Altitude measurement techniques

In short, mass spectrometry is a powerful analytical tool that has been used successfully for a number of years at high altitudes and is now seeing increasing use in the troposphere, including at the earth s surface. A number of different approaches have been developed, including systems that are designed to measure species such as OH, NO, and HNO. They are described in more detail in the sections on measurement techniques for the individual species. 

The most serious possible problem with this measurement technique is the possible loss of OH prior to detection. The primary diagnostic indicator for the presence of contamination is the temperature of the sample air flow after passage through the detection volume. If boundary-layer air has intruded into the detection volume, the sample air temperature will rise above ambient because the walls of the detection module are always warmer than the ambient air temperature. For example, at float altitude the temperature difference is —15 °C. 

Fig. 3-12. Vertical distribution of CIO, HC1, and HF in the stratosphere. Left Filled circles give the averages of eight altitude profiles for CIO measured in 1976-1979 by in situ resonance fluorescence the envelope indicates the range of values (Weinstock etai, 1981) two additional high-mixing-ratio profiles are not included. The open circles are from balloon-borne infrared remote measurements by Waters et al. (1981) and Menzies (1983). Center The envelope encompasses observational data for HC1 obtained by balloon-borne infrared measurement techniques (Farmer et al, 1980 Buijs, 1980 Raper et al., 1977 Eyre and Roscoe, 1977 Williams et al., 1976 Zander, 1981) filled circles represent more recent preliminary data cited...

We would like to mention one further practical application of standard Raman spectroscopy, namely the method of Raman lidar, which is now routinely used to monitor the upper atmosphere for composition (e.g. the presence of water vapour), chemical processes (e.g. the generation or depletion of ozone (O3)), and the determination of temperature profiles at high altitudes. Although absorption and fluorescence lidar systems are also widely used, Raman lidar has the distinct advantage that it is a simultaneous multispecies measurement technique, and that only a single fixed-wavelength laser is required. 

Similarly, Fig. 3.30 shows measurements of i(N02) at an altitude of 7-7.5 km as a function of solar zenith angle compared to a multidirectional model calculation (Volz-Thomas et al., 1996). The agreement in this case is generally good. However, this is not always the case. For example, Fig. 3.31 shows some measurements of 7(N02) as a function of solar zenith angle made by different groups at different locations and using different techniques (Kraus and Hofzumahaus, 1998). 

FIGURE 11.55 Altitude profiles for H02 + R02 in the free troposphere over southern Germany determined by conversion to OH and measuring OH by the mass spectrometric derivatization technique (adapted from Reiner el at., 1998). 

Measurements either from the ground or from satellites have been a major contribution to this effort, and satellite instruments such as LIMS (Limb Infrared Monitor of the Stratosphere) on the Nimbus 7 satellite (I) in 1979 and ATMOS (Atmospheric Trace Molecular Spectroscopy instrument), a Fourier transform infrared spectrometer aboard Spacelab 3 (2) in 1987, have produced valuable data sets that still challenge our models. But these remote techniques are not always adequate for resolving photochemistry on the small scale, particularly in the lower stratosphere. In some cases, the altitude resolution provided by remote techniques has been insufficient to provide unambiguous concentrations of trace gas species at specific altitudes. Insufficient altitude resolution is a handicap particularly for those trace species with large gradients in either altitude or latitude. Often only the most abundant species can be measured. Many of the reactive trace gases, the key species in most chemical transformations, have small abundances that are difficult to detect accurately from remote platforms. 

An example of the use of this technique is the measurement of NO and N02 made near 50°N latitude by Ridley and co-workers. Summertime measurements of NO, N02, 03, temperature, and the photolysis rate of N02 showed that NO and NO were in photochemical steady state (65). However, the abundances of NOx (NO + N02) were observed to be ten times smaller in winter than in summer at altitudes between 20 and 28 km (63). At altitudes above 28 km, the abundances of NOt were similar in both winter and summer. Considering the trajectories of air at different altitudes, they were able to determine that N205 must be the wintertime reservoir species, as was predicted in a number of previous studies. 

Thermal emission spectroscopy can be used in middle- and far-infrared spectral regions to make stratospheric measurements, and it has been applied to a number of important molecules with balloon-borne and satellite-based detection systems. In this approach, the molecules of interest are promoted to excited states through collisions with other molecules. The return to the ground state is accompanied by the release of a photon with energy equal to the difference between the quantum states of the molecule. Therefore, the emission spectrum is characteristic of a given molecule. Calculation of the concentration can be complicated because the emission may have originated from a number of stratospheric altitudes, and this situation may necessitate the use of computer-based inversion techniques (24-27) to retrieve a concentration profile. 

Calculating elevation from climate variables requires measuring an atmospheric quantity that varies with altitude. In other chapters, paleo-pressure are inferred by techniques such as basalt vesicularity (Sahagian and Maus 1994) or measuring cosmogenic nuclide concentrations in exposed rocks. For example, Brown et al. (1991) and Brook et al. (1995) used concentrations of 10Be and 26Al to place constraints on the uplift rate and duration of exposure of rocks in the Transantarctic Mountains, Antarctica. Additional methods and further discussions are available in the present volume. 

A variety of experimental techniques has been applied to the study of 0( D2) deactivation by O2. The total yield of atomic oxygen following (12) followed by (14) has been determined to be in the range 0.5-0.6 (Slanger and Black, 1979 Amimoto and Wiesenfeld, 1980), which suggests that 0.6 < ki2/koj < 1.0. On the basis of these measurements, it has been concluded that these processes may play a role in O3 destruction at high altitudes, reducing its density by as much as 12%. 

Ground-based mm-wave emission techniques have recently provided definitive identification of HO2 in the upper stratosphere, measurements which can be directly compared with previous in situ experiments [13]. Figure 8 presents those recent spectra. These observations can be used both to cross check in situ techniques, for which two have been reported, and to extend measurements of this key free radical above the maximum altitude attainable by balloons, specifically above 45 km. We thus find a critical combination of independent techniques with highly complementary temporal and spatial coverage evolving such that the vertical distribution of a key free radical will soon be established in the scientific literature. 

Most of the ozone molecules in a vertical column are found at altitudes ranging from 10 to 35 km (see Table 5.2). Measurements of the total ozone abundance can be performed from the ground using ultraviolet absorption techniques (see e.g., Dobson, 1963) or from space. Total... 

Meteorological sounding balloons (radiosondes) are used routinely to measure from networks of stations on all continents the vertical distribution of pressure, temperature, wind velocity, and to some extent also relative humidity. Radiosondes reach altitudes of about 30 km. Atmospheric conditions at greater heights have been explored by means of rocket sondes and, in recent years, also by infrared sounding techniques from satellites. 

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