Photochemical Reactions in the Atmosphere

Why does not the solar spectrum at sea level perfectly match the solar spectrum outside the atmosphere  

The more structured curve at sea level is due to gases in the atmosphere absorbing specific wavelengths of light. Flux, the unit on the vertical axis, is light ener per area per unit of time. 

The rupture of a chemical bond resulting from absorption of a photon by a molecule is called photodissociation. No ions are formed when the bond between two atoms is cleaved by photodissociation. Instead, half the bonding electrons stay with one atom and half stay with the other atom. The result is two electrically neutral particles. 

One of the most important processes occurring above an altitude of about 120 km is photodissociation of the oxygen molecule  

The minimum energy required to cause this change is determined by the bond energy (or dissociation energy) of O2,495 kj/mol. 

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Besides these features, the formation of a layer due to an interaction of a stratified fluid with light is itself noteworthy. Analogs to this phenomenon can be found in other media. Examples include photochemical reactions in the atmosphere near the Earth s surface, photochemical reactions in the surface water of the ocean and biological activity near the ocean surface. 

Figure 7-11 and its caption (Crutzen, 1983) depict the most important of the gas phase and photochemical reactions in the atmosphere. Perhaps the single most important interaction involves the hydroxyl free radical, OH-. This extremely reactive radical is produced principally from the reactions of electronically excited atomic oxygen, 0( D), with water vapor. Photo-... 

Interestingly, many of these free radicals are produced from photochemical reactions in the atmosphere of O2 and O3, for example... 

The interest in the reaction still continues and has acquired special importance due to deleterious effect on stratospheric ozone layer of haloalkane free radicals generated by photochemical reactions in the atmosphere. 

Photochemical smog Type of air pollution due photochemical reaction in the atmosphere. 

You pointed out the importance of photochemical reactions in the atmosphere related to pollution, and you described the SO2 oxidation, giving great emphasis to the homogeneous oxidation through free radicals. I definitely agree these are important processes however, I strongly believe and... 

There has been recent interest in a somewhat different aspect of adsorption and reaction on metal oxides photocatalysis. The interest stems partially from that role that some transition-metal oxides can play in photochemical reactions in the atmosphere. Atmospheric aerosol particles can act as substrates to catalyze heterogeneous photochemical reactions in the troposphere. Most tropospheric aerosols are silicates, aluminosilicates and salts whose bandgaps are larger than the cutoff of solar radiation in the troposphere (about 4.3 eV) they are thus unable to participate directly in photoexcited reactions. However, transition-metal oxides that have much smaller bandgaps also occur as aerosols — the most prevalent ones are the oxides of iron and manganese — and these materials may thus undergo charge-transfer excitations (discussed above) in the pres-... 

To relate the initiation of photochemical reactions to the wavelength of light, and survey consequences of photochemical reactions in the atmosphere... 

Automobile exhaust is another source of 2,4- and 2,6-DNPs in air (Nojima et al. 1983). 2,4-DNP is also used as an insecticide, acaricide, and fungicide (HSDB 1994). Therefore, application of this type of pesticide could be a source of 2,4-DNP in air. Photochemical reactions of benzene with nitrogen oxides in air also produce dinitrophenols in the atmosphere (Nojima et al. 1983). Dinitrophenols have been detected in emissions from hazardous waste combustion (James et al. 1984). Dinitrophenols may be present in the aerosol or vapor phase near hazardous waste disposal sites. It has been suggested that the most important origin of dinitrophenols is their formation by photochemical reactions in the atmosphere (Nojima et al. 1983). 

These oxidants, obtained by photochemical reactions in the atmosphere, may be involved in the widespread dieback and decline of forests in both Europe and North America. The O3 and photooxidants theory, and its influence on acid deposition, will be shortly presented and discussed. 

Our study begins in California in the late forties, when the combination of the rapidly increasing population of humans and automobiles with geography and personal expectations focused political and scientific attention onto the photochemical reactions in the atmosphere between the hydrocarbons and nitrogen oxides which were being emitted in automobile exhaust, and on the effects of the reaction products on public health. 

VOC, = [(100- 15-75)/(100- 1.0x75)]xl.0x 1000 = 400 g/1 The calculation permits the deduction of exempt compounds, which are defined as organic compounds which do not participate in photochemical reactions in the atmosphere. 

Atmospheric aerosols are small particulate matter with diameter of a few micrometer, which are caused by sandy dusts, smoke from factories, exhaust gas of cars and their deformed particles by photochemical reactions in the atmosphere. Elemental concentrations in atmospheric aerosol reflect air pollution and its generating process. The elements that may be found in dust samples range from hydrogen to uranium. However, the more common elements are H, C, N, O, F, Na, Mg, Al, Si, P, S, Cl, K, Ca, Ti, V, Cr, Mn, Co, Fe, Cu, Ni, Zn, Br, and Pb. While PIXE is generally used to determine concentrations of all elements with atomic number above Al Z = 13), the... 

Gaseous. Common gaseous atmospheric contaminants are ozone, oxides of sulphur and oxides of nitrogen. They may be present from direct emission from factories, house chimneys, automobile exhausts or as a result of photochemical reaction in the atmosphere. Smogs, for example those in Los Angeles, are due to photochemical reactions of hydro-carbons, principally olefins from automobile exhausts, with oxides of nitrogen and sulphur, these latter also from exhausts. 

Table 5.2 List of X + hv products the most important photochemical reactions in the atmosphere.

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