First generation outdoor marine cloud brightening trial increases aerosol concentration at cloud base height

Regional scale marine cloud brightening (MCB) has been proposed as a novel climate intervention to reduce the impact of global warming and associated marine heatwaves on the Great Barrier Reef. The concept relies on artificially generated sea spray aerosols (SSA) at the ocean surface and their transport in sufficient quantities to low-level maritime clouds. A portion of the SSA that reaches cloud height can act as additional cloud condensation nuclei and modify cloud microphysical properties, potentially reducing the amount of solar radiation reaching the sea surface. Although modelling data supports the MCB concept, field experiments demonstrating the dispersion of artificially produced SSA to clouds have not been previously reported. Here, we show that within a field of low-level trade wind cumulus-type clouds, an aerosol plume generated at the sea surface on board a research vessel was rapidly advected to cloud base height. Aircraft measurements conducted during two different sampling strategies, detected the aerosol plume from the vessel’s sea water atomisation system just below cloud bases at 700-900 m altitudes. For an estimated surface level aerosol production rate of 4 × 10¹⁴ s⁻¹, aerosol concentrations at cloud base were detected with peak number concentrations of up to ∼1 × 10³ cm⁻³. Although such production rates of aerosol particles are orders of magnitude less than what is envisioned for any practical implementation of MCB, our results indicate that cloud perturbation experiments should now be possible using surface-produced SSA.