Stratospheric Ozone Lidar (SOLID)

For more information about Stratospheric Ozone Lidar, please, contact Dr. Alexey Tikhomirov.

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Data

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Ozone is an important gas constitute of the Earth’s atmosphere. It forms a distinctive layer in the stratosphere between ~10 and 50 km above the surface know as ozone layer.  Ozone layer protects life on Earth’s surface by absorbing a large portion of the Sun’s biologically harmful ultraviolet radiation.  Stratospheric ozone trends have been assessed continuously to evaluate the measures implemented by the Montreal Protocol and to observe the reaction of the ozone layer to a changing climate (see Twenty Questions and Answers About the Ozone Layer: 2022 Update).   The ozone quantities in the atmosphere are measured by various instruments installed on the ground as well as mounted on sounding balloons, aircraft, and satellites (McElroy and Fogal, 2010). In-situ instruments measure ozone locally by continuously pumping air samples through a small detection chamber. Remote sensing instruments measure ozone over long distances by utilizing ozone’s unique optical attenuation or emission properties.   In early 1980’s, a differential absorption lidar (DIAL) technique, pioneered by Schotland (1974) to measure atmospheric water vapor, was further developed and improved by several research groups to study many other trace gas species, including NO, SO2, CH4 and stratospheric ozone (Uchino et al., 1978, Megie et al., 1985).  In the lidar, light pulses at two closely spaced wavelengths are generated by means of a laser source and sent into the atmosphere.  As the pulses propagate through the atmosphere, they get scattered or/and absorbed by molecules and aerosol particles.  The backscattered light of both wavelengths is then received with a telescope and fast detectors and converted into electrical signals.  The energy of the light pulses of the first wavelength, coinciding with an absorption line of the atmospheric constituent of interest, is attenuated stronger than the energy of the pulses of the second wavelength, which is in the wing of this absorption line.  Hence, the recorded signals differ from each other. The difference in the signals is used to derive the specific gas concentration.  Since the signals are recorded as a function of time (time-of-flight method), they directly correspond to the range at which the scattering/absorption event occurred. Hence, a dependence of the concentration of atmospheric gas constitute vs range can be retrieved from the measurements.  Over decades, DIAL systems have progressed significantly and become a common remote sensing tool for real time monitoring of the Earth’s atmosphere.

Instrument history and status

Stratospheric Ozone Lidar (SOLID) is a DIAL system. It was installed in Eureka in 1992, when the Arctic Stratospheric Ozone Observatory (AStrO) was established by the Meteorological Service of Canada (currently Environment and Climate Change Canada, ECCC) to conduct research specifically related to stratospheric ozone in the High Arctic.  SOLID was developed and put in place by Optech Inc. and the lidar group from York University (Carswell et al., 1992).  Stratospheric ozone observations with this DIAL system began in 1993 (Carswell et al., 1993).  SOLID along with the double-monochromator version of the Brewer Ozone Spectrophotometer (Bais et al., 1996), BOMEM DA8 FTIR spectrometer (Fast et al., 2011), and Japanese stratospheric aerosol lidar (Nagai et al., 1997) formed an initial measurement suit at AStrO (Donovan et al., 1997) within the framework of a collaborative program between the MSC and the Meteorological Research Institute of Japan.  The lidar has also been used to measure middle atmosphere (10–80 km) temperature profiles (Whiteway and Carswell, 1994) and upper troposphere (1–6 km) water vapour profiles (Moss et al., 2013). The system has been a proven tool to study gravity waves (Duck et al., 1998).   Since 2004, SOLID has participated in the Canadian Arctic ACE/OSIRIS Validation Campaigns (Dupuy et al., 2009).  In 2005, when PEARL was established (Fogal et el., 2013) and AStrO became its main facility under the name PEARL Ridge Laboratory, SOLID has kept providing valuable data to the research community.  The data have been extensively used in a number of satellite validation studies (Kerzenmacher et al., 2005, Sica et al., 2008). In 2009–2015 SOLID underwent a major upgrade (Tikhomirov et al., 2019).  During the upgrade the system received new laser. Additionally, SOLID control and data acquisition system was refurbished. The upgrade also opened the possibility to operate the instrument remotely, which has not been an option before.   Since 1993, SOLID has been a part of the Network for the Detection of Atmospheric Composition Change (NDACC). Currently, it is the only functional stratospheric ozone lidar located above the Arctic circle. The instrument operates in clear or partially clear sky conditions during the nighttime, which in Eureka occurs continuously from late October to early March. Measurement campaigns have an emphasis during polar sunrise (February–March).

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Instrument description

SOLID consists of a transmitter (LightMachinery IPEX-848 XeCl excimer laser, laser beam steering and shaping optics, Raman cell), a receiver (1 m Newtonian telescope, polychromator), and control and data acquisition system, interfaced with a computer.

The transmitter’s laser generates pulses at 308 nm wavelength. Sending the laser beam through a hydrogen Raman cell results in 10% of the 308 nm radiation being converted to 353 nm wavelength. Both wavelengths together then exit the Raman cell and are transmitted vertically into the atmosphere via beam steering and shaping optics.

The laser beam, generated by the transmitter and propagated through the atmosphere, is simultaneously scattered by air molecules and aerosols and absorbed by ozone. The light of 308 nm, an “on line” wavelength, is strongly absorbed by ozone. Conversely, 353 nm light, with an “off line” wavelength, is weakly absorbed by ozone. Part of the light, scattered in the backward direction, is collected by the receiver’s telescope and directed into the polychromator. The polychromator has five measurement channels. It detects elastic Rayleigh returns at both transmitter wavelengths (308 and 353 nm), inelastic returns at 332 and 385 nm from Raman scattering of both transmitter wavelengths on nitrogen molecules, and inelastic returns at 405 nm from Raman scattering of the 353 nm transmitter wavelength on water vapour molecules. The backscattered signals are collected at 150 m vertical resolution and integrated for 5 minutes before being stored.

Traditional data-processing algorithms are used to retrieve ozone and temperature vertical profiles from SOLID backscattered signals (Leblanc et al., 2016). For altitudes above 30 km, where the atmosphere is clean and free from aerosols, the elastic returns are used for the retrievals, since they are proportional to molecular density in this range. For the altitudes below 30 km, where the atmosphere can be contaminated with volcanic aerosol, Raman returns are used since they depend only upon molecular density. There is no aerosol backscattering component in the Raman returns. For the ozone retrieval the derivative of the ratio between the numbers of backscattered photons as a function of range at 308/353 nm and 332/385 nm wavelength pairs is calculated. For the temperature retrieval only the lidar signals collected at 353 and 385 nm wavelengths are used. The temperature is calculated by vertically integrating molecular density downward from the top of the profile assuming hydrostatic balance and the ideal gas law for air. During the data processing the ozone and temperature profiles calculated from elastic and Raman channels are merged and averaged in time to generate either a nightly mean profile, or a mean profile over some other predetermined time interval.

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Useful Links

NDACC measurements at Eureka
NDACC Lidar Working Group web page
Canadian Arctic ACE/OSIRIS Validation Campaign web page

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SOLID nightly mean ozone and temperature

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Data Validation Examples