Measurements on pointing error and field of view of Cimel-318 Sun photometers in the scope of AERONET

Abstract

Abstract. Sensitivity studies indicate that among the diverse error sources of ground-based sky radiometer observations, the pointing error plays an important role in the correct retrieval of aerosol properties. The accurate pointing is specially critical for the characterization of desert dust aerosol. The present work relies on the analysis of two new measurement procedures (cross and matrix) specifically designed for the evaluation of the pointing error in the standard instrument of the Aerosol Robotic Network (AERONET), the Cimel CE-318 Sun photometer. The first part of the analysis contains a preliminary study whose results conclude on the need of a Sun movement correction for an accurate evaluation of the pointing error from both new measurements. Once this correction is applied, both measurements show equivalent results with differences under 0.01° in the pointing error estimations. The second part of the analysis includes the incorporation of the cross procedure in the AERONET routine measurement protocol in order to monitor the pointing error in field instruments. The pointing error was evaluated using the data collected for more than a year, in 7 Sun photometers belonging to AERONET sites. The registered pointing error values were generally smaller than 0.1°, though in some instruments values up to 0.3° have been observed. Moreover, the pointing error analysis shows that this measurement can be useful to detect mechanical problems in the robots or dirtiness in the 4-quadrant detector used to track the Sun. Specifically, these mechanical faults can be detected due to the stable behavior of the values over time and vs. the solar zenith angle. Finally, the matrix procedure can be used to derive the value of the solid view angle of the instruments. The methodology has been implemented and applied for the characterization of 5 Sun photometers. To validate the method, a comparison with solid angles obtained from the vicarious calibration method was developed. The differences between both techniques are below 3%.

Sensitivity of aerosol retrieval to geometrical configuration of ground-based sun/sky radiometer observations

“…The first tests on this topic, done with seven photometers in the work by Torres et al (2013), showed that for most of the instruments the magnitudes of ξ θ and sin θ s ξ ϕ were smaller than 0.1 • . The maximum values obtained for vertical and horizontal error were 0.25 • , while the maximum of the total error, ξ , was 0.3 • .…”

“…The work by Torres et al (2013) (also in Torres, 2012) describes a methodology used for the characterization of these magnitudes and a first evaluation of the results for several individual instruments (all of them Cimel 318 sun photometers). One of the main outcomes of this evaluation is that the magnitudes ξ θ and sin θ s ξ ϕ are constant for each photometer, regardless of θ s .…”

“…The BDRF parameters are basically three -f iso (λ), f vol (λ) and f geom (λ); they characterize the isotropic, volumetric, and geometric optics surface scattering, respectively (Roujean et al, 1992, Wanner et al, 1995and Litvinov et al, 2011. In AERONET version 2 retrievals (Holben et al, 2006, Eck et al, 2008, the land BDRF parameters are adopted from MODIS Ecotype generic BDRF models and mixed with the ecosystem map of Moody et al (2005): geographically and temporally varying (16 day averages throughout the annual cycle) surface albedos are utilized.…”

The CU 2-D-MAX-DOAS instrument – Part 2: Raman scattering probability measurements and retrieval of aerosol optical properties

“…We follow the retrieval strategy introduced in Wagner et al (2009a) and apply the DOAS settings from Wagner et al (2009b) to retrieve the RSP in the fitting window of 426-440 nm. The only atmospheric cross section absorber adjusted to the spectrometer resolution that is included in the analysis is NO 2 (Vandaele et al, 1998). A thirdorder polynomial is fitted to account for broad band spectral structures.…”

Can AERONET data be used to accurately model the monochromatic beam and circumsolar irradiances under cloud-free conditions in desert environment?

“…After matching the measurements, 1067 AERONET and SAM profiles remained. The measurements with the same ξ to the east and west directions of the Sun were averaged to minimize the effects of small pointing errors (Torres et al, 2013). Ideally for these 1067 profiles there should be 5335 measurements of radiance corresponding to the five values from AERONET for ξ < 6 • , where the maximum ξ from the AERONET measurements was found to be 5.8 • .…”