Measurement Guidelines for aerosol high-power lidars

22 December 2025 1 / 21 Measurement Guidelines for aerosol high-power lidars Emitter CARS & ARES Version 01 Revision 01 ACTRIS Centre for Aerosol Remote Sensing C . A . R . S . 22 December 2025 2 / 21 Table of contents APPLICABILITY OF THE DOCUMENT ................................................................................................................. 3 ACRONYMS ...................................................................................................................................................... 3 REFERENCE DOCUMENTS ................................................................................................................................. 3 1 INTRODUCTION ....................................................................................................................................... 4 2 GENERAL CONSIDERATIONS .................................................................................................................... 6 2.1 PRINCIPLES APPLICABLE TO DATA PRODUCTION ................................................................................................. 6 2.2 ACTRIS DATA TYPES ................................................................................................................................... 6 2.3 REQUIREMENTS FOR THE PROVISION OF AEROSOL HIGH-POWER LIDAR COMPLIANT/LABELLED DATA TO THE ACTRIS DATA CENTRE 6 2.4 OTHER CONSIDERATIONS ............................................................................................................................. 7 3 MEASUREMENT SCHEDULE...................................................................................................................... 8 3.1 MEASUREMENT SCHEDULE FOR THE ACTRIS AEROSOL REMOTE SENSING OBSERVATIONAL PLATFORMS ..................... 8 3.2 MEASUREMENT SCHEDULE FOR THE ACTRIS AEROSOL REMOTE SENSING MOBILE PLATFORMS ................................ 8 3.3 MEASUREMENT SCHEDULE FOR THE EARLINET-ACTRIS ASSOCIATED LIDAR STATION ............................................. 8 4 STEP-BY-STEP WORKFLOW FOR PROVISION OF AEROSOL HIGH-POWER DATA TO THE ACTRIS DATA CENTRE ............................................................................................................................................................ 9 4.1 STEPS FOR SETTING UP A VALID OPERATIONAL CONFIGURATION ........................................................................... 9 4.2 LIFE CYCLE OF THE SCC OPERATIONAL CONFIGURATIONS .................................................................................... 9 4.3 LIFE CYCLE OF THE LEVEL 2 DATA PRODUCTION ............................................................................................... 10 ANNEX 1. LIST OF AEROSOL REMOTE SENSING VARIABLES ....................................................................... 12 ANNEX 2. SCHEMATICS OF THE WORKFLOW FOR THE ACTRIS AEROSOL REMOTE SENSING MEASUREMENTS ........................................................................................................................................... 13 ANNEX 3. LIST OF INPUT PARAMETERS FOR THE SCC OPERATIONAL CONFIGURATIONS ........................... 14 ANNEX 4 METHODOLOGY FOR CALCULATING DATA COVERAGE FOR THE LIDAR-RELATED VARIABLES AT STATIONARY STATIONS ................................................................................................................................. 20 ACTRIS Centre for Aerosol Remote Sensing C . A . R . S . 22 December 2025 3 / 21 Applicability of the document These guidelines apply to the ACTRIS National Facilities operating aerosol high-power lidars and to the EARLINET-ACTRIS associated lidar 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]. Guidelines for EARLINET-ACTRIS associated stations to access QA services [10]. Technical requirements for ACTRIS Aerosol Remote Sensing Observational Platforms [11]. Technical requirements for ACTRIS Mobile Platforms operating aerosol high-power lidars [12]. Standard Operation Procedure for aerosol high-power lidars [13]. Quality Assurance Procedure for aerosol high-power lidars ACTRIS Centre for Aerosol Remote Sensing C . A . R . S . 22 December 2025 4 / 21 1 Introduction Aerosol remote sensing instruments as considered by ACTRIS are: a) aerosol high-power lidars; b) automatic sun/sky/lunar photometers; c) automatic low-power lidars and ceilometers. Aerosol high-power lidars are operated by ACTRIS National Facilities either as part of the Observational Platforms (at fixed site, in synergy with collocated automatic sun/sky/lunar photometers), or onboard Mobile Platforms (movable, potentially with other instruments). Measurement guidelines in this document apply specifically to aerosol high-power lidars, operated by Observational Platforms and by Mobile Platforms. In order to be labelled as ACTRIS National Facility, both the Aerosol Remote Sensing Observational Platforms, and the Mobile Platforms operating aerosol remote sensing instruments must undergo the labelling process. The labelling process is described in Labelling of the ACTRIS National Facilities operating Aerosol Remote Sensing instruments. Aerosol high-power lidars can also be operated by other lidar stations or lidar networks, e.g. EARLINET. Although not all of the EARLINET stations are officially part of ACTRIS, they can benefit from CARS and ARES support if they are providing data to the ACTRIS Data Centre and follow the protocols issued by CARS and ARES. In the following, these lidar stations will be referred as "EARLINET-ACTRIS associated lidar station". Measurement guidelines in this document apply also to EARLINET-ACTRIS associated lidar station. In order to be accepted as EARLINET-ACTRIS associated station, and to receive support from CARS and ARES, the EARLINET stations must undergo the technical acceptance process. The acceptance process is described in Guidelines for EARLINET-ACTRIS associated stations to access QA services. Depending on the type of the facility and of the degree of fulfilment of the technical requirements, ACTRIS data products are (ACTRIS vocabulary): - ACTRIS labelled: Data are collected in accordance with ACTRIS procedures, observation is conducted at an ACTRIS National Facility. - ACTRIS compliant: Data are collected in accordance with ACTRIS procedures and comply with the requirements for this variable. The facility component has not necessarily undergone the full ACTRIS labelling process nor is necessarily conducted at an ACTRIS National Facility. - ACTRIS associated: Data partly complying with ACTRIS requirements after start of ACTRIS-ERIC Depending on the level of quality assurance, ACTRIS data products are (ACTRIS Data Management Plan): - Level 0 data: Raw sensor output. Native resolution, metadata necessary for next level. - Level 1 data: Calibrated and quality assured data with minimum level of quality control. - Level 2 data: Approved and fully quality controlled ACTRIS data product or geophysical variable. - Level 3 data: Elaborated ACTRIS data products derived by post-processing of ACTRIS Level 0 - 1 -2 data, and data from other sources. The data can be gridded or not ACTRIS Centre for Aerosol Remote Sensing C . A . R . S . 22 December 2025 5 / 21 This document describes the guidelines for the aerosol high-power lidar data flow, to be implemented by the ACTRIS National Facilities operating aerosol high-power lidars (Observational Platforms and Mobile Platforms), and by the ACTRIS associated lidar stations. ➢ General considerations are presented in Section 2. ➢ The measurements schedule for the aerosol high-power lidars is described in Section 3. ➢ The step-by-step workflow for the provision of data to the ACTRIS Data Centre is described in Section 4. ➢ The list of ACTRIS aerosol remote sensing variables is provided in Annex 1. ➢ Schematics of the workflow for the ACTRIS aerosol remote sensing measurements (lidar and photometer) is described in Annex 2. ➢ The list of input parameters for the SCC operational configurations is provided in Annex 3. ACTRIS Centre for Aerosol Remote Sensing C . A . R . S . 22 December 2025 6 / 21 2 General considerations 2.1 Principles applicable to data production Principle 1: Data products calculated from aerosol high-power lidar measurements are considered "ACTRIS data" only if they are originating from an ACTRIS National Facility or an ACTRIS-associated EARLINET station, and are centrally processed at the ACTRIS DC. Principle 2: ACTRIS National Facilities operating aerosol high-power lidars and the EARLINET-ACTRIS associated lidar station must provide operational data products to the ACTRIS Data Centre, setting up an valid operational configuration in the Single Calculus Chain (SCC). Principle 3: ACTRIS National Facilities operating aerosol high-power lidars and EARLINET-ACTRIS associated lidar station can also setup and use Single Calculus Chain (SCC) experimental configurations for their own experiments and specific needs. Data products calculated with SCC experimental configurations will not be considered as fully quality controlled. Principle 4: ACTRIS Level 2 and Level 3 data products can only be produced if the data is processed with a valid SCC operational configuration, i.e. an SCC operational configuration within its approved validity period. Principle 5: ACTRIS data products produced with an SCC operational configuration outside its approved validity period, as well as data products calculated by using SCC experimental configurations are ACTRIS Level 1 data. 2.2 ACTRIS data types Data provided to the ACTRIS Data Centre by the ACTRIS National Facilities are tagged as: - "ACTRIS compliant data" if all technical requirements are met, the associated SOPs and QAPs issued by CARS and ARES are followed, and the data is submitted to the ACTRIS Data Centre according to the measurement guidelines (Section 3.1 for Observational Platforms / Section 3.2 for Mobile Platforms); - "ACTRIS labelled data" as soon as the ACTRIS label is granted (according to ACTRIS principles for evaluating data provision, service provision and QA/QC - Labelling of the ACTRIS National Facilities operating Aerosol Remote Sensing instruments). Data provided to the ACTRIS Data Centre by the EARLINET-ACTRIS associated lidar station are tagged as: - "ACTRIS associated data" if not all the technical requirements are met but the associated SOPs and QAPs issued by CARS and ARES are followed, and the data is submitted to the ACTRIS Data Centre according to the measurement guidelines (Section 3.3); - "ACTRIS compliant data" if all technical requirements are met, and the associated SOPs and QAPs issued by CARS are followed, and the data is submitted to the ACTRIS Data Centre according to the measurement guidelines (Section 3.3); 2.3 Requirements for the provision of aerosol high-power lidar compliant/labelled data to the ACTRIS Data Centre ACTRIS Centre for Aerosol Remote Sensing C . A . R . S . 22 December 2025 7 / 21 In order to provide ACTRIS compliant or labelled data (as applicable), the ACTRIS National Facilities operating aerosol high-power lidars and the EARLINET-ACTRIS associated lidar station must: • Undergo the labelling (Labelling of the ACTRIS National Facilities operating Aerosol Remote Sensing instruments) or the acceptance process (Guidelines for EARLINET-ACTRIS associated stations to access QA services), as applicable; • Fulfill the technical requirements for the aerosol high-power lidar (Technical requirements for ACTRIS Aerosol Remote Sensing Observational Platforms or Technical requirements for ACTRIS Mobile Platforms operating aerosol high-power lidars, as applicable) • Apply the operation procedures for the aerosol high-power lidar (Standard Operation Procedure for aerosol high-power lidars) • Apply the quality assurance procedures for the aerosol high-power lidar and tools (Quality Assurance Procedure for aerosol high-power lidars) • Implement the general principles for data provision (Section 2.1 of this document) • Follow the measurement schedule (Section 3 of this document) • Implement and follow the workflow for the data provision to the ACTRIS Data Centre (Section 4 of this document) 2.4 Other considerations The ACTRIS National Facilities operating aerosol high-power lidars and the EARLINET-ACTRIS associated lidar station have the responsibility to securely and permanently store and archive native data from the instruments and Level 0 data (raw data in the format to be submitted to the DC unit). In order to guarantee secure store and archive of such data, the National Facilities should organize their archive in two different physical locations. ACTRIS operational data is produced with SCC operational configurations, following the operational workflow described in Section 4 and Annex 2 of this document. The SCC operational configurations are proposed by the Principle Investigator and checked by CARS and ARES. ACTRIS National Facilities operating aerosol high-power lidars and the EARLINET-ACTRIS associated lidar station are responsible for reporting and updating the technical information about the instrument, and setting the appropriate SCC operational configurations to process the data. The list of parameters to be included in the SCC operational configurations is available in Annex 3. The validity period of the SCC operational configurations is proposed by CARS based on the QA activities, is approved by the Principal Investigator, and is implemented in the SCC database by ARES. A valid SCC operational configuration is locked for inhibiting editing. At the request of CARS, ARES can extend the validity of an SCC operational configuration. ACTRIS Centre for Aerosol Remote Sensing C . A . R . S . 22 December 2025 8 / 21 3 Measurement schedule 3.1 Measurement schedule for the ACTRIS Aerosol Remote Sensing Observational Platforms ACTRIS high-power aerosol lidars should be preferably operated continuously, in the absence of precipitation, fog or very low clouds. If the instrument is not automated, it must at least provide unbiased long-term regular observations following the pre-defined schedule with 5 observations per week, each with a duration of minimum 3 hours: Tab. 1 Minimum schedule of measurements to be performed, weather permitting: No. Day of the week Time of the day 1. Monday 11:30 -14:30 LT 2. Monday 18:30 - 23:30 UT 3. Wednesday 11:30 - 14:30 LT 4. Thursday 18:30 - 23:30 UT 5. Friday 18:30 - 23:30 UT In order to allow comprehensive climatology, Saturday daytime measurements are recommended but not mandatory. Furthermore, measurements are to be performed upon alert, e.g., in hazardous situations such as volcanic eruptions, for special events such as dust outbreaks or forest fires, and for dedicated satellite validation purposes. Satellite validation measurements will follow a specific strategy. Alerts and measurement schedules for collocated observations during satellite overpasses will be distributed by the ARES unit of the ACTRIS Data Centre. Denser measurements schedule and continuous measurements are advisable for having a better understanding of the aerosol 4d fields and properties. 3.2 Measurement schedule for the ACTRIS Aerosol Remote Sensing Mobile Platforms The schedule in case of Mobile platforms is defined by the user according to the objectives of the campaign / experiment. Although not mandatory, it is advisable that, while not involved in a campaign or specific experiment, the lidars onboard of Mobile Platforms follow the measurement schedule of the ACTRIS Aerosol Remote Sensing Observational Platforms (Section 3.1). 3.3 Measurement schedule for the EARLINET-ACTRIS associated lidar station In order to allow comprehensive climatology, EARLINET-ACTRIS associated lidar station must follow the schedule for the ACTRIS Aerosol Remote Sensing Observational Platforms (Section 3.1). ACTRIS Centre for Aerosol Remote Sensing C . A . R . S . 22 December 2025 9 / 21 4 Step-by-step workflow for provision of aerosol high-power data to the ACTRIS Data Centre The schematics of the workflow for provision of aerosol high-power data to the ACTRIS Data Centre is presented in Annex 2. 4.1 Steps for setting up a valid operational configuration In order to setup a valid operational configuration, the station must follow the following steps: Preliminary steps (initial acceptance) 1. Contact CARS ([email protected]) to obtain an account for accessing the CARPORT platform 2. Contact ARES ([email protected]a.cnr.it) to obtain a station ID (3-letters code of the station, unique), and an account for accessing the Single Calculus Chain 3. Register the lidar in CARPORT (https://carport.inoe.ro/) – CARS will initially evaluate the instrument, and will accept the support provision 4. Register the lidar to the Single Calculus Chain (htts://scc.imaa.cnr.it). The instrument will be associated to a Lidar ID (for the internal use during data processing) and a lidar PID (for the public landing page) 5. Fill in the instruments settings in the SCC database – telescopes, lasers, channels - and the data products to be calculated. All the information provided is automatically accessible through the Handbook of Instruments (HoI). Regular workflow (performance evaluation) 6. Setup SCC lidar configurations (as many combinations of channels and products as needed) in the SCC database – to each configuration will be assigned a configuration ID 7. Introduce in CARPORT the station ID and the lidar ID provided by the SCC 8. Select the SCC configurations to be used in an operational way (1 daytime + 1 nighttime, or 1 all day) 9. Perform the QA tests for the SCC configurations selected as candidates for the operations, and submit them to CARS (also via CARPORT), as scheduled 10. Read and approve the QA report in CARPORT a. The QA report may contain different values for the instrument parameters than initially set; the PI must approve the new values b. The QA report will contain CARS' recommendation for the start date of validity and end date of validity for each of the SCC operational configurations; the PI must approve the validity periods. Following the approval of the QA report (including the validity period for each operational configuration), CARS will inform ARES, which will check the retrieval parameters (like assumed lidar ratio and angstrom) and if everything is fine will lock the SCC operational configurations. 4.2 Life cycle of the SCC operational configurations SCC operational configuration has a standard validity period of 1 year. ACTRIS Centre for Aerosol Remote Sensing C . A . R . S . 22 December 2025 16 / 21 No. Parameter Description Apply to Value given by Value Source of info / Applicable test Approval / check by polarization calibration (K) possibly by manufacturer 13 Handling of G, H, K parameters Specify how G, H and K should be handled in case multiple values are available (average, take the closest in time to the measurements, take the latest....) all polarization sensitive channels PI Mandatory choice: Take the closest in time before the measurement ARES automatic check Polarisation calibration factor Result from the polarisation calibration measurements. all polarization sensitive channel pairs PI Number ATLAS CARS 14 Maximum statistical error below 2km Maximum statistical error below 2km for each SCC products. Vertical resolution and time integration of each SCC product is fixed to meet this condition. all SCC products PI Default choice: 50% Acceptable range: 10%-50% ARES automatic check 15 Maximum statistical error above 2km Maximum statistical error above 2km for each SCC products. Vertical resolution and time integration of each SCC product is fixed to meet this condition. all SCC products PI Default choice: 50% Acceptable range: 20%-100% ARES automatic check ACTRIS Centre for Aerosol Remote Sensing C . A . R . S . 22 December 2025 17 / 21 No. Parameter Description Apply to Value given by Value Source of info / Applicable test Approval / check by 16 Product detection limit Detection limit of each SCC products at far range. Max. abs. statistical error of products. all SCC products PI Default choice3: 5E-6 for extinction 1E-7 for backscatter ARES automatic check 17 Minimum signal height Minimum height to be used for the calculation of each SCC products. This value should be set to the full overlap height (if no overlap correction or signal ratio is applied) all SCC signals CARS Number Telecover CARS 18 Maximum signal height Maximum height to be used for the calculation of each SCC products. This value should be set to the height at which the lidar signal starts to be dominated by distortions, or at the uppermost height where strong aerosol layers (Cirrus or stratospheric layers can be measured in optimum conditions) all SCC signals CARS Default choice: 20 km Rayleigh fit incl. dark measurement CARS 3 as suggested in the SCC interface ACTRIS Centre for Aerosol Remote Sensing C . A . R . S . 22 December 2025 18 / 21 No. Parameter Description Apply to Value given by Value Source of info / Applicable test Approval / check by 19 Preprocessing integration time Time integration to consider in the pre-processing phase all the low resolution SCC products PI Default choice: 30 min ARES automatic check 20 Preprocessing vertical resolution Vertical resolution at which interpolate pre-processed signal all the low resolution SCC products PI Default choice: 15 m ARES automatic check 21 Error calculation method Method used to compute statistical error of each SCC product. all SCC products PI Mandatory choice: Monte Carlo ARES automatic check 22 Monte Carlo iterations Number of iterations to consider for the statistical error estimation using Monte Carlo simulations all the SCC products for which Monte Carlo simulations are used to compute statistical uncertainties PI Mandatory choice: 30 - ARES automatic check 23 Extinction method Method to use for the extinction retrieval all extinction related SCC products PI Mandatory choice: Non-weighted linear fit - ARES automatic check 24 Particle Angstrom exponent Value of particle Angstrom exponent to use in the extinction retrieval all extinction related SCC products PI Number Site characteristics (climatological values from the photometer measurements) ARES 25 Elastic backscatter method Method to use for the elastic backscatter retrieval all Raman backscatter related SCC products PI Mandatory choice: Fernald-Klett - ARES automatic check ACTRIS Centre for Aerosol Remote Sensing C . A . R . S . 22 December 2025 19 / 21 No. Parameter Description Apply to Value given by Value Source of info / Applicable test Approval / check by 26 Fixed lidar ratio value The fixed value of lidar ratio to use in the elastic backscatter retrieval. all elastic backscatter related SCC products PI Number Site characteristics (climatological values from past Raman measurements) ARES 27 Error on lidar ratio value The uncertainty on fixed value of lidar ratio to use in the elastic backscatter retrieval. all elastic backscatter related SCC products PI Number Site characteristics (climatological values from past Raman measurements) ARES 29 Backscatter calibration options Options to use for the calibration of Raman/elastic backscatter. These options specify the minimum and maximum heights, the width of the sliding window and the reference backscatter value to use in the Raman backscatter calibration. all backscatter related SCC products PI Preferred choice: ID 364 Alternative: ID 395 ARES automatic check 30 Minimum Backscatter ratio for PLDR Minimum backscatter ratio to consider in the calculation of particle linear depolarization ratio. particle linear depolarization ratio SCC products CARS Default choice: 1.1 ATLAS CARS 4 Reference height search: 2-20 km, window width: 500m, reference backscatter ratio: 1.0 5 Reference height search: 2-20 km, window width: 1000m, reference backscatter ratio: 1.0 ACTRIS Centre for Aerosol Remote Sensing C . A . R . S . 22 December 2025 20 / 21 Annex 4 Methodology for calculating data coverage for the lidar-related variables at stationary stations The pre-defined schedule for the ACTRIS aerosol remote sensing National Observational Platforms, and for the EARLINET-ACTRIS associated lidar stations includes 5 observations per week, each with a duration of minimum 3 hours 6 , weather permitted (Section 3.1 Measurement schedule for the ACTRIS Aerosol Remote Sensing Observational Platforms): Not all the variables can be measured in all timeslots. Raman channels can only be operated during night-time due to the low signal-to-noise ratio in daytime conditions. Therefore, the minimum number of expected measurements which can be used to calculate a certain variable with the above mandatory measurement schedule is variable-dependent, as follows: Variable Variable type Constraint No. of expected measurements Attenuated backscatter profile M - 5 per week Volume depolarization profile M - 5 per week Particle backscatter coefficient profile M - 5 per week Particle extinction coefficient profile M Night time 3 per week Lidar ratio profile M Night time 3 per week Ångström exponent profile O Night time 3 per week Backscatter-related Ångström exponent profile O - 5 per week Particle depolarization ratio profile M - 5 per week Column integrated extinction M Night-time 3 per week The above list is valid for each wavelength. The effective number of expected measurements take into account the minimum requirement for high power lidars: 1 wavelength equipped with both Raman and depolarization. Note: Only raw data (input to SCC) are considered in the NF evaluation, as measure of the correct NF operation is terms of measurement performance, collection and submission to the DC. The equation for calculating the data coverage (percentage) over the evaluation period for a variable X is: %𝐷𝑎𝑡𝑎 𝑐𝑜𝑣𝑒𝑟𝑎𝑔𝑒𝑣𝑎𝑟𝑖𝑎𝑏𝑙𝑒𝑋 =𝑁𝑜.𝑜𝑓𝑐𝑜𝑚𝑝𝑙𝑖𝑎𝑛𝑡 𝑚𝑒𝑎𝑠𝑢𝑟𝑒𝑚𝑒𝑛𝑡𝑠 𝑐𝑜𝑙𝑙𝑒𝑐𝑡𝑒𝑑 𝑓𝑜𝑟 𝑣𝑎𝑟𝑖𝑎𝑏𝑙𝑒 𝑋 𝑁𝑜. 𝑜𝑓 𝑒𝑥𝑝𝑒𝑐𝑡𝑒𝑑 𝑚𝑒𝑎𝑠𝑢𝑟𝑒𝑚𝑒𝑛𝑡𝑠 𝑓𝑜𝑟 𝑣𝑎𝑟𝑖𝑎𝑏𝑙𝑒 𝑋−𝑁𝑜. 𝑜𝑓 𝑎𝑐𝑐𝑒𝑝𝑡𝑒𝑑 𝑒𝑥𝑐𝑒𝑝𝑡𝑖𝑜𝑛𝑠, where: • No. of compliant measurements collected is the number of datasets of at least one hour according to the schedule successfully submitted to the Single Calculus Chain and uploaded to the ACTRIS-EARLINET database • No. of expected measurements is 5 or 3 (according to schedule, depending on the variable) multiplied by the number of weeks during the evaluation period • Accepted exceptions are: 6 The 3 hours duration is necessary in order to identify at least 1 hour with clear sky (or at least without low clouds), so that the retrieval of the optical parameters is possible. ACTRIS Centre for Aerosol Remote Sensing C . A . R . S . 22 December 2025 21 / 21 o Scheduled measurements not performed because the instrument is under repair, or on the way to / from being repaired o Scheduled measurements not performed because of very low clouds, precipitation or fog (signal saturation) o Scheduled measurements not performed because of force majeure (an extraordinary event or circumstance beyond the control of the facility, such as a war, strike, riot, crime, epidemic, or sudden legal change preventing the facility from fulfilling its obligations) According to the Standard Operation Procedure for aerosol high-power lidars, the aerosol remote sensing NFs are requested to maintain a log of the measurements not performed and reasons for not performing. This log is hosted by CARS and used by ARES to identify the accepted exceptions above.