Sulfate aerosols, tropospheric sources

Carbonyl sulfide is also the most abundant reduced sulfur gas in Earth s troposphere, but for completely different reasons. Volcanic sources of OCS are negligible by comparison with biogenic emissions, which are important sources of several reduced sulfur gases (e.g., OCS, H2S, (CH3)2S, (CH3)2S2, and CH3SH) in the terrestrial troposphere. Many of these gases are ultimately converted into sulfate aerosols in the troposphere, but OCS is mainly lost by transport into the stratosphere, where it is photochemically oxidized to SO2 and then to sulfuric acid aerosols, which form the Junge layer at —20 km in Earth s stratosphere. 

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. 

The primary sources that are responsible for the presence of this family of compounds in the atmosphere emit NH3, N20, and NO to the troposphere, the lowest level of the atmosphere, which extends to approximately 10 km from the earth s surface. NH3 seems to undergo very little chemistry in the atmosphere except for the formation of aerosols, including ammonium nitrate and sulfates. NH3 and the aerosols are highly soluble and are thus rapidly removed by precipitation and deposition to surfaces. N20 is unreactive in the troposphere. On a time scale of decades it is transported to the stratosphere, the next higher atmospheric layer, which extends to about 50 km. Here N20 either is photodissociated or reacts with excited oxygen atoms, O (lD). The final products from these processes are primarily unreactive N2 and 02, but about 10% NO is also produced. The product NO is the principal source of reactive oxidized nitrogen species in the stratosphere. 

Table VI summarizes aerosol mass concentrations and composition in different regions of the troposphere. It is interesting to note that average total fine particle mass (that associated with particles of diameter less than about 2 /im) in non-urban continental, i.e., regional, aerosols is only a factor of two lower than urban values. This reflects the relatively long residence time of particles (recall the estimate of a lifetime of fine particles by dry deposition of 10 days). Correspondingly, the average composition of non-urban continental and urban aerosols is roughly the same. The average mass concentration of remote aerosols is a factor of three lower than that of non-urban continental aerosols. The elemental carbon component, a direct indicator of anthropogenic combustion sources, drops to 0.3% in the remote aerosols, but sulfate is still a major compo-...

Cloud processing is a major source of sulfate and aerosol mass in general on regional and global scales. Walcek et al. (1990) calculated that, during passage of a midlatitude storm system, over 65% of tropospheric sulfate over the northeastern United States was formed in cloud droplets via aqueous-phase reactions. The same authors estimated that. 

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