| Abstract | dc.description.abstract | Ground-based measurements of ultraviolet (UV) irradiance, carried out by a four-channel
UV radiometer in Santiago de Chile from October 2004 to December 2011, have
been used to estimate daily values of the UV index (UVI). These ground-based data have
been compared with UVI estimates retrieved from the Ozone Measurement Instrument
(OMI) on board the Aura spacecraft. Since the widely used OMI-gridded UVI data may
not be suitable for the complex local morphology and meteorology, a careful screening
of overpass OMI data was applied.
Nevertheless, we found that OMI-derived UVI data overestimate ground-based values;
depending on cloud-cover conditions, the mean bias (MB) and the root mean square
error (RMSE) range from 34.53% to 30.29% and from 35.22% to 43.50%, respectively,
with the lowestMB (and the highest RMSE) values occurring under overcast conditions.
Moreover, the difference between satellite-derived and ground-based UVI data exhibits
a limited seasonality with somewhat larger differences in the fall season. The detected
overestimation seems to be linked with the boundary layer aerosol absorption that is
not accounted for by the OMI algorithm. Indeed, we found that the difference in UVI
increases with the aerosol concentration (which in Santiago shows seasonal variations).
Ceilometer profiles of backscatter intensities, directly related to aerosol concentrations,
and PM10 concentrations correlate with UVI differences (correlation coefficient r of
approximately 0.6 and 0.4, respectively) under cloud-free conditions for time scales
ranging from months to years.
Additional comparisons were performed between UVI estimates retrieved from
our ground-based measurements in Santiago and from the Tropospheric Emission
Monitoring Internet Service (TEMIS) Scanning Imaging Absorption Spectrometer
for Atmospheric Cartography (SCIAMACHY). Under cloudless conditions, also
TEMIS-derived data overestimate ground-based UVI estimations (by about 31%) and
exhibit a small seasonality. | en_US |
