Atmospheric turbidity determined by the annual cycle of the aerosol optical depth over north-center Spain from ground (AERONET) and satellite (MODIS)

The present study focuses on the characterization of aerosol seasonal variability over the northern continental region of the Iberian Peninsula based on remotely sensed aerosol optical properties, and in particular the aerosol optical depth (AOD) and Ångström exponent (Alpha) parameters. For this region, a representative annual cycle of these parameters was built based on data from the AERONET-RIMA station of Palencia (Spain, 42N, 4.5W) for the period 2003–2011. The results also examine in details the interannual variability during the whole period. The ability of satellite to reproduce the seasonal patterns and anomalies, is investigated using the MODIS (Moderate Resolution Imaging Radiometer) for the same period. MODIS instantaneous fields are validated against ground-based sunphotometer data, and the differences between monthly values are estimated. The site of Palencia is characterized by a daily AOD (440 nm) of 0.15 ± 0.10 and Alpha (440–870 nm) of 1.29 ± 0.35 on average, thus presenting values typical of a clean continental background. The seasonal pattern corresponds to mid-high turbidity values of the AOD during a period starting in mid-spring to the end of the summer (max 0.19), and a lower AOD during fall and winter months (min 0.09). When using MODIS data, the overall results for Palencia give a higher (by ∼25%) AOD (470 nm) with 0.19 ± 0.15 and a much lower (by ∼50%) Alpha (470–660 nm) of 0.70 ± 0.20. These numbers reflect substantial differences, with overestimations of the monthly means that can be almost double those of AERONET in the summer months. However MODIS satisfactorily reproduces the increase–decrease cycle in the AOD. These large differences tend to be more attributed to the aerosol models used in the MODIS algorithm rather than to the sampling difference between ground and satellite in this season. Despite the poor sampling in winter and the small AOD (<0.1) observed over the area, the best agreement between satellite and ground is found during this period. The seasonal pattern of the Ångström exponent derived from MODIS was found to be very different from that of AERONET, the former showing apparently no consistency with the latter. Given the aforementioned values and the fact that the AERONET Alpha value for 470–660 nm is 1.41 ± 0.37 (wavelengths used in the comparison with MODIS), we can conclude that the Alpha derived from MODIS is not representative of the aerosol type characterizing this region of the globe.