Tracing vessel-generated sea spray aerosols to support marine cloud brightening

Abstract. The aim of Marine Cloud Brightening (MCB) is to purposely inject artificially generated sea spray aerosols (SSA_a) into low-level marine clouds, increasing their albedo with the objective of cooling the ocean surface beneath the clouds. Over the Great Barrier Reef (GBR), Australia, MCB is under research as a potential intervention to alleviate coral bleaching. Attributing cloud microphysical perturbations to MCB requires the ability to detect and trace SSA_a and distinguish that from other local sources, including the exhaust from research aircraft, research vessel, and generators used to power the MCB technology. Here, we present a novel “tracer” method using in situ airborne measurements of the non-volatile aerosol ratio to identify the aerosol plume emanating from the spraying vessel and apportion aerosols between SSA_a and combustion exhaust. A total of 14 flights were analysed using downwind, upwind, crosswind, or a combination of these sampling legs. Aircraft-sampled plumes of SSA_a and exhaust exhibited contrasting non-volatile properties. Aerosol plume detections were identified using aerosol number concentration, and a mixing ratio model was constructed to apportion the detected plumes between SSA_a and exhaust particulates. The “tracer” method reliably allowed tracking of the vessel-generated SSA_a across all sampling events as the plume spread downwind from the spraying vessel. The majority of sampled plumes were found to be mixed SSA_a and exhaust. This technique enables the tracking and identification of MCB aerosol in the natural marine atmosphere without the addition of a chemical tracer. Reliable plume identification is a prerequisite for studying the transport and dispersion of MCB aerosols and determining how they influence aerosols and clouds in the marine boundary layer.