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

The first multi-instrument nocturnal aerosol optical depth (AOD) intercom-parison campaign was held at the high-mountain Iza ̃na Observatory (Tener-ife, Spain) in June 2017, involving 2-minutes synchronous measurements fromtwo different types of lunar photometers (Cimel CE318-T and Moon Preci-sion Filter Radiometer, LunarPFR) and one stellar photometer. The Robotic Lunar Observatory (ROLO) model developed by the U.S. Geological Survey(USGS) was compared with the open-access ROLO Implementation for Moonphotometry Observation (RIMO) model. Results showed rather small differ-ences at Iza ̃na over a 2-month time period covering June and July, 2017(±0.01 in terms of AOD calculated by means of a day/night/day coherencetest analysis and±2 % in terms of lunar irradiance). The RIMO model hasbeen used in this field campaign to retrieve AOD from lunar photometricmeasurements. No evidence of significant differences with the Moon’s phase angle wasfound when comparing raw signals of the six Cimel photometers involved inthis field campaign.The raw signal comparison of the participating lunar photometers (Cimeland LunarPFR) performed at coincident wavelengths showed consistent mea-surements and AOD differences within their combined uncertainties at 870 nmand 675 nm. Slightly larger AOD deviations were observed at 500 nm, point-ing to some unexpected instrumental variations during the measurement pe-riod.Lunar irradiances retrieved using RIMO for phase angles varying between0◦and 75◦(full Moon to near quarter Moon) were compared to the irradi-ance variations retrieved by Cimel and LunarPFR photometers. Our resultsshowed a relative agreement within±3.5 % between the RIMO model andthe photometer-based lunar irradiances.The AOD retrieved by performing a Langley-plot calibration each nightshowed a remarkable agreement (better than 0.01) between the lunar pho-tometers. However, when applying the Lunar-Langley calibration using RIMO,AOD differences of up to 0.015 (0.040 for 500 nm) were found, with differ-ences increasing with the Moon’s phase angle. These differences are thoughtto be partly due to the uncertainties in the irradiance models, as well asinstrumental deficiencies yet to be fully understood.High AOD variability in stellar measurements was detected during thecampaign. Nevertheless, the observed AOD differences in the Cimel/stellarcomparison were within the expected combined uncertainties of these twophotometric techniques. Our results indicate that lunar photometry is amore reliable technique, especially for low aerosol loading conditions.The uncertainty analysis performed in this paper shows that the com-bined standard AOD uncertainty in lunar photometry is dependent on thecalibration technique (up to 0.014 for Langley-plot with illumination-basedcorrection, 0.012-0.022 for Lunar-Langley calibration, and up to 0.1 for the 2 Sun-Moon Gain Factor method). This analysis also corroborates that theuncertainty of the lunar irradiance model used for AOD calculation is withinthe 5-10 % expected range.This campaign has allowed us to quantify the important technical diffi-culties that still exist when routinely monitoring aerosol optical propertiesat night-time. The small AOD differences observed between the three typesof photometers involved in the campaign are only detectable under pristinesky conditions such as those found in this field campaign. Longer campaignsare necessary to understand the observed discrepancies between instrumentsas well as to provide more conclusive results about the uncertainty involvedin the lunar irradiance model

Climatology of aerosols over the Caribbean islands: aerosol types, synoptic patterns and transport

We present a climatological study of aerosols in four representative Caribbean Sea islands that is based on daily mean values of aerosol optical properties for the period 2008¿16, using the aerosol optical depth (AOD) and Ångström exponent (AE) to classify the dominant aerosol type. A climatological assessment of the spatiotemporal distribution of the main aerosol types, their links with synoptic patterns, and the transport from different sources is provided. Maximum values of AOD occur in the rainy season, coinciding with the minimum in AE and an increased occurrence of dust, whereas the minimum of AOD occurs in the dry season, due to the predominance of marine aerosols. Marine and dust aerosol are more frequent in the easternmost islands and decrease westward because of an increase of continental and mixture dust aerosols. Therefore, the westernmost station displays the most heterogeneous composition of aerosols. Using a weather-type classification, we identify a quantifiable influence of the atmospheric circulation in the distribution of Caribbean aerosols. However, they can occur under relatively weak and/or diverse synoptic patterns, typically involving transient systems and specific configurations of the Azores high that depend on the considered station. Backward trajectories indicate that dry-season marine aerosols and rainy season dust are transported by air parcels traveling within the tropical easterly winds. The main source region for both types of aerosols is the subtropical eastern Atlantic Ocean, except for Cuba, where the largest contributor to dry-season marine aerosols is the subtropical western Atlantic. Different aerosol types follow similar pathways, suggesting a key role of emission sources in determining the spatiotemporal distribution of Caribbean aerosols.. This research was supported by the CSIC (“Consejo Superior de 785 Investigaciones Científicas” of Spain) under project COPA20207. We thank Jack 786 Molinie, Joseph M. Prospero and Brent N. Holben for his effort in establishing and 787 maintaining the Guadeloupe, Ragged Point and La Parguera AERONET sites. The sun 788 photometer at Camagüey was provided by the Grupo de Óptica Atmosférica of the 789 University of Valladolid (UVA), Spain under a cooperation agreement with INSMET, 790 Cuba. The agreement signed in 2007, still in place until the present, has been successful 791 despite limitations and obstacles (Antuña-Marrero et al., 2016; GOAC, 2020). Special 34 792 recognition to Prof. Ángel de Frutos and Victoria Cachorro from UVA for supporting 793 the joint research on atmospheric aerosols. Also, INSMET is recognized by its support 794 until the present. Also, we would like to acknowledge NCEP/NCAR Reanalysis team 795 for making the data publicly available. Version 3.0 of AERONET data were freely 796 downloaded from the AERONET web site (https://aeronet.gsfc.nasa.gov, last access: 8 797 June 2020). NCEP-NCAR Reanalysis data were downloaded from 798 (https://www.esrl.noaa.gov/psd/data/gridded/data.ncep.reanalysis2.html, last access: 25 799 Mayo 2018). We thank the Editor and three anonymous reviewers for their valuable 800 comments and suggestions

Characterizing aerosol optical depth measurements and forecasts of Saharan dust events at Camagüey, Cuba, during July 2009

Saharan dust aerosol optical depth (AOD) sun-photometer measurements conducted at Camagüey, Cuba, during July 2009 are compared with the SKIRON model AOD forecasts and the AOD MODIS measurements. The differences for the daily mean AOD values from SKIRON forecasts and from MODIS measurements (both Terra and Aqua) are lower than the differences of the daily AOD maximums. The results demonstrate the capabilities for developing an alert and tracking system for Saharan dust events across the Atlantic. © Sociedad Española de Óptica