Sink, tropospheric

Apphes to particles released in the lower troposphere only the most important sink is scavenging by precipitation, so in the absence of precipitation, these particles wUl remain suspended longer. 

If the earth did not rotate or if it rotated much more slowly than it does, a meridional (along meridians) circulation would take place in the troposphere (Fig. 17-24). Air would rise over the tropics, move poleward, sink over the poles forming a subsidence inversion, and then stream equa-... 

Release of N2O to atmosphere by variety of sources no significant sinks of N2O in the troposphere have been discovered stratospheric loss estimated by model calculations. 

Tropospheric chemistry is strongly dependent on the concentration of the hydroxyl radical (OH), which reacts very quickly with most trace gases in the atmosphere. Owing to its short boundary layer lifetime ( 1 s), atmospheric concentrations of OH are highly variable and respond rapidly to changes in concentrations of sources and sinks. Photolysis of ozone, followed by reaction of the resulting excited state oxygen atom with water vapour, is the primary source of the OH radical in the clean troposphere ... 

The two major tropospheric sinks of OH are the reactions with CO and CH4. In the clean Southern Hemisphere, CO and CH4 account for up to 50% each of the total OH loss, and HO2 and CH3O2 are the predominant forms of peroxy radicals formed (Reactions 3, 4, respectively). 

Saylor, R. D. An estimate of the potential significance of heterogeneous loss to aerosols as an additional sink for hydroperoxy radicals in the troposphere, Atmos. Environ., 31, 3653-3658,1997. 

Taylor JA, Brasseur GP, Zimmermann PR, Cicerone RJ. 1991. A study of the sources and sinks of methane and methyl chloroform using a global three-dimensional Lagrangian tropospheric tracer transport model. Journal of Geophysical Research 96D 3013-3044. 

FIGURE 14-1 The tropospheric ozone cycle. Reprinted with permission from Corn er al.. Photochemical oxidants Sources, sinks and strategies, J. Air Pbllut. Control Assoc. 25 16-18, 1975. 

Several studies have suggested that NO, thermally decomposes at a sufficient rate to provide a significant sink for NO, in the troposphere (Cantrell et al., 1985 Johnston et al., 1986 Davidson et al., 1990) ... 

Table 8.17 summarizes the rate constants and estimated tropospheric lifetimes of some of these sulfur compounds with respect to reaction with OH or NO-,. The assumed concentrations of these oxidants chosen for the calculations are those characteristic of more remote regions, which are major sources of reduced sulfur compounds such as dimethyl sulfide (DMS). It is seen that OH is expected to be the most important sink for these compounds and that NO, may also be important, for example, for DMS oxidation (see also Chapter 6.J). 

Chebbi, A., and P. Carlier, Carboxylic Acids in the Troposphere, Occurrence, Sources, and Sinks A Review, Atmos. Environ., 30, 4233-4249 (1996). 

The chlorofluorocarbons (CFCs) have very long lifetimes in the troposphere. This is a consequence of the fact that they do not absorb light of wavelengths above 290 nm and do not react at significant rates with 03, OH, or N03. In addition to the lack of chemical sinks, there do not appear to be substantial physical sinks thus they are not very soluble in water and hence are not removed rapidly by rainout. While laboratory studies have shown that some of the CFCs decompose on exposure to visible and near-UV present in the troposphere when the compounds are adsorbed on siliceous materials such as sand (Ausloos et al., 1977 Gab et al., 1977, 1978), the lifetimes for CFC-11 and CFC-12 with respect to these processes have been estimated to be 540 and 1800 years, respectively (National Research Council, 1979). Similarly, an observed thermal decomposition when adsorbed on sand appears to be an insignificant loss process under atmospheric conditions. 

As discussed in other chapters of this book and summarized in Chapter 16, the formation of tropospheric ozone from photochemical reactions of volatile organic compounds (VOC) and oxides of nitrogen (NC/) involves many reactions. Concentrations are therefore quite variable geographically, temporally, and altitudinally. Additional complications come from the fact that there are episodic injections of stratospheric 03 into the troposphere as well as a number of sinks for its removal. Because 03 decomposes thermally, particularly on surfaces, it is not preserved in ice cores. All of these factors make the development of a global climatology for 03 in a manner similar to that for N20 and CH4, for example, much more difficult. In addition, the complexity of the chemistry leading to O, formation from VOC and NOx is such that model-predicted ozone concentrations can vary from model to model (e.g., see Olson et al., 1997). 

In contrast with the typical 1-s lifetime of HO in the cleanest tropospheric air, a significant fraction of the reacting HO can be regenerated via reactions 2 and 3, so the observed decay of HO concentration is prolonged. The importance of the regeneration process depends on the relative concentrations of the various HO radical sinks. One predominant radical sink is N02, which stoichiometrically converts HO to the stable nonradical species HN03 as follows ... 

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. 

In this study we will present aspects of STE in relation with the budget and concentrations of ozone in the troposphere, specifically in the Northern Hemisphere. Firstly, we present ozone observations in the tropopause region from the measurement campaign MOZAIC, and discuss their correlation with potential vorticity. The results have been used to improve the parameterization of stratospheric ozone in a coupled tropospheric chemistry - general circulation model. We will show examples of the performance of the model regarding the simulation of ozone in the tropopause region, and present the simulated seasonality of cross-tropopause ozone transport in relation to other tropospheric ozone sources and sinks. Finally, we will examine and compare the influence of cross-tropopause transports to surface ozone concentrations for simulations with contemporary, pre-industrial, and future emission scenarios. 

The SCIAMACHY utilises near simultaneous limb and nadir measurements of the scattered light in the atmosphere between 240 and 2400 nm to determine the amounts and distributions of tropospheric constituents. The target species and parameters are 03, N02, N20, BrO, CO, H20, S02, CO, C02, CH4, aerosols temperature and pressure. For the long-lived gases such as N20, CH4 and C02 the scientific objective is to measure the small gradients, which define source and sink regions. 

Although the nitric acid molecule is subject to various reactions and to photodissociation, nevertheless it remains, and it becomes the most important of the molecules containing NO (HN04, N205, NO3,. . . ) in the lower stratosphere. However, it cannot accumulate because it crosses the tropopause into the troposphere, where it rapidly disappears because of its solubility in water. Thus, if N20 is the source of the nitrogen oxides in the stratosphere, nitric acid is the sink that prevents their accumulation beyond certain limits. But it is now known that the sequence of reactions (20), (21), and (22) results in a lower concentration of stratospheric ozone than would be possible in a pure oxygen atmosphere. 

Because of the reaction of HC1 with OH, its production in the stratosphere is restricted. It must be remembered that HC1 is the stratospheric sink for chlorine compounds because, after crossing the tropopause and entering the troposphere, it dissolves in water and disappears. 

The atmospheric chemistry of bromine can be regarded as similar to that of chlorine. As far as HF is concerned, it does not react with hydroxyl since it persists, it limits the concentration of the atom F and its oxide FO. Hence, HF is the sink for fluorine in the stratosphere, before it disappears in the troposphere. 

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