Stratospheric trace gases, measurement

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. 

In summary, with the possible exceptions of H20 and O3, trace gas measurements made thus far have exploited these long wavelength ranges exclusively for stratospheric measurements. 

N. D. Sze, M. B. McElroy, S. C. Wofsy, D. Kong, and R. Daesen, Theoretical Models of Stratospheric Chemistry, Perturbations and Trace Gas Measurements, Final report to Manufacturing Chemists Association, Washington, D.C., 1978. 

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. 

In situ measurements of stratospheric reactive trace gas abundances provide an opportunity to test the fundamental photochemical mechanisms (3). The advantage of such measurements is that they are local, so the simultaneous measurements of trace gases place a true constraint on the possible photochemical mechanisms. These measurements are also able to resolve small-scale spatial and temporal structure in the trace constituent fields. The disadvantage of in situ measurements is that they do not capture the global or perhaps even seasonal view of photochemical transformations because they are seldom done frequently enough or in enough places to provide that information. Another disadvantage of in situ measurements is that they must be made from platforms in the stratosphere, and these remote observational outposts have their liabilities. 

The measurements in the midlatitude stratosphere during the last four years have been equally successful because more related species are being measured simultaneously, and these data sets are placing serious constraints on photochemical models (2). This situation is dramatically different from that in the mid-1970s through the mid-1980s, when confirmation that certain trace gas species were present in the stratosphere in approximately the correct abundances was still an issue (6). Analyses of more recent measure-... 

The in situ measurement of several related trace gas species in the stratosphere has for a long time proven to be difficult. Part of the problem stems from the fact that it is difficult to get a larger number of instruments to work well together on the same platform at the same time. This problem has been largely conquered in the 1980s, and a number of multiple instru-... 

These examples illustrate that an increasing number of trace gases must be measured simultaneously if even limited subsets of stratospheric photochemistry and transport are to be understood. The combined uncertainties will also become less of a constraint as simultaneous measurements of trace gas abundances can be compared to values derived from other observed abundances and simple photochemical relationships. As important is the improved measurement of photochemical parameters from laboratory studies as well as the search and study of other mechanisms that may be occurring in the stratosphere. Concerted effort in all of these categories is required to avert future failure in predicting shifts in stratospheric photochemistry, like the Antarctic ozone hole. 

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. 

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. 

Arijs E., Nevejans D., Frederick P. and Ingels J., Stratospheric negative ion composition measurements, ion abundances and related trace gas detection. J. Atmos. Terr. Phys., 44, 681 (1982). 

Arnold F. and Qiu S., Upper stratosphere negative ion composition measurements and inferred trace gas abundances. Planet. Space Sci. , 32, 169 (1984). 

Combined noble gas and trace element measurements on individual stratospheric interplanetary dust particles. Meteorit. Planet. Sci. 3T, 1323-1335. 

Kehm K., Flynn G. J., Sutton S. R., and Hohenberg C. M. (2002) Combined noble gas and trace element measurements on individual stratospheric interplanetary dust particles. Meteorit. Planet. Set 37, 1323-1335. 

Providing the instrumental field of view can be made sufficiently small, measurements tangential to the planetary limb can produce atmospheric profiles with a vertical resolution higher than that achievable from nadir viewing measurements. In addition, the very long path length with a cold space background permits retrieval of trace gas abundances that could not otherwise be accomplished. This approach has been used extensively to determine thermal structure and trace gas distributions in the Earth s stratosphere and mesosphere. 

The Henry s law solubility of trace species such as HNO, HC1, HBr and HOC1 in sulfuric acid solutions is an important issue. Reactions (1) to (4) generate HNO, and its solubility determines whether the product dissolves or is released into the gas phase. As expected from Van t Hoff law, the solubilities of HNO, and HC1 have found to increase with decreasing temperature. In addition, the solubilities for both HC1 and HNO, increase as the concentration of sulfuric acid decreases [49,80]. Both of these factors will work together to predict that the highest solubilities for HC1 and HNO, in stratospheric sulfate aerosols will occur at low temperatures, where the sulfate particles will be most dilute. The measured solubility of HNO, in sulfuric acid is small enough that most of the stratospheric nitric acid will be in the gas phase. Thus the denitrification, which contributes to polar ozone depletion, will not occur on the global sulfate aerosol. 

Our understanding of stratospheric aerosols is still far from being satisfactory. In particular, the gas-to-particle conversion processes involving SO2 oxidation and condensation nuclei formation are still poorly understood. Future research, therefore, should include in situ measurements of aerosols, trace gases and condensation nuclei involved in aerosol formation. Accompanying laboratory studies of relevant chemical processes are also needed. 

Ozone is an essential atmospheric trace substance. This gas plays an important role in the control of the radiation and heat balance of the stratosphere since it absorbs solar radiation with wavelengths shorter than about 0.3 jun. An important consequence of this absorption is that U V radiation lethal to living species does not reach the lower layers of the atmosphere. Because of the importance of atmospheric 03, its study started rather early. Junge (1963) mentions that Dobson and his associates measured total ozone (see later) beginning in the twenties by a European network consisting of six stations. Later, this network became world-wide and even the determination of the vertical distribution of 03 is now a routine measurement. Owing to these studies our knowledge of atmospheric ozone is rather substantial compared to that of other trace constituents. 

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