A Diode Laser Spectrometer for the In Situ ... - AMS Journals

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A Diode Laser Spectrometer for the In Situ Measurement of the HNO 3 Content of Polar Stratospheric Clouds G. TOCI Instituto di Elettronica Quantistica, Florence, Italy

P. MAZZINGHI Instituto Nazionale di Ottica, Florence, Italy

M. VANNINI Instituto di Elettronica Quantistica, Florence, Italy (Manuscript received 21 October 1997, in final form 18 September 1998) ABSTRACT A new instrument realized for measuring the HNO 3 concentration in air is described. The device is a midinfrared absorption spectrometer based on a tunable diode laser and a multipass absorption cell. The instrument is specifically designed for airborne operation on board the M-55 Geophysica, in the frame of the Airborne Polar Experiment project, taking into account all the related environmental and operational constraints. The device is part of a complete measurement package for the measurement of the chemical content of the polar stratospheric clouds and other atmospheric aerosols. Furthermore, it can be used as a stand-alone detector of molecular trace gases. The primary purpose of the instrument is to perform in situ diagnostic measurements in the upper troposphere–lower stratosphere. Design criteria include a new optical setup, one much less sensitive to the vibration and thermal stresses with respect to the conventional diode laser spectrometers. Furthermore, the authors developed a novel detection scheme for quicker acquisition and better signal-to-noise ratio. This paper reports calibration and testing measurements, including a detection lower limit both for the HNO 3 and ammonia. This last gas is used as a wavelength and absorption reference.

1. Introduction The instrument here described is mainly devoted to the measurement of the amount of nitric acid in the aerosol composing the polar stratospheric clouds (PSCs). This quantity, still unknown, is supposed to play a key role in the stratospheric ozone depletion process, which takes place during the polar spring (Solomon 1990; Turco et al. 1989). PSCs are found in the Arctic and Antarctic stratosphere at an altitude of 18–22 km with a temperature of 180–220 K. Depending on the temperature and pressure conditions, the PSC aerosol is composed by droplets or solid crystals sizing from 0.01 to 30 mm in diameter. Recent hypotheses assess that the aerosol is composed by binary solutions of H 2SO 4 /H 2O, ternary solutions of HNO 3 /H 2SO 4 /H 2O, or solid hydrates H 2SO 4-H 2O, HNO 3-H 2O. The diode laser spectrometer, designed and built at

Corresponding author address: Dr. Piero Mazzinghi, Istituto Nazionale di Ottica, Largo Enrico Fermi, 6, 50125 Firenze, Italy. E-mail: [email protected]

q 1999 American Meteorological Society

the Istituto di Elettronica Quantistica–Consiglio Nazionale delle Ricerche (IEQ–CNR), Italy, is a subsystem of an instrument package named Counterflow Virtual Impactor–Polar Stratospheric Cloud Composition (CVI–PSCC), for the chemical characterization of the PSC aerosol. The CVI–PSCC is also composed of an aerodynamic probe [CVI; Department of Meteorology, Stockholm University, Sweden (MISU)] to collect the aerosol (Noone et al. 1988) and a Lyman-a hygrometer (MISU). A computer [Free University of Berlin (FUB), Institute for Experimental Physics] performs systems management and data acquisition from all the instruments. The CVI–PSCC package will be installed in the main bay of the stratospheric aircraft M-55 Geophysica (Myasishchev Design Bureau, Russia). The flight missions will be performed in the frame of the Airborne Platform Experiment, funded by the European Union (Environment and Climate Program), the Italian National Program for Antarctic Research, and the European Science Foundation. The linear absorption spectroscopy by tunable diode lasers (TDLs) for quantitative measurement of trace gases in atmosphere is a well-assessed technique (for a

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review see Webster 1988; Brassington 1994). The concentration of the target chemical species in gas or vapor phase is determined by the absorption lineshape over sharp spectral lines, generally transitions between rotational–vibrational molecular energy levels lying in the midinfrared region (2–10 mm). The transmission at a given optical frequency n is given by the well-known Lambert–Bouguer absorption law: T(n) 5 exp[2a(n)L],

(1)

where L is the absorption path length and a is the linear attenuation coefficient. This latter is linearly related to the concentration of the specie of interest by Beer’s law:

a(n) 5 Nsk(n),

(2)

where N is the molecular number density (in molecules per volume unit) and s is the absorption line intensity for the spectral line under investigation. Here, k(n) is the spectral line profile, which can be Lorentzian, Gaussian, or Voigt-type, depending on the pressure and temperature conditions. Following a widely used convention on the units, the frequency n is expressed in inverse centimeters, s is expressed in centimeters per molecule, and k(n) has the dimension of inverse frequency (cm) and it is normalized so that its integral over the positive frequency axis is 1. In this way the linear attenuation coefficient a is dimensionally in inverse centimeters. This kind of measurement is usually performed at a rather low pressure (20–100 mbar), to have narrow and clearly resolved line profiles with a spectral width close to the Doppler limit, typically of the order of 1022 cm21 . Such a high resolution makes TDL spectroscopy virtually immune to interference by other species. The lead-salt TDLs used for mid-IR spectroscopy are specially designed for narrow emission bandwidth (Brassington 1994), of the order of 1024 cm21 , and usually operate at a low temperature (80–120 K, depending on the emission region). Detectors for this spectral region are usually photodiodes or photoconductors made of low band gap semiconductors, which also operates at cryogenic temperatures to achieve a reasonable thermal noise. A liquid-nitrogen temperature-controlled dewar is then necessary to operate both the TDL and the detector. The laser emission wavelength can be rapidly tuned across the region of interest by modulating the diode current. This procedure allows spectra averaging at a high acquisition frequency (up to several kilohertz), or, alternatively, a synchronous modulation and detection. Both these techniques considerably improve the signal to noise ratio of the detected spectra, allowing the measurement of very small absorption peaks (up to 1025 ). To achieve the required sensitivity when measuring the concentration, the absorption path length is usually several tens of meters long. This is obtained by means of a multipass optical cell. In these conditions, sensitivities up to the order of parts per billions (1029 ) in

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mixing ratios were demonstrated in the laboratory for many molecular gases. Nevertheless, the construction of an instrument preserving the above mentioned performances, when operating unattended and reliably in the environment of a stratospheric aircraft, is a challenging task even in normal flight conditions, due to the strong vibration levels and the wide pressure and temperature operating ranges. Furthermore, the payload must be compliant to all the safety regulations regarding the robustness of the overall assembly. For these reasons, the commercially available TDL spectroscopic systems are not suitable for this application, as recognized by other groups dealing with similar measurements on HNO 3 and other trace gases from balloons (May and Webster 1989; Webster et al. 1990) and from the stratospheric aircraft ER-2 (Webster et al. 1994). The TDL spectrometer for HNO 3 detection described in this paper was designed specifically in view of its installation on the M-55 aircraft. In particular the available experience from other research groups during the installation and in-field operation of their instruments underlined the possible problems and gave us significant hints for their solution in the mechanical and optical design. This paper describes the instrument and its design criteria, discussing the compromises and limitations accepted for a reliable operation on the M-55, including survival of the vibration and shock testing. We also report the laboratory tests for the determination of the instrument sensitivity in the measurement of nitric acid (HNO 3 ) mixing ratio, in air. The absorption sensitivity in the multipass cell resulted less than 1024 , with a consequent HNO 3 sensitivity of 10 ppbv, in agreement with the design specification. 2. Instrument description The optomechanical layout of the instrument is shown in Fig. 1. The laser source is a lead-salt TDL (DHX– SELSM, Mu¨tek Infrared, Germany) chosen for narrowband (linewidth 5 3 1024 cm21 ), single-mode operation at a frequency of 1720 cm21 . The emitted power is very low (about 0.1 mW). The TDL operates at a temperature of about 110 K, and it is installed in a dewar containing liquid nitrogen for cooling. A suitable electronic controller stabilizes the laser temperature within the required range for single-mode operation (62 mK). A commercial controller (Mu¨tek Infrared model TLS 210) was used for laboratory experiments, while for flight operation, a dedicated unit was built by FUB. Ray tracing simulation demonstrated that the commonly used parabolic mirrors for laser beam collimation introduce excessive beam pointing sensitivity to mechanical vibrations. The laser beam was then collimated by an 18-mm focal length ZnSe lens, which increases the tolerance in the beam pointing from 1.5 up to 4

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FIG. 1. Instrument layout.

mrad. The collimated beam coming from the lens is split in a reference and in a measurement beam. The measurement beam enters an astigmatic Herriott cell (McManus et al. 1995) (New Focus model 5611) where 182 reflections on a pair of quasi-confocal, toroidal mirrors set at a distance of 20 cm provide an absorption path of 36 m, with an overall transmission of about 20%. The beam exiting the cell is then focused by a spherical mirror on a HgCdTe detector [Graseby Infrared Co. model HCT 12.5, surface 1 mm 2 , detectivity D* 5 1010 cm (Hz)1/2 W21 , rise time 400 ns] installed in the dewar. The reference beam passes through a reference cell containing a known mixture of air and NH 3 and through a ZnSe etalon with known spectral range for coarse and fine wavelength calibration, respectively. The ZnSe etalon was preferred to the commonly used single crystal Ge etalon because this latter requires a careful thermal stabilization due to the dependence of its refractive index from temperature. The reference detector is a recently developed thermoelectrically cooled InSb photoconductor (Hamamatsu model P6606) having active surface 1 mm 2 , detectivity D* 5 10 9 cm (Hz)1/2 W21 , operating temperature 2708C, rise time 1 ms. In this detector the photosensitive element and the low power (,2 W) Peltier cooler are installed within a power transistor-like TO-3 package. Its small size (less than 40 mm wide) allows some simplification in the optical setup, because it can be installed directly in front of the cell, without any folding mirror, which would be required for a LN 2-cooled detector installed in the dewar. To reduce the vibration sensitivity, we shortened the free paths between the mirrors as much as possible. This results in an assembly filling a small footprint on a aluminum baseplate. All of the system was designed

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with a finite element analysis program to analyze thermal and mechanical behavior. The overall instrument is enclosed in a thermally insulated and pressurized case, to reduce its sensitivity to external temperature changes. A system of constant pressure valves on the dewar exhaust and on the case stabilizes the LN 2 boiling temperature against the external pressure variations. The dry nitrogen evaporating from the dewar is also used to flush the case, thus avoiding moisture condensation effects on the optics. The CVI probe samples the atmosphere, collects the PSC aerosol, and rejects the ambient air (pressure ;50 mbar, temperature ;190 K). Due to the probe aerodynamic action, the aerosol concentration in the collected sample increases of a known factor of about 10 (Noone et al. 1988). The aerosol is then evaporated at a temperature of about 58C, chosen as a compromise between the thermal dissociation of the HNO 3 at high temperatures and its sticking on the metallic surfaces at low temperatures. A continuous flow of a buffer gas at a pressure of ;80 mbar carries the sample through the absorption cell by stream, at a clearance rate of about 1/30 s21 . This sampling method allows the determination of the nitric acid amount contained in the aerosol phase only. 3. HNO 3 spectroscopy The HNO 3 molecule shows a strong rotational–vibrational band (n 2 ) (Rothman et al. 1992; Maki and Wells 1980) centered at 1710 cm21 (5.85 mm). The spectral window around 1720 cm21 , close to the R-band peak, appears particularly suitable for the measurement of the HNO 3 concentration, due to the absence of the almost ubiquitous H 2O absorption lines, otherwise interfering with the measure. In the expected operating conditions (P 5 80 mbar, T 5 273 K) the pressure broadening dominates (g p 5 0.11 cm21 atm21 ), determining only a partial resolution of the rotational fine structure (Fig. 2). Kawa et al. (1992) provided an evaluation of the total HNO 3 amount present in the Arctic and Antarctic PSC aerosol during winter 1989 and summer 1987, respectively, based on the determination of the total aerosol volume made by a forward-scattering spectrometer probe (FSSP), and the indirect evaluation of the amount of the reactive nitrogen (NO y ) in the aerosol by means of a chemioluminescence detector. Both instruments were installed on the National Aeronautic and Space Administration ER-2 stratospheric aircraft. They reported mixing ratios up to 5–10 ppbV at a pressure of 70 mbar and a temperature of 190–195 K. Although these measurements methods exhibit rather large uncertainties and do not agree completely, an evaluation for the HNO 3 density in the aerosol of the order of 1.4– 2.8 mg m23 seems reasonable. Accounting for a CVI probe enrichment factor of 10, this results in a HNO 3 amount in the cell of about 14–28 mg m23 , or a mixing

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FIG. 2. Theoretical HNO 3 absorption cross section spectrum around 1720 cm21 .

ratio of 50–100 ppb, which determines an absorption of the order of 1.8 3 1023–3.6 3 1023 . 4. Detection and calibration protocol The detection scheme relies on rapid sweep integration over the whole absorption region. This is achieved by scanning the laser diode emission frequency and synchronously measuring the transmitted power. The resulting signals are then averaged and stored for the processing. In a typical experiment, the temperature of the diode laser is kept constant at 110 K, whereas its emission wavelength is scanned over an interval of about 0.8 cm21 by modulating the current, covering several HNO 3 absorption lines around 1719.7 cm21 (see Fig. 2). The repetition frequency of the modulation ramp is set at around 1.5 kHz. A fast transient digitizer and averager (realized by FUB), triggered by the ramp start, collects the signals from the detectors at a sampling rate of 5 Msamples s21 with 12 bits resolution, and calculates the average spectrum resulting from a large number (up to 64 000) of individual spectra composed by 1000 points each. An internal antialiasing filter sets the acquisition bandwidth to 1 MHz. The calibration and the measurement spectra are quasi-simultaneously acquired, by interleaving the acquisition of several spectra from the measurement detector with the acquisition of a single spectrum from the reference detector, and averaging the two kind of signals in different memory areas. This procedure ensures that the two spectra are acquired during the same time interval, and therefore it reduces the effects of possible drifts occurring to the laser tuning during the acquisition. Figure 3 shows a typical raw absorption spectrum of the nitric acid (obtained at a rather high concentration which determines a peak absorption of more than 0.1), with its calibration signal.

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FIG. 3. Raw absorption spectrum of the HNO 3 (a) at 50 mbar, 296 K, and wavelength calibration spectrum (b) showing the ammonia reference lines and the etalon fringes. The baseline (thin solid line) is fitted with a fifth-order polynomial.

The intensity calibration of the spectrum is achieved by means of the following procedure. R The dark voltage level of the detectors Vdark is acquired by turning off the laser just before the beginning of the scan; for this purpose, the detectors and the amplifiers are DC coupled. R The laser emission level at the nth channel V b (n) is determined by fitting a high-order polynomial between the points lying away from the absorption lines (the thin line in the upper frame of Fig. 3); this value represents the intensity level that would be present in absence of the absorption lines (that is, with an empty cell). R The absolute absorption level for the nth channel, where we measure a signal level V(n), is calculated by the relation A(n) 5

Vb (n) 2 V(n) , Vb (n) 2 Vdark

(3)

that is, the ratio between the intensity absorbed by the line and the unperturbed laser emission intensity level. This approach simplifies the optical setup with respect to the case of the harmonic detection spectroscopy, where the intensity calibration is usually achieved by a reference cell containing a calibrated mixture of the gas specie of interest. In the case of the nitric acid this can lead to problems of material compatibility for a longlasting storage in the calibration cell. The absolute wavelength calibration is achieved by means of the two absorption lines of the ammonia contained in the reference cell, respectively at 1719.43 and 1719.704 cm21 (Rothman et al. 1992). The interference fringes of the ZnSe etalon (which is 30 mm long and has a free spectral range of 0.07 cm21 ) provide subsidiary wavelength markers. The uneven spacing of the etalon fringes along

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FIG. 4. Calibrated absorption spectrum of the HNO 3 (thick solid line) from the raw data of Fig. 3. The theoretical spectrum (thin solid line) is calculated from the spectral data provided by the HITRAN database (Rothman et al. 1992).

the scan evidences a nonlinearity in the dependence of the emission frequency from the diode current. Ammonia was chosen as calibration gas because it exhibits several strong and easily recognizable lines in the spectral window of interest. On the other hand, HNO 3 itself is somehow inconvenient as a wavelength calibration specie since over a pretty wide region it exhibits equally spaced lines of almost the same intensity, possibly misleading the determination of the wavelength. Figure 4 shows the HNO 3 spectrum obtained after wavelength and intensity calibration of the raw data of Fig. 3. A theoretical spectrum calculated in the same conditions of temperature, pressure, and concentration is shown for reference. The comparison between the experimental and the theoretical spectra shows that the wavelength calibration error is less than 1023 cm21 . The achievement of an accurate wavelength calibration is important since it allows a clearer identification of the weak absorption caused by the gas traces under investigation, and it reduces the possibility of a mistake in the specie identification. The lower sensitivity limit of the system has been checked until now by determining the minimum detectable absorption of the ammonia spectral lines used for the wavelength calibration. Recent results shows that sweep integration can provide in most cases sensitivity levels comparable with those achieved with wavelength modulation techniques (Webster 1988; Brassington 1994; Zahniser et al. 1995; Werle 1996), with the further advantage of a simpler electronic equipment and the direct evaluation of the absorption level. In our case, the acquisition electronics introduces an overall noise of about 1 3 1027 with respect to the overall signal amplitude, when averaging over 64 000 spectra in the conditions described above. In the same conditions, we were able to clearly detect absorption of the order of

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FIG. 5. Weak absorption measurement on the ammonia line at 1719.704 cm21 . The right-hand vertical axis refers to the fitted baseline (thin solid line) and the experimental points (crosses). The lefthand axis refers to the fitted Voigt lineshape (dotted line) and the normalized residuals (solid line with diamonds). The only fit parameter for the Voigt profile is the peak amplitude; the Doppler and Lorenzian line width parameters are calculated from the measurement conditions (pressure 40 mbar, temperature 296 K).

1024 . This is exemplified in Fig. 5. The laser baseline (thin solid line) is fitted with a fifth-order polynomial over the raw data points (crosses). The normalized residuals show the contribution to the absorption of the ammonia line (thick solid line with diamonds). The fit of the Voigt absorption lineshape (dotted line) allows us to determine a peak absorption level of 3.9 3 1024 with an uncertainty 60.4 3 1024 , that is, an S/N ratio of about 10. When referred to the nitric acid, this absorption level would correspond to a concentration of about 10 ppb (number density of about 1.7 3 1010 molecule cm23 ), which is a factor of 5–10 lower than the expected concentration levels in the cell as estimated above. We have found that the formation of interference fringes in the optical setup does not substantially reduce the system sensitivity. The sensitivity lower limit appears to be mainly due to a residual ripple in the laser diode baseline along the frequency scan, which hampers the exact determination of the laser emission baseline. Such a ripple seems to be related to the power versus current emission characteristic of the diode laser used in the tests, since it does not exhibit the periodic behavior that would be determined by spurious interference effects. 5. Mechanical pretests of the optical components The mandatory prerequisite for any airborne scientific payload is that, independently from its capability to produce useful scientific data, it must not, in any case, endanger the aircraft and the crew, even under the more severe flight conditions. A de facto international standard on this subject (recognized by the International Organization for Standardization) is set by an Radio

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FIG. 6. Acceleration power spectral density (PSD) for the functionality tests.

Technical Commission for Aeronautics (RTCA) document (RTCA 1997). This document defines a series of minimum standard environmental test conditions and applicable test procedures to provide a laboratory means of determining the performance characteristics of airborne equipment in environmental conditions representative of those that may be encountered in airborne operation (i.e., vibration, power input, radio frequency susceptibility, lightning, and electrostatic discharge). This standard was applied for the testing of all the instruments installed on the M-55 Geophysica. Besides the safety concern, our main care was to characterize the behavior of the fundamental parts of our instrument when subjected to the environment that will be found on the platform in normal flight situations as experienced by the other instruments during the test and scientific flights, to ensure the proper operation on the platform environment. Until now, we performed a set of functionality vibration tests (Toci et al. 1997) on the multipass absorption cell described above. To ensure proper operation of the instrument, the optical alignment of the cell mirror pair must be preserved against the mechanical vibration and against the thermal stresses. In particular, the nearly confocal configuration of an astigmatic Herriott cell makes it rather insensitive to the mirror’s tilt, but it can be easily affected by the variations in the mirror distance TABLE 1. Vibration-induced beam noise level (normalized to the DC signal level).

Excitation axis Transverse Longitudinal Vertical Transverse Longitudinal Vertical

Vibration frequency range

Normalized rms noise (5 Hz–20 kHz)

0–100 Hz 0–100 Hz 0–100 Hz 100 Hz–2 kHz 100 Hz–2 kHz 100 Hz–2 kHz

0.40% 0.10% 0.40% 2.2% 0.47% 1.7%

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FIG. 7. PSD of the beam noise induced by vibrations (high-frequency spectrum) along the cell axes.

(McManus et al. 1995). The mechanical vibrations can therefore induce a beam wandering in the spot pattern inside the cell, which in turn can determine a random fluctuation in the transmitted beam power, because of small point-to-point variations on the mirror reflectivities, for instance (also due to small scratches or dust particles), or due to beam clipping effects on the mirror boundaries or on the entrance hole edges, and so forth. Therefore, we characterized the noise levels induced by the mechanical vibrations on a laser beam propagating into the cell. For these tests, the cell, in its factory configuration, was firmly screwed on a breadboard that in turn was bolted on the platform of a mechanical shaker (Unholtz– Dickie T4000). The beam of a laser diode (1-mW average power, wavelength 635 nm) was focused in the cell, properly aligned on the 182-pass configuration used for the measurement, as identified by the beam spot pattern on the mirrors. The power of the output beam was measured by a low noise, wide area (8-mm diameter) silicon detector. Both the laser and the detector were rigidly mounted on the breadboard with bulky aluminum bases. When the whole assembly was subjected to the random vibration spectrum of Fig. 6 (one period of 10 min, both for the low- and for the high-frequency portion along each cell axis), the photodiode monitored the mechanically induced laser beam noise. Table 1 reports the mechanically induced rms noise levels for the three axes. The analysis of the power spectral density (PSD) of the beam noise reveals the presence of mechanical resonances in the cell structure affecting the beam propagation (Fig. 7). Further information is obtained by calculating the ratio between the PSD of the beam noise and of the acceleration (transfer function between the vibrations and the beam noise), determining the frequencies that more strongly affects the beam propagation (Fig. 8). By scaling the vibration spectra of Fig. 4 to lower overall intensity level (26 dB, 23 dB) we determined that the beam noise PSD depends almost linearly on the acceleration PSD, so that the transfer

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ly enclosed in the cell housing, and no structural damage occurred. Furthermore, the results of this rather destructive test gave significant insights to modify the mirror mounts in order to improve their mechanical resistance. 6. Conclusions

FIG. 8. Frequency dependence of the ratio between the PSD of the beam noise (Fig. 7) and the PSD of the mechanical vibration along the cell axes (high-frequency spectrum, Fig. 6).

function spectra of Fig. 8 are almost independent from the vibration level. Under the vibration levels adopted in these tests (Fig. 6), the cell remained properly aligned all along and after the tests, without temporary or permanent changes in the beam spot reflection pattern or in the overall alignment. To estimate the effect of the mechanically induced beam noise on the spectroscopic measurements outlined in the section 4, the rms noise levels of Table 1 must be added geometrically (assuming that they are uncorrelated) and divided by the square root of the number of coaveraged spectra of an acquisition (up to 64 000). It results that such a level of mechanical vibration should introduce an additional noise of about 1024 rms, which is comparable with the noise level experienced during the laboratory tests. It is worth noting that this can be considered as a worst-case setup, since the equipment suffered the full vibration intensity present on the aircraft wing (as it was directly bolted on the wing itself ), whereas the assembled instrument will be installed in the aircraft front bay (where the vibration levels are much lower), and the mounting rack will be suspended to the aircraft frame with suitable shock absorbers, which are particularly efficient in damping the highfrequency part of the mechanical vibrations spectrum. These results were used for choosing the proper dampers. The safety tests were performed on the same experimental set up described above (without the laser and the photodiode), according to the specifications of the above mentioned document (RTCA 1997), which applies to the vibration tests for the equipment installed on the wing of turbojet airplanes (3 h for each axis, acceleration 11.9g rms in the frequency bandwidth 10 Hz–1 kHz). Although the functionality of the cell was compromised by this safety test (as the mirror mount suffered some damage), the failure was harmless for the aircraft, because the damaged parts remained complete-

We have described the design and the laboratory tests of a diode laser spectrometer for airborne operation. Our instrument is a subsystem of the instrument package called CVI–PSCC, for the determination of the amount of nitric acid in the PSC aerosol. The instrument is specifically designed in view of its installation on the M-55 Geophysica, taking into account of all the related environmental and operational boundaries, and taking advantage of experience acquired by the other research groups during the installation and in-field operation of their instruments. The problem of the mechanical stability of the system and of its sensitivity to vibrations is solved by adopting a very compact optical setup with very short beam paths, and by a suitable design of the beam focusing and steering optics. On the other hand, after a careful selection based on the preliminary mechanical functionality tests, some commercial components originally intended only for laboratory use, such as the multipass cell, were found to fulfill the safety and functional standards for the construction of the instrument. The sweep integration detection scheme was adopted since it allows the use of a simpler electronic equipment and a simpler optical setup with respect to the widely diffused wavelength modulation and FM modulation spectroscopy. This technique still provides the sensitivity level required for this particular application. During the preliminary tests described in this paper we achieved a sensitivity in absorption of the order of 4 3 1024 with a signal to noise ratio of 10:1 with an integration time of about 60 s. This sets the minimum detectable HNO 3 concentration to the ppbV level, well below the typical expected concentration in the cell. On the other hand, the noise level of the laser diode system is comparable with the beam noise induced by the mechanical vibrations in the multipass cell, which therefore would determine the lower sensitivity limit even with more sensitive electronics. Nevertheless, the system laboratory tests will continue, with the aim to improve the performances described in this paper. In particular we believe that the sensitivity could be further improved by means of suitable techniques of baseline subtraction and numerical signal processing. A further simplification of the optical arrangement could be obtained by substituting the LN2-cooled HgCdTe detector on the measurement arm with a TE-cooled InSb one (of the same type adopted in the reference arm), placed very near to the multipass cell output. Despite its slightly lower detectivity and its slower response time, this would lead to a substantial reduction of the optical path length with a subsequent improvement in the mechanical sensitivity.

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The functionality and the safety test performed until now produced also useful indications for the detailed design of the rest of the instrumentation. In particular the design of the shock absorbing system can be optimized for damping the resonance frequencies, and the tailoring of the data acquisition protocol, avoiding sweep frequencies that correspond to the mechanical resonances and harmonics. Our next activity will be devoted to the implementation of the TDL spectrometer in the CVI–PSSC package and in its installation on the M-55. Acknowledgments. We want to greatly acknowledge Dr. B. Stein and Dr. B. Mielke of the Free University of Berlin for the loan of the transient digitizer. We also want to acknowledge New Focus Inc. (Santa Clara, California), and in particular Dr. C. Hultgren, for the loan of the cell used in the vibration tests. We also gratefully acknowledge the technical assistance of Mr. Mauro Pucci, of IEQ–CNR, for the construction of the instrument optics. Research was funded under the European Community Contract ENV40039 CE and the Italian Space Agency Contract ASI ARS 96-13/264. REFERENCES Brassington, D. J., 1994: Tunable diode laser absorption spectroscopy for the measurement of atmospheric species. Advances in Spectroscopy, R. E. Hester, Ed., Vol. 24, John Wiley, 83–148. Kawa, S. R., and Coauthors, 1992: The artic polar stratospheric cloud aerosol: Aircraft measurement of reactive nitrogen, total water and particles. J. Geophys. Res., 97, 7925–7938. Maki, A. G., and J. S. Wells, 1980: High-resolution measurement and analysis of the infrared spectrum of nitric acid near 1700 cm21 . J. Molec. Spectrosc., 82, 427–434. May, R. D., and C. R. Webster, 1989: In situ stratospheric measurement of HNO3 and HCl near 30 km using the balloon-borne

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laser in situ sensor tunable diode laser spectrometer. J. Geophys. Res., 94, 16 343–16 350. McManus, J. B., P. L. Kebabian, and M. S. Zahniser, 1995: Astigmatic mirror multiple-pass absorption cells for long-path-length spectroscopy. Appl. Opt., 34, 3336–3348. Noone, K. J., J. A. Ogren, J. Heintzenberg, R. J. Charlson, and D. S. Covert, 1988: Design and calibration of a counterflow virtual impactor for sampling of atmospheric fog and cloud droplets. Aerosol Sci Technol., 8, 235–244. Rothman, L. S., and Coauthors, 1992: The HITRAN Molecular Database: Editions of 1991 and 1992. J. Quant. Spectrosc. Radiat. Transfer, 48, 469–507. RTCA, 1997: Environmental conditions and test procedures for airborne equipment. RTCA Note DO-160D, 278 pp. [Available from RTCA Inc., 1140 Connecticut Ave. NW, Suite 1020, Washington, DC 20036.] Solomon, S., 1990: Progress toward a quantitative understanding of Anctartic ozone depletion. Nature, 347, 347–354. Toci, G., P. Mazzinghi, M. Vannini, G. Decanio, and G. Fabrizi, 1997: Design and testing of a high resolution diode laser spectrometer for airborne operation. Proc. Int. Conf. on Lasers ’96, Portland, OR, STS Press, 658–661. Turco, R. P., O. B. Toon, and P. Hamill, 1989: Heterogeneous physicochemistry of the polar ozone hole. J. Geophys. Res., 94, 16 493–16 510. Webster, C. R., R. T. Menzies, and E. D. Winkley, 1988: Infrared laser absorption: Theory and applications. Laser Remote Chemical Analysis, R. M. Measures, Ed., John Wiley, 163–273. , R. D. May, R. Toumi, and J. A. Pyle, 1990: Active nitrogen partitioning and the nighttime formation of N 2O 5 in the stratosphere: Simultaneous in situ measurement of NO, NO 2 , HNO 3 , O 3 , and N 2O using the BLISS diode laser spectrometer. J. Geophys. Res., 95, 13 851–13 866. , , C. A. Trimble, R. G. Chave, and J. Kendall, 1994: Aircraft (ER2) laser infrared absorption spectrometer (ALIAS) for in situ stratospheric measurement of HCl, N 2O, CH 4 , NO 2 , and HNO 3 . Appl. Opt., 33, 454–471. Werle, P., 1996: Spectroscopic trace gas analysis using semiconductor diode lasers. Spectrochim. Acta, A52, 805–822. Zahniser, M. S., D. D. Nelson, J. B. McManus, and P. L. Kebabian, 1995: Measurement of trace gas fluxes using tunable diode laser spectroscopy. Philos. Trans. Roy. Soc. London, Series A, 351, 371–382.