Chemical lifetime

Indirect Measurements of Tropospheric HO. There are two principal means of making indirect measurements of [HO ] or [H02 ] in the troposphere - either by the measurement of the chemical lifetimes of compounds that react uniquely with HO or H02 or by chemical conversion of HO or H02 to more readily measurable species. 

The probabilities, or molar fractions, are equal to the fraction of time a nucleus will spend in a particular environment when observed for a suitably long period of time. They do not reveal how long the nucleus will stay, on average, in each particular position in other words, the average lifetime of the nucleus in each environment cannot be calculated from the molar fractions. As will be seen, the chemical lifetime or its reciprocal, the chemical dissociation rate, are crucial parameters governing the influence of chemical exchange on NMR parameters. 

The vertical profile of DMS in marine air was first determined by Ferek et al (12), over the tropical Atlantic ocean. They found that under stable meteorological conditions, the mixing depth of DMS was about 1 km, with a rapid decline in concentration above this altitude. This distribution was considered consistent with the chemical lifetime of a few days predicted by... 

Making some assumptions on the chemical filiation between some organo-sulfur compounds, it was possible to establish the mathematical variation law for the concentration ratio of the various detected species and consequently to deduce the depletion rate constant of these compounds. From the measurements at the "Pointe de Penmarc h" in September 1983, the DMS lifetime estimations obtained are reported in Table I. This method for determining chemical lifetimes can only be applied for local and intensive sources. The most critical point concerns the chemical relation between the various sulfur compounds which should be verified in order to validate these estimations. However, the other assumptions do not seem to have a significant influence on the lifetime estimation within an order of magnitude. 

The vibrationally excited precursor AB/s/(fs) can decay not only via energy transfer to the bulk but also via a chemical transformation (desorption of B and reaction with the formation of D and C/s/). These chemical processes can be characterized by the chemical lifetime Tch, which can be estimated in the framework of the statistical RRKM theory (see, e.g., Refs. [50, 51]) using the reaction parameters of reagents B and A/s/, precursor AB/s/, and transition complexes determined based on the results of quantum-chemical calculations. Such estimates were performed for many reactions of interest for the growth of metal oxide films [20]. It appeared that in the wide temperature range... 

Air estimated tropospheric chemical lifetimes, x = 5 h, 3 d and > 150 d for reactions with OH, N03 and 03, respectively, under typical remote tropospheric conditions (Falbe-Hansen et al. 2000)... 

The type of calculations are useful as they give an indication of the likely chemical lifetime (i.e. the amount of time it will take before a molecule is reacted away) of a molecule in the atmosphere. Clearly, this form of calculation does not take into account any other chemical loss routes other than reaction with OH or any other physical process that may remove a molecule. 

The species of interest are separated into two categories, according to their chemical lifetimes those exceeding one day, and those less than one hour. The sole exception, H2C=0, does not have a rapidly varying number density and can be included in the latter group. 

The sulfur chemistry has not been included in the model, since it does not appear to have a significant effect on the radical chemistry. If, however, the preliminary estimates of K6a are correct, the oxidation of S02 by OH is an important loss path for both OH and S02 and the sulfur chemistry will then be coupled to the radical chemistry. It will not be the dominant radical loss path but it would be an important one. From what little is known about the gas-phase chemistry of sulfur, chemical lifetimes for H2S and S02 will be estimated from n(OH) and n(H02) profiles calculated separately. 

A second study by McKeen, Liu, and Kiang [7] supports the conclusion that SO2 oxidation does not consume HO. They modelled the oxidation of SO2 in the stratosphere from the 1982 eruption of the El Chichon volcano. Using one and two dimensional models, they examined the photochemical effects on the injection of several megatons of sulfur into the stratosphere and compared the results with the chemical lifetimes of sulfur obtained from observations of stratospheric SO2 and sulfate particles. The SO2 to sulfate conversion scheme they tested assumed that odd hydrogen radicals were consumed (as in Reaction (5)). Under this condition their model predicted that the HO species were significantly suppressed and that the chemical lifetime of SO2 was greater than 100 days. This can be compared with the observations which imply a lifetime... 

To some extent, the ect now under consideration may be considered as a generalization of the Kirkwood-Shumaker mechanism discussed before. A suflBdently small chemical lifetime of protonated states is ensured here by using high concentrations of the proton-carrying agents. 

A convenient measure of the stability of a trace chemical is its local chemical lifetime, which is defined as its local concentration (e.g., in mol L ) divided by its local removal rate (e.g., in mol s ). Concentrations of chemicals whose lifetimes (due to chemical destruction or deposition) are comparable to, or longer than, the above transport times will be profoundly affected by transport. As a simple example, in a steady (constant concentration) state with a horizontal... 

In the above example, if the chemical lifetime ti is much less than the transport time xlu then we could ignore transport and simply consider chemical sources and sinks to determine concentrations. However, for the chemicals controlling oxidation reactions, care must be taken in defining f,. Specifically, sometimes members of a chemical family are converted from one to another on very short timescales relative to transport, whereas the total population of the family is produced and removed on longer timescales. 

The temporal trend of the C/C ratio in soil humus (from prenuclear time to the present) makes it possible to separate the contribution of passive compounds that have very long soil residence times (measured in hundreds to thousands of years) and hence will be little changed by man s activities, and those active compounds which have relatively short chemical lifetimes (measured in decades). It also allows the characterization of the average turnover time of these active components. Further, by conducting such measurements on soils from different climate zones, it is possible to get a handle on how the turnover time of active compounds depends on temperature. Clearly, knowledge of this dependence is critical to the prediction of future global humus inventories. 

The cell is also admirably suited to free radical studies in the liquid phase at low temperatures. Flash photolysis or steady-state photolysis may be used, depending upon the chemical lifetime of the free radical. Pulsed or steady-state x-ray excitation may prove advantageous here, since penetration of the radiation into the solution is required. In these heavy-atom solvents much of the energy can be absorbed by the solvent and gently transferred to the solute. Experiments using liquid neon, hydrogen, or even liquid helimn as solvents may be possible when pulsed electron beams are used. Solutes in these cases may be atomic hydrogen, excited states of helium atoms or molecules, and possibly ionic species. 

The time required for the concentration of A to decrease to f/e of its initial value, if this were the only reaction process occurring, is therefore equal to l/ku. This defines the chemical lifetime (or e-folding lifetime) of species A. The concept of lifetime is an important one in atmospheric applications. If, for example, we wish to know whether an atmospheric species is likely to be directly affected by transport processes, we can compare its chemical lifetime to the time scale appropriate to transport. If chemistry is much faster than transport for that particular species, then the direct effects of transport can be neglected to a first approximation. This will be discussed in more detail in Chapters 3 and 5. 

Second and third order reactions (A + B — products and A + B + C —> products are commonly reduced to a pseudo first order form to examine the chemical lifetime. For example,... 

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