APPENDIX IV - Infrared Instruments (FTIRs)

Infrared spectroscopy is an analytical technique with a long history in environmental science and chemistry. For very-high-resolution spectroscopy, Fourier Transform Infrared Spectroscopy (FTIR) has supplanted other techniques because of its superior performance. This technique has been widely used in atmospheric chemistry and has been validated by exercises such as the Balloon Intercomparison Campaign (BIC) and by various validation efforts for space-based activities such as the Atmospheric Trace Molecule Spectroscopy (ATMOS) experiment. While the FTIR technique needs no further justification as a primary technique for NDACC, individual FTIR instruments, as well as other infrared instruments such as emission spectrometers, still must be validated.

This description is intended to apply to the determination of vertical column amounts of trace gases, primarily by FTIR spectroscopy. High-resolution spectroscopy also can be used for deriving profiles of trace constituents, but additional validation is required for such retrievals. The NDACC Infrared Working Group (IRWG) presently is investigating the capabilities and requirements for profile retrieval; this appendix will be revised subsequently to reflect these validation requirements.

Independent Evaluation of the Instrument Design and Data Analysis

Prior to a formal intercomparison of the new instrument(s) with existing instruments, the Investigator should supply the IRWG with a detailed technical description of the instrument and its general operating parameters, along with sample spectra indicating its performance. It has been agreed that primary instruments will meet the following minimum conditions:

• Maximum optical path difference: >=250 cm
• Spectral range: 700-4100 cm-1 (minimum)
• Continuous spectral coverage (except for the 6-7 micrometer region) in a small number (less than 8) of spectral (filter) bands
• Ability to record full-resolution spectrum (in one filter band) in less than one minute.

Complementary measurement instruments may not fulfill all of these requirements, particularly resolution, but the Investigator must document their performance. Obviously, high signal-to-noise ratio in the spectra is necessary for the detection of weak absorptions, but no specification is provided.

If the proposed instrument is a commercial instrument of the same type as a previously accepted NDACC instrument, the description can be brief, referring to the accepted instrument and highlighting any differences. The IRWG (or a subcommittee thereof) will determine whether the instrument design meets NDACC requirements.

A description of the Investigator's data analysis method should be supplied, along with the sources of any supporting data such as line parameters, constituent profiles, and physical (temperature, pressure) profiles. The data analysis should be intercompared prior to instrument intercomparison by an exercise such as the Second European Stratospheric Monitoring Stations (ESMOS II) Algorithm Intercomparison, or by applying the analysis technique to data supplied from an accepted instrument and comparing results. Documentation of any difference(s) should be provided along with a description of the suspected causes.

The initial data package should include examples of existing measurement data and the results of previous intercomparisons.

Instrument and Data Analysis Intercomparison

Before a new instrument is accepted as part of the NDACC, a formal blind intercomparison must be performed, following the instrument intercomparison protocol. The new instrument(s) will be evaluated by comparison with one or more previously accepted NDACC instruments (reference instruments). It is highly desirable that multiple reference instruments be employed to reduce uncertainty about the origin of observed differences. It is recognized that the difficulty of moving these large delicate instruments may preclude many opportunities for multiple instrument intercomparisons; hence, these opportunities should be planned carefully by the IRWG to maximize the usefulness of the intercomparisons and to minimize cost. If one or more traveling instruments have been compared successfully with several accepted instruments, the latter can subsequently serve as transfer standards. Provisional acceptance of an instrument may be recommended by the IRWG while awaiting finalization of logistical arrangements for the formal blind intercomparison.

The intercomparison should be conducted at a site as dry and as free from tropospheric pollution as practical. The range of solar zenith angles employed should correspond to observations at the target site(s). Observations should be made on at least five clear days. Spectra should be analyzed for the primary NDACC molecules, i.e., hydrogen chloride (HCl), hydrogen fluoride (HF), nitric acid (HNO3), ozone, nitrous oxide (N2O), and molecular nitrogen (N2). Sufficient observing time should be used to ensure that random noise does not limit the retrievals substantially. Spectra should cover the entire observable spectral range. Measurements by the instrument being evaluated and the reference instrument should be as nearly coincident in time as practical. In the analysis, agreed profiles of temperature, pressure, and the constituent profile to be scaled should be used. Agreed spectral regions ('microwindows') and standard line parameters should be used for the analysis.

The intercomparison should include not only solar spectra, but spectra of standard cells containing known concentrations of gases at low pressure. The conformance of the instrument line shape with its expected value can be determined from these spectra.

After the first day of the intercomparison, quick-look data should be submitted to the referee, who may at his/her discretion, advise the participants of any major problems, thereby preventing time wasted from an unsuccessful intercomparison. Following a brief troubleshooting period based on the referee's advice, the comparison will become blind until its conclusion.

The analysis should provide the derived vertical column amount for each of the target gases from the entire day's spectra and the estimated random and systematic errors in the columns. Any additional derived results, such as the instrument resolution or effective apodization, also should also be documented. Spectra encompassing the microwindows used in the analysis should be provided, along with the residuals from the fits. These results should be submitted to the referee within one month of the completion of the data collection, prior to learning the results from other instruments.

Acceptance Criteria for New Primary and Complementary Instruments

The IRWG or a designated subcommittee will examine the results of the intercomparison and make a recommendation to the NDACC Steering Committee. The recommendation will be based in part on the sensitivity of the instrument (random noise in the retrieved columns), the consistency of measurements between the evaluated and reference instruments, and the instrument performance regarding instrument line shape, zero-level errors, phase errors, and line asymmetry.

The Investigator has primary responsibility for ensuring the quality of data from his/her instrument on a continuing basis, and for submitting the data to the NDACC archive in a timely manner. He/she is also responsible for maintaining up-to-date documentation files describing the instrument and its quality control.

Nevertheless, several formal tests are required periodically to ensure the data quality and intercomparability of data from different sites. It is impractical to bring together all of the FTIR instruments for repetitions ofthe instrument intercomparisons; therefore, several methods are planned for continued data evaluation.

For those species (e.g., ozone) that are measured at the NDACC site by techniques other than FTIR, continuous intercomparison of retrieved columns (and profiles, as applicable) should be performed for the purpose of maintaining confidence in both techniques as implemented at the site. Opportunities for intercomparison with satellite measurements should be used, both for mutual evaluation and for enhancing the scientific output.

One or more mobile instruments which have been validated in intercomparison campaigns will be available for transport from site to site for side-by-side comparisons. Agreement of results from these instruments and the permanent site instruments will serve as evidence for the validity of both measuring systems. In the case of disagreement, further experiments will be needed to determine which one is not performing properly and the origin of the difficulties. For this reason, it is useful for two instruments to travel to a site for comparison with the fixed instrument. Results from all of these intercomparisons must be documented in the NDACC archive. Because of the difficulty and cost of such comparisons, they will be relatively infrequent, perhaps every three to five years at a given site.

Standard cells with known long-term stability (such as the HCl cell used inthe Halogen Occultation Experiment (HALOE) and the similar cells constructed for NDACC) can circulate on a more frequent basis. Careful measurements of the cell spectrum and analysis to determine the column amount provide a useful check on the overall system performance. One or more of these cells should be circulating among the sites all the time; it is anticipated that each site should measure them at least annually. Again, results will be documented in the NDACC archive.

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 indetectable 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 withone of the traveling comparison instruments.

Investigators routinely should analyze the data for the column of gases with known concentration such as carbon dioxide (CO2) and N2 (and molecular oxygen (O2) when possible); these data should be reported along with the trace gas columns. The FTIR columns should be compared regularly with the column amounts determined by other NDACC instruments at the same site where there are common species and the measurements are comparable.

There should be an ongoing exchange of spectra and analysis results among the groups. This will help prevent systematic differences in the analysis methods and provide early detection of any data quality problems which may develop. A careful and defensible way of assessing the random and systematic errors in the retrieved columns that is consistent among the groups must be developed by the IRWG.

It is anticipated that the processes of data collection and analysis will become more and more automated in the future. It is the Investigator's responsibility to ensure that all data archived are examined in such a way that high data quality is maintained and that undetected errors do not enter by the automation process.

Since one of the major goals of the NDACC is the detection of long-term trends, care should be used in any modifications of the instrument or data analysis which may affect the results. Once the regular operation of a primary or complementary instrument has begun, such changes should not be undertaken lightly; consultation with the IRWG is recommended. The primary data (interferograms or spectra) should be retained by the Investigator indefinitely (although not deposited in the NDACC archive), so that improved data-retrieval processes, including improved spectral line parameters, can be applied retrospectively to the earlier data. In such cases,the entire dataset should be reprocessed and archived, along with (at least) reference to earlier versions.