Tropospheric abundance

Montzka, S. A R. C. Myers, J. H. Butler, and J. W. Elkins, Early Trends in the Global Tropospheric Abundance of Hydrochlo-rofluorocarbon-141b and 142b, Geophys. Res. Lett., 21, 2483-2486 (1994). 

Plots of all available data points in relative units are shown in Figs. 9-11. Average reference profiles of the different species were obtained by means of a spline fit procedure applied to the respective data points. Table 1 contains pertinent information for every individual halocarbon on tropospheric abundances. With the help of these, absolute profiles can be calculated for any of the substances shown. 

Figure 9. Average vertical profiles of fully halogenated methanes plotted in raltive units with respect to tropospheric abundances measured between ground surface and 8 km altitude. All available data points are shown along with "average curves" spline fitted to these individual points. All data were measured at 44°N.

If our database obtained so far is considered representative for middle, high and low northern latitudes, latitude-height cross-sections of halocarbons can be derived based on the assumption of smooth meridional mixing on sloped surfaces and zonal symmetry. By applying a spline fit to the data obtained at the 3 discrete latitudes, isolines of 90, 80, 70%, - with respect to tropospheric abundance for CFC-12, CFC-11 and CH3CCI3 are shown in figs. 14-16. 

These isolines derived from out balloon data have been combined with those computed from aircraft data of project AIRSTREAM measured, up to about 20 km, over the American continent, between 10°S and 80°N [26], These aircraft data covering the time span 1973-1983 were averaged in relative units with respect to tropospheric abundances, in the same fashion as described for the balloon data. No seasonal effects were considered. As figs. 14-16 demonstrate, both data sets match fairly well, at least for middle and higher latitudes, yielding complete "hemispheric" distributions of CFC-12, CFC-11 and CH3CCI3. It should be mentioned that no intercalibration of the balloon and aircraft data sets has been carried out so far. 

Montzka S.A., Butler J.H., Meyers R.C., Thompson T.M., Swanson T.H., Clarke A.D., Lock L.T., ElkinsJ.W. (1996a) Decline in the tropospheric abundance of halogen from halocarbons implications for stratospheric ozone depletion. Science 212,1318-22. 

A likely source of active iodine is provided by the photolysis of methyl iodide (CH3f) and perhaps of other iodocarbons. As the atmospheric lifetime of CH3f is relatively short (a few days), the tropospheric abundance of this compound is generally lower than 10 pptv (Moyers and Duce, 1972 Singh et al., 1983 Atlas et al., 1993) although local maxima are found over the productive regions of the ocean (Oram and Penkett, 1994). If iodine atoms are released above the tropopause, they react with ozone to form the iodine monoxide radical... 

Ehhalt, D.H., U Schmidt, R. Zander, P. Demouhn, and C.P. Rinsland, Seasonal cycle and secular trend of the total and tropospheric abundance of ethane above the Jungfraujoch. J Geophys Res 96, 4985, 1991. 

Montzka, S.A., J.H. Butler, R.C. Myers, T.M. Thompson, T.H. Swanson, A.D. Clarke, L.T. Lock, and J.W. Elkins, Decline in the tropospheric abundance of halogen from halocarbons Implications for stratospheric ozone depletion. Science 272, 1318, 1996. 

The oxidation of N02 eventually leads to the formation of nitric acid and aerosol nitrate, which are deposited at the earth surface. The relevant oxidation pathways are indicated in Fig. 9-6. The following discussion deals first with observations of reaction intermediates then with tropospheric abundances of N02, PAN, and HN03/ aerosol nitrate, and finally with the budget of nitrogen oxides and their oxidation products in the troposphere. 

The four giant planets have hydrogen- and helium-rich compositions reminiscent of the Sun, but all of them clearly depart from strict solar composition in that their densities are too high and the few heavier elements whose tropospheric abundances can be measured all show clear evidence of enrichment. For all four giant planets we have spectroscopic compositional data on the few compounds that remain uncondensed in the visible portion of their atmospheres, above their main cloud layers. These include ammonia, methane, phosphine, and germane. For Jupiter, these volatile elements (C, N, S, P and Ge) are enriched relative to their solar abundances by about a factor of five. For Saturn, with no detection of germane, the enhancement of C, N, and P is about a factor of 10. For Uranus and Neptune the methane enrichment factor is at least 60, consonant with their much higher uncompressed densities. 

FIGURE 2.5 Globa] mean CFC-11 (CFC13) and CFC-12 (CF2C12) tropospheric abundance from 1950 to 1998 based on smoothed measurements and emission models (IPCC 2001). The radiative forcing of each CFC is shown on the right axis (see Chapter 23). 

The efficiency with which these gases absorb IR radiation is not equal, as shown by the greenhouse factors in Table 6.2. Some gases are orders of magnitude more efficient at absorbing IR radiation. We are fortunate that the concentration of these efficient absorbers in our atmosphere (the tropospheric abundance) is quite low. Since the seriousness of the problem is related both to the efficiency with which the gas absorbs IR and its concentration, we will focus on GOg and methane, both present in relatively high and increasing concentrations. 

Convert the tropospheric abundance values listed in Table 6.2 into ppm... 

TOS 6.2 ) Relative Efficiencies of IR Absorption by Some Key Greenhouse Gases Compared with Their Tropospheric Abundance ... 

N2O is one of the most important tropospheric "greenhouse" gases, and its transport into the stratosphere is considered to be the dominant source of stratospheric oxides of nitrogen. The tropospheric abundance of N2O has been shown to be increasing by approximately 0.2 % per year in recent times. The anaerobic denitrification/reduction of N03 and the nitrification NH/ in soils have been postulated as important microbial sources of N2O. 

To study the UV absorption spectra as well as the kinetics and products of the self-reactions and reactions with HO2 for simple peroxy radicals derived from tropospherically abundant VOCs (e,g, CH4 and C2H4). 

---