Ammonia, tropospheric

The evolution of the emissions of some atmospheric pollutants in Europe (EU-15) in the period 1990-1999 has been presented in the report of Goodwin and Mareckova (2002). The report includes acidifying pollutants (ammonia, sulfur dioxide, and nitrogen oxides), tropospheric ozone precursors, NMVOCs, carbon monoxide, and particulate matter... 

The aerosols of sulfuric acid so formed increase the reflectivity (albedo) of the Earth s atmosphere, cutting down the solar radiation that reaches the Earth s surface and so counteracting to some extent the greenhouse warming due to CO2 emissions that accompany the SO2, as mentioned earlier. Airborne sulfuric acid may be neutralized by traces of ammonia in the air, giving particulate NH4HSO4 and (NH4)2S04 hazes, but in the absence of such neutralization the aqueous sulfuric acid droplets in tropospheric clouds may reach pH 1.5 or lower. 

Nonfluorine CFC substitutes have been considered, but few are fully satisfactory. For example, we could go back 50 years to the use of anhydrous ammonia as a refrigerant, but NH3 is as toxic now as it ever was. Cyclopentane could be used as a foam-blowing agent, but it is less effective than HCFC-141b and besides would contribute to the volatile organic compound load in the troposphere, which is the root cause of ozone pollution (Section 8.3.2). On the other hand, supercritical CO2 is emerging as an alternative to CFCs in various steps in the preparation of fluorocarbon polymers (Section 8.1.3). 

Society is facing several crucial issues involving atmospheric chemistry, Species containing nitrogen are major players in each. In the troposphere, nitrogen species are catalysts in the photochemical cycles that form ozone, a major urban and rural pollutant, as well as other oxidants (references 1 and 2, and references cited therein), and they are involved in acid precipitation, both as one of the two major acids (nitric acid) and as a base (ammonia) (3, 4). In the stratosphere, where ozone acts as a shield for the... 

Ziereis, H and F. Arnold, Gaseous ammonia and ammonium ions in tire free troposphere, Nature, 321,... 

The observed positive ions are protonated clusters containing water and high proton affinity species such as acetonitrile in the lower stratosphere (26) or ammonia in the lower troposphere (20). Other high proton affinity species such as pyridine and picolines may enter into the positive ion chemistry of the lower troposphere (27,28). Further discussion of these studies and the experimental techniques can be found elsewhere (28,29). 

The temperature and density structure of the troposphere, along with the concentrations of major constituents, are well documented and altitude profiles have been measured over a wide range of seasons and latitudes for the minor species water, carbon dioxide, and ozone. A few profiles are available for carbon monoxide, nitrous oxide, methane, and molecular hydrogen, while only surface or low-altitude measurements have been made for nitric oxide, nitrogen dioxide, ammonia, sulfur dioxide, hydrogen sulfide, and nonmethane hydrocarbons. No direct measurements of nitric acid and formaldehyde are available, though indirect information does exist. The concentrations of a number of other important species, such as peroxides and oxy and peroxy radicals, have never been determined. Therefore, while considerable information concerning trace constituent concentrations is available, the picture is far from complete. 

The scope of this paper is to provide an overview of methods used to study properties of electrically neutral molecular clusters initiating particle formation in the troposphere, with focus on quantum chemistry. The review of results is intended to be complete with regard to water-sulfuric acid-ammonia clusters. Concerning studies on clusters including other molecular species, we review representative examples and newest publications. Ionic clusters and clusters involving iodine, related to coastal nucleation, are mentioned in passing. 

Another application of quantum chemical methods is the investigation of the fundamental chemical behaviour of molecular systems potentially relevant to nu-cleation. Within the field of tropospheric nucleation mechanisms, two questions which have merited considerable study under the last decade are the modeling of the hydration of sulfuric acid, and the role of ammonia in sulfuric acid-water nucleation. 

Other potential reactant trace gases besides H2O and EhS04 are NIL,. CILCN, and acids such as HNO3 and HCl. Ammonia, for example, is highly soluble in water and therefore may become depleted from the gas phase. According to in situ measurements, tropospheric ammonia vapor abundances greatly exceed the critical reactant trace gas level. Consequently, NHj may markedly influence the positive ion chemistry. The same may be true for CILCN which seems to originate from the troposphere. 

Another important tropospheric trace gas which should be detectable via ion composition measurements is ammonia. It reacts, as already mentioned, with positive ions yielding NH4 cores. So far, however, such cores could not be detected in the upper troposphere which sets an upper limit to the ammonia vapour abundance (mole fraction) of only about 2 10 around the tropopause [29]. 

Hauck G. and Arnold F., Positive ion composition measurements in the upper troposphere - Implications for acetonitrile and ammonia vapours. Nature (1984) submitted. 

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. 

Nitric oxide (NO), nitrous oxide (N2O), dinitrogen (N2), and ammonia (NH3) are constituents of the Earth s atmosphere. They play important roles in the chemistry and climate of the present-day Earth. Moreover, they are intermediates of the oceanic nitrogen cycle. In contrast to most of the other components of the oceanic nitrogen cycle, they exist as dissolved gaseous molecules. Being gases they can be transferred across the seasurface-troposphere interface. 

The reported measured rate constant for reaction of hydrazine with atmospheric hydroxyl (OH) radicals producing ammonia and nitrogen gas was 6.lx 10 cm molecule s (Harris et al. 1979). The rate constant for 1,1-dimethylhydrazine was not measured since the chemical decomposed rapidly in the test system, but the value was estimated at 5 /10 cm molecule s . Assuming an average OH radical concentration of about 10 molecLile/cm . the tropospheric half-lives ofboth chemicals due to reaction with OH were estimated to be about 3 hours. The half-lives are expected to range from less than 1 hour in polluted urban air to 3-6 hours in less polluted atmospheres (Tuazon et al. 1981). 

Those oxides with wide band gaps, such as y-Al203, MgO, etc., are also known to be efficient photocatalysts for the oxidation of ammonia, nitrogen oxides, and hydrocarbons [23-26]. When evaluating the possible role of these most abundant components of continental aerosols in the tropospheric photochemistry, their importance may be due to their photoactivity under irradiation not only in their main absorption bands, but also in the bands of surface chemisorbed species. For example, for dispersed Sn02, it was found that surface carbonates were formed upon CO2 adsorption, which in turn sensitized some surface photoreactions to light quanta with energies less than 2eV (below 600 nm) [27]. 

It should be stressed that the activity of man also produce atmospheric N20. Several authors argue (e.g. Bremner and Blackmer, 1978) that N20 can be released to the atmosphere through denitrification of fertilizer-derived nitrate and ammonia. This concept seems to be supported by the possibility that the N20 mixing ratio is increasing in the troposphere. It is obvious, however, that much work remains to be done to clarify this problem. 

It was also proposed in the literature (e.g. Crutzen, 1974) that a non-negligible part of NHj is converted in the troposphere into nitrogen oxides. The first step of this conversion is the reaction between ammonia and OH free radicals ... 

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. 

We can thus conclude that the spring maximum cannot be explained either by the annual variation of source intensity at the Earth s surface or by the variation of the quantity of precipitation. It has been postulated (E. Meszaros, 1974a) that this maximum is due to the oxidation effects of tropospheric ozone, the concentration of which also has a maximum during the spring (see Fig. 13). Ozone oxidizes S02 and N02 in atmospheric liquid water (see Subsection 5.3.2) which leads to the lowering of the pH. The increase in the concentration of hydrogen ions promotes the absorption of ammonia gas from the air, as well as the transformation of insoluble mineral components (e.g. calcium carbonate) into water-soluble materials. If this speculation is correct, this process provides a non-negligible ozone sink in the... 

Aneja VP, Murthy AB, Battye W, et al. 1998. Analysis of ammonia and aerosol concentrations and deposition near the free troposphere at Mt. Mtichell, NC, USA. Atmos Environ 32(3) 353-358. 

Levine JS, Augustsson TR, Hoell JM. 1980. The vertical distribution of tropospheric ammonia. Geophys Res Lett 7(5) 317-320. 

Sprenger U, Bachmann K. 1987. Determination of ammonium in aerosols, cloud and rain water, and of gaseous ammonia in the troposphere. Fresenius Z Anal Chem 327 16. 

Sulfuric acid exists essentially totally in the particulate phase under atmospheric conditions, and it is generally neutralized by reaction with metal cations or ammonia, to form salts such as Na2S04, CaS04, (NH3)2S04, and NH3HS04. Wet and dry deposition lead to sulfate removal from the troposphere. 

Table 9-3. Estimates for the Strengths of the Sources and Sinks of Ammonia (TgN/yr) in the Troposphere According to Various Authors... 

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