A sensitivity study on the retrieval of aerosol vertical profiles using the oxygen A-band
Atmospheric absorption in the O<sub>2</sub> A-band (12 950–13 200 cm<sup>−1</sup>) offers a unique opportunity to retrieve aerosol extinction profiles from space-borne measurements due to the large dynamic range of optical thickness in that spectral region. Absorptions in str...
| Main Authors: | , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2016-04-01
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| Series: | Atmospheric Measurement Techniques |
| Online Access: | http://www.atmos-meas-tech.net/9/1889/2016/amt-9-1889-2016.pdf |
| Summary: | Atmospheric absorption in the O<sub>2</sub> A-band
(12 950–13 200 cm<sup>−1</sup>) offers a unique opportunity to retrieve
aerosol extinction profiles from space-borne measurements due to the large
dynamic range of optical thickness in that spectral region. Absorptions in
strong O<sub>2</sub> lines are saturated; therefore, any radiance measured in
these lines originates from scattering in the upper part of the atmosphere.
Outside of O<sub>2</sub> lines, or in weak lines, the atmospheric column
absorption is small, and light penetrates to lower atmospheric layers,
allowing for the quantification of aerosols and other scatterers near the
surface.</p><p class="p">While the principle of aerosol profile retrieval using O<sub>2</sub>
A-band absorption from space is well-known, a thorough quantification
of the information content, i.e., the amount of vertical profile
information that can be obtained, and the dependence of the
information content on the spectral resolution of the measurements,
has not been thoroughly conducted. Here, we use the linearized vector
radiative transfer model VLIDORT to perform spectrally resolved
simulations of atmospheric radiation in the O<sub>2</sub> A-band for
four different aerosol extinction profile scenarios: urban
(urban–rural areas), highly polluted (megacity areas with large
aerosol extinction), elevated layer (identifying elevated plumes, for example for
biomass burning) and low extinction (representative of small aerosol
extinction, such as vegetated, marine and arctic areas).
<br><br>
The high-resolution radiances emerging from the top of the atmosphere measurements
are degraded to different spectral resolutions, simulating spectrometers
with different resolving powers. We use optimal estimation theory to
quantify the information content in the aerosol profile retrieval with
respect to different aerosol parameters and instrument spectral
resolutions.
<br><br>
The simulations show that better spectral resolution generally leads
to an increase in the total amount of information that can be
retrieved, with the number of degrees of freedom (DoF) varying between
0.34–2.01 at low resolution (5 cm<sup>−1</sup>) to 3.43–5.38 at high
resolution (0.05 cm<sup>−1</sup>) among all the different
cases. A particularly strong improvement was found in the retrieval of
tropospheric aerosol extinction profiles in the lowest 5 km of the
atmosphere. At high spectral resolutions (0.05 cm<sup>−1</sup>),
1.18–1.48 and 1.31–1.96 DoF can be obtained in the lower (0–2 km)
and middle (2–5 km) troposphere, respectively, for the different
cases. Consequently, a separation of lower and mid tropospheric
aerosols is possible, implying the feasibility of identification of
elevated biomass burning aerosol plumes (elevated layer scenario). We
find that a higher single scattering albedo (SSA) allows for the
retrieval of more aerosol information. However, the dependence on SSA
is weaker at higher spectral resolutions.
<br><br>
The vegetation (surface albedo 0.3), marine (surface albedo 0.05)
and arctic (surface albedo 0.9) cases show that the dependence of
DoF on the surface albedo decreases with higher resolution. At low
resolution (5 cm<sup>−1</sup>), the DoF are 1.19 for the marine case,
0.73 for the vegetation case and 0.34 for the arctic case, but
increase considerably at 0.05 cm<sup>−1</sup> resolution to 3.84 (marine)
and 3.43 (both vegetation and arctic), showing an improvement of a factor
of 10 for the arctic case. Vegetation and arctic case also show the same
DoF at higher resolution, showing that an increase of albedo beyond a
certain value, i.e., 0.3 in our case, does not lead to a larger information content.
<br><br>
The simulations also reveal a moderate dependence of information content on the
integration time of the measurements, i.e., the noise of the spectra.
However, our results indicate that a larger increase in DoF is obtained by an
increase in spectral resolution despite lower signal-to-noise ratios. |
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| ISSN: | 1867-1381 1867-8548 |