Atmospheric aerosol concentration, contribution

Particle emissions resulting from natural processes should be roughly distributed according to the relative amounts of land and ocean areas in various latitudinal bands. Assuming this, natural amissions are estimated to be a function of latitude, based on the amount of land and water distributed in various latitude zones. When this is done for the northern hemisphere and compared with an estimated zonal distribution of pollutants based on Figure 3, the relative contribution of pollutant sources to atmospheric aerosol concentrations as a function of latitude is estimated. 

Figure 4. Relative contribution of pollutant aerosols to total atmospheric aerosol concentrations in the Northern Hemisphere...

Atmospheric particles influence the Earth climate indirectly by affecting cloud properties and precipitation [1,2], The indirect effect of aerosols on climate is currently a major source of uncertainties in the assessment of climate changes. New particle formation is an important source of atmospheric aerosols [3]. While the contribution of secondary particles to total mass of the particulate matter is insignificant, they usually dominate the particle number concentration of atmospheric aerosols and cloud condensation nuclei (CCN) [4]. Another important detail is that high concentrations of ultrafine particles associated with traffic observed on and near roadways [5-7] lead, according to a number of recent medical studies [8-11] to adverse health effects. 

Estimates of the power of natural sources of SDA injected in to the atmosphere through wind erosion mechanism vary within (1-5) 10 t/year [2, 15, 40]. However, as is noted by Ivlev [4], these figures are probably very much underestimated, and should be considered as the amount of the SDA contribution to the background global concentration of the atmospheric aerosol. 

As shown in Figure 1, within an atmospheric aerosol the smallest particles usually dominate the total number of particles, while the accumulation and coarse modes often determine the total surface area and volume (i.e., mass), respectively. For example. Figure 3 shows results from a study in Atlanta where nanoparticles (Dp = 3-10 nm) and nano- and ultrafme particles (Dp = 10-100 nm) contributed approximately 30 and 60%, respectively, to the total particle number concentration (Dp < 2 pm). However, in terms of particle mass, the accumulation mode particles were dominant, and nanoparticles with Dp < 10 nm contributed insignificantly. 

The concentration of atmospheric aerosols varies considerably in space and time. This variability of the aerosol concentration field is determined by meteorology and the emissions of aerosols and their precursors. For example, the annual average concentration of PM2.5 in North America varies by more than an order of magnitude as one moves from the clean remote to the polluted urban areas of Mexico City and southern California (Figure 8.24). Sulfate dominates the fine aerosol composition in the eastern United States, while organics are major contributors to the aerosol mass everywhere. Nitrates are major components of the PM2.5 in the western United States. The EC makes a relatively small contribution to the particle mass in many areas, but because of its ability to absorb light and its toxicity, it is an important component of atmospheric particulate matter. 

Measurement of the Aitken nuclei or Aitken particles can be made by a pneumatic expansion type particle counter. Aitken particles is the name given to those atmospheric particles whose size lies between a few angstroms (1 angstrom is 10 cm or 10 " pm) and 0.1 pm. They constitute the greatest contribution to the spectrum of sizes in the atmospheric aerosol. The measurement of the concentration of these particles is based on their ability to act as condensation nuclei for water vapour, when this is supersaturated. 

For environmental samples, such as atmospheric aerosols, sediments, and snow, Pb from several sources may contribute to the total Pb concentration and therefore the isotopic signature wUl be a mixture of those of the various contributions. When there is mixing between Pb from two sources, the Pb isotope ratio results plotted on a three-isotope plot, that is, one isotope ratio of Pb, plotted as a function of another with a common denominator (Figure 1.5), will fall on a straight mixing line between the two end-member compositions, and the extent to which each of the two sources contributes to the sample s signature can be calculated [27]. If one Pb isotope ratio is plotted as a function of the Pb... 

There is a large variety of atmospheric sulfur compounds, in the gas, solid, and liquid phases. Table 7-3 lists a number of gaseous compounds, range of concentration, source, and sink (where known). As this list illustrates, a significant number of these gases contribute to the existence of oxidized sulfur in the forms of SO2 and sulfate aerosol particles. Table 7-4 lists the oxy-acids of sulfur and their ionized forms that could exist in the atmosphere. Of these the sulfates certainly are dominant, with H2SO4 and its products of neutralization with NH3 as the most frequently reported forms. 

Threshold concentrations for Ct+ alkenes are in the parts-per-hundred-million to parts-per-million range. Determination of their gas-phase concentration is required for assessing their contribution to atmospheric organic aerosol. 

It should be noted that Pb concentrations in this polluted air are about 100 times lower than the aerosol S levels, or are only a few tens of ng Pb/m. In the Beijing area the contribution of automotive emissions to atmospheric Pb at Xinglong is expected to be minor owing to the small amounts of leaded gasoline used privately owned automobiles, trucks, and other gasoline powered vehicles are nonexistent in China. Instead, coal combustion may be the principal source of fine particle Pb as well as K and S. 

One of the important properties of particles that contributes to both the observed size distribution and the number concentration of aerosols in the atmosphere is the motion they undergo when suspended in air. This includes gravitational settling and Brownian diffusion. 

As discussed in Section C.la, sea salt particles in the marine boundary layer have been shown to likely play a major role in backscattering of solar radiation (Murphy et al., 1998), i.e., to the direct effect of aerosol particles. However, they also contribute to the indirect effect involving cloud formation, since they can also act as CCN. Since such particles are a natural component of the marine atmosphere, their contribution will not play a role in climate change, unless their concentration were somehow to be changed by anthropogenic activities, e.g., through changes in wind speed over the... 

---