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  • View in gallery

    Block diagram of coherent differential absorption and Doppler wind lidar [isolator (ISO), piezoelectric translator with mirror (PZT)].

  • View in gallery

    Block diagram for λ-center laser (MO-I) stabilization.

  • View in gallery

    Error signal and sweeping signal from PID controller to adjust PZT driver’s DC voltage.

  • View in gallery

    Long-term laser frequency fluctuation of λ-center laser.

  • View in gallery

    Block diagram for λ-on laser (MO-II) stabilization.

  • View in gallery

    Relation between wavelengths of three CW Tm,Ho:YLF lasers and one-way transmission over 1 km in atmosphere with HITRAN 2008 database, 400-ppm CO2 concentration, 1013-hPa pressure, and 298.15-K temperature.

  • View in gallery

    SNR in atmosphere for offline and online lasers at three frequency offsets: 2.5 (thick gray line), 4.8 (dashed gray line), and 5.8 GHz (thin gray line) from center of R30 absorption line of CO2.

  • View in gallery

    Temporal variations of CO2 concentrations measured by Co2DiaWiL and in situ instrument on 22 Oct 2009. Laser frequency offset was 4.8 GHz for horizontal CO2 measurement.

  • View in gallery

    Vertical profiles of DAOD and SNR obtained from (a) 0408 to 0433 UTC 14 Feb, (b) 0325 to 0427 UTC 20 Feb, and (c) 0314 to 0413 UTC 23 Feb 2010. Vertical profiles of temperature, pressure, and relative humidity measured by radiosonde launched at around 0300 UTC on (d) 14, (e) 20, and (f) 23 Feb 2010. SNR of offline (black) and online (gray) laser measured at 90° (solid lines) and 16° (dashed lines) elevation angles is shown. DAOD at 90° (open circles) and 16° (closed circles) elevation angles.

  • View in gallery

    As in Fig. 9, but from (a) 0339 to 0439 UTC 28 Jan, (b) 0242 to 0342 UTC 31 Jan, (c) 0302 to 0402 UTC 3 Feb, and (d) 0235 to 0335 UTC 7 Feb 2011, and at around 0300 UTC (e) 28 and (f) 31 Jan, and (g) 3 and (h) 7 Feb 2011.

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Partial CO2 Column-Averaged Dry-Air Mixing Ratio from Measurements by Coherent 2-μm Differential Absorption and Wind Lidar with Laser Frequency Offset Locking

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  • 1 * National Institute of Information and Communications Technology, Koganei, Tokyo, Japan
  • | 2 Hamamatsu Photonics K. K., Hamamatsu, Shizuoka, Japan
  • | 3 Nippon Aleph Co., Yokohama, Kanagawa, Japan
  • | 4 Tokyo Metropolitan University, Hino, Tokyo, Japan
  • | 5 National Institute for Environmental Studies, Tsukuba, Ibaraki, Japan
  • | 6 ** Tohoku Institute of Technology, Sendai, Miyagi, Japan
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Abstract

A coherent 2-μm differential absorption and wind lidar (Co2DiaWiL) with a 2-μm single-frequency Q-switched laser with laser frequency offset locking was used for long-range CO2 measurement. The frequency stabilization of the single-frequency λ on pulsed laser was 1.0 MHz. Experimental horizontal CO2 measurement over a column range of 2.6–5.6 km and 900 shot pairs (1-min integration time) was conducted on 22 October 2009 to examine the detection sensitivity of the Co2DiaWiL. The achieved precision was less than 2.1%. The root-mean-square of the differences between the 30-min CO2 averages measured by the Co2DiaWiL and a ground-based in situ instrument was 0.9% (3.5 ppm). Experimental vertical CO2 measurements were conducted in February 2010 and January and February 2011. The partial CO2 column-averaged dry-air mixing ratios (XCO2) for an altitude between 0.4 and 1.0 km in 2010 and 2011 were 403.2 ± 4.2 and 405.6 ± 3.4 ppm, respectively. In the paper, the Co2DiaWiL results were well validated carefully against those of the airborne in situ instrument; they agreed well within the margin of error. The values of XCO2 measured in presence of cirrus clouds near the tropopause (hard target cases) show a difference of less than 4.1 ppm with the airborne measurements performed on 14 February 2010. This result demonstrates the capability of the Co2DiaWiL to measure XCO2 within a precision better than 1%.

Current affiliation: Chiba Institute of Technology, Chiba, Japan.

Current affiliation: Japan Aerospace Exploration Agency, Tokyo, Japan.

Corresponding author address: Shoken Ishii, National Institute of Information and Communications Technology, 4-2-1 Nukuikitamachi, Kogenei, Tokyo 184-8795, Japan. E-mail: sishii@nict.go.jp

Abstract

A coherent 2-μm differential absorption and wind lidar (Co2DiaWiL) with a 2-μm single-frequency Q-switched laser with laser frequency offset locking was used for long-range CO2 measurement. The frequency stabilization of the single-frequency λ on pulsed laser was 1.0 MHz. Experimental horizontal CO2 measurement over a column range of 2.6–5.6 km and 900 shot pairs (1-min integration time) was conducted on 22 October 2009 to examine the detection sensitivity of the Co2DiaWiL. The achieved precision was less than 2.1%. The root-mean-square of the differences between the 30-min CO2 averages measured by the Co2DiaWiL and a ground-based in situ instrument was 0.9% (3.5 ppm). Experimental vertical CO2 measurements were conducted in February 2010 and January and February 2011. The partial CO2 column-averaged dry-air mixing ratios (XCO2) for an altitude between 0.4 and 1.0 km in 2010 and 2011 were 403.2 ± 4.2 and 405.6 ± 3.4 ppm, respectively. In the paper, the Co2DiaWiL results were well validated carefully against those of the airborne in situ instrument; they agreed well within the margin of error. The values of XCO2 measured in presence of cirrus clouds near the tropopause (hard target cases) show a difference of less than 4.1 ppm with the airborne measurements performed on 14 February 2010. This result demonstrates the capability of the Co2DiaWiL to measure XCO2 within a precision better than 1%.

Current affiliation: Chiba Institute of Technology, Chiba, Japan.

Current affiliation: Japan Aerospace Exploration Agency, Tokyo, Japan.

Corresponding author address: Shoken Ishii, National Institute of Information and Communications Technology, 4-2-1 Nukuikitamachi, Kogenei, Tokyo 184-8795, Japan. E-mail: sishii@nict.go.jp

1. Introduction

Atmospheric carbon dioxide (CO2) is a greenhouse gas playing a very important role in climate change. Global spatial and temporal variations of CO2 concentration are important for understanding the carbon cycle and estimating the carbon flux. Spaceborne measurement is a promising approach to globally measure the temporal and spatial distribution of the CO2 column-averaged dry-air mixing ratio (XCO2). In January 2009, the Greenhouse Gases Observing Satellite (GOSAT; Kuze et al. 2009), equipped with spaceborne passive sensors, was launched to continuously monitor the global total CO2 column concentration. The Orbiting Carbon Observatory 2 (Crisp et al. 2004) will be launched for the same purpose no later than February 2013. A passive sensor is affected by the presence of aerosols and thin clouds; it tends to overestimate the optical depth in the presence of aerosols and underestimate the optical depth in the presence of thin clouds, and the CO2 data are easily biased (e.g., Morino et al. 2011).

A differential absorption lidar (DIAL) technique is regarded as one of the next-generation spaceborne sensors. DIAL has the potential advantage of providing high measurement accuracy (with bias close to zero), high precision (a few parts per million), ranging capability, and high sensitivity for detecting aerosol and clouds. Observations can be performed during daytime and nighttime at all latitudes, irrespective of season. Many research groups are developing various types of DIAL to measure the atmospheric CO2 concentration (Browell et al. 2010; Kameyama et al. 2009; Abshire et al. 2010; Amediek et al. 2008; Sakaizawa et al. 2009; Spiers et al. 2011; Koch et al. 2004, 2008; Gilbert et al. 2006, 2008; Ishii et al. 2010). National Aeronautics and Space Administration’s (NASA’s) Langley Research Center (LaRC) and the Japan Aerospace Exploration Agency (JAXA) developed a 1.57-μm laser absorption spectrometer (LAS) with modulated continuous wave (CW) and direct detection (Browell et al. 2010; Kameyama et al. 2009). NASA Goddard Space Flight Center (Abshire et al. 2010), Deutsches Zentrum für Luft- und Raumfahrt (Amediek et al. 2008), and Tokyo Metropolitan University (Sakaizawa et al. 2009) used a 1.57-μm pulse laser and direct detection. NASA Jet Propulsion Laboratory (JPL; Phillips et al. 2003; Spiers et al. 2011) is developing a 2.05-μm LAS with a continuous wave laser and heterodyne detection. LaRC (Koch et al. 2004, 2008), L’Institut Pierre-Simon Laplace (IPSL) École Polytechnique (Gilbert et al. 2006, 2008), and the National Institute of Information and Communications Technology (NICT) reported 2.05-μm DIALs with a pulse laser and heterodyne detection (Ishii et al. 2010). The 2.05-μm spectral region is more sensitive for CO2 distribution in the lower troposphere where CO2 sinks and sources interact with the atmosphere than the 1.57-μm one, as shown by the simulated weighting function of a CO2 absorption cross section (Menzies and Tratt 2003; Ehret et al. 2008). A 2.05-μm integrated path differential absorption (IPDA) lidar is thus one of the most promising next-generation spaceborne sensors.

We previously developed a 2.05-μm conductively cooled laser diode–pumped single-frequency Q-switched solid-state laser with 2.4-W power (80 mJ, 30 Hz) and a coherent 2-μm differential absorption and wind lidar (Co2DiaWiL) equipped with a two-axis scanning device (Ishii et al. 2010). The R30 absorption line of the (20°1)III←(00°0) band of CO2 was selected because of its suitable absorption depth and low temperature dependence for the 2-μm DIAL. The Co2DiaWiL used the 2050.967-nm (online) and 2051.125-nm (offline) wavelengths, which correspond to the center and far wing, respectively, of the R30 absorption line. One-way transmission of the online laser is 0.7 over 1 km in an atmosphere of 400-ppm CO2 concentration, 1013-hPa pressure, and 298-K temperature. The signal-to-noise ratio of the online laser decreases rapidly with increase in the range resulting from the strong CO2 absorption, so the wavelength of the online laser should be shifted from the R30 absorption line center in order to make long-range CO2 measurements. In previous work, we used frequency modulation to stabilize the wavelength of the on-liner laser. It was tuned finely with the piezoelectric translator (PZT) of the output coupler, and the PZT was driven with a 100-Hz sinusoidal wave. The frequency stabilization of the online laser was locked within 9 MHz. A change of 9 MHz is negligible at the line center. Because the absorption cross section is more changeable at the wing than at the line center, it is very important to control the frequency of the online laser with high precision when adapting the online laser wavelength in the wing of the CO2 absorption line. NASA LaRC (Koch et al. 2008) and JPL (Spiers et al. 2011) used a laser frequency offset locking and controlled the wavelength of the online laser with a precision of about 2 MHz. This precision level leads to an error of ~0.1% in a DIAL CO2 measurement with online laser frequency offset. NICT aimed to lock the online laser within 1 MHz to reduce the error caused by the uncertainty of the online laser frequency. In the next section, details of the coherent 2-μm DIAL with laser frequency offset locking are presented. The retrieval method of XCO2 and the error analysis are described in section 3. Ground-based and upper-atmospheric measurements are presented in section 4. In section 5, experimental horizontal measurements made to examine the detection sensitivity of the Co2DiaWiL are presented. The results of the XCO2 derived from slant and vertical measurements are shown in the next section, and the comparison between the vertical CO2 measurements and the airborne in situ measurements is described.

2. Co2DiaWiL and wavelength control of single-frequency laser

The Co2DiaWiL specifications are listed in Table 1, and a block diagram of the Co2DiaWiL is shown in Fig. 1. The Co2DiaWiL has three diode-pumped single-frequency CW Tm,Ho:YLF lasers, a Q-switched Tm,Ho:yttrium lithium fluoride (YLF) laser, a Mersenne off-axis telescope with a 10-cm aperture, a two-axis scanning device, two heterodyne detectors, and signal processing devices. The single-frequency Tm,Ho:YLF laser with a 2.05-μm operating wavelength demonstrates 80-mJ output energy with a 150-ns pulse width [full width at half maximum (FWHM)] at a 30-Hz pulse repetition frequency. The Q-switched laser, which is described in detail in previous work (Ishii et al. 2010), has a ring configuration and uses an acousto-optic Q switch (AO Q-sw) to generate Q-switch pulses. The Tm,Ho:YLF laser rod in the pumping cavity is placed in a vacuum container and is side pumped by 12 InGaAs/GaAs laser diode arrays. The laser rod and diode arrays in the pumping cavity are conductively cooled to −80° and 16°C, respectively.

Table 1.

Specifications of coherent 2-μm differential absorption and Doppler wind lidar.

Table 1.
Fig. 1.
Fig. 1.

Block diagram of coherent differential absorption and Doppler wind lidar [isolator (ISO), piezoelectric translator with mirror (PZT)].

Citation: Journal of Atmospheric and Oceanic Technology 29, 9; 10.1175/JTECH-D-11-00180.1

The Q-switched laser is injection seeded to obtain single-frequency operation. Three commercial diode-pumped single-frequency CW Tm,Ho:YLF lasers are used for the injection seeding and are respectively referred to as the λ-center (MO-I), online (MO-II), and offline (MO-III) lasers. The wavelength of the λ-center laser is set at 2050.967 nm, corresponding to the R30 absorption line center of CO2. Because the frequency stabilization of the λ-center laser determines that of the online laser, as described below, bad frequency stabilization of the λ-center laser affects the precision of the DIAL measurement. We applied a method similar to that used by Phillips et al. (2003) to lock the λ-center laser within 1 MHz. A block diagram of the λ-center laser (MO-I) stabilization is shown in Fig. 2. The laser beam is phase modulated by an electro-optic modulator (EOM) after passing an optical isolator (ISO) and a polarizing beam splitter (PBS). The PBS adjusts the polarization of the laser beam to the EOM and the polarization-maintaining optical fiber. The laser beam is modulated by the EOM, and the spectrum of the modulated beam contains a strong carrier (zero order) and sets of sidebands determined by the nth-order harmonic of an oscillator (OSC) of frequency Ω. The Ω was determined by simulating the sensitivity to the deviation from the line center, and a 150-MHz phase modulation was selected. Zero-order plus and minus first-order laser frequencies are important, and sets of higher-order sidebands can be negligible because of their low intensity. After passing a single-pass, 40-cm-long cell containing pure CO2 at 2666.4 Pa three times, the modulated beam is focused on a detector (DET0, InGaAs-PIN photodiode). The signal is mixed with a 150-MHz sinusoidal wave and passes a low-pass filter (LPF) for phase-sensitive detection. The output error signal changes monotonically with deviation from the line center. Zero crossing at the line center is shown in Fig. 3. A proportional integral derivative (PID) controller adjusts the PZT driver’s DC voltage to maintain the zero error signal. The long-term laser frequency fluctuation of the λ-center laser is shown in Fig. 4, estimated from the amount of frequency shift of the λ-center laser for the error signal output. The upper and lower frequency fluctuations are +138 and −157 kHz in the error signal. The frequency stabilization of the λ-center laser is locked within ~160 kHz. The frequency fluctuations are not result of an independent measurement, and there is a possibility that unknown interferences affect the stabilization, which cannot be identified in the error signal itself. It means that the real frequency stability could be worse.

Fig. 2.
Fig. 2.

Block diagram for λ-center laser (MO-I) stabilization.

Citation: Journal of Atmospheric and Oceanic Technology 29, 9; 10.1175/JTECH-D-11-00180.1

Fig. 3.
Fig. 3.

Error signal and sweeping signal from PID controller to adjust PZT driver’s DC voltage.

Citation: Journal of Atmospheric and Oceanic Technology 29, 9; 10.1175/JTECH-D-11-00180.1

Fig. 4.
Fig. 4.

Long-term laser frequency fluctuation of λ-center laser.

Citation: Journal of Atmospheric and Oceanic Technology 29, 9; 10.1175/JTECH-D-11-00180.1

The online wavelength is controlled by laser frequency offsetting the online laser from the λ-center laser. A block diagram of the frequency offset locking of the online laser (MO-II) is shown in Fig. 5. The laser frequency was offset by mixing the λ-center laser with the online laser, that is, the heterodyne technique, and the desired laser frequency offset, was controlled using a phase-locked loop (PLL). The frequency characteristic of the frequency-modulated online laser must be known to investigate the transfer characteristics of the PLL. These transfer characteristics depend on the mass of the output coupler, generative force, spring constant, capacitance of the piezoelectric element, current supply of the PZT drive, and so on. The PLL circuit was designed using an electric circuit simulator considering those characteristics. Laser frequency offset can be used in the range of 2.5–6.5 GHz (Fig. 6). The λ-on laser is locked within ~100 kHz to a selected offset from the λ-center laser. The sum of the frequency errors of the λ-center and online lasers is within 190 kHz.

Fig. 5.
Fig. 5.

Block diagram for λ-on laser (MO-II) stabilization.

Citation: Journal of Atmospheric and Oceanic Technology 29, 9; 10.1175/JTECH-D-11-00180.1

Fig. 6.
Fig. 6.

Relation between wavelengths of three CW Tm,Ho:YLF lasers and one-way transmission over 1 km in atmosphere with HITRAN 2008 database, 400-ppm CO2 concentration, 1013-hPa pressure, and 298.15-K temperature.

Citation: Journal of Atmospheric and Oceanic Technology 29, 9; 10.1175/JTECH-D-11-00180.1

The wavelength of the offline laser (MO-III) is set at 2051.250 nm in the far wing of the R30 absorption line. The offline laser is controlled by simply adjusting the resonator temperature and the piezoelectric movement of the output coupler element. Its wavelength drift is smaller than 7 pm and does not affect calculation of the differential on–off absorption. The interference resulting from the presence of other atmospheric gases is negligible. The online and offline lasers entering a LiNbO3 electro-optic switch (EOS) are horizontal and vertical linearly polarized, respectively. The master oscillator (MO) transmitted by the EOS and the PBS, is chosen between the online (MO-II) and offline (MO-III) laser according to their polarization. The selection of polarization is controlled by the applied voltage to the EOS. A major portion of the online or offline lasers enters an acousto-optic modulator (AOM) to upshift the frequency of the laser beam by 105 MHz and is injected into the AO Q-sw slave oscillator (SO) with a ring configuration resonator. Single-frequency Q-switched laser pulses are obtained by injection seeding of each upshifted laser beam to which the resonator is matched by the ramp-and-fire technique (Henderson et al. 1986). The resonant length of the SO is tuned finely with a PZT having a ~2-μm sweep range. An InGaAs PIN photodiode (resonance detector) detects the injected pulse twice during the sweep motion of the PZT. When the detector senses the second resonance, the AO Q-sw opens and the injected pulse builds rapidly in the SO. Mechanical fluctuations of the PZT motion result in ~1 MHz of the frequency deviation between the MO and the injected laser pulse. Therefore, the absolute frequency stability of the injected pulsed laser beam is dominated by this fluctuation and contributed a little by the variance of the λ-center laser and the online laser, which is 1 MHz [≈√(190 kHz)2 + (1 MHz)2] at most.

The online and offline laser pulses are alternately switched every one shot. The pulsed laser beam is emitted into the atmosphere by using a 10-cm off-axis telescope and a waterproof two-axis scanning device, and the signal that is backscattered by moving aerosol particles is photomixed with a portion of the MO-II or MO-III output on an InGaAs-PIN photodiode (DET1). Heterodyne detection is operated under shot noise–limited conditions, where shot noise resulting from a portion of the MO-II or MO-III power dominates all other noise by about 9 dB. A small portion of the pulsed laser beam is also mixed with the MO-II or MO-III output on a balanced InGaAs-PIN photodiode (DET2) to monitor the frequency of the outgoing laser pulse. The data acquisition and signal processing are conducted as described in Ishii et al. (2010). The outputs of DET1 and DET2 are digitized at 500 MHz by using 8-bit analog-to-digital (A/D) converters. The on- and offline backscattered signals are obtained using an algorithm proposed by Frehlich et al. (1997).

3. Estimation of XCO2 and error analysis

The power Pi = On,Off(Rj) of the backscattered signal received at a range Rj can be expressed as
e1
where Ki is an instrument constant (heterodyne efficiency, quantum efficiency, receiver area, etc.) for the wavelength i, P0,i is the laser output power, βi(Rj) is the backscattering coefficient of the target atmosphere, and αi(r) is the extinction coefficient of the atmosphere. Here, αi(r) is defined as αi(r) = αatm(r) + σi(r)nCO2, where σi(r) is the absorption cross section of CO2 and nCO2 is the CO2 number density; αatm(r) is the extinction coefficient associated with any other extinction processes. Because the on- and offline wavelengths are sufficiently close, we can neglect the wavelength dependence of the instrument constant, the backscattering coefficient of the target atmosphere, and the extinction coefficient except for the CO2 absorption. By applying Eq. (1) to the ranges R1 and R2, and to the on and off wavelengths, we can obtain the differential absorption optical depth (DAOD) τ (R1, R2) resulting from the CO2 absorption in the range between R1 and R2 as follows:
e2
The relative error Δτ(R1, R2)/τ(R1, R2) of the DAOD is given by
e3
The relative error Δτ(0, Rj)/τ(0, Rj) of the DAOD is calculated using Eq. (3) of Killinger and Menyuk (1981),
e4
where is the mean power of the backscattered signal, SNRi(Rj) is on- and offline signal-to-noise ratios, and ρ is the temporal cross-correlation coefficient between POn(Rj) and POff(Rj). The SNRi(Rj) is given by
e5
where is the mean noise power, NC is the number of coherent cells (Gilbert et al. 2006), and NL is the number of on- and offline laser shots. The coefficient ρ ranges between 0 and 1. Although the relative error of the optical depth can be reduced by setting ρ closer to 1, ρ is set to 0 to avoid the practical difficulties involved in determining ρ. The relative error Δτ(R1, R2)/τ(R1, R2) of the DAOD is reduced to
e6
By using the DAOD and the weighting function (WF), the XCO2 for an altitude between R1 and R2 is given by the following equation:
e7
e8
e9
where PRi=1,2 is the pressure at the altitude Ri=1,2, p is the pressure, T is the temperature, RH is the relative humidity, Δσ(p, T) [=σOn(p, T) − σOff(p, T)] is the difference between the absorption cross sections corresponding to the wavelengths of the on- and offline lasers, g is the gravity constant, mair and mH2O are the molecular mass of dry-air and water vapor, and ρH2O (p, T, RH) is the water vapor volume mixing ratio. To calculate σi(p, T) we used the absorption line parameters in the High-Resolution Transmission Molecular Absorption Database (HITRAN) 2008 (Rothman et al. 2009) database; these values were modified by considering the parameters reported by Régalia-Jarlot et al. (2006) and meteorological elements. The τH2O is the DAOD resulting from the H2O absorption in the altitude range between R1 and R2 and is calculated using meteorological data measured by a radiosonde.
A profile of the DAOD from the surface to the free troposphere is obtained by joining slant (DAODS) and vertical (DAODV) measurements. The CO2 concentration is assumed to be horizontally uniform and well mixed in the boundary layer. To obtain the vertical profile of the DAOD, we used the DAODS for the boundary layer and the DAODV for the free troposphere. An elevation angle of the slant measurement is 16°. Both DAODS and DAODV were linked with each other at the altitude of ~1 km. The error ΔXCO2 of the XCO2 is obtained using Eq. (3), the DAOD, the WF, and the meteorological data as follows:
e10
Experimental horizontal CO2 measurements were carried out to examine the detection sensitivity of the Co2DiaWiL with respect to the laser frequency offset locking. The CO2 volume mixing ratio and error were obtained using the slope method (Gilbert et al. 2006) under assumptions that CO2 concentration and CO2 absorption cross sections did not change between R1 and R2.

4. Ground-based and upper-atmospheric measurements

The ground-based meteorological and CO2 measurements are described in detail in the previous work (Ishii et al. 2010). An automatic weather station (Vaisala WXT510) measures barometric pressure, temperature, relative humidity, liquid precipitation, wind speed, and wind direction from the roof of a four-story building (83 m above sea level) at NICT. These meteorological parameters are averaged in 1-min intervals. The accuracies of the barometric pressure, temperature, and relative humidity values are better than ±0.5 hPa, ±0.3°C (at a temperature of 0°–20°C), and ±3% (at a relative humidity of 0%–90%), respectively. These uncertainties on these three parameters lead to errors of 0.0%, <0.2%, and <0.1% in the CO2 concentration retrieved from the DIAL measurement, respectively. Ground-based measurements of the CO2 and H2O concentrations were carried out at 1-s intervals by an in situ instrument (a LI-COR Model LI-840 nondispersive infrared CO2/H2O gas analyzer). This instrument was installed in an observation room on the upper roof of the same building at NICT. The inlet for the in situ instrument was situated about 86 m above sea level. Temporal calibrations were carried out with N2 and CO2 mixed gases at concentrations of 0, 350, and 528 ppm. Final calibrations were made with standard CO2 gases at concentrations of 0, 353, and 399 ppm at the National Institute for Environmental Studies (NIES). The Vaisala GPS radiosonde RS92-SGP was launched at NICT to measure meteorological elements (pressure P, temperature T, and relative humidity RH) in the troposphere and lower stratosphere. The accuracies of the barometric pressure, temperature, and relative humidity values are better than ±1 hPa, ±0.5°C (two-sigma confidence level), and ±5% (two-sigma confidence level), respectively. These correspond to errors of 0.0%, <0.1%, and <0.1% in the integrated weighing function. The ΔIWF/IWF of Eq. (4) is estimated to be 0.0014, so Eq. (10) can be rewritten as
e11

5. Results

a. Experimental horizontal CO2 measurements

Figure 7 shows an example of atmospheric returns corresponding to the offline laser (black line) and three laser frequency offsets of the online laser, that is, 2.5 (thick gray line), 4.8 (dashed gray line), and 5.8 GHz (thin gray line), from the center of the R30 absorption line of CO2. The laser beam was directed horizontally westward by the two-axis scanning device. Averaged pulsed number t was 9000 shots (10-min integration time). The strong scattering of the outgoing laser pulses from the receiver optics affects return signals in the near range. The SNR of the online laser decreased more slowly with increasing laser frequency offset. The observable range depends strongly on atmospheric transmission. In this example, the range limits of the measurements with laser frequency offsets of 2.5, 4.8, and 5.8 GHz were up to approximately 7, 12, and 14 km, respectively. The increase of the laser frequency offset enables longer-range CO2 measurement. The SNR of the offline laser pulse indicates that the Co2DiaWiL has a potential to detect atmospheric return at a longer range than ~20 km. In this study, the laser frequency offset of 4.8 GHz was used in consideration of smaller error dependence of the cross section on atmospheric pressure and temperature uncertainties (Ishii et al. 2007).

Fig. 7.
Fig. 7.

SNR in atmosphere for offline and online lasers at three frequency offsets: 2.5 (thick gray line), 4.8 (dashed gray line), and 5.8 GHz (thin gray line) from center of R30 absorption line of CO2.

Citation: Journal of Atmospheric and Oceanic Technology 29, 9; 10.1175/JTECH-D-11-00180.1

Experimental horizontal CO2 measurements were conducted to compare results of the Co2DiaWiL with those of the in situ instrument on 22 October 2009. The data from the Automated Meteorological Data Acquisition System (AMeDAS) provided by the Japan Meteorological Agency (JMA) indicate that the weather was fine in the morning but there was complete cloud cover in the afternoon. Convection was active in the boundary layer in the morning, but less sunlight suppressed convection in the afternoon, which causes calm conditions similar to nighttime. The temporal variations of the CO2 concentrations measured by the Co2DiaWiL and the in situ instrument are shown in Fig. 8. The Co2DiaWIL was run continuously to stabilize the laser and detection systems, and then horizontal measurements were made. The open circles show the CO2 concentrations for 900 shot pairs over a column range from 2.616 to 5.614 km measured by the Co2DiaWiL. The black line shows the 30-min running average of 1-min Co2DiaWiL results, and the gray line shows the data obtained from the in situ instrument averaged over 10 min. There are very large different sampling volumes between the two sensors. Both the Co2DiaWiL and in situ instrument measured CO2 concentrations at almost the same height. Because the root-mean-square of the difference between the surface temperature measured by our automatic weather station and that measured by two surface monitoring stations 4- and 7-km westward of NICT was less than 0.6°C, the error in the CO2 concentration resulting from the temperature was <0.3%. The differences between the pressure measured by our automatic weather station and that measured by the two surface monitoring stations were smaller than 1 hPa, and the CO2 concentration error about the pressure was negligibly small. The temporal variations of the CO2 concentrations measured by the Co2DiaWil and in situ instrument agree well with each other. The results of the horizontal experimental measurements showed the precision of 1.1%–2.1% for the column range of 2.616–5.614 km and 900 shot pairs. The root-mean-square of the absolute values of the differences between the 30-min running averages of the two sensors was 0.9% (3.5 ppm). The results show that the Co2DiaWiL can measure CO2 with high accuracy for a 3-km column range.

Fig. 8.
Fig. 8.

Temporal variations of CO2 concentrations measured by Co2DiaWiL and in situ instrument on 22 Oct 2009. Laser frequency offset was 4.8 GHz for horizontal CO2 measurement.

Citation: Journal of Atmospheric and Oceanic Technology 29, 9; 10.1175/JTECH-D-11-00180.1

b. Experimental vertical CO2 measurements in 2010 and 2011

The NIES and JAXA made airborne CO2 measurements for the GOSAT data product validation in February 2010 and in January and February 2011. NIES installed an in situ instrument (LI-COR Model LI-840) and hand-operated air-sampling equipment on an aircraft and made airborne CO2 measurements using the in situ instrument at altitudes of 2–7 km over Kumagaya, Japan (139.25°N, 36.2°E), on 14, 20, and 23 February 2010, and at altitudes of 0.5–3 km over NICT (35.71°E, 139.495°N) on 14 and 20 February 2010. The CO2 concentration was measured continuously using the airborne in situ instrument during flight, and its mixing ratio was calibrated by using two standard gases. The airborne in situ instrument can measure CO2 with a high precision of 0.2 ppm for an average of 10 s (Machida et al. 2008). NIES made airborne CO2 measurements using the in situ instrument over Kumagaya in January and February 2011, but airborne CO2 measurements were not conducted over NICT in that period.

NICT made ground-based DIAL slant and vertical CO2 measurements on 14, 20, and 23 February 2010. Radiosondes were launched to obtain profiles of meteorological elements at 0100, 0300, and 0500 UTC on the same days. The XCO2 is obtained by using the signal backscattered by aerosol particles (atmospheric return) or by clouds (hard target) observed by the Co2DiaWiL. Figures 9a–c show vertical profiles of the DAOD and the SNR obtained on 14, 20, and 23 February 2010, respectively. The solid and dashed black lines show the SNR of the offline laser measured at elevation angles of 90° and 16°, respectively. The solid and dashed gray lines show the same as the black lines, but those for the online laser. The open and closed circles are the DAOD at elevation angles of 90° and 16°, respectively. Figures 9d–f show vertical profiles of the temperature (solid black line), pressure (gray line), and relative humidity (dashed black line) measured by the radiosonde on 14, 20, and 23 February 2010, respectively. The DAOD and SNR were obtained using seven 75-m range bins average and a 4.8-GHz laser frequency offset. The numbers of on- and offline laser shot pairs for the slant and vertical CO2 measurements made on 14 February were 4500 and 18 000, respectively, and those for 20 and 23 February were 9000 and 36 000, respectively. Figures 9d–f indicate that the top of the boundary layer was at an altitude of ~1–1.5 km and the bottom of the free troposphere was at an altitude of ~2 km. Although Fig. 9d indicates air masses being rich in water vapor at altitudes of 1.9 and 6–11 km, the synoptic situations for the data were dry atmosphere and clear sky. The estimated partial XCO2 is listed in Table 2. Airborne in situ CO2 measurements were made over NICT and compared with the results of the Co2DiaWiL. The partial XCO2 derived on 14 February from the airborne measurements and from the Co2DiaWiL between 0.4 and 1.0 km were 406.2 and 407.2 ppm, respectively. The partial XCO2 measured by the two sensors on 20 February were 402.4 and 401.7 ppm, respectively.

Fig. 9.
Fig. 9.

Vertical profiles of DAOD and SNR obtained from (a) 0408 to 0433 UTC 14 Feb, (b) 0325 to 0427 UTC 20 Feb, and (c) 0314 to 0413 UTC 23 Feb 2010. Vertical profiles of temperature, pressure, and relative humidity measured by radiosonde launched at around 0300 UTC on (d) 14, (e) 20, and (f) 23 Feb 2010. SNR of offline (black) and online (gray) laser measured at 90° (solid lines) and 16° (dashed lines) elevation angles is shown. DAOD at 90° (open circles) and 16° (closed circles) elevation angles.

Citation: Journal of Atmospheric and Oceanic Technology 29, 9; 10.1175/JTECH-D-11-00180.1

Table 2.

Partial XCO2 concentration and measurement error estimated using Eqs. (7) and (10), and meteorological data measured by radiosonde on 14, 20, and 23 Feb 2010. Error of XCO2 given by statistical random error, spectroscopic data error of meteorological data, and on- and offline laser wavelengths.

Table 2.

The precision of the Co2DiaWiL measurements were in the range of 1.0%–1.1%. The results of the two sensors agree well with each other within the margin of error. When the SNR of the online laser was below ~24 dB, the DAOD began to show small fluctuations. The DAOD did not show continuity for the SNR of the online laser lower than ~20 dB. The decrease of the SNR of the online laser affects the reliability of the DAOD estimation, resulting in inaccurate estimation of the partial XCO2. The results indicate that when the SNR of the online laser is larger than 24 dB, the Co2DiaWiL has the potential of measuring with high accuracy. To increase the 20-dB SNR to 24 dB, the SNR of the online laser must be improved by a factor of ~2.5. The SNR depends on the atmospheric conditions and the systematic parameters of the DIAL. The atmospheric conditions are changeable, and improvement of the systematic parameters of the DIAL is limited. Therefore, the number of laser shot pairs is often increased to improve the SNR. Equation (5) indicates that the required number of laser shot pairs is 6.3 times as many as in this experimental measurement.

On 14 February 2010, the Co2DiaWiL received strong backscattered signals from an altitude of ~10.7 km. The meteorological data indicate the presence of cirrus clouds. Clouds are regarded as a hard target, and they can help to estimate the DAOD. As a first hard target case (a second hard target case is discussed later), we attempted to estimate the partial XCO2 in the altitude range between 0.4 and 10.485 km, to be 390.6 ± 5.1 ppm. The partial XCO2 between 0.4 and 10.485 km was also estimated from the airborne in situ measurement with the assumption that the CO2 concentration was constant above the highest observational point up (~7 km) to the tropopause. The estimated partial XCO2 was 394.7 ppm, which is consistent with the Co2DiaWiL observation.

NICT made ground-based slant and vertical CO2 measurements for the GOSAT data product validation on 28 and 31 January, and on 3 and 7 February 2011. On those days, radiosondes were launched to obtain profiles of meteorological elements at 0130, 0330, and 0530 UTC. The numbers of on- and offline laser shot pairs for slant and vertical CO2 measurements were 9000 and 36 000, respectively. The partial XCO2 was estimated for the online laser SNR larger than 24 dB. Figures 10a–h show the vertical profiles of DAOD, SNR, and meteorological data obtained on the mentioned dates. Figures 10a,e and b,f show data for 28 and 31 January 2011, respectively. Figures 10c,g and d,h show data for 3 and 7 February 2011, respectively. The estimated partial XCO2 is listed in Table 3. The averages of the partial XCO2 for the altitude ranges between 0.4 and 1.0 and between 0.4 and 2.0 km estimated during the period were 405.6 and 400.8 ppm, respectively, and the total measurement errors of the XCO2 were 0.8% and 1.6%, respectively. The Co2DiaWiL observed the signals of cirrus clouds at an altitude of ~9 km on 7 February 2011. As the second hard target case, the XCO2 for the altitude between 0.4 and 8.986 km was estimated, with the result of 395.9 ± 4.0 ppm.

Fig. 10.
Fig. 10.

As in Fig. 9, but from (a) 0339 to 0439 UTC 28 Jan, (b) 0242 to 0342 UTC 31 Jan, (c) 0302 to 0402 UTC 3 Feb, and (d) 0235 to 0335 UTC 7 Feb 2011, and at around 0300 UTC (e) 28 and (f) 31 Jan, and (g) 3 and (h) 7 Feb 2011.

Citation: Journal of Atmospheric and Oceanic Technology 29, 9; 10.1175/JTECH-D-11-00180.1

Table 3.

As in Table 2, but for 28 and 31 Jan, and 3 and 7 Feb 2011.

Table 3.

6. Conclusions

A powerful 2-μm conductively cooled laser diode–pumped single-frequency Q-switched solid-state laser with laser frequency offset locking was developed for long-range CO2 measurements and was installed into the Co2DiaWiL. Experimental horizontal CO2 measurements were made to examine the detection sensitivity of the Co2DiaWiL with respect to the laser frequency offset locking. The λ-center laser is set at the R30 absorption line center of CO2 by using an EOM, a CO2 cell, a PID controller for PZT, and the phase-sensitive detection technique. The PID controller adjusts the DC voltage supplied to the PZT, which changes the cavity length of the λ-center laser. The long-term frequency stabilization of the λ-center laser was locked within ~160 kHz for more than 13 h. The online laser is locked to the λ-center laser. The difference in the frequency between the λ-center and online lasers is determined by heterodyne and phase-sensitive detections. The frequency of the online laser is controlled by changing the cavity length of the online laser through adjusting the PZT driver’s DC voltage and can be set to 2.5–6.5 GHz from the λ-center laser frequency. The frequency stabilization of the online laser was within ~100 kHz to a selected offset from the λ-center laser. We used the ramp-and-fire technique to establish the single-frequency pulsed laser. The frequency jitter was found to be within ~1 MHz. The total frequency stability of the online pulsed laser depends mainly on the frequency jitter of the ramp-and-fire technique and secondarily on the stability of the λ-center and online lasers. The absolute frequency stability of the injected online pulsed laser beam was within ~1 MHz. The horizontal column from 2.6 to 5 km has been retrieved from 900 shot pairs (1-min integration time) with a precision of less than 2.1%. The root-mean-square of the differences between the 30-min CO2 averages measured by the two sensors was 0.9% (3.5 ppm).

The Co2DiaWiL slant and vertical CO2 measurements with high precision were made synchronously with the GOSAT data products validation experiments. These comparisons between the Co2DiaWiL slant and vertical CO2 measurements and the airborne in situ measurements are the first validation of ground-based vertical CO2 measurements up to aircraft altitude. The partial XCO2 derived from both an airborne in situ CO2 measurements and DIAL measurements for an altitude between 0.4 and 1.0 km on 14 and 20 February agreed well with each other within the margin of error. Tropospheric partial XCO2 of 390.6 ± 5.1 ppm for the altitude between 0.4 and 10.485 km on 14 February 2010 have been retrieved from the strong backscatter of cirrus clouds (hard target) and agree with that derived from an airborne in situ measurement. We also estimated a partial XCO2 of 395.9 ± 4.0 ppm for the altitude between 0.4 and 8.986 km on 7 February 2011. The Co2DiaWiL has the potential for measuring XCO2 at much higher altitudes with a high precision of <1% for hard target returns.

Although it is difficult to frequently make the airborne in situ measurement, the ground-based DIAL vertical CO2 measurement can be made continuously as long as the weather condition permitted. It is a very useful technique to obtain the CO2 concentration. A bias-free high precision of <1 ppm is needed to provide a useful tool capable of supporting carbon studies. To enable reliable CO2 measurement, a combination of temporal or spatial scales, range resolution, and number of laser shots should be considered. We could achieve the XCO2 measurement with the bias-free high precision if the number of shot pairs used in this paper increased by a factor of 10. The estimation of a correct CO2 absorption cross section is very important to achieve the XCO2 measurement with the bias-free high precision. The wavelength of the offline laser is controlled by simply adjusting the resonator temperature and the piezoelectric movement of the output coupler element. Controlling the wavelength of the λ-offline laser is also necessary for measuring CO2 with higher precision.

Acknowledgments

A part of this research was conducted as selected research in the GOSAT Research Announcement in the field of GOSAT data products validation. The Vaisala sounding system borrowed from the Hydrospheric Atmospheric Research Center, Nagoya University. We would like to express our appreciation to the Hydrospheric Atmospheric Research Center for supporting for radiosonde observation in 2010.

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