Chemistry atmospheric

Atmospheric chemistry is a vast subject, and it was one of the first areas of environmental chemistry to be developed with some scientific rigor. Part of the motivation for this field was early problems with smog in Los Angeles and with stratospheric ozone depletion. This chapter presents only a quick survey of some of these areas for more details, one should consult the excellent textbooks by Seinfeld and Pandis1 or by Finlayson-Pitts and Pitts.2 

The atmospheric chemistry of nitrogen is quite complex and involves literally hundreds or thousands of chemical reactions. Although the fluxes are much smaller than the biological fluxes, these processes are important for a variety of reasons, including impacts on climate, stratospheric ozone, and photochemical smog. In this section we present an overview of the most important processes. 

Photochemistry plays a significant role in nitrogen s atmospheric chemistry by producing reactive species (such as OH radicals). These radicals are primarily responsible for all atmospheric oxidations. However, since the photochemistry of the atmosphere is quite complex, it will not be dealt with in detail here. For an in-depth review on tropospheric photochemistry, the reader is referred to Logan et al. (1981), Finlayson-Pitts and Pitts (1986), Crutzen and Gidel (1983) or Crutzen (1988). 

In most cases, the direct reaction of N2 with O2 is slow under ambient conditions. It is the presence of numerous odd electron species (for example, OH, HO2, and RO2 radicals) that are photochemically produced and responsible for most of the oxidizing reactions of nitrogen species in the atmosphere. Some of the important reactions are shown below  

0( D) is an electronically excited oxygen atom. It can decay back to a ground state oxygen atom ( P) (which will regenerate an ozone molecule), or else it can react with water to produce two OH radicals  

The OH radical is a primary oxidizer in the atmosphere, oxidizing CO to CO2 and CH4 and higher hydrocarbons to CH2O, CO, and eventually CO2. OH and other radical intermediates can oxidize CH4 and NO in the following sequence of reactions  

The initial photochemical processes for CH4 are complex, with five competing reaction channels. The amount of each possible reaction, i.e. the number of products appearing in each reaction channel, is called the branching ratio and is a matter of 

The formation of the 1,3CH2 methylene radial in either the triplet or singlet electronic states depends on collisions with other molecules such as N2 in the Titan 

It is then possible to construct reaction schemes to build all of the hydrocarbon molecules observed to date. The reactions with N2 require much shorter wavelength photons to break the N=N triple bond and the chemistry is initiated by cosmic ray (cr) ionisation, with the reactions leading to HCN  

Having initiated the chemistry within the atmospheric model, the elongation of the carbon-chain polyyne species occurs by successive addition of the C2H radical  

Polymerisation of HCN species is also possible once the initial monomers have been formed by the reactions with nitrogen HCN polymers have been postulated in many places in the solar system, from the clouds of Jupiter and Saturn to the dark colour of the surface of comet Halley, not to mention its possible role in the formation of the prebiotic molecule adenine. Photolysis of HCN produces CN and then the formation of nitrile polymers  

Concentrations of trace gases and particles in the atmosphere can be expressed as mass per unit volume, typically pg m . The difficulty with this unit is that it is not independent of temperature and pressure. Thus, as an airmass becomes warmer or colder, or changes in pressure, so its volume will change, but the mass of the trace gas will not. Therefore, air containing 1 pg m of sulfur dioxide in air at 0°C will contain less than 1 pg m of sulfur dioxide in air if heated to 25°C. For gases (but not particles), this difficulty is overcome by expressing the concentration of 

John E., and Williams, David A. (1997). The Physics of the Interstellar Medium. Philadelphia Institute of Physics Publishing. 

Greenberg, J. Mayo (2000). The Secrets of Stardust. Scientific American 283(6) 70-75. 

Astrobiology. New Scientist. Available from http //www.newscientist.com/hottopics /astrobiology . 

The format here is as before, with the addition of a short section emphasizing potential ozone-loss mechanisms in the stratosphere. 

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Gas-phase reactions play a fundamental role in nature, for example atmospheric chemistry [1, 2, 3, 4 and 5] and interstellar chemistry [6], as well as in many teclmical processes, for example combustion and exliaust fiime cleansing [7, 8 and 9], Apart from such practical aspects the study of gas-phase reactions has provided the basis for our understanding of chemical reaction mechanisms on a microscopic level. The typically small particle densities in the gas phase mean that reactions occur in well defined elementary steps, usually not involving more than three particles. 

Squires R R 1997 Atmospheric chemistry and the flowing afterglow technique J. Mass Spectrum. 32 1271-72... 

Optical metiiods, in both bulb and beam expermrents, have been employed to detemiine tlie relative populations of individual internal quantum states of products of chemical reactions. Most connnonly, such methods employ a transition to an excited electronic, rather than vibrational, level of tlie molecule. Molecular electronic transitions occur in the visible and ultraviolet, and detection of emission in these spectral regions can be accomplished much more sensitively than in the infrared, where vibrational transitions occur. In addition to their use in the study of collisional reaction dynamics, laser spectroscopic methods have been widely applied for the measurement of temperature and species concentrations in many different kinds of reaction media, including combustion media [31] and atmospheric chemistry [32]. 

References 4, 5, 6, 7, 8, 11, 12, 45, 56, 61, 64, and 70 and the foUowiag books and reports constitute an excellent Hst for additional study. Reference 7 is an especially useful resource for global atmospheric chemistry. 

J. H. Seiafeld, Atmospheric Chemistry and Physics of Air Pollution, John Wiley Sons, Inc., New York, 1986. 

Representation of Atmospheric Chemistry Through Chemical Mechanisms. A complete description of atmospheric chemistry within an air quaUty model would require tracking the kinetics of many hundreds of compounds through thousands of chemical reactions. Fortunately, in modeling the dynamics of reactive compounds such as peroxyacetyl nitrate [2278-22-0] (PAN), C2H2NO, O, and NO2, it is not necessary to foUow every compound. Instead, a compact representation of the atmospheric chemistry is used. Chemical mechanisms represent a compromise between an exhaustive description of the chemistry and computational tractabiUty. The level of chemical detail is balanced against computational time, which increases as the number of species and reactions increases. Instead of the hundreds of species present in the atmosphere, chemical mechanisms include on the order of 50 species and 100 reactions. 

Three different types of chemical mechanisms have evolved as attempts to simplify organic atmospheric chemistry surrogate (58,59), lumped (60—63), and carbon bond (64—66). These mechanisms were developed primarily to study the formation of and NO2 in photochemical smog, but can be extended to compute the concentrations of other pollutants, such as those leading to acid deposition (40,42). 

The mechanisms by which a jurisdiction develops its air pollution control strategies and episode control tactics are outlined in Fig. 5-1. Most of the boxes in the figure have already been discussed—sources, pollutant emitted, transport and diffusion, atmospheric chemistry, pollutant half-life, air quality, and air pollution effects. To complete an analysis of the elements of the air pollution system, it is necessary to explain the several boxes not vet discussed. 

Sloane, C. S., and Tesche, T. W., "Atmospheric Chemistry Models and Predictions for Climate and Air Quality." Lewis Publishers, Chelsea, Ml, 1991. 

Atmospheric chemistry influences human health, climate, food production, and through its impact on visibility, our view of the world. Chemicals in the air affect us with each breath we take. Suspended particulate matter that form from gas-phase reactions affect the amount of solar energy reaching the earth s surface. 

P. Crutzen (Max Planck Institute for Chemistry, Mainz), M. Molina (Massachusetts Institute of Technology) andF. S. Rowland (Irvine, California) work in atmospheric chemistry, particularly concerning the formation and decomposition of ozone. 

Self-Test 2.9A Write a Lewis structure for the hydrogenperoxyl radical, HOO-, which plays an important role in atmospheric chemistry and which, in the body, has been implicated in the degeneration of neurons. 

Nitrogen forms several oxides, with oxidation numbers ranging from - -l to +5. All nitrogen oxides are acidic oxides and some are the acid anhydrides of the nitrogen oxoacids (Table 15.2). In atmospheric chemistry, where the oxides play an important two-edged role in both maintaining and polluting the atmosphere, the) are referred to collectively as NO (read nox ). 

Sulfur forms several oxides that in atmospheric chemistry are referred to collectively as SOx (read sox ). The most important oxides and oxoacids of sulfur are the dioxide and trioxide and the corresponding sulfurous and sulfuric acids. Sulfur burns in air to form sulfur dioxide, S02 (11), a colorless, choking, poisonous gas (recall Fig. C.1). About 7 X 1010 kg of sulfur dioxide is produced annually from the decomposition of vegetation and from volcanic emissions. In addition, approximately 1 X 1011 kg of naturally occurring hydrogen sulfide is oxidized each year to the dioxide by atmospheric oxygen ... 

NOx An oxide, or mixture of oxides, of nitrogen, typically in atmospheric chemistry, noble gas A member of Group 18/VIlI of the periodic table (the helium family). 

The choice of a particular type of gas discharge for quantitative studies of ion-molecule reactions is essential if useful information is to be obtained from ion abundance measurements. Generally, two types of systems have been used to study ion-molecule reactions. The pulsed afterglow technique has been used successfully by Fite et al. (3) and Sayers et al. (1) to obtain information on several exothermic reactions including simple charge transfer processes important in upper atmosphere chemistry. The use of a continuous d.c. discharge was initiated in our laboratories and has been successful in both exothermic and endothermic ion-molecule reactions which occur widely within these systems. 

Clean and Polluted Air. In the development of atmospheric chemistry, there has been an historic separation between those studying processes in the natural or unpolluted atmosphere, and those more concerned with air pollution chemistry. As the field has matured, these distinctions have begun to disappear, and with this disappearance has come the realization that few regions of the troposphere are completely unaffected by anthropogenic emissions. An operational definition of clean air could be based upon either the NMHC concentration, or upon the NOjj concentration. 

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