Automatic Ship-Based Photometers for Enhanced Aerosol Characterization Over the Open Ocean: Towards a Global Network

Oceans remain severely under-sampled in aerosol optical properties, limiting our ability to assess marine aerosol–climate feedbacks, long-range transport and to validate satellite products over two-thirds of Earth’s surface. The Sun–sky–lunar photometer CIMEL CE318-T has been successfully adapted for autonomous shipborne operation through the PHOTONS Observation Service (ACTRIS, Univ. Lille, CNRS) in collaboration with CIMEL and within ACTRIS-CARS activities. The system is fully compatible with AERONET processing. Here we summarize recent developments and multi-year results supporting the deployment of a global network of automatic ship-based photometers.

Since 2021, a CE318-T has operated continuously on the research vessel (R.V.) Marion Dufresne under the framework of MAP-IO program. Between July 2021 and June 2024, it collected >25,000 Level 1.5 measurements in the southwestern Indian Ocean, with mean AOD (Aerosol Optical Depth) of 0.09 ± 0.07 (440 nm) and 0.05 ± 0.03 (870 nm), and EAE (Extinction Angstrom Exponent) of 0.7 ± 0.4, typical of clean marine conditions. A biomass-burning aerosol (BBA) event allowed retrieval of microphysical properties (size distribution, refractive index), showing the capability of the system under real ship-motion.

During TRANSAMA campaign (La Réunion–Barbados, Apr–May 2023), two CE318-T photometers combined with a micropulse lidar aboard R.V. Marion Dufresne provided complementary column and vertical information. Despite low AOD (0.08 ± 0.04 at 440 nm) in the remote South Atlantic, the lidar detected transported continental layers not evident from column-integrated data alone. Photometer intercomparison showed excellent agreement (R > 0.96; RMSE = 0.005–0.008). Motion-induced degradation on data quality highlighted the need for the ongoing instrumental tests, using a motion-simulation hexapode platform.

A second permanent site aboard the R.V. Gaia Blu (CNR, Italy) has collected >20,000 measurements mainly in the Mediterranean since Feb 2024. Mean AOD (0.19 ± 0.14 at 440 nm) and EAE (1 ± 0.4) reflect more variable aerosol regimes, including BBA and Saharan dust. Comparisons with nearby AERONET ground-based stations validated retrieval quality. In addition, the installation of a 3D scanning lidar (Sep 2025) further enhances observations of aerosol vertical structure and air–sea interactions.

These first results demonstrate the capability of ship-based photometers, and their synergy with lidar, to fill critical observational gaps over the oceans. Improved data acquisition strategies addressing vessel structure and motion are ongoing. To date, five ship-photometers operate across major maritime regions: R.V. Marion Dufresne-France (Indian Ocean), R.V. Gaia Blu-Italy (Mediterranean Sea), R.V. Sarmiento de Gamboa-Spain (Atlantic Ocean), R.V. Perseverance-France (currently Pacific Ocean), and MS Richard With-Norway (Arctic/Norwegian coast). These installations represent a major European step toward the first global network of automatic shipborne photometers to improve aerosol characterization over remote oceans and support satellite CAL/VAL.