Group 12. Photochemical oxidation mechanisms (Sievering, Saltzman)

Intercomparison of NSS results by 2 Moudi and 1 Sierra was reviewed in context of implications for the contribution of homogeneous, in-cloud and in-sea-salt hetero. conv. mechanisms. 3 of 4 ACE-1 intensive impactor data sets considered indicate bimodal fine and coarse NSS( Cainey's shows trimodal - 3rd being very fine aerosol) character with Quinn's not showing bimodality. Possibility that fine is dominated by in-cloud hetero. and coarse by in-sea-salt hetero.( w. very fine, tho a minor contributor to NSS mass, due to homo.) and that distinct "mechanism modes" were observed during very clean air ACE-1 conditions was discussed.

The Cainey seasonal NSS data - w. an average 1.2 nmol NSS m-3 of total 2.2-2.3 nmol NSS m-3 in coarse( > 0.7 um ambient dia.) were reviewed w. great interest. It appeared these data will be very useful to integrative analysis of the ACE-1 data set in a seasonal context. These data should be fully available upon completion of her PhD thesis.

Saltzman described an SO2 budget for ACE-1 conditions. A trend in SO2 yield, considering especially the ACE ship transit data, with 70-90% in equatorial, 30-60% in 30S range, and 20-40% in Southern Ocean area is evident.

Sievering described assessment of in-sea-salt conv. of SO2 to date. This included fact that more NSS appears to be present due to this mechanism than the sea salt alkalinity would allow. Added CO3=/HCO3- may have been present due to biogenic( esp. CaCO3) source. Data from SEM indiv. particle analysis( Jim Anderson) support this view - indirectly. Combined SEM/TEM results need to be considered to answer this question.

The need for an aerosol S budget analysis was underscored by the discussion of an "S topsoil layer" due to rambunctious NSS production. A rough calc. of the SO2 lifetime during ACE-1( > 0.5 days) suggests this may not be a concern. One outcome of S Oxidation Group discussions is that a paper on "Aerosol S Budget in the Southern Ocean MBL during ACE-1" will be forthcoming w. Sievering, Anderson, Cainey, Clarke, and others on author list.