Canadian Aerosol Module (CAM): A size-segregated simulation of atmospheric aerosol processes for climate and air quality models - 2. Global sea-salt aerosol and its budgets

Size-segregated sea-salt aerosols were included as dynamic constituents in the third generation of the Canadian general circulation model (GCMIII) using the Canadian Aerosol Module (CAM). A 12-bin sectional model was used to simulate wind-dependent sea-salt aerosol generation at the surface, diffusive, and advective transports, as well as wet and dry removals as a function of particle size. The dependence of model-predicted atmospheric sea-salt concentration on wind speed is weaker than the u(3.41) dependence of sea-salt emissions but nevertheless still significant. A comparison of GCMIII surface winds with European Centre for Medium-Range Weather Forecasting reanalyzed winds was used to identify a globally averaged bias of modeled surface marine winds of 18%. After taking this into account, two-year simulations were performed to study the atmospheric sea-salt cycle. Comparisons of seasonal mean model predictions with observed sea-salt mass concentrations at 24 marine observatories were in reasonable agreement (generally better than a factor of 2). The annual global sea-salt flux is estimated to be 1.01x10(13) kg with similar to32% from the Northern Hemisphere and with 98% in supermicron particles. Although emissions to the atmosphere of submicron sea-salt particles larger than 0.2 mum radius make up only 2% of the total global emissions, they contribute significantly to the background aerosol mass and number concentrations in the marine atmosphere. Submicron sea-salt aerosols contribute substantially to the total optical depth of the atmosphere over the open oceans. They account for 5-15% of total sea-salt mass in air at the surface and 20% at 700 hPa. The global mass mean dry diameter of sea-salt aerosols over oceans is 2.8 mum at the surface and 1.9 mum in the midtroposphere. Concentrations are highest in the roaring forties of the Southern Hemisphere and over the northern oceans from October to March. Residence times of sea-salt aerosols were highly variable depending on size, vertical dispersion, and removal processes. The mean residence time for 7.7 mum and 0.4 mum particles in the marine boundary layer were in the range 0.3-10 hours and 80-360 hours, respectively.