Upper troposphere

FIGURE 6.35 Measured OH concentrations at an altitude of 11.8 km near Hawaii and concentrations predicted using simple chemistry (adapted from Wennberg et al., 1998). 

In general, for a compound to reach the upper troposphere in sufficient concentrations to impact the chemistry, it must not react rapidly in the lower troposphere. However, more reactive compounds can be rapidly transported (on the time scale of a few minutes) from the surface to the upper troposphere through convective events (e.g., thunderstorms) (e.g., Gidel, 1983 Chatfield and Crutzen, 1984 Chatfield and Alkezweeny, 1990 Pickering et al., 1992 Wang et al., 1995 Kley et al., 1996 Mahlman, 1997 Kley, 1997 Jaegle et al., 1998a Talbot et al., 1998). As a result, some compounds with relatively short tropospheric lifetimes can be carried into the upper troposphere and act as free radical sources. 

FIGURE 6.36 (a) Measured OH concentrations as a function of altitude and model-predicted concentrations without acetone photolysis and with acetone photolysis, respectively, (b) Calculated rates of HOx production from O, and acetone photolysis, respectively, as a function of altitude. (Adapted from Wennberg et at., 1998.) 

Based on this chemistry, the production rate of 03 is expected to be very sensitive to the NO concentration, increasing with NO (see also Chapter 16 for a discussion of the dependence of 03 generation on NOx). In this context, Folkins et al. (1998) suggest that acetone is likely the major contributor to enhanced ozone production in the upper troposphere, since increased CH3OOH and H202 concentrations at 9- to 12-km altitude were observed only at very small NO concentrations, indicative of clean marine boundary layer air under such low NOx conditions, destruction rather than production of 03 is expected. 

1997 Jaegle et al., 1998b Brunner et al., 1998 Dias-Lalcaca et al., 1998), or by uptake of HN03 in clouds. Another possibility is errors in the kinetics for NOx and NOy reactions in the models. For example, those for the OH + N02 and OH + HN03 reactions have been recently revised and bring the models and measurements into better agreement (see Chapters 7.B.1 and 7.E.2 and Problem 7.9). 

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Hauck G and Arnold F 1984 Improved positive-ion composition measurements in the upper troposphere and lower stratosphere and the detection of acetone Nature 311 547-50... 

Clouds cover roughly two-thirds of our earth s surface and play an important role in influencing global climate by affecting the radiation budget. Cirrus clouds are one example of a cloud type whose optical properties are not accurately known. Cirrus clouds form in the upper troposphere and are composed almost exclusively of non-spherical ice crystal particles. The impact of cloud coverage on dispersion of pollution in the atmosphere is an area of great concern and intensive study. 

Since feedbacks may have a large potential for control of albedo and therefore temperature, it seems necessary to highlight them as targets for study and research. Besides the simple example above of cloud area or cloud extent, there are others that can be identified. High-altitude ice clouds, for example, (cirrus) have both an albedo effect and a greenhouse effect. Their occurrence is very sensitive to the amount of water vapor in the upper troposphere and to the thermal structure of the atmosphere. There may also be missing feedbacks. 

The warming climate is likely to induce changes in the hydrological cycle that will lead to further climate change. Increased heating should increase the rate of evaporation and, hence, the amount of water vapor, which is a GHG. The IPCC s Fourth Assessment Report, published in 2007, finds that the average atmospheric water vapor content has increased since at least the 1980s over land and ocean as well as in the upper troposphere. ... 

As discussed in Chapter 6.J, acetone photochemistry is of interest because this ketone is distributed globally, has both biogenic and anthropogenic sources, and has been proposed to be a significant source of free radicals in the upper troposphere. The absorption cross sections of acetone (as well as other aldehydes and ketones) are temperature dependent at the longer wavelenths, which is important for application to the colder upper troposphere. Figure 4.29, for example, shows the absorption cross sections of acetone at 298 and 261 K, respectively (Hynes et al., 1992 see also Gierczak et al., 1998). 

Photolysis of acetone also contributes significantly to its loss and is the dominant loss process in the upper troposphere (Gierczak et al., 1998 see Section J.3). 

The peroxynitrate CH3C0CH200N02 formed from the N02 reaction thermally decomposes, with a rate constant of 3 s-1 at 700 Torr and 295 K. Sehested et al. (1998b) suggest that its lifetime with respect to thermal decomposition is sufficiently small even at the lower temperatures of the upper troposphere that this species cannot participate in long-range transport of NOx, as is the case for PAN, and that in the upper troposphere, reaction of CH3C0CH200N02 with OH and photolysis will be major fates. 

There has been a great deal of research activity on the effects of subsonic aircraft in the upper troposphere, with respect to impacts both on the chemistry and on the radiation balance through effects on clouds and 03 (e.g., see April 15, May 1, and May 15, 1998, issues of Geophysical Research Letters and the July 27, 1998, issue of Atmospheric Environment). Aircraft emit a variety of pollutants, including NOx, S02, and particles whose concentrations have provided exhaust signatures in some studies (e.g., Schlager et al., 1997 Hofmann et al., 1998). 

There is evidence from laboratory studies that heterogeneous reactions on sulfate particles may be important in the upper troposphere as well. For example, HCHO uptake into sulfuric acid solutions or ternary mixtures of sulfuric and nitric acids and water has been observed in laboratory studies (e.g., Tolbert et al., 1993 Jayne et al., 1996 Iraci and Tolbert, 1997). In sulfuric acid, the effective Henry s law constant at the low... 

These products were observed at room temperature, although their formation at the lower temperatures found in the upper troposphere could not be confirmed (Iraci and Tolbert, 1997). Such reactions may contribute to a conversion of 1IN03 to NOx proposed by Chatfield (1994). 

In short, although relatively little is known about the possibility of heterogeneous chemistry of organics in the upper troposphere, the results of initial laboratory studies suggest that this may be important. 

Folkins et al. (1997) directly measured OH and H02 using an aircraft platform while descending through a plume from biomass burning. The measured concentrations of HOx and the associated calculated rate of production of 03 were much larger than expected based on the known chemistry. They suggested that, as for the clean upper troposphere, there must be an additional source of HOx in biomass plumes. 

Arnold, F., J. Schneider, K. Gollinger, H. Schlager, P. Schulte, D. E. Hagen, P. D. Whitefield, and P. van Velthoven, Observation of Upper Tropospheric Sulfur Dioxide- and Acetone-Pollution Po-... 

Arnold, F., V. Burger, B. Droste-Fanke, F. Grimm, A. Krieger, J. Schneider, and T. Stilp, Acetone in the Upper Troposphere and Lower Stratosphere Impact on Trace Gases and Aerosols, Geophys. Res. Lett., 24, 3017-3020 (1997b). 

Folkins, I R. Chatfield, H. Singh, Y. Chen, and B. Heikes, Ozone Production Efficiencies of Acetone and Peroxides in the Upper Troposphere, Geophys. Res. Lett., 25, 1305-1308 (1998). 

Iraci, L. T and M. A. Tolbert, Heterogeneous Interaction of Formaldehyde with Cold Sulfuric Acid Implications for the Upper Troposphere and Lower Stratosphere, . /. Geophys. Res., 102, 16099-16107 (1997). 

Mahlman, J. D., Dynamics of Transport Processes in the Upper Troposphere, Science, 276, 1079-1083 (1997). 

The 1997 recommendations for the OH + N02 rate constants (DeMore et al., 1997 Atkinson et al., 1997a, 1997b) may be systematically high (e.g., Donahue et al., 1997) at temperatures below 240 K. Thus, recent measurements at temperatures characteristic of the upper troposphere give rate constants that are smaller than the recommendations by 10-30% (Brown et al., 1999a Dransfield et al., 1999). In addition, 02 appears to be only about 70% as efficient a third body as N2 in the termolecular reaction. Using a modified form of the semiempirical equation for the rate constant in the falloff region (Chapter 5, Eq. (C)), which takes into account the variable collision efficiency /3,... 

Given the sticky nature of HN03, it is likely that it is also taken up on other types of particles in the atmosphere, and depending on the nature of the surface, may react further. For example, Tabazadeh etal. (1998) have proposed that uptake on mineral particles and particles from biomass burning could be important in the upper troposphere, leading to reactions such as... 

As discussed in Chapter 6.J.3, the ratio NO /NOy measured in the upper troposphere is larger than predicted by many models. One potential factor is uncertainties in the reaction kinetics involving these species. Consider only the following two reactions ... 

Assume HN03 is in a steady state. Calculate the ratio [N02]/[HN03] at a temperature of 300 K and 1 atm pressure using the DeMore et al. (1997) recommendations and those of Brown et al. (1999a, 1999b). For (a) T = 300 K and P = 1 atm and (b) T = 220 K and P = 150 Torr, characteristic of the lower stratosphere/ upper troposphere, would the revised kinetics be expected to bring the measurements and models into better agreement ... 

Choi, W., and M.-T. Leu, Nitric Acid Uptake and Decomposition on Black Carbon (Soot) Surfaces Its Implications for the Upper Troposphere and Lower Stratosphere, J. Phys. Chem. A, 102, 7618-7630 (1998). 

Sulfate is a ubiquitous component of particles in the troposphere in both polluted urban areas and remote regions such as the Mauna Loa Observatory (e.g., see Johnson and Kumar, 1991). Indeed, sulfates can sometimes be the major component of tropospheric particles. For example, more than 90% of the total particles sampled in the upper troposphere were observed to... 

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