Global distribution of sea salt aerosols: new constraints from in situ and remote sensing observations
We combine in situ measurements of sea salt aerosols (SS) from open\nocean cruises and ground-based stations together with aerosol optical\ndepth (AOD) observations from MODIS and AERONET, and the GEOS-Chem\nglobal chemical transport model to provide new constraints on SS\nemissions over the world's oceans. We find that the GEOS-Chem model\nusing the Gong (2003) source function overestimates cruise observations\nof coarse mode SS mass concentrations by factors of 2-3 at high wind\nspeeds over the cold waters of the Southern, North Pacific and North\nAtlantic Oceans. Furthermore, the model systematically underestimates SS\nover the warm tropical waters of the Central Pacific, Atlantic, and\nIndian Oceans. This pattern is confirmed by SS measurements from a\nglobal network of 15 island and coastal stations. The model discrepancy\nat high wind speeds (>6 m s(-1)) has a clear dependence on sea surface\ntemperature (SST). We use the cruise observations to derive an empirical\nSS source function depending on both wind speed and SST. Implementing\nthis new source function in GEOS-Chem results in improved agreement with\nin situ observations, with a decrease in the model bias from +64% to\n+33% for the cruises and from +32% to -5% for the ground-based sites.\nWe also show that the wind speed-SST source function significantly\nimproves agreement with MODIS and AERONET AOD, and provides an\nexplanation for the high AOD observed over the tropical oceans. With the\nwind speed-SST formulation, global SS emissions show a small decrease\nfrom 5200 Mg yr(-1) to 4600 Mg yr(-1), while the SS burden decreases\nfrom 9.1 to 8.5 mg m(-2). The spatial distribution of SS, however, is\ngreatly affected, with the SS burden increasing by 50% in the tropics\nand decreasing by 40% at mid- and high-latitudes. Our results imply a\nstronger than expected halogen source from SS in the tropical marine\nboundary layer. They also imply stronger radiative forcing of SS in the\ntropics and a larger response of SS emissions to climate change than\npreviously thought.