Coral reef exposure increases aerosol and cloud condensation nuclei over the Great Barrier Reef

Abstract. The Great Barrier Reef (GBR) is the largest reef ecosystem in the world and a home to diverse marine life. Lower troposphere aerosol concentrations and dynamics over the GBR are important for cloud and radiative processes, however their in-situ characterisation is lacking in the literature. In this study, we present analysis of multi-year in situ aerosol measurements over the GBR, showing for the first time direct observations of coral reefs contributing to aerosol loading over the reef. Our results show that aerosol concentrations over the GBR are typical of a clean coastal environment, and the aerosol loading over the GBR is primarily influenced by long-range transport of aerosol particles. However, a non-negligible effect from local sources is also observed. The fraction of ultrafine particles in the aerosol population increases in air masses that pass over the coral reef ecosystem. Our statistical modelling shows that cloud condensation nuclei concentrations over the GBR are dominantly driven by availability of accumulation and Aitken mode aerosol particles with negligible effects from local meteorology. While accumulation mode particle concentrations have the strongest impact on cloud condensation nuclei concentrations, counterfactual modelling shows that Aitken mode concentrations can contribute up to 6 % of cloud condensation nuclei over the reef.