Atmosphere photochemical reactions

Area sources of either a selected chemical or a precursor present a common problem for modeling. In particular, the rich and complex patterns of hydrocarbon emissions from general urban and industrial sources either include or might produce through atmospheric photochemical reactions some of the species on the analysis list. The treatment of such species in photochemical airshed modeling is difficult (8, 9). The effort required for any one such exercise is substantial, and the effort required for a comprehensive analysis of all urban regions relevant to this program would be prohibitive. 

Chemical radicals—such as hydroxyl, peroxyhydroxyl, and various alkyl and aryl species—have either been observed in laboratory studies or have been postulated as photochemical reaction intermediates. Atmospheric photochemical reactions also result in the formation of finely divided suspended particles (secondary aerosols), which create atmospheric haze. Their chemical content is enriched with sulfates (from sulfur dioxide), nitrates (from nitrogen dioxide, nitric oxide, and peroxyacylnitrates), ammonium (from ammonia), chloride (from sea salt), water, and oxygenated, sulfiirated, and nitrated organic compounds (from chemical combination of ozone and oxygen with hydrocarbon, sulfur oxide, and nitrogen oxide fragments). ... 

Altshuller, A.P. Measurements of the products of atmospheric photochemical reactions in laboratory studies and in ambient air-relationships between ozone and other products, Atmos. Environ., 17(12) 2383-2427, 1983. 

Flagan, R. C., S.-C. Wang, F. Yin, J. H. Seinfeld, G. Reischi, W. Winklniayr, and R. Karch, Electrical Mobility Measurements of Fine-Particle Formation during Chamber Studies of Atmospheric Photochemical Reactions, Environ. Sci. Technol., 25, 883-890 (1991). 

Gaseous hydrogen peroxide is a key component and product of the earth s lower atmospheric photochemical reactions, in both clean and polluted atmospheres. Atmospheric hydrogen peroxide is believed to be generated exclusively by gas-phase photochemical reactions (lARC, 1985). Low concentrations of hydrogen peroxide have been measured in the gas-phase and in cloud water in the United States (United States National Library of Medicine, 1998). It has been found in rain and surface water, in human and plant tissues, in foods and beverages and in bacteria (lARC, 1985). 

The main source of oxygen in prebiotic planetary atmospheres is ablation of cometary material. The primary source of various oxygen species in all atmospheric photochemical reactions is water. Water molecules are easily photolyzed by the solar Lyman aline (121.6nm) and on irradiation within the water UV absorption bands (140-190 nm).The main products are hydroxyl radicals, oxygen atoms in their ground (3P) and excited ( D) states, and hydrogen atoms [14],... 

Oxidants present in the atmosphere thermochemically capable of oxidizing SO2 or NO2 include not only molecular O2 but also the trace, highly reactive constituents O3 and H2O2 that are the products of secondary atmospheric photochemical reactions. Despite... 

They are used for chemical synthesis, water and waste treatment, disinfection, neutralizations, and the like. They can also be produced in atmospheric photochemical reactions and as undesirable products, for example, in the combustion of fuels and industrial processes. The most important inorganic gases are ammonia, carbon oxides, nitrogen oxides, ozone and sulfur oxides. 

The reflection principle approach (see article by Jost in this issue) produces essentially perfect agreement with experiment but it is not able to provide vibrational structure. Therefore the time-dependent techniques are important for indirect photodissociation, of which there are many examples in atmospheric chemistry, including HCHO, SO2, NO2,02, CO, HCl, H2O and O3. Many studies have shown how isotopic analysis is able to provide valuable insight concerning atmospheric photochemical reactions, and emissions sources and loss mechanisms. One of the largest uncertainties in projections of future climate is the ability to predict greenhouse gas concentrations, which depend on accurate knowledge of their sources, sinks and atmospheric photochemistry. 

The biosphere is a major contributor to the atmosphere of heavier hydrocarbons. Fritz Went (8, 9), who first recognized the global extent of smog, pointed out the general importance of natural emissions from vegetation. He estimated that sources in the biosphere annually emit between 170 X 10 and 10 tons of hydrocarbon material to the atmosphere. Went also observed that these materials are mainly in the terpene class and that, because they are photochemically reactive, these materials are polymerized in atmospheric photochemical reactions to form an organic aerosol. He attributes the blue haze found in many forested areas to the optical effects of this aerosol. 

PCBTF received an exemption from VOC regulations based on the fact that its atmospheric hydroxyl radical reaction rate is slower than that of ethane [25]. A VOC is defined as any compound of carbon, excluding carbon monoxide, carbon dioxide, carbonic add, metallic carbides or carbonates, and ammoniiun carbonate, which partidpates in atmospheric photochemical reactions [26]. A VOC exemption petition for BTF was filed with the EPA on March 11,1997. Volatile organic compounds (VOCs) emission is controlled by regulation in efforts to reduce the tropospheric air concentrations of ozone. 

To compute atmospheric photochemical reaction rates it is necessary to determine the total light intensity incident on a given volume of air, from all directions. The light... 

Heterogeneous reactions on solid particles may also play a role in the removal of sulphur dioxide from the atmosphere. In atmospheric photochemical reactions, such particles may function as nucleation centres. Thus, they act as catalysts and grow in size by accumulating reaction products. The final result would be the production of an aerosol with a composition unlike that of the original particle. Little research has been done on the role that solid particles play in the oxidation of sulphur dioxide under conditions like those found in the atmosphere. Soot particles, which consist of elemental carbon contaminated with polycyclic aromatic hydrocarbons produced in the incomplete combustion of carbonacetous fuels, have been shown to catalyse the oxidation of sulphur dioxide to sulphates. 

Any compound of carbon, excluding carbon monoxide, carbon dioxide, carbonic acid, metal carbides or carbonates and ammonium carbonates which participates in atmospheric photochemical reactions [1]... 

To compute atmospheric photochemical reaction rates it is necessary to determine the total light intensity incident on a given volume of air, from all directions. The light intensity impacting a volume of air includes not only direct solar radiation, but light, either direct from the Sun or reflected from the Earth s surface, that is scattered into the volume by gases and particles, as well as light reflected directly from the Earth s surface. 

Volatile organic compound (VOC) Any organic compound that participates in atmospheric photochemical reactions, except for those designated by the EPA Administrator as having negligible photochemical reactivity. A subset of VOCs that include one or more chlorine atoms covalently bonded to carbon and are abbreviated, at least in this book, as CIVOCs. Examples of CIVOCs are vinyl chloride, trichloroethene (TCE), and per-chloroethylene or tetrachloroethylene (PCE). 

NAAQS Volatile organic compounds (VOC) are essentially considered the same as the criteria pollutant ozone. VOCs are very broadly defined by the U.S. EPA (40 CFR 51.100) any compound of carbon, excluding carbon monoxide, carbon dioxide, carbonic acid, metallic carbides or carbonates, and ammonium carbonate, that participates in atmospheric photochemical reactions. This includes any organic compound other than those specifically listed as having been determined to have negligible photochemical reactivity. Reactive VOCs are essentially all fiiose judged to be clearly more reactive than ethane - the most reactive member of the neghgibly reactive class. C4 - paraffins are of relatively... 

Certain organic compounds that may be released under the specified bake conditions are not classified as VOC as they do not participate in atmospheric photochemical reactions. Suchnon-photochemically active compounds are referred to as exempt volatile compounds in the practice. 

O Classified as a volatile organic compound (VOC). VOC can react in the lower atmosphere to form ozone and other oxidants. VOC means any compound of carbon, excluding carbon monoxide, carbon dioxide, carbonic acid, metaUic carbides or carbonates, and ammonium carbonate, which participates in atmospheric photochemical reactions. Some compounds are specifically exempted firom this definition which is found in 40 C.F.R. 51.100(s). T Considered a hazardous air pollutant (HAP) and listed in Title III of the Clean Air Act Amendments of 1990. D A regulated stratospheric ozone layer depleter. 

Forest destruction and changes in the earth s biomass may add 20-30 X 10 Tg C02/year to atmosphere Photochemical reactions in stratosphere may impact on O3 layer Volcanoes may release 10-20 TgCl/year> ... 

Secondary products, formed from in situ atmospheric photochemical reactions, include the alkyl nitrates, peroxyacyl nitrate, and mat r other compounds. In general, the contribution of these species is usually fairly modest (a few percents) compared to the inoiganic species already mentioned. However, because some of these compounds have well-documented health implications (e g., PAN, ni-trosamines, lutroarenes) and because some of these can play roles in regulating the chemistiy of ozone (eg., PAN),... 

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