Troposphere equilibria

One goal of tropospheric [HO ] or [H02 ] measurements is the generation of data for comparison with model calculations-to test or validate the models. Due to its high reactivity, HO comes into rapid photochemical equilibrium with its surroundings. Thus a test of a photochemical model, which compares measured and calculated HO concentrations, is mainly a test of the chemical mechanism that the model contains, and is relatively independent of... 

As shown in Fig. 3, CHEMGL considers 10 major well-mixed compartments air boundary layer, free troposphere, stratosphere, surface water, surface soil, vadose soil, sediment, ground water zone, plant foliage and plant route. In each compartment, several phases are included, for example, air, water and solids (organic matter, mineral matter). A volume fraction is used to express the ratio of the phase volume to the bulk compartment volume. Furthermore, each compartment is assumed to be a completely mixed box, which means all environmental properties and the chemical concentrations are uniform in a compartment. In addition, the environmental properties are assumed to not change with time. Other assumptions made in the model include continuous emissions to the compartments, equilibrium between different phases within each compartment and first-order irreversible loss rate within each compartment [38]. 

Hydrogen sulfide occurs in natural gas. It also is found in many sewer gases. It is a by-product of many industrial processes. Trace amounts of dissolved H2S are found in wastewaters in equilibrium with dissolved sulfides and hydrosulfides. It also is found in volcanic eruptions, hot springs and in troposphere. The average concentration of H2S in the air is about 0.05 ppb. 

In this section, we use another chain reaction to show the relation between the steady-state treatment and the quasi-equilibrium treatment. The former is more general than the latter, and leads to more complete but also more complicated results. Ozone, O3, is present in the stratosphere as the ozone layer, and in the troposphere as a pollutant. Ozone production and destruction in the atmosphere is primarily controlled by photochemical reactions, which are discussed in a later section. Ozone may also be thermally decomposed into oxygen, O, although... 

To date, there are no direct tropospheric measurements of N2Os at the levels predicted to be in natural or polluted air masses. However, concentrations of n2o5 as high as 10-15 ppb have been calculated for the Los Angeles area using simultaneous measurements of ambient N03 and N02 and the equilibrium constant for reaction (24) (e.g., see Atkinson et al., 1986). 

It is interesting that while N2Os is believed to play a significant role in tropospheric chemistry, it has never been directly measured in the troposphere. However, using the measured concentrations of N02 and N03 and the equilibrium constant for the reaction in which it is formed,... 

Evidence for formation of this adduct has been obtained in laboratory studies between 216 and 258 K, where CH3S is observed to come to equilibrium in the presence of 02 (Turnipseed et al., 1992). A contribution from the back reaction is difficult to avoid in experimental systems, making measurements of the true forward rate constant somewhat uncertain. Extrapolation of the measured kinetics to 298 K suggests that approximately 30-75% of the CH3S would be in the form of the adduct at typical tropospheric temperatures of 298-275 K and 1 atm pressure (Turnipseed et al., 1992). 

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. 

Two examples serve to illustrate these photochemical relationships. One member of the family of atmospheric peroxy radicals is the peroxy acetyl radical, CH3C(0)02. In at least the warm portion of the troposphere, PAN is near thermal equilibrium with the peroxy acetyl radical and NO . The equilibrium constant for this reaction has been measured in laboratory studies. Therefore, if concentrations of both PAN and N02 are measured, the concentration of these radicals can be calculated from the equilibrium constant and the ratio of the two nitrogen species as shown in Figure 2. The... 

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... 

PAN is often used as an unambiguous marker for tropospheric chemistry. The lifetime of PAN in the troposphere is very much dependant on the temperature dependence of the equilibrium in reaction (2.50), the lifetime varying from 30 min sAT— 298 K to 8 h at T = 273 K. At midtroposphere temperature and pressures PAN has thermal decomposition... 

A parameterised approach for the calculation of clear-sky NO2 - photolysis Jno2 rates was used based on the band scheme by Landgraf and Crutzen (1998) in combination with actinic fluxes parameterised following Krol and Van Weele (1997). The diagnostic operator does not reflect the influence of clouds on ji o2 and the adjustments to the equilibrium because of hydro-carbons in the lower troposphere and abundant 0-radical in the higher stratosphere and mesosphere. 

We do not measure water vapor directly in the stratosphere. Again we can possibly infer it from the hydrate ion distribution, but this is more difficult because here you have to rely on a thermodynamic equilibrium distribution, which can be altered due to the electric field. Therefore we feel that this method is not very useful because there exist better methods for measuring water vapor. Therefore I simply cannot argue the question. There are no parallel water vapor measurements. As far as the troposphere is concerned, the dew point is measured on the earth in parallel to the nitric acid measurements. But here, as you have seen, we have only so far obtained four data points and that would be premature to discuss anything. 

An order of magnitude estimate of the OH concentration in the remote troposphere may be obtained by considering OH to be in a photochemical equilibrium established by reactions (R3), (R4), (R5), (R6), (R7) and that no regeneration of OH from HO2 occurs. Under these assumptions. 

Because tropospheric OH is photochemically controlled, a complete understanding of OH requires an understanding of the processes which control the distributions of the species which influence the OH photochemical equilibrium. Of these species the most important are O3, H2O, CH4, CO, and NOx. The levels of atmospheric EhO are largely controlled by the processes of evaporation and condensation and are not discussed here brief discussions of the atmospheric cycles of the other species are presented below. 

The variety of reactions which have been described indicate the large number of chemical species which enter in the troposphere and which may have an impact upon the environment. It is necessary to emphasize that all species are in a dynamic equilibrium for the meteorological turbulence so that in some cases redox and acid-base reactions occur with the formation of stable compounds with high formation constants but nevertheless the reagent species are detectable in the atmosphere. A typical example is the evaluation of the atmospheric aerosol where besides the presence of ammonium sulphates, free ammonia and sulphuric acid may coexist. 

Another important application of the CO2 isotopic measurements is their use as a measure of stratosphere-troposphere mixing. Stratospheric CO2 is mass-independently fractionated due to its coupling with ozone, while tropospheric is mass dependent because of its equilibrium with water. As originally described by Urey (1947), this is a purely mass-dependent process and has been confirmed by laboratory measurements (Thiemens et al., 1995). This feature provides an ideal marker for the two individual atmospheric reservoirs. 

At present in the troposphere the contents of many minor gases are monitoring in concentrations well in excess of what is predicted under equilibrium geochemistry (Table 4). 

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