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Atmosphere
ОглавлениеThe atmosphere is the envelope of gases surrounding the earth, and it is subdivided into regions depending on the altitude. The constituents of the atmosphere are primarily nitrogen (N2, 78.08% v/v), oxygen (O2, 20.95% w/w), and water vapor (0 to 0.25% w/w), although the concentration of water vapor (H2O) is highly variable, especially near the surface, where volume fractions can be as high as 4% in the tropics. There are many minor constituents or trace gases such as (alphabetically rather than by abundance, which is variable) argon (Ar), carbon dioxide (CO2), helium (He), hydrogen (H2), krypton (Kr), methane (CH4). (neon, Ne), nitrous oxide (N2O), ozone (O3), and water vapor (H2O) (Table A-26).
Table A-26 Approximate Composition of the Atmosphere.
| Component, % v/v | Amount, % v/v |
|---|---|
| Major components: | |
| Nitrogen (N2) | 78.08 |
| Oxygen (O2) | 20.95 |
| Minor/trace components: | |
| Argon (Ar) | 0.93 |
| Water vapor (H2O) | 0.025* |
| Carbon dioxide (CO2) | 0.035 |
| Neon (Ne) | 0.018 |
| Helium (He) | 0.00052 |
| Methane (CH4) | 0.00014 |
| Krypton (Kr) | 0.00010 |
| Nitrous oxide (N2O) | 0.00005 |
| Hydrogen (H2) | 0.00005 |
| Ozone (O3) | 0.0000007 |
| *Variable; can be as high as 4% v/v in humid areas. |
In addition to the gaseous constituents, the atmosphere also contains suspended solid and liquid particles. Aerosols are particulate matter usually less than 1 micron in diameter (also called 1 µm which is equivalent to 1 m x 10-6 in diameter, i.e., one millionth of a meter or one thousandth of a millimeter, 0.001 mm, or approximately 0.000039 of an inch) that are created by gas-to-particle reactions and are lifted from the surface by the winds. A portion of these aerosols can become centers of condensation or deposition in the growth of water and ice clouds. Cloud droplets and ice crystals are made primarily of water with some trace amounts of particles and dissolved gases. Their diameters range up to 100 µm. Water or ice particles larger than approximately 100 microns begin to fall because of gravity and may result in precipitation at the surface.
Ozone is found in trace quantities throughout the atmosphere, the largest concentrations being location in a layer in the lower stratosphere between the altitudes of 9 and 18 mi (15 and 30 km). This ozone results from the dissociation by solar ultraviolet radiation of molecular oxygen in the upper atmosphere and nitrogen dioxide in the lower atmosphere. Ozone also plays an important role in the formation of photochemical smog and in the purging of trace species from the lower atmosphere.
The chemistry of ozone formation can be explained in relatively simple terms, although the reactions are believed to be much more complex. Thus, above approximately 19 mi (30 km), oxygen is dissociated during the daytime by energy (hv) from ultraviolet light:
The oxygen atoms produced then form ozone:
In this equation, M is an arbitrary molecule required to conserve energy and momentum in the reaction that produces ozone. Although present in only trace quantities (Table A-26), atmospheric ozone plays a critical role for the biosphere by absorbing the ultraviolet radiation with a wavelength from 240 to 320 nm (nm, 1 nm = 1 meter x 109, which would otherwise be transmitted to the surface of the Earth.
The atmospheric ozone should not be confused with the ozone layer which acts is a region of stratosphere of the Earth that absorbs most of the ultraviolet radiation from the Sun. The ozone layer is mainly found in the lower portion of the stratosphere, from approximately 9 to 22 mi above the Earth, although the thickness of the layer varies seasonally and geographically. This layer is so-named because it contains a high concentration of ozone (O3) in relation to other parts of the atmosphere, although still small in relation to other gases in the stratosphere. The ozone layer contains up to 10 parts per million of ozone, while the average ozone concentration in the atmosphere of the Earth as a whole is on the order of 0.3 parts per million.
The ultraviolet radiation is lethal to simple unicellular organisms (algae, bacteria, protozoa) and to the surface cells of higher plants and animals. It also damages the genetic material of cells (deoxyribonucleic acid, DNA) and is responsible for sunburn in human skin. In addition, the incidence of skin cancer has been statistically correlated with the observed surface intensities of the ultraviolet wavelengths from 290 to 320 nm, which are not totally absorbed by the ozone layer.
Atmospheric pressure decreases as an approximately exponential function of altitude, which largely determines the characteristics of the atmosphere. Thus:
In this equation, Ph is the pressure at any given height, Po is the pressure at zero altitude (sea level); m is the average gram molecular mass of air (28.97 g/mole in the troposphere); g is the acceleration of gravity (981 cm x sec-1 at sea level); h is the altitude (in cm or meters or kilometers), and R is the gas constant (8.314 x 107 erg x deg-1 x mole-1), and T is the absolute temperature. Furthermore:
At sea level where the pressure is 1 atm:
The characteristics of the atmosphere vary widely with altitude, time (season), location (latitude), and even solar activity. At a high altitude, normally reactive species, such as atomic oxygen, persist for long periods of time. At such altitudes, the pressure is low and the distance traveled by a reactive species before it collides with a potential reactant (the mean free path) is high.
An important effect noted as a result of the changes in the constituents of the atmosphere is the tendency for the temperature close to the surface of the Earth to rise, a phenomenon referred to as the greenhouse effect. This term is used to describe the rise in the temperature of the earth, analogous to the rise in temperature in a greenhouse when the energy from the sun is trapped and cannot escape from the enclosed space. Although the term greenhouse effect has generally been used for the role of the whole atmosphere (mainly water vapor and clouds) in keeping the surface of the earth warm, it has been increasingly associated (perhaps erroneously) with the contribution of carbon dioxide. However, there are various types of gases that are the result of industrial and domestic activities that can contribute to this effect and, thus, the continual rise in the surface temperature of the earth.
The arguments related to the magnitude of the greenhouse effect have ranged back and forth for some time with carbon dioxide as the main (if not the sole) cause without recognition of other causes, several of which are natural causes or events. There are those who believe that the Earth is doomed to a rise in temperature and serious harm to the human race is imminent. On the other hand, there are those observers who believe that there is no cause for concern related to the environment and humans can go on merrily as has been the case for centuries and polluting the atmosphere without any concerns related to the consequences of such actions. Whatever the correct side on which to base an argument, there is no doubt that the emissions, which can give rise to such an effect, must be limited. The continuous pollution of the atmosphere with the so-called greenhouse gases can be of no advantage to life on earth, even if the effects of these gases are not manifested in a temperature rise but in the form of aggravating pollutants to flora and fauna.
Both of these opposite opinions are of some concern because they may mask the reality of the situation. It is analogous to other situations that have arisen in the last several decades. For example, in the late 1960s and early 1970s, there were warnings related to an approaching ice age. In fact, they were those observers who would have us believe that, upon looking out of the window, an observer(s) would see glaciers approaching from the end of the street! In fact, it is entirely likely that several important analysts who then warned of global cooling and imminent glaciation of northern societies are now warning of global warming. The glaciers did not arrive and now, in a little more fifty years later, the frantic warnings are not related to a rise in global temperature! There are also advisories that the rise in temperature that is the basis of global warming is being accompanied (perceived or real) by the emergence (or reemergence) of a variety of infectious diseases. If, as has been suggested, the Earth has warmed 0.3 to 0.6°C (approximately l°F, or less) during this century, the perception is that a higher rise in the temperature of the earth may lead to a series of global catastrophes.
These types of contradictory reports and arguments add much confusion to an already difficult area of technology. It seems that every time a government appropriates money to study an issue, the heretofore unheard-of-experts spring into action. What is really needed is a careful study of the data, the generation of new data, and less enthusiasm for catching the headlines.
Chemicals can be emitted directly into the atmosphere or formed by chemical conversion through chemical reactions of precursors species. In these reactions, highly toxic chemicals can be converted into less toxic products, but the result of the reactions can also be products having a higher toxicity than the starting chemicals. In order to understand these reactions, it is also necessary to understand the chemical composition of the natural atmosphere, the way gases, liquids, and solids in the atmosphere interact with each other and with the surface of the Earth and associated biota, and how human activities may be changing the chemical and physical characteristics of the atmosphere.
There are a number of critical environmental issues associated with a changing atmosphere, including photochemical smog, global climate change, toxic air pollutants, acidic deposition, and stratospheric ozone depletion. Much of the anthropogenic (human) impact on the atmosphere is associated with the increasing use of fossil fuels as an energy source – for things such as heating, transportation, and electric power production. Photochemical smog/ tropospheric ozone is a serious environmental problem that has been associated with burning such fuels, and the result has been the formation and deposition of acid rain.
Acid rain is formed when sulfur oxides and nitrogen oxides react with water vapor and other chemicals in the presence of sunlight to form various acidic compounds in the atmosphere. The principal source of acid rain-causing pollutants, sulfur oxides and nitrogen oxides, is from fuel combustion – specifically from fuels that contain sulfur and nitrogen:
Two of the pollutants that are emitted are hydrocarbon derivatives (e.g., unburned fuel) and nitric oxide (NO). When these pollutants build up to sufficiently high levels, a chain reaction occurs from their interaction with sunlight in which the NO is converted to nitrogen dioxide (NO2) – a brown gas and at sufficiently high levels can contribute to urban haze. However, a more serious problem is that nitrogen dioxide (NO2) can absorb sunlight and break apart to produce oxygen atoms that combine with the oxygen in the air to produce ozone (O3), a powerful oxidizing agent, and a toxic gas.
In addition, as a result of a variety of human activities (e.g., agriculture, transportation, industrial processes) a large number of different toxic chemical pollutants are emitted into the atmosphere. Among the chemicals that may pose a human health risk are pesticides, polychlorobiphenyl derivatives (PCBs), polycyclic aromatic hydrocarbon derivatives (PAHs), dioxin derivatives, and volatile compounds (e.g., benzene, carbon tetrachloride).
Polychlorobiphenyl derivatives; n and m can by any number from 1 to 5.
Polychlorinated dibenzo-p-dioxin derivatives; n and m can by any number from 1 to 5.
Many of the more environmentally persistent compounds (such as the polychlorobiphenyl derivatives) have been measured in various floral and faunal species.
See also: Condensation.
