Light absorption from particulate impurities in snow and ice determined by spectrophotometric analysis of filters

Thomas C. Grenfell; Sarah J. Doherty; Antony D. Clarke; Stephen G. Warren

doi:10.1364/AO.50.002037

Applied Optics
Vol. 50,
Issue 14,
pp. 2037-2048
(2011)
•https://doi.org/10.1364/AO.50.002037

Light absorption from particulate impurities in snow and ice determined by spectrophotometric analysis of filters

Thomas C. Grenfell, Sarah J. Doherty, Antony D. Clarke, and Stephen G. Warren

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Thomas C. Grenfell, Sarah J. Doherty, Antony D. Clarke, and Stephen G. Warren, "Light absorption from particulate impurities in snow and ice determined by spectrophotometric analysis of filters," Appl. Opt. 50, 2037-2048 (2011)

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Table of Contents Category
- Instrumentation, Measurement, and Metrology
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About this Article
History
- Original Manuscript: June 3, 2010
- Revised Manuscript: January 26, 2011
- Manuscript Accepted: January 27, 2011
- Published: May 5, 2011

Abstract

Light absorption by particulate impurities in snow and ice can affect the surface albedo and is important for the climate. The absorption properties of these particles can be determined by collecting and melting snow samples and extracting the particulate material by filtration of the meltwater. This paper describes the optical design and testing of a new instrument to measure the absorption spectrum from 400 to $750 nm$ wavelength of the particles collected on filters using an “integrating-sandwich” configuration. The measured absorption is shown to be unaffected by scattering of light from the deposited particulates. A set of calibration standards is used to derive an upper limit for the concentration of black carbon (BC) in the snow. The wavelength dependence of the absorption spectra from 450 to $600 nm$ is used to calculate an absorption $Å ngstrom$ exponent for the aerosol. This exponent is used to estimate the actual BC concentration in the snow samples as well as the relative contributions of BC and non-BC constituents to the absorption of solar radiation integrated over the wavelength band 300 to $750 nm$ .

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Symbol	Description of Subscripts
est	Estimated; e.g., best estimate of BC level
equiv	Equivalent; based on ascribing all absorption on filter to an equivalent loading of BC
MAX	Maximum; based on assuming that all absorption from 650 to $700 nm$ is due to BC
NBC	Non-black-carbon component of absorption
tot	Wavelength integrated from 300 to $750 nm$
Symbol	Description of Variables
$Å_{i}$	Absorption $Å ngstrom$ exponent describing the wavelength dependence of absorption by a particular type of absorber in a specified spectral range via $τ (λ) \propto λ^{- Å}$ . Index i denotes tot, BC, or NBC as required.
$β (λ)$	Mass-absorption efficiency, $m^{2} / g$ . For our standard M71 soot samples $β (525 nm) = 6 m^{2} / g$ .
$τ_{i}^{k}$	Absorption optical depth (dimensionless) of the material deposited onto the filter. Equal to filter loading multiplied by β, and in general depends on wavelength. Index i denotes tot, BC, or NBC as required. The index k denotes MAX or estimated (est); it is absent if $i = tot$ .
$L_{BC}^{k}$	BC loading of filter. Index k denotes MAX, est, or equiv.
$C_{BC}^{k}$	Concentration of BC in snow. Index k denotes MAX, est, or equiv.
$r_{j}$	The ratio $τ_{j} (λ_{0}) / τ_{tot} (λ_{0})$ , where j denotes BC or NBC as indicated. Here $λ_{0} = 525 nm$ .
$f_{BC}^{k}$	The ratio of BC absorption to total absorption from 300 to $750 nm$ . Index k denotes MAX or est.
$f_{NBC}^{k}$	$[1 - f_{BC}^{k}]$ . Index k denotes MIN or est.
$E_{BC}^{k}$	Maximum wavelength-integrated absorption of solar energy by BC from 300 to $750 nm$ . Dimensionless. Index k denotes MAX or est.
$E_{TOT}$	Wavelength-integrated absorption of solar energy from 300 to $750 nm$ by all material on filter. Dimensionless.
$S_{0} (λ)$	Detected signal for an unloaded filter, relative voltage
$S (λ)$	Detected signal for a loaded filter, relative voltage
$χ_{λ}$	Relative spectral attenuation, $\ln [S_{0} (λ) / S (λ)]$ . Dimensionless.
$F_{net}$	Net spectral irradiance in the vicinity of the filter and integrating sandwich, $W / m^{2}$
$F_{s}$	Scalar irradiance in the vicinity of the filter and integrating sandwich, $W / m^{2}$
$κ_{abs}$	Volume absorption coefficient of the material deposited onto the filter for single transit of a plane wave, $m^{- 1}$
$R_{1}$	Reflectivity of the integrating sphere-filter combination
$R_{2}$	Reflectivity of the integrating sandwich
A/M	Ratio of exposed area on filter (A, $m^{2}$ ) to the total mass of snow filtered (M, g).

Light absorption from particulate impurities in snow and ice determined by spectrophotometric analysis of filters

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