Info

0.075 0.08

0.01

40-320

2.2-3.0(1.4/1.1) 15-25(8.4/11.6)* 0.01-2 (0.8/0.2) 0.25-0.78 (0.3/0.2)c

25-40

98-120

"Numbers in parentheses are fluxes from Northern Hemisphere/Southern Hemisphere.

''Excluding seasalt contributions.

'Excluding soil dust contributions.

''Andreae and Crutzen (1997).

Source: Berresheim et al. (1995).

Hemisphere. Figure 2.1 shows estimates of global anthropogenic sulfur emissions since 1850, and Table 2.3 summarizes observed mixing ratios and atmospheric lifetimes of atmospheric sulfur gases.

2.2.1 Dimethyl Sulfide (CH3SCH3)

Dimethyl sulfide (DMS) is the dominant sulfur compound emitted from the world's oceans. DMS was first measured in the surface ocean by Lovelock et al. (1972), who showed that DMS, produced by marine phytoplankton, was ubiquitous in the surface ocean waters. It had been known for a number of years that the global sulfur budget could not be balanced without a substantial flux of a sulfur-containing compound from the oceans to the atmosphere. Lovelock's measurements pointed to DMS as that compound. Since Lovelock's pioneering data, many expeditions have measured DMS concentrations in both ocean waters and the marine atmosphere. Kettle et al. (1999) summarize all the oceanic DMS data available as of 1999, comprising 134 cruises and over 15,000 individual measurements. There is a rough correspondence between areas of high DMS concentrations and blooming of marine phytoplankton that produce DMS. In January, the highest ocean water DMS concentrations occur in the Southern Hemisphere,

TABLE 2.3 Average Lifetimes and Observed Mixing Ratios of Tropospheric Sulfur Compounds

Mixing Ratio, ppt

TABLE 2.3 Average Lifetimes and Observed Mixing Ratios of Tropospheric Sulfur Compounds

Mixing Ratio, ppt

Average

Marine

Clean

Polluted

Free

Species

Lifetime

air

Continental

Continental

Troposphere

H2S

2 days

0-110

15-340

0-800

1-13

OCS

7 years

530

510

520

510

cs2

1 week

30^15

15^*5

80-300

< 5

CH3SCH3

0.5 day

5^100

7-100

2-400

< 2

so2

2 days

10-200

70-200

100-10,000

30-260

SO2"

5 days

5-300°

10-120

100-10,000

5-70

"Nonseasalt sulfate. Source-. Lelieveld et al. (1997).

"Nonseasalt sulfate. Source-. Lelieveld et al. (1997).

especially in Antarctic waters; in July, the highest concentrations are in the Northern Hemisphere oceans.

DMS is thought to originate from the decomposition of dimethyl sulfoniopropionate produced by marine organisms, in particular, phytoplankton (Andreae 1990). Its concentration in the upper layer of the ocean varies between a few nanograms of S per liter to a few micrograms of S per liter. The DMS surface seawater concentration is highly nonuniform; its average concentration is approximately 100 nanograms (ng) of S per liter. It has been observed that the concentration of DMS is dependent on diurnal (Andreae and Barnard 1984) and seasonal variations (Turner and Liss 1985), and on depth and location (Andreae and Raemdonck 1983). On the basis of DMS concentrations in the atmosphere and its Henry's law constant in seawater (see Chapter 7), oceanic DMS concentrations are greatly in excess of those that would be in equilibrium with atmospheric values. The result of this lack of equilibrium is a flux of DMS from the ocean to the atmosphere.

Once the importance of DMS to the global sulfur cycle was established, numerous measurements of DMS concentrations in the marine atmosphere have been conducted. The average DMS mixing ratio in the marine boundary layer (MBL) is in the range of 80-1 lOppt but can reach values as high as 1 ppb over entrophic (e.g., coastal, upwelling) waters. DMS mixing ratios fall rapidly with altitude to a few parts per trillion in the free troposphere. After transfer across the air-sea interface into the atmosphere, DMS reacts predominantly with the hydroxyl radical and also with the nitrate (N03) radical. Oxidation of DMS is the exclusive source of methane sulfonic acid (MSA) in the atmosphere, and the dominant source of S02 in the marine atmosphere. We will return to the atmospheric chemistry of DMS in Chapter 6.

2.2.2 Carbonyl Sulfide (OCS)

Carbonyl sulfide is the most abundant sulfur gas in the global background atmosphere because of its low reactivity in the troposphere and its correspondingly long residence time. It is the only sulfur compound that survives to enter the stratosphere. (An exception is the direct injection of S02 into the stratosphere in volcanic eruptions.) In fact, the input of OCS into the stratosphere is considered to be responsible for the maintenance of the normal stratospheric sulfate aerosol layer.

The estimated global budget of OCS is given in Table 2.4. The main atmospheric sources of OCS are OCS emissions at the surface and conversion of CS2 and dimethyl sulfide (DMS) in the atmosphere. Because of its long tropospheric lifetime, much of the OCS reaches the stratosphere, where photolysis and oxidation lead to S02 and eventually to sulfate particles. The OCS mixing ratio in the tropospheric is relatively constant at about 500 ppt (Chin and Davis 1995). Enhanced OCS mixing ratios up to 600 ppt from biomass burning emissions have been found below the tropical tropopause, at altitudes between 10 and 18 km (Notholt, et al. 2003). The other sulfur compounds, CS2 and DMS, have much shorter tropospheric lifetimes than OCS and therefore do not contribute appreciably to the sulfur budget of the stratosphere under volcanically quiescient periods.

On the basis of atmospheric measurements, Chin and Davis (1995) estimated the total quantity of OCS in the atmosphere to be 5.2 Tg, of which 4.63 Tg is in the troposphere and 0.57 Tg in the stratosphere. Based on the estimated global OCS source strength of 0.86 Tgyr"1, the global atmospheric lifetime of OCS is estimated to be about 6 years. We will return to the global cycle and chemistry of OCS in Chapter 5 in connection with the stratospheric aerosol layer.

TABLE 2.4 Global Budget of Carbonyl Sulfide (OCS)

Sources, 109

g S yr~la

Direct OCS flux from oceans

41

(154)

Indirect OCS flux as CS2 from oceans

84

(54)

Indirect OCS flux as DMS from oceans

154

(37)

Direct anthropogenic OCS flux

64

(32)

Indirect OCS flux as anthropogenic CS2

116

(58)

Total sources

459

Sinks, 109 g

; S yr"la

OCS uptake by soils

130

(56)

OCS uptake by plants

238

(30)

OCS reaction with OH radical

94

Total sinks

462

"Estimated uncertainties are shown in parentheses. Source: Kettle et al. (2002).

"Estimated uncertainties are shown in parentheses. Source: Kettle et al. (2002).

2.2.3 Sulfur Dioxide (S02)

Sulfur dioxide is the predominant anthropogenic sulfur-containing air pollutant. Mixing ratios of S02 in continental background air range from 20 ppt to over 1 ppb; in the unpolluted marine boundary layer levels range between 20 and 50 ppt. Urban S02 mixing ratios can attain values of several hundred parts per billion. We will consider the atmospheric chemistry of S02 in Chapter 6.

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