Since the 1950s, it has been widely known that coal-burning power plants emit dangerous levels of particulate and gaseous pollutants into the atmosphere, including ash, soot, SO2, NO2, and NO. Over the ensuing 70 years, there has been much progress on reducing emissions to ensure cleaner air, particularly in the United States and Europe. For example, the United States passed the Clean Air Act (CAA) in 1963, giving the Environmental Protection Agency (EPA) the authority to take action to improve air quality and promote public health.
Yet even though the CAA has been modified several times (in 1970, 1977, 1982, and 1990) to extend the EPA’s power, according to a 2013 study conducted at the Massachusetts Institute of Technology (MIT), air pollution is still responsible for roughly 200,000 deaths each year in the United States alone .
As the developing world has continued to industrialize in its transition into modernity, the demand for energy is skyrocketing, particularly in places in places like China and India. While cleaner technologies are meeting some of the worldwide markets’ demands, roughly 40% of the electricity generated around the world is form coal-burning power plants. In countries such as China and India, this is as high as 70 to 75% . As a result, environmental monitoring systems are as relevant today as they have ever been.
Continuous Emission Monitoring Systems (CEMS)
From the early days of the CAA, engineers have been developing ways to purify smoke stack exhaust with technologies such as electrostatic precipitators [3, 4]. But, real-time monitoring of gaseous pollutants has historically been far more of a challenge. Fortunately, with the advances in optical sensing technologies over the past 20 years, engineers are now capable of developing a continuous emission-monitoring system (CEMS), using absorption spectroscopy.
Like any other covalently-bound molecule, gasses have absorption bands in both the infrared (vibrionic) and UV/vis (electronic) regions of the spectrum. There are pros and cons to operating in both of these regimes.
Standoff infrared absorption techniques both in the near-infrared (NIR) and mid-infrared (MIR), require the use of tuneable laser sources such as distributed Bragg reflectors (DBRs) and quantum cascade lasers (QCLs) used in conjugation with InGaAs and HgCdTe photodetectors.
Through processes like tuneable diode laser absorption spectroscopy (TLDAS) and differential absorption lidar (DIAL), NIR and MIR absorption spectroscopy can provide highly accurate quantization of molecular species. The major downside of this approach, however, is that in most cases, each species of interest will require a different excitation laser, further compounding the already expensive component cost. By contrast, using a methodology called differential optical absorption spectroscopy (DOAS), UV/vis absorption can be used to measure a wide range of molecular species with a single broadband light source. Not only does DOAS decrease the cost of multi-species detection, but it is also often far less expensive for single-species detection as well. This is because of its use of inexpensive CCD spectrometers and a constant current D2 emission lamp. In many cases, DOAS can even be performed using sunlight, although this is typically not advisable for CEMS due to the inherent fluctuations of sunlight levels.
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Differential Optical Absorption Spectroscopy (DOAS)
The earliest reported use of DOAS for atmospheric measurements was in 1973, for the monitoring of NO2 concentrations by monitoring absorption bands between 430 and 450 nm . Yet, as stated above, it wasn’t until the advent of modern CCD detector-based spectrometers that the technique gained commercial viability.
Today, DOAS is performed by recording two absorption spectra, one being a reference scan through a column of atmosphere devoid of analyte (ideally) and one through an identical pathlength containing the analyte. A detailed mathematical treatment of DOAS is beyond the scope of this application note. Still, it is essential to point out that by taking the difference of these two spectra and applying the Beer-Lambert Law, the resultant differential spectrum is now dependent solely on the absorption cross-section of the species in the analyte and the difference between the column densities of the species and reference spectrum .
Furthermore, since the cross-section can be represented as the sum of a slowly varying and rapidly verifying function, the differential spectrum allows for the separation of these two components. Since both Rayleigh and Mie scattering are slowly varying functions, the differential spectrum in conjunction with mathematical tools, such as a Fourier Transform filter, eliminates both attenuations and scattering from the data .
Figure 1 shows the absorption bands of a wide range of common atmospheric pollutants, including the SO2, NO, and NO2 mentioned above. From this chart, it is clear that the vast majority of molecular species, with the notable exceptions of CO and CO2, can be detected from 200 to 460 nm. As a result, it is possible to use a single compact fixed grating spectrometer in a DOAS-based CEMS.
It is important to note that when choosing a spectrometer to measure in the UV region of the spectrum, it is critical to use an ultra-low stray light configuration to minimize stray light in the system. Cross Czerny-Turner spectrograph designs, while compact tend to have inherently higher stray light levels than transitional Czerny-Turner spectrographs. Examples of ultra-low stray light spectrometers include the AvaSpec-ULS2048XL-EVO and AvaSpec-ULS2048x64-EVO. Both of these instruments feature an ultra-low stray light design and back-thinned CCD detectors for superior UV sensitivity and signal to noise ratio.
Figure 2 shows a typical CEMS setup for DOAS where a concave mirror is used to collimate a broadband light source (typically a D2 lamp) across the monitoring path, where a second concave mirror is then used to couple the transmitted light into a fibre-optic cable, which then directs it into the spectrometer.
Additional DOAS Applications
In addition to CEMs, sunlight-based DOAS is also widely used for general environmental monitoring. For example, a team of Greek scientists presented data at the 2015 conference Advances in Atmospheric Science and Applications, where they used a mini multi-axis (MAX) DOAS system to measure tropospheric column densities of NO2, SO2 and HCHO in urban, rural, and suburban locations around Thessaloniki, Greece . The mini MAX DOAS system utilized an AvaSpec-ULS2048x64TEC-EVO spectrometer, configured from 300 to 450 nm. Of the three systems deployed, one used a 25 micron entrance slit with a ~0.25 nm resolution, and the other two utilized a 50 micron entrance slit with ~0.38 nm resolution.
A second group at the Max Planck Institute in Germany also used a similar mini MAX-DOAS system based upon an Avantes spectrometer to collect similar measurements in Romania and Germany .
As societies around the world continue to grow, so too will the need for clean technologies, including the monitoring of environmental pollutants both in the form of CEMS and field-based DOAS systems. In both cases, these applications require sensitive, stable, high-resolution, low-stray- light spectrometers; to ensure accurate and reliable DOAS measurements with low detection limits and high signal-to-noise ratios. The AvaSpec-ULS optical benches offer the highest degree of stability and smallest stray light of any miniature spectrometer on the market, and the option of either CMOS or back-thinned CCD detectors provides the flexibility to manage cost and sensitivity trade-offs.
Among the most highly recommended back-thinned instruments, the Avantes ULS2048 series spectrometers offer incredible performance. The AvaSpec-ULS2048XL-EVO features extra-large 14×500 micron pixels that deliver exceptional efficiency in the UV range (200-400 nm) and the NIR range (950-1160 nm). The internal shutter allows in-line dark capture integration times as low as 2 microseconds. The uncooled AvaSpec-ULS2048x64-EVO is another instrument with a proven track record for DOAS applications due to its high UV response, 0.9 mm detector height and fast integration times.
All of the AvaSpec-ULS series spectrometers discussed above are also available as OEM modules. They can be integrated into turn-key industrial or field-sensing devices, in addition to functioning as add-ons to existing laboratory equipment setups. These instruments can communicate via USB, Ethernet and the native digital and analogue input/output capabilities of the Avantes AS-7010 electronics board, which provides for a superior interface with other devices.
Additionally, the Avantes AvaSpec DLL software development application, with sample programs in Delphi, Visual Basic, C#, C++, LabView, MatLab, and other programming environments, enables users to develop code for their own applications.
For more information about the full range of laboratory and OEM spectrometer options available from Avantes, please contact an Avantes sales engineer today!
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