Tropospheric chemical composition

Crntzen, PJ., A.C. Delany, J. Greenberg, P. Haagenson, L. Heidt, R. Lueb, W. Pollock, W. Seiler, A. Wartburg, and P. Zimmerman (1985) Tropospheric chemical composition measurements in Brazil during the dry season, J. Atmos. Chem., 2,233. 

Lehrer, E., D. Wagenbach, and U. Platt, Aerosol Chemical Composition during Tropospheric Ozone Depletion at Ny Alesund/Svalbard, Tellus, 49B, 486-495 (1997). 

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. 

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). 

The observational program included getting information about the content and properties of atmospheric aerosol and most substantial optically active MGCs (03, CO, NOx, S02, etc.), but concentrated on aerosol studies to retrieve data on direct and indirect aerosol RF (ARF). The most interesting (and in many respects unexpected) results were connected with detection of a thick aerosol layer in the troposphere (an important feature of aerosol chemical composition consisted in the presence of a considerable black carbon component) and distinct manifestations of the long-range transport of both aerosol and MGCs. 

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. 

Lehrer E., Wagenbach D., and Platt U. (1997) Aerosol chemical composition during tropospheric ozone depletion at Ny Alesund/Svalbard. Tellus 49B, 486-495. 

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