assessment of the sensitivity of core / shell parameters derived using the single-particle soot photometer to density and refractive index
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2015
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Abstract
Black carbon (BC) is the dominant absorbing aerosol in the atmosphere, and
plays an important role in climate and human health. The optical properties
and cloud condensation nuclei (CCN) activity of soot depend on the amounts
(both relative and absolute) of BC and non-refractory material in the
particles. Mixing between these two components is often represented in
models by a core / shell coated sphere. The single-particle soot photometer
(SP2) is one of, if not the only, instrument capable of reporting
distributions of both core size and coating thickness. Most studies combine
the SP2's incandescence and 1064 nm scattering data to report coating
properties, but to date there is no consistency in the assumed values of
density and refractive index of the core that are used in these
calculations, which can greatly affect the reported parameters such as
coating thickness. Given that such data are providing an important constraint for
model comparisons and comparison between large data sets, it is important that
this lack of consistency is addressed.
In this study we explore the sensitivity of the reported coatings to these parameters. An assessment of the coating properties of freshly emitted, thermodenuded ambient particles demonstrated that a core density of 1.8 g cm−3 and refractive index of (2.26–1.26i) were the most appropriate to use with ambient soot in the Los Angeles area. Using these parameters generated a distribution with median shell / core ratio of 1.02 ± 0.11, corresponding to a median absolute coating thickness of 2 ± 8 nm. The main source of statistical error in the single-particle data was random variation in the incandescence signals. Other than the sensitivity to core refractive index, the incandescence calibration was the main source of uncertainty when optically determining the average coatings. The refractive index of coatings was found to have only a minor influence.
This work demonstrates that using this technique the SP2 can accurately determine the average mixing state (externally or internally mixed) of ambient soot within the precision of the instrument calibration. Ambient coatings were measured up to a median shell / core ratio of 1.50 ± 0.11, meaning that this technique is able to resolve absolute changes in mixing state.
However, when different core parameters were used, the core / shell ratio and the coating thickness were shown to be offset by amounts that could be larger than the atmospheric variability in these parameters, though the results have a similar precision. For comparison, using the core parameters that resulted in the thickest coatings, on the same thermodenuded fresh particles as before, generated a median shell / core ratio of 1.39 ± 0.11, corresponding to a median absolute coating thickness of 30 ± 8 nm. These results must be taken into account when comparing BC coatings measured using this technique, or if using these data for optical or CCN calculations.
We have determined the most appropriate values of BC density and refractive index to use to measure mixing state at 1064 nm where particle morphology has only a minor effect, but appropriate values to use for optical calculations of nonspherical particles at visible wavelengths will also be subject to similar, significant uncertainties. Without similar constraints as those provided here, constraining the behaviour of BC particles in models using field data will be subject to large systematic measurement uncertainties.
In this study we explore the sensitivity of the reported coatings to these parameters. An assessment of the coating properties of freshly emitted, thermodenuded ambient particles demonstrated that a core density of 1.8 g cm−3 and refractive index of (2.26–1.26i) were the most appropriate to use with ambient soot in the Los Angeles area. Using these parameters generated a distribution with median shell / core ratio of 1.02 ± 0.11, corresponding to a median absolute coating thickness of 2 ± 8 nm. The main source of statistical error in the single-particle data was random variation in the incandescence signals. Other than the sensitivity to core refractive index, the incandescence calibration was the main source of uncertainty when optically determining the average coatings. The refractive index of coatings was found to have only a minor influence.
This work demonstrates that using this technique the SP2 can accurately determine the average mixing state (externally or internally mixed) of ambient soot within the precision of the instrument calibration. Ambient coatings were measured up to a median shell / core ratio of 1.50 ± 0.11, meaning that this technique is able to resolve absolute changes in mixing state.
However, when different core parameters were used, the core / shell ratio and the coating thickness were shown to be offset by amounts that could be larger than the atmospheric variability in these parameters, though the results have a similar precision. For comparison, using the core parameters that resulted in the thickest coatings, on the same thermodenuded fresh particles as before, generated a median shell / core ratio of 1.39 ± 0.11, corresponding to a median absolute coating thickness of 30 ± 8 nm. These results must be taken into account when comparing BC coatings measured using this technique, or if using these data for optical or CCN calculations.
We have determined the most appropriate values of BC density and refractive index to use to measure mixing state at 1064 nm where particle morphology has only a minor effect, but appropriate values to use for optical calculations of nonspherical particles at visible wavelengths will also be subject to similar, significant uncertainties. Without similar constraints as those provided here, constraining the behaviour of BC particles in models using field data will be subject to large systematic measurement uncertainties.
| Reference Key |
taylor2015atmosphericassessment
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|---|---|
| Authors | ;J. W. Taylor;J. D. Allan;D. Liu;M. Flynn;R. Weber;X. Zhang;B. L. Lefer;N. Grossberg;J. Flynn;H. Coe |
| Journal | bioorganic & medicinal chemistry |
| Year | 2015 |
| DOI |
10.5194/amt-8-1701-2015
|
| URL | |
| Keywords |
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