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Advances in Space Research 42 (2008) 1167–1171

Ground-based millimeter-wave observations of water vaporemission (183 GHz) at Atacama, Chile

T. Kuwahara a,*, A. Mizuno a, T. Nagahama a, H. Maezawa a, A. Morihira b,N. Toriyama a, S. Murayama a, M. Matsuura a, T. Sugimoto a, S. Asayama c,

N. Mizuno d, T. Onishi d, Y. Fukui d

a Solar-terrestrial Environment Laboratory, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8601, Japanb Ulvac Inc., 1-1 Yada, Hachiya-cho, Minokamo, Gifu 505-0009, Japan

c National Astronomical Observatory Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japand Department of Astrophysics, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8601, Japan

Received 14 December 2006; received in revised form 16 November 2007; accepted 20 November 2007

Abstract

We report the first results of ground-based millimeter-wave measurements of 183 GHz atmospheric water vapor spectra fromAtacama highland (4800 m alt.), Chile. The measurements were carried out in December 2005 by using a spectroscopic radiometerequipped with a superconductive heterodyne receiver. A conspicuous H2O spectrum at 183 GHz was detected with an integration timeof only 1.5 min, and this is the first high frequency-resolution H2O spectrum at 183 GHz obtained in the southern subtropical region. Thevertical profile of H2O volume mixing ratio between 40 and 64 km were retrieved from the spectrum by using the modified optimalestimation method.� 2007 COSPAR. Published by Elsevier Ltd. All rights reserved.

Keywords: Ground-based remote sensing; Stratosphere; Mesosphere; Water vapor; Microwave radiometer; Spectroscopy

1. Introduction

It is well known that water vapor plays important rolesin chemistry and energy balance in the middle atmosphere.Water vapor is a source of hydroxyl radical that causes cat-alytic ozone destruction cycles in the upper stratosphereand mesosphere and is an efficient green house gas thataffects the cooling budget of atmosphere through its mid-and far-infrared rotational band radiation.

The stratospheric water vapor had increased by �about1% per year for more than decades since 1950s based onvarious observational datasets (Nedoluha et al., 1998,SPARC Report No. 2, 2000 and references therein), whilethe increasing rate slowed down or stopped around 2000

0273-1177/$34.00 � 2007 COSPAR. Published by Elsevier Ltd. All rights rese

doi:10.1016/j.asr.2007.11.030

* Corresponding author.E-mail address: kuwahara@stelab.nagoya-u.ac.jp (T. Kuwahara).

(e.g., Nedoluha et al., 2003). It is suggested that theincrease of water vapor may enhance the stratosphericcooling and possibly more activates the formation of polarstratospheric clouds (PSCs) that cause the heterogeneousreactions leading to massive ozone depletion in the polarregions (e.g., Kirk-Davidoff et al., 1999). It is well knownthat the major origins of water vapor in the stratosphereare oxidation of methane and entering through the tropicaltropopause, but it is not fully understood which is the keyprocess and how controls the inter-annual and decadalvariations of the stratospheric water vapor. Thus, it is ofvital importance to continue the monitoring of water vaporto elucidate the long-term trend observationally. Watervapor in the middle atmosphere is measured from bothground and space. The satellite observations provide data-sets of global three-dimensional distribution of watervapor, but typical life time of space mission is shorter thana decade. Ground-based observations can be continued for

rved.

Table 1Specifications of the radiometer system

Target frequency 183.3101170 GHzLocal frequency 181.210117 GHzType of SIS mixer Double sideband mixerNoise temperature of SIS Mixer 150 K (@LO = 181.1201170 GHz)Spectrometer Acousto-optical spectrometer (AOS)Center frequency of AOS 2100 MHzNumber of CCD channels 2048 chBandwidth of AOS 1000 MHzResolution of AOS 1 MHz

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more than decades, and they are suitable to monitor thelong-term variation of atmospheric minor constituents witha single instrument. However, the ground-based observa-tions provide only one-dimensional and local information,thus collaborative networks of ground-based observationssuch as the Network Detection of Atmospheric Composi-tion Change (NDACC) are essentially important.

In order to make future contribution to the ground-based observing network, we made a plan to develop anew millimeter-wave water vapor radiometer. We intendto make a simultaneous monitoring of three water vaporisotopes, H2O, H2

18O, and HDO whose intense lines arelocated in 183.3, 203.4, and 225.9 GHz, respectively. Theisotopic ratios are different between the water vapor iso-topes entering from tropopause and those produced bymethane oxidation (e.g., Moyer et al., 1996). One ofour motivations is to provide new information to studyof formation and transportation processes of water vaporthrough the time and altitudinal changes of the isotopicratio of water vapor. Some pioneering works of isotopicmeasurements of water vapor have been done by John-son et al. (2001) in the mid-latitude region by usingballoon-borne FTIR and by Moyer et al. (1996) byspace-borne FTIR, whereas the infrared instruments arenecessary to bring up above troposphere because of theopaque lower atmosphere. In contrast, the millimeter-wave water vapor lines are observable from ground,and it allows us to make continuous monitoring of watervapor isotopes.

In this paper, we report the radiometer developmentand the results of test observation of 183 GHz watervapor spectra (313–220) as a part of the water vaporisotopic observational project. At present, most of theground-based stratospheric and mesospheric water vaporobservations are carried out at 22 GHz because of its hightropospheric transmittance and its altitudinal coverage upto 85 km determined by the saturation height of pressurebroadening linewidth. However, the intensity of the22 GHz line is very weak. The intensity is about twoorders of magnitude weaker than that of 183 GHz, andthe 183 GHz observations have potential to improve thesignal-to-noise ratio. The upper limit of the altitudinalcoverage defined by the saturation height is 75 km forthe 183 GHz line. But we should note that the atmo-spheric absorption of the 183 GHz line due to the tropo-spheric water vapor is more severe than that of the22 GHz line, and it is suggested that the 183 GHz obser-vations are only possible from high altitude sites undervery dry conditions (e.g., Siegenthaler et al., 2001). The22 GHz measurements are therefore efficient from sealevel sites, whereas the 183 GHz measurements from suit-able sites may be of great advantage in gaining the signal-to-noise ratio or in obtaining much higher time-resolutiondata. Therefore, in order to evaluate the feasibility of the183 GHz measurements, we installed a superconductiveradiometer and make a test observation at the Atacamahighland (4800 m alt.), Chile.

2. Instruments and observations

The millimeter-wave spectroscopic radiometer isequipped with a superconductor–insulator–superconduc-tive (SIS) double sideband heterodyne mixer receiver. Theradiometer has been developed by Nagoya Universityand Ulvac Inc. The superconductive tunnel junction forthe SIS mixer is made by ourselves in the clean room facil-ity at Nobeyama Radio Observatory (NRO) of theNational Astronomical Observatory Japan (NAOJ). Thebackend spectrometer is an acousto-optical spectrometer(AOS). Specifications of the radiometer system are summa-rized in Table 1.

We made a test observation of the water vapor spectrum(313–220) at 183.310117 GHz from Atacama highland inDecember 2005. Atacama highland is located in a desertarea in the northern part of Chile, and we installed theradiometer in the Pampa la Bola area (22� 58 0S, 67�42 0W, 4800 m alt.) close to the border to Bolivia. Typicalatmospheric pressure at the site is 570–580 hPa, The rela-tive humidity is generally below 30% in daytime except dur-ing the summer (Sakamoto et al., 2000), and theprecipitable water vapor is lower than 2 mm in 2/3 yearin Chajnantor (23� 00 0S, 67� 45 0W, 5100 m alt.) nearPampa la Bola (http://www.apex-telescope.org/weather/year_weather/index.htm). In summer, the relative humiditysometimes rises up to 100% when the winds coming fromAmazon basin bring moist air, which is so-called ‘‘BolivianWinter’’ condition.

In the millimeter-wave radiometer observations, so-called ‘‘switching’’ procedure is necessary to compensatefor the temporal drift of the receiver gain and to removethe background noise. Various switching methods such asbeam switching with reference sky (e.g., Parrish et al.,1988; Mizuno et al., 2002), beam switching with a referenceload (e.g., Maier et al., 2001), and frequency switching(e.g., Nagahama et al., 1999) have been used for millime-ter-wave atmospheric observations. We applied ‘‘cold-loadswitching’’ method to the present observation. As a coldreference load, we use a millimeter-wave absorber soakedin the liquid nitrogen in a styrene foam vessel, and thereceiver input is switched between the cold-load and theobserving sky alternately every 15 s. In the present observa-tion, the elevation angle of the observing sky is fixed at 70�from the horizon.

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In order to establish the intensity scale of the observedspectra, we have to determine the conversion factorbetween the receiver output voltage and the brightness tem-perature of the spectra. In addition, we have to make thecorrection for the signal attenuation due to the tropo-spheric absorption. The conversion factor to the brightnesstemperature is determined by measuring the continuum sig-nal levels of the cold-load at the liquid nitrogen tempera-ture (77 K) and the hot load at the ambient roomtemperature (300 K). The absorption correction factor,i.e., atmospheric optical depth, is derived from the eleva-tion dependence of the atmospheric continuum signal byusing so-called ‘‘sky tipping’’ procedure (e.g., Ulich et al.,1980).

Fig. 2. The retrieved altitude profile of the H2O volume mixing ratio onDecember 7, 2005. The optical depth at 183 GHz is assumed to be 1.1. Thehorizontal error bars show the random error estimated from the noisefluctuation of the spectral data.

3. Results

In Fig. 1, we show the stratospheric water vapor spec-trum obtained on December 7, 2005 with an integrationtime of 1.5 min. The vertical profile of H2O volume mix-ing ratio was retrieved by using the weighted-dampedleast squares fitting algorithm (Nagahama et al., 1999)based on the optimal estimation method (Rodgers,1976) (Fig. 2). The forward model spectrum reproducedfrom the retrieval result and the fitting residual are shownin Fig. 1. Temperature and pressure data necessary for the

Fig. 1. Top: The H2O spectrum observed on December 7, 2005 at theAtacama station with an integration time of 1.5 min. A linear baseline issubtracted. The forward model spectrum is superposed in red line.Bottom: The residual of the forward model fitting. (For interpretation ofthe references to colour in this figure legend, the reader is referred to theweb version of this article.)

retrieval calculation are obtained from the COSPARInternational Reference Atmosphere (CIRA86) (Reeset al., 1990). A priori profile of the H2O volume mixingratio is taken from the mid-latitude daytime profile ofMIPAS reference atmospheres (2001). Since a standingwave originated between the mirrors in the optics deformthe spectral base line, so we used only the central 30 MHzpart of the spectrum for the current data analysis. Thisreduction of the frequency window determines the loweraltitude boundary of the retrieval to be 40 km. The upperlimit is determined by the frequency resolution of thespectrometer, 1 MHz. Finally the vertical volume mixingratio profile was derived for an altitude range from 40to 64 km at 2 km grid spacing as shown in Fig. 2. Typicalaveraging kernels are shown in Fig. 3. The vertical resolu-tions are 15 and 20 km at an altitude of 40 and 60 km,respectively.

From the numerical simulations results, the randomerror of the retrieved H2O volume mixing ratio due tothe rms noise fluctuation of the spectral data is estimatedto be �6%. On the other hand, the major factor of sys-tematic errors is the uncertainty in the intensity calibra-tion because of the following reasons. Since we use adouble-sideband receiver in our radiometer system, theoptical depth measured by the sky tipping procedure isan average value of the two different frequency bands,the signal-band and the image-band. According to theradiative model calculation, the average optical depth isexpected to be four or five times smaller than the actualoptical depth at the water vapor frequency. Siegenthaleret al. (2001) observed the 183 GHz water vapor line fromJungfraujoch (47�N, 8�E, 3580 m alt.) and presented a ser-ies of spectra obtained in various conditions of atmo-spheric optical depths. By comparing the intensitiesbetween the spectra in Siegenthaler et al. (2001) and our

Fig. 3. The averaging kernels for the retrieval calculations at selectedaltitudes.

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present spectrum, we estimated more appropriate opticaldepth at 183 GHz to be �1.1 and applied this value forthe present retrieval analysis. If we assume of 10% errorin the optical depth, the resulting systematic error is esti-mated to be �15%.

In order to reduce the systematic error, we need tomake the sky tipping measurements by using a singlesideband receiver that enables us to determine the sig-nal-band optical depth precisely. We have developed asideband separating receiver with a waveguide filter. In90–110 GHz band, we succeeded in obtaining the side-band separation ratio better than 10 dB (Asayamaet al., 2004), and this receiver is being used for thestratospheric ozone observation at 111 GHz in Tsukuba,Japan. For the 180 GHz band, we have fabricated a pro-totype sideband separating receiver designed with thesame concept and just started the evaluation of its per-formance. This new sideband separating receiver will pro-vide more accurate estimates for the water vapor volumemixing ratio.

4. Summary and future perspective

We succeeded in obtaining a 183 GHz water vapor spec-trum in December 2005 at the Atacama highland, Chile,and retrieved the vertical profile of the H2O volume mixingratio between 40 and 64 km in altitude. The random errorof the volume mixing ratio is estimated to be smaller than�6%, suggesting that continuous monitoring of 183 GHzwater vapor from Atacama is feasible enough, but the sys-tematic error due to the uncertainty in determining theatmospheric optical depth is still large in the current doublesideband receiver system. To reduce the systematic error,more accurate measurements of the optical depth by using

a single sideband receiver is necessary. We are now evalu-ating a new sideband separation receiver in the laboratoryfor this purpose.

Acknowledgments

This work is supported by Solution Oriented Researchfor Science and Technology (SORST) program promotedby Japanese Science and Technology Agency (JST). Wethank to H. Yamamoto, T. Minamidani, T. Kawase, L.Siales, and R. Salzar for their support in setting up theinstruments at Atacama NANTEN2 site.

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