Figure 7. Spatial
distributions of color composite (RGB: Bands 4–3–2) images and
corresponding AOD retrievals at 550 nm (30-m resolution) for full-scene
and zoomed-in core-region views (areas outlined by red boxes) over two
typical dark-surface regions: (a-c) Denver, USA and (d-f) Madrid, Spain.
Identified unsuitable pixels for aerosol retrievals (e.g., clouds and
snow/ice) are in black in the two rightmost mages, and the top and
bottom annotations indicate acquisition times (mm/dd/yyyy, where yyyy =
year, mm = month, and dd = day) and orbital records (path-row) of the
Landsat images.
Figure 8 displays true-color
images and corresponding AOD retrievals (at 550 nm) from two regions
characterized by bright surfaces: Beijing, China, and the Sahara Desert,
on different dates. Beijing, a
typical city with a high population density, exhibits complex surface
structures and faces substantial air pollution. A comparison of
retrievals from different periods clearly indicates significant
variations in pollution levels, ranging from clear to highly polluted
conditions (Figure 8a-c).
Spatially, a noticeable trend of increasing AOD from the northwest to
the southeast is observed. This aligns with changes in land-use cover,
transitioning from mountainous and forested terrain in the northwest to
urban and croplands in the southeast (Figure S6c).
The central core urban region
experiences much higher air pollution levels than the surrounding areas
due to frequent human activities that release a large amount of
pollutants. In the Sahara Desert,
located in northern Africa, the predominant land surface type is barren,
comprising rocky deserts and sand dunes, with low vegetation cover and
limited human activities (Figure S6d).
The region experiences relatively
high aerosol concentrations primarily attributed to frequent dust
storms. Our model effectively generates smooth and spatially continuous
fine-scale aerosol distributions, even over highly reflective desert
surfaces, and the AOD features bear closer resemblance to surface
reflectance in Sahara (Figure 8d-f).
We also conducted validations of
the retrievals against measurements at 8 AERONET and SONET sites in
these two regions (Table S7), showing high correlations (R = 0.976 and
0.752). Approximately 52% to 76% of the retrievals met the criteria
for adequate EEs. These results
underscore the strong robustness of our model in aerosol retrievals over
both dark and bright surfaces over land.
However, it is important to note
that AOD retrievals can be susceptible to interference at cloud edges or
in areas with thin clouds due to inaccuracies in Landsat’s official
cloud mask. This remains a challenging task for high-resolution
satellites with fewer spectral channels, requiring future improvements
(J. Wei et al., 2020a).