This RAPID project supports the participation of US scientists in the international HALO-south field campaign to measure clouds and aerosols over the Southern Ocean in austral spring. The HALO-south mission is a large collaborative project with partners in both Germany and New Zealand. The Southern Ocean is one of the cloudiest areas on Earth and is key for hemispheric constraints on Earth’s radiation budget and global cloud feedbacks. Currently, atmospheric models poorly represent both aerosols and clouds in this region. This effort will lead to a better understanding of the sources and sinks of cloud-nucleating aerosols over the Southern Ocean. This project will make a substantial contribution to guiding flight planning and mission science during the HALO-south field campaign. Aerosol number concentrations will be forecast, with a focus on predicting new particle formation events and aerosol-cloud interactions. Data collected during the field campaign will be analyzed to better understand aerosol sources and sinks in the Southern Ocean and why most general circulation models fail to accurately predict Southern Ocean cloud concentration nuclei and cloud droplet concentrations. Several graduate students will participate in the project. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.