Radiatively active gas

Chemical vapor deposition (CVD) is a process whereby a thin solid film is synthesized from the gaseous phase by a chemical reaction. It is this reactive process that distinguishes CVD from physical deposition processes, such as evaporation, sputtering, and sublimation.8 This process is well known and is used to generate inorganic thin films of high purity and quality as well as form polyimides by a step-polymerization process.9-11 Vapor deposition polymerization (VDP) is the method in which the chemical reaction in question is the polymerization of a reactive species generated in the gas phase by thermal (or radiative) activation. 

Nitrous oxide (N2O) is a long-lived (120 yr) trace component of the atmosphere (Prinn et al., 1990). It is a climate-active gas as it has a radiative forcing 300 times that of CO2, although N2O presently contributes only 5% to the total greenhouse effect (Schimel, 1996). N2O also acts as a source of nitric oxide in the stratosphere and therefore participates in the catalytic removal of ozone (Crutzen, 1970). It is produced as a reaction intermediate in both microbial denitrification and nitrification processes and at greater rates under conditions of low O2 (Law and Owens, 1990) (see Chapter 6.11 by Emerson and Hedges for more details). 

The previous discussion is based on the assumption that collisional excitation and de-excitation is the dominant process determining the populations of the vibrational-rotational levels of all radiatively active molecules. This is usually the case in the lower atmosphere, where the pressure is high and molecular collisions are frequent. The relative populations of the upper and lower states of a vibrational-rotational transition are then described by the Boltzmann distribution at the local kinetic temperature T, the gas is considered to be in local thermodynamic equilibrium (LTE), and Kirchoff s law can be applied locally. If we consider a two-level system, then the relative populations of the lower and upper states, no and ni, respectively, are given by... 

Although the basic principles of the retrieval of vertical composition profiles from infrared measurements by inversion of the radiative transfer equation are the same as the retrieval of temperature profiles discussed in Section 8.2, the composition problem is usually more difficult to deal with in practice. The optical depth at a given level in the atmosphere is determined by an integration over the optically active gas profile from that level to the effective top of the atmosphere. Calculation of the radiance at the top of the atmosphere then requires an integration of the source function over all optical depths from the lower boundary to the top of the atmosphere. Thus the desired abundance profile is embedded within a double integration. 

The lifetime of triplet acetone at 25° in the vapor phase, as measured from the rate of decay of phosphorescence, is 0.0002 sec,318 so that the rate of decay is 5 x 103 sec-1. This figure represents the sum of the rates of all decay processes. Since the data at 40° 308 indicate that decomposition and internal conversion of triplet acetone occur approximately 40 times as fast as emission, the radiative lifetime must be on the order of 0.01 sec. Measurements of the rate of phosphorescence decay from solid acetone at 77°K, where all activated fragmentation and most radiationless decay normally disappear, have actually yielded values approximately one-tenth as large as that obtained in the gas phase at room temperature.319 The most recent measurements of the lifetime of triplet acetone at 77°K in frozen glasses does indeed yield an estimate of 0.01 sec for the radiative lifetime of triplet acetone.318... 

The possibility of deactivation of vibrationally excited molecules by spontaneous radiation is always present for infrared-active vibrational modes, but this is usually much slower than collisional deactivation and plays no significant role (this is obviously not the case for infrared gas lasers). CO is a particular exception in possessing an infrared-active vibration of high frequency (2144 cm-1). The probability of spontaneous emission depends on the cube of the frequency, so that the radiative life decreases as the third power of the frequency, and is, of course, independent of both pressure and temperature the collisional life, in contrast, increases exponentially with the frequency. Reference to the vibrational relaxation times given in Table 2, where CO has the highest vibrational frequency and shortest radiative lifetime of the polar molecules listed, shows that most vibrational relaxation times are much shorter than the 3 x 104 /isec radiative lifetime of CO. For CO itself radiative deactivation only becomes important at lower temperatures, where collisional deactivation is very slow indeed, and the specific heat contribution of vibrational energy is infinitesimal. Radiative processes do play an important role in reactions in the upper atmosphere, where collision rates are extremely slow. 

The observed trends in the NAM towards higher indices may have resulted from human-induced changes in the temperature structure of the lower stratosphere in response to greenhouse gas emissions and ozone depletion (Shindell et al., 2001). The radiative effects of solar activity and of volcanic eruptions may also have produced NAM- like signatures detectable at the Earth s surface. The response of the Earth system to climate forcing may therefore involve changes in particular dynamical modes, and hence the human influence on climate at the Earth surface may occur in part by way of the stratosphere. 

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