SO2 oxidation in Seasalt Aerosols: Enhanced NSS Production due to Biogenic Alkalinity and Aerosol Indirect Effect RF
Abstract
Marine boundary layer (MBL) size-distributed aerosol ion measurements during SEAREX, ACE, and other open ocean field campaigns confirm that O3-oxidation of SO2 in seasalt aerosols must be considered in global MBL sulfur cycling, budgets, and modeling. More recent field work at Baring Head, New Zealand and at Cape Grim, Australia show that alkalinity derived from upwind surface waters' primary productivity contributes substantial additional alkalinity to seasalt aerosols emitted from open ocean surface waters (beyond bulk seawater alkalinity). The total alkalinity present in these seasalt aerosols supports the uptake, conversion and removal of SO2 from the MBL back to ocean surface waters. For example, four years of Cape Grim ion data for 2.5-10 ìm dia. aerosols show that 35% of the total aerosol non-seasalt SO4= (NSS) is found in these large seasalt aerosols at this site. Essentially all of the NSS in this size range is due to O3-oxidation of SO2 in the water that dominates these aerosols' volume. Two-thirds of the alkalinity that was found in these 2.5-10 ìm aerosols is excess Ca++ (beyond seawater derived Ca++) derived from planktonic primary productivity in the ocean's surface waters upwind of Cape Grim. Large seasalt aerosols generally have dry deposition velocities significantly greater than that of SO2 itself. We used the MISTRA MBL model to estimate effective SO2 dry deposition velocities (effVd); ie., once accounting for removal in large seasalt aerosols. It was concluded that effVd may be ~50% greater to several times that of direct SO2 gaseous dry deposition velocities - for a range of typical open ocean conditions. effVd values are most sensitive to the prevailing wind speed regime. These large effVd indicate that less SO2 and less NSS aerosol mass is available to over-ocean cloud processing. As a result, the positive radiative forcing by the aerosol indirect effect (AIE) considered in many global climate model (GCM) simulations is reduced in its magnitude. This presentation will include a discussion of possible impacts on AIE radiative forcing estimates that have been generated by several GCM models.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2006
- Bibcode:
- 2006AGUFM.A41H..06S
- Keywords:
-
- 0305 Aerosols and particles (0345;
- 4801;
- 4906);
- 0312 Air/sea constituent fluxes (3339;
- 4504);
- 0315 Biosphere/atmosphere interactions (0426;
- 1610);
- 0330 Geochemical cycles (1030);
- 1626 Global climate models (3337;
- 4928)