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High-resolution spectrometer for atmospheric studies
ARTICLE IN PRESS Journal of Atmospheric and Solar-Terrestrial Physics 71 (2009) 1383–1388
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High-resolution spectrometer for atmospheric studies Piero Di Carlo a,b,, Massimiliano Barone a, Alfonso D’Altorio a, Cesare Dari-Salisburgo b, Ermanno Pietropaolo a a b
` Degli Studi di L’Aquila, L’Aquila, Italy Dipartimento di Fisica, Universita ` Degli Studi di L’Aquila, L’Aquila, Italy Centre of Excellence—CETEMPS, Universita
a r t i c l e in fo
abstract
Article history: Received 7 November 2008 Received in revised form 19 May 2009 Accepted 22 May 2009 Available online 21 June 2009
A high-resolution spectrometer (0.0014 nm at 313 nm) has been developed at the University of L’Aquila (Italy) for atmospheric spectroscopic studies. The layout, optics and software for the instrument control are described. Measurements of the mercury low-pressure lamp lines from 200 to 600 nm show the high performances of the spectrometer. Laboratory measurements of OH and NO2 spectrums demonstrate that the system could be used for cross-section measurements and to detect these species in the atmosphere. The first atmospheric application of the system was the observation of direct solar and sky spectrums that shows a filling-in of the sky lines due to rotational Raman scattering. The measurements have been done with clear and cloudy sky and in both there was a strong dependence of the filling-in from the solar zenith angle whereas no dependence from the wavelengths was evident at low solar zenith angles (less than 851). & 2009 Elsevier Ltd. All rights reserved.
Keyword: Remote sensing Atmospheric spectroscopy Fraunhofer lines Ring effect
1. Introduction A direct technique such as differential optical absorption spectroscopy (DOAS) is a powerful method to measure the column contents of target species in the atmosphere. This technique does not need a calibration and is based on the analysis of spectral signatures of the molecules. In the last decades DOAS has been used to detect almost all the species of interest for climate change, air quality and chemistry in the troposphere and stratosphere. Species like ozone (O3), nitrogen dioxide (NO2), iodine (I2), bromine oxide (BrO), nitrate radical (NO3), formaldehyde (HCHO), glyoxal (CHOCHO) and hydroxyl radical (OH) were studied using DOAS on ground, aircraft and satellite platforms (Platt, 1994; Plane and Smith, 1995; Plane and Saiz-Lopez, 2006). DOAS using moderate spectral resolution spectrometers (between 0.1 and 1 nm) are usually used to detect all the species excluding OH that needs high-resolution spectrometer since its absorption lines have features in a spectral window of about 0.10 at 308 nm (Mount, 1992; Dorn et al., 1995). Spectrometers with high spectral resolution (better than 0.01 nm) are usually used in laboratory experiments to determine molecular cross section but they could improve the selectivity, precision and accuracy of the measurements of some atmospheric minor constituents. For example, the cross section of atmospheric chemistry key species, such as NO2,
Corresponding author at: Dipartimento di Fisica, Universita` Degli Studi di L’Aquila, 67010L’Aquila, Italy. E-mail address: [email protected] (P. Di Carlo).
1364-6826/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jastp.2009.05.012
iodine oxides (IO, OIO) and molecular iodine (I2) shows large variation of the shape and the intensity with temperature and pressure; these could be a limitation in their observations. IO, OIO and I2 are central in the chemistry of marine boundary layer and they are usually detected with DOAS moderate resolution (0.5–0.27 nm) spectrometers (Alicke et al., 1999; Allan et al., 2001, Read et al., 2008). To reduce measurements errors and artifacts of I2, an high-resolution spectrometer (at least for laboratory cross-section measurements) could be used to detect the variations of I2 shapes lines and intensity with temperature and pressure (Spietz, et al., 2006). High-resolution spectrometers (better than 0.010 nm) allow the exact identification of the spectral lines of CHOCHO, an important species in the ozonecontrol strategies, because is an indicator of the oxidation rate of numerous volatile organic compounds (Volkamer, et al., 2005). Finally, an high-resolution spectrometer that allows the detection of small changes (less than 0.1%) in the absorption lines, could be used to retrieve the NO2 vertical distribution since atmospheric temperature and pressure changes with altitude (Wennberg, et al, 1997; Nizkorodov et al., 2004). High-resolution spectrometers could help to advance our knowledge of the physical phenomena that reduces the lines depth (‘filling-in’) when atmospheric scattered light is measured. This effect, observed since early sixties and also known as Ring effect (RE) (Grainger and Ring, 1962), is a filling-in of Fraunhofer lines in the spectrum of the scattered solar radiation compared to the spectrum of direct solar light. Inaccurate evaluation of the RE can generate systematic errors of the measurements of the trace gases like NO2, O3, OClO, BrO, SO2, CO and HCHO, made with
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absorption spectroscopy techniques (Vountas et al., 1998; Wagner et al., 2002). The observations of the RE have had a long and tortuous history with controversial interpretations of the origin: now it is generally accepted that is mainly due to the rotational Raman scattering (Brinkmann, 1968; Joiner et al., 1995; Sioris and Evans, 1999). Big differences are observed in the dependence of the lines depth of the RE on solar zenith angle (SZA) and wavelength (Brinkmann, 1968; Kattawar et al., 1981; Pallamraju et al., 2000). The variations of the RE with solar zenith angle show contradictory results since some measurements report an increase of the filling-in as SZA increases (Pavlov et al., 1973; Harrison, 1976; Pallamraju et al., 2000), whereas other measurements show constant filling-in for small SZA and an abrupt increase just at twilling Conde et al. (1992). Finally, few others report a decrease of RE as SZA increases (Noxon and Goody, 1965; Harrison, 1976). Most of the RE studies have been done just at one wavelength and the few done at more than one wavelength show incongruous filling-in dependence on wavelength: few observations report an increase of the RE with wavelength (Harrison, 1976; Chanin, 1975), whereas others a decrease (Noxon and Goody, 1965; Pavlov et al., 1973). A more systematic study, looking at 11 wavelengths, in two different spectral regions (428.6 and 558.6 nm) have been done by Pallamraju et al. (2000) with the highest resolution spectrometer (0.029 at 589.3 nm) used so far for this kind of observations at 558.6 nm. In this work, we describe the characteristics of a very highresolution spectrometer (0.0014 nm at 313 nm) setup, mainly, for OH DOAS detection at 308 nm. Laboratory observations have been conducted to test the resolution of the instrument using mercury lamp. Further laboratory tests have been done generating OH and NO2 in a flow reactor to test directly if the system is able to observe the cross-section lines of these important atmospheric species. The first atmospheric use of the system was the measurement of the solar and sky spectrum at 558.6 nm to observe the RE in a spectral window where comparisons could be done with previous measurements. Results of the atmospheric measurements of the RE with our high-resolution spectrometer are reported as well as the dependence of the filling-in sky Fraunhofer lines on solar angle and wavelength.
2. Instrumentation and laboratory measurements The high-resolution spectrometer developed at the University of L’Aquila uses optics and mechanics manufactured by SOPRA and includes a pre-monochromator (pre-mono) to select the order
of the spectrometer. The spectrometer has a focal length of 2 m and could be used in a single-pass or in a double-pass configuration (Fig. 1). In this work, the double-pass setup has been used, because it allows to double the performance of the system in terms of resolution. In the double-pass configuration the light from the entrance slit hits the concave mirror M1 (Fig. 1) and it is sent to the grating that reflects the dispersed light again in the direction of the mirror M1. From the 27 cm diameter concave mirror the light is sent to a couple of 451 small mirrors M2 and M3 (3 1.5 cm) and it is shifted about 2 cm above the entrance plane of the light. After the mirror M3 the light hits again the mirror M1 and the grating, travelling parallel to the first path, to be detected by the CCD on the exit plane. With this configuration the spectrometer is like 2 spectrometers superimposed with only one grating and one field mirror (M1), this allows to double the dispersion and the resolution and to reduce of a factor of 100 the stray light. On the other hand, the luminosity will be reduced of a factor of two. The spectrometer includes an Echelle grating with 316 grooves/mm and the highest efficiency (86%) is at 594 nm. The grating and the spectrometer optics have a wide spectral window (from 180 to 2600 nm with an optimized reflectivity in the interval 270–850 nm, higher than 90%) and it can work at several spectral orders; these characteristics of the spectrometer are quite unique compared with spectrometers previously developed (Pallamraju et al., 2000; Chakrabarti et al., 2001). In the UV it works at the 19th order, whereas in the visible it works at the 10th order. The M1 efficiency at 594 nm is 92% and in a double-pass setup the transmission of all the system (pre-mono plus spectrometer) is about 30%. The spectrometer uses 3 bars of Super Invar to form a coaxial triangle to carry the grading, the mirror and the slits. The Super Invar guarantees a high thermal stability of the spectrometer and from several tests no effects were evident on the spectral dispersion for variations in temperature within 2 1C. To avoid any temperature effects on the prism and diffraction grating the spectrometer is used in a temperature-controlled laboratory with temperature changes below 71 1C. The pre-mono is a 50 cm focal length monochromator in a Czerny–Turner setup (Fig. 1). It allows high efficiency of the spectrometer in all his spectral window selecting the spectral order. The light is dispersed twice by a quartz prism, which has a spectral band that covers the spectrometer window and an efficiency of 70% peaked around 400 nm. Its input aperture is f/5, whereas the output aperture is adapted to the spectrometer input. The pre-mono reduces also the stray light of the spectrometer: tests using a Yag laser (emission at 532.072 nm and the
340.0 mm
2080.0 mm
M3
CCD M1
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Pre-mono PC Controller Pre-mono & Spectrometer
Controller CCD Camera
Fig. 1. Schematic of the spectrometer and pre-mono used to select the order and reduce the stray light. M2 and M3 are 451 mirrors allow the double-pass configuration to simulate a spectrometer with a double focal length (only one light path is reported).
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full-width at half-maximum (FWHM) ¼ 0.044 nm) as a source show that the stray light increases from 106 to 105 when the pre-mono is removed. These measurements have been done moving the grating to detect light at about 71 nm from the laser line peak. The theoretical spectral resolving power (R) of the grating is given by: R ¼ mNW
(1)
where m is the order, N the lines per mm of the grating (in our case 316) and W the grating width (here, 265 mm). For example, at 313 nm the spectrometer grating works at the 19th order so R ¼ 1,590,000, whereas at 558 nm the order m ¼ 10 so R ¼ 837,400. This is the theoretical spectral resolving power of the grating; the spectral resolving power of all the system (premono plus spectrometer) is lower and it is measured from experimental mercury lines analysis (see below). The entrance of the pre-mono is coupled with two fiber systems: (1) a bundle of fibers (20 fibers of 200 mm each) with one end round shaped to collect the light and the other end linear (200 mm 6 mm) to match the slit entrance, (2) a bundle of 5 separate fibers (60 mm each) to detect 5 different signals, in the other end the fibers are aligned to match the slit entrance, here the distance between each fiber is 400 mm to avoid the mixing of the signals on the CCD. The fibers are 10 m long and tests, using a 532.072 nm Yag laser and 632.820 nm He–Ne laser light, show that they completely scramble the polarization information in the incoming light. The detector is a CCD manufactured by Princeton with 1024 256 pixels. The pixel dimension is 26 26 mm, the full well capacity is 1.3 106 electrons in spectroscopy mode. The CCD is front illuminated with a UV extension and the quantum efficiency is 45% at 650 nm, 30% at 500 nm, 10% at 960 nm and 12% from 200 to 400 nm. The controller has a dynamic range of 16 bit. The cooling system uses liquid nitrogen and the dark current is 0.5 e-/piixel/h at 120 1C. A custom software to control the spectrometer has been developed using LabVIEW of National Instruments and it has been designed for the pre-mono prism movements (to select the order), the spectrometer grating movements (for wavelength selection),
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the aperture of the shutter and for data collection of the CCD. The software allows automatic scan from UV to IR with fixed or variable CCD acquisition time for each scan and automatic measurements and compensation for the flat field and subtraction of the background. To test the performance of the system a mercury lamp from Avantes (model Avalight-cal) as light source was coupled with the bundle of fibers. In Fig. 2 are reported some Hg lines observed scanning the spectrometer from UV to visible. This highresolution system allows to resolve in the UV as well as in the visible the satellites of several mercury lines originated by a magnetic hyperfine structures of isotopes of Hg contained in the lamp (Sansonetti et al., 1996). The wavelength difference between these satellites is 0.0028 nm for the 312.5674 nm line and 0.013 nm for the 579.067 nm line. To experimentally estimate the resolution of the spectrometer, we found two close mercury lines centered at 313.1555 and 313.1844 nm (see Fig. 2). Using the slit aperture of 25 mm, the FWHM of the lines is 5 pixels and their separation is 102 pixels. The linear dispersion is 0.00028 nm/ pixel and the resulting resolution is 223,000 that corresponds to 0.0014 nm (FWHM), whereas the linear dispersion at 558 nm is 0.0008 nm/pixel and, the resolution estimated using the solar lines centered at 558.875 and 558.676 nm, is 111,000 that is about 0.005 nm (FWHM). To reach the above resolution values the aperture of the monochromator slit must be kept very small (about 25 mm). In this configuration, the instrument can be used with a laser source or a lamp, because the luminosity is very low. For atmospheric observations, to have enough light through the monochromator, a wider slit must be used, and that implies a poorer resolution. To test the spectrometer and the CCD performance with species of atmospheric interest, we measured the absorption spectrum of OH and NO2 in the laboratory at ambient pressure. OH is produced in flow reactor cell (1 m long) where 3 lowpressure mercury lamps (Heraeus, mod. G36TSVH/4) emit about 3 W at 185 nm. The mercury line at 185 nm photodissociates water vapor creating OH and H. Water vapor is generated flowing nitrogen through distilled water in the reactor. The source used for the measurements is a calibrated xenon lamp (Hamamatsu model L7810). In Fig. 3a, is shown the OH absorption spectrum calculated
x 104 0.003 nm
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5
4
3
2
1
0 296.7282 312.5674
313.1555 313.1844 404.6564 435.8334 579.0670 Wavelength (nm)
Fig. 2. Overview of a mercury lamp spectrum measured with the spectrometer system. The wavelength of the central lines is reported but their distances are not in scale.
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1.001 1 I/Io
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Wavelength (nm) Fig. 3. Panel A: absorption lines of OH measured in a humidified flow cell (see text). Panel B: absorption lines measured in a cell filled with NO2 at ambient pressure (see text) compared with the cross section of NO2 measured by Vandaele et al. (2002).
from the ratio of the signal measured when in the reactor is generated OH (I) and the signal without OH (IO). The relative intensity and wavelength displacement of the OH lines (Q1(2), Q1(3) and P1(1)) agree with previous observations (Mount, 1992; Dorn et al., 1995). The system is able to detect signal differences less than 0.1%. The absorption spectrum of another species of atmospheric interest (NO2) is reported in Fig. 3b. The measurements have been done with the technique used for the OH measurements. NO2 is flushed in the cell mixed with nitrogen and oxygen. NO2 cross-section measurements are in agreement with Vandaele et al. (2002) observations.
3. Atmospheric observations Field measurements in the atmosphere have been done observing the Fraunhofer lines to find how their depth changes when zenith-scattered solar spectrum are compared with direct solar sunlight. The band identified for this investigation was centered at 558.7 nm, because, looking at previous works, the lines in this band show a big difference of the filling-in as function of the solar zenith angle and wavelength (Kattawar et al., 1981; Pallamraju et al., 2000). The observations were carried at The Gran Sasso National Laboratories (LNGS) near L’Aquila (Italy) (42.351N, 13.381E, elevation 980 m). The measurements were done during clear sky as well as during cloudy days. Each observation consists of switching between measurements pointing the zenith to observe the scattered light and measurements tracking the sun for the observation of the direct sunlight. The resolution has been degraded (increasing the slit aperture) to about 0.015 nm to keep a high signal-to-noise ratio in a short-time exposure. The CCD exposure was few seconds during direct sunlight measurements and 100 s during observation of scattered light. The time exposure for the scattered light has been chosen in order to have counts close to the full well capacity of the CCD (to improve the signal-tonoise ratio) and at the same time not too long to have the lower possible variation of the sun zenith angle (less than 0.51). The RE that is the difference between zenith-scattered solar spectrum and direct solar sunlight spectrum (also called absolute RE contribution), is affected by the background continuum level
that varies with SZA and by the wavelength. To remove the background continuum level, the RE is computed in terms of the fractional Ring effect (FRE) (Kattawar et al., 1981) FRE ¼ RE=ðRE þ BCÞ
(2)
where BC is the background continuum obtained with a linear fit between the continuum regions on each side of the Fraunhofer lines under consideration. Fig. 4a shows an example of a skylight and direct sunlight measured and of the background continuum. The big difference between the scattered light (skylight) at SZA higher than 831 and the direct sunlight is due to the filling-in calculated in terms of FRE (Fig. 4b). The error on the FRE evaluation is due to shot noise, spectrum alignment and background continuum evaluation, and globally is estimated to be between 2% and 3%. The variation of the FRE with SZA and wavelength is reported in the Fig. 5a for measurements with clear sky during all periods of the observations. The sky conditions were monitored with a pyrheliometer (model MS-53, Eko instrument) pointed to the zenith controlling the scattered light and a pyranometer (MS-802, Eko instrument) mounted on a sun tracker (STR-O1, Eko instrument) to monitor the direct sun light. For SZA higher than 851, we observed a dependence of the FRE on the wavelength and at SZA ¼ 901 the FRE is 8%, 10%, 14%, 15% and 16% for the lines at 559.04, 558.78, 558.75, 558.67 and 558.87 nm, respectively. At SZA ¼ 911 all the lines are in the FRE range between 17% and 20%. The variations of the RE as function of SZA (reported in Fig. 5a) is very low (less than 5% for SZA lower than 801), and an abrupt increase up to 30% for SZA higher than 901 are reported by previous observations. Fig. 5a shows no wavelength dependence of the RE for all the 5 lines measured in the 558.6 nm band (SZA lower than 851), whereas in a previous work with a spectrometer with a poorer resolution (factor of 2) was found a big difference of the FRE. Pallamraju et al. (2000) report a difference of the FRE between the lines 558.67 and 558.87 nm of more than 5% at almost all the solar zenith angle. Besides the difference in the spectrometer resolution, Pallamraju et al. (2000) observations were carried out in Boston, US, while we took our measurements in a quite clean environment. Joiner et al. (1995) show that degrading the resolution the filling-in becomes lower, this could
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1 0.8 0.6 Scattered skylight, SZA = 87.72° Direct sunlight Background continuum
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Fig. 4. Panel A: an example of direct sunlight and scattered skylight measured when the solar zenith angle (SZA) was 83.521. Panel B: the fractional Ring Effect of the spectrums reported in the upper panel.
50 558.67nm 558.75nm 558.78nm 558.87nm 559.04nm
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Fig. 5. Panel A: fractional Ring effect observed during clear sky. The measurements started at 16:10 LT until 20:02 LT. Panel B: as for Panel A but with partly cloudy sky.
suggest a possible instrumental effect to explain the difference between Pallamraju et al. (2000) observations and ours. For example, Conde et al. (1992) report the same RE dependence on SZA of our study, observing the sodium D2 line at 589 nm in Antartica with a Fabry–Perot spectrometer having higher resolution (0.00134 nm) than Pallamraju et al. (2000) as well as of this work. Fig. 5b shows the FRE as function of the solar zenith angle observed during a cloudy day that was mainly
cumulonimbus. The qualitative variation of the FRE with SZA is similar to that detected during the clear day reported in Fig. 5a. Looking at the data of the pyrheliometer the solar radiation of the cloudy day was, globally, about 20% lower than what measured during the clear day, but the variation of the RE for low SZA was very small. On the other hand, the filling-in for SZA higher than 851 was bigger during cloudy day compared to what observed during the clear sky for all the lines: at SZA ¼ 901 the FRE rises
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from 10%, for the observations with clear sky to 14% for cloudy sky (line 558.78 nm), the FRE of the line 558.87 and 558.67 nm increases of 9% and 5%, respectively. No effect of the clouds is observed for the line 558.75 nm. For SZA ¼ 911 the increment of the filling-in ranges from 1% of the lines 558.75 and 558.78 nm to 13% of the line 558.87 nm. The cloud effect on the filling-in has been studied first by Harrison and Kendall (1974): they suggested that overcast skies increase filling-in whereas bright sunlit clouds have the effect of weakening the filling-in. These results are confirmed by observations and models made by de Beek et al. (2001) and our observations are consistent with the results of their model for cumulonimbus clouds.
4. Conclusions In this work, we described a new spectrometer with a resolution of 0.0014 nm for atmospheric spectroscopy observations. The spectrometer allows to detect high-resolution spectra from UV to IR, so that the system could be used for cross-section measurements of wide range of species of atmospheric interest. Laboratory measurements of OH and NO2 absorption spectrums prove that it could be used for the detection of the atmospheric hydroxyl radicals with DOAS techniques, the retrieval of atmospheric vertical distribution of NO2 and for the detection of column content of other atmospheric gases. The measurements of the filling-in of the Fraunhofer lines in the visible show a strong dependence on the solar zenith angle only near the twilight with a quite flat RE (5%) during the all day. The wavelength dependence of the filling-in is evident only for solar zenith angle higher than 851. Cloudy sky increases the filling-in at all the wavelength (excluding the 558.75 nm) at high solar zenith angle as expected from models using low (0.2 nm)resolution spectra.
Acknowledgements We thank the support of Francesco Del Grande (Dipartimento di Fisica Universita` di L’Aquila), Leo Tatananni, Antonello Rotilio, Bruno Romualdi and Angelo Corsi (Laboratori Nazionali del Gran Sasso, INFN) for the realization of all the mechanical parts. P. Di Carlo work is supported by Fondazione CARISPAQ. This work has been supported by the Center of Excellence CETEMPS and we thank Prof. G. Visconti for the helpful comments and his helps in setting up this instrument. References Alicke, B., Hebestreit, K., Stutz, J., Platt, U., 1999. Iodine oxide in the marine boundary layer. Nature 397 (6720), 572–573. Allan, B.J., Plane, J.M.C., McFiggans, G., 2001. Observations of OIO in the remote marine boundary layer. Geophysical Research Letters 28 (10), 1945–1948. Brinkmann, R.T., 1968. Rotational Raman scattering in planetary atmospheres. Astrophysics Journal 154, 1087–1093. Chakrabarti, S., Pallamraju, D., Baumgardner, J., Vaillancourt, J., 2001. HiTIES: a high throughput imaging echelle spectrograph for ground-based visible airglow and auroral studies. Journal of Geophysical Research 106, 30337–30348.
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Journal of Atmospheric and Solar-Terrestrial Physics journal homepage: www.elsevier.com/locate/jastp
High-resolution spectrometer for atmospheric studies Piero Di Carlo a,b,, Massimiliano Barone a, Alfonso D’Altorio a, Cesare Dari-Salisburgo b, Ermanno Pietropaolo a a b
` Degli Studi di L’Aquila, L’Aquila, Italy Dipartimento di Fisica, Universita ` Degli Studi di L’Aquila, L’Aquila, Italy Centre of Excellence—CETEMPS, Universita
a r t i c l e in fo
abstract
Article history: Received 7 November 2008 Received in revised form 19 May 2009 Accepted 22 May 2009 Available online 21 June 2009
A high-resolution spectrometer (0.0014 nm at 313 nm) has been developed at the University of L’Aquila (Italy) for atmospheric spectroscopic studies. The layout, optics and software for the instrument control are described. Measurements of the mercury low-pressure lamp lines from 200 to 600 nm show the high performances of the spectrometer. Laboratory measurements of OH and NO2 spectrums demonstrate that the system could be used for cross-section measurements and to detect these species in the atmosphere. The first atmospheric application of the system was the observation of direct solar and sky spectrums that shows a filling-in of the sky lines due to rotational Raman scattering. The measurements have been done with clear and cloudy sky and in both there was a strong dependence of the filling-in from the solar zenith angle whereas no dependence from the wavelengths was evident at low solar zenith angles (less than 851). & 2009 Elsevier Ltd. All rights reserved.
Keyword: Remote sensing Atmospheric spectroscopy Fraunhofer lines Ring effect
1. Introduction A direct technique such as differential optical absorption spectroscopy (DOAS) is a powerful method to measure the column contents of target species in the atmosphere. This technique does not need a calibration and is based on the analysis of spectral signatures of the molecules. In the last decades DOAS has been used to detect almost all the species of interest for climate change, air quality and chemistry in the troposphere and stratosphere. Species like ozone (O3), nitrogen dioxide (NO2), iodine (I2), bromine oxide (BrO), nitrate radical (NO3), formaldehyde (HCHO), glyoxal (CHOCHO) and hydroxyl radical (OH) were studied using DOAS on ground, aircraft and satellite platforms (Platt, 1994; Plane and Smith, 1995; Plane and Saiz-Lopez, 2006). DOAS using moderate spectral resolution spectrometers (between 0.1 and 1 nm) are usually used to detect all the species excluding OH that needs high-resolution spectrometer since its absorption lines have features in a spectral window of about 0.10 at 308 nm (Mount, 1992; Dorn et al., 1995). Spectrometers with high spectral resolution (better than 0.01 nm) are usually used in laboratory experiments to determine molecular cross section but they could improve the selectivity, precision and accuracy of the measurements of some atmospheric minor constituents. For example, the cross section of atmospheric chemistry key species, such as NO2,
Corresponding author at: Dipartimento di Fisica, Universita` Degli Studi di L’Aquila, 67010L’Aquila, Italy. E-mail address: [email protected] (P. Di Carlo).
1364-6826/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jastp.2009.05.012
iodine oxides (IO, OIO) and molecular iodine (I2) shows large variation of the shape and the intensity with temperature and pressure; these could be a limitation in their observations. IO, OIO and I2 are central in the chemistry of marine boundary layer and they are usually detected with DOAS moderate resolution (0.5–0.27 nm) spectrometers (Alicke et al., 1999; Allan et al., 2001, Read et al., 2008). To reduce measurements errors and artifacts of I2, an high-resolution spectrometer (at least for laboratory cross-section measurements) could be used to detect the variations of I2 shapes lines and intensity with temperature and pressure (Spietz, et al., 2006). High-resolution spectrometers (better than 0.010 nm) allow the exact identification of the spectral lines of CHOCHO, an important species in the ozonecontrol strategies, because is an indicator of the oxidation rate of numerous volatile organic compounds (Volkamer, et al., 2005). Finally, an high-resolution spectrometer that allows the detection of small changes (less than 0.1%) in the absorption lines, could be used to retrieve the NO2 vertical distribution since atmospheric temperature and pressure changes with altitude (Wennberg, et al, 1997; Nizkorodov et al., 2004). High-resolution spectrometers could help to advance our knowledge of the physical phenomena that reduces the lines depth (‘filling-in’) when atmospheric scattered light is measured. This effect, observed since early sixties and also known as Ring effect (RE) (Grainger and Ring, 1962), is a filling-in of Fraunhofer lines in the spectrum of the scattered solar radiation compared to the spectrum of direct solar light. Inaccurate evaluation of the RE can generate systematic errors of the measurements of the trace gases like NO2, O3, OClO, BrO, SO2, CO and HCHO, made with
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absorption spectroscopy techniques (Vountas et al., 1998; Wagner et al., 2002). The observations of the RE have had a long and tortuous history with controversial interpretations of the origin: now it is generally accepted that is mainly due to the rotational Raman scattering (Brinkmann, 1968; Joiner et al., 1995; Sioris and Evans, 1999). Big differences are observed in the dependence of the lines depth of the RE on solar zenith angle (SZA) and wavelength (Brinkmann, 1968; Kattawar et al., 1981; Pallamraju et al., 2000). The variations of the RE with solar zenith angle show contradictory results since some measurements report an increase of the filling-in as SZA increases (Pavlov et al., 1973; Harrison, 1976; Pallamraju et al., 2000), whereas other measurements show constant filling-in for small SZA and an abrupt increase just at twilling Conde et al. (1992). Finally, few others report a decrease of RE as SZA increases (Noxon and Goody, 1965; Harrison, 1976). Most of the RE studies have been done just at one wavelength and the few done at more than one wavelength show incongruous filling-in dependence on wavelength: few observations report an increase of the RE with wavelength (Harrison, 1976; Chanin, 1975), whereas others a decrease (Noxon and Goody, 1965; Pavlov et al., 1973). A more systematic study, looking at 11 wavelengths, in two different spectral regions (428.6 and 558.6 nm) have been done by Pallamraju et al. (2000) with the highest resolution spectrometer (0.029 at 589.3 nm) used so far for this kind of observations at 558.6 nm. In this work, we describe the characteristics of a very highresolution spectrometer (0.0014 nm at 313 nm) setup, mainly, for OH DOAS detection at 308 nm. Laboratory observations have been conducted to test the resolution of the instrument using mercury lamp. Further laboratory tests have been done generating OH and NO2 in a flow reactor to test directly if the system is able to observe the cross-section lines of these important atmospheric species. The first atmospheric use of the system was the measurement of the solar and sky spectrum at 558.6 nm to observe the RE in a spectral window where comparisons could be done with previous measurements. Results of the atmospheric measurements of the RE with our high-resolution spectrometer are reported as well as the dependence of the filling-in sky Fraunhofer lines on solar angle and wavelength.
2. Instrumentation and laboratory measurements The high-resolution spectrometer developed at the University of L’Aquila uses optics and mechanics manufactured by SOPRA and includes a pre-monochromator (pre-mono) to select the order
of the spectrometer. The spectrometer has a focal length of 2 m and could be used in a single-pass or in a double-pass configuration (Fig. 1). In this work, the double-pass setup has been used, because it allows to double the performance of the system in terms of resolution. In the double-pass configuration the light from the entrance slit hits the concave mirror M1 (Fig. 1) and it is sent to the grating that reflects the dispersed light again in the direction of the mirror M1. From the 27 cm diameter concave mirror the light is sent to a couple of 451 small mirrors M2 and M3 (3 1.5 cm) and it is shifted about 2 cm above the entrance plane of the light. After the mirror M3 the light hits again the mirror M1 and the grating, travelling parallel to the first path, to be detected by the CCD on the exit plane. With this configuration the spectrometer is like 2 spectrometers superimposed with only one grating and one field mirror (M1), this allows to double the dispersion and the resolution and to reduce of a factor of 100 the stray light. On the other hand, the luminosity will be reduced of a factor of two. The spectrometer includes an Echelle grating with 316 grooves/mm and the highest efficiency (86%) is at 594 nm. The grating and the spectrometer optics have a wide spectral window (from 180 to 2600 nm with an optimized reflectivity in the interval 270–850 nm, higher than 90%) and it can work at several spectral orders; these characteristics of the spectrometer are quite unique compared with spectrometers previously developed (Pallamraju et al., 2000; Chakrabarti et al., 2001). In the UV it works at the 19th order, whereas in the visible it works at the 10th order. The M1 efficiency at 594 nm is 92% and in a double-pass setup the transmission of all the system (pre-mono plus spectrometer) is about 30%. The spectrometer uses 3 bars of Super Invar to form a coaxial triangle to carry the grading, the mirror and the slits. The Super Invar guarantees a high thermal stability of the spectrometer and from several tests no effects were evident on the spectral dispersion for variations in temperature within 2 1C. To avoid any temperature effects on the prism and diffraction grating the spectrometer is used in a temperature-controlled laboratory with temperature changes below 71 1C. The pre-mono is a 50 cm focal length monochromator in a Czerny–Turner setup (Fig. 1). It allows high efficiency of the spectrometer in all his spectral window selecting the spectral order. The light is dispersed twice by a quartz prism, which has a spectral band that covers the spectrometer window and an efficiency of 70% peaked around 400 nm. Its input aperture is f/5, whereas the output aperture is adapted to the spectrometer input. The pre-mono reduces also the stray light of the spectrometer: tests using a Yag laser (emission at 532.072 nm and the
340.0 mm
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Fig. 1. Schematic of the spectrometer and pre-mono used to select the order and reduce the stray light. M2 and M3 are 451 mirrors allow the double-pass configuration to simulate a spectrometer with a double focal length (only one light path is reported).
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full-width at half-maximum (FWHM) ¼ 0.044 nm) as a source show that the stray light increases from 106 to 105 when the pre-mono is removed. These measurements have been done moving the grating to detect light at about 71 nm from the laser line peak. The theoretical spectral resolving power (R) of the grating is given by: R ¼ mNW
(1)
where m is the order, N the lines per mm of the grating (in our case 316) and W the grating width (here, 265 mm). For example, at 313 nm the spectrometer grating works at the 19th order so R ¼ 1,590,000, whereas at 558 nm the order m ¼ 10 so R ¼ 837,400. This is the theoretical spectral resolving power of the grating; the spectral resolving power of all the system (premono plus spectrometer) is lower and it is measured from experimental mercury lines analysis (see below). The entrance of the pre-mono is coupled with two fiber systems: (1) a bundle of fibers (20 fibers of 200 mm each) with one end round shaped to collect the light and the other end linear (200 mm 6 mm) to match the slit entrance, (2) a bundle of 5 separate fibers (60 mm each) to detect 5 different signals, in the other end the fibers are aligned to match the slit entrance, here the distance between each fiber is 400 mm to avoid the mixing of the signals on the CCD. The fibers are 10 m long and tests, using a 532.072 nm Yag laser and 632.820 nm He–Ne laser light, show that they completely scramble the polarization information in the incoming light. The detector is a CCD manufactured by Princeton with 1024 256 pixels. The pixel dimension is 26 26 mm, the full well capacity is 1.3 106 electrons in spectroscopy mode. The CCD is front illuminated with a UV extension and the quantum efficiency is 45% at 650 nm, 30% at 500 nm, 10% at 960 nm and 12% from 200 to 400 nm. The controller has a dynamic range of 16 bit. The cooling system uses liquid nitrogen and the dark current is 0.5 e-/piixel/h at 120 1C. A custom software to control the spectrometer has been developed using LabVIEW of National Instruments and it has been designed for the pre-mono prism movements (to select the order), the spectrometer grating movements (for wavelength selection),
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the aperture of the shutter and for data collection of the CCD. The software allows automatic scan from UV to IR with fixed or variable CCD acquisition time for each scan and automatic measurements and compensation for the flat field and subtraction of the background. To test the performance of the system a mercury lamp from Avantes (model Avalight-cal) as light source was coupled with the bundle of fibers. In Fig. 2 are reported some Hg lines observed scanning the spectrometer from UV to visible. This highresolution system allows to resolve in the UV as well as in the visible the satellites of several mercury lines originated by a magnetic hyperfine structures of isotopes of Hg contained in the lamp (Sansonetti et al., 1996). The wavelength difference between these satellites is 0.0028 nm for the 312.5674 nm line and 0.013 nm for the 579.067 nm line. To experimentally estimate the resolution of the spectrometer, we found two close mercury lines centered at 313.1555 and 313.1844 nm (see Fig. 2). Using the slit aperture of 25 mm, the FWHM of the lines is 5 pixels and their separation is 102 pixels. The linear dispersion is 0.00028 nm/ pixel and the resulting resolution is 223,000 that corresponds to 0.0014 nm (FWHM), whereas the linear dispersion at 558 nm is 0.0008 nm/pixel and, the resolution estimated using the solar lines centered at 558.875 and 558.676 nm, is 111,000 that is about 0.005 nm (FWHM). To reach the above resolution values the aperture of the monochromator slit must be kept very small (about 25 mm). In this configuration, the instrument can be used with a laser source or a lamp, because the luminosity is very low. For atmospheric observations, to have enough light through the monochromator, a wider slit must be used, and that implies a poorer resolution. To test the spectrometer and the CCD performance with species of atmospheric interest, we measured the absorption spectrum of OH and NO2 in the laboratory at ambient pressure. OH is produced in flow reactor cell (1 m long) where 3 lowpressure mercury lamps (Heraeus, mod. G36TSVH/4) emit about 3 W at 185 nm. The mercury line at 185 nm photodissociates water vapor creating OH and H. Water vapor is generated flowing nitrogen through distilled water in the reactor. The source used for the measurements is a calibrated xenon lamp (Hamamatsu model L7810). In Fig. 3a, is shown the OH absorption spectrum calculated
x 104 0.003 nm
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4
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0 296.7282 312.5674
313.1555 313.1844 404.6564 435.8334 579.0670 Wavelength (nm)
Fig. 2. Overview of a mercury lamp spectrum measured with the spectrometer system. The wavelength of the central lines is reported but their distances are not in scale.
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1.001 1 I/Io
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Wavelength (nm) Fig. 3. Panel A: absorption lines of OH measured in a humidified flow cell (see text). Panel B: absorption lines measured in a cell filled with NO2 at ambient pressure (see text) compared with the cross section of NO2 measured by Vandaele et al. (2002).
from the ratio of the signal measured when in the reactor is generated OH (I) and the signal without OH (IO). The relative intensity and wavelength displacement of the OH lines (Q1(2), Q1(3) and P1(1)) agree with previous observations (Mount, 1992; Dorn et al., 1995). The system is able to detect signal differences less than 0.1%. The absorption spectrum of another species of atmospheric interest (NO2) is reported in Fig. 3b. The measurements have been done with the technique used for the OH measurements. NO2 is flushed in the cell mixed with nitrogen and oxygen. NO2 cross-section measurements are in agreement with Vandaele et al. (2002) observations.
3. Atmospheric observations Field measurements in the atmosphere have been done observing the Fraunhofer lines to find how their depth changes when zenith-scattered solar spectrum are compared with direct solar sunlight. The band identified for this investigation was centered at 558.7 nm, because, looking at previous works, the lines in this band show a big difference of the filling-in as function of the solar zenith angle and wavelength (Kattawar et al., 1981; Pallamraju et al., 2000). The observations were carried at The Gran Sasso National Laboratories (LNGS) near L’Aquila (Italy) (42.351N, 13.381E, elevation 980 m). The measurements were done during clear sky as well as during cloudy days. Each observation consists of switching between measurements pointing the zenith to observe the scattered light and measurements tracking the sun for the observation of the direct sunlight. The resolution has been degraded (increasing the slit aperture) to about 0.015 nm to keep a high signal-to-noise ratio in a short-time exposure. The CCD exposure was few seconds during direct sunlight measurements and 100 s during observation of scattered light. The time exposure for the scattered light has been chosen in order to have counts close to the full well capacity of the CCD (to improve the signal-tonoise ratio) and at the same time not too long to have the lower possible variation of the sun zenith angle (less than 0.51). The RE that is the difference between zenith-scattered solar spectrum and direct solar sunlight spectrum (also called absolute RE contribution), is affected by the background continuum level
that varies with SZA and by the wavelength. To remove the background continuum level, the RE is computed in terms of the fractional Ring effect (FRE) (Kattawar et al., 1981) FRE ¼ RE=ðRE þ BCÞ
(2)
where BC is the background continuum obtained with a linear fit between the continuum regions on each side of the Fraunhofer lines under consideration. Fig. 4a shows an example of a skylight and direct sunlight measured and of the background continuum. The big difference between the scattered light (skylight) at SZA higher than 831 and the direct sunlight is due to the filling-in calculated in terms of FRE (Fig. 4b). The error on the FRE evaluation is due to shot noise, spectrum alignment and background continuum evaluation, and globally is estimated to be between 2% and 3%. The variation of the FRE with SZA and wavelength is reported in the Fig. 5a for measurements with clear sky during all periods of the observations. The sky conditions were monitored with a pyrheliometer (model MS-53, Eko instrument) pointed to the zenith controlling the scattered light and a pyranometer (MS-802, Eko instrument) mounted on a sun tracker (STR-O1, Eko instrument) to monitor the direct sun light. For SZA higher than 851, we observed a dependence of the FRE on the wavelength and at SZA ¼ 901 the FRE is 8%, 10%, 14%, 15% and 16% for the lines at 559.04, 558.78, 558.75, 558.67 and 558.87 nm, respectively. At SZA ¼ 911 all the lines are in the FRE range between 17% and 20%. The variations of the RE as function of SZA (reported in Fig. 5a) is very low (less than 5% for SZA lower than 801), and an abrupt increase up to 30% for SZA higher than 901 are reported by previous observations. Fig. 5a shows no wavelength dependence of the RE for all the 5 lines measured in the 558.6 nm band (SZA lower than 851), whereas in a previous work with a spectrometer with a poorer resolution (factor of 2) was found a big difference of the FRE. Pallamraju et al. (2000) report a difference of the FRE between the lines 558.67 and 558.87 nm of more than 5% at almost all the solar zenith angle. Besides the difference in the spectrometer resolution, Pallamraju et al. (2000) observations were carried out in Boston, US, while we took our measurements in a quite clean environment. Joiner et al. (1995) show that degrading the resolution the filling-in becomes lower, this could
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Fig. 4. Panel A: an example of direct sunlight and scattered skylight measured when the solar zenith angle (SZA) was 83.521. Panel B: the fractional Ring Effect of the spectrums reported in the upper panel.
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Fig. 5. Panel A: fractional Ring effect observed during clear sky. The measurements started at 16:10 LT until 20:02 LT. Panel B: as for Panel A but with partly cloudy sky.
suggest a possible instrumental effect to explain the difference between Pallamraju et al. (2000) observations and ours. For example, Conde et al. (1992) report the same RE dependence on SZA of our study, observing the sodium D2 line at 589 nm in Antartica with a Fabry–Perot spectrometer having higher resolution (0.00134 nm) than Pallamraju et al. (2000) as well as of this work. Fig. 5b shows the FRE as function of the solar zenith angle observed during a cloudy day that was mainly
cumulonimbus. The qualitative variation of the FRE with SZA is similar to that detected during the clear day reported in Fig. 5a. Looking at the data of the pyrheliometer the solar radiation of the cloudy day was, globally, about 20% lower than what measured during the clear day, but the variation of the RE for low SZA was very small. On the other hand, the filling-in for SZA higher than 851 was bigger during cloudy day compared to what observed during the clear sky for all the lines: at SZA ¼ 901 the FRE rises
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from 10%, for the observations with clear sky to 14% for cloudy sky (line 558.78 nm), the FRE of the line 558.87 and 558.67 nm increases of 9% and 5%, respectively. No effect of the clouds is observed for the line 558.75 nm. For SZA ¼ 911 the increment of the filling-in ranges from 1% of the lines 558.75 and 558.78 nm to 13% of the line 558.87 nm. The cloud effect on the filling-in has been studied first by Harrison and Kendall (1974): they suggested that overcast skies increase filling-in whereas bright sunlit clouds have the effect of weakening the filling-in. These results are confirmed by observations and models made by de Beek et al. (2001) and our observations are consistent with the results of their model for cumulonimbus clouds.
4. Conclusions In this work, we described a new spectrometer with a resolution of 0.0014 nm for atmospheric spectroscopy observations. The spectrometer allows to detect high-resolution spectra from UV to IR, so that the system could be used for cross-section measurements of wide range of species of atmospheric interest. Laboratory measurements of OH and NO2 absorption spectrums prove that it could be used for the detection of the atmospheric hydroxyl radicals with DOAS techniques, the retrieval of atmospheric vertical distribution of NO2 and for the detection of column content of other atmospheric gases. The measurements of the filling-in of the Fraunhofer lines in the visible show a strong dependence on the solar zenith angle only near the twilight with a quite flat RE (5%) during the all day. The wavelength dependence of the filling-in is evident only for solar zenith angle higher than 851. Cloudy sky increases the filling-in at all the wavelength (excluding the 558.75 nm) at high solar zenith angle as expected from models using low (0.2 nm)resolution spectra.
Acknowledgements We thank the support of Francesco Del Grande (Dipartimento di Fisica Universita` di L’Aquila), Leo Tatananni, Antonello Rotilio, Bruno Romualdi and Angelo Corsi (Laboratori Nazionali del Gran Sasso, INFN) for the realization of all the mechanical parts. P. Di Carlo work is supported by Fondazione CARISPAQ. This work has been supported by the Center of Excellence CETEMPS and we thank Prof. G. Visconti for the helpful comments and his helps in setting up this instrument. References Alicke, B., Hebestreit, K., Stutz, J., Platt, U., 1999. Iodine oxide in the marine boundary layer. Nature 397 (6720), 572–573. Allan, B.J., Plane, J.M.C., McFiggans, G., 2001. Observations of OIO in the remote marine boundary layer. Geophysical Research Letters 28 (10), 1945–1948. Brinkmann, R.T., 1968. Rotational Raman scattering in planetary atmospheres. Astrophysics Journal 154, 1087–1093. Chakrabarti, S., Pallamraju, D., Baumgardner, J., Vaillancourt, J., 2001. HiTIES: a high throughput imaging echelle spectrograph for ground-based visible airglow and auroral studies. Journal of Geophysical Research 106, 30337–30348.
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