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A new cavity based absorption instrument for detection of water isotopologues in the upper troposphere and lower stratosphere
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Image of FIG. 1.
FIG. 1.

A spectra recorded by the ICOS instrument at with approximately 30 ppmv of water vapor in the cell. The strong absorption features are labeled in the top plot. The insert shows the skewing effect of the ICOS spectrum. The bottom plot is the etalon spectrum, which is used to determine the tuning rate of the laser. The FSR or distance between the peaks of the etalons is .

Image of FIG. 2.
FIG. 2.

Plot a shows the spacing or FSR of the longitudinal cavity modes for a 90 cm cavity in blue. Also shown are the LLW and typical width of an absorption line. The LLW is narrow compared to the cavity mode spacing, resulting in large amplitude fluctuations whenever the frequency is resonant with one of these modes as shown in plot b. If the laser is aligned into the cavity to form a multipass pattern, then the cavity behaves like one that is times longer. For a pass pattern the FSR of the cavity is reduced by 200 (plot c). The LLW is now broad compared to the FSR of the cavity (inset of plot c) but still narrow compared to an absorption line resulting in a smooth absorption feature as shown in plot d.

Image of FIG. 3.
FIG. 3.

Calculated spot pattern for a cavity composed of spherical mirrors with a radius of curvature of 140 cm, cavity length of 90.57 cm, and for light of wavelength . Only the first 30 spots are shown. The light becomes re-entrant after 300 round trips. However, even after 27th pass there is some overlap between spots as shown in the enlarged portion of the figure. Pictured on the right is the spot pattern formed using a visible light. Due to slight astigmatism in the mirrors, the spots form a Lissajous pattern.

Image of FIG. 4.
FIG. 4.

Schematic of the ICOS laser and optics system. The laser (QCL) is collimated by an internal lens and then passes through a beam splitter with a fraction diverted to a Ge-etalon used to reference the frequency scale. The majority of the light is focused via a telescope and steering mirrors into the ICOS optical cavity. The light transmitted through the rear mirror is focused onto the detector. Details of each component are discussed in the text.

Image of FIG. 5.
FIG. 5.

Schematic (left) and drawing (right) of the major components of the ICOS flow system. Details are discussed in the text.

Image of FIG. 6.
FIG. 6.

Timing diagram of an ICOS waveform. A zero is recorded before and after each spectrum. The laser is on for during which time the current is ramped from 625 to 845 mA. Between scans there is a period when the QCLI resets and the data are processed by the CPCI computer.

Image of FIG. 7.
FIG. 7.

Line fitting using two types of baselines. The two sets of plots on the left show the data in red and the fit to the data in green. The residual (data minus fit) is shown above each plot. The top plot is fit using a cubic polynomial to represent the baseline. The bottom plot shows the same data fit using a baseline composed of a cubic polynomial and frequencies that represent the dominant etalon modes of the spectrum. In order to allow for random phase, each etalon is modeled as the sum of a sine and cosine. The right hand plot shows the absorption coefficient as calculated by the cubic and etalon fits in cyan and blue, respectively.

Image of FIG. 8.
FIG. 8.

Fits of data at 4 ppmv for spectra taken during the CR-AVE mission. Top plot shows the raw data, the fit to the data, and the baseline in blue, green, and red, respectively. Plots in the next panel show the baseline removed from the data with the data plotted in units of percent transmission in blue and the fit to the data in green. Two different baseline fits are shown. The first uses just a fourth order polynomial, marked 4p0e, and the second uses a fourth order polynomial and 12 frequencies to represent oscillations in the baseline, marked 4p12e (see text for details). The second fit is offset by 1% for visual clarity. The bottom four plots show the residuals from the fit in the same units as the second plot. The residuals are from four different baseline representations as discussed in the text.

Image of FIG. 9.
FIG. 9.

DFT of the residuals from a fit using a fourth order polynomial to represent the baseline. Shown is the average of all the DFTs from the data plotted in Fig. 10.

Image of FIG. 10.
FIG. 10.

Top plot shows the water vapor mixing ratio derived from spectra using four different baselines that have a different number of sine waves added to represent etalons and other artifacts in the baseline power curve. The baselines all use a fourth order polynomial but have either zero, four, six, or 12 frequencies added and are plotted in blue, cyan, green, and red, respectively. The bottom plot shows the HDO mixing ratio derived from the same spectra as in the top plot.

Image of FIG. 11.
FIG. 11.

Schematic of the two types of water addition systems used to calibrate ICOS. The bubbler system is shown on the left and the microdroplet injector is shown on the right.

Image of FIG. 12.
FIG. 12.

Schematic of the bubbler system. Dry air or nitrogen is mixed with saturated air to create air with a known water vapor mixing ratio. Details of the system are discussed in the text.

Image of FIG. 13.
FIG. 13.

Calibration run using the bubbler system. The plot on the left shows the mixing ratio of water added to the system calculated from the bubbler and the mixing ratio calculated from the fit in black and red, respectively. The plot on the right shows the water vapor added via the bubbler plotted vs the water vapor calculated from the fit. The slope of the line is the correction to the HITRAN absorption cross-sections.

Image of FIG. 14.
FIG. 14.

Time evolution of contamination from water desorbing off the ICOS cell and inlet tubing walls. High water added during a pulse and during a 20 min continuous flow. Residual contamination is observed by comparing water vapor mixing ratio measured by the Lyman- instrument and by ICOS.


Generic image for table
Table I.

Spectroscopic line parameters from the HITRAN database. Line intensities and air broadening widths have been corrected as described in the text, and the values that have been changed appear in italics.

Generic image for table
Table II.

Uncertainties for the water species measured by ICOS. The first column gives the average wintertime tropical stratospheric value of each species. Precision is determined by calculating the equivalent mixing ratio of a line with depth equal to the residual of the fit. This number gives the same result as calculating the standard deviation of the data during a period with constant mean mixing ratio. The laboratory accuracy is the accuracy of the spectral cross-sections and line widths determined by calibration. The maximum bias uncertainty is the uncertainty due to etalons in the spectrum baseline and other artifacts that bias the data.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: A new cavity based absorption instrument for detection of water isotopologues in the upper troposphere and lower stratosphere