Differential optical absorption spectroscopy ... - OSA Publishing

Differential optical absorption spectroscopy instrument for stratospheric balloonborne trace-gas studies Frieder Ferlemann, Nadine Bauer, Richard Fitzenberger, Hartwig Harder, Hartmut Osterkamp, Dieter Perner, Ulrich Platt, Matthias Schneider, Paul Vradelis, and Klaus Pfeilsticker

A newly developed UV–visible instrument for differential optical absorption spectroscopic measurements of atmospheric trace gases from balloon platforms is described. Direct solar light at daytime in the near-ultraviolet 共320.6 – 422.6-nm兲 and the visible 共417.6 – 670.7-nm兲 spectral ranges can be simultaneously analyzed for the atmospheric column abundances or profiles of O3, NO2, NO3, BrO, OClO, O4, H2O, and possibly other species 共HNO2, IO, CH2O兲. Compared with previously used balloonborne UV–visible spectrometers, the instrument has the superior properties of low mass 共42 kg兲, low power consumption 共30 W兲, decreased spectral drift that is caused by temperature and pressure changes, low detector dark current, and low spectrometer stray light. The three last-named characteristics are achieved by enclosure of the entire spectrometer in a pressurized and thermostated container and by inclusion of separately thermostated photodiode array detectors. The optical setup is simplified to reduce its weight. The spectral stray light is reduced by suppression of the higher-order and zero-order grating reflections by use of light traps and in the UV by addition of a dispersive prism preanalyzer. The major instrumental design characteristics and the instrumental performance as tested in the laboratory and during several stratospheric balloon flights are reported. © 2000 Optical Society of America OCIS code: 010.0010.

1. Introduction

The need to study the photochemical and dynamic processes that influence the stratospheric ozone layer has been recognized for many years. Potential changes in stratospheric ozone that arise from anthropogenic effects have been discussed and investigated since the recognition in recent decades of the importance of hydrogen, nitrogen, and halogen chemistry in the ozone budget.1– 6 Impetus for new research was provided by the discovery of the ozone hole over Antarctica in the spring, as reported in 1985

When this research was performed, F. Ferlemann, N. Bauer, R. Fitzenberger, H. Osterkamp, U. Platt, M. Schneider, P. Vradelis, and K. Pfeilsticker 共[email protected]兲 were with the Institut fu¨r Umweltphysik, Universita¨t Heidelberg, Im Neuenheiner Feld 229, D-69120 Heidelberg, Germany. N. Bauer and P. Vradelis are now with Systems, Application and Products AG, D-69190 Walldorf, Germany. M. Schneider is now with Forschungszentrum Karlsruhe, D-76021 Karlsruhe, Germany. D. Perner is with the Max-Planck Institut fu¨r Chemie, Abteilung Luftchemie, Saarstrasse 23, D-55122 Mainz, Germany. H. Harder is with the Universita¨t Heidelberg and with the MaxPlanck Institut fu¨r Chemie. Received 20 July 1999. 0003-6935兾00兾152377-10$15.00兾0 © 2000 Optical Society of America

by Farman et al.,7 by the findings of the International Ozone Trends Panel8 of a significant and as yet unexplained ozone decline in the winter–spring midlatitude and high latitude, and by the recent discovery of a dramatic ozone loss in the Arctic stratosphere in late winter. Although many basic processes that govern the chemistry of the ozone layer seem to be understood, several important questions are still open. For example, 共a兲 How well do we understand the stratospheric Bry chemistry?9 共b兲 What are the processes that cause the so-called renoxification 共i.e., the conversion of HNO3 to NO and NO2兲 in the lower stratosphere?10 共c兲 How well do we understand the nighttime production of lower-stratospheric NO3?11 共d兲 What are the mechanisms and extent of the potential effects of iodine-containing species on stratospheric ozone?12 共e兲 Is the heterogeneous formation of HNO2 at night and its subsequent photolysis in the early morning hours an important stratospheric OH source?13 共f 兲 What is the stratospheric abundance of CH2O? An important step toward answering these questions can be made by measurement of the vertical profiles of the involved species for various geophysical conditions. Clearly, UV–visible differential optical absorption spectroscopy 共DOAS兲 solar occultation 20 May 2000 兾 Vol. 39, No. 15 兾 APPLIED OPTICS

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measurements from balloons14 are a powerful tool for this purpose. The solar occultation technique relies on the spectroscopic analysis of atmospheric tracegas absorption features observed in direct sunlight. The optical paths are increasing 共decreasing兲 during sunset 共sunrise兲 and the balloon descent 共ascent兲. In principle, the profile information on the atmospheric absorbers is retrieved from these series of measured column densities by inversion. In practice this inversion is achieved by either the so-called onionpeeling technique or the matrix-inversion technique.15 UV兾visible differential optical absorption spectroscopy of atmospheric trace gases relies on the detection of the electronic, vibrational, and rotational transitions that produce spectra with typical widths of several nanometers and typical optical densities of the order of 10⫺3 or even less. An important feature of the technique is to remove the strong Fraunhofer lines 共typical optical density about unity兲 in the recorded sunlight that would otherwise mask the much weaker absorption signatures in the atmosphere.16,17 These lines are removed by ratioing of the measured spectra taken at large and small solar zenith angles 共SZA’s兲. This property of the DOAS technique, however, requires that the spectrometer be extremely stable with respect to spectral registration; also, detector parameters must be insensitive to changes in ambient conditions. Recent improvements in optical as well as electronic components allow us to meet such requirements more easily than previously and have led us to design a new instrument for aircraftborne and balloonborne atmospheric DOAS measurements. 2. Description of the Instrument

Any reliable DOAS instrument has to meet some specific design criteria. For balloonborne applications these criteria are the following: 共a兲 stable tracking of the solar disk to avoid any significant change in recording of the optical density of the Fraunhofer lines emitted from the solar photosphere, 共b兲 insignificant thermal drift of the spectroscopic system with changing ambient temperature, 共c兲 stability of spectral imaging with respect to variations in the ambient pressure, 共d兲 small photoelectron detection noise that is stable over the measurement period, 共e兲 insignificant contribution by the spectrometer’s stray light to the signal, and finally 共f 兲 stable and reliable working analog electronics. Other instrumental criteria that are important for stratospheric balloonborne applications are light weight and low power consumption. Our novel balloon spectrometer basically consists of six components 共Figs. 1–3兲: a sun tracker, which conducts the sunlight into two small telescopes 共not shown in Fig. 1兲; two small telescopes 共Figs. 2 and 3兲; two quartz fiber bundles, which conduct the light into the spectrometer housing; two spectrometers, in which the UV and the visible parts of the sunlight are analyzed separately; two photodiode array detectors; and the electronics to drive and control the instrument. 2378

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Fig. 1. Schematic drawing of the instrument: Two holographic grating spectrometers 共denoted UV and Visible兲 are integrated into a vacuum-sealed stainless-steel container. The light enters the spectrometers through two quartz fiber bundles. The exits of the fiber bundles form an apparent rectangular slit 共column of individual fiber exits兲 and direct the light onto the holographic gratings. Light detection is performed with two photodiode array detectors 共see text兲. The whole spectrometer container is thermostated by a liquid-water–ice-filled vessel, which surrounds it. In addition, a refrigerant is circulated in a cooling circuit to regulate the temperature of the optical components and to cool the warm side of the photodiode Peltier elements. The total weight of the instrument, including 10 L of a water–ice mixture, is approximately 42 kg, and the total size is 450 mm ⫻ 450 mm ⫻ 550 mm.

The direct sunlight is guided by an optical system 共sun tracker兲 into the spectrometer’s entrance telescopes. The sun tracker is mounted upon an azimuthally controlled Laboratoire de Physique Molećulaire et Applications 共LPMA兲 balloon gondola.18 Overall, a pointing accuracy of the solar disk’s center within approximately 1兾60 of a degree is guaranteed.19 Slightly off-axis mounting of the entrance telescopes is required so the inner core of the beam can be directed into the IR instrument, which is also operated onboard the gondola.18,19 The two entrance optics are intended to average the light received from the solar disk, to limit the

Fig. 2. Schematic of the visible spectrometer’s entrance optics 共inner diameter, 10 mm兲. The solar light enters from the right and subsequently passes through two filters 关共a兲 GG385 and 共b兲 BG38兴 and a diffuser 共c兲. The entrance of the quartz fiber bundle 共 f兾2.2兲 is mounted 22 mm from the diffuser, which allows the diffuser exit 共diameter, 10 mm兲 to be imaged properly.

Table 1. Stratospheric Gases That Can Be Detected by Our Instrumenta

Center Wavelength Differential Cross Detection Limit Gas Species 共nm兲 Section 共cm2兲b 共molecules兾cm2兲c

Fig. 3. Schematic of the UV spectrometer’s entrance optics. The solar light enters from the right 共a兲 onto a quartz prism 共b兲 and subsequently passes through a quartz lens 共c兲, an aperture slit 共e兲, a lens 共 f ⫽ 57 mm兲共f 兲, two quartz diffusers 共g兲 and 共h兲, and an aperture orifice 共diameter, 2.3 mm兲共h兲, which is mounted at the proper distance 共19 mm兲 from the entrance of the quartz fiber bundle 共i兲 to match the spectrometer’s f-number 共 f兾3.3兲. Component 共d兲 is a two-dimensional displacement tool that is necessary for adjustment of the preanalyzer 关components 共a兲–共c兲兴 to the telescope 关components 共e兲–共i兲兴.

spectral transmission range of the incoming light, and to match the f-number of each spectrometer. For the visible-light channel, only a telescope is used 共Fig. 2兲, but for the UV light a preanalyzer–telescope combination 共Fig. 3兲 is used. The visible telescope consists of two Schott filters 关GG385 and BG58, parts 共a兲 and 共b兲 in Fig. 2兲 and a diffuser 关part 共c兲兴 mounted 22 mm in front of the entrance to the glass fiber bundle. This arrangement of the diffuser and the fiber guarantees matching of the f-number 共 f兾3.5兲 of the visible spectrometer. The two filters limit the optical transmission of the visible telescope to the range 共385– 680 nm兲 that is necessary to reduce the spectrometer stray light. The UV entrance optics is composed of two components: a preanalyzer consisting of a prism, a lens, and an aperture slit 关parts 共b兲–共e兲 in Fig. 3兴 arranged in front of an UV telescope 关parts 共f 兲–共h兲兴. The preanalyzer is intended to limit the spectral range to wavelengths lower than 450 nm, which is necessary to reduce the spectrometer stray light. Because of the defocused image of the Sun on the diffuser plate, the transmission of the preanalyzer is only ⬃20% at 360 nm according to our measurements. The UV telescope consists of an entrance quartz lens 共diameter, 10 mm; focal length, 57 mm兲, a light-diffuser plate, an aperture plate, a filter, and the circular entrances of the quartz fiber bundles 共Fig. 3兲. The incoming solar light is focused by the lens onto the diffuser plates, which are mounted a distance of 8 mm from the lens. This arrangement of lens and diffuser plates is intended primarily to collimate the incoming solar photon flux while making light detection insensitive to possible misalignments of the sun-tracker– telescope orientation. We achieve this stability by allowing the Sun to be viewed with a field of view 共10°

NO2 NO3 BrO OClO HNO2 SO2 IO O3 O4 H2O CH2O

448 663 348 360 354 304 428 505 577 650 338

3 ⫻ 10⫺19 2.2 ⫻ 10⫺17 1.5 ⫻ 10⫺17 9 ⫻ 10⫺18 5 ⫻ 10⫺19 6 ⫻ 10⫺19 3 ⫻ 10⫺17 4.5 ⫻ 10⫺22 1.1 ⫻ 10⫺46b 1.6 ⫻ 10⫺25 3.7 ⫻ 10⫺19

6.6 ⫻ 1014 9.0 ⫻ 1012 1.3 ⫻ 1013 2.1 ⫻ 1013 4.0 ⫻ 1014 3.3 ⫻ 1014 6.6 ⫻ 1012 4.0 ⫻ 1016 1.8 ⫻ 1042c 1.25 ⫻ 1021 5.4 ⫻ 1014

a We assume a detection limit of 2 ⫻ 10⫺4 in optical density for the direct-sunlight observation mode. b cm3兾molecule2. c Molecules兾cm⫺2.

and f兾5.7 in the UV; 16° and f兾3.5 in the visible spectral ranges兲 that is larger than the solar disk 共0.55°, f兾55兲. Wide-angle observation of the Sun is desirable to smooth out any variation in the optical densities of the solar Fraunhofer lines that otherwise would occur for small changes in the observation geometry of the solar photosphere. However, viewing the whole Sun limits the achievable height resolution in a measured profile to approximately 1 km when the instrument observes the Sun from a 30-km altitude when the SZA is 91.0°. Both quartz fiber bundles consist of a set 共14兲 of individual quartz fibers, each with a 125-␮m outer diameter. The bundles are arranged to form round orifices at the fiber entrance and rectangular columns 共125 ␮m in width and 2.5 mm in height兲 at the fiber exit, which serve as entrance slits for the spectrometers. The total length of each quartz fiber bundle is 5 m, facilitating negligible light attenuation of 0.04 dB兾m 共loss 5%兲 and 0.025 dB兾m 共loss 3.5%兲 at 320 and 500 nm, respectively. From the entrance slit of either spectrometer, the incoming light reaches a holographic grating, which disperses the solar light onto the detector arrays in the respective wavelength interval 共UV, 320.6 – 422.6 nm; visible, 417.6 – 670.7 nm兲. The spectral ranges are chosen to permit sensitive detection of the electronic, vibrational, and rotational absorption bands of the atmospheric gases listed in Table 1. The light is detected by two state-of-the-art 1024element photodiode array detectors cooled to ⫺10 °C with on-chip integrated Peltier elements 共Hamamatsu S5931-1024N兲. The frequently used sapphire window that is usually included to protect the photodiode array surface was removed to prevent the occurrence of disturbing reflections of the incoming light, a process that increases spectrometer stray light. The photodiode outputs are preamplified and fed into two 16-bit analog-to-digital converters and read out by two 68332-CPU-driven controller devices. The total read20 May 2000 兾 Vol. 39, No. 15 兾 APPLIED OPTICS

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Table 2. Light Throughput of Individual Optical Components and Total Light Throughput 共at 360 nm兲 for the UV Instrument

Component

Number of Components

Relative Efficiency

Sun-tracker mirrors Preanalyzer Adjusting aperture

2 1 1

⬃0.9 0.2 1.3 ⫻ 10⫺4

Quartz fiber Holographic grating Quantum efficiency of photodiode array Total

1 1

0.95 0.5 共0.4..0.8兲 0.4

See Ref. 19 14 fibers with aperture 3.3 calculated for 8-mm-distance diffuser–fiber entrance Entrance and exit reflection and internal losses In the first order For the Hamamatsu S5931 photodiode array at 400 nm

4.4 ⫻ 10⫺6 or 共3.3 ⫻ 10⫺7兾pixel兲

out time of the electronics is ⬃60 ms for 1024 diodes, allowing us to record individual spectra within ⬃100 ms. Both 68332-CPU controller devices are controlled by a 486-PC single board, which also controls the onboard data storage as well as the communication to the ground station by means of telemetry– telecommand. We removed the heat generated at the warm sides of the Peltier elements that cool the photodiode arrays by cycling a refrigerant 共a 50%:50% mixture of ethylene glycol and water held at 0 °C兲 through a cooling system that consists of a reservoir containing ⬃10 kg of a liquid-water–ice mixture. We stabilized the spectrometer’s temperature within 共0 ⫾ 0.2兲 °C by integrating the whole spectrometer housing into the water–ice reservoir 共see Fig. 1兲 and by cycling the refrigerant through the cooling circuit with a twinhead membrane pump at a rate of 5 mL兾s. Also, the refrigerant is circulated through tubes mounted at the warm side of the photodiode Peltier elements and onto the middle plate of the spectrometer setup 共Fig. 1兲. The heat flux to the ambient is reduced by the instrument’s insulation with Styrodur foam plates of at least 5-cm wall thickness. Because of the large melting energy of the ice 共333 kJ兾kg兲, the water–ice reservoir buffers a total thermal energy of ⬃1.6 MJ flowing in either direction, which is sufficient to keep the instrument at constant temperature for the duration of a balloon flight 共maximum of 14 h兲. All optical, electrical, and liquid connections are fed through a flange of the spectrometer container to which the optical setup is attached. During preflight preparation the sealed spectrometer container is evacuated to ⬃10⫺6 mbars 共1 bar ⫽ 1 ⫻ 10⫺5 Pa兲. The leakage rate of the container is lower than ⬃1.3 ⫻ 10⫺6 mbars ⫻ L兾s, which prevents sizable amounts of ambient air and water vapor from entering the spectrometer. The evacuation of the spectrometers thus prevents any change in the refractive index of the spectrometer interior, which would change the spectrometer’s imaging and wavelength registration. For example, because during a balloon flight the ambient pressure changes from 1000 mbars to practically vacuum 共3 mbars at 40 km兲, for unsealed spectrometers the refractive index of air changes from 1.00027 to 1 and the wavelengths of the 2380

Comments


For a dispersion of 0.1 nm兾pixel

analyzed light change accordingly 共⬃0.094 nm at 350 nm and ⬃1.48 nm at 550 nm兲. Not sealing the spectrometer would thus cause a pressure-dependent change in spectral alignment 共⬃1 pixel兾1000 mbars at 350 nm兲 and in the spectrometer resolution 共⬃1 pixel兾1000-mbar full width at half-maximum; see Subsection 3.C below兲. Both effects are rather disturbing when one is attempting to measure gases with low optical densities 共e.g., stratospheric BrO with a typical optical density of 0.001兲 in a highly structured but constant light source such as the Sun. The sealing of the spectrometer also prevents the condensation of water vapor onto the cooled photodiode array detectors. Such a layer of water and ice would cause unwanted Fabry–Perot etalon modulation of the light that is illuminating the detector surface and also change the photon-detection characteristics and the dark current of the photodiode array. The total mass of the instrument is 42 kg, and its electrical power consumption is ⬃30 W, supplied by Ni–Cd batteries integrated onboard the gondola. 3. Instrumental Characteristics A.

Light Throughput

We estimate the instrumental light throughput by using the efficiencies of the spectrometer’s aperture, transmission, and detector quantum yield 共Table 2兲. Accordingly, for a solar photon flux of 3.3 ⫻ 1014 photons兾共s ⫻ cm2 ⫻ nm兲 at 400 nm,20 a signal 共for the UV spectrometer兲 of ⬃2 ⫻ 108 electrons兾共s ⫻ pixel兲 can be achieved. Inasmuch as the photodiode array saturation level is ⬃1.3 ⫻ 108 electrons 共see below兲, an overall integration time of ⬍1 s can be expected, a result that is in agreement with the measured signal at 30-km altitude 共and a SZA of 88°兲. B.

Detector Performance

1. Detector Noise The trace-gas detection sensitivity 共given by the smallest measurable optical densities兲 of the spectrometer is limited by the noise of photoelectron detection. It is given by the root-mean-square sum of

individual noise contributions 共a兲–共e兲 below to the total noise: 共a兲 The photoelectron shot noise is due to the statistical variance of the number of electrons generated by the photons illuminating the detector pixel. From the semiconductor capacity 共Cdiode ⫽ 10 pF兲 and its charge voltage 共U ⫽ 2.06 V兲, for a given fraction of saturation 共␣兲 the total number of electrons 共S兲 on a single photodiode array pixel is given by S⫽

Cdiode ⫻ U ⫻ ␣ . e

(1)

Accordingly, the photoelectron shot noise 共Nph兲 is given by Nph ⫽ 冑S,

(2)

where e is the elementary charge. For ␣ ⫽ 0.8, S is 1.04 ⫻ 108 electrons and Nph is ⬃104 electrons, or approximately five binary units 共BU兲 of the 16-bit analog-to-digital converter. 共b兲 The dark-current noise of a detector photodiode is due to the statistical variance of the dark current across the junction. For a detector temperature of ⫺10 °C, the dark current for the Hamamatsu S5931 chip is approximately SD ⫽ 8296 electrons兾s, or 4 BU兾s. Therefore the contribution of the darkcurrent noise to the total noise is approximately 公SD兾s ⫽ 91兾s, or 0.04 BU兾s. 共c兲 The dominant contribution to the preamplifier noise is given by Npr ⫽ 共1兾e兲关共in ⫻ tr兲2 ⫹ 共un ⫻ Ct兲2兴1兾2,

(3)

where in and un are the current and voltage noises of the preamplifier, respectively, tr is the integration time of the readout for one pixel 共tr ⫽ 20 ␮s兲, and Ct is the total capacity at the input of the preamplifier 共40 pF兲. For the low-noise preamplifier that we used 共operational preamplifier type OPA 627 from BurrBrown兲, in is 3.5 ⫻ 10⫺13 A and un is 1.0 ⫻ 10⫺6 V for a bandwidth of 5 ⫻ 104 Hz, and accordingly Npr is ⬃249 electrons, or 0.14 BU. 共d兲 The readout noise of a diode pixel that results from switching electronic capacities is given by21 Nr ⫽ 共1兾e兲兵k ⫻ T ⫻ 关2共Cdiode ⫹ Cvc兲兴其1兾2, where k is the Boltzmann constant, T 共⫽⫺10 °C兲 is the temperature, and Cvc 共⬃2 pF兲 is the capacity of the video and the clock line signal cross talk 共the other symbols are defined as above兲. Accordingly Nr is ⬃4124 electrons, or ⬃1.99 BU. 共e兲 The analog-to-digital converter noise is 1.3 BU, according to the data given by the manufacturer. Because usually contribution 共b兲 is negligible for short integration times, we can test contributions 共c兲– 共e兲 to the total noise by monitoring the signals of individual photodiodes in the fastest scanning mode 共100 ms兲 at zero illumination. For the sum 关共c兲–共e兲兴, ⬃3940 electrons, or 1.9 BU, are measured, in good

Fig. 4. Performance of the spectrometer: noise-to-signal ratio 共1兾公S兲 as a function of the residual structures 共1 ⫺ ␴ noise and peak-to-peak noise兲 detected in a ratioed halogen lamp spectra.

agreement with expectation 共4148 electrons, or ⬃2.0 BU兲. In summary, for a single spectrum, contributions 共a兲–共e兲 are 6.0 BU 共for a signal integration time of 1 s and an illumination of ␣ ⫽ 0.8 of the maximum兲, and thus a signal-to-noise ratio of ⬃104 is obtained. Taking into account that the signal integration times are typically several seconds for SZA’s of ⬍90° 共e.g., during the ascent of the gondola兲 and at most 15 s during sunset or sunrise, the photoelectron detection noise is given primarily by the photoelectron shot noise 关contribution 共a兲兴. Because this contribution depends on the square root of the total signal 关Eq. 共2兲兴, coadding the signal 共from subsequently recorded spectra兲 should correspondingly result in a squareroot increase of the signal-to-noise ratio. As can be seen clearly from Fig. 4, the instrument can be operated at the photoelectron shot noise, and thus we can further increase the detection sensitivity only by increasing the integrated photon flux. 2. Linearity The linearity of the spectrometer is the most crucial requirement for reliable DOAS measurements, because otherwise any change in illumination would inevitably cause residual structures that would be much larger than the absorptions to be measured. For atmospheric DOAS applications a rigorous test of an instrument’s overall linearity consists in its ability to record an individual solar Fraunhofer line at constant optical density for different fractions of detector saturation ␣. Accordingly, we tested the instrument’s linearity by observing direct Sun spectra from the ground near local noon 共SZA, 25°兲 at various saturations of the photodiodes. For such conditions, changes in the recorded optical density of Fraunhofer lines as a result of overlapping and temporally changing atmospheric absorptions 共e.g., owing to the presence of NO2兲 are expected to be negligible. For the test, two regions of the spectrum, one from the line center 共9 pixels wide兲 and one from the line wing 共11 pixels wide兲 of the Ca Fraunhofer line centers 共at 393 20 May 2000 兾 Vol. 39, No. 15 兾 APPLIED OPTICS

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Fig. 5. Test of the detector’s linear response 共the photodiode and the electronics兲. Shown are the ratios of two sums of photodiode readouts 共added signals of 11 photodiodes from the far wing of a Fraunhofer line, denoted as the upper channel cluster, and accordingly 9 added signals from the centers of two calcium Fraunhofer lines at 393 and 396 nm, denoted as the lower channel cluster兲 for direct Sun observation near local noon as a function of the average saturation level of the upper channel cluster. The error bars are due to the photoelectron shot noise 共Subsection 3.B兲. The thick solid line represents a least-squares regression to all data points of a signal saturation levels of ⬍80%. The resultant slope that represents the linearity of the detector electronics combination is 共2.7 ⫾ 3.3兲 ⫻ 10⫺5. Also shown are 共the two dashed– dotted vertical lines兲 the typical maximum signals at which the instrument is operated during a balloon flight 共at a saturation level of ⬃72%兲 and the level where the saturation effects of the photodiode array start to become important 共at ⬃90% of the total saturation level兲.

nm兲 were used. The ratio of these averaged lower and upper clusters is plotted in Fig. 5 as a function of signal saturation. Clearly, except for signals larger than ⬃90% of the total saturation, the instrument responds linearly to the illumination 关within 共2.7 ⫾ 3.3兲 ⫻ 10⫺5兴. For signal fractional saturations 共␣兲 larger than ⬃90%, however, individual photodiodes start to saturate, with a corresponding loss in linear response. Accordingly, if the signal is held below that limit, nonlinear effects of the photon detection are not expected to play a significant role in the measurements. C.

shift of solar spectra recorded during a balloon flight as a function of ambient pressure 共Fig. 7兲. Clearly, for the UV spectrometer and 共not shown here兲 for the visible spectrometer the largest wavelength shifts are of the order of 0.1 detector pixel width 共corresponding to a 1 ⫻ 10⫺2 nm shift in the wavelength calibration兲. The remaining change in the spectral imaging requires shifting and squeezing the reference spectra by fractions of a photodiode pixel width. As a consequence the spectra have to be numerically interpolated between the actual data points. Whether the effect of a shifting wavelength calibration can be properly accounted for was studied in a numerical simulation. A high-resolution solar spectrum23 was convoluted with the measured instrumental slit function to yield a simulated solar Fraunhofer reference spectrum. In a second calculation the highresolution solar spectrum was shifted by a given amount and again convoluted. The original and shifted spectra were then evaluated by use of the nonlinear fitting algorithm of Stutz and Platt.24 The simulations were carried out in a spectral range that

Spectral Characteristics

Following the sensitivity test of Roscoe et al.,22 proper line sampling for DOAS requires recording absorption lines with more than 4.5 detector pixels per FWHM. Accordingly, slit widths of 125 ␮m 共corresponding to five photodiode pixels兲 were chosen, which—in practice—lead to a FWHM of 0.45 nm 共or ⬃0.1 nm兾pixel兲 and of 1.48 nm 共or ⬃0.257 nm兾pixel兲 for the UV and the visible instruments, respectively. The mechanical setup has been designed to minimize the effect of changing ambient pressure and temperature on wavelength calibration. However, because of the mechanical stress caused by the changing ambient pressure, that effect cannot completely be avoided, and thus the wavelength calibration still changes during a balloon flight 共Fig. 6兲. We quantified the effect by inspecting the wavelength 2382

Fig. 6. Inferred spectral shifts for the UV instrument as a function of time for flights 1–3 共locations of León, Kiruna, and Gap given in text兲.


Fig. 7. Inferred peak-to-peak residual structure as a function of prescribed shifts for high-resolution solar spectra convoluted to the instrument’s resolution 共for details, see text兲.

Fig. 8. Ratio of retrieved and prescribed amounts of NO2 as a function of a prescribed spectral shift. For the simulation a peakto-peak differential absorption of NO2 of 10⫺4 in the UV 共346 –360 nm兲 is assumed. For a maximum wavelength shift 关10⫺2 nm; see Fig. 2共c兲兴 encountered in a balloon flight the error in the retrieved amount of NO2 is ⬃3%.

is used for the retrieval of BrO for two spectral resolutions 共5 and 9 detector pixels at FWHM兲 because the absorption features of BrO 共approximately 345– 360 nm兲 are expected to be most sensitive to any poorly corrected spectral shifts. As can be seen, the residual structures that are due to the remaining interpolation errors of the highly structured solar spectrum can be as large as 5 ⫻ 10⫺4 for shifts of half of a pixel width. For the typical spectral shifts encountered during a balloon flight 共below 0.1 pixel兲, however, the interpolation error results in residual structures of ⬃1 ⫻ 10⫺4, which are smaller than those that are due to other errors 共cf. the photoelectron shot noise or other spectral retrieval errors兲. However, not only the magnitude but also the position and the form of the spectral residual may interfere with trace-gas absorption. We simulated that influence by adding a high-resolution absorption signature of NO2 共with 10⫺4 peak-to-peak absorption兲25 to the high-resolution solar spectrum, which was then convoluted and again shifted with respect to the reference spectrum. Then the recovered amount of NO2 was investigated as a function of the spectral shift. Clearly, in that case significant errors in the retrieved amount of NO2 occur for shifts larger than 0.1 pixel width 共Fig. 8兲. In turn, because the instrument is operated at wavelength shifts smaller than 0.1 pixel, that effect is not likely to disturb our measurements. D.

Spectrometer Stray Light

For DOAS measurements, spectrometer stray light is one of the most disturbing effects.17 Because the DOAS method relies on an investigation of spectra recorded under different geophysical conditions 共for example, measured at low SZA at balloon float and at high SZA during solar occultation兲 a possible changing contribution of the spectrometer stray light to the signal can result in residual structures from individ-

ual Fraunhofer lines as large as 共or larger兲 than the trace-gas absorption to be investigated. During the first three flights a small but significant contribution of spectrometer stray light was observed in spectra recorded by the UV spectrometer. By subsequently blocking off the IR part of the solar spectrum with a set of filters 共Schott OG and RG series兲 we could identify most of the spectrometer stray light as originating from the 550 –1000-nm wavelength interval. As the spectrometer stray light did not display any narrow-band structures, which could mask or mimic trace-gas absorptions, that contribution could be corrected for by inclusion of a stray-light correction spectrum into the spectral retrieval 共i.e., an inverted Fraunhofer spectrum; for details see Refs. 16 and 26兲. To reduce the spectrometer stray light further, beginning with flight number 4 共see Section 4 below兲 we equipped the UV telescope with a spectroscopic preanalyzer designed to block photons with wavelengths larger than ⬃550 nm. A similar test with different visible and near-IR cutoff filters showed that the visible spectrometer is not affected by stray light in the same manner, and a further performance test showed that the stray-light contribution from the visible wavelength band is reasonably low 共⬍10⫺3兲. 4. Spectrometer Performance during Stratospheric Flight

Six successful balloon flights have already been conducted with the new DOAS instrument: flights 1 and 4 on 23 November 1996 and 19 March 1998 from Leoń, Spain 共42.5 °N, 5.8 °W兲; flights 2, 5, and 6 on 14 February 1997, 19 –20 August 1998, and 10 February 1999 from Esrange–Kiruna, Sweden 共68.9 °N, 22 °E兲; and flight 3 on 21 June 1997 from Gap, France 共44.5 °N, 6.1 °E兲. During all flights the instrument performed well and proved its reliability and its high sensitivity for the detection of the atmospheric species O3, NO2, OClO, BrO, H2O, and O4. Beginning with flight 4, a preselector was included in the instrumentation. The spectral retrieval was performed as already described in detail by Stutz and Platt24 and Sanders.27 For the retrieval in the visible spectral range the absorption signatures of O3, NO2 共for two different atmospheric temperatures兲, O4, and H2O were simultaneously nonlinear least-squares fitted to the ratio of measured spectra and a high Sun 共Fraunhofer reference兲 spectrum. The high Sun reference was taken from a measurement at balloon float altitude 共40 km兲. At this altitude small trace-gas absorptions were expected for small SZA’s. Accordingly, for the retrieval in the UV, the spectra of O3 共for two different temperatures兲, NO2, BrO, OClO, and O4 were simultaneously fitted. For a proper fit, the spectral signatures of the reference spectra 共O3, NO2, and OClO兲 were recorded with the instrument at several temperatures 共0, ⫺20, ⫺40, ⫺60, and ⫺80 °C for ozone兲 in the laboratory before the flights. The reference spectra of O4 and H2O were taken from Greenblatt et al.28 and Rothmann29 and convoluted with the actual instrumental slit function. Because 20 May 2000 兾 Vol. 39, No. 15 兾 APPLIED OPTICS

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Fig. 9. Retrieved trace-gas absorptions from spectra recorded during the ascent of the Leoń flight on 23 November 1996: 共a兲 450 –555-nm wavelength portion of two visible spectra taken at altitudes of 30 km 共thick curve兲 and 5 km 共thin curve兲; 共b兲 retrieved ozone absorption 共thin curve兲 compared with the laboratory ozone absorption signature 共thick curve兲; 共c兲 same as 共b兲 but for NO2; 共d兲 same as 共b兲 but for O4; 共e兲 same as 共b兲 but for H2O; 共f 兲 residual absorption after all known absorptions are removed from the ratio of the spectra shown in 共a兲.

the contribution of scattered photons to the signal was expected to be negligible for direct Sun observations, atmospheric ring spectra, which are due to the atmospheric rotational Raman scattering, were not included in the fitting procedure.30 Reasonably low spectral shifts 共Fig. 6兲 and spectral squeezes 共less than 0.1 pixel, or ⬃0.01 nm兲 were found, which demonstrated the high degree of spectral stability of the instrument even for the extreme changes in ambient conditions 共⌬T ⫽ 80 K and ⌬P ⫽ 1000 mbars兲 encountered during the balloon flights. Figure 9 shows the spectral signatures of the absorbers retrieved in the wavelength range 450 –555 nm 共for three scans added兲. All relevant absorbers 共O3, NO2, O4, and H2O兲 with differential absorption signatures larger than the detection limit 共⬃10⫺4兲 were detected, and the systematic residual structures 关Fig. 9, trace 共f 兲兴 were of the order of 3 ⫻ 10⫺4, as large as expected from the photoelectron shot noise. In Figure 10 the corresponding spectral features for the retrieval in the UV spectral range 共346 –360 nm兲 are shown. This spectral range is particularly sensitive for the detection of stratospheric BrO 关see Fig. 10, trace 共e兲兴, of which further details have recently been reported by Ferlemann,31 Ferlemann et al.,32 and Harder et al.33 In this case the residual structures are larger than expected purely from photoelec2384


Fig. 10. Same as Fig. 9 but for retrieval in the UV 共346 –360 nm兲: 共a兲 direct Sun spectra measured at 5.19 km 共SZA, 82°兲 and at 30.74 km 共SZA, 88°兲 over Spain on 23 November 1996, 共b兲 ozone, 共c兲 NO2, 共d兲 O4, 共e兲 BrO, 共f 兲 residual absorption structure.

tron detection noise. A comparison of successively recorded spectra shows similar residuals,32 which suggests that either unknown spectrometer structures or shortcomings in describing the atmospheric absorptions are most likely their cause. Finally, the utility of the instrument for stratospheric measurements is demonstrated by an intercomparison of the DOAS ozone measurement and a simultaneously launched electrochemical cell ozonesonde. For the test, the ozone Chappuis absorption band was chosen. The two ozone profiles 共the DOAS and the electrochemical cell兲 compare well, although the instruments did not probe exactly the same air masses, primarily because of the spatial averaging of the measured ozone absorption along the line of sight of the DOAS instrument. For NO2 measurements similarly good agreement between the DOAS instrument and the LPMA IR instrument 共which was also operated onboard the gondola and probes the atmosphere with the same observation geometry兲 was found. In addition, on the Kiruna flight 共14 February 1997兲, good agreement between NO2 measurements performed both from the LPMA–DOAS gondola and the ILAS 共Improved Limb Atmospheric Spectrometer兲 instrument operated on the Japanese ADEOS 共Advanced Earth Observing Satellite兲 was found. 5. Conclusion

A new state-of-the art UV–visible differential optical absorption spectrometer for the detection of strato-

The project described here is funded by the Bundes Ministerium fu¨r Bildung und Forschung 共01LO9316兾5兲 and the European Union 共contract ENV4-CT-95-0178兲. We are grateful to C. CamyPeyret of the LPMA, Paris, and his colleagues as well as to D. Huguenin of the Observatoire de Gene`ve and his team for their great support in integrating the DOAS instrument on the LPMA gondola and for their assistance in the balloon launches already conducted. References

Fig. 11. Comparison of ozone profiles measured with the DOAS instrument by solar occultation during the balloon ascent with an electrochemical cell 共ECC兲 in situ ozone sonde with measurements performed from Kiruna, Esrange, on 14 February 1997. Because the DOAS instrument probed air masses in the atmospheric line of sight, which is several tens to several hundreds of kilometers long, but the electrochemical cell sonde measures ozone in situ, spatial inhomogeneities in the ozone field are expected to yield different profiles. Nevertheless, the major features of the ozone layer, such as the laminae near 18 km, were detected by both instruments.

spheric trace-gas profiles by direct Sun observation from balloons has been described. It was demonstrated that the instrument meets important design criteria for sensitive DOAS balloonborne solar occultation measurements. These include a high signalto-noise ratio, low dark current of the detector, light weight, lower power consumption, low spectrometer stray light, and low spectral shifts at changing atmospheric pressures and temperatures. Also, we have argued that the instrument’s detection limit is due to the photoelectron shot noise; in the visible region this limit 共⬃3 ⫻ 10⫺4兲 is almost reached. However, either because of yet unresolved problems in properly describing the atmospheric absorption features from trace gases and aerosols or because of still unknown spectrometer structures, the trace-gas detection limit 共in terms of optical density兲 given by the peak-to-peak residuals is still ⬃8 ⫻ 10⫺4 in the UV. Recent balloon flights undertaken with the instrument demonstrated its reliability. High-quality absorption spectra of stratospheric O3, NO2, BrO, OClO, O4, and H2O were recorded. Vertical profiles of O3, NO2, BrO, and O4 could already be derived.32,33 From these data and results yet to come, new and exciting insights into stratospheric ozone chemistry can be obtained.

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