bruce E. Anderson2, Robert F. Martin2, Michael A. Shook2, Luke D. Ziemba2, Andreas J. Beyersdorf2,6, Claire E. Robinson2,5, Chelsea A. Corr2,7, and Maria A. Tzortziou3,4 Carolyn E. Jordan et al.
  • 1National Institute of Aerospace, Hampton, Virginia, USA
  • 2NASA Langley Research Center, Hampton, Virginia, USA
  • 3NASA Goddard Space Flight Center, Greenbelt, Maryland, USA
  • 4City University of New York, New York, New York, USA
  • 5Science Systems and Applications Inc., Hampton, Virginia, USA
  • 6California State University, San Bernardino, California, USA
  • 7Springfield College, Springfield, Massachusetts, USA

Abstract. In situ observations of spectrally-resolved aerosol extinction coefficients (300–700 nm at ~ 0.8 nm resolution) from the May–June 2016 Korea U.S. – Ocean Color (KORUS-OC) oceanographic field campaign are reported. Measurements were made with the custom-built Spectral Aerosol Extinction (SpEx) instrument that previously has been characterized only using laboratory-generated aerosols of known size and composition. Here, the performance of SpEx under realistic operating conditions in the field was assessed by comparison to extinction coefficients derived from commercial instruments that measured scattering and filter-based absorption coefficients at three discrete visible wavelengths. Good agreement was found between these two sets of extinction coefficients with slopes near unity for all 3 wavelengths within the SpEx measurement error (±5 Mm−1). The meteorological conditions encountered during the cruise fostered diverse ambient aerosol populations with varying sizes and composition at concentrations spanning two orders of magnitude. The sampling inlet had a 50 % size cut of 1.3 µm diameter particles such that the in situ aerosol sampling suite deployed aboard ship measured fine mode aerosols only. The extensive hyperspectral extinction data set acquired revealed that nearly all measured spectra exhibited curvature in logarithmic space, such that Ångström exponent (α) power law fits led to large errors compared to measured values, especially in the ultraviolet (UV) wavelength range. This problem was particularly acute for α values calculated over only visible wavelengths, then extrapolated to the UV, highlighting the need for measurements in this wavelength range. Second-order polynomial fits to the logarithmically-transformed data provided a much better fit to the measured spectra than the linear fits of power laws. Building on previous studies that used total column AOD observations to examine the information content of spectral curvature, the relationship between α and the second order polynomial fit coefficients (a1 and a2) was shown to depend on the characteristic wavelength (λch) of any given spectral measurement, such that differing curvature among aerosol size distributions with the same α will map to a line in (a1,a2) space with a slope related to λch. Thus, spectral curvature represented by (a1,a2) may provide more detailed aerosol size distribution information than α alone.

Carolyn E. Jordan et al.

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Carolyn E. Jordan et al.

Carolyn E. Jordan et al.

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Short summary
The first field data from a custom-built in situ instrument to measure hyperspectral (300–700 nm, 0.7 nm resolution) ambient atmospheric aerosol extinction is presented. The advantage of this capability is that it can be directly linked to other in situ techniques that measure physical & chemical properties of atmospheric aerosols. 2nd order polynomials provided a better fit to the data than traditional power law fits yielding greater discrimination among distinct ambient aerosol populations.
The first field data from a custom-built in situ instrument to measure hyperspectral...
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