A Dark Target research aerosol algorithm for MODIS observations over eastern China: increasing coverage while maintaining accuracy at high aerosol loading
<p>Satellite aerosol products such as the Dark Target (DT) produced from the MODerate resolution Imaging Spectroradiometer (MODIS) are useful for monitoring the progress of air pollution. Unfortunately, the DT often fails to retrieve during the heaviest aerosol events as well as the more moder...
Main Authors: | , , , , , |
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Format: | Article |
Language: | English |
Published: |
Copernicus Publications
2021-05-01
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Series: | Atmospheric Measurement Techniques |
Online Access: | https://amt.copernicus.org/articles/14/3449/2021/amt-14-3449-2021.pdf |
Summary: | <p>Satellite aerosol products such as the Dark Target (DT)
produced from the MODerate resolution Imaging Spectroradiometer (MODIS) are
useful for monitoring the progress of air pollution. Unfortunately, the DT
often fails to retrieve during the heaviest aerosol events as well as the
more moderate events in winter. Some of the literature attributes this lack of
retrieval to the cloud mask. However, we found this lack of retrieval is mainly
traced to thresholds used for masking of inland water and snow.
Modifications to these two masks greatly increase 50 % of the retrievals
of aerosol optical depth at 0.55 <span class="inline-formula">µ</span>m (AOD) greater than 1.0. The
“extra”-high-AOD retrievals tend to be biased when compared with
a ground-based sun photometer (AErosol RObotic NETwork, AERONET). Reducing bias in new retrievals
requires two additional steps. One is an update to the assumed aerosol
optical properties (aerosol model); the haze in this region is both less
absorbing and lower in altitude than what is assumed in the global
algorithm. The second is accounting for the scale height of the aerosol,
specifically that the heavy-aerosol events in the region are much closer to
the surface than what is assumed by the global DT algorithm. The resulting
combination of modified masking thresholds, new aerosol model, and lower
aerosol layer scale height was applied to 3 months of MODIS observations
(January–March 2013) over eastern China. After these two additional steps are
implemented, the significant increase in new retrievals introduces no
overall bias at a high-AOD regime but does degrade other overall validation
statistics. We also find that the research algorithm is able to identify
additional pollution events that AERONET instruments may not due to
different spatial sampling. Mean AOD retrieved from the research algorithm
increases from 0.11 to 0.18 compared to values calculated from the
operational DT algorithm during January to March of 2013 over the study
area. But near Beijing, where the severe pollution occurs, the new algorithm
increases AOD by as much as 3.0 for each 0.5<span class="inline-formula"><sup>∘</sup></span> grid box over the
previous operational-algorithm values.</p> |
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ISSN: | 1867-1381 1867-8548 |