Evaluating representation of marine aerosols in Norwegian Earth System Model

Oceans contribute to atmospheric aerosol concentration by directly emitting particles and releasing aerosol precursor gases which later reacts and condenses to form secondary aerosols. Marine aerosols modulate the Earth’s radiative budget by scattering and absorbing radiation and influencing the cloud microphysics. Aerosol–cloud interactions are one of the largest sources of uncertainty in climate projections, and understanding the natural, pre-industrial aerosol background is essential for constraining anthropogenic influences. Marine aerosols constitute a substantial fraction of this natural aerosol burden.
In this study, we assess the marine aerosols predictions and their effects on climate using the Norwegian Earth System Model version 2 (NorESM2). NorESM2 uses OsloAero6, a production tagged aerosol module to simulate aerosol size distribution, aerosol mass, detailed aerosol physical, chemical and optical properties. OsloAero6 includes marine aerosols as sea salt emissions driven by wind speed and sea surface temperature (SST), primary organic aerosols (POA) emissions linked to wind speed, SST and chlorophyll concentrations, secondary organic (SOA) and sulphate aerosols formed from the oxidation of dimethyl sulphide (DMS).
Ten-year simulations from 2009-2019, nudged to ERA-Interim reanalysis data are analyzed. Predicted total aerosol number concentration (N_total) and the number concentration of particles larger than 100 nm (N₁₀₀) are evaluated against long-term surface observations from four marine sites: Ascension Island, Zeppelin, Graciosa Island, and La Réunion.
At Ascension Island, the model overestimates N_total by up to 500% and N₁₀₀ by up to 200% compared to observation, largely due to long-range transport of aerosols from African continent. At Zeppelin, N₁₀₀ is underestimated by up to 200% and N_total by up to 100% compared to observation, maybe due to underestimated long-range transport, missing aerosol sources, or an underrepresented condensation sink that limits particle growth. At Graciosa Island and La Réunion, predicted aerosol number concentrations agree with observations within 30%. Overall, predicted N_total agrees reasonably well with observations across sites but underestimates N₁₀₀, while capturing the observed seasonal variability.
To quantify radiative impacts, sensitivity simulations were performed by removing marine aerosol sources to study the direct radiative effect (DRE) and aerosol indirect effect (AIE). The removal of sea salt results in a warmer climate, with decrease in the magnitude of globally averaged DRE and AIE by 0.013 Wm⁻² and 0.003 Wm⁻², respectively, relative to the control simulation. This demonstrates the net cooling effect of sea salt through radiative scattering and cloud interactions. Removing POA and DMS also leads to warming driven by decrease in the magnitude of AIE, with negligible changes in DRE, consistent with their weaker direct radiative influence. These results highlight the importance of prediction of marine aerosol size and composition, and their role in regulating Earth’s radiative balance and cloud properties.

Acknowledgements. We acknowledge the U.S. DOE ARM user facility for providing data from the Graciosa (ENA) and Ascension Island (ASI) sites. We also thank the NILU EBAS database and the contributing networks (ACTRIS, GAW-WDCA, EMEP) for the chemical composition, size distribution, and meteorological data from the Zeppelin (Ny-Ålesund) and Maïdo (La Réunion) observatories.