Nitrogen dioxide lifetime

Reactions 2-1 through 2-3 show the most common chemical reactions that occur in the polluted atmosphere. The reason is that nitrogen dioxide is the strongest absorber of sunlight. At a latitude of 40 the typical turnover lifetime for nitrogen dioxide is about 1.4 min. This means that, every 1.4 min on the average, half the nitn en dioxide molecules are photodissociated (Reaction 2-1) and reformed (Reaction 2-3). No other molecule in smog is so active. 

Thus the lifetime of a constituent with a first order removal process is equal to the inverse of the first order rate constant for its removal. Taking an example from atmospheric chemistry, the major removal mechanism for many trace gases is reaction with hydroxyl radical, OH. Considering two substances with very different rate constants for this reaction, methane and nitrogen dioxide... 

The nitrate radical (NO3) is formed in the atmosphere primarily by the reaction of nitrogen dioxide (NO2) with ozone (O3). At the outset of this project the potential importance of the role of NO3 as an oxidant in the troposphere had just been recognised. In order to assess the latter accurate physico-chemical models, describing the behaviour of NO3 in the troposphere, are needed. These require a detailed understanding of the elementary photochemical or chemical reactions and physical processes such as deposition or transport, which determine the tropospheric lifetime of NO3. 

The same authors also observed an interesting difference in the behavior of the a- and (3-tetralone silyl enol ethers 3 and 4, providing a further indication for the presence of radical ions as reactive intermediates in this reaction. The a-tetralone silyl enol ether radical cation 36 reacted with nitrogen dioxide to form cation 37, whereas the (3-tetralone based radical cation 38 reacted much more slowly and gave a mixture of products (Scheme 8). Due to the mesomeric stabilization of the radical cation 38, its lifetime increased dramatically as observed by time resolved spectroscopy. This favors a cage escape of the radical cation and opens the possibility for further reactions. 

NO , is removed from the atmosphere by several mechanisms including via the formation of low-volatility organic nitrates in reaction (3b) which are incorporated into aerosol and then undergo wet and dry deposition. In addition, nitrogen dioxide reacts with OH radicals with a rate coefficient of 1.19 x 10 cm molecule" s in 1 atmosphere of air at 298 K (Atkinson et al., 2004) and gives HNO3 which is removed by wet and dry deposition. In the presence of [OH] = 10 molecule cm the lifetime of NO2 with respect to reaction with OH is approximately 1 day. NO , has a relatively short atmospheric lifetime and is not transported directly from polluted to remote areas. 

A fiber-optic oxygen sensor with the fluorescence decay time (rather than its intensity) as the information carrier has been described by two groups [119, 120]. In the former work, a ruthenium complex is immobilized in silicone-rubber, and quenching by oxygen is measured by either lifetime or intensity measurements. The 337-nm line of a nitrogen laser served as the excitation line, and the dye was dissolved in a silicone-rubber membrane placed in the fluorimeter. This sensing membrane is reported to be highly specific, and chlorine and sulfur dioxide were the only interferents. 

The nitrogen laser is pumped with a high-vollagc spark source that provides a momentary (1 to 5 ns) puLse of current through the gas. I he excitation creates a population inversion that decays very quickly by spontaneous emission because the lifetime of the excited stale is quite short relative to the lifetime of the lower level. The result is a short (a few nanoseconds) pulse of intense (up to I MW) radiaiion at. 337,1 nm. This output is used for exciting fluorescence in a variety of molecules and for pumping dye lasers. I he carbon dioxide gas laser is used to produce monochromatic Infrared radiation at 10.6 pm. 

The lifetime of these species in the atmosphere is relatively short and if they were distributed evenly their harmful effects would be minimal. Unfortunately these man-made effluents are usually concentrated in localized areas and their dispersion is limited by both meteorological and topographical factors. Furthermore, synergistic effects mean that the pollutants interact with each other in the presence of sunlight, carbon monoxide, nitrogen oxide(s), and unburned hydrocarbons lead to photochemical smog, while when sulfur dioxide concentrations become appreciable, sulfur oxide-based smog is formed. 

In his 1956 paper on radioactive fallout (7) Libby pointed out that neutrons released in the explosions of nuclear weapons react with nitrogen nuclei in the air to make carbon-14, which has a half-life of about 5600 years. In his discussion of bomb-test carbon-14 he said that Fortunately, this radioactivity is essentially safe because of its long lifetime and the enormous amount of diluting carbon dioxide in the atmosphere. He pointed out that 5.2 tons of neutrons would be needed to double the feeble natural radioactivity of living matter due to radiocarbon. Such an increase would have no significance from the standpoint of health. He mentioned that, for a given energy release, thermonuclear weapons produce more neutrons than fission weapons, and concluded that the essential point is that the atmosphere is difficult to activate and the activities produced are safe, ... 

Many photochemical reactions are carried out at low temperatures as low as 4 K to slow down the reaction rate for the study of the lifetimes of the reactive intermediates. The most useful matrix materials are solid argon, solid neon and solid nitrogen. The initial photoproduct is trapped within a rigid matrix that inhibits the decay of the reactive species in diffusion process. For example, 5-hydroxy-a, P, y, 5-unsaturated valerolactone 1 on photochemical decomposition gives cyclobutadiene 2 and carbon dioxide. The intermediate and the products of this reaction are characterized in low-temperature matrix isolation process. 

---