Absorption in the atmosphere

Rotational spectroscopy and microwave astronomy are the most accurate way to identify a molecule in space but there are two atmospheric windows for infrared astronomy in the region 1-5 im between the H2O and CO2 absorptions in the atmosphere and in the region 8-20 xrn. Identification of small molecules is possible by IR but this places some requirements on the resolution of the telescope and the spacing of rotational and vibrational levels within the molecule. The best IR telescopes, such as the UK Infrared Telescope on Mauna Kea in Hawaii (Figure 3.13), are dedicated to the 1-30 xm region of the spectrum and have a spatial resolution very close to the diffraction limit at these wavelengths. 

Most commercial spectrometers report absorbance, as defined in Eq. (Q), versus wavelength. This is very important to recognize, since as we will see later, calculations of the rate of light absorption in the atmosphere require the use of absorption coefficients to the base e rather than to the base 10. While the recent atmospheric chemistry literature reports absorption cross sections to the base e, most measurements of absorption coefficients reported in the general chemical literature are to the base 10. If these are to be used in calculating photolysis rates in the atmosphere, the factor of 2.303 must be taken into account. 

L. Trafton, Observational studies of collision-induced absorption in the atmospheres of the major planets. In J. Szudy, ed., Spectral Line Shapes 5, p. 755, Ossolineum, Warsaw, 1989. 

Infrared absorption in the atmosphere can have the same effect. Over the last century the concentration of carbon dioxide in the atmosphere has risen dramatically because of combustion. As a result, the atmosphere now absorbs more infrared radiation than it did in the past, and cooling into space is less efficient. A likely consequence is global warming, although a detailed calculation of the magnitude of the expected effect is far from simple. For example, while is it not difficult to estimate total CO2 emissions from combustion, most of these molecules end up in the ocean as carbonates or bicarbonates, and do not directly contribute to global warming. Nonetheless, there is broad consensus in the scientific community that carbon dioxide emissions will tend to increase the Earth s temperature over the next few decades, with environmental consequences which may be severe. 

We are now going to calculate the attenuation of direct (directional) solar radiation through scattering and absorption in the atmosphere. The atmosphere is assumed to be cloudless for details on the complicated effect of clouds, [5.34] is suggested. We will consider a bundle of rays that goes through an optically turbid, namely absorbent and scattering medium, Fig. 5.42. The reduction dLx of its spectral... 

Qo is the solar radiation at the top of the atmosphere and Tu = 0.95 is the transmissivity of the upper atmosphere. The albedo a is calculated from the Fresnel formula (Kondratyev, 1969). The quantities Tr and Ab are transmissivity and absorption in the atmospheric boundary layer. 

H.J. Liebe, Updated Model for MMW Propagation in Moist Air Data on Oxygen and Water Absorption in the Atmosphere, Radio Sci., 1985, 20, 1069-1089. 

Bauer, B. Duterage and M. Godon, Atmospheric Pressure Broadening of the 183 GHz Water Line, J. Quant Spec. Rad. Trans., 1992, 48, 629-643 ibid., 1985, 33, 167-175. H.J. Liebe, Updated Model for MMW Propagation in Moist Air Data on Oxygen and Water Absorption in the Atmosphere, Radio Sci. 1985, 20, 1069-1089. 

Between 0.3 and 0.7 pm (visible range) and 8-12 pm, with the exception of ozone bands, there is virtually no absorption in the atmosphere - therefore these ranges are called atmospheric windows, solar and terrestrial radiation can penetrate the atmosphere unopposed. Between 1 and 8 pm H2O (2.5-3.5 pm and 4.5-7.S pm) and CO2 (2.2-3.5 pm and 4-4.5 pm as well as 15-20 pm) absorb terrestrial radiation partially, and at > 15 pm nearly completely. 

Fig. 5.112 (a-b) Reabsorption spectrum of apatite (a) and water and oxygen absorption in the atmosphere (Kuze and Chance 1994)... 

Measurements of ozone (O3) concentrations in the atmosphere are of particular importance. Ozone absorbs strongly in the ultraviolet region and it is this absorption which protects us from a dangerously high dose of ultraviolet radiation from the sun. The vitally important ozone layer lies in the stratosphere and is typically about 10 km thick with a maximum concentration about 25 km above the surface of the earth. Extreme depletion of ozone in a localised part of the atmosphere creates what is known as an ozone hole. 

Fig. 9. Genesis of acid tain (13). From the oxidation of C, S, and N during the combustion of fossil fuels, there is a buildup in the atmosphere (gas phase, aerosol particles, raindrops, snowflakes, and fog) of CO2 and the oxides of S and N, which leads to acid—base interaction. The importance of absorption of gases into the various phases of gas, aerosol, and atmospheric water depends on a number of factors. The genesis of acid rain is shown on the upper right as an acid—base titration. The data given are representative of the environment in the vicinity of Zurich, Switzedand.

Toxicological studies conducted on DCPD indicate that it is a moderately toxic material and, to some extent, an irritant and a narcotic. By oral administration in the rat, the LD q is 0.82 g/kg of body weight, and by skin absorption in the rabbit, the LD q is 6.72 ml,/kg. An atmospheric concentration of 2000 ppm causes death in rats exposed for a period of 4 hours. 

Let s consider the influence of gases and particles on the optical properties of the atmosphere. Reduction in visibility is caused by the following interactions in the atmosphere light scattering by gaseous molecules and particles, and light absorption by gases and particles (2). 

The extinction coefficient is dependent on the presence of gases and molecules that scatter and absorb light in the atmosphere. The extinction coefficient may be considered as the sum of the air and pollutant scattering and absorption interactions, as shown in Eq. (10-4) ... 

The secondary source of fine particles in the atmosphere is gas-to-particle conversion processes, considered to be the more important source of particles contributing to atmospheric haze. In gas-to-particle conversion, gaseous molecules become transformed to liquid or solid particles. This phase transformation can occur by three processes absortion, nucleation, and condensation. Absorption is the process by which a gas goes into solution in a liquid phase. Absorption of a specific gas is dependent on the solubility of the gas in a particular liquid, e.g., SO2 in liquid H2O droplets. Nucleation and condensation are terms associated with aerosol dynamics. 

Toxic chemicals can enter the body in various ways, in particular by swallowing, inhalation and skin absorption. Skin absorption may lead to dermatitis and this can be a most annoying complaint. Whereas some chemicals may have an almost universal effect on human beings, others may attack only a few persons. A person who has worked with a given chemical for some years may suddenly become sensitised to it and from then on be unable to withstand the slightest trace of that material in the atmosphere. He may as a result also be sensitised not only to the specific chemical that caused the initial trouble but to a host of related products. Unfortunately a number of chemicals used in the plastics industry have a tendency to be dermatitic, including certain halogenated aromatic materials, formaldehyde and aliphatic amines. 

Applications The differential optical absorption spectrometer has been used to monitor concentrations of gases or intermediate compounds such as SO, NO, O5, HCHO, HNO, CS, NO, and OH in the atmosphere.In atmospheric measurements with open paths of 100 to 1000 m, a detection limit of about 1 ppb can be achieved. In the emission measurements, the path length across the duct or the plume can range in meters. 

Both factors depend on the respective partial vapor pressures of water and carbon dioxide and upon the distance to the radiation source. The partial vapor pressure of carbon dioxide in the atmosphere is fairly constant (30 Pa), but the partial vapor pressure of water varies with atmospheric relative humidity. Duiser (1989) published graphs plotting absorption factors (a) against the product of partial vapor pressure and distance to flame (Px) for flame temperatures ranging from 800 to 1800 K. 

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