Evaluation of the new ESR network software for the retrieval of direct sun products from CIMEL CE318 and PREDE POM01 sun-sky radiometers

The European Skynet Radiometers network (EuroSkyRad
or ESR) has been recently established as a research
network of European PREDE sun-sky radiometers.
Moreover, ESR is federated with SKYNET, an international
network of PREDE sun-sky radiometers mostly present in
East Asia. In contrast to SKYNET, the European network
also integrates users of the CIMEL CE318 sky-sun photometer.
Keeping instrumental duality in mind, a set of open
source algorithms has been developed consisting of two
modules for (1) the retrieval of direct sun products (aerosol
optical depth, wavelength exponent and water vapor) from
the sun extinction measurements; and (2) the inversion of
the sky radiance to derive other aerosol optical properties
such as size distribution, single scattering albedo or refractive
index. In this study we evaluate the ESR direct sun
products in comparison with the AERosol RObotic NETwork
(AERONET) products. Specifically, we have applied
the ESR algorithm to a CIMEL CE318 and PREDE POM simultaneously
for a 4-yr database measured at the Burjassot
site (Valencia, Spain), and compared the resultant products
with the AERONET direct sun measurements obtained with
the same CIMEL CE318 sky-sun photometer. The comparison
shows that aerosol optical depth differences are mostly
within the nominal uncertainty of 0.003 for a standard calibration
instrument, and fall within the nominal AERONET
uncertainty of 0.01-0.02 for a field instrument in the spectral
range 340 to 1020 nm. In the cases of the A° ngstro¨m exponent
and the columnar water vapor, the differences are lower
than 0.02 and 0.15 cm, respectively. Therefore, we present an
open source code program that can be used with both CIMEL
and PREDE sky radiometers and whose results are equivalent
to AERONET and SKYNET retrievals