Ammonium, atmospheric aerosols

All of these species are very soluble in a rain or cloud drop and are an important source of atmospheric aerosols. For ammonia and ammonium, the condensed phases (I and s) represent approximately two-thirds of the total atmospheric burden, whereas for nitric acid and nitrates, about two-thirds is in the gas phase (Soderlund and Svensson, 1976). 

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]. 

Apart from NO, ammonia also occurs in the atmosphere which is largely formed by the natural ecosystem. In industrial regions it can undergo a series of reactions to produce ammonium sulphate aerosol in presence of sulphuric acid, or alternatively form NH2, N2O and NO. These species are responsible for the destruction of ozone in the troposphere9. 

Atmospheric aerosols are hygroscopic, taking up and releasing water as the RH changes (see also Section C.l) because some of the chemical components are themselves deliquescent in pure form. For example, sodium chloride, the major component of sea salt, deliquesces at 298 K at an RH of 75%, whereas ammonium sulfate, (NH4)2S04, and ammonium nitrate, NH4N03, deliquesce at 80 and 62% RH, respectively. (See Table 9.16 for the deliquescence points of some common constituents of atmospheric particles.) De-... 

Oatis, S., D. Imre, R. McGraw, and J. Xu, Heterogeneous Nucleation of a Common Atmospheric Aerosol Ammonium Sulfate, Geophys. Res. Lett., 25, 4469-4472 (1998). 

Toon, O. B., J. B. Pollack, and B. N. Khare, 1976. The optical constants of several atmospheric aerosol species ammonium sulfate, aluminum oxide and sodium chloride, J. Geophys. Res.. 81, 5733-5748. 

The study described here demonstrates that ESCA provides information regarding the chemical nature of the surface of an unperturbed sample which would be difficult to acquire by other methods. A major weakness of ESCA, the necessity of exposing the sample to vacuum, together with its attendant problem of sample volatilization, can also be one of its strengths. The volatility of some nitrogenous species in atmospheric aerosol particles can be used to provide strong evidence for chemical identity of ionic compounds (e.g., ammonium nitrate) rather than simply ionic identities as provided by wet chemical methods. This volatility is accelerated by x-ray irradiation, so that similar results could be achieved only by extended vacuum exposure alone if another analytical technique were used. Also, with ESCA, volatile losses can be conveniently monitored since the sample remains in the spectrometer throughout the process. 

Accomplishment of the complex observational experiment LACE-98 made it possible to obtain extensive information about atmospheric aerosol (aircraft measurements of the size distribution and number density of fine aerosols, coefficients of aerosol absorption, backscattering and depolarization, chemical composition of aerosol, as well as surface observations of the spectral optical thickness of the atmosphere, coefficients of extinction and backscattering). Fiebig et al. (2002) compared the observational data on optical parameters obtained from the results of numerical modeling for total H2S04 aerosol near the tropopause as well as for the ammonium sulfate/soot mixture in the remainder of the air column (Osborne et al., 2004). 

Recent estimates testify to the very strong impact of the phase state of atmospheric ammonium sulfate aerosol (with radiative humidity 80%) on the level of... 

Dod, R.L., Gundel, L.A., Benner, W.H. and Novakov, T. (1984) Non-ammonium reduced nitrogen species in atmospheric aerosol-particles. Sci. Total Environ., 36, 277-282. 

Figure 1 shows a schematic of a typical atmospheric aerosol particle (if such an entity can be assumed to exist). The particle consists of sulfates, nitrates, water, ammonium, elemental and organic carbon, metals, and dust. After a primary particle is emitted, gas-phase reactions occur, converting oxides of nitrogen to nitric acid, sulfur dioxide to sulfuric acid, and hydrocarbons to oxidized, low-vapor-pressure condensable organics. 

The variety of reactions which have been described indicate the large number of chemical species which enter in the troposphere and which may have an impact upon the environment. It is necessary to emphasize that all species are in a dynamic equilibrium for the meteorological turbulence so that in some cases redox and acid-base reactions occur with the formation of stable compounds with high formation constants but nevertheless the reagent species are detectable in the atmosphere. A typical example is the evaluation of the atmospheric aerosol where besides the presence of ammonium sulphates, free ammonia and sulphuric acid may coexist. 

Tang I. N. and Munkelwitz H. R. (1977) Aerosol growth studies III. Ammonium bisulfate aerosols in a moist atmosphere. J. Aerosol Sci. 8, 321-330. 

NH3 and to a lesser extent mono-, di-, and trimethylamines are the only significant gaseous bases in the atmosphere, and there has been considerable interest in whether the oceans are a source or sink of these gases. Early attempt to assess the air-sea flux from concentration measurements are probably suspect because of the ease with which sample contamination can occur during laboratory processing and analysis. It should be noted here that due to its high solubihty (low value of Henry s law constant), the air-water transfer of NH3 (and the methylamines for the same reason) is under gas phase control (see Section 6.03.2.1.1). The first reliable measurements were probably from the North and South Pacific and indicated that the flux of NH3 from sea to air is of a size similar to that for emission of DMS (Quinn et al., 1990, 1988). Indeed, the authors showed that this similarity was mirrored in the molar ratio of (non-sea-salt) sulfate to ammonium (1.3 0.7) in atmospheric aerosol particles collected on the cruise, indicating that for clean marine air remote from terrestrial sources, the emission of DMS and NH3 from the sea appears to control the composition of the aerosol. 

The formation of ammonium sulfate and ammonium nitrate aerosols is an acid-base reaction in the atmosphere. Ammonia neutralizes the acids. Sulfuric acid has a very low vapor pressure (< 10 atm) and thus exists in the atmosphere as liquid particles, which react with NH3 and H2O (reaction 22). 

FIGURE 4-39 The acid deposition process. Acid precursors, notably oxides of nitrogen and sulfur, are emitted to the atmosphere, primarily by fuel-burning equipment. Acid precursors are oxidized in the atmosphere to nitric and sulfuric acids by a variety of homogeneous and heterogeneous reactions. The acids are deposited by precipitation-related processes such as washout and rainout, by sorption of nitric acid vapor, and by dry deposition of acidic particulate material such as ammonium sulfate aerosol. (Stern et ah, 1984.)... 

Total and soluble Si in rain and atmospheric aerosols were determined using FIA system and ammonium molybdate as a reagent merged with oxalic acid and l-amino-2-naphthol-4-sulfonic acid as a reductant [ 1 ]. The molybdosilicic acid makes a basis of the determination of Si in airborne particulate matter [2]. 

An important example related lo the atmospheric aerosol is the droplet containing dissolved sulfates that form as a result of the oxidation of SO2 in solution. The sulfates may be present a.s sulfuric acid or in a partially neutralized form as ammonium salts or metallic salts from sources such as flyash. The droplet size distribution and chemical composition are determined by a combination of thermodynamic and rate processes. In this section, we consider only equilibrium thermodynamics as it affects the vapor pressure of the drop. 

A significant fraction of NH3 is converted into ammonium containing aerosol particles in the atmosphere. These particles are generally composed of ammonium sulfate, the formation of which will be discussed later (Subsection 3.5.3). We only note here that the concentration of NH4 in the lower troposphere is comparable to that of NH3 gas. Even, in the upper troposphere the particulate concentration may be greater than the level of gaseous NH3. For this reason Soderlund and Svensson (1976) speculate that the atmospheric NH4 burden is twice the global mass of NH3 (both expressed as nitrogen). 

Wet-chemical analyses of aqueous extracts of aerosol samples have established the presence of anions such as sulfate, nitrate, and the halides, and of cations such as ammonium and the ions of the alkali and alkaline earth elements. Table 7-13 shows selected data to illustrate the abundances of important inorganic components in the urban, continental, arctic, and marine aerosols. Included for comparison are the concentrations of silicon, aluminum, and iron, which are the major elements of crustal origin. They occur in oxidized form, such as in aluminosilicates, which are practically insoluble. Taken together, the elements listed in Table 7-13 account for 90% of all inorganic constituents of the atmospheric aerosol. 

Atmospheric aerosols at high relative humidities are aqueous solutions of species such as ammonium, nitrate, sulfate, chloride, and sodium. Cloud droplets, rain, and so on are also aqueous solutions of a variety of chemical compounds. 

Stelson and Seinfeld (1981) have shown that solution concentrations of 8-26 M can be expected in wetted atmospheric aerosol. At such concentrations the solutions are strongly nonideal, and appropriate thermodynamic activity coefficients are necessary for thermodynamic calculations. Tang (1980), Stelson and Seinfeld (1982a-c), and Stelson et al. (1984) have developed activity coefficient expressions for aqueous systems of nitrate, sulfate, ammonium, nitric acid, and sulfuric acid at concentrations exceeding 1M. 

Solution of (10.96) for a given temperature requires calculation of the corresponding molalities. These concentrations depend not only on the aerosol nitrate and ammonium but also on the amount of water in the aerosol phase. Therefore calculation of the aerosol solution composition requires estimation of the aerosol water content. As we have seen in Section 10.2.1, the water activity will be equal to the relative humidity (expressed in the 0-1 scale). While this is very useful information, it is not sufficient for the water calculation. One needs to relate the tendency of the aerosol components to absorb moisture with their availability and the availability of water given by the relative humidity. In atmospheric aerosol models (Hanel and Zankl 1979 Cohen et al. 1987 Pilinis and Seinfeld 1987 Wexler and Seinfeld 1991) the water content of aerosols is usually predicted using the ZSR relationship (Zdanovskii 1948 Stokes and Robinson 1966)... 

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