Troposphere nitrogen oxide chemistry

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). 

Nitrogen oxides also play a significant role in regulating the chemistry of the stratosphere. In the stratosphere, ozone is formed by the same reaction as in the troposphere, the reaction of O2 with an oxygen atom. However, since the concentration of O atoms in the stratosphere is much higher (O is produced from photolysis of O2 at wavelengths less than 242 nm), the concentration of O3 in the stratosphere is much higher. 

Jaffe, D The Relationship between Anthropogenic Nitrogen Oxides and Ozone Trends in the Arctic Troposphere, in The Tropospheric Chemistry of Ozone in the Polar Regions (H. Niki and... 

Stevenson D.S., W.J. Collins, C.A. Johnson and RG. Derwent, 1997 The impact of nitrogen oxide emissions on tropospheric ozone studied with a 3-D Lagrangian model including foil diurnal chemistry, Atmos. Env.,31, 1837-1850. 

Globally, the oxides of nitrogen, NO (nitric oxide), NO2 (nitrogen oxide), and N2O (nitrous oxide), are key species involved in the chemistry of the troposphere and stratosphere. NO and N2O are produced mostly by microbial soil activity, whereas biomass burning is also an important source of NO. Nitric oxide is a species involved in the photochemical production of ozone in the troposphere, is involved in the chemical produaion of nitric acid, and is an important component of acid precipitation. Nitrous oxide plays a key role in stratospheric ozone depletion and is an important greenhouse gas, with a global warming potential more than 200 times that of CO2. 

The hydroxyl radical so produced is the major oxidising species in the troposphere, and a complete picture of its chemistry holds the key to furthering progress in understanding tropospheric chemistry. The chemistry discussed in detail elsewhere, is of course very complex. To take, for example, the cycle of reactions with carbon monoxide, which may be net producers or destroyers of tropospheric ozone depending upon the concentration of oxides of nitrogen present. In the presence of NO, the cycle (16)-(20) occurs, without loss of OH or NO, whereas at low NO concentrations, the cycle (17), (18) and (21), again without loss of OH. 

Logan J.A., Nitrogen oxides in the troposphere Global and regional budgets. Advances in Chemistry, in press (1983). 

A chemical mechanism is the set of chemical reactions and associated rate constants that describes the conversion of emitted species into products. From the point of view of tropospheric chemistry, the starting compounds are generally the oxides of nitrogen and sulfur and organic compounds, and ozone is a product species of major interest. Chemical mechanisms are a component of atmospheric models that simulate emissions, transport, dispersion, chemical reactions, and removal processes (Seinfeld, 1986, 1988). 

Heterogeneous catalysis has a role in the atmospheric chemistry of ozone in the troposphere as well. Catalytic converters in automobiles are filled with a porous ceramic material, which provides a surface that catalyzes the removal of CO and NO (nitrogen oxides) ftom the exhaust. (Nitrogen oxides initiate the formation of ozone and other lung irritants in photochemical smog. We will examine this process in detail in Section 11.8.) The process by which catalytic converters operate is shown in Figure 11.14. The types of steps shown there are found in most examples of heterogeneous catalysis. 

FIGURE 7 Compounds participating in the tropospheric nitrogen oxides cycle and their interrelations. [From Warneck, R (1999). Chemistry of the Natural Atmosphere, Academic Press, San Diego.]... 

Although we wish to preserve the ozone in the stratosphere, we want to minimize its production in the troposphere, that part of the atmosphere where we live. Photochemical smog produced as a result of the action of solar radiation on the effluents from automobiles is the primary source of ozone in the troposphere. The trigger for photochemical smog is based on nitrogen oxide chemistry, so we defer our discussion of this problem until Chapter 16 on Group 5A chemistry. 

As the influence of human activities to the atmosphere is enhanced, the atmospheric concentrations of anthropogenic species such as nitrogen oxides (NOx = NO +NO2), volatile organic compounds (VOCs), etc. increase over a certain level. Tropospheric chemistry then exceeds the range of perturbation to the natural atmosphere, a chemical reaction system characteristic to the polluted atmosphere, sometimes called smog reactions, is brought about. Oxidation reaction mechanisms for the NOx-VOC mixtures directly related to the OH chain reaction are described in this section. 

This type of approach was used in the study of mechanisms describing tropospheric chemistry by Tomlin et al. (2001). A simple mechanism describing CO oxidation and the interaction between ozone and nitrogen species is first used as an illustrative example. Figure 6.2 illustrates the relationship between timescale modes and species for this simple system as determined by the left eigenvectors. In the figure, mode 1 is the fastest mode (X —8 x 10 ) and can be seen to be... 

The family method was applied within an atmospheric chemistry box model to NOy, HOx, Cly, Ox and Bry families in order to study the effect of increases in ground level bromine emissions on stratospheric ozone by Ramarosmi et al. (1992), and for simulations of lower stratospheric HCl in Douglass and Kawa (1999). The nonlinear features of tropospheric ozone production from nitrogen oxides and VOCs were reproduced using a numerical method based on family methods in Elliott et al. (1996). 

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