Evaluation of night-time aerosol measurements and lunar irradiance models in the frame of the first multi-instrument nocturnal intercomparison campaign

Photometric measurements have been used during decades to provide reliable and valuable information to compile long-term global AOD and other aerosol properties records in order to increase our knowledge on the role of aerosols in the Earth’s radiative balance. However, AOD data provided by sun photometers is severely restricted since it is limited to daytime, which seriously constrain the study of atmospheric processes in which day-to-night variations play an important role, such as in the case of polar regions, aerosol nucleation during the daytime, convective processes or other processes with a well-defined time pattern (traffic,domestic heating among others).

Figure 1.- Cimel CE318-T photometers operating in Izaña during the field campaign.

The first multi-instrumental comparison campaign of nocturnal measurements held at Izaña Observatory in 2017 was the first time that nocturnal measurements were performed coincidentally with different photometers (Figure 1). Three different instruments were involved in this field campaign: two different lunar photometers (Lunar PFR, from PMOD /WRC, and Cimel CE318-T) and one stellar photometer. The stellar photometer that participated in this campaign is a non-commercial instrument which measures direct star irradiance and it belongs to University of Granada. The review of the different calibration techniques for lunar photometry as well as the uncertainty estimation of the nocturnal AOD are an important outcome of the work recently published by Barreto et al. (2019). In addition, the new free and open-access ROLO Implementation for Moon-photometry Observation (RIMO) model has been released which will allow the scientific community to obtain reliable lunar exo-atmospheric irradiance values for the analysis of photometric data.

The result of this field campaign suggest that lunar photometry is a reliable technique, especially for low aerosol loading conditions (see Figure 2). Small instrumental differences between the two lunar photometers were observed during the field campaign. These discrepancies were only appreciated under pristine sky conditions and are hypothesized to be caused by the different pointing process or dark current correction performed by the two lunar photometers.

Figure 2.- AOD time series from the two lunar photometers involved in the field campaign (at 674.8 nm), Lunar PFR (at 675.6 nm) and the stellar photometer (at 670.0 nm), including daytime AERONET (at 675.7 nm) in the period 2-17 June 2017. AOD from lunar photometers have been extracted by means of (a) Lunar-Langley calibration with the RIMO model, and (b) Langley-plot calibration with V0c values. The black line and right y axis correspond to the evolution of the Moon’s illumination factor (IF, in %) in this period. Reprinted from Barreto et al. (2019).