It is assumed that the majority of carbon monoxide is removed from the atmosphere by these reactions. Seiler (1974) hypothesizes that the yearly CO loss in the troposphere due to [3.5] and [3.7], is (1940-5000) x 106 t yr-1 The corresponding figure fofr the stratosphere is estimated to be 110 x 106 t yr"1 (see Table 7). In contrast, Warneck (1974) speculates that the global atmospheric strength of this sink is much smaller than the Seiler's figure. Finally, according to the calculations of Ehhalt and Schmidt (1978) about (1500-2900) x 106 t C02 is produced yearly from CH4 by reaction steps [3.4], [3.2], [3.5] and [3.7]. On the basis of these data for the schematic representation of the atmospheric pathways of carbon a value of 2800 x 106 t yr "1 expressed in C02 will be accepted (see Fig. 8, p. 46) for this sink term.
This subsection surveys our knowledge of tropospheric non-methane organic compounds. The concentration and budget of organic substances containing no halogen will first be discussed. The gaseous phase in this case will be termed "organic vapours", while the particulate phase will be called "particulate organic carbon". After this short discussion the tropospheric concentration and sources of anthropogenic halocarbons will be mentioned. The role of halocarbons in stratospheric photochemistry is discussed in Subsection 3.4.3.
The importance of organic vapours in the chemistry of the clean atmosphere was stressed in several papers by Went (e.g. Went, 1966) in the sixties. He recognized that natural volatile organic vapours (mostly terpenes) play an important part in the formation of atmospheric aerosol particles. In spite of the importance of these materials in the budget of the particulate matter, "perhaps less is known about the global sources, distribution and fluxes of organic matter than any other major class of chemical substances in the atmosphere" (Duce, 1978). This situation is at least partially explained by the complexity of the problem. Thus, many organic compounds in vapour and particulate form can be found in the atmosphere as a result of different natural and anthropogenic sources as well as complicated atmospheric transformation processes. In the following, the results on organic vapours and particulate organic carbon will be presented as recently reviewed by Duce (1978). For further details the reader is referred to the original paper and to its references.
The concentration of vapour phase organic carbon in surface air over the oceans is 5-10 ng m "3 STP. The corresponding range over non-urban North America was found to be 10-50 n% m"3 STP. It should be noted, however, that much higher continental concentrations were measured in the Amazon jungle of Brazil.
Accepting average concentrations of 8 fig m ~3 STP and 50 fig m ~3 for oceanic and continental areas, respectively, the global tropospheric burden is calculated to be 52 x 106 t if we assume a relatively constant mixing ratio for the tropopause. The source of these organic compounds is primarily plants. Unfortunately the value of the global plant emission is very uncertain; Went (1966) has estimated it at 1000 x 1061 yr ~while Duce (1978) suggests for terpenoid hydrocarbons a value of (10-350) x 106 t yr-1, calculated as carbon. The strength of total non-methane hydrocarbon sources due to human activity is around 65 x 106 t yr-1 in carbon equivalents.
The particulate organic carbon level varies from 0.5 fig m~3 STP (oceanic air) to about 3 fig m~3 STP (clean continental air). The results of atmospheric measurements show that about 80 % of this carbon can be identified on aerosol particles with radii smaller than 0.5/im. Using 0.5 fig m ~3 STP as the average level in oceanic air and 1.5 fig m "3 STP as the mean continental concentration, Duce (1978) has calculated that the burden of particulate organic carbon in the troposphere is 2.2 x 106 t, which is about 2 % of the corresponding value for organic vapours. For his calculation he has assumed that particles with radii larger than 0.5 fim have a constant mixing ratio from the surface up to 1000 m and then the concentration drops to one-third of its surface value from that level to the tropopause. On the other hand Duce has speculated that the mixing ratio of smaller organic particles is essentially constant to the top of the troposphere. This latter assumption was based partly on the particulate organic carbon measurements of Ketseridis et al. (1976) carried out at a mountain station (Jungfraujoch, Switzerland) and partly on the aircraft measurements of Gillette and Blifford (1971). It should be mentioned that these latter authors did not measure the organic compounds in particulate matter. They determined, however, among other things the vertical profile of sulfur particles of non-maritime origin having a size distribution similar to that of organic compounds (see Section 4.4).
An interesting consequence of Duce's speculations is the fact that he could calculate a reasonable residence time for smaller organic aerosol particles (4-7 days) only by assuming that a large quantity of these materials is formed from vapours in the atmosphere by gas-to-particle conversion. This argument leads to an atmospheric source intensity of (80-160) x 106 t yr-1. He has also estimated that about 20 x 1061 of anthropogenic hydrocarbons (expressed in carbon equivalents) are converted annually to aerosol particles in the troposphere. These anthropogenic organic compounds are mostly olefinic hydrocarbons emitted with automotive exhaust gases (see also Subsection 3.6.3).
Duce (1978) found an acceptable residence time (2 days) for larger organic particles when he took into account only primary sources. These primary aerosol particle sources are partly natural (e.g. materials from ocean waters, wind erosion of soil5, biospheric matter, forest wildfires) and partly anthropogenic (mostly from
5 The formation of sea salt and mineral particles is discussed in Subsection 4.2.1.
combustion process). Annually they produce 36 x 1061 of organic carbon particles with radii greater than 0.5 nm. About 40 % of this quantity is due to pollution sources.
We can conclude from this short discussion that our knowledge of atmospheric non-methane organic vapours and particles is rather scanty. Much more research is needed to determine the source strength and atmospheric concentration of these materials. The measurement of the concentration of organic vapours and particles in the air over a wide variety of vegetation as well as the observation of vertical profile of the concentration would be of crucial interest. The more detailed investigation of removal mechanisms of organic vapours is also an important future task.6 This research is indispensable to explain the role of organic compounds in the formation of aerosol particles and ozone in non-urban areas as well as to estimate the importance of non-methane hydrocarbons in the atmospheric cycle of carbon monoxide. Furthermore, it is not excluded that some petroleum hydrocarbons are transported to the ocean through the atmosphere. Clarification of this problem is also of interest for global environmental pollution studies.
Another class of organic compounds in the atmosphere comprises the various halocarbons. It can be demonstrated (Graedel and Allara, 1976) that these substances cannot be produced in the atmosphere by chemical transformations. While the majority of methyl chloride (CH3C1) is of maritime origin, the other halocarbons arise from man's use of these compounds as refrigerants, solvents and propellants. According to McCarthy et al. (1977) the yearly release of anthropogenic fluorocarbon-11 (CC13F) and fluorocarbon-12 (CC12F2) to the atmosphere in 1975 were 0.34 x 106 t and 0.41 x 106 t, respectively.
Measurement of atmospheric halogenated hydrocarbon started rather recently. The early measurements of Lovelock etal.( 1973) show, among other things, that the background concentration of CC13F, observed over the Atlantic ocean from the United Kingdom to the Antarctic and back, depends upon the geographical latitude. At mid-latitudes in the Northern Hemisphere the CC13F level is about 25 % higher than the mean value (0.50 ppb) calculated on the basis of all observations.7 The results of very numerous measurements carried out since about 1970 are reviewed by Jesson et al. (1977) and Graedel and Allara (1976). Graedel and Allara's estimates of mean tropospheric concentrations are tabulated in Table 8. These estimates are based on analyses of air samples carried out before 1975. The data suggest that the level of chlorofluoromethanes is steadily increasing in the troposphere even in the air over the Southern Hemisphere. Thus, over Australia (Fraser and Pearman, 1978a) the CC13F concentration increased at a rate of
6 The particulate organic carbon is removed from the atmosphere by dry and wet deposition (see Chapter 5).
7 More recent data suggest smaller interhemispheric gradient. For example, Rasmussen and his co-workers (see: Fraser and Pearman, 1978b) observed an 11 % decrease in concentration from 30° N to 15° S.
Tropospheric concentration of halocarbons according to Graedel and Allara (1976)
Tropospheric concentration of halocarbons according to Graedel and Allara (1976)
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