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
Authors: Jordan Carolyn E. , Stauffer Ryan M. , Lamb Brian T. , Hudgins Charles H. , Thornhill Kenneth L. , Schuster Gregory L. , Moore Richard H. , Crosbie Ewan C. , Winstead Edward L. , Anderson Bruce E. , Martin Robert F. , Shook Michael A. , Ziemba Luke D. , Beyersdorf Andreas J. , Robinson Claire E. , Corr Chelsea A. , Tzortziou Maria A. 
Published: January 2021

Abstract. In situ observations of spectrally resolved aerosol extinction coefficients (300–700 nm at ∼ 0.8 nm resolution) from the
May–June 2016 Korea–United States 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 three 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 2 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 could lead to large errors compared to measured values. This problem was particularly acute for α values calculated over only
visible wavelengths and 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 aerosol optical depth observations to examine the information content of spectral curvature, the relationship between α and the second-order polynomial fit coefficients (a1 and a2) was found to depend on the wavelength range of the spectral measurement such that any given α maps into a line in (a1, a2) coefficient space with a slope of −2LN(λch), where
λch is defined as the single wavelength that characterizes the wavelength range of the measured spectrum (i.e., the
“characteristic wavelength”). Since the curvature coefficient values depend on λch, it must be taken into account when comparing
values from spectra obtained from measurement techniques with different λch. Previously published work has shown that different
bimodal size distributions of aerosols can exhibit the same α yet have differing spectral curvature with different (a1, a2). This
implies that (a1, a2) contain more information about size distributions than α alone. Aerosol size distributions were not measured
during KORUS-OC, and the data reported here were limited to the fine fraction, but the (a1, a2) maps obtained from the SpEx data set are
consistent with the expectation that (a1, a2) may contain more information than α – a result that will be explored further with
future SpEx and size distribution data sets.