Hydrocarbons, atmosphere

Methane and the Nonmethane Hydrocarbons. It is traditional to distinguish CH4 from all other atmospheric hydrocarbons. Methane is by far the most abundant atmospheric hydrocarbon and has very large natural emissions. Its abundance in auto exhaust but low atmospheric reactivity has led air pollution scientists to enact controls on nonmethane hydrocarbons NMHC (also called VOC for volatile organic compounds, which include oxygenated hydrocarbons). 

Methane is the atmospheric hydrocarbon least reactive with HO, losing a hydrogen after an atmospheric lifetime of about a decade ... 

Damall, K.R., Lloyd, A.C., Winer, A.M., Pitts, J.N. (1976) Reactivity scale for atmospheric hydrocarbons based on reaction with hydroxyl radicals. Environ. Sci. Technol. 10, 692-696. 

Currie, L. A., Noakes, J., Breiter, D., Measurment of Small Radiocarbon Samples Power of Alternative Methods for Tracing Atmospheric Hydrocarbons, Ninth International Radiocarbon Conference, University of California, Los Angeles and San Diego, 1976. 

Octafining A process for isomerizing m-xylene to o- and p-xylene, developed by the Atlantic Richfield Company in 1960. The catalyst was originally platinum on an aluminum silicate base now a zeolite base is used. The reaction takes place in a hydrogen atmosphere. Hydrocarbon Research installed units in Argentina and the USSR. 

Some pollutants fall in both categories. Nitrogen dioxide, which is emitted directly from auto exhaust, is also formed in the atmosphere photochemically from NO. Aldehydes, which are released in auto exhausts, are also formed in the photochemical oxidation of hydrocarbons. Carbon monoxide, which arises primarily from autos and stationary sources, is likewise a product of atmospheric hydrocarbon oxidation. 

More than 100 compounds are released in the atmosphere of urban areas by automobiles, and there is a close relation between the atmospheric hydrocarbon composition and the composition of gasolines and automobile exhausts. The full range of compositions of gasolines has been reported by Sanders and Maynard.They identified 180 of the 240 compounds separated by capillary-column gas chromatography. Detailed fiiel compositions were reported by other investigators, and exhaust hydrocarbon compositions were reported by Neligan et a/.,McEwen, and, more recently. Papa et Jacobs, ... 

Rudolph, J., and F. J. Johnen, Measurements of Light Atmospheric Hydrocarbons over the Atlantic in Regions of Low Biological Activity, . /. Geophys. Res., 95, 20583-20591 (1990). 

Darnall, K. R., A. C. Lloyd, A. M. Winer, and J. N. Pitts, Jr., Reactivity Scale for Atmospheric Hydrocarbons Based on Reaction with Hydroxyl Radical, Environ. Sci. TechnoL, 10, 692-696 (1976). 

All of the methods used to determine atmospheric hydrocarbon concentrations include the three distinct steps of... 

Simons, J.H., Mausteller, J.W. (1952) The properties of w-butforane and its mixtures with n-butanc. J. Chem. Phys. 20, 1516-1519. Singh, H.B. (1977) Atmospheric hydrocarbons Evidence in favor of reduced average hydroxyl radical concentrations in troposphere. Geophys. Res. Lett. 4, 101-104. 

The most abundant hydrocarbon in the atmosphere is methane, CH4. This gas is released from underground sources as natural gas and produced by the fermentation of organic matter. Methane is one of the least reactive atmospheric hydrocarbons and is produced by diffuse sources, so that its participation in the formation of pollutant photochemical reaction products is minimal. 

The only practical method for the measurement of specific hydrocarbons is by gas chromatography (GC), which can be automated, but is expensive and often used as a laboratory analyzer. Certain specially prepared columns can be used to adsorb specific hydrocarbon classes. For atmospheric hydrocarbons, a common method is to pass a sample of air through a small freeze-out trap, sweep out the air with helium, and then warm the trap and introduce the condensables into the GC column in one concentrated slug. 

These few examples show the desperate need for further research and assessment in the chemistry and all aspects of air pollution. One might question whether we have knowledge of all urban sources of hydrocarbons that are important to atmospheric interactions. Does all rubber dust from automotive tires remain as dust What is the contribution of asphalt roadways to atmospheric hydrocarbons Is the contribution from such unassessed and other unknown anthropogenic sources sufficient in itself to account for health-damaging levels of ozone These and other questions require immediate attention if we are to realize the goal of effective pollution control. 

Another example12 is the separation of atmospheric hydrocarbons on an FID and a photoionization detector (PID), which is more sensitive for unsaturated hydrocarbons (Figure 6.7). The identities of the peaks are given in Table 1 along with the PID/FID response ratios. It can be seen that the ratios can be used as additional information in assigning peak identities. In fact, Figure 6.8 shows that the hydrocarbon type (saturates, olefins, or aromatics) can be assigned from the detector ratio in many cases. 

Oxidation of hydrocarbons has long been considered as a fundamental problem to atmospheric chemists, both from experimental and theoretical points of view, because of the inherent complexity. The reaction kinetics and mechanism of atmospheric hydrocarbons have been the focuses of numerous researches in both experimental and theoretical aspects. Although advances have been made in elucidation of the VOC oxidation mechanisms, large uncertainty and tremendous numbers of unexplored reactions still remain. Several review articles on the atmospheric degeneration of VOCs have been published [4,11-14]. In this review, recent advances in the application of theoretical methods to the atmospheric oxidation of biogenic hydrocarbons are discussed. We will introduce the backgrounds on the quantum chemical calculations and kinetic rate theories, recent progress on theoretical studies of isoprene and a-, y3-pinenes, and studies on other monoter-penes and sesquiterpenes. 

THEORETICAL APPROACHES IN ATMOSPHERIC HYDROCARBON OXIDATION RESEARCH... 

Duce R.A., Quinn J.G. and Wade T.L., Atmospheric hydrocarbons and the ocean. In Int. Symp. on the Chemistry of Sea/Air Particulate Exchange Process, 1973. Nice (France) BuU. Union Oceanogr. France, Spec. Issue, 1973, 133 pp. 

It should be noted that this quantifleation approach assumes that the sample is homogeneous in depth. If this were the case however, the use of a surface analysis technique would not be Justifled. The approximation involved is applied in the absence of other information that can be used to describe the depth distribution of the elements. In particular, if a surface contamination layer (for example, atmospheric hydrocarbons) is present on the sample this will influence the intensity of the peaks to an extent which depends on the energy of the pho-toclcctron (through the dependence of X on the kinetic energy) and thus the clement. 

Equation (7.10)] by atmospheric hydrocarbons is the principal cause of photochemical smog. 

Fall R. and S.D. Copley Bacterial sources and sinks of isoprene, a reactive atmospheric hydrocarbon. Environ. Microbiol. 2 (2000) 123-130. 

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