Troposphere chemical reactions

What are the principal chemical reactions that take place in the chemosphere to give it its name How do they influence stratospheric and tropospheric chemical reactions ... 

Quantitative understanding of the sources, sinks and atmospheric lifetime for CHa is an important future goal for several reasons. The direct increase in tropospheric CHa concentrations adds another important infrared absorbing contributor to the greenhouse effect. The calculated contribution from a CHa increase of 0.18 ppmv in a decade is a tropospheric temperature increase of 0.04 C [N.A.S., 1983], about 1/3 as large as that calculated for the observed 12 ppmv increase for CO2 over the decade from 1970-1980. As described earlier, increasing concentrations of CHa in the stratosphere have an influence on ozone-depletion by ClOx through diversion of Cl into HCl, and should in addition after oxidation increase the upper stratospheric concentrations of H2O. Methane is also a participant in tropospheric chemical reaction sequences which lead under some conditions to the formation of ozone. 

The above mentioned urban air pollution in Asian cities drives the tropospheric chemical reactions. This tropospheric chemistry is dominated by the oxidation of trace atmospheric components, as aresult ofwhich organic compounds such as methane and other hydrocarbons are converted into carbon dioxide and water. The consequences of these chemical transformations are known as photochemical smog (photosmog) and the associated problem of ground level ozone. Here we should consider also the effects of particulate matter, one of the major pollutants of urban air in Asia. 

From these results we conclude that the systematic reduction techniques discussed can be usefully applied to tropospheric chemical reaction schemes. The process is totally objective and easily accomplished. It is also clear from the results that the... 

Air pollution (qv) problems are characteri2ed by their scale and the types of pollutants involved. Pollutants are classified as being either primary, that is emitted direcdy, or secondary, ie, formed in the atmosphere through chemical or physical processes. Examples of primary pollutants are carbon monoxide [630-08-0] (qv), CO, lead [7439-92-1] (qv), Pb, chlorofluorocarbons, and many toxic compounds. Notable secondary pollutants include o2one [10028-15-6] (qv), O, which is formed in the troposphere by reactions of nitrogen oxides (NO ) and reactive organic gases (ROG), and sulfuric and nitric acids. 

Trace-gas Lifetimes. The time scales for tropospheric chemical reactivity depend upon the hydroxyl radical concentration [HO ] and upon the rate of the HO/trace gas reaction, which generally represents the slowest or rate-determining chemical step in the removal of an individual, insoluble, molecular species. These rates are determined by the rate constant, e,g. k2s for the fundamental reaction with HO, a quantity that in general must be determined experimentally. The average lifetime of a trace gas T removed solely by its reaction with HO,... 

Altshuller, A. P. (1991) Chemical reactions and transport of alkanes and their products in the troposphere. J. Atmos. Chem. 12, 19-61. 

On a global scale the air layers within a few kilometers of the earth s surface are rapidly mixed by wind action. This region is called the troposphere. Natural and manmade sources of chemicals such as CH4 and other hydrocarbons, CO, SO, NO, ozone, and chlorine are emitted into the troposphere. Most of these are removed or reacted away to form harmless products by dissolving in rain, adsorption on solids, and chemical reactions. 

As discussed in detail in Sections C.3.d and C.3.e, the fastest atmospheric reactions of S02 are believed to be with H202 and perhaps with Os at higher pH values. Under extreme conditions of large fog droplets (—10 yu,m) and very high oxidant concentrations, the chemical reaction times may approach those of diffusion, particularly in the aqueous phase. In this case, mass transport may become limiting. However, it is believed that under most conditions typical of the troposphere, this will not be the case and the chemical reaction rate will be rate determining in the S(IV) aqueous-phase oxidation. 

Tropospheric chemistry models have to take into account a significant number of chemical reactions required to simulate correctly tropospheric chemistry. In the global background marine troposphere, it seems reasonable to consider a simplified chemistry scheme based on O3/ NOx/ CH, and CO photochemical reactions. However, natural emissions of organic compounds from oceans (mainly alkenes and dimethyl sulphide-DMS) might significantly affect the marine boundary layer chemistry and in particular OH concentrations. Over continental areas both under clean and polluted conditions,... 

Note that careful evaluation and minimization of uncertainties and errors in CTMs is requested to enable the application of these CTMs to the study of observed changes in 03 as small as < 1.5 %/yr. However, actually 03 concentrations are simulated by the models within 20-50%. Chemical reaction rates are also uncertain, for instance in the 90 s determinations of the rates of CH4 and CH3CC13 reactions with OH suggested that these reactions are about 20% slower than believed. Similarly OH reaction with N02 which is an important sink for NOx in the troposphere is measured to be 10-30% lower than earlier estimates [23]. Thus, the past years a number of studies (mainly based on Monte Carlo simulations) focused on the identification and evaluation of the importance of various chemical reactions on oxidant levels to highlight topics crucial for error minimization. Temperature dependence of reaction rates can also introduce a 20-40% uncertainty in 03 and H20 computations in the upper troposphere. It has been also shown that 03 simulations are particularly sensitive to the photolysis rates of N02 and 03 and to PAN chemistry. 

The model tropopause is defined by a PV level of 3.5 pvu poleward of 20° latitude, and by a -2 K km 1 temperature lapse rate equatorward of 20° latitude. Consequently, in this study the troposphere is defined as the volume between the surface and the simulated tropopause. Because the model does not consider typical stratospheric chemical reactions explicitly, ozone concentrations are prescribed from 1-2 levels above the model tropopause up to the top of the model domain at 10 hPa. In both hemispheres we apply monthly and zonally averaged distributions from a 2D stratospheric chemistry model [31]. In the present version of the model, we use the simulated PV and the regression analysis of the MOZAIC data (Section 2) to prescribe ozone in the NH extratropical lower stratosphere, which improves the representation of ozone distributions influenced by synoptic scale disturbances [32, 33]. Furthermore, the present model contains updated reaction rates and photodissociation data [34]. 

Tropospheric 03 is a secondary constituent formed by chemical reactions in the atmosphere involving several precursors (NOx, hydrocarbons and CO). The concentrations of these precursors are controlled by the atmospheric oxidation processes, which are regulated by hydrogen radicals OH and H02. 

The term "heterogeneous" as applied to the atmosphere refers to chemistry that occurs in or on ambient condensed phases that are in contact with the gas phase aerosols, clouds, surface waters, etc. It is important to distinguish between heterogeneous processes that occur on the surface of the solid, and multiphase chemical reactions that take place in the bulk of the liquid medium. In the latter case, it is assumed that the reaction takes place after the molecule has been incorporated in the bulk liquid medium, such as occurs by wet deposition, where a species is ultimately removed from the atmosphere, especially in the troposphere. 

In addition to photolysis (Chapter 15) and chemical reactions (see the next section), wet and dry deposition also can remove gas- and particle-phase chemical compounds from the troposphere (Eisenreich et al., 1981 Bidleman, 1988). Thus to completely characterize the atmospheric loss processes and overall lifetime of a chemical, we must understand its atmospheric lifetime due to dry and/or wet deposition. Wet deposition refers to the removal of the chemical (or particle-associated chemical) from the atmosphere by precipitation of rain, fog, or snow to earth s surface). Dry deposition refers to the removal of the chemical or particle-assodated chemical from the atmosphere to the Earth s surface by diffusion and / or sedimentation. 

Air estimated tropospheric chemical lifetimes, x = 5 h, 3 d and > 150 d for reactions with OH, N03 and 03, respectively, under typical remote tropospheric conditions (Falbe-Hansen et al. 2000)... 

There are several mechanisms whereby organic compounds released into the atmosphere may be removed (i) physical removal by precipitation ( rain-out ) (ii) chemical reaction in the troposphere (in) transport into the stratosphere (iv) chemical reaction in the stratosphere. The physical and dynamic conditions of the different atmospheric regions will usually dictate the type of mechanism that occurs2,3. 

A few key (i.e., primary or direct) photochemical reactions are the principal drivers of overall chemical reactions in the troposphere. These reactions involve primarily O3 and NO2 photolysis. Other reactions presented in Table 1 will be discussed later. 

Thus taking the boundary layer as a discrete compartment, the lifetime of ozone with respect to dry deposition is around 1 day. The lifetime in the free troposphere (the section of the atmosphere above the boundary layer) is longer, being controlled by transfer processes in and out, and chemical reactions. The stratosphere lifetime of ozone is controlled by photochemical and chemical reaction processes. 

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