Tropospheric background aerosol

An important step in the understanding of the formation and composition of tropospheric background aerosol was provided by the work of Fenn et al. (1963) who demonstrated that in aerosol samples collected in Greenland13, 40 % of the large particles consisted of sulfate. This finding was confirmed by American authors... 

Because the mass of ammonium sulfate and sulfuric acid particles is mainly in the size range of active condensation nuclei, it is believed that this process provides a very effective removal mechanism for the tropospheric background aerosol. However, we have to emphasize that other processes are also operating in the cloud to remove small aerosol particles, of which the most important process is the coagulation of particles and cloud drops. As we have seen (Subsection 4.1.1), thermal coagulation is particularly effective in the range of very small particles inactive in condensation. To estimate this process, let us consider a cloud in which the number concentration of drops with radius rc is Nc. If the number concentration... 

Hoffman et al. (1974) found the same procedure applicable to data obtained from measurements on board of ships in the central Atlantic Ocean. Table 7-15 includes mean (X)/(Na) ratios from their work. Shown in parentheses are the values derived from the slopes of regression lines. They are distinctly lower than the averaged data. Hoffman et al. (1974) measured also the abundance of iron in the aerosols. Since the samples were taken in a region partly affected by fallout from the Saharan dust plume, iron serves as a convenient indicator for the contribution of material from continental sources. Not surprisingly, the enrichment of the elements Mg, Ca, K, and Sr was well correlated with the iron content. The (X)/(Na) ratios approached those of sea water only when the Fe concentrations were very low. These results demonstrate that materials from both marine and crustal sources are present over the open ocean. In addition, they provide some verification for the existence of a tropospheric background aerosol having the continents as a source, and they confirm the absence of a significant fractionation of alkali and alkaline earth elements in the production of sea salt. 

Fig. 7-26. Model for the vertical distribution of particulate matter in the troposphere. The model assumes a superposition of the tropospheric background aerosol with boundary layer aerosol over the continents, and with sea-salt aerosol over the oceans.

Table 10-12 includes concentrations of particulate sulfate in the free troposphere at elevations above 4 km. Although the data come from various parts of the world, the concentrations are fairly uniform, ranging from 20 to 130 ng S/m3 STP. Thus, sulfate is an important constituent of the tropospheric background aerosol. If, as in Section 7.6, we assume a mixing ratio for the background aerosol of 1 xg/m3 STP, sulfate is found to contribute roughly 25% by mass. 

It follows from this discussion that the study of properties and effects of atmospheric aerosol particles exceeds the scope of air chemistry. Considering the complexity of the problem, we restrict our discussion in the following to the presentation of formation processes and principal physical and chemical properties of background aerosol in the troposphere and stratosphere. 

It should be mentioned that the size distributions presented in Figs 26 and 27 are typical of a polluted atmosphere. Unfortunately, very little information is available about the background aerosol filling 80-90 % of the troposphere. These... 

On the basis of the foregoing discussion it is concluded that tropospheric background particles consist mainly of sulfur compounds. Sulfate particles contain hydrogen or ammonium ions as a function of the ammonia gas available. These mostly Aitken size particles are (externally or internally) mixed with some organic material at least near the Earth s surface.16 More research is needed to know in more detail the chemistry of this aerosol. 

Background aerosols that occur in the free troposphere (above the clouds). For practical purposes, these are sometimes taken to be the aerosols that occur at altitudes above 5 km. 

Thus, while there are copious data showing that ultrafine aerosols nucleate in a wide range of situations (e.g., boundary layer air [3], cloud outflows [27], upper troposphere[31,32]), and that particle production is often coherent over regional scales, and not confined to local sources, the underlying nucleation processes have not yet been identified. An alternative explanation for some of these observations, outlined in the following sections, considers the role of background ionization in the generation of new particles in the troposphere [19,33]. 

Simoneit, B.R.T., and Mazurek, M.A. (1982) Organic matter of the troposphere—II. Natural background of biogenic lipid matter in aerosols over the rural western United States. Atmos. Environ. 16, 2139-2159. 

Due to the considerably different nature of aerosols and their living conditions in different layers of the atmosphere (which are nearly isolated in the sense of energy- and mass-exchange), the atmospheric aerosol is subdivided into tropospheric, stratospheric, mesospheric, etc., aerosols. Its background and disturbing components are, accordingly, differentiated. 

The atmospheric, climatic, environmental, and health effects of volcanic volatile emissions depend on several factors but importantly on fluxes of sulfur and halogens. As discussed in Section 3.04.5.1, intermittent explosive emptions can pump >10 kg of sulfur into the stratosphere, against a background of continuous fiimarolic and open-vent emission into the troposphere. The episodic, large explosive eruptions are the principal perturbation to stratospheric aerosol levels (e.g., 30 Mt of sulfate due to the 1991 emption of... 

Particle size and surface area as well as vertical distribution are also important for heterogeneous reactions. The majority of the mass of atmospheric aerosol matter is represented by particles of size 10 -10 m [5], i.e. their surface area must be 10 m g. The specific surface area A of solid aerosols near the Earth surface is considered to be 10 m dm of air under background conditions and can increase by a factor of 100 in urban areas [3]. The vertical distribution of A is shown in Fig. 1(B). Due to sedimentation, almost all of aerosols are located in the lower layer of the troposphere. 

The optical and microphysical properties of aerosol allowed the identification of the days during which the aerosol concentration in separate layers exceeded the background 10. .. 10 times. Two such layers were identified the Atmospheric Brown Cloud (ABC) in the upper troposphere (5.5-9.1 km) and ABC in the lower troposphere (1.7-4.3 km). The highest aerosol concentrations in the separate layers of the troposphere were observed most frequently in warm periods (May - September), and much less often in cold seasons. 

Atmospheric chemistry is dominated by trace species, ranging in mixing ratios (mole fractions) from a few parts per million, for methane in the troposphere and ozone in the stratosphere, to hundredths of parts per trillion, or less, for highly reactive species such as the hydroxyl radical. It is also surprising that atmospheric condensed-phase material plays very important roles in atmospheric chemistry, since there is relatively so little of it. Atmospheric condensed-phase volume to gas-phase volume ratios range from about 3 x KT7 for tropospheric clouds to 3 x ICE14 for background stratospheric sulfate aerosol. 

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