Atmospheric chemistry, “normal

The Unit Sheet contains a number of conversions necessary to accommodate the variety of units used in experimental atmospheric chemistry. The Variable Sheet is arranged so that die variables at the top are the ones normally chosen as input variaWes. Since the usual goal of running the model is to determine the pH of the raindrop, the variable pH is chosen as the one on which to iterate. 

Singlet molecular oxygen is of interest in connection with atmospheric chemistry with respect both to its mode of excitation and to the consequences of its presence in the upper or lower atmosphere. The first part of this section deals with processes of importance in normal, unpolluted atmospheres, while the second part examines the possibility, only recently appreciated, that singlet molecular oxygen may play a part in the chemistry of polluted urban atmospheres. 

Other common ways of expressing abundances, particularly of solid or liquid particles, is to express them as concentrations in units of micrograms per cubic meter or nanomoles per cubic meter. For purposes of consistency, concentrations expressed in these units should be normalized to standard conditions of temperature and pressure. Because there is some confusion as to what constitutes standard conditions in atmospheric chemistry (273 K and 1.013 bar are commonly used in chemistry and physics and 293 K and 1.013 bar are used in engineering), it is important to define the standard conditions that are assumed when reporting data. This explicit definition is frequently not done. Concentrations expressed in these units can be easily converted to mixing ratios by use of the ideal gas law ... 

The tropospheric sulfur chemistry is different. Unlike the nitrogen and carbon chemistry, where combustion is an insignificant source, the combustion source of SO2 appears to be very important. While OH reactions can be shown to convert sulfides to SO2, it is not clear that normal atmospheric chemistry is important in the next step—the conversion of S02 to H2SO, which is then removed from the atmosphere by rainout. It has also been suggested that a large amount of SO2 is removed directly by rainout. Unfortunately we have the fewest data, both kinetic and atmospheric, on sulfur compounds. Most of the kinetic data we do have are at high temperatures, and most of the atmospheric data are for polluted environments. 

Atmospheric measurements are also challenging because they must deal with low to extremely low concentrations of trace chemical species. The major components (>99.999%) of the lowest portions of the atmosphere (the troposphere up to 10 km in altitude and the stratosphere between 10 and 50 km) are molecular nitrogen, molecular oxygen, argon, water vapor, and carbon dioxide. Chemists will recognize that all of these species are very stable, strongly bonded molecules or atoms that are essentially inert gases at normal atmospheric temperatures (190-310 K). Indeed, without solar photons to break up selected molecules, atmospheric chemistry would be very dull indeed. 

The consequences of local peculiarities for global atmospheric chemistry are still uncertain but if the levels of hydroxyl radical would be decreased, many pollutants that are normally oxidized by OH radical could build up, leading to significant relevant... 

A growing body of data on Antarctic ice cores shows that the history of volcanic eruptions is resolvable year-by-year (38). The record includes dated explosive events such as Cerro Hudson in 1991, Pinatubo-1991, Agung-1963, Krakatoa-1883, Tambora-1815, and a 1259 AD eruption of unknown location. Analysis of the oxygen and sulfur isotope composition of aerosol particles frozen in the polar refrigerator provides a proxy of atmospheric chemistry applicable to early Earth. Aerosols from Cerro Hudson, the smallest of the explosive eruptions, have normal sulfur isotope compositions with no anomalous fractionation of (39). Cerro Hudson s eruption cloud did not break... 

Table 5.1 classifies how chemical regimes meet in the climate system. We see that almost normal conditions occur and extreme low and high temperatures border the climate system. The chemistry described in the following chapters concerns almost these normal conditions of the climate system. We focus on the troposphere and the interfaces. For example, aqueous phase chemistry in cloud droplets does not differ principally from surface water chemistry (aquatic chemistry) and much soil chemistry does not differ from aerosol chemistry (colloidal chemistry). Plant chemistry, however, is different and only by using the generic terms (Chapter 2.2.2.S) of inorganic interfacial chemistry can we link it. The chemistry of the atmosphere is widely described (Seinfeld and Pandis 1998, Wameck 1999, Finlay-son-Pitts and Pitts 2000, Wayne 2000, Brasseur et al. 2003) and the branches in atmospheric chemistry are well defined (Fig. 5.2). 

The order of the compounds determined by the magnitude of the mass emissions is not intended to reflect the relative influence of these compounds on atmospheric chemistry, greenhouse effect, or other atmospheric phenomena. To illustrate this point, compare in table I-C-5 the orders of importance of the top 31 species in mass emissions in table I-C-2 as estimated from a consideration of criteria other than mass emissions. We list the normalized relative values for molecular emissions (proportional to mass emissions/molecular weight), relative rate of attack by OH radicals (proportional to molecular emissions x oh), the relative rate of ozone generation as estimated for a polluted urban atmosphere (proportional to mass emissions x MIR, the maximum incremental reactivity factor Carter, 1998), and the relative number of CO2 molecules that atmospheric oxidation of this species will ultimately generate (proportional to molecular emissions x number of C-atoms molecule" ). The latter comparison... 

The ozone hole would almost certainly be much worse if chemists had not studied the reactions of CFCs with atmospheric gases before ozone depletion was discovered. The 1995 Nobel Prize in chemistry was awarded to the three pioneers in this effort. A German chemist, Paul Crutzen, discovered how ozone concentration is regulated in a normal stratosphere, while two Americans, F. Sherwood Rowland and Mario Molina, showed that CFCs can destroy ozone. These studies of molecular reactions allowed quick determination that CFCs are a likely cause of ozone depletion and led to the international restrictions described above. 

Whether the prediction scheme is a simple chart, a formula, or a complex numerical procedure, there are three basic elements that must be considered meteorology, source emissions, and atmospheric chemical interactions. Despite the diversity of methodologies available for relating emissions to ambient air quality, there are two basic types of models. Those based on a fundamental description of the physics and chemistry occurring in the atmosphere are classified as a priori approaches. Such methods normally incorporate a mathematical treatment of the meteorological and chemical processes and, in addition, utilize information about the distribution of source emissions. Another class of methods involves the use of a posteriori models in which empirical relationships are deduced from laboratory or atmospheric measurements. These models are usually quite simple and typically bear a close relationship to the actual data upon which they are based. The latter feature is a basic weakness. Because the models do not explicitly quantify the causal phenomena, they cannot be reliably extrapolated beyond the bounds of the data from which they were derived. As a result, a posteriori models are not ideally suited to the task of predicting the impacts of substantial changes in emissions. 

The tungsten-halogen lamp operates in a very specific mode, dependent on the detailed chemistry of the atmosphere within the lamp enclosure. The lamp envelope is normally a thin quartz bulb, filled with an inert... 

The mercury barometer (Fig. 10-4) indicates directly the absolute pressure of the atmosphere in terms of height of the mercury column. Normal (standard) barometric pressure is 101.325 kPa by definition. Equivalents of this pressure in other units are 760 mm mercury (at 0°C), 29.921 inHg (at 0°C), 14.696 Ibf/in, and 1 atm. For cases in which barometer readings, when expressed by the height of a mercury column, must be corrected to standard temperature (usually 0°C), appropriate temperature correction factors are given in ASME PTC, op. cit., pp. 23-26 and Weast, Handbook of Chemistry and Physics, 62d ed.. Chemical Rubber, Cleveland, 1984, pp. E36-E37. 

The hydroxyl radical is normally present only in low concentrations in the troposphere, as it reacts with further ozone to form the hydroperoxy radical HOO- which in turn gives hydrogen peroxide H202. Ozone, the hydroxyl radical, and hydrogen peroxide are the main oxidizing species in the troposphere, from the standpoint of environmental chemistry. The hydroxyl radical in particular performs an important function as a natural cleansing agent for the atmosphere.26 In elevated concentrations, however,... 

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