Tropospheric definition

It is quite ambitious for a scientist to describe a natural phenomenon in terms of a specific reaction. The situation in the atmospheric environment is however more complicated as a variety of reactions are occurring simultaneously and a certain species may take part in different reactions affecting the relative equilibria. Most data are coming from laboratory work and experimental conditions are definitely different from the ones observed in the troposphere. As an example the mechanism of oxidation of sulphur dioxide, in gas phase is usually reported occur to a large extent through free radicals. If the presence of humidity and of particulated matter is considered, specifically in the lower part of the troposphere, definitely also heterogeneous reactions play a very important role. I feel that experiments carried on in the atmosphere yield more consistent results to elucidate the chemistry of the atmospheric environment. 

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. 

A definition of clean air based upon NOj concentrations is more difficult to obtain, because of the ubiquitous natural source in lightning (56,154), and because NO may influence HO concentration even at the lowest tropospheric NO levels (a few ppt) that have been observed. An operational definition might require that the rate of the H02 self-reaction R33 be much faster than its reaction with NO, R13. At NO concentrations below about 1 ppb, [H02 ] is relatively independent of [NO ], while [HO ] may increase with [NO ]. Above this level, both [HO2 ] and [HO ] fall with increasing [NOJ. These [HOJ dependencies on [NO ] are shown in Figure 4 (58) and discussed further in the last section. 

Definitely yes. In the troposphere, to which I think you may be referring, N2O is relatively inert. It is in the stratosphere where reaction (18) plays such an important role releasing the nitrogen oxides which dominate the catalytic destruction of ozone. 

In this book air chemistry is defined as a branch of atmospheric science dealing with the atmospheric part of the biogeochemical cycle of different constituents. In other words this means that we will deal mainly with the atmospheric pathways of those components that are involved in the mass flow between the atmosphere and biosphere, as well as in chemical interactions between the air and the other media of our environment (soils, oceans etc.). It follows from this definition that, on the one hand, our discussion will be restricted to the troposphere and the stratosphere4 and, on the other hand, the photochemistry of the upper layers, the subject matter of the aeronomy (e.g. Nicolet, 1964), will be omitted. This separation of the (photo) chemistry of the lower (troposphere and stratosphere) and upper atmosphere makes it possible to give a more compact treatment of our problem, including the global anthropogenic effects due to the increase of air pollution. 

The photodissociation of N02 is the only definitely established process for the formation of ozone in the troposphere and must be held responsible also for the generation of ozone in photochemical smog. The reaction of alkylperoxy radicals with oxygen, ROO + 02— R0 + 03 has occasionally been invoked—for example, by Cadle and Allen (1970)—but according to... 

The OH radical is the most important species in the atmosphere. This is often referred to as the detergent of the troposphere because it reacts with many pollutants, often acting as the first step in their removal. Since OH (this is the neutral form of the hydroxyl ion OH , i. e. OH is an extremely weak acid) is from H2O, it returns to water by the abstraction of H from nearly all hydrocarbons RH this pathway is the definitive sink for OH. However, the organic radical R combines with O2 (reaction 5.4) and in many subsequent reactions produces organic oxygen radicals and recycles HO2 (Chapter 5.3.2.2) ... 

Peroxyacetylnitrate (PAN) is an important species for transport of NOy because of its relatively low reactivity, particularly in the cold regions of the troposphere. Work in LACTOZ provided the first definitive rate data for the formation from its precursor species and decomposition of this molecule ... 

It is worthwhile noting here that Reactions (7.20) and (7.21) are the only definitely established chemical mechanism for producing ozone in the troposphere the other source for tropospheric ozone is downward transport from the stratosphere. Together these two sources maintain a background concentration of ozone in the troposphere of about 0.03 to 0.05 parts per million of the air by volume (ppmv). Ozone is of critical importance in the chemistry of the troposphere because, not only is it a powerful oxidant itself, it is the primary source of the hydroxyl radical (OH), which is highly reactive and of paramount importance in tropospheric chemistry. Also, as can be seen from Eq. (7.23), the concentration of ozone in the air determines the ratio of [N02(g)l to [NO(g)]. Nitric oxide, NO(g), is also a very reactive gas and of great importance in atmospheric chemistry. ... 

---