The CANDAC Rayleigh-Mie-Raman Lidar (CRL) is located at the ØPAL Laboratory of the Polar Environment Atmospheric Research Laboratory (PEARL). CRL was developed at Dalhousie University (PI: Tom Duck) to address the need for high-resolution profile measurements through the troposphere to the lower stratosphere. CRL was installed at Eureka in 2007 (Nott et al., 2012), regular measurements began in January 2009. An upgrade to allow depolarization measurements was made in 2010, with calibrated depolarization measurements available as of 2013. The lidar has been used to study aerosols (Perro 2010, Baibakov et. al. 2015), water vapour (Doyle et al. 2011), stratospheric optical depth particularly in the context of volcanic eruptions (O’Neill et. al. 2012), and clouds (McCullough et al. 2019). These data are critical for understanding weather and climate processes in the High Arctic region known to be particularly sensitive to climate change.
The CRL consists of a transmitter (two lasers and associated optics), a receiver (1-m Dal-Kirkham telescope and polychromator including photomultiplier tube (PMT) detectors), an electronics system, and a computer. All the optical components of both the transmitter and the receiver are mounted on an optical table, equipped with vibration isolation system. The instrument is housed in a customized 6.1 m (20 ft.), high-cube shipping container built by Container and Trailer Services (Dartmouth, Nova Scotia, Canada).
Directly above the telescope is a 1-m-diameter roof window for access to the sky and bad weather protection. It has a laser-quality insert in the center for transmission of the outgoing laser beams. The telescope is also protected from direct sunlight by a 1.5-m-high box with a motorized hatch, located on top of the container.
The CRL transmitter has been constructed around two transmitted wavelengths, 532 nm and 355 nm. Each wavelength is generated by its own Nd:YAG Surelite III-10 laser from Continuum (Santa Clara, California), which produces 1064 nm light with a repetition rate of 10 Hz. One laser is frequency doubled to 532 nm (visible green, 380 mJ/pulse, 5 ns pulse duration), and the other frequency tripled to 355 nm (UV, 240 mJ/pulse, 5 ns pulse duration). There is additionally a fiber laser available for injection seeding of the 532 nm laser for wavelength stability. The 532 and 355 nm beams are co-aligned using steering optics. The beams are then expanded to 10 cm diameter, 0.1 mrad divergence, and directed vertically to the sky through the roof window.
Both laser beams interact with the atmosphere above the lidar and are scattered in many directions by oxygen, nitrogen, and water vapour molecules, and particulate. The laser light which scatters directly back down towards the ground are captured by the telescope and focused into the polychromator. There, beam splitters separate the returned light by wavelength. The delay between laser emission and photon detection is recorded, allowing the altitude of the scatterers to be calculated. From every laser shot, CRL collects photons for a delay time which corresponds to distances of 0 to 120 km altitude. Most of the lidar beam is scattered away (either back into the lidar or in other directions) well before 120 km, so the highest altitude returns are background measurements. Signals suitable for calculation of atmospheric quantities are generally returned from altitudes up to 12 km, and with longer measurement integration times can extend up to about 30 km for some data products.
CRL’s polychromator includes eight measurement channels. It was built by Spectral Applied Research (Richmond Hill, ON). Elastic (Rayleigh) measurements are taken in both the visible (532.08 nm) and ultraviolet (354.72 nm). Nitrogen is measured at two wavelengths: 607.47 nm from Raman scattering of the visible beam, and 386.67 from Raman scattering of the UV beam. The weak Raman profiles of water vapor are measured in the ultraviolet at 407.52 nm to maximize the scattering cross section and thus signal levels (Avila et al. 2004). Two channels close to 532 nm (531.16 nm and 528.63 nm) are used to measure the anti-Stokes branch of the overlapping rotational Raman spectra of nitrogen and oxygen, which are used for tropospheric temperature retrievals. Linear depolarization measurements are made by directing a small portion of the returned 532 nm light into a Licel Polarotor rotating Glan–Thomson prism. This allows both parallel-polarized measurements (from clear air and spherical particles such as cloud water droplets) and perpendicular-polarized measurements (from ice particles, aerosols, dust, etc.) to be made using a single detector on alternate laser shots. Depolarization measurements are used to distinguish liquid water from ice particles within Arctic clouds.
All eight measurement channels use Licel GmbH transient recorders to produce signals in photon counting mode, which is ideal for counting individual photons when signal levels are very low (e.g. from weak scattering as from water vapour, or from high altitude returns). The Elastic channels (532 nm, 532 nm depolarization, 355 nm) additionally have a simultaneous analogue detection mode to allow measurements at much higher signal rates (e.g. from strong scatterers like clouds, and from low altitude returns). Signals from both modes are combined (“glued”) to make a single measurement profile for each Elastic channel. This gives CRL a large dynamic range.
Measurements at CRL are collected at 60 second x 7.5 metre resolution in all channels via onboard averaging in the Licel units. The measurement data are compressed and sent south via satellite internet link for processing and archiving. For most final data products (e.g. backscatter cross section, water vapour mixing ratio) the measurements are co-added to lower resolution to improve signal-to-noise.
The instrument operates in all fair weather, 24/7, year-round and can tolerate light precipitation and windy conditions. The observation emphasis is placed on making measurements during the polar sunrise season (February – March) to coincide with major multi-instrument measurement campaigns at Eureka. The CRL is a 100% remotely controlled instrument. It is operated semi-autonomously via satellite internet link by an operator in Southern Canada.