Regions of the Atmosphere

Eigure 6.2 shows the stratification of the atmosphere and the influence of electromagnetic solar radiation in forming the layers of the atmosphere. Photochemical reactions in which energetic photons of UV solar radiation (represented as hv, where h is Planck s constant and v is the frequency of the radiation) may break chanical bonds in atmospheric air molecules to produce O atoms as well as ions. 

Above the troposphere is the stratosphere in which the average temperature increases from about -56°C at its lower boundary that ranges between 10 and 16 km in altitude to about -2°C at its upper limit at around 50 km altitude. The stratosphere is warmed by the energy of intense 

A critical part of the stratosphere is the UV-absorbing ozone that it contains. Often referred to as the ozone layer, stratospheric ozone is dispersed widely over several kilometers of altitude in the stratosphere. Stratospheric ozone actually constitutes only a minuscule fraction of the atmosphere s total mass, although it is absolutely essential in protecting humans and other organisms on Earth from deadly solar UV radiation. Stratospheric ozone and pollutant threats to it are discussed in more detail in Chapter 7, Section 7.9. 

As anyone who has traveled from sea level to the top of a mountain knows, the atmosphere becomes thinner with increasing elevation, which means that atmospheric pressure and density both decrease with increasing elevation. (The temperature also decreases with elevation.) 

For our purposes we can divide the atmosphere into three regions based on elevation. In order from Earth s surface to the highest elevation, these three regions are the troposphere, the stratosphere, and the ionosphere. 

The units used to measure atmospheric pressure are as follows  

On average, atmospheric pressure is 1 atm at sea level (hence the unit atmospheres), or 760 mm Hg. At 5,000 feet (1,500 meters), atmospheric pressure drops to 630 mm Hg, and at 10,000 feet (3,000 meters), atmospheric pressure drops to 520 mm Hg. At the top of Mt. Everest in Nepal, which is at an elevation of 29,035 feet (8,850 meters), the pressure is only 250 mm Hg. The summit of Mt. Everest is poking into the stratosphere. 

The air is much colder on top of a 14,000-foot (4,270-meter) peak in Colorado than it is at mile-high Denver, Colorado (1 mile is 5,280 feet, or 1,609 meters), which in turn is usually cooler than the Great Plains, which is much closer to sea level. 

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This rate law tells us that the reaction is slower in regions of the atmosphere where 02 molecules are abundant than where they are not so abundant. In Example 13.7 in Section 13.8, we shall use this rate law as a source of insight into how the reaction takes place. 

Three regions of the atmosphere are seen to have significant zonal components of flow and thus of advection. The mid-latitude troposphere at the surface tends to exhibit westerly flow (i.e., flow from west to east) on the average. This region contains the familiar high- and low-pressure systems that cause periodicity in mid-latitude weather. Depending on the lifetime of the substances of concern, the motion in these weather systems may be important. 

Why is the ozone layer confined to one region of the atmosphere The production of ozone requires both a source of oxygen atoms and frequent collisions between the atoms and the molecules that make up the atmosphere. At altitudes lower than 20 km, all the light energetic enough to split oxygen molecules into oxygen atoms has already... 

C07-0091. List the region of the atmosphere and the atmospheric gases that absorb light in each of the following spectral regions (a) less than 200 nm (b) 240 to 310 nm. 

C07-0093. A high-altitude balloon equipped with a transmitter and pressure sensor reports a pressure of 10 atm (see Figure 7-24L (a) What altitude has the balloon reached (b) In what region of the atmosphere is the balloon (c) What chemical processes take place in this region ... 

C07-0109. Refer to Figure 7-24 to answer the following questions (a) What is the pressure at an altitude of 60 km (b) What atomic and molecular species are present at that altitude (c) At what altitude is the pressure 8 torr (d) What region of the atmosphere is this ... 

G. Mitri and co-workers calculated the minimum area of hydrocarbon lakes which would be necessary to preserve the relative methane humidity in the lower regions of the atmosphere. The result was surprising the calculations indicated that only between 0.002 and 0.2% of the total surface area of Titan would be required (Mitri et al., 2007). 

The thin-film model is the simplest and, therefore, most commonly used approach to estimate air-sea fluxes of gases. In this model, molecular diffusion is assumed to present a barrier to gas exchange in each of two layers. As illustrated in Figure 6.5, one layer is composed of a shallow region of the atmosphere that lies in direct contact with the sea surface. The second is a shallow layer of seawater tliat lies at the sea surface. These layers have depths less than 100 (am and, hence, are referred to as thin films. 

Figure 1.1 shows the different regions of the atmosphere. (See also Appendix V for typical pressures and... 

The transition zones between the various regions of the atmosphere are known as the tropopause, stratopause, and mesopause, respectively. Their locations, of course, are not fixed, but vary with latitude, season, and year. Thus Fig. 1.1 represents an average profile for mid-latitudes. Specific temperatures, pressures, densities, winds, and the concentrations of some atmospheric constituents as a function of altitude, geographic position, and time are incorporated into a NASA model, the Global. Reference Atmosphere Model (GRAM) information on obtaining this model and data is included in Appendix IV. 

The stratosphere is often referred to as the ozone layer, because of the relatively high concentrations produced by photochemical reactions in this region of the atmosphere. Ozone, derived from the Greek word meaning to smell, was first discovered by Schonbein in 1839. It has a rather pungent smell, which is sometimes noticeable around copy machines and laser printers that use discharge processes. 

The implications of reaction (47) in the stratosphere are discussed in detail in Chapter 12. However, suffice it to say that it also plays a critical role in that region of the atmosphere as well. 

Although the focus of this chapter is tropospheric HO measurements, it is worthwhile to mention techniques that have proven useful in the laboratory or in other regions of the atmosphere. As a small molecule in the gas phase, HO has a much-studied and well-understood discrete absorption spectrum in the near UV (29), shown in Figure 1, that lends itself to a variety of absorption and fluorescence techniques. The total atmospheric HO column density has been measured (30-32) from absorption of solar UV radiation, observed with a high-resolution scanning Fabry-Perot spectrometer. Long-path measurements of stratospheric HO from its thermal emission spectra in the far infrared have been reported (33-35). Long absorption paths in the atmospheric boundary layer have been used for HO detection from its UV absorption (36-42). 

Notice how the temperature of the atmosphere changes with altitude. Close to the surface of the Earth, the temperature is about 20°C. The temperature falls to about — 55°C at 15 km and then rises again at higher altitudes. One reason that the stratosphere is warmer than lower regions of the atmosphere is the fact that solar radiation causes different chemical reactions at different altitudes. For example, these reactions result in a relatively stable concentration of ozone we call the ozone layer in the stratosphere. The reactions that produce ozone also release energy and, as a result, the temperature rises with altitude. 

The region of the atmosphere above the stratopause in which Tg continuously reduces towards a second minima, the mesopause, is termed the mesosphere. The mesopause occurs at an altitude of about 80 km and is the lowest temperature point in the atmosphere, values of Tg approaching 100 K having been recorded, although temperatures nearer to 200 K are more typical. Significantly, it is the relatively high pressure, low temperature mesosphere in which cluster ions were first detected in the atmosphere (Sect. 2.2). 

As previously mentioned, ternary (or 3-body) ion-molecule reactions are only significant in the Earth s atmosphere below the mesopause ( 80 km) where they play a crucial part in the ion chemistry. In this region of the atmosphere, the major problem to be solved is to determine the ionic reaction paths which convert the primary positive and negative ions to the dominant positively and negatively charged water cluster ions [see reactions (6) and (11)]. Most progress has been made in elucidating the positive ion chemistry, so this will be considered first. 

The current understanding of the ion chemistry of the atmosphere has been achieved by co-ordinating the data obtained from in-situ ion composition measurements with the data obtained from appropriate laboratory experiments. This review has largely been concerned with the elementary ionic reaction processes involved in the overall chemistry and detailed chemical models of the ion chemistry of the atmosphere have been deliberately excluded since such have recently appeared in the literature8,73,74,, 47. However, it is appropriate here to summarise, through block diagrams, the chains of ionic reactions via which ions are formed, evolve and are finally lost from the atmosphere. To this end, it is convenient to consider separately three regions of the atmosphere ... 

Loss of ions occurs via the processes of mutual neutralization, ternary ionic recombination and attachment to aerosol surfaces, processes which urgently need further study in the laboratory. It is an interesting fact that the ion chemistry directly accelerates the loss of ionization from all regions of the atmospheric plasma. Atomic ions are converted into molecular ions, molecular ions into larger cluster ions which recombine more rapidly. The larger ions also act as nucleation sites for the formation of aerosols, thus involving a transition from the molecular to the liquid state. 

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