Adaptation of the CIMEL-318T to Shipborne Use: Three Years of Automated AERONET-Compatible Aerosol Measurements Onboard the Research Vessel Marion Dufresne

Abstract. The Earth's oceans play a critical role in regulating the global climate and atmospheric processes, with marine aerosols significantly influencing weather patterns, air quality, and climate dynamics. Despite extensive land-based aerosol monitoring through networks like AERONET (AErosol RObotic NETwork), marine aerosol characterization remains a critical gap, due in part to the logistical challenges of conducting measurements in remote oceanic environments. To address this, robust, automated, and precise monitoring systems adapted for research vessels are essential.

This study reports on the first three years (July 2021 to June 2024) of continuous aerosol optical depth (AOD) measurements collected aboard the R.V. Marion Dufresne using a ship-adapted CIMEL 318-T automatic photometer in the frame of MAP-IO (Marion Dufresne Atmospheric Program – Indian Ocean) program. The dataset comprises over 25,000 quality-assured AOD measurements, primarily from the South-West Indian Ocean region, revealing mid-range AOD and Angström exponent values consistent with previous studies. The reliability and precision of the system were validated through dual-instrument comparisons conducted during the Amaryllis-Amagas/Transama campaign, yielding strong correlations (R > 0.96 for different wavelengths) and low root-mean-squared errors (RMSE < 0.01), within the expected error margins for AERONET ground-based sites, and benefiting from the continue tracking system implemented for ship-adapted version. Additionally, recurrent comparisons with the ground-based AERONET site at Saint-Denis (La Réunion) further confirm the system's accuracy, presenting good correlations despite differences in altitude and the greater influence of local urban aerosols in Saint-Denis.

Retrievals from spectral AOD and sky radiance data collected over the Indian Ocean during a biomass burning event (October 2023) demonstrate the feasibility of deriving detailed aerosol properties, including size distribution and optical characteristics, from shipborne platforms adapted for marine conditions, following the protocols of the AERONET standard algorithm. Observed SSA values, ranging from 0.88 to 0.95 with higher absorption at longer wavelengths, align with those recorded at the Saint-Denis site during the event and are consistent with expectations for a mixture of biomass burning (at the end of the dry season) and sea salt aerosols. These preliminary results underscore the potential of shipborne systems to provide comprehensive aerosol characterization in remote marine environments.