Measurement Guidelines for automatic sun/sky/lunar photometers

22 December 2025 1 / 10 Measurement Guidelines for automatic sun/sky/lunar photometers Emitter CARS & ARES Version 01 Revision 01 ACTRIS Centre for Aerosol Remote Sensing C . A . R . S . 22 December 2025 2 / 10 Table of contents APPLICABILITY OF THE DOCUMENT ................................................................................................................. 3 ACRONYMS ...................................................................................................................................................... 3 REFERENCE DOCUMENTS ................................................................................................................................. 3 1 INTRODUCTION ....................................................................................................................................... 4 2 MEASUREMENT GUIDELINES ................................................................................................................... 4 3 ADDITIONAL RESOURCES ......................................................................................................................... 6 ANNEX 1. LIST OF AEROSOL REMOTE SENSING VARIABLES ......................................................................... 8 ANNEX 2. SCHEMATICS OF THE WORKFLOW FOR THE ACTRIS AEROSOL REMOTE SENSING MEASUREMENTS ............................................................................................................................................. 9 ANNEX 3 METHODOLOGY FOR CALCULATING DATA COVERAGE FOR THE PHOTOMETER-RELATED VARIABLES AT STATIONARY STATIONS ............................................................................................................................ 10 ACTRIS Centre for Aerosol Remote Sensing C . A . R . S . 22 December 2025 3 / 10 Applicability of the document These guidelines apply to the ACTRIS National Facilities operating automatic sun/sky/lunar photometers and to the AERONET-ACTRIS associated photometer stations. Acronyms • ACTRIS - Aerosol, Clouds and Trace gases Research InfraStructure • ACTRIS GA – ACTRIS General Assembly • ARES – Aerosol Remote Sensing unit of the ACTRIS Data Centre • ARS – Aerosol Remote Sensing • CARPORT – CARS workflow management portal • CARS - Centre for Aerosol Remote Sensing • CLU – Cloud Remote Sensing unit of the ACTRIS Data Centre • CCRES - Centre for Cloud Remote Sensing • DC – Data Centre • MP – Mobile Platform • NF - National Facility • NRT – Near-real time (less than 3 days from the measurements) • OP – Observational Platform • PI – Principal Investigator • RI Comm – Research Infrastructure Committee • RRT – Real-real time (less than 3 hours form the measurement) • SCC – Single Calculus Chain Reference documents [1]. Documentation on technical concepts and requirements for ACTRIS Observational Platforms [2]. ACTRIS NF Labelling Plan [3]. Descriptions of the workflows between ACTRIS components [4]. ACTRIS Data Management Plan [5]. CARS implementation plan [6]. ACTRIS vocabulary [7]. ACTRIS principles for evaluating data provision, service provision and QA/QC [8]. Labelling of the ACTRIS National Facilities operating Aerosol Remote Sensing instruments [9]. Technical requirements for ACTRIS Aerosol Remote Sensing Observational Platforms [10]. Technical requirements for ACTRIS Mobile Platforms operating automatic sun/sky/lunar photometers [11]. Standard Operation Procedure for automatic sun/sky/lunar photometers [12]. Quality Assurance Procedure for automatic sun/sky/lunar photometers [13]. ACTRIS CIMEL Photometer Guide for Users ACTRIS Centre for Aerosol Remote Sensing C . A . R . S . 22 December 2025 4 / 10 1 Introduction Detailed instructions are provided for the deployment, operation, quality control, and calibration of CIMEL photometers within the AERONET-ACTRIS network, for both stationary and mobile/marine sites. Through rigorous facility-based calibration, weekly local and remote QC, and network-level verification, every CIMEL CE318T photometer remains a reliable and accurate instrument for climate research, atmospheric monitoring, and environmental policy applications. This Guide summarizes the main relevant information all users / instruments PI must be aware of. 2 Measurement Guidelines Automatic sun, sky, and lunar photometers form the backbone of high-quality atmospheric aerosol observations. In the ACTRIS network, CIMEL CE318T photometers are operated according to precise measurement principles that ensure consistency, traceability, and scientific validity across all sites. The core objective is to produce a reliable time series of aerosol optical depth (AOD), sky radiance, and derived aerosol optical and microphysical properties, critical for climate research, air quality monitoring, and satellite validation. For stationary sites, continuous measurements are recommended throughout daylight hours for sun observations and during moon visibility for lunar measurements (Fig. 1). Sky radiance measurements are conducted at regular angular intervals, depending on the photometer configuration (Fig. 2). Mobile or marine deployments follow similar principles, but the observation schedule may be adapted to vessel movement, weather conditions, and mission requirements. Data collected by the instrument are recorded in standardized formats (k8 files) and have to be transmitted regularly to the processing center using PhotoGetData software. Timely data transmission ensures integration into network databases and supports near-real-time monitoring. Observers must maintain complete metadata describing site conditions, instrument configuration, and any deviations from standard operating conditions, ensuring transparency and scientific reproducibility. Direct measurements are performed at day (SUN scenario) and at night (MOON scenario). In this type of measurement, the photometer points the Sun and the Moon using an ephemeris, performing a subsequent refinement by means of a four-quadrant sensor. With this refinement, the photometer points to the center of the illuminated part of the Sun or the Moon. For direct measurements, the CIMEL CE318-T performs a sequence of 3 consecutive measurements of the 10 spectral bands (nine nominal wavelengths plus an additional measurement at 1020 nm with the InGaAs detector). These measurements (performed every 30 s), called triplets, are repeated with a given frequency, between 3 and 15 minutes. Triplets are useful to check the stability in measurements as well as to screen clouds. Sky observations are performed only at daytime to measure sky radiance at different scattering angles to retrieve aerosol properties. These observations are made keeping constant the solar zenith angle and varying the azimuth angle (almucantar, ALM) and also keeping constant the solar azimuth angle and varying the zenith angle (principal plane, PP) (Fig. 2). The new Hybrid (HYB) scan protocol is a mixture of ALM and PP scans. The combination of these two scans allows us to maximize the range of scattering angles and achieve scan symmetry. ACTRIS Centre for Aerosol Remote Sensing C . A . R . S . 22 December 2025 5 / 10 In the binary k8 files, there exist other types of measurements, as described in Table 1. Table 1: Description of each CIMEL CE318 scenario in the binary k8 files. SCENARIO File extension DESCRIPTION BLACK BLK The black scenario consists in measuring the electronic noise of the device. The filter-wheel is positioned in between two filters so that the signal coming through the two channels is cut. HYBRID HYL, HYR Mix of PPLAN and ALMUCANTAR ALMUCANTAR ALL, ALR The ALMUcantar technique consists in measuring the sky radiance in aerosol channels, keeping a constant zenith angle equals to the zenith solar angle with varying azimuth angle. PPLAN PPL. PPR The Principal Plane technique also consists in measuring the sky radiance in the aerosol channels. Unlike the almucantar, a constant azimuth angle is kept with varying zenith angle to make radiance measurements. CURVE CROSS CCS Sky observations in the solar almucantar between 3 and 7.5 deg. for 1020nm channel. Used for cloud screening. SUN/MOON NSU, NLU The SUN / MOON scenario consists in repeating 3 times a direct measurement in all channels (triplet). Temperature is recorded once at the end of the scenario. CROSS SUN/MOON CSU, CLU The Cross Sun / Moon scenario consists in measuring sun or moon irradiance for several angles close to the sun / moon using a cross scheme. In Cross Sun / Moon scenario: - Zenithal angles vary from -1° to 1° using SUN gain with constant azimuthal angle - Azimuthal angles vary from -1° to 1° using SUN gain with constant zenithal angle. POL LUN PLU Direct Moon Polarized measurements. The guiding principle is that all measurements must be traceable to internationally recognized standards. Observations should be planned to maximize temporal coverage while minimizing ACTRIS Centre for Aerosol Remote Sensing C . A . R . S . 22 December 2025 6 / 10 systematic uncertainties, and any environmental factor affecting optical measurements, such as shading or reflections, must be carefully controlled. These guidelines serve as the foundation for all subsequent operational and quality assurance activities. Fig 1: (Left) Photometer and robot setup. (Right) setup of the control box Figure 2 : (a) Almucantar geometry is conducted with viewing zenith angle equals to solar zenith angle = constant and azimuthal angle varying (0-360°); (b) Solar Principal Plane geometry is conducted in the solar plane (azimuthal angle = 0 and 180°) and viewing zenith angle varying from 0° to 85°; (c) Hybrid geometry: azimuth and zenith angles are varied to scan fixed scattering angles up to 75° zenith, then only azimuthal scanning is performed. Figure reprinted from González et al. (2020). 3 Additional resources More detailed information at: Multiband photometer CE318-N. User’s manual (http://support.cimel.fr/photo/pdf/man_ce318_us.pdf). Goloub et al., Good practices : a report on the requirements for SI-traceable calibrations of sunphotometers from aerosol remote sensing monitoring networks. Metrology for Aerosol oPtical Properties., 2023. Gónzalez, Ramiro. Desarrollo de nuevos métodos de procesado de datos de redes fotométricas para el análisis de propiedades del aerosol atmosférico. 2021. (a) (b) (c) ACTRIS Centre for Aerosol Remote Sensing C . A . R . S . 22 December 2025 7 / 10 Giles, D. M., Sinyuk, A., Sorokin, M. G., Schafer, J. S., Smirnov, A., Slutsker, I., Eck, T. F., Holben, B. N., Lewis, J. R., Campbell, J. R., Welton, E. J., Korkin, S. V., and Lyapustin, A. I. (2019): Advancements in the Aerosol Robotic Network (AERONET) Version 3 database – automated near-real-time quality control algorithm with improved cloud screening for Sun photometer aerosol optical depth (AOD) measurements, Atmos. Meas. Tech., 12, 169-209, https://doi.org/10.5194/amt-12-169-2019. ACTRIS Centre for Aerosol Remote Sensing C . A . R . S . 22 December 2025 8 / 10 Annex 1. List of aerosol remote sensing variables Variable Data product level Requirement Aerosol highpower lidar Sun/sky/lunar photometer Attenuated backscatter profile L1, L2 Minimum Volume depolarization profile L1, L2 Minimum Particle backscatter coefficient profile L1, L2 Minimum Particle extinction coefficient profile L1, L2 Minimum Lidar ratio profile L1, L2 Minimum Ångström exponent profile L1, L2 Optimum Backscatter-related Ångström exponent profile L1, L2 Optimum Particle depolarization ratio profile L1, L2 Minimum Particle layer geometrical properties (height and thickness) L1, L2 Minimum Particle layer optical properties (extinction, backscatter, lidar ratio, Ångström exponent, depolarization ratio, optical depth) L1, L2 Minimum Column integrated extinction L1, L2 Minimum Spectral Downward Sky Radiances L1 Minimum Direct Sun/Moon Extinction Aerosol Optical Depth (column) L1 Minimum Aerosol columnar properties L2 Minimum Aerosol profile microphysical and optical properties L2 Minimum ACTRIS Centre for Aerosol Remote Sensing C . A . R . S . 22 December 2025 9 / 10 Annex 2. Schematics of the workflow for the ACTRIS aerosol remote sensing measurements