1.3.5 Clx Chemistry

Since the development of chlorofluorocarbons (CFCs) in the 1930s, chlorine has played an increasingly important role in determining the ozone concentrations in the stratosphere. Chorine was present in the stratosphere prior to the introduction of CFCs due to the breakdown of methyl chloride emitted from the oceans (Solomon & Brasseur [1985], IPCC [1995]) but the levels are now many times this natural background (Peter [1994], WMO [1994]).

CFCs were originally developed as a refrigerant, and believed to be inert and harmless; indeed work in the early seventies (Lovelock et al. [1973]) showed that they were not being broken down in the troposphere. This led to the conclusion that CFCs could only be destroyed in the upper stratosphere (Molina & Rowland [1974], Stolarski & Cicerone [1974], Rowland & Molina [1975]).

It should be noted that when CFCs are photolysed much of the fluorine contained therein does not contribute to ozone depletion, or participate in the chemistry of the stratosphere in any other way, but remains in the form of hydrogen fluoride. Since this has the same source as the chlorine, hydrogen fluoride may be used as a tracer to differentiate between dynamical and chemical effects.

Methyl chloride and anthropogeninc chlorofluorocarbons, such as CFC-12 (CF2Cl2), are broken down by photolysis in the stratosphere (Brasseur et al. [1999]):

CH3Cl + hn ® CH3 + Cl (1.22)

CF2Cl2 + hn ® CF2Cl + Cl (1.23)

Methyl chloride can also react with the hydroxyl radical :

CH3Cl + OH ® CH2Cl + H2O (1.24)

which, after a series of reactions, yields active chlorine by photolysis of HCOCl:

HCOCl + hn ® HCO + Cl (1.25)

The main catalytic cycle involving chlorine in ozone destruction in the upper stratosphere is:

1) Cl + O3 ® ClO + O2 (1.26)
ClO + O ® Cl + O2 (1.27)
Net: O + O3 ® 2O2

Other cycles are also possible in the presence of NOx or HOx species:

2) Cl + O3 ® ClO + O2 (1.26)
OH + O3 ® HO2 + O2 (1.14)
ClO + HO2 ® HOCl + O2 (1.28)
HOCl + hn ® HO + Cl (1.29)
Net: 2O3 ® 3O2

3) Cl + O3 ® ClO + O2 (1.26)
NO + O3 ® NO2 + O2 (1.18)
ClO + NO2 + M ® ClONO2 + M (1.30)
ClONO2 + hn ® Cl + NO3 (1.31)
NO3 + hn ® NO + O2 (1.32)
Net: 2O3 ® 3O2

4) Cl + O3 ® ClO + O2 (1.26)
ClO + NO ® Cl + NO2 (1.33)
NO2 + O ® NO + O2 (1.19)
Net: O + O3 ® 2O2

Cycle 1) is the main depletion mechanism involving chlorine above about 20 km; cycles 2) and 3) become important at lower altitudes but cycle 4) is not a major source of ozone loss.

1.3.6 Reservoir Species

It should be noted that the catalytic species can be temporarily prevented from participating in the ozone depleting cycles by their conversion to the relatively stable reservoir species nitric acid, hydrogen chloride and chlorine nitrate:

NO2 + OH + M ® HNO3 + M (1.34)

Cl + HO2 ® HCl + O2 (1.35)

ClO + NO2 + M ® ClONO2 + M (1.30)

These species contribute greatly to the chemical stability of the stratosphere and hence are of importance in the study of the ozone layer.

1.3.7 Chemistry in the Polar Vortex

During the polar night (primarily in the southern hemisphere), within the vortex (section 1.1.3) the reservoir gases mentioned above may break down, releasing chlorine. This activation of chlorine through the breakdown of hydrogen chloride and chlorine nitrate, occurs in early winter on the surface of sulphate aerosol and polar stratospheric cloud particles as follows:

ClONO2 + HCl ® Cl2 + HNO3 (1.36)

ClONO2 + H2O ® HOCl + HNO3 (1.37)

HOCl + HCl ® Cl2 + H2O. (1.38)

In the dark, the active chlorine is in the form of HOCl, Cl2 and ClO and its dimer, but when the sun returns in the spring the proportion of chlorine monoxide rises due to the photolysis of the chlorine reservoirs, and ozone is rapidly destroyed by the large amounts of reactive chlorine released in the polar night:

1) 2(Cl + O3 ® ClO + O2) (1.26)
ClO + ClO + M ® Cl2O2 + M (1.39)
Cl2O2 + hn ® Cl + ClOO (1.40)
ClOO + M ® Cl + O2 + M (1.41)
Net: 2O3 ® 3O2

As the pole warms and the vortex breaks up and mixes with mid latitude air, the active chlorine returns to the reservoirs.

As stated above, hydrogen chloride and hydrogen fluoride are dynamically equivalent and so it follows that a loss of HCl and chlorine nitrate relative to HF indicates an increase in the proportion of active chlorine. Since many of the processes concerned with the ozone depletion are dependent on height it the processes concerned with the ozone depletion are dependent on height it is important to our understanding of ozone depletion that vertical profiles be retrieved where possible.

1.4 Use of Fourier Spectrometry for Stratospheric Monitoring

Although there is little of interest that cannot be measured by other means, Fourier spectrometry offers significant benefits in that it can measure a greater range of important species from the ground than others. Instruments permanently deployed as part of Network for the Detection of Stratospheric Change (NDSC) routinely monitor the levels of hydrogen chloride, hydrogen fluoride, nitric acid, chlorine nitrate, ozone, nitrous oxide, methane, CFC-12 (CF2Cl2) and CFC-22 (CF2HCl). In measurement campaigns it is usual to rely on ground based Fourier spectrometry to provide data on the first four of these.

Ground based FTIR spectrometry also has the capability to provide measurements with high temporal and spatial resolution which, combined with the ability to detect both the main chlorine reservoir species and hydrogen fluoride makes the system well suited to the detection of relatively small scale features such as filaments of the polar vortex (Bell et al. [1998]).

The fact that such a wide range of compounds can be measured with one instrument, and the established nature of the technique also make it a useful tool for validation of satellite instruments and numerical models.

1.4.1 Importance of Height Resolution

In the main, FTIR measurements have been used for the derivation of total column amounts but this is not a straightforward process since it is relative, rather than absolute absorption that is measured. The process of column retrieval relies on the use of an assumed vertical profile in a forward model to simulate the observed spectrum. Errors will be introduced at this point since the true profile is not known and is likely to differ from the assumed one; it follows that any improvement in the quality of vertical profiles retrieved from FTIR spectra will lead to an improvement in the accuracy of the column totals, in addition to being of value in itself.

In addition to the above, the retrieval of more reliable profiles would further facilitate the validation of space-borne instruments and could be used to further constrain numerical models of the atmosphere.

1.4.2 Vertical Profile Retrieval

Although vertical profiles have been retrieved from FTIR spectra for some time (Marché [1983] and references therein) the vertical resolution currently available is only of the order of a pressure scale height (Mellqvist et al., [1997], Connor et al., [1996], Liu et al., [1996]) for hydrogen chloride. Most methods implement Rodgers’ optimal estimation method (Rodgers [1976]) although Mellqvist et al., employ the Chahine-Twomey algorithm (Twomey [1977]) in their HCl retrieval scheme.

1.4.3 Measurement of Instrument Response

The property of atmospheric spectra that permits the retrieval of vertical profile information is Lorentz, or pressure, broadening.

The shape of an absorption line varies according to the pressure of the gas (section 4.1), and the depth according to its quantity, and so it follows that the shape and depth of an atmospheric absorption line will contain information about the vertical distribution of that gas along the line of sight of the measuring instrument. If this information is to be retrieved it is of crucial importance that any factors affecting the shape of the spectral line be accurately known; the main factor in this regard is the instrument line shape.

Although the mathematics of the instrument line shape is well understood in the case of a well-aligned instrument, and is presented in standard texts (e.g. Griffiths & de Haseth [1986], Chamberlain [1979]), there is less understanding of the line shapes of misaligned instruments, and relatively little work has been reported on the measurement of the response of high resolution instruments for atmospheric observation, although this is acknowledged to be a problem (Morris et al., [1996], Park [1983], Pougachev et al., [1995]).

A monitoring system does exist under the auspices of the NDSC based on the analysis of spectra of standard low pressure gas cells containing nitrous oxide. While these may provide a check on overall system performance, they are of limited use in assessing the instrument line shape, since a stated requirement for all primary instruments is that they have an optical path difference in excess of 250 cm. This sets the theoretical maximum resolution of such instruments at better than 0.0027 cm-1 compared with the the Doppler width of nitrous oxide of 0.003 cm-1 (section 4.1). Another problem with the NDSC cell system is that since nitrous oxide is present in the atmosphere in significant quantities, it cannot be used to assess the line shape during atmospheric measurements and so any changes in the response of the instrument that occurs between the use of the cell and the recording of the atmospheric spectra will go undetected.

Attempts were made at the National Physical Laboratory to measure the line shape of their Bruker 120M in November 1994 using a single mode, stabilised carbon dioxide laser (NPL internal report) but these were not successful. The output from the laser was passed through a X10 beam-expanding telescope followed by spatial filters, intended to flatten the beam, and attenuators before being sent to the spectrometer. A number of the resulting spectra showed a sinc function with a full width at half maximum (FWHM) of 0.0024 cm-1. Since this value exceeds the maximum resolution of the Bruker 120M (0.0026 cm-1 2) doubt has been cast on their validity. Subsequent comparison of the measured spectra with simulations showed that the laser beam was too highly collimated and hence was under-filling the field stop. This situation is obviously unacceptable as the aperture is one of the main factors determining the instrument line shape (section 3.4.3) and hence a significant potential source of error (section 4.4). The report also notes that “the instrument line shape was found to vary considerably from one scan to the next”, emphasising the importance of developing a system whereby the line shape can be accurately measured simultaneously with atmospheric measurements.

There have been more recent, unpublished, reports of attempts to measure line shape using single and multimode helium:neon laser systems but these are believed to have been unsuccessful to date (Bell (private communication), Blumenstock (private communication)).

1.5 Outline of Thesis

In view of the above, the aim of the work described in this thesis is threefold:

-to quantify the errors in retrieved vertical profiles that may result from common deviations of the instrument line shape from the theoretical;

- to develop a portable means of measuring the instrument line shape of Fourier spectrometers simultaneously with making atmospheric measurements, for routine quality assurance and to reduce the errors on retrieved vertical profiles;

- to use the above as part of the National Physical Laboratory's programme of instrument intercomparisons for the validation of FTIR measurements at NDSC monitoring sites.

The importance of the second and third points is emphasised by the NDSC Steering Committee [1998] who state, under the heading of “Quality Criteria for the Evaluation of Continuing Primary and Complementary Instruments and Instrument Teams” that:

“Each site should have one or more cells containing known amounts of gases at low pressure for routine evaluation of the instrument performance, especially the instrument line shape. The gases in question should (if possible) be linear molecules (for well-separated lines), heavy (for narrow Doppler widths), easy to handle (for convenience), and not present in detectable quantity in the atmosphere (so the cell can be used in the direct solar beam to evaluate the performance during actual data collection). This test should be performed approximately weekly, and the results included in the archive. Provision for measuring the temperature of the gas in the cell during the operation should be available.

“If possible, monochromatic laser sources should be used to evaluate the instrument line shape. A suitable laser source perhaps could circulate with one of the travelling comparison instruments.”

The results of this work shall then be discussed and recommendations made for future developments.

[...chapter 2]

Summary
Acknowlegements
Contents
Chapters: 1, 2, 3, 4, 5, 6, 7, 8
Appendices
References

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