66 S. BHASKARAN ET AL.
5.2. Spatio-temporal Distribution of AOD in
Birdsville, 2005
For Birsdville in general MODIS retrievals overestimate
AODs. For instance for 12th March, 2005 MODIS more
than Aeronet retrievals by 0.02 except at 440 nm - 470
nm where MODIS retrievals are underestimated to the
Aeronet AODs. 28thMarch, 2005: MODIS retrievals
overestimate AOD at 470 nm but run close to Aeronet
retrievals for the other wavelengths. 13thApril, 2006
MODIS retrievals overestimate AODs at 412 nm and
470 nm but underestimate at 659 nm. 29th April:
MODIS retrievals underestimate AODs at 412 nm and
659 nm, but overestimate at 470 nm.15thMay, 2005:
MODIS retrievals underestimate AODs at 412 nm and
659 nm but overestimate at 470 nm 31stMay, 2005:
MODIS retrievals underestimate AOD at 412 nm and
659 nm but overestimate at 470 nm MODIS retrievals
underestimate at all wavelengths. 2ndJuly,2005: MODIS
underestimates at 412 nm and 470 nm but overestimates
at 675 nm marginally. 18th July,2005: MODIS retrievals
underestimate AODs at 412 nm and 659 nm but overes-
timate at 470 nm. For the months of August, September,
and October MODIS retrievals underestimate Aeronet
retrievals at all the wavelengths.
5.3. Spatio-temporal Distribution of AOD in
Tinga Tingana – 2005
MODIS retrievals overestimate Aeronet retrievals for all
wavelengths but for certain months the opposite trend is
observed particularly for in the months of October and
November, 2005 where the MODIS retrievals are under-
estimated by Aeronet estimates. Some months the
MODIS and Aeronet estimates underestimate at some
wavelengths but overestimate at others. For instance on
12th March, 2005 the MODIS retrievals match Aeronet at
380 - 412 nm (Figure 3 (e)) but overestimates at 440 -
470 nm and underestimates at 659 - 675 nm wavelength.
On 28th March, 2005 the MODIS retrievals overestimate
the Aeronet retrievals at all wavelengths. On the 13th
April, 2005 the MODIS retrievals underestimate the
Aeronet estimates at 380 - 412 nm and 659 - 675 nm
wavelengths, but between 440 – 470 nm wavelengths the
MODIS retrievals match the Aeronet retrievals. On 15th
May,2005 the MODIS retrievals overestimate the Aer-
onet retrievals at all wavelengths and on the 2ndJuly,
2005 the MODIS retrievals underestimate at 380 - 412
wavelength, but overestimate the Aeronet retrievals at
the 440 - 470 nm and 659 - 675 nm wavelengths. On the
18thJuly, 2005 the MODIS retrievals underestimate
AODs at 412 nm and 659 nm but overestimate at 470 nm.
On 3rdAugust, 2005 the MODIS retrievals overestimate
Aeronet retrievals for all the wavelengths.
On the 4thSeptember, 2005 the MODIS retrievals un-
derestimate the Aeronet retrievals for all wavelengths.
On 22ndOctober and 7thNovember, 2005 the MODIS re-
trievals underestimate the Aeronet retrievals for all
wavelengths.
6. Conclusions
Yearly data sets were compared in this study over three
sites in Australia using level 1.5 cloud free data. The
MODIS retrieved AOD overestimates compared with the
Aeronet retrievals. The results also may be affected by
the numerous problems posed to aerosol retrieval by the
surface background over land and the potential sampling
mismatch in comparing averages from MODIS spatially
variable data space and AERONET temporally variable
data space. There were some limitations that may have
contributed to the accuracy of validation. AOD is gener-
ally less than unity, a good calibration of the radiometer
is essential for the successful retrieval of aerosol optical
density from space. Calibrated Aeronet data (Level 2.0)
was not available over the 3 study sites which further
reduced the accuracy of the study. It is important to un-
derstand the proportion of trace gases like water vapour,
carbon dioxide which absorb solar radiation in the 1600,
2100 nm wavelengths. Estimating AODs is a challenging
research investigation due to the highly variable spatial
and temporal dimensions of AODs. Some changes in the
internal instrument temperature are also some cause of
false signals in sun photometers. Calibration of sites is a
complex process and efforts are underway in Australia to
provide calibrated AOD data which we hope this will
further enhance the accuracy of results presented in this
study.
Despite these limitations in the assessment a general
conclusion is that MODIS AODs tend to overestimate
the AODs particularly over land.
7. Acknowledgements
The authors would like to thank the PI and staff at
CSIRO for establishing and maintaining the AERONET
sites used in this study.
8. References
[1] Griggs, M., 1975: Measurement of atmospheric aerosol
optical thickness over water using ERTS-1 data. J.Air
Pollut.Contr. Assoc., 25, 622-626.
[2] Hauser, A., Oesch, D., Foppa, N., Aerosol optical depth
over land: Comparing AERONET, AVHRR and MODIS.
Geophysical Research Letters, Vol. 32, LXXXXX, doi:
10.1029/2005GL023579, 2005
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