Evaluating the assumptions of surface reflectance and aerosol type selection within the MODIS aerosol retrieval over land: The problem of dust type selection

Evaluating the assumptions of surface reflectance and aerosol type selection within the MODIS aerosol retrieval over land: The problem of dust type selection

Mielonen, T.
Levy, R.C.
Aaltonen, V.
Komppula, M.
de Leeuw, G.
Huttunen, J.
Lihavainen, H.
Kolmonen, P.
Lehtinen, K.E.J.
Arola, A.

2011

Aerosol Optical Depth (AOD) and Ångström exponent (AE) values derived with the MODIS retrieval algorithm over land (Collection 5) are compared with ground based sun photometer measurements at eleven sites spanning the globe. Although, in general, total AOD compares well at these sites (R 2 values generally over 0.8), there are cases (from 2 to 67% of the measurements depending on the site) where MODIS clearly retrieves the wrong spectral dependence, and hence, an unrealistic AE value. Some of these poor AE retrievals are due to the aerosol signal being too small (total AOD < 0.3) but in other cases the AOD should have been high enough to derive accurate AE. However, in these cases, MODIS indicates AE values close to 0.6 and zero fine model weighting (FMW), i.e. dust model provides the best fitting to the MODIS observed reflectance. Yet, according to evidence from the collocated sun photometer measurements and backtrajectory analyses, there should be no dust present. This indicates that the assumptions about aerosol model and surface properties made by the MODIS algorithm may have been incorrect. Here we focus on problems related to parameterization of the land-surface optical properties in the algorithm, in particular the relationship between the surface reflectance at 660 and 2130 nm. The retrieval assumes that there is a linear equation that relates the reflectance in these two channels, with the value of the slope (slope660/2130) determined, in part, by the infrared Normalized Difference Vegetation Index, (NDVISWIR). However, the assumed dependence of the slope on the NDVISWIR is not supported by a MODIS based surface albedo climatology. The use of a modified relationship based on the albedo data improves the AE retrieval at the studied sites. The increase in the AE agreement fraction between MODIS and AERONET measurements is between 3 and 22 percentage units depending on the site. These results indicate that the surface reflectance assumptions, especially the slope660/2130 in the MODIS algorithm is the major reason for the inaccurate AE values and the flawed aerosol model combining in the retrieval. However, at some of these sites, the new relationship slightly reduces the correlation between the MODIS and AERONET AOD. This decrease indicates that the combination of the assumed surface and aerosol properties still do not match the actual properties under investigation. © Author(s) 2011.

Earth & Environment
CAS - Climate, Air and Sustainability
EELS - Earth, Environmental and Life Sciences
Urban Development
Environment
Built Environment
accuracy assessment
aerosol property
albedo
algorithm
atmospheric modeling
climatology
comparative study
dust
land surface
MODIS
NDVI
optical depth
parameterization
photometer
surface reflectance

http://resolver.tudelft.nl/uuid:8c231c6f-f336-4770-a7f1-d422972cacfc

447067

1867-1381

Atmospheric Measurement Techniques, 4 (2), 201-214

article