52
J. M. Pro spero
Measurements of soi- over the oceans are complicated by that fact that sea water
spray contains sulphate. Indeed, in aerosols over many remote regions of the ocean,
the mass ratio of total SO~- to Na + is quite close to that of bulk sea water, 0.2517. However, as stated before, most of the sea salt aerosol surface area and mass is in the size
range between 1-10 /lm diameter (Fig. 2.1), while nss-SO~- aerosol over the ocean is
predominantly in the submicrometre size fraction (see for example, Li-Jones and
Prospero 1998; Li et al. 1996).
2.3.1
Global Sulphur Budgets
The concentration of nss-SO~- in the marine environment is principally derived from
two gaseous precursors: DMS produced by marine organisms and S02 from continental
pollution sources and volcanoes. The global budget of sulphur from these sources is
shown in Table 2.2 (Graf et al. 1997, Table 6; see also Benkovitz et al. 1996). The total
global emissions of sulphur are 100 Tg S yr-'. Of this total, two-thirds is produced from
anthropogenic sources. Thus, anthropogenic sulphur has dramatically impacted the
global atmospheric sulphur cycle. It is for this reason that so much attention is focused
on the possible implications of pollution emissions on climate processes. Prior to the
human era, the atmospheric sulphur cycle was dominated by DMS from the oceans.
Volcanic inputs are also important, but they are sporadic. Also, in most cases, the impact of volcanic eruptions is limited to regional scales; this is especially true for volcanoes wJlOse emissions are largely confined to the troposphere, where aerosol lifetimes are relatively short. In contrast, highly explosive volcanoes that inject material
into the stratosphere can have long-range effects that can persist for years. In some
cases, the effects can be global; for example, the eruption of Pinatubo in 1991 emitted
20 Tg S02, sharply increased the concentration of sulphate aerosol in the stratosphere
and caused temperatures in the Northern Hemisphere to drop 0.2°C (Hansen et al.
1992). The effects of Pinatubo aerosols could be detected for several years after the
eruption. In contrast, oceanic DMS sources are widely distributed, and emissions are
relatively steady from year to year (Kettle et al. 1999). Note in Table 2.2 that biomass
burning is a minor source of sulphur emissions but it is a major source of nitrogen
species emissions (see below). In the following sections, we briefly consider some of the
more important aspects of the chemistry of S02 and DMS in the marine atmosphere.
2.3.2
50 2 and nss-SO:Anthropogenic sulphur is emitted primarily in the form of S02' The principal sources
are fossil fuel combustion (especially coal) and the smelting of ores. In the atmosphere,
S02 is oxidized in the gas phase by the OH radical to produce H 2 S0 4 , This reaction is
rather slow; it yields an S02 atmospheric lifetime of 1-2 weeks. If the reaction with
OH were the major controlling reaction for S02 in the atmosphere, then S02 would be
transported over greater distances than is typically observed; indeed, under most conditions the concentration of pollutant-derived S02 over the oceans is generally quite
J. M. Pro spero
Measurements of soi- over the oceans are complicated by that fact that sea water
spray contains sulphate. Indeed, in aerosols over many remote regions of the ocean,
the mass ratio of total SO~- to Na + is quite close to that of bulk sea water, 0.2517. However, as stated before, most of the sea salt aerosol surface area and mass is in the size
range between 1-10 /lm diameter (Fig. 2.1), while nss-SO~- aerosol over the ocean is
predominantly in the submicrometre size fraction (see for example, Li-Jones and
Prospero 1998; Li et al. 1996).
2.3.1
Global Sulphur Budgets
The concentration of nss-SO~- in the marine environment is principally derived from
two gaseous precursors: DMS produced by marine organisms and S02 from continental
pollution sources and volcanoes. The global budget of sulphur from these sources is
shown in Table 2.2 (Graf et al. 1997, Table 6; see also Benkovitz et al. 1996). The total
global emissions of sulphur are 100 Tg S yr-'. Of this total, two-thirds is produced from
anthropogenic sources. Thus, anthropogenic sulphur has dramatically impacted the
global atmospheric sulphur cycle. It is for this reason that so much attention is focused
on the possible implications of pollution emissions on climate processes. Prior to the
human era, the atmospheric sulphur cycle was dominated by DMS from the oceans.
Volcanic inputs are also important, but they are sporadic. Also, in most cases, the impact of volcanic eruptions is limited to regional scales; this is especially true for volcanoes wJlOse emissions are largely confined to the troposphere, where aerosol lifetimes are relatively short. In contrast, highly explosive volcanoes that inject material
into the stratosphere can have long-range effects that can persist for years. In some
cases, the effects can be global; for example, the eruption of Pinatubo in 1991 emitted
20 Tg S02, sharply increased the concentration of sulphate aerosol in the stratosphere
and caused temperatures in the Northern Hemisphere to drop 0.2°C (Hansen et al.
1992). The effects of Pinatubo aerosols could be detected for several years after the
eruption. In contrast, oceanic DMS sources are widely distributed, and emissions are
relatively steady from year to year (Kettle et al. 1999). Note in Table 2.2 that biomass
burning is a minor source of sulphur emissions but it is a major source of nitrogen
species emissions (see below). In the following sections, we briefly consider some of the
more important aspects of the chemistry of S02 and DMS in the marine atmosphere.
2.3.2
50 2 and nss-SO:Anthropogenic sulphur is emitted primarily in the form of S02' The principal sources
are fossil fuel combustion (especially coal) and the smelting of ores. In the atmosphere,
S02 is oxidized in the gas phase by the OH radical to produce H 2 S0 4 , This reaction is
rather slow; it yields an S02 atmospheric lifetime of 1-2 weeks. If the reaction with
OH were the major controlling reaction for S02 in the atmosphere, then S02 would be
transported over greater distances than is typically observed; indeed, under most conditions the concentration of pollutant-derived S02 over the oceans is generally quite