Measuring Arctic Ozone with a Differential Absorption Lidar

Author: Ghazal Farhani, PhD Student at The University of Western Ontario

Ozone is a minor constituent of the atmosphere, but it plays an important role by absorbing harmful ultraviolet (UV) radiation emitted by the Sun. The bulk of this protective ozone resides in the stratosphere at an altitude range between 15 and 50 km. A small amount resides near the surface, but it is a pollutant. Significant chemical depletion of the total ozone during late winter and spring has been observed in both Antarctica and, more recently, in the Arctic. This depletion is commonly known as an ozone hole. The greatest changes in stratospheric ozone occur during polar sunrise (the transition from continuous night to continuous day). In order to detect and quantify the ozone hole, continuous measurements during this period are required.

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The ultraviolet beam of the Ozone DIAL at the Ridge Lab in Eureka, Nunavut. Image Credit: Ghazal Farhani

Many different instruments have been used to make ozone measurements. A LiDAR (LIght Detection And Ranging) is one of these instruments. A Lidar is a ground-based active remote sensing instrument that is similar to radar but operates in the ultraviolet, optical, or infrared range. During lidar measurements, laser pulses are sent into the atmosphere, the backscattered photons are collected by a telescope and then detected by photomultiplier tubes. Measurements of the ozone vertical distribution are carried out using a DIfferential Absorption Lidar (DIAL). The high spatial resolution of the receiving signal gives us the opportunity to collect more data to retrieve the ozone profile, and is the big advantage of lidars over satellites.

Observations of stratospheric ozone have been carried out using the DIAL for more than 20 years in Eureka, Nunavut at the Polar Environment Atmospheric Research Laboratory (PEARL). The DIAL system measures ozone by obtaining back-scattered light from two laser wavelengths. One wavelength is in a spectral region with a high absorption for ozone (308 nm), and the other with a low absorption (353 nm). The transmitted laser beam undergoes Rayleigh scattering in the atmosphere and the back-scattered photons are detected by the lidar receiver. Ozone density profiles can be derived from the back-scattered photon measurements.

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PhD Student Ghazal Farhani operating the Ozone DIAL. Image Credit: Dr. Emily McCullough

During 2009 and 2010, the laser component of the lidar was replaced and several other components of the lidar were upgraded. As of fall 2014, many of these components have been reinstalled. The refurbished DIAL’s first measurements were made by Dr. Alexey Thikhorimov (Dalhousie University) in February/March 2015 during the ACE/OSIRIS Arctic Validation Campaign.

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PhD student Ghazal Farhani checking the Ozone DIAL’s telescope and making sure it’s ready for work.

In the February/March 2016 ACE/OSIRIS Arctic Validation Campaign, Dr. Emily McCullough and I joined Dr. Alexey Thikhomirov to take more measurements. As mentioned above, the DIAL system has been gone through hardware and software replacements. As a result, the data collected from the upgraded system should be validated as well. My PhD thesis will involve learning how to operate the system, and validating the new data. We hope this upgrade will give us better insight into how Ozone concentrations are evolving in the Arctic.

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About createarcticscience

The CREATE program for Arctic Atmospheric Science supports researchers and students across Canada. This blog provides a venue for sharing our experiences.
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