We present a comparison of 1064 nm aerosol optical depth (AOD) and aerosol extinction profiles from the Cloud-Aerosol Transport System (CATS) level 2 aerosol product with collocated Aerosol Robotic Network (AERONET) AOD, Moderate Imaging Spectroradiometer (MODIS) Aqua and Terra Dark Target AOD and Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) AOD and extinction data for the period of March 2015–October 2017. Upon quality-assurance checks of CATS data, reasonable agreement is found between aerosol data from CATS and other sensors. Using quality-assured CATS aerosol data, for the first time, variations in AODs and aerosol extinction profiles are evaluated at 00:00, 06:00, 12:00 and 18:00 UTC (and/or 00:00, 06:00, 12:00 and 18:00 local time or LT) on both regional and global scales. This study suggests that marginal variations are found in AOD from a global mean perspective, with the minimum aerosol extinction values found at 18:00 LT near the surface layer for global oceans, for both the June–November and December–May seasons. Over land, below 500 m, the daily minimum and maximum aerosol extinction values are found at 12:00 and 00:00/06:00 LT, respectively. Strong diurnal variations are also found over north Africa, the Middle East and India for the December–May season, and over north Africa, south Africa, the Middle East and India for the June–November season.
Aerosol measurement through the Sun-synchronous orbits of Terra and Aqua by nature encourages a larger-scale daily average point of view. Yet, we know that pollution (e.g., Zhao et al., 2009; Tiwari et al., 2013; Kaku et al., 2018), fires and smoke properties (e.g., Reid et al., 1999; Giglio et al., 2003; Hyer et al., 2013) and dust (e.g., Mbourou et al., 1997; Fiedler et al., 2013; Heinold et al., 2013) can exhibit strong diurnal behavior. Sun-synchronous passive satellite aerosol observations from the solar spectrum only provide a small sampling of the full diurnal cycle. Geostationary sensors such as the Advanced Himawari Imager (AHI) on Himawari 8 (Yoshida et al., 2018) and Advanced Baseline Imager on GOES-16/17 (Aerosol Product Application Team of the AWG Aerosols/Air Quality/Atmospheric Chemistry Team, 2012) satellites, while an improvement over their predecessors, must overcome the broader range of scattering and zenith angles (Wang et al., 2003; Christopher and Zhang, 2002) with no nighttime retrievals. Aerosol Robotic Network (AERONET; Holben et al., 1998) based Sun photometer studies improve sampling but until very recently with the development of a prototype lunar photometry mode, are also limited to daylight hours. The critical early morning and evening are largely missed in solar-observation-based approaches.
Observations of the diurnal variations of aerosol properties are needed for improving chemical transport modeling, geochemical cycles and ultimately climate. The measurement of diurnal variations of aerosol properties resolved in the vertical is especially crucial for visibility and particulate matter forecasts. Indeed, the periods around sunrise and sunset show significant near-surface variability that is difficult to detect with passive sensors. While lidar data from Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) provide early afternoon and morning observations, two temporal points and a 16 d repeat cycle are insufficient to evaluate the critical morning and evening hours where many key aerosol life-cycle processes take place.
Some of the limiting factors in previous studies can be addressed by the
Cloud-Aerosol Transport System (CATS) lidar that flew aboard the
International Space Station (ISS) from 2015 to 2017 (McGill et al., 2015).
The ISS's precessing orbit with a 51.6
Use of CATS has its own challenges. Most importantly, CATS retrievals must cope with variable solar noise around the solar terminator where we expect some of the strongest diurnal variability to exist. Further, CATS lost its 532 nm channel early in its deployment, leaving only a 1064 nm channel functioning. The availability of only one wavelength limited the CATS cloud–aerosol discrimination algorithm, which can cause a loss of accuracy compared to CALIPSO, which has two wavelengths. This deficiency is in part overcome by using the feature type score (CATS algorithm theoretical basis document). Using 2 years of observations from CATS, in this paper, we focus on understanding of the following questions: how well do CATS-derived aerosol optical depth (AOD) and aerosol vertical distributions compare with aerosol properties derived from other ground-based and satellite observations such as AERONET, MODIS and CALIOP? Do differences exhibit a diurnal cycle? What are the diurnal variations of aerosol optical depth on a global domain? What are the diurnal variations of aerosol vertical distribution on both regional and global scales?
Four datasets, including ground-based AERONET data, as well as satellite-retrieved aerosol properties from MODIS and CALIOP, are used for intercomparison with AOD and aerosol vertical distributions from CATS. Upon thorough evaluation and quality-assurance procedures, CATS data are further used for studying diurnal variations of AOD and aerosol vertical distributions for the period of March 2015–October 2017.
CATS level 2 (L2) version 3-00 5 km aerosol profile products
(L2O_D-M7.2-V3-00_05kmPro, L2O_N-M7.2-V3-00_05kmPro) were used in this study for nearly the
entire period of CATS operation on the ISS (
CATS data are quality assured following a manner similar to Campbell et al. (2012), which was applied to CALIOP. Quality assurance (QA) thresholds (including extinction quality control (QC) flag, feature type score and uncertainty in extinction coefficient) are
listed below:
Extinction_QC_Flag_1064_Fore_FOV Feature_Type_Fore_FOV Extinction_Coefficient_Uncertainty_1064_ Fore_FOV
Extinction was also constrained using a threshold as provided in the CATS
data catalog (Extinction_Coefficient_1064_Fore_FOV
NASA's CALIOP is an elastic backscatter lidar that operates at both 532
and 1064 nm wavelengths (Winker et al., 2009). Being a part of the A-Train
constellation (Stephens et al., 2002), CALIOP provides both day- and
nighttime observations of Earth's atmospheric system, at a Sun-synchronous
orbit, with a laser spot size of around 70 m and a temporal resolution of
L2_05kmAProf data are available at 5 km along-track horizontal resolution
and include aerosol retrievals at both 532 and 1064 nm
wavelengths. The vertical resolution is 60 m near the surface, degrading to 180 m above 20.2 km in mean sea level (MSL) altitude. As only 1064 nm CATS data are used in this
study as mentioned above, likewise only those CALIOP parameters relating to
1064 nm are used in this study (Vaughan et al., 2019; Omar et al., 2013).
Note that as suggested by Rajapakshe et al. (2017), lower signal-to-noise
ratio (SNR) and higher minimum detectable backscatter are found for the
CALIOP 1064 nm data in comparison with the CALIOP 532 nm data. Also, the
CALIOP aerosol layers are detected at 532 nm and the 1064 nm extinction is
only computed for the bins within these layers. This may introduce a bias
for aerosol above-cloud studies. The uncertainties in retrieved aerosol
extinction, as suggested by Young et al. (2013), are around 0.05–0.5 km
In this study, Extinction_Coefficient_1064 and
Column_Optical_Depth_Tropospheric_Aerosols_1064 are used for CALIOP
extinction and AOD retrievals, respectively (Vaughan et al., 2019; Omar et
al., 2013). As with the CATS data, CALIOP data are quality assured following
the quality-assurance steps as mentioned in a few previous studies (e.g.,
Campbell et al., 2012; Toth et al., 2016, 2018). These QA thresholds are
listed below:
Extinction_QC_Flag_1064 Atmospheric_Volume_Description Extinction_Coefficient_Uncertainty_1064
Furthermore, as in Campbell et al. (2012), only those profiles with AOD > 0 were retained in order to avoid profiles composed of only
retrieval fill values. Extinction was also constrained to the nominal range
provided in the CALIOP data catalog (Extinction_1064
Moderate Resolution Imaging Spectroradiometer (MODIS) Collection 6.1 Aqua and Terra Dark Target over-ocean AOD data (Levy et al., 2013) were used
for comparison to CATS AOD. The data field of Effective_Optical_Depth_Best_Ocean was
used, and only those data flagged as “good” or “very good” by the
Quality_Assurance_Ocean runtime QA flags were
selected for this study, similar to Toth et al. (2018). Because MODIS does
not provide AOD in the 1064 nm wavelength, AOD retrievals from 860 and 1240 nm spectral channels are used to logarithmically interpolate AODs at 1064 nm. Here, we assume the Ångström exponent value, computed using
instantaneous AOD retrievals at the 860 and 1240 nm, remains the same for
the 860 to 1064 nm wavelength range, similar to what has been suggested by
Shi et al. (2011, 2013). Mean and standard deviation of Ångström
exponents using this method were 0.69 and 0.55, respectively. Only totally
cloud-free (or cloud fraction equal to zero) retrievals, as indicated by the
Aerosol_Cloud_Fraction_Ocean
parameter, are used. While the uncertainties in MODIS infrared (e.g., 1240 nm) retrievals are less explored, the reported over-ocean MODIS DT AOD
retrievals are (
By measuring direct and diffuse solar energy, AERONET observations are used
for retrieving AOD and other ancillary aerosol properties such as size
distributions (Holben et al., 1998). AERONET data are considered as the
ground truth for evaluating CATS retrievals in this study. Only cloud-screened and quality-assured version 3 level 2 AERONET data at the 1020 nm
spectrum are selected and are used for intercomparison with CATS AOD
retrievals at the 1064 nm wavelength. AERONET does not have specific
guidance on error in the 1020 nm channel, as it is known to have some
thermal sensitivities. However, they do report significantly more confidence
in version 3 of the data, which has temperature correction (Giles et al.,
2019). Error models are ongoing, and for this study we assume double the
root mean square error (RMSE) or
Note that most evaluation efforts for passive and active sensor AOD retrievals are focused on the visible spectrum and the performance of AOD retrievals at the 1064 nm channel is less explored. Thus, in this subsection, the performance of over-land and over-ocean CATS AOD retrievals is compared against AERONET and C6.1 over-ocean MODIS Dark Target (DT) aerosol products. In AOD-related studies, CATS- and CALIOP-reported AOD values are used. However, only AOD values with corresponding aerosol vertical extinction that meet the QA criteria as mentioned in Sect. 2.1 and 2.2 were used. CATS-derived aerosol extinction vertical distributions are also cross-compared against collocated CALIOP aerosol extinction vertical distributions.
As the initial check, CATS data from nearly the entire mission (March 2015–October 2017) were spatially (within 0.4
Collocated AERONET 1020 nm AOD vs. CATS 1064 nm AOD
As shown in Fig. 1a, without quality-assurance procedures, high spikes in
CATS AOD of above 1 (1064 nm) can be found for collocated AERONET data with
AOD less than 0.4 (1020 nm). Still, those high spikes in CATS AOD are much
reduced compared to the V2-01 CATS aerosol products (e.g., a similar plot to
Fig. 1 is included in Appendix A with the use of V2-01 CATS aerosol
data). Upon completion of the QA steps as outlined in Sect. 2.1, a
reasonable agreement is found between quality-assured CATS (1064 nm) vs.
AERONET (1020 nm) AODs with a correlation of 0.65 (Fig. 1b). Comparing
Fig. 1a with b, with the loss of only
To examine over-ocean performance, column-integrated CATS AODs are
intercompared with collocated MODIS C6.1 Terra and Aqua DT over-ocean AODs,
interpolated to 1064 nm. Over-ocean MODIS C6.1 DT data are selected due to
the fact that higher accuracies are reported for over-ocean vs. over-land
MODIS DT AOD retrievals (Levy et al., 2013). In addition, compared to over-land MODIS DT data, which provide AOD retrievals at three discrete
wavelengths (0.47, 0.55 and 0.65
Collocated MODIS C6.1
MODIS and CATS AOD retrievals are collocated for the study period of March 2015–October 2017 (Fig. 2). Pairs of CATS and MODIS data were first selected
for both retrievals that fall within
In the previous two sections, AODs from CATS were intercompared with
retrievals from passive-based sensors such as MODIS and AERONET. In this
section, AOD data from CALIOP, which is an active sensor, are evaluated
against AOD retrievals from CATS. Note that despite difference in
instrumental designs, CALIOP and CATS are both elastic backscatter lidars.
Again, for each collocation incident, pairs of CALIOP and CATS data are
selected in which both retrievals fall within
Figure 3a shows the comparison of CATS and CALIOP AODs for all collocated pairs including both day- and nighttime. A reasonable correlation of 0.74, with a slope of 0.73, is found for a total of 2762 collocated data pairs. Further breaking down the comparison into day and night cases, a much better agreement is found between the two datasets during nighttime with correlations of 0.83 and 0.81 for over-ocean and over-land cases, respectively. In comparison, a lower correlation of 0.64, with a slope of 0.49, is found between the two datasets, using over-land daytime data only, for a total of 170 collocated pairs. Correspondingly, a lower correlation of 0.55, with a slope of 0.57, is found between the two datasets, using over-ocean daytime data only, for a total of 1180 collocated pairs. This result is not surprising, as daytime data from both CALIOP and CATS are nosier due to solar contamination (e.g., Omar et al., 2013; Toth et al., 2016).
Collocated CALIOP 1064 nm AOD vs. CATS 1064 nm AOD with CATS QA
applied for
Note that based on the slopes of the regression lines shown in Figs. 1–3,
AODs retrieved by CATS are less than AERONET, CALIOP and MODIS Aqua DT AOD
retrievals. As shown in Table 1, however, for the 1-to-1 collocated
datasets, mean CATS AODs (1064 nm) are
Descriptive statistical properties between collocated CATS and
AERONET, CALIOP and MODIS AOD retrievals. Here, SD indicates
standard deviation of AOD and
Also note that as suggested by Omar et al. (2013), the choices of spatial
and temporal collocation windows have an effect on collocation results.
Thus, we repeated the exercises in Figs. 1–3 by doubling the spatial and
temporal collocation windows as well as reducing the collocation windows by
half. The descriptive statistics of this sensitivity study are included in
Table 2. While the number of collocated data pairs is drastically affected
by the spatial and temporal collocation window sizes, less significant
changes are found in descriptive statistics such as mean, median and
standard deviations of AODs, as well as slopes and correlation values. The
slope of MODIS Aqua DT and CATS AODs, however, seems sensitive to changes in
collocation methods. Changes in slope of 0.61 to 0.78 are found for the
change of temporal collocation window from 15 to 60 min with a
fixed spatial collocation window of 0.4
Sensitivity study of descriptive statistical properties between
collocated CATS and AERONET, CALIOP and MODIS Aqua AOD retrievals by varying
spatial and temporal collocation windows. Here, SD indicates standard
deviation of AOD and
Still, larger discrepancies between CATS and CALIOP AODs during daytime
indicate that both sensors are susceptible to solar contamination. To
overcome solar contamination and more accurately detect aerosol layers,
CALIOP and CATS data products are averaged up to 80 and 60 km,
respectively. Noel et al. (2018) found that the feature type score can be
used for cloud screening throughout the diurnal envelope of solar angles. To
further evaluate impact of the solar-contamination-introduced bias in the
diurnal analysis in aerosol detection or products, CATS AODs are evaluated
as a function of local time. For each CATS observation of a given location
and UTC time, the associated local time is computed by adding the UTC time
by 1 h per 15
CATS 1064 nm AOD
Figure 4c shows the difference between AERONET (1020 nm) and CATS (1064 nm)
AOD (
One advantage of CATS is its ability to retrieve both column-integrated AOD
and vertical distributions of aerosol extinction. Therefore, in this
section, extinction profiles from CATS are compared with that from CALIOP.
Again, similar to Sect. 3.1.3, collocated profiles for CATS and CALIOP
are first found for both retrievals that are close in space and time (within
The CATS cloud–aerosol discrimination (CAD) algorithm is a multi-dimensional probability density function (PDF) technique that is based on the CALIPSO algorithm (Liu et al., 2009). The PDFs were developed based on cloud physics lidar (CPL) measurements obtained during over 11 field campaigns and 10 years. As shown in Fig. 5e, a reasonable agreement is found between CATS V3-00 aerosol extinction with CALIOP for over land. However, CATS overestimates aerosol extinction around 1 km compared to CALIOP over ocean (Fig. 5d). This can also be seen on a plot of the difference between CATS and CALIOP 1064 nm extinction for all collocated profiles, included in Fig. 5f, where there is an overall positive difference around 1 km.
CATS and CALIOP vertical profiles of 1064 nm extinction for
Due to the precessing orbit of the ISS, the CATS sampling is irregular and
very different compared to the Sun-synchronous orbits of the A-Train
sensors. These orbital differences between CATS and CALIOP make comparing
the data from these two sensors challenging since they are fundamentally
observing different locations of the Earth at different times. Thus, we
should not expect the extinction profiles and AOD from these two sensors to
completely agree. Additionally, there are other algorithm and instrument
differences that can lead to differences in extinction coefficients and AOD.
Over land where dust is the dominant aerosol type, differences in lidar
ratios between the two retrieval algorithms (CATS uses 40 sr while CALIOP
uses 44 sr) can cause CATS extinction coefficients that are up to 10 %
lower than CALIOP, potentially explaining the higher CALIOP extinction
values in Fig. 5e. Over ocean, especially during daytime, differences in
CATS and CALIOP lidar ratios for marine and smoke aerosols can introduce a
difference between CATS and CALIOP extinction coefficients (Fig. 5d).
These differences in over-ocean data (Fig. 5d) could also be attributed to
differences in CATS and CALIOP 1064 nm backscatter calibration. For example,
Pauly et al. (2019) reports that CATS attenuated total backscatter is about
19.7 % lower than Polly
Also, differences in the lowest 250 m between CATS and CALIOP extinction profiles are observable, which are due to how the instrument algorithms detect the surface and near-surface aerosols. Both the CATS and CALIOP feature detection algorithms create a gap between the surface and near-surface aerosol base altitude, despite the possible presence of aerosols in this altitude region. CALIOP has an aerosol base extension algorithm that is designed to (1) detect scenarios when aerosols are present in the bins just above the surface and (2) extend the near-surface aerosol layer base down to the surface (Tackett et al., 2018). However, CATS does not use such an algorithm, so false regions of “clear air” exist between the surface and near-surface aerosol layers.
Mean AOD (1064 nm) by season for
Vertical profiles of collocated CATS and CALIOP extinction for daytime-only
profiles and nighttime-only profiles are shown in Fig. 5b and c,
respectively. Compared to a total collocated pair count of 2748 in the
overall profile data, day and night profiles have 1311 and 1437 collocated
pairs, respectively. Again, the shapes of the CATS and the CALIOP
extinction vertical profiles are similar for all three cases, despite the
abovementioned offsets in altitude. Figure 5d and e show the mean of those
extinction profiles which occurred over water and over land, as defined by
the CATS surface type flag. Again, in both cases, CATS and CALIOP have similar
shapes in their vertical extinction profiles. The vertical structure of
over-water extinction is also very similar to that of all profiles, day and
night, which is perhaps not surprising as water profiles made up 2142 of
2748 (
Mean CATS AOD (1064 nm) by season for
Using the quality-assured CATS data, seasonal variations as well as diurnal variations in CATS AODs are derived in this section. Diurnal variations in the vertical distributions of CATS aerosol extinction are also examined at both global and regional scales.
Figure 6a–b show the spatial distributions of CATS AODs at the 1064 nm
spectral channel for boreal winter–spring (December–May, DJFMAM) and boreal
summer–fall (June–November, JJASON) seasons, for the period of March 2015–October 2017. To construct Fig. 6a and b, quality-assured CATS AODs are first
binned on a 5
Seasonal mean AOD (1064 nm) binned by every 6 h for
In the DJFMAM season, significant aerosol features are found over north Africa, the Middle East, India and eastern China. For the JJASON season, besides the abovementioned regions, aerosol plumes are also observable over south Africa, related to summer biomass burning of the region (e.g., Eck et al., 2013). The seasonal-based spatial distributions of AODs from CATS, although reported at the 1064 nm channel which is different from the 550 nm channel that is conventionally used, are similar to some published results (e.g., Lynch et al., 2016).
Maximum minus minimum mean seasonal AOD (1064 nm) for
For comparison purposes, Fig. 6c–d show similar plots to Fig. 6a–b but with the use of CALIOP AOD at the 1064 nm spectral channel. Note that those are climatological means rather than pairwise comparisons. While patterns are similar in general, at regions with peak AODs of 0.4 or above for CALIOP, such as north Africa for the DJFMAM season and north Africa, the Middle East and India for the JJASON season, much lower AODs are found for CATS. However, in some other regions, such as over south Africa for the JJASON season, higher CATS AOD values are observed. A table of mean AOD across each of these regions as well as over the globe (within the latitude range where CATS has data) has been included for reference (Tables 3). Figure 6e and f show similar spatial plots to Fig. 6a and b but with the use of MODIS Aqua AODs from the DT products (using all available MODIS DT retrievals that passed QA steps as described in Sect. 2.3). For the MODIS Aqua DT products, aerosol retrievals at the shortwave infrared channels are only available over oceans, and thus Fig. 6e–f show only over-ocean retrievals. Again, while general AOD patterns look similar, discrepancies are also visible, such as over the coast of southwest Africa for the JJASON season and over the west coast of Africa for the DJFMAM season. Those discrepancies may result from biases in each product, but it is also possibly due to the differences in satellite overpass times, as CALIOP provides early morning and afternoon overpasses, and MODIS Aqua has an overpass time after local noon, while CATS is able to report atmospheric aerosol distributions at multiple times during a day.
CALIOP and CATS mean AODs/AOD standard deviations for regions as
highlighted in Fig. 6 and globally within
Similar to Fig. 6a and b, Fig. 7a and b show the spatial distribution of CATS AODs but for CATS extinction values that are below 1 km above ground level (a.g.l.) only, for the DJFMAM and JJASON seasons, respectively. Figure 7c and d show the CATS mean AOD plots for extinction values from 1 to 2 km a.g.l., while Fig. 7e and f show CATS mean AOD for extinction values above 2 km a.g.l.. For the DJFMAM season, elevated aerosol plumes with altitude above 2 km a.g.l. are found over north Africa. For the JJASON season, elevated dust plumes (> 2 km a.g.l.) are found over the north African and the Middle Eastern regions, while elevated smoke plumes are found over the west coast of south Africa where above-cloud smoke plumes are often observed during the northern hemispheric summer season (e.g., Alfaro-Contreras et al., 2016).
CATS has a non-Sun-synchronized orbit, which enables measurements at nearly
all solar angles. Thus, we also constructed 5
Still, strong diurnal variations with the maximum averaged diurnal AOD changes of above 0.10 can be observed for regions with significant aerosol events such as north Africa, the Middle East and India for the DJFMAM season and north Africa, south Africa, the Middle East and India for the JJASON season, as illustrated in Fig. 9. Note that Fig. 9a shows the maximum minus minimum seasonal mean AODs for the four different times, as shown in Fig. 8a, c, e, g. Similarly, Fig. 9b shows the maximum minus minimum seasonal mean AODs for the four different times as shown in Fig. 8b, d, f, h. Interestingly but not unexpectedly, regions with maximum diurnal variations match well with locations of heavy aerosol plumes as shown in Figs. 6 and 8.
Geographic ranges, height above ground level of maximum extinction, diurnal extinction range at height of maximum extinction and time (local) of peak extinction for the boxed red regions in Fig. 6 and vertical profiles shown in Figs. 12 and 13. Note that only the JJASON season is analyzed for the South Africa region.
Using quality-assured CATS-derived aerosol vertical distributions, mean global CATS extinction vertical profiles are also generated as shown in Fig. 10. Similar to steps as described in Sect. 3.2.1, CATS extinction profiles are binned into 00:00, 06:00, 12:00 and 18:00 UTC times based on the closest match in time for the JJASON and DJFMAM seasons. Figure 10a shows the daily averaged CATS extinction profiles on a black line, and 00:00, 06:00, 12:00 and 18:00 UTC averaged on blue, green, yellow and red lines, respectively, for the DJFMAM season. A similar plot is shown in Fig. 10d for the JJASON season. CATS extinction profiles for the daily average as well averages for the four selected times are similar, suggesting that minor temporal variations in CATS extinctions can be expected for global averages.
Global mean 6 h vertical profiles of CATS 1064 nm extinction
for
Those global averages are dominated by CATS profiles from global oceans
(Fig. 10b and e), which also have small diurnal variations, as
However, if we examine the mean global CATS extinction vertical profiles with respect to local time, as shown in Fig. 11, some distinct features appear. For example, Fig. 11a and d suggest that, on global average, the minimum aerosol extinction below 1 km is found for 18:00 local time (LT), for both JJASON and DJFMAM seasons. Similar patterns are also observed for over global oceans. However, for over-land cases, for both seasons, the minimum and maximum aerosol extinction below 500 m is found for 12:00 and 00:00/06:00 LT.
Global mean 6 h (00:00, 06:00, 12:00 and
18:00 LT) vertical profiles of CATS 1064 nm extinction for
DJFMAM 6 h average (00:00, 06:00, 12:00
and 18:00 LT) vertical profiles of CATS 1064 nm for locations shown in Fig. 6a;
In this section, the diurnal variations of aerosol vertical distributions are studied as a function of local solar time for selected regions with high mean AODs as highlighted in Fig. 6. Note a near-1-to-1 transformation can be achieved between UTC and local solar time. Also, as learned from the previous section, aerosol features are likely to have a local time dependency. A total of four regions, including north Africa, the Middle East, India and northeast China, which show significant seasonal mean AODs in Fig. 6, are selected for the DJFMAM season (Fig. 12). For the JJASON season (Fig. 13), in addition to the abovementioned four regions, the south African region is also included due to biomass burning in the region during Northern Hemisphere summertime. The latitude/longitude boundary of each selected region is described in Table 4. Regional-based analyses are also conducted for four selected regions for the DJFMAM season and five selected regions for the JJASON season at four local times: 00:00 (midnight), 06:00, 12:00 and 18:00 LT, using quality-assured CATS profiles. Generally, the maximum diurnal change in aerosol extinction is found at the altitude of below 1 km for all regions as well for both seasons. Also, larger diurnal variations in vertical distributions of aerosol extinction are found for the JJASON season, in comparison to the DJFMAM season, while regional-based differences are apparent.
JJASON 6 h average (00:00, 06:00, 12:00
and 18:00 LT) vertical profiles of CATS 1064 nm for locations shown in Fig. 6b;
For the north African region, the dominant aerosol types are dust and smoke
aerosol for the DJFMAM season and dust for the JJASON season (e.g., Remer et
al., 2008). Interestingly, the maximum aerosol extinction below 500 m is
found at 06:00 LT for the DJFMAM season. While for the JJASON season, the
maximum aerosol extinctions are found at 0:00/06:00 LT for the 100–500 m
layer, with a significant
For the Middle Eastern region, for the JJASON season, a daily maximum in
aerosol extinction of
For the India region, for the JJASON season, a large peak in aerosol
extinction of up to 10 % higher than daily mean is found at 06:00 LT below
500 m. The minimum aerosol extinction is found at 12:00/18:00 LT for the
layer below 500 m and is overall
For the northeast China region, a significant peak is found at the 500 m–1 km layer for local afternoon (18:00 LT) for the DJFMAM season. A similar feature is also found for the JJASON season, while the peak extinction for the JJASON season happens at 06:00 LT for the aerosol layer below 500 m. Lastly, for the south African region, biomass burning aerosols are prevalent during the summertime, and thus only the JJASON season is analyzed. As shown in Fig. 13b, below 500 m in altitude, lower extinction values are found for local afternoon (18:00 LT) and higher extinction values are found for local morning or early morning (00:00 and 06:00 LT).
Using CALIOP, MODIS and AERONET data, we evaluated CATS-derived AODs as well
as vertical distributions of aerosol extinctions for the study period of
March 2015–October 2017. CATS data (at 1064 nm) were further used to study
variations in AODs and aerosol vertical distributions diurnally. We found the following:
Quality-assurance steps are critical for applying CATS data in aerosol-related applications. With a less than 2 % data loss due to QA steps, an
improvement in correlation from 0.51 to 0.65 is found for the collocated
CATS and AERONET AOD comparisons. Using quality-assured CATS data,
reasonable agreement is found between CATS-derived AODs and AODs from
CALIOP, MODIS Aqua DT and MODIS Terra DT at the same local times, with
correlations of 0.74, 0.74 and 0.72, respectively. While the averaged vertical distributions from CATS compare reasonably well
with that from CALIOP, differences in peak extinction altitudes are present.
This may due to sampling difference as well as algorithm and instrument
differences such as different lidar ratios used. From the global mean perspective, minor changes are found for AODs at four
selected times, namely 00:00, 06:00, 12:00 and 18:00 UTC. Yet, noticeable diurnal
variations in AODs of above 0.10 (at 1064 nm) are found for regions with
extensive aerosol events, such as over north Africa, the Middle East and India
for the DJFMAM season, and over north and south Africa, India and the Middle
East for the JJASON season. From the global mean perspective, changes are less noticeable for the
averaged aerosol extinction profiles at 00:00, 06:00, 12:00 and 18:00 UTC. Yet, if the
study is repeated with respect to local time, a peak in aerosol extinction
is found for local noon (12:00 LT) for the DJFMAM season and the minimum
value in aerosol extinction is found at 18:00 LT for both the JJASON
and DJFMAM seasons. While the over-water aerosol vertical distributions are
similar to the global means, for over-land cases, the minimum and maximum
extinctions are found at local noon (12:00 LT) and local morning or early
morning (06:00 and 00:00 LT) for the layer below 500 m for both seasons. Larger diurnal variations are found in regions with heavy aerosol plumes
such as north and south (summer season only) Africa, the Middle East, India
and eastern China. In particular, aerosol extinctions from 06:00 LT over
north Africa are Still, readers should be aware that AOD retrievals at the 1064 nm are less
sensitive to fine-mode aerosols such as smoke and pollutant aerosols
compared to coarse-mode aerosols such as dust aerosols (e.g., Dubovik et al.,
2000). Thus, an investigation of diurnal variations of aerosol properties at
the visible channel may be also needed for a future study.
This paper suggests that strong regional diurnal variations exist for both AOD and aerosol extinction profiles. Still, at present, these conclusions are tentative and will remain so until a comprehensive analysis of the CATS calibration accuracy and stability is completed. These results demonstrate the need for global aerosol measurements throughout the entire diurnal cycle to improve visibility and particulate matter forecasts as well as studies focused on aerosol climate applications.
All data used in this study are publicly available. The CATS (
Collocated AERONET 1020 nm AOD vs. CATS 1064 nm AOD
JZ, JSR and LL designed the study. LL worked on data processing for the project. JEY guided LL on data processing. The manuscript was written with inputs from all coauthors.
The authors declare that they have no conflict of interest.
This article is part of the special issue “Holistic Analysis of Aerosol in Littoral Environments – A Multidisciplinary University Research Initiative (ACP/AMT inter-journal SI)”. It is not associated with a conference.
We thank the NASA AERONET team for the AERONET data used in this study. CATS and CALIOP data were obtained from the NASA Langley Research Center Atmospheric Science Data Center. MODIS aerosol products were obtained from the NASA Goddard Space Flight Center’s MODAPS site. We thank Mark Vaughan and two other anonymous reviewers for their constructive suggestions and comments.
This research has been supported by the ONR (grant no. N00014-16-1-2040), NASA (grant no. NNX17AG52G), NASA NESSF (grant no. NNX16A066H) and the Office of Naval Research (codes 322 and 33).
This paper was edited by S. D. Miller and reviewed by Mark Vaughan and two anonymous referees.