Aerosols, Bubbles and Sea Spray Production Studies during the Red Experiments

Aerosols have a large effect on climate and weather. They modulate the surface irradiance in clear sky conditions, but also modify the radiative properties of clouds. To assess the consequences of reduced surface irradiation due to aerosols for weather forecasting and climate, data on aerosol radiative effects and their spatial and temporal variations are required over a large area. The necessary 4-dimensional distribution of aerosol properties can be provided by integrating groundbased, airborne and satellite observations through data assimilation with advanced models. The use of satellite data to constrain Global Climate Models (GCM) or chemical transport models (CTM) has been demonstrated [Builtjes et al., 2001; Verver et al., 2002]. This leads to a new era of high potential for providing continuous availability of high quality data for use in, e.g., weather forecasting, climate studies and air quality assessment. With 70% of the Earth covered by oceans, the occurrence of sea salt, produced primarily by breaking wind waves, is an important factor. Sea salt aerosol production has been observed in the size range from 20nm [Mårtensson et al., 2002] to over 100μm [De Leeuw, 1993]. Sea salt is the dominant submicrometer scatterer in most ocean regions and dominates the marine boundary layer particulate mass concentration in remote oceanic regions, with a significant fraction occurring in the submicrometer size range [IPCC, 2001]. Sea salt contributes 44% to the global aerosol optical depth. Estimates for top-ofatmosphere, global-annual radiative forcing due to sea salt are -1.51 and -5.03 Wm -2 for low and high emission values, respectively [IPCC, 2001]. At lower levels in the atmosphere, sea spray particles can dominate atmospheric propagation at visible and infrared wavelengths. The main contribution to the indirect aerosol effect (IAE), the most uncertain forcing mechanism in the prediction of climate change, comes from marine stratocumulus clouds. Boers et al. [1998] report an example of IAE related to changes in the distributions of natural CCN over the ocean, and O’Dowd et al. [1999a, b] have demonstrated from aircraft observations that sea spray particles can play a significant role in marine stratocumulus microphysics and chemistry. In addition, sea spray particles can provide a significant sink for natural and anthropogenic trace gases [O’Dowd et al, 2000]. These examples show the importance of an accurate knowledge of sea spray aerosol emissions and resulting concentrations. Global circulation models (GCM) and chemistry transport models (CTM) need to incorporate source functions for all significant aerosol (and aerosol precursor) emissions, plus treatments of heterogeneous chemistry to deal with the transformations and removal of anthropogenic pollutant gases.