Tropospheric concentration

Direct Measurement of HO, in the Troposphere. Techniques to measure tropospheric concentrations of HO have been reviewed (O Brien Hard, submitted to Advances in Chemistry, 1991) so only a summary will be given here. The most extensively researched technique for [HO ] measurement in the troposphere is based on laser-induced fluorescence (LIF) of HO. This approach has been developed in many configurations directing the laser into the free atmosphere and collecting fluorescence back scatter (LIDAR) (105,106,107) LIF of air sampled at atmospheric pressure... 

The principles behind these and other techniques used to measure a variety of trace gases in the atmosphere, including the criteria pollutants and free radicals such as N03, OH, H02, and ROz, are described in the following sections. In addition, typical tropospheric concentrations in regions from remote to urban areas are given. 

Typical tropospheric concentrations of HN03. Given the difficulties in measuring atmospheric nitric acid,... 

Measurements of HN03 in the marine boundary layer are typically of the order of tens to hundreds of ppt. For example, Heikes et al. (1996) reported average concentrations of 160 ppt, with a range from 30 to 280 ppt. In the middle and upper troposphere, concentrations of 100-400 ppt have been reported (e.g., Singh et al., 1998). 

Typical tropospheric concentrations. Studies carried out in a remote region, at Izana de Tenerife in the Canary Islands, showed average nighttime N03 concentrations in clean air from the mid-Atlantic to be 8 ppt, with a maximum of 20 ppt (Carslaw et al., 1997b), and the concentrations are often below the detection limits (e.g., <3 ppt at Loop Head, Ireland Platt and Janssen, 1995). 

FIGURE 13.4 Fits to monthly mean tropospheric concentrations of some CFCs and other chlorinated organics falling under the Montreal Protocol and its amendments in the Northern Hemisphere... 

Midgley, 1998). It might be expected that the tropospheric concentrations of these CFC alternatives would therefore increase, and this is indeed the case (e.g., see Montzka et al., 1993, 1994, 1996a, 1996b Irion et al., 1994 Schauffler et al., 1995 Simmonds et al., 1998b and Miller et al., 1998). 

FIGURE 13.6 Fits to monthly mean tropospheric concentrations... 

Figure 13.7 shows the effective total tropospheric concentration of chlorine from halocarbons from 1992 to 1996 (Montzka et al., 1996a). The concentration peaked in 1994 at 3.0 ppb, but when methyl chloride (CH-,C1) and other chlorinated organics are taken into account, the peak was likely 3.7 ppb. The total tropospheric chlorine concentration in mid-1995 decreased at a rate of approximately 25 ppt per year, in contrast to increases of 110 ppt per year in 1989 (Montzka et al., 1996a Cunnold et al., 1997). Bromine compounds show the same trend. As a result, the stratospheric levels of chlorine and bromine are expected to peak around the year 2000 (Montzka et al., 1996a World Meteorological Organization, 1995,1999). 

FIGURE 13.7 Effective total tropospheric concentration of chlorine from halocarbons from f992 to f996 in the Northern (------)... 

In short, the trends in the tropospheric concentrations of CFCs, halons, and their substitutes follow trends in their emissions. The effects of the controls imposed by the Montreal Protocol and its subsequent amendments are evident in the trends and have been used to show that the associated impact on ozone destruction is expected to begin about the turn of the century. The following section briefly describes the observed trends in stratospheric ozone. 

It is clear from the data presented in this chapter that the effects of control strategies developed for CFCs and halons are already measurable. Although loss of stratospheric ozone with accompanying increases in ultraviolet radiation in some locations have clearly occurred, the tropospheric concentrations of CFCs are not increasing nearly as fast as in the past. Indeed, the concentrations of CFC-11 and CFC-113 appear to have peaked and have started to decline. The equivalent effective stratospheric chlorine concentrations are predicted to have peaked about 1997 and to return to levels found around 1980 at about the year 2050 (World Meteorological Organization, 1995). The significance of the 1980 level is that these levels resulted in detectable Antarctic ozone depletion. 

While the tropospheric concentrations of CFCs and halons are not increasing as rapidly in the past due to controls outlined in the Montreal Protocol and subsequent amendments, those of the CFC replacements are increasing. However, due to their different structures and reactivities, the ozone depletion potentials associated with these compounds are significantly less that those of the compounds they replace. This truly represents a success story in terms of application of atmospheric chemistry to the development of effective control strategies. 

The rate constants for the reactions of cis- and IransA, 3-dichloropropene with 03 have been measured to be 1.5 X 10 19 and 6.7 X 10 19 cm3 molecule-1 s-1, respectively (Tuazon et al., 1984). Show that the loss of these compounds by reaction with 03 is not important compared to their loss by reaction with OH under typical daytime tropospheric concentrations where 03 is 80 ppb and OH is 5 X 106 cm-3. 

Dimmer CH, McCulloch A, Simmonds PG, Nickless G, Bassford MR, Smythe-Wright D (2001) Tropospheric Concentrations of the Chlorinated Solvents, Tetrachloroethene and Trichloroethene, Measured in the Remote Northern Hemisphere. Atmos Environ 35 1171... 

Rate constants have been measured for the gas-phase reactions of a large number of organic compounds with OH radicals (Atkinson, 1989,1994,1997), N03 radicals (Atkinson, 1991, 1994, 1997), and 03 (Atkinson and Carter, 1984 Atkinson, 1994, 1997). These measured rate constants can be combined with measured or estimated ambient tropospheric concentrations of OH radicals, N03 radicals, and 03 to provide tropospheric lifetimes with respect to the various loss processes (see, for example, Atkinson, 1995). 

Rate constants for the reaction of O, OH, Cl, NO3, and O3 with DMS and DMSO are listed in Table II. With the exception of the rate constants for the reactions of O, OH, NO , and O3 with DMS which are from references (16.18. 29-311. respectively, all other values are either results of this study or of unpublished work from this laboratoiy. The value quoted for OH + DMS is the average of the two latest studies on this reaction (16.181. Apart from the reaction of OH with DMSO which is a factor of 10 faster than the reaction of OH with DMS, DMSO is less reactive towards the other atmospheric species O, Cl, NO3, and O3 than DMS. Of the radicals listed in Table II Cl snows the highest reactivity towards both DMS and DMSO2. No Held measurements of the tropospheric concentration of Cl have been reported. Its tropospheric concentration is almost certainly lower than the typical atmospheric OH concentration of lxl06 molecules cm 3. However, a Cl concentration of 104 molecules cm 3, which may occur in industrial regions, would suffice to make the Cl + DMS reaction a significant atmospheric sink for DMS in addition to... 

TABLE 1. Tropospheric concentrations (parts per trillion by volume) of principal organohalide source gases... 

The measured rate constants, or upper limits thereof, for the gas-phase reactions of OH radicals, N03 radicals and 03 can be combined with the estimated ambient tropospheric concentrations of OH radicals, N03 radicals and 03 to calculate the tropospheric lifetimes of the gas-phase PCBs, PCDDs and PCDFs due to each of these reactions. The lifetime, tx, for reaction with species X is given by tx = (fcx[X]) Ambient concentrations (molecule cm- 3) of OH radicals, a 24 h, seasonal, annual and global tropospheric average of 9.7 x 105 91 N03 radicals, a 12h nighttime average of 5 x 108 92 and 03, a 24h averageof7 x 1011(30x 10 9 mixing ratio)8 7 are used to calculate lifetimes due to these gas-phase reactions. 

Sulfate aerosols are the major form of atmospheric sulfur Robinson and Robbins recommended an average tropospheric concentration of 2 vg m... 

Concentrations of Po were measured in the North Atlantic troposphere. Concentrations of Po were measured in rain water, surface seawater, and the marine microlayer. An excess of Po activity was measured in the aerosol relative to what was expected on the basis of Pb and aerosol residence times. In surface seawaters, deficiencies of Po were observed. The mechanism of Po enrichment in the atmospheric aerosol was attributed to possible enrichments from the organic components of the marine microlayer or air sea exchange of organic polonium species. However, no organic compounds of polonium were actually isolated or characterized. 

Fig. 1. Tropospheric Concentrations of CCljF Measured in Remote Locations during Summer 1979.

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