DMS route. On the other hand, if its measured d^34 S is close to the 0 to +5‰
CDT range of fossil fuels, then its contained sulphur is likely to be from this
source. Samples with intermediate values will have sulphur from both sources,
the ratio being directly calculable by simple mass balance.
There are, of course, several assumptions behind this apparently simple
description. One is that the d^34 S signal of all fossil fuel is in the above range. At
the moment only a rather small number of samples of power-station flue gases
from limited locations have been analysed. A second assumption is that the d^34 S
signal of seawater SO 42 - is not altered significantly when DMS crosses the air–
sea interface and is oxidized to SO 2 and SO 42 - in the atmosphere. The evidence
to date indicates that neither of these assumptions introduces much error, but
more work is required to prove this approach.
As might be expected, urban aerosols have a d^34 S signature overlapping to
somewhat higher than that from fossil fuels (Fig. 7.21). By contrast, the very few
aerosol samples obtained from locations remote from human influence in the
South Pacific have a d^34 S value which can approach that of seawater SO 42 -. Results
from detailed sampling conducted over a full yearly cycle at Mace Head, a remote
site on the west coast of Eire (Fig. 7.21), show almost the whole range of d^34 S.
Because of the large number of samples collected it has been possible to calcu-
late the percentage of sulphur from the two main sources for different seasons.
Thus, in spring and summer approximately 30% of the sulphur in the aerosols
at Mace Head comes from DMS (very probably produced by phytoplankton in
the northeastern Atlantic, which are only active in any substantial way at these270 Chapter Seven
0 +10 +20 +30 d^34 S‰ CDTFossil fuel emissions
SeawaterUK urban aerosolsMace Head aerosolsSouth Pacific aerosolsFig. 7.21Sulphur isotope ratios (d^34 S) for various sources of sulphur and in atmospheric
aerosols for several localities. Mace Head (western Ireland) data, courtesy of Nicola McArdle,
UEA.