Troposphere table

In this overview and review of tropospheric photochemistry, we will examine a limited set of important homogeneous and heterogeneous photochemical reactions of relevance in the troposphere (Table 1). An expanded array of photochemical reactions is considered viable in the upper atmosphere (e.g., stratosphere) due to exposure to actinic radiation at wavelengths below 290 nm. A brief summary of a limited subset of this array of possible photochemical reactions will be provided in this review. 

Present (see text) 490 -530 — From gradient in the upper troposphere, Table 5-4... 

The gaseous composition of unpolluted tropospheric air is given in Table 2-1. Unpolluted air is a concept, i.e., what the composition of the air would be if humans and their works were not on earth. We will never know the... 

Has the composition of the unpolluted air of the troposphere most probably always been the same as in Tables 2-1 and 2-2 Will Tables 2-1 and 2-2 most probably define unpolluted air in the year 2085 Discuss your answer. 

In addition to reactions with HO, tropospheric organic compounds may be oxidized by ozone (via ozonation of non-aromatic carbon/carbon double bonds, Atkinson 1990) and in some cases by reaction with nitrate radical, described below. Table I gives representative trace-gas removal rates for these three processes. In spite of these competing reactions, HO largely serves as... 

HO oxidation of CO is much faster than the reaction with methane, resulting in a mean CO lifetime of about two months, but considerably slower than reaction with the majority of the nonmethane hydrocarbons. Table I gives representative removal rates for a number of atmospheric organic compounds their atmospheric lifetimes are the reciprocals of these removal rates (see Equation E4, below). The reaction sequence R31, R13, R14, R15 constitutes one of many tropospheric chain reactions that use CO or hydrocarbons as fuel in the production of tropospheric ozone. These four reactions (if not diverted through other pathways) produce the net reaction... 

Table IV. Predicted HO concentrations near the surface of the Northern Hemisphere. Average values obtained from tropospheric models. Taken from Altshuller (147) (see that paper for table references 20 and 24).

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... 

Table 7-2 includes most of the main gaseous constituents of the troposphere with observed concentrations. In addition to gaseous species, the condensed phases of the atmosphere (i.e. aerosol particles and clouds) contain numerous other species. The physical characteristics and transformations of the aerosol state will be discussed later in Section 7.10. The list of major gaseous species can be organized in several different ways. In the table, it is in order of decreasing concentration. We can see that there are five approximate categories based simply on concentration ... 

In addition to the criteria pollutants, a wide variety of trace gaseous and particulate species are present in the polluted troposphere (Finlayson-Pitts and Pitts, 1997). Table 2.9 shows some of these gaseous noncriteria pollutants identified in photochemical air pollution and gives typical concentrations under conditions ranging from those in remote areas to severely polluted urban air (see also Chapter 11). 

Figure 4.34 compares the absorption spectra of the diatomic halogens, F2, Cl2, Br2, and I2. Cl2 is of particular recent interest in the troposphere in that levels up to 150 ppt have been observed in marine areas (Keene et al., 1993 Pszenny et al., 1993 Spicer et al., 1998). Table 4.30 summarizes the absorption cross sections of Cl2, Br2, and BrCl (DeMore et al., 1997 Marie et al., 1994 Hubinger and Nee, 1995). These diatomics all dissociated with a photodissociation quantum yield of 1 (Calvert and Pitts, 1966). 

Finally, alkyl iodides, some of which have natural sources, are of interest for both the troposphere and stratosphere. Figure 4.46 shows the absorption spectra of some simple alkyl iodides and Table 4.42 the absorption cross sections for CH3I (Roehl et al., 1997). 

The first thing that stands out in Table 6.2 is that the OH-CH4 rate constant, 6.2 X 10 15 cm3 molecule 1 s-1, is much smaller than those for the higher alkanes, a factor of 40 below that for ethane. This relatively slow reaction between OH and CH4 is the reason that the focus is on non-methane hydrocarbons (NMHC) in terms of ozone control in urban areas. Thus, even at a typical peak OH concentration of 5 X 106 molecules cm 3, the calculated lifetime of CH4 at 298 K is 373 days, far too long to play a significant role on urban and even regional scales. Clearly, however, this reaction is important in the global troposphere (see Chapter 14.B.2b). 

While these reactions are much slower than the corresponding OH reactions, the nighttime peak concentrations of NO, under some conditions are much larger than those of OH during the day, 400 ppt vs 0.4 ppt. Even given the differences in concentration, however, as seen from the lifetimes in Table 6.1, the nitrate radical reaction is still relatively slow. While the removal of the alkanes by NO, is thus not expected to be very significant under most tropospheric conditions, reaction (20) can contribute to HNO, formation and the removal of NOx from the atmosphere. 

The room temperature rate constants for the reactions of 03 with some alkenes are given in Table 6.9. While the values are many orders of magnitude smaller than those for the corresponding OH reactions, the fact that tropospheric ozone concentrations are so much larger makes these reactions a significant removal process for the alkenes. 

Table 6.10 gives the ranges of observed yields of the stabilized Criegee intermediates at 1 atm pressure in air and at room temperature. Clearly, significant decomposition of the intermediates occurs under typical tropospheric conditions. 

TABLE 6.12 Range of Reported Rate Constants for the Reactions of the Criegee Intermediate with Some Gases" and Associated Lifetimes of the Criegee Intermediate under Polluted Tropospheric Conditions... 

As seen in Table 6.1, the reactions of the nitrate radical with the simple aromatic hydrocarbons are generally too slow to be important in the tropospheric decay of the organic. However, one of the products of the aromatic reactions, the cresols, reacts quite rapidly with NO,. o-Cresol, for example, reacts with N03 with a room temperature rate constant of 1.4 X 10 " cm3 molecule-1 s-1, giving a lifetime for the cresol of only 1 min at 50 ppt N03. This rapid reaction is effectively an overall hydrogen abstraction from the pheno-... 

Table 8.1 shows the values of these constants for some species of tropospheric interest. The most soluble gases have Henry s law constants of approximately 105 M atm-1, whereas the least soluble have values about eight orders of magnitude smaller. 

For complex formation between aldehydes and S(IV) to be important in the troposphere, the aldehydes not only must have high solubility but also be present in air at significant concentrations and form stable adducts with S(IV) at a sufficiently fast rate that it can occur during the lifetime of a typical cloud or fog event. Table 8.4 gives the rate constants /c,4 and kt5 for formation of the S(IV) complexes as well as the stability constants Ku and apparent stability constant K p, defined as... 

Table 8.5 shows the mass accommodation coefficients for S02, as well as for some other gases of tropospheric interest, on liquid water. It is seen that the uptake of most gases into liquid water is quite efficient. Interactions of gas molecules at the air-liquid interface may have additional implications other than the rate at which it is transferred into the aqueous... 

TABLE 8.5 Some Mass Accommodation Coefficients (a) for Gases of Tropospheric Interest on a Liquid Water Surface"... 

While the Henry s law constant for ozone is fairly small (Table 8.1), there is sufficient ozone present in the troposphere globally to dissolve in clouds and fogs, hence presenting the potential for it to act as a S(IV) oxidant. Kinetic and mechanistic studies for the 03-S(IV) reaction in aqueous solutions have been reviewed and evaluated by Hoffmann (1986), who shows that it can be treated in terms of individual reactions of the various forms of S(IV) in solution. That is, S02 H20, HSOJ, and SO2- each react with 03 by unique mechanisms and with unique rate constants, although in all cases the reactions can be considered to be a nucleophilic attack by the sulfur species on 03. 

The aqueous-phase and gas-phase chemistries of HO, are sufficiently closely coupled that the chemistry shown in Tables 8.11 and 8.12 can affect gas-phase concentrations as well. For example, including the aqueous-phase chemistry in models of tropospheric ozone formation alters predicted 03 concentrations, although whether the perturbation is significant is subject to some controversy (e.g., see Lelieveld and Crutzen, 1990 Jonson and Isaksen, 1993 Walcek et al., 1997 Liang and Jacob, 1997). 

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). 

Based on numerous size distributions measured in air, various categories of tropospheric aerosols have been proposed. Table 9.2, for example, shows a typical set of categories and some of their associated characteristics. However, these should be taken merely as examples rather than as fixed categories since many aerosols will display characteristics of more than one category. 

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