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Abstract
We developed a mobile ozone differential absorption lidar system to simultaneously measure the vertical profiles of tropospheric and stratospheric ozone from an altitude of ~5 to 50 km. The system emits four laser beams at wavelength of 289 nm, 299 nm, 308 nm and 355 nm and receives their corresponding Mie/Rayleigh backscattering return signals, and two N2 Raman return signals at 332 nm and 387 nm shifted from 308 nm and 355 nm, respectively. An assembled telescope array with four 1.25-m telescopes (effective diameter > 2 m) collects the Rayleigh and Raman backscattering signals at 308/332 and 355/387 nm. This system is currently deployed at the Yangbajing Observatory in Tibet (~4300 m elevation) and has begun observations in regular campaign mode since October 2017. The lidar results agree very well with those observed by the Aura/MLS satellite. This novel ozone lidar system operates at the highest elevation of any such system in the world. The higher elevation and larger receiver aperture of this system yield a higher signal-to-noise ratio and lower statistical uncertainty.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Ozone is a very important trace gas in Earth’s atmosphere that protects life on Earth from harmful solar ultraviolet radiation [1]. The stratosphere contains 90% of the Earth’s ozone, with a peak height of approximately 20-25 km and a density of up to 6 × 1012 molecules/cm3 [2,3]. A small amount of ozone also exists in the troposphere and mesosphere. Tropospheric ozone is a polluting gas, and excessive tropospheric ozone is detrimental to human and biological health [4,5]. Ozone is an important tracer for studying atmospheric dynamics, and also plays an important role in radiative balance and climatology change [1]. At the same time, ozone actively participates in atmospheric chemical processes and affects changes in atmospheric chemical composition.
Many ground-based and space-borne instruments have been developed to measure atmospheric ozone concentrations and total content, including the Dobson spectrometer [6], ultraviolet absorption ozone analyzer [7], lidar [8], balloons with ozonesondes [9,10] and satellites with instruments [11–15]. Dobson spectrometers can detect the total ozone column by measuring the amount of ultraviolet radiation that passes through the atmosphere, and an ozone analyzer is able to detect single-point ozone concentrations near the ground. Balloons with ozonesondes can in situ measure tropospheric and lower stratospheric ozone profiles at a specific location. Satellites can detect global ozone profiles, but with a coarse vertical resolution for tropospheric ozone. Lidar has advantages for long-term ozone profile observations with high temporal and vertical resolutions at a location. Generally, Raman shifted wavelengths from Nd:YAG lasers are widely used for tropospheric ozone detection and excimer lasers or excimer laser combined with a third harmonic Nd:YAG laser are common used for stratospheric ozone detection [16–22]. Young et.al. recently developed an all-solid-state UV laser source for tropospheric ozone detection [23].
However, until now, there have only been a few stations in the world where ozone differential absorption lidars (DIAL) can simultaneously detect ozone in both the troposphere and stratosphere [24–27], such as the Haute-Provence Observatory, Table Mountain Observatory, La Reunion Island Observatory and Arctic Lidar Observatory. Simultaneous detection of tropospheric ozone and stratospheric ozone is scientifically important for studying the material exchange between the troposphere and stratosphere as well as the dynamic coupling of the troposphere and stratosphere. Due to the special large topography, Qinghai-Tibet plateau accompanying Asian monsoon circulation is significantly different from that of the surrounding areas in dynamic and thermal properties and has important influences on the formation and development of atmospheric circulation and climate change in Asia and even the world. In this paper, we present a mobile ozone DIAL lidar system that emits multiple laser beams and receives signals through multiple telescopes to simultaneously measure the tropospheric and stratospheric ozone over Tibet. The second section of this paper specifically introduces the design and implementation of the ozone detection lidar. The third section provides the typical results of the lidar and a comparison of the lidar and satellite results. The fourth section provides a summary of the study.
2. Ozone lidar system architecture
To simultaneously measure the tropospheric and stratospheric ozone, we designed a system that combines differential absorption with different backscatter mechanisms: Mie/Rayleigh backscattering in the troposphere, Raman backscattering between the upper troposphere and middle stratosphere, and Rayleigh backscattering in the upper stratosphere. Figure 1 shows a schematic diagram of the Tibetan mobile ozone lidar system developed by the University of Science and Technology of China (USTC). The main parameters of the system are shown in Table 1. Below, we separately describe the transmitter (1), receiver (2), and timing control and acquisition (3) subsystems in detail.
Table 1. Main parameters of the mobile ozone lidar
(1) Transmitter
Four laser beams with wavelengths of 289 nm, 299 nm, 308 nm, and 355 nm are emitted. The 289 nm and 299 nm lasers are separately generated from D2 and H2 Raman cells by using stimulated Raman scattering as a Raman laser source [28,37–42]. Each of the Raman cells is pumped by a 266 nm laser beam that is equally split from the 4th harmonic frequency of a Nd:YAG laser (Powerlite DLS 9050, Continuum Inc.). Generally, a longer optical interaction path with a buffer gas in Raman cells is optimal to generate a first-order Stokes shift with higher efficiency [28–32]. A lens with a 1.2 m focal length focuses each 266 nm laser beam into a 2 m length Raman cell to generate a Raman laser wavelength. One Raman cell is filled with pure D2 with 99.999% purity, and the other is filled with pure H2 with 99.999% purity. Low-pressure gas was used due to its advantages of greater safety and less leakage. After a sufficient number of laboratory experiments under different pressures, we were able to obtain 289 nm with ~8 mJ energy at 1.93 × 105 Pa and 299 nm with ~9.5 mJ energy at 1.0 × 105 Pa under pumping by ~35 mJ 266 nm laser beam, corresponding to ~23% and ~27% conversion efficiencies, respectively, in the forward direction for the D2 and H2 first-order Stokes Raman shifts.
The pair of 289 nm (on-line) and 299 nm (off-line) beams serves as differential absorption wavelengths for the lower atmosphere (upper troposphere and lower stratosphere) ozone measurements. The 308 nm beam is generated from an XeCl excimer laser (Coherent LPX 220) with a 200 Hz repetition rate and 35 W of power. The 355 nm beam is generated from an all-semiconductor pumped Nd:YAG laser with type NO. DPSS EVO II from the INNOLAS Corporation. At the time of this study, the DPSS EVO II is the most advanced all-solid-diode pumped Nd:YAG laser, with the highest power and repetition rate in the world. Its repetition rate is 200 Hz, and the energy of the 3rd harmonic of 1064 nm (355 nm) can reach 120 mJ, corresponding to an average power of 24 W. The wavelengths of 308 nm and 355 nm form one differential absorption wavelength pair for upper stratospheric ozone detection, while N2 Raman returned signals with wavelengths of 332 nm and 387 nm serve as another wavelength pair for ozone measurements between the upper troposphere and middle stratosphere to minimize stratospheric aerosol impact [33]. All of the laser beams are optimized for beam divergence by their respective self-designed beam expanders with an ~3X beam expansion factor. They are then emitted into the atmosphere by an optical antenna subsystem with four motor driven gimbal mounts, which can scan in azimuth and zenith directions to optimize the return signals similar to what is used in our sodium temperature/wind lidar system [34].
(2) Receiver subsystem
For the detection of upper stratospheric ozone (~30 km-50 km), we use four independent 1.25 m diameter telescopes to form an assembled telescope array with an effective diameter of >2 m to improve the signal-to-noise ratio, as shown in Fig. 2. A similar telescope array of for lidar measurements has also been implemented at the Haute-Provence Observatory and later at La Reunion Island [24]. Two fibers with a core diameter of 1000 μm are arranged at the focal plane of each independent telescope. One fiber is used to transmit backscattering signals for 308 nm Rayleigh and 332 nm N2 Raman; the other is used to transmit the backscattering signals for 355 nm Rayleigh and 387 nm N2 Raman. The telescope array has four fibers for the 308nm and 332nm signals and four fibers for the 355nm and 387nm signals. One end is composed of four separate cylinder connectors that mount each of four fiber cores, and on the other end is single cylinder connector that bundles four fiber cores together.
For tropospheric and lower stratospheric ozone detection (~5 km-19 km), one main focus type telescope with a diameter of 1 m for the ~8-19 km range and two Newtonian telescopes with diameters of 210 mm for the 5-10 km range are used. Similar to setup of a single telescope in the telescope array, two optical fibers are installed at the focal plane of the 1 m telescope. The connectors used for the two optical fibers are custom-made with two SMA (Sub - Miniature - A) connectors on one end and a cylindrical connector with two fibers packed together on the other end. One of the two fibers used for higher altitude 289 nm signals is marked as 289H, and the other used for higher altitude 299 nm signals is marked as 299H. Two 210 telescopes are attached on the cylinder wall of the 1 m telescope, one of which is used to receive the lower altitude 289 nm return signal (marked as 289L), and the other is used to receive the lower altitude 299 nm return signal (299L). The fiber is connected to the focus of each 210 mm telescope to transmit the optical signal to the subsequent optical coupling unit.
We specifically designed an integrated subsequent optical unit to effectively couple and transmit the received signals from all fibers to photomultipliers. Figure 3(a) shows the front view of the subsequent optical unit. The subsequent optical unit has six input ports labeled 289L, 299L, 289H, 299H, 308&332, and 355&387. A 360 mm diameter chopper disc rotating at a high speed of 6000 rpm is used in the optical unit to mechanically block the strong backscattered signals from the near field of view and to minimize the signal induced noise, as shown in Fig. 3(b). The four slots evenly distributed on the outermost circle with 1/8 circumference are for 308, 355, 332, 387, 289L and 299L channels. The two slots symmetrically distributed on the middle circle with 1/8 circumference are for 289H and 299H channels. The two minimum slots on the innermost circle are used to generate the main system synchronization pulse of the lidar system. The backscattering signals at 308 nm and 332 nm from the telescope array transmitted by bundled optical fibers are first collimated into a quasi-parallel light beam through a self-designed air-gap achromatic lens collimator, and then split into 308 nm and 332 nm channels by a color beam splitter (R332T308), finally pass through their respective interference filters, as shown in Fig. 3(c). The two channels are nearly symmetrical to the optical axis of the collimator, but their beam positions on the chopper disc can be independently adjusted over a small range (approximately ± 5°) around the collimated optical axis, resulting in the 332 nm signal passing through the chopper ahead of the 308 nm signal. The 332 nm signal is coupled into the photomultiplier tube after passing through the chopper disc, whereas the 308 nm signal is divided into two beams at 10% (308-2) and 90% (308) after the chopper disc and then directed into their respective photomultipliers. The channel setting of the 355 nm and 387 nm signals is similar to that of the 308 nm and 332 nm signals. The 289L and 299L channels are symmetrically arranged on the outer slots of the disc, whereas the 289H and 299H channels are symmetrically arranged on the inner slots of the disc.
Fig. 3 (a) Layout of the subsequent optical units followed telescopes, (b) specific designed chopper, (c) optical setup for 308/332 channel.
We note here that to eliminate the influence of lens color differences for the 308/332 nm and 355/387 nm channels, the achromatic lens collimator uses two different glass materials with different refractive indexes in the UV band. The first lens is made of fused silica, and the second lens is made of calcium fluoride. The curvature of the two lenses and the distance between them are optimized by Zemax software to obtain the best collimation, which improves the transmission through the filter.
Photomultiplier tubes (PMTs) are used to convert optical signals into electrical signals. Considering that the backscattering signals are all within the UV band, we chose a photomultiplier tube module consisting of a UV-sensitive multiplier tube (Hamamatsu R9880U) along with a high voltage and a pulsed sharp circuit. The multiplier tube module directly outputs a positive-level pulse signal with a width of ~10 ns and a resolution of ~20 ns. The R9880U multiplier tube has an ultra bialkali photocathode that is very sensitive to the ultraviolet band with a quantum efficiency of ~30% from 280 to 400 nm.
(3) Timing and acquisition subsystem
The timing, as shown in Fig. 4, plays a key role in coordinating all of the components of the system to work in the predesigned sequence. The main system synchronization pulse S1 is generated by a photodiode (PD) in front of the wheel when the light beam emitted from a light emitting diode (LED) behind the wheel is chopped through two small symmetric trigger slots (see Fig. 3). The chopper wheel rotates at a speed of 6000 rpm, corresponding to a repetition rate of 200 Hz for the main sync pulse. This main pulse is connected as an external triggering signal to a digital delay pulse generator (DG645), which then generates four pulse signals. One signal with the same frequency is used as feedback to stabilize the rotating speed of the chopper. The second signal, S6, with a 4-frequency division and some delay time, is used to trigger the 266 nm laser. The third and fourth signals (S2, S4) directly trigger the 308 nm and 355 nm lasers after the proper time delay. To minimize the mutual interference among the four laser beams, we intentionally set an ~1 ms delay for each laser trigger in sequence. The time separating method can significantly reduce interference due to the insufficient OD value of the filters and the poor sealing of the internal optical path of the subsequent optical unit.
Considering the actual time delay between the Q synchronous pulse and the real laser pulse, we use a photodiode to detect laser light emission and generate a simultaneous trigger pulse for data acquisition to minimize the lidar detection height error (S3, S5, S7). The three multichannel photon counters (MCS6A) finally record the backscattered photons. Each photon counter has five counting channels with a counting rate of 10 GHz and a FIFO memory of 1 GB and transfers data between the USB interface and the computer. Three MCS6A counters are used to separately acquire signals for the three lasers (the 266 nm, 308 nm, and 355 nm lasers).
To facilitate mobile capability, all components of our ozone lidar system are installed in two standard containers with dimensions of 6 × 2.4 × 2.4 m (length × width × height). One container is equipped with the 266 nm and 355 nm lasers, the Raman source and auxiliary optics, the 1 meter telescope, and the subsequent optical unit and acquisition control parts, and the other container is equipped with the 308 nm laser and monitoring unit. The telescope array is installed in a facility building. Figure 5(a) shows the layout of the ozone lidar system, and Fig. 5(b) is a photograph of the ozone lidar system.
3. Signals and initial results
The mobile ozone lidar developed at the USTC was initially deployed to Huainan, Anhui Province in June 2016 to be integrated with the assembled telescope array. In October 2017, it was finally deployed at Yangbajing Observatory in Tibet at an elevation of ~4,300 meters and becomes one of the highest ozone lidars operating in the world. Generally, the lidar saves a raw signal file integrated for one minute and binned in a 61.44 meter range with 2048 data points. In the raw data file, the site information, start and end times of the acquisition, integration time, and range-resolved signals of the multiple channels are all stored.
Before retrieval of the ozone vertical profile, we first preprocess the raw signals. The preprocessing includes a time integration of the raw signal, subtraction of the background noise, saturation correction using Donowan’s method [35], and smoothing of the data. We also checked the background noise in different integration time and found that there was no obvious decay at the tail end of the lidar return signals. These test results of intermediate process are not shown. Figure 6 shows the return signals and signal-to noise-ratio (SNR) after integration for 30 minutes. It can be seen that the signal from channel 289L can reach 18 km with an SNR greater than or equal to 10, and a similar SNR in the signals can be found at 22 km from 289H, 21 km from 299L, 26 km from 299H, 70 km from 308H, 81 km from 355H, 40 km from 332 and 50 km from 387. Background noise averaged over the range of 115 to 125 km is subtracted from the raw signals. Then, the background-removed signal is smoothed by the segmental triangle weight smoothing method for different altitude range. The normal vertical resolution is 1 km for Mie/Rayleigh signals and Rayleigh signals below 42 km, but 3 km for the Raman signal and the Rayleigh signal above 42 km.
Fig. 6 Integrated raw signals of ozone lidar in 30 minutes of (a)289L,299L,289H,299H, and (b)308, 355, 332, 387, 308-2,355-2, and their signal to noise ratios (c) and (d).
After the signal is preprocessed, the ozone profile is derived using the differential absorption inversion equations, including the Rayleigh/Mie differential absorption inversion equation and the Raman differential absorption inversion equation [19,33]. The absorption cross section is downloaded from IGACO-O3 website (http://igaco-o3.fmi.fi/ACSO/cross_sections.html, SER) [36]. In the present inversion, we use temperature independent cross-sections for different wavelengths to derive the ozone profile (cross-sections at 283K for 289nm and 299nm and cross-sections at 233K for 308nm, 355nm, 332nm and 387nm). In the future, we will improve our inversion algorithm considering the temperature effect on cross section in the both tropospheric and stratospheric ozone derivation. The aerosol correction follows the Klett method to calculate aerosol profiles under assumed Ångström exponents of 1 and lidar ratio of 50 sr [37]. Rayleigh molecule correction is performed based on Naval Research Laboratory (NRL) mass spectrometer and incoherent scatter radar (MSIS-00) model.
Figure 7 shows the vertical profiles of the ozone number density and relative error detected by our lidar at 17:30 UT on December 24, 2017. The ozone profiles below 20 km, between 20 and 30 km, and above 30 km are derived from the 289 and 299 nm Mie/Rayleigh signals with a 1km vertical resolution, 332 and 387 nm N2 Raman signals with a 3km vertical resolution, and 308 and 355 nm Rayleigh signals with a 1km vertical resolution below 42km and 3km above 42km, respectively. From the figure, it is clear that the ozone number density at 10-17 km is approximately 5 × 1011 molecules/cm3, and that the variation is small. The ozone density rapidly increases from 17 km, reaching a maximum value of 4.5 × 1012 molecules/cm3 at approximately 24 km, and the ozone density then gradually decreases with height to a very small amount above 50 km. The relative error of the ozone concentration detection mainly depends on the SNR and the absolute value of the ozone concentration. The overall performance shows that the relative error gradually increases as the signals attenuate. Because the Raman signal is much weaker than the Mie/Rayleigh signals, the relative error derived from the Raman signals in the middle stratosphere is relatively large. Below 46 km, the relative error of the ozone concentration ranges between 3% and 30%. Above 46 km, the measurement error can easily reach 100% due to much lower ozone concentration in this region.
Fig. 7 Observation result of ozone lidar on Dec., 24, 2017 in Yangbajing, Tibet (a) ozone number density and (b) its relative error.
To validate our ozone lidar results, we compared the lidar results with Aura/MLS satellite observations for three different days, as shown in Fig. 8. The blue dash lines are the ozone number density profiles of the satellite observations near the Yangbajing Observatory in Tibet. The height of the MLS observation is log-pressure height calculated from the zonal mean temperature and pressure. The red solid lines are the lidar results within two hours when the satellite passed over the location. The lidar results are very consistent with the satellite observations, in general. The relative difference is within 20% in most altitude ranges. Main differences are near the tropopause and the stratopause. The maximum relative differences are respectively greater than 100%, ~60%, and greater than 100% near the tropopause in the three days. Stratosphere-Troposphere exchange usually occurs in the Upper Troposphere-Lower Stratosphere region. Normally there is a large variability in this region. So possible reasons for the difference near tropopause may be co-location error and different temporal resolution between lidar and satellite. We will further study the interesting ozone exchange characteristics for this region in the future using long term data. The reasons for the differences in the stratopause region are mainly due to small ozone concentration and low SNR.
Fig. 8 Ozone number density comparison between ozone lidar and Aura/MLS (left), and relative difference (right) in three different days.
4. Summary
In this paper, we describe in detail a mobile ozone lidar for the simultaneous observation of the tropospheric and stratospheric ozone, which was developed at the USTC and deployed at the Yangbajing Observatory in Tibet. The system combines Mie/Rayleigh, Raman and Rayleigh backscatter channels. In this lidar system, three lasers and Raman cells are used to generate four laser beams with wavelengths of 289 nm, 299 nm, 308 nm and 355 nm. An assembled telescope array with four 1.25 m telescopes, one 1 m telescope and two 210 mm telescopes compose the lidar receiver. For signal transmission, we specifically designed the fiber connectors to meet the demands of the assembled telescope and the 1 m telescope. To greatly reduce the interference among the multiple backscattering signals, we used time-division during laser emission and space-separation in the receiving field of view. We designed a highly integrated subsequent optical unit with a rotating chopper wheel at 6000 rpm to simultaneously transmit all of the signals to PMTs from the fibers.
Using this lidar system, we obtain an ozone number density profile with high spatial and temporal resolution from 9 to 55 km over Tibet (at an altitude of 4300 km) with a relative error of less than 30% in most of the range and greater than 30% above 45 km. The observed lidar ozone results agree very well with those observed by the Aura/MLS satellite. However, there are some differences near tropopause and stratopause region. In future work, we plan to upgrade the lidar system to extend the observation range starting from the near surface and study the anomalous troposphere and stratosphere exchange over Tibet.
Funding
National Natural Science Foundation of China (41674149, 41127901), and the Open Research Project of Large Research Infrastructures of CAS – “Study on the interaction between low/mid-latitude atmosphere and ionosphere based on the Chinese Meridian Project.”
Acknowledgments
We thank all other members in APSOS team for their contribution to the system installation. The Aura/MLS data is downloaded from https://earthdata.nasa.gov. We also thank editors and three anonymous reviewers for their constructive comments and suggestions.
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