Carbon compounds, tropospheric

Most studies of the chemical composition of particles in the troposphere to date have used analysis of bulk samples, which are usually collected in the boundary layer close to the earth s surface. As discussed in Chapter 6. J.3, there is a great deal of interest in the chemistry of the upper troposphere. Much less is known about the chemical composition in this region, particularly of particles. However, it appears that organics are also important constituents of particles in this region as well. For example, Novakov et al. (1997) in studies of particles both onshore and offshore of the eastern United States found that the mass fraction of the particles due to carbon compounds increased as a function of altitude. In the boundary layer, the fraction was typically 10-40%, increasing to 50-90%atan altitude of 2-3 km. 

Analysis of aerosol samples obtained at several locations in Western Europe has shown that about 60% of the content of organic carbon in tropospheric aerosol is the share of water-soluble organic compounds. According to observational data, at a rural location in Austria, mono- and dicarboxylic acids constitute about 11 % (with respect to the total content of organic carbon in cloud water). While insoluble organic compounds hamper the assimilation of water by aerosol, soluble organic matter, as a rule, favors water assimilation. 

Many of HCFC or HFC alternatives to CFCs are two-carbon compounds, and are therefore haloethanes of general formula CX3CX2H, where each X may be any of H, F or Cl. These compounds should be susceptible to tropospheric degradation by hydroxyl-mediated abstraction of the hydrogen atom which each one contains, and should not reach the stratosphere. This strategy is effective if the tropospheric lifetime of the HFC or HCFC is short relative to its rate of transport into the stratosphere. Obviously it is important to consider whether tropospheric oxidation of these compounds may lead to the production of any other halogenated species which might themselves be transported into the stratosphere. 

The tropospheric chemistry of carbon compounds is summarized in Figure 5. CH4 is the dominant input of reactive carbon in the atmosphere, and the main action of atmospheric chemistry is to oxidize it to C02. Atmospheric H2C=0 and CO are produced along the way. Combustion is only a small source of CO, and the total CH contribution to atmospheric carbon is less than 1% of the atmospheric carbon exchanged by the C02 cycle. The contribution of reactive hydrocarbons such as terpenes is not clear, although they may provide a significant source of CO and... 

The tropospheric chemistry of reactive carbon compounds has the anaerobic production of CH as its major source. The OH reactions then convert CH by a series of reactions to CO2 and produce atmospheric H2C=0, CO, and H2 in the process. Atmospheric chemistry is the major source and sink for these three species, as... 

Air pollution (qv) problems are characteri2ed by their scale and the types of pollutants involved. Pollutants are classified as being either primary, that is emitted direcdy, or secondary, ie, formed in the atmosphere through chemical or physical processes. Examples of primary pollutants are carbon monoxide [630-08-0] (qv), CO, lead [7439-92-1] (qv), Pb, chlorofluorocarbons, and many toxic compounds. Notable secondary pollutants include o2one [10028-15-6] (qv), O, which is formed in the troposphere by reactions of nitrogen oxides (NO ) and reactive organic gases (ROG), and sulfuric and nitric acids. 

The chemical transformations occurring in the atmosphere are best characterized as oxidation processes. Reactions involving compounds of carbon (C), nitrogen (N), and sulfur (S) are of most interest. The chemical processes in the troposphere involve oxidation of hydrocarbons, NO, and SO2 to... 

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... 

Trees and shrubs contain a group of fragrant compounds called terpenes. The simplest terpene is isoprene. All other terpenes are built around carbon skeletons constructed from one or more isoprene units. Plants emit terpenes into the atmosphere, as anyone who has walked in a pine or eucalyptus forest will have noticed. The possible effect of terpenes on the concentration of ozone in the troposphere has been the subject of much debate and has led to careful measurements of rates of reaction with ozone. 

Atmospheric particles in the troposphere are composed of a complex mixture of highly water-soluble inorganic salts, insoluble mineral dust, and carbonaceous material (which includes organic compounds plus elemental carbon) (Jacobson et al., 2000). Studies in which the chemical composition has been determined as a function of particle size demonstrate a correlation between the chemical composition and the size mode of atmospheric aerosols (Meszaros et al., 1997 Krivacsy and Molnar, 1998 Alves et al.,2000 Maenhaut et al.,2002 Smolik et al., 2003 Samara andVoutsa, 2005). 

Biogenic Sulfur Emissions from the Ocean. The ocean is a source of many reduced sulfur compounds to the atmosphere. These include dimethylsulfide (DMS) (2.4.51. carbon disulfide (CS2) (28). hydrogen sulfide (H2S) (291. carbonyl sulfide (OCS) (30.311. and methyl mercaptan (CH3SH) ( ). The oxidation of DMS leads to sulfate formation. CS2 and OCS are relatively unreactive in the troposphere and are transported to the stratosphere where they undergo photochemical oxidation (22). Marine H2S and CH3SH probably contribute to sulfate formation over the remote oceans, yet the sea-air transfer of these compounds is only a few percent that of DMS (2). 

Although only 10% of atmospheric ozone resides in the troposphere (0-15 km altitude) it has a profound impact on tropospheric chemistry. Ozone concentrations in the troposphere vary from typically 20-40 ppb for a remote pristine site to 100-200 ppb in a highly polluted urban environment. Ozone is a reactive molecule, which readily adds to carbon-carbon double bonds [8]. Reaction with ozone provides an important removal mechanism for many unsaturated reactive organic compounds. 

The tropospheric sulfur chemistry is different. Unlike the nitrogen and carbon chemistry, where combustion is an insignificant source, the combustion source of SO2 appears to be very important. While OH reactions can be shown to convert sulfides to SO2, it is not clear that normal atmospheric chemistry is important in the next step—the conversion of S02 to H2SO, which is then removed from the atmosphere by rainout. It has also been suggested that a large amount of SO2 is removed directly by rainout. Unfortunately we have the fewest data, both kinetic and atmospheric, on sulfur compounds. Most of the kinetic data we do have are at high temperatures, and most of the atmospheric data are for polluted environments. 

There are over 70 alcohols in the atmosphere as a result of biogenic and anthropogenic emissions [67]. For example methanol and ethanol [68-70] have been used as fuels additives to reduce automobile emissions of carbon monoxide and hydrocarbons [71], in particular ethanol has been used in Brazil as a fuel for over 20 years [72]. 1-Propanol is widely used as a solvent in the manufacturing of different electronic components. The high volatility of these compounds causes their relative abundance in the troposphere and makes it relevant to determine their degradation pathways. During daytime the major loss process for alcohols is their reaction with OH radicals [68]. Accordingly, several experimental [69,70,73-84] and theoretical [85-88] kinetic studies of alcohols -F OH reactions have been performed. 

---