Relative errors in derived multi-wavelength intensive aerosol optical properties using cavity attenuated phase shift single-scattering albedo monitors, a nephelometer, and tricolour absorption photometer measurements
<p>Aerosol intensive optical properties, including Ångström exponents for aerosol light extinction (EAEs), scattering (SAEs) and absorption (AAEs) as well as and the single-scattering albedo (SSA), are indicators for aerosol size, chemical composition, radiative behaviour and particle sources....
| Main Authors: | , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2022-06-01
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| Series: | Atmospheric Measurement Techniques |
| Online Access: | https://amt.copernicus.org/articles/15/3279/2022/amt-15-3279-2022.pdf |
| Summary: | <p>Aerosol intensive optical properties, including Ångström
exponents for aerosol light extinction (EAEs), scattering (SAEs) and
absorption (AAEs) as well as and the single-scattering albedo (SSA), are indicators for
aerosol size, chemical composition, radiative behaviour and particle
sources. Derivation of these parameters requires the measurement of aerosol
optical properties at multiple wavelengths, which usually involves the use
of several instruments. Our study aims to quantify the uncertainties in the
determination of these intensive properties using an optical closure
approach. In our laboratory closure study, we measured the full set of
optical properties for a range of light-absorbing particles with different
properties externally mixed with ammonium sulfate to generate aerosols with
controlled SSA values. The investigated absorbing particle types were fresh
combustion soot emitted by an inverted flame soot generator (SOOT; fractal
agglomerates), Aquadag (AQ; compact aggregates), Cabot black (BC;
compact agglomerates) and an acrylic paint (magic black, shape unknown).
The instruments used in this study were two cavity attenuated phase
shift particle monitors for single-scattering albedo (CAPS PM<span class="inline-formula"><sub>SSA</sub></span>'s; <span class="inline-formula"><i>λ</i>=450</span>, 630 nm) for measuring light-extinction and light-scattering coefficients, one
integrating nephelometer (<span class="inline-formula"><i>λ</i>=450</span>, 550, 700 nm) for light-scattering coefficients, and one tricolour absorption photometer (TAP;
<span class="inline-formula"><i>λ</i>=467</span>, 528, 652 nm) for filter-based light-absorption
coefficients.</p>
<p>One key finding is that the coefficients of light absorption, scattering
and extinction derived from combing the measurements of two independent
instruments agree with measurements from single instruments; the slopes of
regression lines are equal within reported uncertainties (i.e. closure is
observed). Despite closure for measured absorption coefficients, we caution
that the estimated uncertainties for absorption coefficients, propagated for
the differential method (DM; absorption <span class="inline-formula">=</span> extinction minus scattering),
can exceed 100 % for atmospherically relevant SSA values (<span class="inline-formula">>0.9</span>).
This increasing estimated uncertainty with increasing SSA yields AAE values
that may be too uncertain for measurements in the range of atmospheric
aerosol loadings. We recommend using the DM for measuring AAE values when the
SSA <span class="inline-formula"><</span> 0.9. EAE- and SAE-derived values achieved closure during this
study within stated uncertainties for extinction coefficients greater than
15 Mm<span class="inline-formula"><sup>−1</sup></span>. SSA values for 450 and 630 nm wavelengths internally agreed
with each other within 10 % uncertainty for all instrument combinations
and sampled aerosol types, which fulfils the defined goals for measurement
uncertainty of 10 % proposed by Laj et al. (2020) for GCOS (Global Climate
Observing System) applications.</p> |
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| ISSN: | 1867-1381 1867-8548 |