54
J. M. Pro spero
2.3.3
OMS and the Atmospheric Sulphur Cycle
Charlson et al. (1987), in their classic paper, suggested that in the pre-human era, the
radiative properties of the marine atmosphere were strongly modulated by nss-SO~that was derived from the oxidation of DMS emitted by various marine organisms.
Even today, in remote ocean regions, the production of new ultrafine sulphate particles
is linked to DMS production. The precursor of DMS is DMSP (dimethylsulphonium
propionate), an osmolyte produced by many phytoplankton species, especially dinoflagellates, prymnesiophytes (including coccolithophores), and chrysophytes. Two
species in particular, Phaeocystis pouchetii and Emiliania huxleyi, are known to be
very strong producers of DMSP. DMSP is released by these organisms during senescence or when grazed; in water DMSP is enzymatically cleaved to produce a variety of
compounds including DMS. The concentration of DMS in the ocean follows in a general way the seasonal cycle of oceanic primary productivity. Accordingly in the mid
and high latitudes of the Northern Hemisphere, DMS concentrations increase in March
or April and peak in Mayor June then decrease rapidly; in the Southern Hemisphere,
the cycle is shifted by six months (Kettle et al.1999). In contrast, in the tropics there is
little evidence of a seasonal cycle. The concentration of MSA in the atmosphere mimics the seasonal cycle of DMS in the ocean. In Fig. 2.5, at Heimaey, Iceland, MSA shows
a strong peak in June-July and very low concentrations during the remainder of the
year. At Bermuda, the maximum MSA concentrations also occur during summer, but
the peak is broader; winter concentrations are very low. At Barbados, the seasonal MSA
cycle is even less evident; it is much broader and there is no well-defined winter minimum.
The cycle of DMS production and emission to the atmosphere is quite complex, and
there is much that is not understood about the processes that affect DMS distributions in and over the oceans. Global surface-water DMS concentration data (over
10 000 measurements) were recently compiled and interpolated into a 1 x 1 monthly
data set (Kettle et al. 1999), and the results compared to published fields of geophysical and biological parameters. Kettle et al. could not find any correlation between DMS
and these parameters, and they could find no simple algorithm to create monthly fields
of sea surface DMS concentrations based on these parameters. There clearly is much
research to be done before we can understand the linkage between biological processes
and the emission rate of DMS to the atmosphere.
The atmospheric chemistry of DMS is complex, and there are many unresolved issues (Berresheim et al. 1995; Ravishankara et al. 1997). Nonetheless, studies show that
OH is the dominant oxidizing agent for DMS in unpolluted marine atmospheres where
DMS has a lifetime of about 1 day. Reaction with OH can proceed by two dominant
routes. Hydrogen can be abstracted by OH from the methyl group to form the radical
group CH3SCHz, or OH can be added to the sulphur atom. The principal products
are methanesulphonic acid (CH3S03H, MSA) and SO~-. Sulphate production is the
higher energy route, and as a result, the relative yields of MSA and SO~- would be expected to be temperature dependent, yielding higher ratios of nss-SO;-/MSA in warmer
climates.
J. M. Pro spero
2.3.3
OMS and the Atmospheric Sulphur Cycle
Charlson et al. (1987), in their classic paper, suggested that in the pre-human era, the
radiative properties of the marine atmosphere were strongly modulated by nss-SO~that was derived from the oxidation of DMS emitted by various marine organisms.
Even today, in remote ocean regions, the production of new ultrafine sulphate particles
is linked to DMS production. The precursor of DMS is DMSP (dimethylsulphonium
propionate), an osmolyte produced by many phytoplankton species, especially dinoflagellates, prymnesiophytes (including coccolithophores), and chrysophytes. Two
species in particular, Phaeocystis pouchetii and Emiliania huxleyi, are known to be
very strong producers of DMSP. DMSP is released by these organisms during senescence or when grazed; in water DMSP is enzymatically cleaved to produce a variety of
compounds including DMS. The concentration of DMS in the ocean follows in a general way the seasonal cycle of oceanic primary productivity. Accordingly in the mid
and high latitudes of the Northern Hemisphere, DMS concentrations increase in March
or April and peak in Mayor June then decrease rapidly; in the Southern Hemisphere,
the cycle is shifted by six months (Kettle et al.1999). In contrast, in the tropics there is
little evidence of a seasonal cycle. The concentration of MSA in the atmosphere mimics the seasonal cycle of DMS in the ocean. In Fig. 2.5, at Heimaey, Iceland, MSA shows
a strong peak in June-July and very low concentrations during the remainder of the
year. At Bermuda, the maximum MSA concentrations also occur during summer, but
the peak is broader; winter concentrations are very low. At Barbados, the seasonal MSA
cycle is even less evident; it is much broader and there is no well-defined winter minimum.
The cycle of DMS production and emission to the atmosphere is quite complex, and
there is much that is not understood about the processes that affect DMS distributions in and over the oceans. Global surface-water DMS concentration data (over
10 000 measurements) were recently compiled and interpolated into a 1 x 1 monthly
data set (Kettle et al. 1999), and the results compared to published fields of geophysical and biological parameters. Kettle et al. could not find any correlation between DMS
and these parameters, and they could find no simple algorithm to create monthly fields
of sea surface DMS concentrations based on these parameters. There clearly is much
research to be done before we can understand the linkage between biological processes
and the emission rate of DMS to the atmosphere.
The atmospheric chemistry of DMS is complex, and there are many unresolved issues (Berresheim et al. 1995; Ravishankara et al. 1997). Nonetheless, studies show that
OH is the dominant oxidizing agent for DMS in unpolluted marine atmospheres where
DMS has a lifetime of about 1 day. Reaction with OH can proceed by two dominant
routes. Hydrogen can be abstracted by OH from the methyl group to form the radical
group CH3SCHz, or OH can be added to the sulphur atom. The principal products
are methanesulphonic acid (CH3S03H, MSA) and SO~-. Sulphate production is the
higher energy route, and as a result, the relative yields of MSA and SO~- would be expected to be temperature dependent, yielding higher ratios of nss-SO;-/MSA in warmer
climates.