Water vapor, tropospheric

About 51 percent of solar energy incident at the top of the atmosphere reaches Earth s surface. Energetic solar ultraviolet radiation affects the chemistry of the atmosphere, especially the stratosphere where, through a series of photochemical reactions, it is responsible for the creation of ozone (O,). Ozone in the stratosphere absorbs most of the short-wave solar ultraviolet (UV) radiation, and some long-wave infrared radiation. Water vapor and carbon dioxide in the troposphere also absorb infrared radiation. 

Water vapor is the most abundant of the greenhouse gases and is the dominant contributor to the natural greenhouse effect. About 99 percent of all the moisture in the atmosphere is found in the troposphere, which extends about 1(1 to 16 kilometers above sea level. Only about one-tliird of the precipi-... 

Upward diffusion of water vapor through the cold temperatures of the tropopause is very inefficient in fact, the upper limit of cloud formation often occurs at the tropopause. Thus the stratosphere is so dry as to prevent rain formation, and particles and gases have very much longer residence times there than in the troposphere. Stratospheric removal requires diffusion back through the tropopause, which then may be followed by precipitation scavenging. 

The turnover time of water vapor in the atmosphere obviously is a function of latitude and altitude. In the equatorial regions, its turnover time in the atmosphere is a few days, while water in the stratosphere has a turnover time of one year or more. Table 7-1 Qunge, 1963) provides an estimate of the average residence time for water vapor for various latitude ranges in the troposphere. Given this simple picture of vertical structure, motion, transport, and diffusion, we can proceed to examine the behavior of... 

Only two possibilities exist for explaining the existence of cloud formation in the atmosphere. If there were no particles to act as cloud condensation nuclei (CCN), water would condense into clouds at relative humidities (RH) of around 300%. That is, air can remain supersaturated below 300% with water vapor for long periods of fime. If this were to occur, condensation would occur on surface objects and the hydrologic cycle would be very different from what is observed. Thus, a second possibility must be the case particles are present in the air and act as CCN at much lower RH. These particles must be small enough to have small settling velocity, stay in the air for long periods of time and be lofted to the top of the troposphere by ordinary updrafts of cm/s velocity. Two further possibilities exist - the particles can either be water soluble or insoluble. In order to understand why it is likely that CCN are soluble, we examine the consequences of the effect of curvature on the saturation water pressure of water. 

Since feedbacks may have a large potential for control of albedo and therefore temperature, it seems necessary to highlight them as targets for study and research. Besides the simple example above of cloud area or cloud extent, there are others that can be identified. High-altitude ice clouds, for example, (cirrus) have both an albedo effect and a greenhouse effect. Their occurrence is very sensitive to the amount of water vapor in the upper troposphere and to the thermal structure of the atmosphere. There may also be missing feedbacks. 

The warming climate is likely to induce changes in the hydrological cycle that will lead to further climate change. Increased heating should increase the rate of evaporation and, hence, the amount of water vapor, which is a GHG. The IPCC s Fourth Assessment Report, published in 2007, finds that the average atmospheric water vapor content has increased since at least the 1980s over land and ocean as well as in the upper troposphere. ... 

The most important aspect of 03 photochemistry for the troposphere is the yield and wavelength dependence of ( D) production in reaction (5) since it is a source of hydroxyl free radicals via its reaction with water vapor ... 

Water vapor concentrations have also been used to show that stratospheric air in the midlatitudes cannot all have originated via the tropical pump, i.e., path I in Fig. 12.3. For example, Dessler et al. (1995b) have shown that water vapor concentrations in the lowermost stratosphere at 37.4°N, 122.1°W are higher than expected for an air mass that has passed through the cold tropical tropopause. Their data are consistent with path II, although as they point out, these measurements do not exclude path III, which represents convective transport from the troposphere to the stratosphere at mid and high latitudes. Lelieveld et al. (1997) report aircraft measurements of CO, 03, and HNO-, over western Europe that suggest that tropospheric air can be mixed into the lower stratosphere. 

The importance of the reaction of CF30 with HzO is less certain an upper limit to the rate constant at 298 K is 1 X 10 16 cm3 molecule 1 s 1 (Wallington et al., 1993b Turnipseed et al., 1995). However, because of the relatively large amounts of water vapor in the troposphere, application of this upper limit gives a lifetime for CF30 with respect to reaction with water vapor of > 30 ms at 50% RH. This can be compared to a lifetime with respect to reaction (10) with 100 ppt NO of 7 s. 

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. 

While water is a major component of tropospheric particles, and hence largely determines the surface tension (y), organics found in particles may act as surfactants (see Chapter 9.C.2). In this case, their segregation at the air-water interface could potentially lead to a substantial surface tension lowering of such particles, which would lead to a lower equilibrium water vapor pressure over the droplet (Eq. (BB)) and hence activation at smaller supersaturations. This possibility is discussed in more detail in the next section. 

Table 7.3 lists examples of common mineral aerosol particles in the Earth s troposphere based on estimates by Leinen et al. [64] and Claquin et al. [65]. The surfaces of these particles interact with water vapor and atmospheric gases (e.g., S02 and NOJ, and thus serve as cloud condensation nuclei, which can have dramatic effects on global temperatures [66]. 

Reaction 8.6 produces atomic oxygen in an excited (lD) electronic state, singlet oxygen. This is important in the troposphere too, as 0( D) reacts with water vapor to produce chemically reactive hydroxyl radicals, -OH ... 

At 298 K and atmospheric pressure with 50% relative humidity, about 0.2 HO" are produced per O( D) atom formed. Photolysis of 03 in the presence of water vapor is the major tropospheric source of HO", particularly in the lower troposphere where water vapor mixing ratios are high (for an explanation of the term mixing ratio see below). Other sources of HO" in the troposphere include the photolysis of nitrous acid (HONO), the photolysis of formaldehyde and other carbonyls in the presence of NO, and the dark reactions of 03 with alkanes. Note that all these processes involve quite complicated reaction schemes. For a discussion of these reaction schemes we refer to the literature (e.g., Atkinson, 2000). 

Scientists classify the atmosphere by dividing it into layers, each layer distinct in its characteristics. The lowest layer is the troposphere, which contains 90 percent of the atmospheric mass and essentially all of the atmosphere s water vapor and clouds, as Figure 17.2 shows. This is where weather occurs. 

Troposphere The atmospheric layer closest to Earths surface, containing 90 percent of the atmospheres mass and essentially all water vapor and clouds. 

From Table VIII-2 one can see that this is not the case. The mixing ratios of CO and 02 are only about 0.1% of the amount of C02 that would be produced in only 2 years. Considerable efforts have been devoted to explain this unusual stability of C02 in Mars [see Hunten (491)]. Based on the abundant water vapor and HO (H, HO, H02) in the Martian atmosphere, McElroy et al. (675, 677) present a mechanism involving an HO cycle for the catalytic oxidation of CO to C02 similar to the one proposed for NO oxidation in the troposphere [see Section (VII 1-2.3), p. 333],... 

In the troposphere reaction (2a) is approximately 10 times slower than (2b). However, it is of great importance because excited atomic oxygen (0 D) reaction with water vapor is the major source of hydroxyl radical (OH), the main oxidant in the troposphere. 

The model results obtained in this study suggest that future aircraft emissions will have a significant impact on levels of NOx, stratospheric water vapor, and ozone in both the troposphere and the stratosphere. The effect of future supersonic aircraft depends strongly on the cruising altitude assumed for the supersonic fleet. 

Although the troposphere has the characteristic of containing a high relative concentration of water vapor (10 5-10-2), the stratosphere is dry and the water vapor concentration is only a few parts in a million. However, the oxidation of methane by hydroxyl radical must be intro-... 

The presence of 03 in the troposphere leads to the formation of OH radicals through the photolysis of 03 at wavelengths 290-350 nm to form the electronically excited oxygen atom, 0(JD), which either reacts with water vapor or is deactivated by reaction with 02 and N2 to the ground state oxygen atom, (03P) (Atkinson, 1995 Atkinson et al., 1997). 

Gases, such as water vapor, carbon dioxide, tropospheric ozone, nitrous oxide, methane, and chloroflurocarbons (CFCs), are largely transparent to solar radiation... 

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