Comparison of aerosol optical depth from satellite (MODIS), sun photometer and broadband pyrheliometer ground-based observations in Cuba
Authors: Antuña-Marrero Juan Carlos , Cachorro Revilla Victoria , García Parrado Frank , de Frutos Baraja Ángel , Rodríguez Vega Albeth , Mateos David , Estevan Arredondo René , Toledano Carlos 
Published: April 2018

Abstract. In the present study, we report the first comparison between the aerosol
optical depth (AOD) and Ångström exponent (AE) of the Moderate Resolution
Imaging Spectroradiometer (MODIS) instruments on the Terra (AODt) and Aqua
(AODa) satellites and those measured using a sun photometer (AODSP) at
Camagüey, Cuba, for the period 2008 to 2014. The comparison of Terra and
Aqua data includes AOD derived with both deep blue (DB) and dark target (DT)
algorithms from MODIS Collection 6. Combined Terra and Aqua (AODta) data were
also considered. Assuming an interval of ±30 min around the
overpass time and an area of 25 km around the sun photometer site, two
coincidence criteria were considered: individual pairs of observations and
both spatial and temporal mean values, which we call collocated daily means.
The usual statistics (root mean square error, RMSE; mean
absolute error, MAE; median bias, BIAS), together with linear regression
analysis, are used for this comparison. Results show very similar values for
both coincidence criteria: the DT algorithm generally displays better statistics
and higher homogeneity than the DB algorithm in the behaviour of AODt, AODa,
AODta compared to AODSP. For collocated daily means, (a) RMSEs of 0.060 and 0.062 were obtained for Terra and Aqua with the DT
algorithm and 0.084 and 0.065 for the DB algorithm, (b) MAE follows the same
patterns, (c) BIAS for both Terra and Aqua presents positive and negative
values but its absolute values are lower for the DT algorithm; (d) combined
AODta data also give lower values of these three statistical indicators for
the DT algorithm; (e) both algorithms present good correlations for comparing
AODt, AODa, AODta vs. AODSP, with a slight overestimation of satellite data
compared to AODSP, (f). The DT algorithm yields better figures with slopes of
0.96 (Terra), 0.96 (Aqua) and 0.96 (Terra + Aqua) compared to the DB
algorithm (1.07, 0.90, 0.99), which displays greater variability. Multi-annual
monthly means of AODta establish a first climatology that is more comparable
to that given by the sun photometer and their statistical evaluation reveals
better agreement with AODSP for the DT algorithm. Results of the AE
comparison showed similar results to those reported in the literature
concerning the two algorithms' capacity for retrieval. A comparison between
broadband aerosol optical depth (BAOD), derived from broadband pyrheliometer
observations at the Camagüey site and three other meteorological stations
in Cuba, and AOD observations from MODIS on board Terra and Aqua show a poor
correlation with slopes below 0.4 for both algorithms. Aqua (Terra) showed
RMSE values of 0.073 (0.080) and 0.088 (0.087) for the DB and DT algorithms. As
expected, RMSE values are higher than those from the MODIS–sun photometer
comparison, but within the same order of magnitude. Results from the BAOD
derived from solar radiation measurements demonstrate its reliability in
describing climatological AOD series estimates.