Characterizing the Crosswind Structure of Artificial Seawater Droplet Plumes during a Sea Trial on the Great Barrier Reef

Shading coral from high solar irradiance has been suggested as an effective approach to reduce the risk of coral mortality due to bleaching. Spraying seawater droplets is potentially an effective way to provide shading at the scale of a single reef or a collection of reefs. For the first time, we provide in situ characterizations of artificially made seawater droplet plumes over coral reefs. Here, we analyzed 21 crosswind transects at the surface level carried out at multiple downwind distances of 1.5 km or less from the prototype seawater droplet generator. The average particle number size distribution of the sprayed seawater droplets showed a primary mode of 124 nm dry diameter and agreed with that collected in laboratory experiments. The observed particle number concentration of seawater droplet aerosols ranged from 10³ to 10⁵ cm⁻³ and exhibited a monotonic decreasing trend with increasing plume transport time downwind of the seawater droplet generator. With a downwind transport time scale of up to 200 s, the estimated plume width can expand from 50 m to nearly 500 m. The results of this study will assist in estimating the shading provided by the seawater droplet plume and inform future scale-up of the technique on target reef areas.