Global distribution of sea salt aerosols: new constraints from in situ and remote sensing observations
1Department of Atmospheric Sciences, University of Washington, Seattle, Washington, USA
2Pacific Marine Environmental Laboratory, National Oceanic & Atmospheric Administration, Seattle, Washington, USA
3School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts, USA
Abstract. We combine in situ measurements of sea salt aerosols (SSA) from open ocean cruises and ground-based stations together with aerosol optical depth (AOD) observations from MODIS and AERONET, and the GEOS-Chem global chemical transport model to provide new constraints on SSA emissions over the world's oceans. We find that the GEOS-Chem model using the Gong (2003) SSA source function overestimates cruise observations of coarse mode SSA mass concentrations by factors of 2–3 at high wind speeds over the cold waters of the Southern Ocean and North Atlantic. Furthermore, the model systematically underestimates SSA over the warm tropical waters of the Central Pacific, Atlantic, and Indian Oceans. This pattern is confirmed by SSA measurements from a global network of 15 island and coastal stations. The model discrepancy at high wind speeds (>6 m s−1) has a clear dependence on sea surface temperature (SST). We use the cruise observations to derive an empirical SSA source function depending on both wind speed and SST. Implementing this new source function in GEOS-Chem results in improved agreement with in situ observations, with a decrease in the model bias from +64% to +33% for the cruises and from +32% to −5% for the ground-based sites. We also show that the wind speed-SST source function significantly improves agreement with MODIS and AERONET AOD, and provides an explanation for the high AOD observed over the tropical oceans. With the wind speed-SST formulation, global SSA emissions show a small decrease from 5212 Mg/year to 4545 Mg/yr, while the SSA burden decreases from 9.1 to 8.5 mg/m2. The spatial distribution of SSA, however, is greatly affected, with the SSA burden increasing by 50% in the tropics and decreasing by 40% at mid- and high-latitudes. Our results imply a stronger than expected halogen source from SSA in the tropical marine boundary layer. They also imply stronger radiative forcing of SSA in the tropics and a larger response of SSA emissions to climate change than previously believed.