Cloud sulfate source

DMS has been observed in the marine atmosphere since the early 1970s, but it was not until the mid-1980s that there was interest in this gas as being a natural source for sulfate CCN. Sulfate aerosols are, in number terms, the dominant source of CCN. The major role clouds play in the climate system leads to possible climatic implications if changes to DMS production occurred. Furthermore, the dependence of this production on environment conditions means that scope for a feedback process arises this feedback is called the Charlson hypothesis. ... 

Atmospheric aerosols have a direct impact on earth s radiation balance, fog formation and cloud physics, and visibility degradation as well as human health effect[l]. Both natural and anthropogenic sources contribute to the formation of ambient aerosol, which are composed mostly of sulfates, nitrates and ammoniums in either pure or mixed forms[2]. These inorganic salt aerosols are hygroscopic by nature and exhibit the properties of deliquescence and efflorescence in humid air. That is, relative humidity(RH) history and chemical composition determine whether atmospheric aerosols are liquid or solid. Aerosol physical state affects climate and environmental phenomena such as radiative transfer, visibility, and heterogeneous chemistry. Here we present a mathematical model that considers the relative humidity history and chemical composition dependence of deliquescence and efflorescence for describing the dynamic and transport behavior of ambient aerosols[3]. 

Husain and co-workers, for example (Husain, 1989 Husain et al., 1991 Husain and Dutkiewicz, 1992 Burkhard et al., 1995 Dutkiewicz et al., 1995), have developed techniques using Se, As, and Sb as tracers to follow the oxidation of S02 with time in clouds at Whiteface Mountain in New York State. The principle is based on the fact that the major sources of these metals are high-temperature combustion, e.g., of coal and oil. Thus these metals are found in particles (see Chapter 9) that act as condensation nuclei for cloud formation. These particles also contain sulfate formed from the gas-phase oxidation of S02. Because of the low vapor pressure of H2S04, it becomes associated with particles, either by homogeneous condensation or by condensing out on preexisting particles (see Chapter 9.C). As a result, the metals can be used as tracers for sulfate formed in the gas phase, as opposed to sulfate formed by the uptake of S02 into cloudwater followed by oxidation. 

Anastasio, C., B. C. Faust, and C. J. Rao, Aromatic Carbonyl Compounds as Aqueous-Phase Photochemical Sources of Hydrogen Peroxide in Acidic Sulfate Aerosols, Fogs, and Clouds. 1. Non-Phenolic Methoxybenzaldehydes and Methoxyacetophe-nones with Reductants (Phenols), Environ. Sci. Techno ., 31, 218-232 (1997). 

The seasonal cycle of CCN has also been shown to be correlated with that of cloud optical depth in one remote marine area (Boers et al., 1994), and the isotope composition of non-sea salt sulfate over remote regions of the southern Pacific Ocean has been shown to be consistent with a DMS source (Calhoun et al., 1991). 

Sulfuric acid is a stronger acid than sulfurous [pAa(l) < 0, p7fa(2) = 1.99 at 25 °C and infinite dilution] rain as acidic as pH 2.1 has been recorded at Hubbard Brook, New Hampshire, and the pH of water droplets in clouds can be as low as 1.5 (for comparison, the pH of rainwater saturated with atmospheric CO2 is about 5.6 at 15 °C). Acid rain destroys building materials (especially marble), kills fish and vegetation, accelerates metallic corrosion (Sections 16.5 and 16.7), and can be directly harmful to humans (e.g., it causes the alligator skin condition reported in Cubatao, Brazil). Sulfate rain is not completely without redeeming features, as many soils (e.g., in southern Alberta, Canada) are sulfur-deficient. On balance, however, its acidity is unacceptable, and sulfur oxide emissions must be controlled at the source. Several control measures are possible ... 

CCN). Changes in the concentrations of CCN may alter the cloud droplet concentration, the droplet surface reflectivity, the radiative properties of clouds (cloud albedo) (2), and hence, the earth s climate (8-101. This mechanism has been proposed for the remote atmosphere, where the radiative properties of clouds are theoretically predicted to be extremely sensitive to the number of CCN present (ID). Additionally, these sulfate particles enhance the acidity of precipitation due to the formation of sulfuric acid after cloud water dissolution (11). The importance of sulfate aerosol particles to both radiative climate and rainwater acidity illustrates the need to document the sources of sulfur to the remote atmosphere. 

Atmospheric Oxidation of SOo to Sulfate. Regardless of the source, sulfur dioxide is oxidized under atmospheric conditions in the gas phase, cloud droplets or on the surface of wet aerosol particles. The gas phase reaction (Equation 4)... 

The discovery of substantial amounts of volatile organosulfides in the oceans was one of the major additions to the sulfur cycle in the second half of the twentieth century. The largest flux of reduced sulfur to the atmosphere from the oceans is as DMS. The importance of this compound that was largely unknown in nature until the 1970s was revealed by Lovelock et al. (1972) as a potential explanation for the imbalance in the sulfur cycle. Over time it has become clear that this process has important implications to the atmosphere and offers a source of sulfate to form cloud condensation nuclei. 

Table 8-6 presents an overview on the concentrations of the major ions in rainwater observed at various locations. Table 8-7 provides some information on cloud and fog waters. In maritime regions seasalt is an important source of cloud condensation nuclei, and it undergoes effective below-cloud scavenging as well. Sodium chloride accordingly contributes the largest fraction of all ions in rainwater. Some of the other ions usually are somewhat enriched in comparison with their relative abundances in seasalt. The enrichment of potassium and calcium is due to the admixture of aerosol from continental sources, and that of sulfate arises from the oxidation of gaseous precursors such as dimethyl sulfide of S02- This excess sulfate is associated almost exclusively with submicrometer-sized particles (see Section 7.5.1). 

Mineral dust is emitted from both natural and anthropogenic activities. Natural emissions arise by wind acting on undisturbed source regions. Anthropogenic emissions result from human activity, including (1) land-use changes that modify soil surface conditions and (2) climate modifications that, in turn, alter dust emissions. Such modifications include changes in windspeeds, clouds and precipitation, and the amounts of airborne soluble material, such as sulfate, that may become attached to mineral dust particles and render them more susceptible to wet removal. 

A raindrop in a pollutant-free atmosphere has, as we have seen, a pH of 5.6 as a result of the dissolution of C02. However, emissions of S02 and NO lead to conversion of these species during transport from their sources to acidic sulfate and nitrate, and their incorporation into cloud and rainwater. Addition of these acids lowers the rainwater pH, and the rain reaching the ground is acidic. 

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