Troposphere trace gases

Fig. 7-8 Inverse relationship between relative standard deviation of concentration, a /c, and residence time, T, for important trace chemicals in the troposphere. (Modified with permission from C. E. Junge (1974). Residence variability of tropospheric trace gases, Tellus 26, 477-488, Swedish Geophysical Society, Stockholm.)...

Junge, C. E. (1974). Residence variability of tropospheric trace gases. Tellus 26,477-488. 

Walcek, C. J., W. R. Stockwell, and J. S. Chang, Theoretical Estimates of the Dynamic, Radiative, and Chemical Effects of Clouds on Tropospheric Trace Gases, Atmos. Res., 25, 53-69 (1990). 

Harder, J. W., R. O. Jakoubek, and G. H. Mount, Measurement of Tropospheric Trace Gases by Long-Path Differential Absorption Spectroscopy during the f993 OH Photochemistry Experiment, J. Geophys. Res., 102, 6215-6226 (1997a). 

Mount, G. H., and J. W. Harder, The Measurement of Tropospheric Trace Gases at Fritz Peak Observatory, Colorado, by Long-Path Absorption OH and Ancillary Gases, . /. Atmos. Sci., 52, 3342-3353 (1995). 

Stratospheric and Tropospheric Trace Gases. In J.A. Pyle and N.R.P. Harris (Eds.) Polar stratospheric ozone. Proceedings of the 1" European Workshop, 3-5 October 1990, Schliersee, Germany. CEC Air Pollution Research Report 34, CEC, Brussels, pp. 99-102. 

Table 2 Global turnover of tropospheric trace gases and the fraction removed by reaction with OH according to Ehhalt (1999). The mean global OH concentration was taken as 1X10 cm 3 (Prinn et al., 1995) (1 Tg = lO g).

Mount G. H. and Harder J. W. (1995) The measurement of tropospheric trace gases at Fritz Peak Observatory by long-path absorption OH and ancillary gases. J. Atmos. Sci. 52, 3342-3353. 

R. Zellner, J. Hagele, A double-beam UV-laser differential absorption method for monitoring tropospheric trace gases. Opt. Laser Technol. 17, 79 (1985)... 

In this lecture the scientific requirements for stratospheric and tropospheric remote sensing are discussed and recent developments are reviewed. The development of atmospheric trace constituent remote sounding is discussed. An emphasis is placed on stratospheric and tropospheric trace gas measurements from satellites orbiting the earth. The current and next generation of instrumentation to make global measurements is then discussed. 

Another important tropospheric trace gas which should be detectable via ion composition measurements is ammonia. It reacts, as already mentioned, with positive ions yielding NH4 cores. So far, however, such cores could not be detected in the upper troposphere which sets an upper limit to the ammonia vapour abundance (mole fraction) of only about 2 10 around the tropopause [29]. 

Now a possible role of free ions in tropospheric trace gas and aerosol processes will be discussed. 

Concerning a possible role of ions in tropospheric trace gas processes little can be said at present. Besides ion-molecule reactions, ion-ion recombination and ion-catalyzed reactions may in this respect be important. The latter may also include quasi liquid phase reactions occurring in relatively large cluster ions or polyions. Maximum rates for trace gas destruction and formation are similar to those estimated for the stratosphere (see section on Potential Role of Ions in Stratospheric Trace Gas and Aerosol Processes). 

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... 

Trace-gas Lifetimes. The time scales for tropospheric chemical reactivity depend upon the hydroxyl radical concentration [HO ] and upon the rate of the HO/trace gas reaction, which generally represents the slowest or rate-determining chemical step in the removal of an individual, insoluble, molecular species. These rates are determined by the rate constant, e,g. k2s for the fundamental reaction with HO, a quantity that in general must be determined experimentally. The average lifetime of a trace gas T removed solely by its reaction with HO,... 

P. Matuska, R. Schmitt, D. Mihelcic, P. Muesgen, H.-W. Paetz, M. Schultz, and A. Volz-Thomas, Trace Gas Measurements during the Oxidizing Capacity of the Tropospheric Atmosphere Campaign 1993 at Izana, J. Geophys. Res., 103, 13505-13518 (1998). 

In the global troposphere, the role of HO has been to maintain the low and relatively constant trace gas composition that apparently has persisted for at least 10,000 years (2). More recently, however, measured increases in tropospheric methane (3-9) have led to concern about humanity s possible influence on the natural abundance of HO (10). 

Keywords satellite - remote sensing - troposphere - stratosphere - trace gas -atmospheric chemistry... 

Though atmospheric composition is dominated by both oxygen and nitrogen, it is not the amount of oxygen that defines the capacity of the troposphere to oxidise a trace gas. The oxidising capacity of the troposphere is a somewhat nebulous term probably best described by Thompson. 

There is a variety of processes that act to remove a trace gas from either the troposphere or the stratosphere. For oxygenated VOCs the main tropospheric loss occurs via gas phase oxidation reactions involving OH, O3, NO and Cl radicals, and photolysis. However, the hydroxyl radical is the most important oxidizing species in the global troposphere [16-19]. As a result of its role in initiating the majority of oxidation reaction chains, the OH radical is the primary cleansing agent for the lower atmosphere and has been called the "tropospheric vacuum cleaner" [20]. The dominant production cycle for tropospheric OH involves the reaction of 0( D), produced from the photolysis of O3, with H2O ... 

The nature of tropospheric ions and their possible role in trace gas and aerosol processes is, as already mentioned, largely unknown. Building on recent progress in our understanding of stratospheric ion processes and first in situ ion composition measurements which were recently made in the upper troposphere, an assessment of tropospheric ion chemistry will be attempted in the following section. 

Other potential reactant trace gases besides H2O and EhS04 are NIL,. CILCN, and acids such as HNO3 and HCl. Ammonia, for example, is highly soluble in water and therefore may become depleted from the gas phase. According to in situ measurements, tropospheric ammonia vapor abundances greatly exceed the critical reactant trace gas level. Consequently, NHj may markedly influence the positive ion chemistry. The same may be true for CILCN which seems to originate from the troposphere. 

As in the stratosphere, in situ ion composition measurements should offer an enormous potential for neutral trace gas detection. Likely candidates for PACIMS trace gas detection are the reactant trace gases discussed above. Diagnostic applications for probing aerosol properties also seem promising. Since tropospheric cluster ions are relatively large, they should resemble aerosol solution droplets. 

Several authors have been able to show experimentally that natural air containing trace amounts of SO2, exposed to sunlight, is photochemical-ly oxidized and that the rate of SO2 oxidation was a complex function of the trace gas composition of the air mixture (NO, NO2 - hydrocarbons level and composition). The oxidation of SO2 within the natural troposphere is therefore expected to occur largely through reactions 7,8 and 9 of table I. The main mechanism is through radicals which might be formed in various ways such as from ozone photodissociation and from the reaction ... 

Methane is an important atmospheric trace gas which affects the chemistry of the troposphere [10] and of the stratosphere [11]. It is... 

The key species in tropospheric chemistry is believed to be the free radical species, OH. It is this species that is hypothesized to initiate the oxidation of many of the reduced compounds emitted into the atmosphere and thus has a major role in controlling the trace gas composition of the atmosphere. While photochemical models predict OH to be present in the lower atmosphere, this prediction has yet to be confirmed by direct atmospheric measurements. A major goal of any research program in global tropospheric chemistry should be a test of photochemical theory for OH. This will require not only measurements of OH levels in the atmosphere but also simultaneous measurements of the concentrations of the species which control OH. Concurrently with the test of the OH photochemical theory, more detailed studies of tropospheric chemical transformation and reactions should be prepared. 

In contrast to Venus, SO2 is only a trace gas in Earth s atmosphere and it is generally less abundant than OCS and other reduced sulfur gases in Earth s troposphere. The average SO2... 

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