Sulfate atmospheric residence times

Although hydrogen sulfide does not react photochemically, it may be transformed to sulfur dioxide and sulfate by nonphotochemical oxidation reactions in the atmosphere. Its atmospheric residence time is typically less than 1 day (Hill 1973), but may be as high as 42 days in winter (Bottenheim and Strausz 1980). 

Figure 13-5 is the box model of the remote marine sulfur cycle that results from these assumptions. Many different data sets are displayed (and compared) as follows. Each box shows a measured concentration and an estimated residence time for a particular species. Fluxes adjoining a box are calculated from these two pieces of information using the simple formula, S-M/x. The flux of DMS out of the ocean surface and of nss-SOl back to the ocean surface are also quantities estimated from measurements. These are converted from surface to volume fluxes (i.e., from /ig S/(m h) to ng S/(m h)) by assuming the effective scale height of the atmosphere is 2.5 km (which corresponds to a reasonable thickness of the marine planetary boundary layer, within which most precipitation and sulfur cycling should take place). Finally, other data are used to estimate the factors for partitioning oxidized DMS between the MSA and SO2 boxes, for SO2 between dry deposition and oxidation to sulfate, and for nss-SO4 between wet and dry deposition. 

Sulfur-35 is formed from the spallation of argon in the atmosphere. It has a half-life of 87 d (Lai and Peters, 1967). After oxidation to sulfate, it is deposited on the land surface. Because of its relatively long residence time in the upper atmosphere and its short half-life, specific activities of S are low at the time of deposition. Due to the anionic form of SO it is relatively conservative in soil water and groundwater and... 

During their residence time in the atmosphere, mineral dusts become coated by sulfates, nitrates, and other species (Dentener et al. 1996 Buseck and P6 sfai 1999 Zhang and Carmichael 1999 Song and Carmichael 1999 Buseck et al. 2000). These coatings are formed through chemical reactions such as the oxidation of SO2 and NO2 at the gas-solid interface, as well as by condensation of sulfuric and nitric acids. Once coated, the hygroscopic dusts act as cloud condensation nuclei and further oxidation reactions can take place in the aqueous medium (Wurzler et al. 2000). Subsequent evaporation of the cloud droplet yields a coated particle. 

Sulfur occurs mostly in the solid Earth reservoirs and in oceans and is only a minor component of the atmosphere. Volcanic, sulfur-bearing gases emitted into the atmosphere have only a short residence time and are quickly transferred into the oceans. Because sulfur occurs in both the oxidized form -chiefly as the sulfate mineral gypsum, and in a reduced state - principally as the sulfide... 

Fig. 13-5 The sulfur cycle in the remote marine boundary layer. Within the 2500 m boundary layer, burden units are ng S/m and flux units are ng S/m h. Fluxes within the atmospheric layer are calculated from the burden and the residence time. Dots indicate that calculations based on independent measurements are being compared. The measured wet deposition of nss-SO " (not shown) is 13 7 //g S/m /h Inputs and outputs roughly balance, suggesting that a consistent model of the remote marine sulfur cycle within the planetary boundary layer can be constructed based on biogenic DMS inputs alone. Data (1) Andreae (1986) (2) Galloway (1985) (3) Saltzman et al. (1983) (4) sulfate aerosol lifetime calculated earlier in this chapter based on marine rainwater pH the same lifetime is applied to MSA aerosol. Modified from Crutzen et al. (1983) with the permission of Kluwer Academic Publishers.

This oxidation is of third order and its reaction rate is independent of the temperature. Using reaction rate values measured under laboratory conditions and the concentrations of M and O for different levels of the atmosphere, Cadle and Powers calculated that this process can be significant only above 10 km if the S02 concentration is 1 /ig m 3 STP. The residence time of sulfur dioxide molecules is estimated to be 103 hr at an altitude of 10 km, while at 30 km the corresponding figure ranges from 5 hr to 10 hr. Hence it seems probable that this reaction is not important in the troposphere. However, it may play an important role in the formation of the stratospheric sulfate layer (Subsection 4.4.3). 

We have to emphasize here that the majority of the sulfate-sulfur in the tropospheric reservoir in not sea salt. Friend (1973) estimated that the atmospheric sea-salt burden is around 0.1 x 106 t. By subtracting this value from the sulfate-sulfur loading given in Fig. 19 and considering only the strength of chemical sources (62 x 106 t yr 1), a residence time of more than 4 days is obtained for the excess sulfate. 

From the foregoing parts of this book it is clear that solar radiation in the stratosphere is primarily attenuated by ozone (see Subsection 3.4.3) and at a lesser extent by the stratospheric sulfate aerosol layer (see Subsection 4.4.3). This means that any change in the stratospheric 03 burden or aerosol concentration involves modification of radiative transfer in this atmospheric domain. We should remember that the residence time of trace constituents above the tropopause is rather long because of the thermal structure and the absence of wet removal. Furthermore at these altitudes the density of the air is low as compared to that of lower layers. For this reason even an insignificant quantity of pollutants can produce relatively long and significant effects. 

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. 

Schwartz, S. E. (1979) Residence times in reservoirs under non-steady-state conditions Application to atmospheric S02 and aerosol sulfate, Tellus 31, 520-547. 

Aerosol residence times in the troposphere are roughly 1-2 weeks if all S02 sources were shut off today, anthropogenic sulfate aerosols would disappear from the planet in 2 weeks. By contrast, not only are GHG residence times measured in decades to centuries, but because of the great inertia of the climate system, as noted in the previous chapter, the effect of GHG forcing takes decades to be fully transformed into equilibrium climate warming. As a result, if both C02 and aerosol emissions were to cease today, the Earth would continue to warm as the climate system continues to respond to the accumulated amount of C02 already in the atmosphere. 

As a result, the difference between cases B and C increases compared to the larger droplet case. The pH attained by the 0.2 mm droplet is much lower for all cases than for the 5 mm, reaching values as low as 1 for the case of zero ammonia. Due to the higher acidity, the H2O2 reaction is the main sulfate formation mechanism. The long residence time of the small droplets in the atmosphere results in lower pH values and much higher sulfate concentrations. 

The column burden of sulfate aerosol, m Q2, can be estimated from the global source strength of SO2, 0sO i the average fractional conversion of SO2 to sulfate in the atmosphere, vso, the mean residence time of sulfate aerosol in the atmosphere, t q , and the area, A, of the geographical region over which the estimate is performed (e.g. the entire globe, the Northern Hemisphere, etc.), as... 

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