The growing interest in night-time observations of aerosol optical depth (AOD) has led to the development of a lunar Precision Filter Radiometer (PFR) at PMOD/WRC. In addition to the retrieval of nocturnal AOD, the radiometric calibration of the lunar PFR aims to provide absolute lunar irradiance measurements with an expanded uncertainty of less than 5%.
Atmospheric aerosols are known to impact the climate, but they still represent one of the largest uncertainties in climate change studies. Night-time AOD measurements could provide valuable information on the climatology of aerosols at high latitude stations, where direct sun measurements are not possible throughout the year. For example, in the northern hemisphere they can be used to monitor the arctic haze during polar winter. In passive remote sensing techniques, the moon or the stars are used as the light source. The high intensity of the moon in comparison to stars (more than 5 orders of magnitude) makes it suitable for nocturnal observation without the necessity of using complex systems. However, the transition from Sun to lunar photometry is challenging due to the continuous change in lunar brightness, depending on the viewing geometry, as well as the fact that lunar irradiance is 5 to 6 orders of magnitude less than the Sun’s.
Figure 1. The PMOD/WRC Lunar PFR being calibrated in the lab.
PMOD/WRC participated in the Lunar Arctic Project (March 2014 – June 2015) funded by the Svalbard Science Foundation, that aimed to close the gap in the annual cycle of the arctic aerosol climatology and to develop Svalbard as a satellite validation site. The role of PMOD/WRC in the project was the modification of an existing PFR and its deployment at Ny Ålesund. The lunar PFR is a standard PFR instrument that is used in the GAW-PFR network, with enhanced sensitivity, which measures at four wavelengths 412, 500, 675, and 862 nm with sensor stabilisation at 20°C. The instrument was installed at Ny-Ålesund (Norway; 78.9°N, 11.9°E), from October 2014 to February 2015, and measured six lunar cycles. During the Arctic Summer, it was installed at Izaña (Spain) to perform Langley calibrations, investigate its stability, and to evaluate its measurement limitations. A valuable data-set was retrieved during this period, providing the opportunity to improve a number of technical and measurement issues.
The prototype lunar PFR was upgraded in Oct. 2015 whereby the sensitivity was increased by a factor 10 in three of the four channels and a 22-bit data acquisition system (SACRAM) was installed. The instrument was characterised at PMOD/WRC: the filter functions and instrument linearity were measured using the PMOD/WRC ATLAS tunable laser system. Differences of ~0.5 nm were found from the initial nominal central wavelength values. The absolute responsivity of the lunar PFR was measured on the direct irradiance calibration setup, consisting of a reference irradiance source (1000 W FEL type lamp source). The calibration factors were determined with an expanded uncertainty of 3.5% (k = 2) including uncertainties in the reference plane determination, non-linearity correction, and the reference irradiance used.
The lunar PFR was deployed at Ny Ålesund from December 2015 to February 2016 and November 2016 to February 2017 to monitor AOD during the Arctic winter, in collaboration with partners from NILU (Norway), ISAC/CNR (Italy), AWI (Germany), NOAA/CIRES (US), and IGF/PAS (Poland). A calibration of the lunar PFR was performed in Izaña, Spain, measuring one lunar cycle in October 2016, alongside the Triplet-Cimel belonging to AERONET.