Gas absorption optical depth in O2 A and B bands
Figure 1a–b show the spectral absorption optical depth in O2 A and B
bands for a typical midlatitude summer atmosphere (McClatchey et al.,
1972). The maximum absorption optical depth can reach 100 and 7 in the
O2 A and B bands, respectively. Therefore, the sunlight can be fully
attenuated before it reaches the surface at wavelengths of strong O2
absorption. The large variability of absorption optical depth in the O2
A and B bands enables sunlight from the TOA to penetrate
the atmosphere at different depths (before it completely attenuated in the
atmosphere; Fig. 1c–d). For instance, the penetration altitude is about
30 km above the surface in the center of O2 A band and decreases to 20 km, 10 km
and near the surface as the wavelength moves from the center to
the edge of O2 A band (Fig. 1c). Similarly, for wavelengths at the
O2 B band, the penetration altitude can vary from 15 km to the surface
depending on the wavelength (Fig. 1d). Hence, for a given atmospheric
profile with well-characterized vertical profile of O2 absorption, the
spectral contrast of reflected sunlight in terms of their intensity and
polarization in these bands contains information of atmospheric scattering
(including aerosol scattering) at different altitudes of the atmosphere.
To quantitatively illustrate the sensitivity of backscattered intensity and
polarization with respect to the vertical profile of scattering, we first
conduct the UNL-VRTM calculation for aerosol-free conditions. Considering that the
absorption optical depth highly varies with wavelength in both O2 A and
B bands, we plot in Fig. 1e–f the DOLP at the top of an aerosol-free atmosphere
over various surface types (with albedo As of 0.0, 0.05, 0.2 and 0.5)
as a function of O2 absorption depth. It should be noted that the
O2 absorption (often larger than 1) is much larger than the Rayleigh
scattering optical depth (0.026–0.024 and 0.040–0.037, respectively) in
O2 A and B bands, and hence the spectral variation of DOLP in these bands has visually
no dependence on spectral variation of Rayleigh optical depth (figures now
shown). As shown in Fig. 1e–f, the magnitude of O2 absorption
optical depth and its effect on DOLP can be categorized into the following
three distinct regions.
(a) The profile of aerosol optical depth for each layer.
(b) Difference of DOLP between 759.98 (absorption channel in O2 A band) and
757 nm (continuum channel) as a function the aerosol peak height for
different surface albedos. (c) Same as (b) but for 689.78 mm (inside O2 B
band) and 686 nm (outside O2 A band). (d) The profile of aerosol
optical depth for different half width. (e–f) Same as (b–c) but as a
function of half width at different absorption wavelengths. The solar zenith angle and view zenith angle are 66 and 0.0∘, respectively.
The first region has the gaseous absorption optical depth less than 0.1.
Because of the strong interaction between atmospheric scattering and surface
reflection (with As > 0), DOLP is less than 20 % when
absorption optical depth is very small (for example ≪ 0.1)
in Fig. 1e–f. As surface reflectance increases from 0.05 (grey line) to
0.2 (blue line), the DOLP decreases because the stronger multiple scattering
occurring between the surface and atmosphere decreases the DOLP. Indeed,
DOLP is nearly 0 when surface reflectance is very high (As of 0.5).
However, when surface is black (As = 0), DOLP is a result of the Rayleigh
scattering only (as O2 optical depth ≪ 0.1) and
therefore is the largest (up to 57 %).
The second region has the gaseous absorption optical depth larger than 20.
The DOLP in this region is mainly determined by scattering in the upper
atmospheric layer where molecular scattering dominates and leads to larger
DOLP (up to 57 %) regardless of the surface reflectance, since the
incident solar light cannot reach lower altitude and light scattered in the
lower atmosphere cannot easily reach the TOA (because of strong O2
absorption). In other words, O2 layer in the lower part of atmosphere
now acts as a black surface.
The third region is located between the above defined first and second
regions. In this region, light can penetrate through and be reflected by the
lower atmosphere (and surface depending on absorption optical depth), and
thus intensity and polarization of the upwelling radiation at TOA have the
sensitivity to the vertical distribution of scattering in the whole
atmospheric column when surface is not black. However, for black surfaces
(with As = 0), Rayleigh scattering becomes the only source for DOLP and,
hence, DOLP is also largest (up to 57 %).
In summary, when surface is black and/or O2 absorption in the lower
part of the atmosphere is large enough to completely prevent the light
reaching the surface, the backscattered DOLP is from the Rayleigh scattering
only and is always close to 57 %. For black surface only, there is a
slight (∼ 1 %) increase of DOLP as O2 optical depth
increases (from nearly 0 to 10). This small increase of DOLP is due to a
slight decrease of effective Rayleigh optical depth that interacts with the
light; the slight decrease is because path length of light is getting
shorter with the increase of O2 absorption. As atmospheric
aerosols can affect scattering optical depth at various altitudes, it is
expected that aerosols can change DOLP in O2 A and B bands, and the
spectral variation of DOLP within these bands should have information on
vertical distribution of aerosol scattering.
Sensitivity of DOLP to the aerosol vertical distribution
To investigate the sensitivity of DOLP and intensity to aerosol vertical
distribution, we focus mainly on two parameters, the aerosol peak height
(H) and half width (γ) at half maximum, which are essential to
determine the aerosol vertical profile as mentioned in the previous section.
To explain the physics underlying the results, we first present the results
for four different aerosol profile examples (Fig. 2a) in which the aerosol
peak height H varies from 4 to 13 km with an increment of 3 km. Here, the
column aerosol optical depth is assumed to be 0.2 and γ is 1 km.
Figure 2b shows the differences of DOLP (ΔDOLP) from one wavelength
(759.98 nm) within the O2 A band to the nearby continuum band (757 nm)
for surface reflectances of 0.0, 0.05, 0.2 and 0.5. The solar zenith angle
and view zenith angle are 66 and 0∘, respectively. It
can be seen that ΔDOLP increases as the surface reflectance
increases, which is caused by relatively larger increase of multiple
scattering between the surface and atmosphere in the continuum band (than in
the O2 absorption band).
The ΔDOLP values for As of 0.2 and 0.5 show much smaller differences
(within 0.05 in Fig. 2b), suggesting the advantage of using DOLP to characterize
aerosol properties over the bright surfaces. Indeed, the variation of
ΔDOLP with peak height appears similar regardless of the
surface reflectance. For a given surface reflectance, ΔDOLP
decreases as the aerosol peak height increases (Fig. 2b). While the DOLP
of Rayleigh scattering is positive and strong, the DOLP of spherical
aerosols is negative and weaker compared to that of Rayleigh scattering.
Consequently, the increase of scattering by aerosols, as aerosol gets aloft,
can partially offset the effects of Rayleigh scattering, resulting in less
positive DOLP in O2 A band (Wang et al., 2014). In the continuum band,
however, Zeng et al. (2008) showed that DOLP is not sensitive to the aerosol
height as the scattering is primarily due to aerosol optical depth.
It is noted that the lower the aerosol layer, the increased absorption in
O2 A band suppresses the aerosol scattering. This explains the increase
of DOLP (and ΔDOLP) in the O2 A band as the aerosol peak height
decreases (closer to the surface). The same patterns can be seen in the
O2 B band (Fig. 2c). The change of DOLP with respect to the aerosol peak height also has wavelength dependence. Indeed, in some absorption wavelengths, the O2 absorption
optical depths can be sufficiently large (∼ 7.7 at 689.78 nm)
that aerosols near the surface have much less chance to interact with the
incident light. Thus, the DOLP differences at these wavelengths are more
sensitive to high-altitude aerosol layers (7–10 km, Fig. 1c). However,
at some other wavelengths, the O2 absorption optical depths (such as
∼ 0.88 at 759.98 nm) are relatively smaller and the light can
reach lower atmosphere. The DOLP differences at these wavelengths are more
sensitive to low-altitude aerosol layers (4–7 km, Fig. 1b).
Figure 2d is the profile of aerosol optical depth for different γ
values, where H is 5 km and the column aerosol optical depth is assumed to be
0.2. Similar to Fig. 2b–c, e–f show the ΔDOLP
for various γ values (with the same H). Consistent with Fig. 2b and c,
ΔDOLP decreases as the surface reflectance increases. Also,
it can be seen that for given surface reflectance, as γ increases,
ΔDOLP decreases. Because O2 absorption coefficient above the
aerosol peak height is smaller than that below the aerosol peak height, the
elevated portion of aerosols above the peak height (due to the increase of
profile half width) has a larger effect on DOLP than the counterpart below
the aerosol peak height, counteracting more positive DOLP from Rayleigh
scattering.
To further evaluate whether the findings revealed in Fig. 2 can be generalized,
we compute the DOLP at 759.98 nm at TOA as a function of H and γ for As
of 0, 0.2 and 0.5, respectively, representing black, bright and highly
bright surfaces (Fig. 3). Top panels of Fig. 3 show the results at the scattering angle
Θ= 120∘ (a nadir view with solar zenith angle of
60∘). Bottom panels correspond to scattering angle Θ= 150∘. These results (and their contrasts with their
counterparts in continuum band; Fig. S1 in the Supplement) confirm that (a) an increase of
surface albedo leads to more depolarization (e.g., decrease the positive
DOLP for Θ= 120∘ and make less negative DOLP for Θ= 150∘) at TOA, and (b) DOLP in O2 A band decreases as the
peak height H increases for the same γ value. Moreover, these results
also suggest that DOLP in O2 A band at TOA has a strong angular
dependence. It can be seen that the DOLP at Θ= 120∘ is
positive for all chosen H and γ. The maximum of DOLP at TOA occurs
when aerosols appear in the near surface. In contrast, DOLP at Θ= 150∘ is found to be negative for all combinations of aerosol
peak height and half width (bottom three panels in Fig. 3). This contrast
can be explained by the different angular dependence of DOLP between
Rayleigh scattering and aerosol scattering. As discussed in Wang et al. (2014),
the DOLP of Rayleigh scattering (Fig. 8 in Wang et al., 2014) is
positive at all scattering angles (with peak value at Θ= 90∘ and 0 at Θ= 180∘); aerosol
particles often yield negative DOLP at all backscattering angles, with the
largest negative DOLP at around Θ= 150∘. Regardless of
angle, however, it is expected that the increase of aloft aerosol
scattering, either through increase of peak height or through increase of
half width or both, can lead to a decrease of DOLP for the same surface
reflectance (Fig. 3).
Calculation of the DFSs
The actual observations inevitably contain instrumental error and
measurement noise. It is critical to properly treat these experimental
errors in the design of retrieval methods. In the following analysis we
assume that aerosol properties including optical depth are known with some
uncertainties, and the parameters to be retrieved are related to
aerosol vertical profile only. Hence, the measurement errors defined here also
include the model errors.
To compare the information content of the DOLP and intensity at TOA for
retrieving aerosol vertical distribution, we calculate the DFSs for aerosol
peak height H at each individual wavelength (or channel at 0.01 nm spectral
resolution across O2 A and B bands) using (a) only radiances in
O2 A band, (b) only DOLP in O2 A band and (c) only DOLP in the
O2 B band. It can be seen in Fig. 4a that the maximum DFS for
using radiance is only about 0.35 at the wavelength near 759.7 nm in the
O2 A band. However, the DFSs for using DOLP are often larger than 0.4,
with maxima values close to unity, in both O2 A (Fig. 4b) and O2
B bands (Fig. 4c). Figure 4 hence suggests that the polarization of light
contains more information on the aerosol peak height than that of intensity.
Contours of the DOLP at TOA as functions of aerosol peak height
and half width at two different scattering angles, Θ= 120∘
(upper panels) and Θ= 150∘ (lower panels), for three
different surface albedo, 0.0, 0.2 and 0.5. The wavelength is 759.98 nm.
Degree of freedom of signal of individual channel within O2
A and B bands, calculated from (a) radiance in O2 A band, (b) DOLP in
O2 A band and (c) DOLP in O2 B band. (d) Sorted degree of freedom
of signal (blue) calculated from radiance in O2 A band, and
corresponding gas absorption optical depth (red) for both O2 A and B
band. (e) and (f) are the same as (d) but calculated from DOLP in O2 A and B
bands, respectively.
Contours of degree of freedom of signals of DOLP for retrieving
aerosol peak height as a function of aerosol peak height H and half width at
half maximum γ at three difference wavelengths in both O2 A and
B bands for different surface albedo, 0.0, 0.2 and 0.5. The gas absorption
optical depths at wavelength 759.98, 762.68 and 689.78 nm are 0.88, 2.73
and 0.34, respectively.
Same as Fig. 5 but for the DFSs for retrieving profile half
width.
The DFS values in Fig. 4d–f are sorted in descending order for three
cases in Fig. 4a–c. The x axis in Fig. 4d–f indicates
the number of spectral wavelengths (or channels). The left-side y axis
indicates DFSs and the right-side y axis represents the corresponding
gaseous absorption optical depth for an individual wavelength. As shown in
Fig. 4d, the gaseous absorption optical depths corresponding to the first
ten maximum DFSs of radiance in the O2 A band are above 5.0. In
contrast, the counterparts for the DFSs of DOLP in O2 A and B bands are
above 1.0 and 0.35, respectively (see Fig. 4e and f). Furthermore, there
are no less than 10 wavelengths (simulated at 0.01 nm interval) with DFS
values close to 1 in DOLP measurements in both O2 A and B bands, but
the DFS values of radiance measurements are always below 0.4. These findings
from Fig. 4 further suggest that DOLP can have more information content
for inferring aerosol peak height H than intensity.
Figure 5 shows the DFS in DOLP measurements for inferring H as a function of aerosol peak height H and half width at half maximum
γ for three wavelengths in both O2 bands for different
surface reflectance values of 0.0, 0.2 and 0.5. We can find that, at some
wavelengths where the O2 absorption optical depths are smaller
relatively (such as 759.98 nm), the DFS values are more influenced by the
change of surface reflectance than at other wavelengths where the O2
absorption optical depths are larger (such as 762.68 nm with optical depth
of 2.7). Indeed, DFS approaches 0 at 762.68 nm for aerosol peak height
less than 2 km. For the same wavelength (e.g., each row in Fig. 5), DFS
values decrease as the surface reflectance increases; for surface
reflectance of 0.5, the DFS is less than 0.5 for aerosol peak height H below
2 km, regardless of γ.
Similarly, we calculate the DFS in DOLP measurements for γ (Fig. 6). It can be seen that although the DFS values are not as sensitive to the
change of surface reflectance as that shown in Fig. 5 for inferring
aerosol peak height, they have remarkable spectral dependence. At some
wavelengths such as 759.98 and 689.78 nm, the DFS values for γ are
larger (than 0.5) for low-level aerosol layer H below 2 km except for γ
less than 1 km; however, at other wavelengths such as 762.68 nm, the DFS
values for γ are more sensitive to higher aerosol layer. While
results in Figs. 5 and 6 are calculated for measurement error of
5 %, a smaller measurement error yields larger DFS values for H and γ
(Figs. S2 and S3) and measurement error of 8 % gives
results similar to those from 5 % (with DFS difference often less than 0.05).
The following question arises: can the two pieces of information for aerosol vertical
profile (e.g., peak height and half width) be retrieved from combined use of
several different wavelengths in both O2 A and B bands? To address this
question, we select the most informative channels following
Rodgers (1996). We first sorted DFS values in DOLP measurements at each
wavelength (or channel at 0.01 nm resolution) in a descending order in both
O2 A and B bands for a baseline aerosol vertical distribution with peak
height H of 8 km (the middle value in 0–16 km range of peak height values
we analyzed), half width at half maximum of 1 km and surface reflectance of
0.2. The first channel is selected with the maximum DFS for H. Subsequently,
DFSs are examined for a combination of the first channel to any of the rest
channels, and the second channel is identified when the largest DFS value is
achieved. More channels are further selected with the same process applied
and thereby, each additional channel leads to the increase of DFS values at
the possibly largest increasing rate. Once the sequence and the group of
these channels are defined for the baseline case, these channels are used to
evaluate DFS for all other cases with different combinations of H and γ.
DFSs of DOLP for retrieving aerosol peak height calculated from
various number (1, 2, 4, 8, 16 and 32) of selected channels in O2 A
band as a function of aerosol peak height and half width. The surface
reflectance is assumed to be 0.2.
Figure 7a shows the DFSs of the first channel DOLP in the O2 A band for
the retrieval of aerosol peak height H (e.g., 764.76 nm, the channel that
maximizes DFS for the baseline vertical profile) as a function of various
combinations of H and γ values. It reveals that aerosol peak height
H can be easily retrieved by using the first channel (regardless of γ
values), given that other aerosol parameters are well characterized, for those
aerosol layers higher than 5.5 km. However, DFS decreases rapidly and is
nearly 0 when most aerosols start to concentrate near the surface (e.g.,
as H and γ values approach from 4 km to 0 or approach the
bottom corner of the panel in Fig. 7a). As expected, DFS value for H
increases as more channels are used (2, 4, 8, 16 and 32 shown in Fig. 7b–f),
especially when the aerosol layer is close the surface. For example,
the white zone with 0 DFS in the bottom left corner in Fig. 7a is
slowly filled up with colors of DFS > 0.1 from Fig. 7b–f.
Even with 32 channels, however, DFS is still smaller than 0.4 when H is
smaller than 2 km (Fig. 7f). Therefore, a greater number of channels or
better DOLP accuracy and measurement accuracy (uncertainty less than 5 %)
are required to gain a sufficient signal for retrieving aerosol distribution
below the boundary layer. It is noted, however, that current measurements of DOLP
often have an uncertainty less than 1 % (and sometimes around 0.2 %) and
measurements of intensity have an uncertainty of 2 % (Zeng et al., 2008).
Therefore, there is still potential in using DOLP in O2 A and B bands to characterize
aerosol distribution even within 2 km above the surface.
Same as Fig. 7 but for DFSs for retrieving profile half width.
Contours of degree of freedom of signals of DOLP for
simultaneously retrieving aerosol profile H and half width at half maximum
γ calculated from various number (2, 4, 8, 16, 32 and 64) of
pre-selected channels in O2 A band as a function of aerosol peak height
and half width. The surface albedo is assumed to be 0.2. The pre-selected
channels are a result of optimization for the case with H of 8 km and γ of 1 km.
Degree of freedom of signal of DOLP for retrieving both aerosol
peak height H and half width at half maximum γ as a function of the
number selected channels from O2 A band only (left column), O2 B
band only (middle column) and from O2 A and B bands together (right
column). The top row is for the cases with H of 8 km and γ of 1 km,
while the bottom row is for H of 5 km and γ of 1 km. See the text
for details.
Similar to Fig. 7, we calculated the DFS values for γ for a
different number of channels in O2 A and B bands and the results are
presented in Fig. 8. The pattern of DFS in Fig. 8 is different from that
in Fig. 7; Fig. 8 has the maxima of DFS in the bottom right corner of
each panel while the latter has the maxima of DFS on the top. In other
words, for larger H, DOLP has less sensitivity to large γ.
Regardless, the physics remains the same: DFS increases when there are more
aerosols in the upper layer (e.g., either lager H or larger γ width
or both). It is interesting to note that DFS seems to increase with altitude
up to about 10–12 km but decreases very quickly above that because the
O2 absorption is much weaker there. As shown in Fig. 8, we find
that using 32 channels can retrieve γ well for most possible
vertical distributions of aerosols, except when aerosols present within a
low-altitude thin layer or highly aloft.
After we calculated the DFS values for a single parameter of aerosol vertical
distribution (either H or γ, respectively) for various number of
selected channels, we calculated the DFS values in DOLP measurements from
various selected channels in the O2 A band for retrieving H and γ
simultaneously. Hence, the maximum DFS value in this kind of analysis is
2. Figure 9 shows these DFS values as a function of H and γ. As we
can see from Fig. 9, the retrievable regions are very limited when only a
few number of channels (< 8) are used. However, with more channels
used, DFS values can approach 2 when aerosol peaks in the middle troposphere
with large γ values. As shown in Fig. 9f, when 64 wavelengths are
used to retrieve H and γ values simultaneously, the retrievable
regions cover most possible vertical distributions of aerosols, except for
aerosols near the surface or aerosols within a very thin layer (e.g.,
γ less than 1 km).
Figure 10a–c show the DFS values for simultaneously retrieving H and
γ with the DOLP in (a) O2 A band, (b) O2 B band and (c) the combination
of A and B bands. It is evident that the combined use of
O2 A and B bands (Fig. 10c) can significantly enhance the
information content compared to the use of only the A (Fig. 10a) or B band
(Fig. 10b) and can decrease the number of required channels for a
sufficiently accurate retrieval. Figure 10c shows that 10 channels from
O2 A and B bands, as listed in Table 2, are sufficient to resolve H and
γ for the cases studied (where H and γ are assumed to be 5.0
and 1.0 km, respectively).
The wavelength that is used to define the baseline case and therefore the
sequence of channels does have effects on the final results, but the overall
conclusion remains the same. Results similar to Figs. 10a–c and 9
but using H of 5 km instead of 8 km to select the channels as the baseline
case are presented in Figs. 10d–e and 11, respectively. The smaller
the H, the larger number of channels is needed to make DFS values for H and
γ both reach unity (e.g., as seen from the contrast between Fig. 10a–c and d–f). However, as expected, for the group of channels that are
optimally selected for smaller H (such as 5 km), they also have more
information characterizing H for cases where H is also lower (e.g., 3 km).
For example, only 16 channels are needed in Fig. 11 (in which baseline
case has H of 5 km) to make total DFS for H and γ up to 1 for the case
with H of 2.5 km and γ of 1.8 km; in contrast, in Fig. 9 (in which
baseline case has H of 8 km), the first 16 channels yield less than 0.2 DFS
values for the same case with H of 2.5 km and γ of 1.8 km. For the
completeness, we also show the first 10 channels that are optimal for
characterizing H of 5 km and γ of 1 km in Table S1 in the Supplement. Also shown in Fig. S4 are results similar to those in Fig. 9, but using the channels for the baseline case with H of 1 km and γ of 1 km. Overall, from all
the cases we examined, we found that DOLP in O2 A and B bands have rich
and fine-structure information for retrieving a wide range of H and γ
values. Such fine-structure information can only be obtained with
measurements of DOLP taken at hyperspectral resolution. DFS needed for
simultaneous retrieval of H and γ for high-lofted, or near-surface (with H of 1 km, for example, in Fig. S4), aerosol profiles can be obtained from a combined use of DOLP
measurements at ∼ 10–100 O2 A and B absorption
wavelengths, depending on the specific values of H. Future studies are needed
to reveal how the spectral resolution of DOLP measurements may affect the
retrieval accuracy of H and γ.
Same as Fig. 9 except that the pre-selected channels are a
result of optimization for H of 5 km and γ of 1 km.
The first ten wavelengths selected for maximum contribution to the
total information content from O2 A and B bands for the case with H of 8
km and γ of 1 km.
No.
Wavelength
DFS*
(nm)
H
γ
1
762.68
0.9998
0.0002
2
761.04
0.9998
0.0035
3
687.84
1.0000
0.9453
4
692.60
1.0000
0.9720
5
687.34
1.0000
0.9805
6
760.82
1.0000
0.9806
7
692.08
1.0000
0.9841
8
694.94
1.0000
0.9866
9
760.96
1.0000
0.9866
10
686.98
1.0000
0.9882
* DFS values for both peak height and half width corresponding to the channel
number n are the result of information content analysis from a combined use
of the first channel to channel number n.
The results presented above are based on the calculations that assume
spherical dust particles. As a comparison, we also investigate other types
of aerosols listed in Table 1, including soot (absorbing) and sulfate
(non-absorbing) aerosols. Figure 12 shows the comparison of the DOLP at TOA
and their corresponding DFS values for three types of aerosols, dust,
sulfate and soot, as functions of aerosol peak height and half width at
the scattering angle 120∘. The wavelength is 759.98 nm. The
figure shows that highly absorbing aerosols (such as soot; Fig. 12c) can
lead to a smaller decrease of DOLP (or larger DOLP) as compared to the
counterparts by less absorbing (such as dust; Fig. 12a) and pure
scattering aerosols (such as sulfate; Fig. 12b); this can be understood
because absorbing aerosols suppress the multiple scattering at the
atmosphere. Correspondingly, the information content in DOLP (in terms of
DFS values) is generally smaller for characterizing vertical profile of
absorbing aerosols (as shown by the contrast between Fig. 12f and d–e and the
contrast between Fig. 12i and g–h). However,
while these difference of DFS exist between various type of aerosols because
of their different physical and optical properties, the patterns of DFS
variations generally appear the same regardless of aerosol type,
revealing that DFS is higher when aerosols are concentrated at high levels
due to larger H, larger γ or both. Therefore, a combination
of DOLP measurements in O2 A and B bands enables the retrieval of
aerosol vertical distribution for a wide-range of aerosol types. The results
between dust and sulfate particles remain similar, except that DFS values
for both H and γ are slightly reduced for H < 4 km and
γ > 1 km (e.g., from the contrast between Fig. 12d and e and between Fig. 12g and h).
However, for soot particles, the DFS
values for H are largely reduced except for high-elevated cases with
smallerγ (Fig. 12f). These results suggest that the information
content in DOLP in O2 A and B band decreases with aerosol absorption.
Hence, a combination of using both UV radiance and DOLP in O2
absorption band may enhance the information content for characterizing
profiles of absorbing aerosols.
Contours of the DOLP at TOA and DFSs for three types of aerosols,
dust, sulfate and soot, as functions of aerosol peak height and half width
at the scattering angle 120∘. The wavelength is 759.98 nm.