Fitzgerald [1991] reviews the status of marine aerosols and suggests that
the background aerosol in the boundary layer over remote oceans is of marine
origin, and not aged continental aerosol. The particle concentrations were
quite uniform throughout the tropical trade wind regions and normally ranged
between 100 - 300 cm
, except in the region near clouds where CCN
concentrations are reduced (Hudson [1993b]). The particle concentrations for
sizes less that 0.3 microns contain 90-95% of the particles and consist of
non-sea-salt sulfates. These particles are most likely produced by a
gas-to-particle conversion of the oxidation products of organosulfur gases,
primary dimethylsulfide (DMS), emitted by phytoplankton in the ocean. The
suggestion that phytoplankton in the ocean may regulate the amount of
radiation reaching the ocean surface via the control of CCN concentrations
by the production of DMS has received increased attention during the last four
years. The study by Berresheim et al. [1993] showed a positive correlation
between the concentration of individual sulfur species and CCN, supporting the
proposed relationship between DMS and CCN concentrations. At high
supersaturations, (0.9%), they show that other compounds less soluble than
sulfur may become important in marine CCN formation. Hegg et al. [1991a and
b] also showed a significant correlation between non-sea-salt mass and
concentration of CCN active at 1% supersaturation. On the other hand, more
recent measurements by Quinn et al. [1993] suggest that the production of
CCN by oxidation products of DMS is relatively inefficient due to the
condensation of gas phase precursors onto existing aerosols. In addition,
the modeling study by Lin et al. [1992] showed that the growth of freshly
nucleated sulfuric acid particles to CCN size can only occur efficiently
when pre-existing CCN concentrations are less than 1 cm
for typical
SO
concentrations in the marine boundary layer. Considering that
typical CCN concentrations are on the order of 100--300 cm
, production
of CCN by DMS is unlikely to occur through this mechanism. Lin and Chameides
[1993] have recently suggested an alternate path for the production of CCN
by DMS oxidation which does not involve SO
gas as an intermediate,
which they suggest is capable of producing significant new CCN in the Marine
Boundary Layer within a two to three day period. Clearly, more studies
are needed to help resolve some of the conflicting results concerning the
production of CCN by DMS and the potential impacts on climate.