Oxidative capacity troposphere

P. Matuska, R. Schmitt, D. Mihelcic, P. Muesgen, H.-W. Paetz, M. Schultz, and A. Volz-Thomas, Trace Gas Measurements during the Oxidizing Capacity of the Tropospheric Atmosphere Campaign 1993 at Izana, J. Geophys. Res., 103, 13505-13518 (1998). 

Kourtidis K., I. Ziomas, C. Zerefos, D. Balis, P. Suppan, A. Vasaras, V. Kosmidis and A. Kentarchos (1997) On the background ozone values in Greece, In Proceedings of the 7th European Symposium on Physico-Chemical Behaviour of Atmospheric Pollutants the Oxidizing Capacity of the Troposphere, B. Larsen, B. Versino, and G. Angeletti (eds.), European Commission, Brussels, p. 387-390. 

Formaldehyde is formed in the atmospheric degradation of virtually all hydrocarbons. Its photolysis has an important effect on the atmosphere s oxidation capacity since it is a significant source of HOx radicals in the middle and upper troposphere, and in polluted regions [135] ... 

Aldehydes are emitted directly into the atmosphere from a variety of natural and anthropogenic sources and are also formed in situ from the atmospheric degradation of volatile organic compounds (VOCs). The atmospheric fate of aldehydes is controlled by photolysis and reaction with hydroxyl (OH) or nitrate (NO3) radicals and, in the case of unsaturated compounds, reaction with ozone (Atkinson, 1994). The photolysis of aldehydes is of particular importance because it is a source of free radicals in the troposphere, and thus may significantly influence the oxidizing capacity of the lower atmosphere (Finlayson-Pitts and Pitts, 1986). 

Herrmann, H., H.-W. Jacobi, G. Raabe, A. Reese, Th. Umschlag and R. Zellner Free radical reactions in the tropospheric aqueous phase, in B. Larsen, B. Versino, G. Angeletti, (eds). The Oxidizing Capacity of the... 

Spatial scales characteristic of various atmospheric chemical phenomena are given in Table 1.1. Many of the phenomena in Table 1.1 overlap for example, there is more or less of a continuum between (1) urban and regional air pollution, (2) the aerosol haze associated with regional air pollution and aerosol-climate interactions, (3) greenhouse gas increases and stratospheric ozone depletion, and (4) tropospheric oxidative capacity and stratospheric ozone depletion. The lifetime of a species is the average time that a molecule of that species resides in the atmosphere before removal (chemical transformation to another species counts as removal). Atmospheric lifetimes vary from less than a second for... 

In spite of the fact that the atmosphere is composed predominantly of relatively inert molecules such as N2 and O2, it is actually a rather efficient oxidizing medium. One reason for the atmosphere s oxidizing capacity arises because the atmosphere contains minute amounts of very reactive molecular fragments, called free radicals. The most important free radical in the chemistry of the troposphere is the hydroxyl (OH) radical, which reacts with nearly every molecular species in the atmosphere. In addition, the atmosphere contains trace amounts of species less reactive than free radicals but nonetheless reactive enough to attack a variety of airborne compounds. Ozone (O3) is one important oxidizer, which also participates in the formation of the hydroxyl radical. 

Nitric oxide (NO) plays a central role in atmospheric chemistry, influencing both ozone cycling and the tropospheric oxidation capacity through reactions with hydroperoxy- and organic peroxy-radicals. When the NO concentration exceeds 40 pptv (pptv = parts per trillion by volume) it catalyzes the production of ozone (O3) ... 

At high concentrations (>50ppbv ppbv = parts per billion by volume), O3 in the atmospheric boundary layer becomes a toxic pollutant that also has important radiative transfer properties. The production of nitric acid from NO influences atmospheric pH, and contributes to acid rain formation. In addition, the oxidation of NO to the nitrate (NO3) radical at night influences the oxidizing capacity of the lower troposphere. Determination of the magnitude and location of NO sources is critical to modeling boundary layer and free tropospheric chemistry. 

CH4 is the most abundant organic volatile in the atmosphere and, next to CO2, is responsible for 15% of the current greenhouse radiative forcing, with a direct radiative forcing of 0. 5 Wm. CH4 reacts with OH and so limits the tropospheric oxidation capacity and influences ozone and other greenhouse... 

This leads to a reaction chain in which O3 is destroyed and not formed as under usual conditions in the troposphere. This type of chemistry is important in very clean environments such as in the unpolluted marine boundary layer in the southern hemisphere. Ozone also strongly regulates the oxidant capacity of the troposphere, because photolysis of O3 is the most dominant tropospheric source of OH radicals. These radicals are most important for the chemical degradation of the gaseous reduced compounds which include important greenhouse gases such as, e.g., methane. 

These processes are important for the chemistry of the troposphere, as the production of 0( D) controls the oxidative capacity of this region through its reaction... 

Cmtzen, P.J., 1997 Problems in Global Atmospheric Chemistry , in Larsen, B. Versino, B. Angeletti, G. (Eds.) The Oxidizing Capacity of the Troposphere, Proceedings of the 7th European Symposium on Physico-chemical Behaviour of Atmospheric Pollutants , Venice, Italy, 2-4 Ocotber 1996 (Bmssels European Commission) 1-13. 

The chemistry of the troposphere (the layer of the atmosphere closest to earth s surface) overlaps with low-temperature combustion, as one would expect for an oxidative environment. Consequently, the concerns of atmospheric chemistry overlap with those of combustion chemistry. Monks recently published a tutorial review of radical chemistry in the troposphere. Atkinson and Arey have compiled a thorough database of atmospheric degradation reactions of volatile organic compounds (VOCs), while Atkinson et al. have generated a database of reactions for several reactive species with atmospheric implications. Also, Sandler et al. have contributed to the Jet Propulsion Laboratory s extensive database of chemical kinetic and photochemical data. These reviews address reactions with atmospheric implications in far greater detail than is possible for the scope of this review. For our purposes, we can extend the low-temperature combustion reactions [Equations (4) and (5)], whereby peroxy radicals would have the capacity to react with prevalent atmospheric radicals, such as HO2, NO, NO2, and NO3 (the latter three of which are collectively known as NOy) ... 

CFCs are the most important, hut by no means the only, chemicals capable of destroying ozone molecules. For many years, researchers have recognized that oxides of nitrogen have the capacity both to increase and to decrease the concentration of ozone in the stratosphere. They can increase ozone concentrations in the presence of ultraviolet (uv) radiation by undergoing uv-mediated reactions similar to those that occur in the lower troposphere. For example ... 

As we noted in Section 4.01.1, the ability of the troposphere to chemically transform and remove trace gases depends on complex chemistry driven by the relatively small flux of energetic solar UV radiation that penetrates through the stratospheric O3 layer (Levy, 1971 Chameides and Walker, 1973 Crutzen, 1979 Ehhalt et al., 1991 Logan et al, 1981 Ehhalt, 1999 Crutzen and Zimmerman, 1991). This chemistry is also driven by emissions of NO, CO, and hydrocarbons and leads to the production of O3, which is one of the important indicators of the oxidizing power of the atmosphere. But the most important oxidizer is the hydroxyl free radical (OH), and a key measure of the capacity of the atmosphere to oxidize trace gases injected into it is the local concentration of hydroxyl radicals. 

Paul continued to make major contributions to stratospheric chemistry. For example, he explained how nitric acid clouds cause the Antarctic ozone hole. At the same time, he also turned his attention to the troposphere, which is the air layer that connects with the biosphere and where weather and climate take place. The troposphere is also prone to air pollution, while it is cleaned by oxidation reactions. The self-cleaning capacity relies on the presence of reactive hydroxyl radicals that convert pollutant gases into more soluble compounds that are removed by rain. The primary formation of hydroxyl radicals in turn is from ozone. While most ozone is located in the stratosphere, protecting life on Earth against harmful ultraviolet radiation from the Sun, a small amount is needed in the troposphere to support the self-cleaning capacity. While previous theories had assumed that tropospheric ozone originates in the stratosphere, Paul discovered that much of it is actually chemically formed within the troposphere. The formation mechanism is similar to the creation of ozone pollution in photochemical smog . 

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