Troposphere ozone generation

The primary photochemical loss process for O3 arises from the sequence of reactions which produces OH, i.e. (R3) and (R5). Tropospheric ozone generation by photochemistry can arise from the oxidation CO and hydrocarbons via reaction sequences such as... 

In a clean troposphere, ozone would react with NO molecules, resulting in no net generation of ozone. However, in a dirty atmosphere, excess ozone is generated, thereby resulting in the formation of an oxidant which can lead to health effects, an especially hazardous condition for children and the elders who suffer from asthma and other respiratory challenges. 

Recent estimates indicate that the level of carbon dioxide in the atmosphere has increased by a third since the beginning of the industrial age, and that it contributes significantly to global warming. Other major contributors include methane, tropospheric ozone, and nitrous oxide. Methane is the principal component of natural gas, but it is also produced by other sources such as rice paddies and farm animals. Tropospheric ozone is generated naturally and by the sunlight-... 

We conclude by illustrating how our understanding of these chemical processes in our clean and polluted troposphere and stratosphere plays a crucial role in generating the exposure portions of scientific health risk assessments. Such assessments provide the foundation for sound, health-protective and cost-effective strategies for the control of tropospheric ozone, particles, acids, and a spectrum of hazardous air pollutants (including carcinogens and pesticides)—as well as for the mitigation of stratospheric ozone depletion. 

Table 14.4 summarizes the estimated total direct radiative forcing calculated for the period from preindustrial times to 1992 for C02, CH4, N20, and O, (IPCC, 1996). The estimate for CH4 includes the effects due to its impacts on tropospheric ozone levels or on stratospheric water vapor, both of which are generated during the oxidation of methane. That shown for 03 is based on the assumption that its concentration increased from 25 to 50 ppb over the Northern Flemi-sphere. The total radiative forcing due to the increase in these four gases from preindustrial times to the present is estimated to be 2.57 W m 2. 

Conversely, generating hydrogen from sustainable sources would reduce emissions of carbon monoxide and NOx, with a consequent fall in tropospheric ozone levels. This would improve air quality in many regions of the world. Furthermore, C02 emissions would be reduced, thereby slowing the global warming trend. 

The last reaction is responsible for generation of the ozone layer at 2(M0 km above sea level. In the troposphere ozone is also synthesized by reaction between 02... 

The data base has been used to establish structure-reactivity relationships and to provide quantitative rate coefficients and reliable reaction pathways for reactions for which no experimental data are available or have proved to be experimentally inaccessible. Such a strategy was adopted in view of the very large number of VOCs involved in the generation of tropospheric ozone. These structure-reactivity relationships are discussed in this report and in other publications. 

The rate of ozone generation in a particular region of the troposphere is determined by the perturbation of the local photostationary state of NOx due to oxidation of NO to NO2, primarily by peroxy radicals, RO2, in the reaction ... 

NO, or NO2 is generated—the latter two are related to formation of tropospheric ozone, which is deleterious to human health. 

The absorption regions of the alkyl nitrites overlaps well the distribution of solar flux within the lower troposphere see figure IX-H-1. The characteristic band structure that appears in each spectrum, like that in HONO, originates from the excitation to the different vibrational energy levels (an N-O stretching mode, V2) in the Si excited state. Like HONO, the alkyl nitrites (RONO) photodissociate readily to form free radicals RONO+hv RO+NO, that can drive the chemistry that results in ozone generation within the troposphere. The alkyl nitrites (RONO) are often used as convenient sources of alkoxy radicals (RO) in laboratory experiments. 

The photodecomposition of the various oxidation products of the alkanes, alkenes, and the aromatic hydrocarbons play important roles in the chemistry of the urban, mral, and remote atmospheres. These processes provide radical and other reachve products that help drive the chemistry that leads to ozone generation and other important chemistty in the troposphere. In this chapter, we have reviewed the evidence for the nature of the primary processes that occur in the aldehydes, ketones, alkyl nitrites, nittoalkanes, alkyl nitrates, peroxyacyl nitrates, alkyl peroxides, and some representative, ttopospheric, sunlight-absorbing aromatic compounds. Where sufficient data exist, estimates have been made of the rate of the photolytic processes that occur in these molecules by calculation of the photolysis frequencies ory-values. These rate coefficients allow estimation of the photochemical lifetimes of the various compounds in the atmosphere as well as the rates at which various reactive products are formed through photolysis. 

The most important class of these molecules in promoting ozone generation within the troposphere is the aldehydes, and formaldehyde leads this list. Photodecomposition of the other higher aldehydes is somewhat less competitive with the OH reaction, but still important. This is especially true at the higher altitudes of the troposphere where the photodissociation is often enhanced by the lowered collisional quenching of the exited aldehyde molecules and the increased solar flux. 

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