Tropospheric water vapour

Taylor C. B. (1972) The Vertical Distribution of the Isotopic Concentrations of Tropospheric Water Vapour over Continental Europe and their Relationship to Tropospheric Structure. Report INS-R, 107. N. Z. Dep. Sci. Ind. Res., Inst. Nucl. Sci. 

Kley, D. Smit, H.G.J. Vomel, H. Grassl, H. Ramanathan, V. Cmtzen, P.J. Williams, S. Meyweik, J. Oilmans, S.J., 1997 Tropospheric Water-Vapour and Ozone Cross-Sections in a Zonal Plane over the Central Equatorial Pacific Ocean , in Quarterly Journal of the Royal Meteorological Society, 123 2009-2040. 

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 ... 

Therefore, ground-based measurements are restricted to atmospheric windows where the water vapour absorptions are relatively small. For limb space-based atmospheric sounding, this is less of a problem but retrieval of trace gases in the upper troposphere are restricted to a limited number of candidates. For many gases such measurements are optimal for sounding the stratosphere and mesosphere. For the lower stratosphere and upper troposphere, some interesting possibilities exist, but the range of application is more restricted. 

The gases in the air are held in an envelope around the Earth by its gravity. The atmosphere is approximately 100 km thick (Figure 11.1), and about 75% of the mass of the atmosphere is found in the layer nearest the Earth called the troposphere (Figure 11.2). Beyond this layer, the atmosphere reaches into space but becomes extremely thin. Nearly all atmospheric water vapour (or moisture) is found in the troposphere, which also contains the liquid water in the oceans, rivers and lakes. 

Harteck (1954) measured the amount of T in the lower atmosphere before H-bomb tests had added significantly to natural activity, and found 4000 and 3 TU in hydrogen and water vapour respectively. The amount of H2 in the atmosphere has increased in recent years due to industrial production (Schmidt, 1974). Circa 1950 there were about 1.5 x 1011 kg of H as H2, compared with 1.4 x 1015 kg as H20. Thus Harteck s values for natural T correspond to tropospheric inventories of 1.8 g as HT and 13 g as HTO. Because HTO is deposited in rain and by vapour transfer to the sea, its residence time in the troposphere is only about a week, similar to that of 137Cs (Fig. 2.8), and more than 90% of the atmospheric inventory is in the stratosphere. By contrast, the residence time of HT in the troposphere is several years (Mason Ostlund, 1979), and most of the atmospheric inventory of HT is in the troposphere. It is only necessary for a small fraction of naturally produced T to form HT to account for the high specific activity of hydrogen gas. 

Turnover times of 137Cs and HTO in the troposphere and removal rates of natural aerosol particles and water vapour. Journal of Geophysical Research, 78, 7076-86. 

Such replacement reactions would be most important in the troposphere. However, the large relative concentrations of water vapour in this region would tend to shift the equilibrium in the generalised reaction... 

The troposphere contains 87% of the total air mass, while the stratosphere accounts for essentially all the remainder. The higher regions of the atmosphere, including the mesosphere and the thermosphere, do not need to be considered for the purposes of this review. The concentrations of the chief atmospheric constituents, N2(78.1%), O2(20.9%), Ar (0.93%), C02 (0.33%), Ne (0.0018%), He (0.00052%) and CH4 (0.000165%), expressed here by volume, remain remarkably fixed over time water vapour, however, has a variable concentration, typically 1%. 

Atmospheric photochemistry produces a variety of radicals that exert a substantial influence on the ultimate composition of the atmosphere. Probably the most important of these in terms of its reactivity is the hydroxyl radical, OH. The formation of OH is the initiator of radical-chain oxidation. Photolysis of ozone by UV light in the presence of water vapour is the main source of hydroxyl radicals in the troposphere, viz... 

The troposphere receives the thermal energy from the Earth s surface which absorbs the Sun s radiation. Because of the heating of the air by the infrared radiation emitted by the surface, intensive vertical (convective) motions can be generated. This convection transports heat, water vapour and other trace constituents of surface origin to the higher levels of the troposphere. In such an... 

According to the above concept of ozone destruction, the troposphere is an inert medium concerning ozone chemistry. However, as Crutzen (1974) pointed out, there are several possible reaction steps for tropospheric 03. Thus ozone can be removed chemically from the air by transformation processes tabulated in Table 10. One reaction chain starts with the photolysis of 03, which is caused by radiations in the Hartley and Chappuis bands. The excited oxygen atoms, formed by Rl, are partly transformed to ground state atomic oxygen by R4. However, they also react with water vapour to give OH radicals (R5). The sum of reactions 1-5 can be written in the following way ... 

Finally, it should be mentioned that cirrus clouds formed in the upper troposphere can also control the radiation balance of the atmosphere. On occasion these clouds are certainly caused by the growth of condensation trails from highflying aircraft. This problem is important from the point of view of anthropogenic modification of the atmospheric composition since a significant quantity of water vapour (and ice nuclei ) is emitted in aircraft exhaust. Hence it is not surprising that in recent years the quantity of cirrus clouds has increased. It is estimated by experts (see SMIC, 1971) that in day-time the albedo increase caused by these clouds generally exceeds the effects of absorption of infrared radiation by ice crystals. This means that cirrus clouds cool the troposphere in day-time. At night, however, cirrus clouds produce the inverse effect on the tropospheric temperature in the majority of cases. 

In the troposphere, from the standpoint of the microclime and bioclime, it is possible to identify a limit layer or a friction layer, where the friction of the flowing air with the earth s surface is manifested, and which may extend up to 1 to 2 km depending on conditions. Inside the limit layer, a layer adjacent to the earth s surface of several tens of metres may be considered. It is characterized by vertical currents of the momentum, heat and water vapour independent of the height and by diurnal as well as seasonal variations of the temperature. 

W/m are absorbed in the troposphere, mainly by tropospheric ozone and water vapour,... 

Spectroscopic, kinetic, photolytic and mechanistic studies have been carried out on simple peroxy radicals such as HO2, CH3O2, C2H5O2 and CH3C(0)02 using a variety of laboratory techniques. The photo-oxidation studies of selected carbonyl compounds was performed and quantum yields established in order to assess their photolytic lifetime in the troposphere. The product distribution of the ozonolysis of selected alkenes was determined in the presence of water vapour. 

The objective of present research was to provide a better understanding of the chemical processes involved in production and loss of ozone in the troposphere. This was achieved by providing kinetic and mechanistic data for several reactions of peroxy radicals involved in the photo-oxidation of volatile organic compounds (VOC). Additional aims were to determine the product quantum yields in the photolysis of carbonyl compounds, and to investigate the mechanism in the ozonolysis of alkenes, especially in the presence of water vapour. 

The (indirect) photodegradation - reaction with reactive species formed by photochemical processes - has been recognized as the major transformation pathway for chemicals in the troposphere. The electrophilic addition of tropospheric radicals constitutes the principal degradation pathway. The relevant reactive species are hydroxyl radicals (OH ) and ozone (O3) during day time and N03 radicals at night. The hydroxyl radicals result from reactions of oxygen atoms with water vapour, photolysis of HNO2 and reactions of HO2 (a... 

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