Our Instruments

An essential component of the ACE Arctic Validation Campaign are the instruments which we are using to make our measurements. As you read through our daily reports you might be wondering a little about what we're talking about. Here are some of the stars of our show:

Fourier Transform Spectrometers

The Fourier Transform Spectrometers (FTSs) being used during the Arctic Campaign are all direct-sun, infrared absorption instruments. Sunlight enters the instrument by way of a tracker mounted on the roof and several mirrors inside the FTS lab. Once inside the instrument, the light is split into two beams by a beam-splitter. A changing path difference between the two beams is introduced by the use of a moving mirror in one beam. The constructive and destructive interference that results when the beams are recombined is measured by a detector. This signal is called an interferogram. A mathematical transformation, the Fourier transform, of the interferogram produces a spectrum, which is essentially a 'fingerprint' of the atmosphere that the sunlight has passed through on its way to the instrument. Absorption features in the spectrum are produced by specific molecules in the atmosphere, providing information about the concentration and location of these gases.

EC DA8At PEARL there are currently three FTS instruments, each of slightly different design and spectral resolution. All of them are capable of measuring more than 15 different gases, including O3, NO2, NO, HNO3, N2O, CO, CH4, HCl, HF, CO2, COF2 and ClONO2.

Environment Canada DA8

The EC DA8 infrared FTS was installed at the Arctic STRatospheric Ozone (ASTRO) Observatory, now PEARL, in 1993 and has provided atmospheric solar absorption spectra each spring over the past fifteen years.  The DA8 FTS is equipped with two separate liquid nitrogen-cooled detectors, an Indium Antimonide (InSb) detector and a Mercury-Cadmium-Telluride (MCT) detector, covering different spectral regions.  The spectral range from 700 cm-1 to 5000 cm-1 is recorded at an apodized resolution of 0.004 cm-1 using a sequence of eight narrow-band interference filters in turn.  

EC DA8 (left) and Bruker IFS 125 HR (right). Photos by R. Lindemaier

Bruker FTS


The Bruker IFS125HR is the most recent addition to PEARL, installed in July 2006, and is part of the CANDAC suite of instruments. It has very high resolution (0.0035cm-1) and, like the DA8, has two detectors - an InSb for the 1800-8500 cm-1 region and an MCT for the 500-5000cm-1 region - which are cooled with liquid nitrogen. This is the Bruker's second Arctic spring campaign. The Bruker operates year-round (when the Sun is up), giving us a long-term dataset to help us to better understand the composition and changes occurring in the Arctic atmosphere.

University of Waterloo PARIS-IR

PARISThe Portable Atmospheric Research Interferometric Spectrometer for the Infrared (PARIS-IR) is a terrestrial version of the ACE-FTS, the high-resolution FTS on board SCISAT-1 (http://www.ace.uwaterloo.ca).   It is small and portable compared to the other FTS instruments, and is only at Eureka during the ACE validation campaign each spring.

Like the other FTSs, PARIS-IR uses infrared radiation from the Sun to measure absorption spectra of the gases in the Earth's atmosphere. While PARIS-IR has a lower resolution (of 0.02 cm-1) than the other FTSs, each spectrum measured by PARIS-IR covers the full spectral range (750 - 4400 cm-1). This allows changes in the concentrations of all of the trace gases to be measured simultaneously throughout the day.   

PARIS-IR in the PEARL FTS lab. Photo by A. Harrett

UV-Vis spectrometers

As their name implies, UV-Vis spectrometers are instruments which directly collect spectra in the UV and visible regions of the electromagnetic spectrum. UV-Vis spectrometers use a grating to split light into spectra (much as a prism divides white light into a 'rainbow'), which are recorded directly by the detector and electronics. For this campaign there is a suite of UV-Vis instruments, each recording over a similar wavelength region, but with differing resolution.


UT_GBSPEARL-GBS. Photo by A. Fraser

During the campaign, there will be two University of Toronto ground-based instruments in operation at PEARLThese are zenith-sky (looking straight up) UV-Visible spectrometers which measure stratospheric trace gases from the ground using a well-established method of retrieval called
Differential Optical Absorption Spectroscopy (DOAS) to determine vertical columns of ozone and NO2, and slant column densities of BrO and OClO.  One of these instruments, the UT-GBS, records zenith scattered sunlight spectra. It was deployed at Eureka eight previous times for polar sunrise between 1999 and 2007.  The other instrument, the PEARL-GBS, was installed permanently in Eureka in August 2006 for year-round operation, and will record direct-sun spectra, using a new sun-tracker developed at the University of Toronto.  With two instruments working together, one in zenith mode and one in direct-sun mode, information on the vertical distribution and diurnal variation of BrO can be retrieved using an optimal estimation method.


SAOZ (Systeme d'Analyse par Observation Zenithale) is an automated, zenith-sky-viewing UV-Visible spectrometer on loan from Service d'Aeronomie, CNRS  (France), for the ACE Validation Campaign. SAOZ  instruments are widely used around the world, and are specifically designed for making stratospheric column measurements in polar regions. SAOZ measurements are made in the 300-600 nm wavelength range with a resolution of about 0.8 nm, primarily at solar zenith angles greater than 81 degrees. The DOAS method is used for retrieving stratospheric gas column amounts of O3 and NO2 in a similar way to the UT-GBS systems.

SAOZ. Photo by A. Fraser


SPS (Sun Photo Spectrometer) is a light-weight, compact, photodiode array spectrophotometer. It has a holographic diffraction grating which focuses the spectra, in the range of 290-775 nm in two orders, onto a detector. It has a spectral resolution of approximately 1.2 nm in the second order and 2.5 nm in the first order.  A double filter-wheel assembly inside the instrument allows a selection of filter combinations for different purposes, including order selection and polarizing filters. The SPS has flown on  the Space Shuttle and several ER-2 aircraft campaigns, and is a temporary visitor at Eureka for the spring campaigns. Here its primary focus is measurements of atmospheric NO2 and O3 concentrations. It can also measure sky brightness and aerosols.

MAESTRO (left) and SPS (right) on the roof of PEARL. Photo by T. Kerzenmacher

MAESTRO-G (Measurement of Aerosol Extinction in the Stratosphere and Troposphere Retrieved by Occultation) is the ground-based equivalent of ACE-MAESTRO, which is currently flying on the ACE satellite. It is composed of two independent diode-array spectrometers measuring in the UV-Visible (285-565 nm) and the visible-near-infrared (515 -1015 nm) spectral regions, each with a resolution of approximately 2 nm. Each spectrometer consists of a lens, slit, concave holographic grating and a photodiode array detector. It can measure various  atmospheric constituents, including O3, NO2, BrO, NO3, and aerosols.

MAESTRO-G is mounted on one side of a solar tracker (the left, in the picture), and the SPS is mounted on the other. Measurements are made in zenith sky mode (pointing straight up) most of the time, but for a short period during each degree of zenith angle the instruments make measurements while pointed directly at the sun. This is important for the process used to retrieve gas amounts from the recorded spectra.

BrewerThe Brewer Spectrophotometer

The Brewer spectrophotometer is an internationally recognized WMO standard instrument for the measurement of ozone. In simple terms, it uses the fact that ozone has a large difference in its absorption strength at different wavelengths to determine the amount of ozone in the atmosphere between the instrument and the sun. Unlike the older, manual Dobson instrument, which uses prisms to break the light into spectra, the Brewer uses a holographic grating and is completely automated.

There are three Brewer spectrophotometers in operation at Eureka - one at PEARL and two at the weather station. The three instruments are all different models with slightly different gratings and operating in different orders (which gives them different wavelength regions to look in). As well as ozone data, NO2 can be determined. The data from these instruments goes into the world-wide network to monitor ozone change.

Brewer Spectrophotomer, Photo T. Kerzenmacher
Lidar over PEARL

Ozone Profiling Instruments


A LIDAR (LIght Detection and Ranging) is an instrument that uses laser light to detect particles and gases in the atmosphere. Light pulses are sent into the atmosphere where they are scattered back by aerosol particles and air molecules and absorbed by atmospheric constituents. The returned light is captured in a telescope, and is used to study the location and nature of particles and molecules in the atmosphere. Environment Canada has had a 
DIfferential Absorption LIDAR (DIAL)  at Eureka since 1993. DIAL is a special type of lidar that uses more than one wavelength to detect differences in absorption. For example, by studying the difference in absorption at wavelengths where ozone absorbs strongly and weakly, a very accurate profile of the ozone distribution can be obtained. Because the returned light is very weak (it is being scattered back from very high altitudes by molecules and particles), the DIAL operates only at night when the background light is less intense. In addition to measuring ozone, DIAL measurements can also produce temperature profiles.

DIAL LIDAR over PEARL, Courtesy T. Nagai
Balloon filling for an ozonesonde launch. Photo by T. Kerzenmacher


Unlike the other campaign instruments, which measure remotely (either from the ground or a satellite), an ozonesonde actually travels through the atmosphere measuring the local concentration of ozone. To do this, the instrument catches a ride on a hydrogen-filled balloon which rises up through the atmosphere (just like those helium filled ones you had as a kid - but a lot bigger!) The instrument itself is a small chemical cell. Air is sucked into one chamber of the cell. Ozone in the air sample reacts with the chemical, causing a current to flow. The concentration of ozone is determined from the measured current.  The ozone amount, along with temperature, pressure and humidity information that are also measured along the way, is transmitted back to earth as the balloon goes up, using a radio signal.  The ozonesonde gives us a very high-precision profile of the ozone through the part of the atmosphere that it measures. As you can imagine, this is very useful for helping verify the data that we get with the other instruments. Unfortunately, because ozonesondes are expensive (those balloons get to as big as a hockey arena by the time they pop at around 30km!),  measurements are not made as frequently as with the other instruments. Thus, an important part of this campaign is the daily launch of ozonesondes during the campaign's intensive phase.

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