Nitrogen species atmospheric 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). 

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... 

The nitrogen species enter the atmosphere from a variety of natural and anthropogenic sources (7). The largest sources are concentrated in urban and industrialized areas. The levels of the species in the atmosphere vary from hundreds of parts per billion by volume (ppbv, that is, 10 9 mole fraction) in these source regions to below one part per trillion by volume (pptrv, 10"12 mole fraction) in remote areas. Even at the pptrv level, these species can play significant roles in atmospheric chemistry, and measurements of species at the sub-pptrv level can yield useful information concerning atmospheric photochemistry. 

The primary sources that are responsible for the presence of this family of compounds in the atmosphere emit NH3, N20, and NO to the troposphere, the lowest level of the atmosphere, which extends to approximately 10 km from the earth s surface. NH3 seems to undergo very little chemistry in the atmosphere except for the formation of aerosols, including ammonium nitrate and sulfates. NH3 and the aerosols are highly soluble and are thus rapidly removed by precipitation and deposition to surfaces. N20 is unreactive in the troposphere. On a time scale of decades it is transported to the stratosphere, the next higher atmospheric layer, which extends to about 50 km. Here N20 either is photodissociated or reacts with excited oxygen atoms, O (lD). The final products from these processes are primarily unreactive N2 and 02, but about 10% NO is also produced. The product NO is the principal source of reactive oxidized nitrogen species in the stratosphere. 

Studies of nitrogen oxide radicals in various condensed media by means of the EPR technique started about 45 years ago. Initial results were collected in [88, 28]. NxOy radicals are of interest first of all because of their toxicity and a key role in atmospheric chemistry. From this point of view, formation, stability and reactivity of these species adsorbed on the surface of nanosized metal-oxide semiconductor particles, which are photoactive and widely presented in atmosphere, are of essential importance. Principal values of g- and A-tensors for some cases are picked up in the following Table 8.4. 

Lee, Y.-N. Atmospheric aqueous-phase reactions of nitrogen species Kinetics of some aqueous-phase reactions of peroxy-acetyl nitrate. Conference on Gas-Liquid Chemistry of Natural Waters Brookhaven National Laboratory, 1984 BNL 51757,... 

In this chapter we will examine the atmospheric degradation mechanisms of the following important classes of anthropogenic molecules alkanes, alkenes, aromatics, nitrogen oxides, S()2, CFCs and Halons, and finally HFCs and HCFCs. Our intent is not to give an exhaustive account of the photochemical oxidation of every man-made chemical species but rather to present examples of the degradation mechanisms of a few representative members of each class of pollutant. First, we need to consider the general features of atmospheric chemistry. 

The atmospheric chemistry of nitrogen can be divided into four groupings of nitrogen species... 

The N( D) + H2 reaction has been less studied than 0( D) + H2. However atomic nitrogen is of fundainental int( r( st in combustion and astrophysical and atmospheric chemistry. For instance, reactions involving this species with simple hydrocarbons play a role in the atmosphere of Saturn s moon Titan. The N( D) - -H2 reaction is perhaj)s a better prototype of an insertion reaction than 0( D)- -H2. Here, there is no abstraction mechanism due to an excited PES [34]. 

Nitrogen oxide, formed by photofragmentation of N02 or chlorine nitrate (C10N02), hydroxyl radicals and some other reactive species are also responsible for stratospheric ozone depletion. These compounds may have both natural and anthropogenic (combustion, etc.) origin. Atmospheric chemistry also takes place in aerosol particles, cloud droplets1370 and ice crystals.1371,1372... 

Atmospheric measurements are also challenging because they must deal with low to extremely low concentrations of trace chemical species. The major components (>99.999%) of the lowest portions of the atmosphere (the troposphere up to 10 km in altitude and the stratosphere between 10 and 50 km) are molecular nitrogen, molecular oxygen, argon, water vapor, and carbon dioxide. Chemists will recognize that all of these species are very stable, strongly bonded molecules or atoms that are essentially inert gases at normal atmospheric temperatures (190-310 K). Indeed, without solar photons to break up selected molecules, atmospheric chemistry would be very dull indeed. 

Table II gives typical ozone and oxides of nitrogen levels in these four regions. Urban- and regional-scale atmospheric chemistry is characterized by the definitive influence of anthropogenic emissions. The goals of a study of urban- and regional-scale atmospheric chemistry are to understand the atmospheric transformations of emitted species to be able to predict the formation of ozone and other pollutants, and to predict the pathways of removal of emitted species and their transformation products from the atmosphere.

Nitrogen Oxides and Atmospheric Chemistry. Over the past few years it has become clear that nitrogen oxides play a significant role in the chemistry of our atmosphere. It therefore seems pertinent to include a brief summary of relevant research in this Report. Two features require careful examination the detection of the reactive species (often in very low concentrations) and the measurement of the rates at which they react. 

The atmospheric chemistry of sulfur is simpler than that of nitrogen in two aspects. First, the number of stable species in air is smaller and second, the variety of interactions in the climate system is less - almost all effects come from sulfate, such as acidity and radiation scattering. In the oxidation line to sulfate, oxidants are consumed ... 

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