Sulfate particles

Particles are the major cause of the ha2e and the brown color that is often associated with smog. The three most important types of particles produced in smog are composed of organics, sulfates, and nitrates. Organic particles are formed when large VOC molecules, especially aromatics and cycHc alkenes, react with each other and form condensable products. Sulfate particles are formed by a series of reactions initiated by the attack of OH on SO2 in the gas phase or by Hquid-phase reactions. Nitrate particles are formed by... 

Acid Deposition. Acid deposition, the deposition of acids from the atmosphere to the surface of the earth, can be dry or wet. Dry deposition involves acid gases or their precursors or acid particles coming in contact with the earth s surface and thence being retained. The principal species associated with dry acid deposition are S02(g), acid sulfate particles, ie, H2SO4 and NH HSO, and HN02(g). Measurements of dry deposition are quite sparse, however, and usually only speciated as total and total NO3. In general, dry acid deposition is estimated to be a small fraction of the total... 

Because the regions of the alimentary tract vary widely ia pH and chemical composition, many different commercial formulations of barium sulfate are available. The final preparations of varyiag viscosity, density, and formulation stabiUty levels are controlled by the different size, shape, uniformity and concentration of barium sulfate particles and the presence of additives. The most important additives are suspending and dispersiag agents used to maintain the suspension stabiUty. Commercial preparations of barium sulfate iaclude bulk and unit-dose powders and suspensions and principal manufacturers are E-Z-EM (Westbury, New York), Lafayette-Pharmacol, Inc. (Lafayette, Indiana), and Picker International, Inc. (Cleveland, Ohio). 

Heterogeneous chemistry occurring on polar stratospheric cloud particles of ice and nitric acid trihydrate has been estabUshed as a dorninant factor in the aggravated seasonal depletion of o2one observed to occur over Antarctica. Preliminary attempts have been made to parameterize this chemistry and incorporate it in models to study ozone depletion over the poles (91) as well as the potential role of sulfate particles throughout the stratosphere (92). 

Sulfate particles formed in the gas phase can condense. In addition, sulfate can become bound to metals and can be adsorbed on unburned carbon particles. 

Acid deposition and the associated particulate nitrates and sulfates are implicated in the deterioration of certain sensitive ecosystems, decreased visibility, negative human health effects, and increased degradation of certain stone building materials and cultural resources, especially those made of limestone and marble. Fine particulate nitrate and sulfate particles... 

Climate is often viewed as the aggregate of all of the elements of weather, with quantitative definitions being purely physical. However, because of couplings of carbon dioxide and many other atmospheric species to both physical climate and to the biosphere, the stability of the climate system depends in principle on the nature of feedbacks involving the biosphere. For example, the notion that sulfate particles originating from the oxidation of dimethylsulfide emitted by marine phytoplankton can affect the albedo (reflectivity) of clouds (Charlson et ai, 1987). At this point these feedbacks are mostly unidentified, and poorly quantified. 

The most oxidized form of sulfur, S(+VI), is predominantly sulfate, SOl. Sulfate particles ranging in composition from pure sulfuric acid (H2SO4) to fully neutralized ammonium sulfate ((NH4)2S04) are ubiquitous constituents of the atmosphere (see Chapter 7). 

For both polluted and remote conditions, therefore, the cycling of sulfur from low oxidation state gas to sulfate particles and then back to the surface in rain takes place on a time scale of a few days. 

Measurements of the photophoretic force on crystalline ammonium sulfate particles were made by Lin and Campillo (1985) using an electrodynamic balance. The measurement procedure is identical to that for any such force, that is, the levitation voltage is measured in the absence of the photophoretic force and then when the force is exerted. A force balance yields an equation for the photophoretic force similar to Eq. (31) ... 

Fig. 12, Photophoretic force data of Lin and Campillo (1985) for crystalline ammonium sulfate particles levitated in an electrodynamic balance. Reprinted with permission from Lin, H.-B., and Campillo, A. J., Applied Optics 24, 244, Copyright 1985, The Optical Society of America.

The refined source profiles that best reproduced the coarse fraction are listed in table 7. The calculated profiles of the two crustal components follow those of Mason ( ), though the calcium concentration of 20 in the limestone factor is less than the reported value. The paint pigment profile strongly resembles that calculated for the fine-fraction data. The only major difference is that unlike the fine fraction, the coarse-fraction profile does not associate barium with the paint-pigment factor. The calculated sulfur concentration in the coarse-fraction sulfate factor is much less than that in the fine-fraction and there are sizable concentrations of elements such as aluminum, iron, and lead not found in the fine-fraction profile. The origin of this factor is not clear although as described earlier a possible explanation is that a small part of the sulfate particles in the fine fraction ended up in the coarse samples. 

There is evidence from laboratory studies that heterogeneous reactions on sulfate particles may be important in the upper troposphere as well. For example, HCHO uptake into sulfuric acid solutions or ternary mixtures of sulfuric and nitric acids and water has been observed in laboratory studies (e.g., Tolbert et al., 1993 Jayne et al., 1996 Iraci and Tolbert, 1997). In sulfuric acid, the effective Henry s law constant at the low... 

FIGURE 9.10 Modified particle modes and growth processes for sulfate particles involving aqueous-phase reactions in low altitude fogs and in higher altitude clouds upon advection of boundary-layer air upwards. (Adapted with permission from Ondov and Wexler, 1998. Copyright 1998 American Chemical Society.)... 

In short, growth of sulfate particles at least in the accumulation mode and the presence of two peaks are both believed to be largely controlled by interactions with water in the atmosphere, including the aqueous phase oxidation of SOz to sulfate. 

Table 9.8 summarizes some measurements of the contribution of light absorption by particles, b, for different types of locations from urban residential to remote. In urban areas, bap varies from 13 to 42% of 6cxl, whereas in very remote areas, the contribution of absorption is much less. This is not surprising since the combustion sources producing graphitic carbon tend to be in urban-industrial areas. However, it is noteworthy that soot has been found associated with many sulfate particles even over the remote oceans (Buseck and Posfai, 1999). 

Sulfur 10-200% increase6 in surface area in sulfate particles Increased aerosol surface area, enhanced ozone depletion by CIO, decreased ozone depletion by NO,... 

HSCTs also emit particles and S02, with the latter being oxidized to H2S04 and sulfate particles. Measurements of particle concentrations in the plume of the Concorde SST showed much larger particle concentrations than anticipated (Fahey et al., 1995a). Furthermore, a much larger portion of the S02 in the exhaust was oxidized to H2S04 particles than expected based on the OH levels measured in the exhaust plume (Hanisco et al., 1997), suggesting that there are some as yet unknown mechanisms of S02 oxidation in the plume. 

Some field measurements of HN03 suggest that the formation of liquid or solid Type I PSCs depends on the initial background sulfate aerosols on which the PSCs form. If they are liquid, then liquid ternary solution PSCs tend to form first as the temperature drops below 192 K, whereas if the sulfate particles are initially solids, solid Type lc PSCs may be generated (Santee et al., 1998). 

Additional sulfates continue to form after the eruption as gaseous S02 is oxidized to sulfuric acid and sulfates. While we shall focus here on the effects of these sulfate particles on the heterogeneous chemistry of the stratosphere, there may be other important effects on the homogeneous chemistry as well. For example, model calculations by Bekki (1995) indicate that this oxidation of S02 by OH leads to reduced OH levels, which alters its associated chemistry. 

There are a number of measurements documenting changes in NO and NO. in the stratosphere after the Mount Pinatubo eruption and which have been attributed to the removal of oxides of nitrogen due to reactions on aerosol particles. For example, a decrease in stratospheric NOz after the eruption followed by a return to normal levels has been reported (e.g., see Van Roozendael et al., 1997 and De Maziere et al., 1998). Similarly, NO decreases of up to 70% were reported, as well as increases in gaseous HN03 (much of that produced on the sulfate particles is released to the gas phase) (e.g., see Coffey and Mankin, 1993 Koike et al., 1993, 1994 David et al., 1994 Webster et al., 1994 and Rinsland et al., 1994). 

The chemistry in the midlatitude stratosphere follows that discussed throughout this chapter. As seen in the previous sections, the heterogeneous chemistry that was once thought to be unique to PSCs also occurs in and on the liquid solutions characteristic of sulfate particles distributed globally, with their relative importance being determined by the temperature, composition, and phase of the condensed phase. 

Toumi et al. (1994) also suggested there is a feedback between reduced stratospheric ozone and particles in that the increased UV due to ozone depletion may increase sulfate particle formation by increasing the concentrations of tropospheric OH. 

It should be noted that the magnitude of the predicted forcing is quite sensitive to treatment of relative humidity (RH) in the model because of the effects on particle size and optical properties (e.g., Haywood and Shine, 1995 Haywood and Ramaswamy, 1998 Ghan and Easter, 1998 Haywood et al., 1998a Penner et al., 1998). For example, in the calculations by Penner et al. (1998), when the particle properties were held fixed at the values for 90% RH for 90-99% RH, the predicted direct radiative forcing for sulfate particles decreased from - 1.18 W m-2 to -0.88 W m 2 for the Northern Hemisphere and from -0.81 to -0.55 W m-2 globally. 

Volcanic eruptions provide one test of the relationship between light scattering by sulfate particles and the resulting change in temperature, since they generate large concentrations of sulfate aerosol in the lower stratosphere and upper troposphere. These aerosol... 

Modeling studies by Langner et al. (1992) suggest that at most 6% of the anthropogenic S02 emissions can form new particles, since removal of S02 by direct deposition is large ( 50%) and the portion that is oxidized in clouds does not lead to new particles. Taking these factors into account, Langner et al. (1992) estimate that new sulfate particles may have doubled since preindustrial times. 

In addition to the differences in geographical distribution of the greenhouse gases compared to the aerosol particles and the day-night differences, there are also differences in their temporal behavior. As discussed earlier, typical residence times for sulfate particles are about a week, whereas that of C02 is about 100 years. As a result, the impacts of sulfate aerosols are almost immediately manifested, whereas those due to C02 occur over decades to centuries (Schwartz, 1993). 

Table 5.1.1 Typical Conditions for the Formation of Monodispersed Basic Iron Sulfate Particles...

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