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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.

      Atmospheric Equivalent Boiling Point

      The atmospheric equivalent boiling point is the boiling point at which a sample or fraction of a fuel would boil under atmospheric pressure if it was stable and would not decompose. This provides a common basis for the categorization and direct comparison of petroleum components across the entire volatility range accessible by atmospheric and vacuum distillation.

      Distillation at lower pressure allows high-boiling fractions to distill at lower temperatures, thereby foregoing the potential of thermal decomposition if attempts were made to distill the fraction at the higher temperatures required at atmospheric pressure. This allows the collection of distillates with an atmospheric equivalent temperature cut point of as high as 560°C (1050°F). The actual observed boiling points during this distillation are, of course, much lower. Despite the low pressure, the reboiler may have to be heated as high as 370°C (700°F) for such high-boiling distillates.

      Molecular distillation is a non-equilibrium process and the atmospheric equivalent boiling range of a fuel or a fraction of a fuel that cannot be measured directly. The atmospheric equivalent boiling point (AEBP) concept was developed to compensate for this and can be estimated from one of the following equations:

      The mid-AEBP is the temperature for the 50% mass point on the distillation curve of the fraction, and M_n is the molecular weight. Either the specific gravity (sp. gr.) or atomic hydrogen/carbon (H/C) ratio, which can be readily measured in the fractions, can also be used. Using the mid-AEBP concept, even fractions of the non-distillable residue can be included in the boiling range curves and relationships such as the variation of sulfur and nitrogen

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