As optical technology becomes more sophisticated, spectroscopic techniques are taking on a much more significant role in analytical chemistry. Infrared spectroscopy is one of many common analytical methods used for the detection and measurement of atmospheric pollution.
Much of the fundamental research on atmospheric pollution used spectroscopic techniques, as this technology helps us to comprehend various threats to both public health and the environment. For example, open-path Fourier transform infrared (FT-IR) spectrometers are being used to detect accidental releases of toxic gases at industrial sites.
FT-IR Spectroscopy for Air Pollution
FT-IR spectrometers can determine levels of several gases concurrently over a pathlength extending between 50 and 1000 meters. In the field, the FT-IR sends beams of IR radiation towards retro-mirrors that send each beam back to its receiver. Gases travel through these beams and take in some energy as they do, and so will show up in the measured absorbance.
The gap between the receivers and the reflectors is selected based on the substances being targeted for identification, anticipated concentrations, and the physical layout location. Once put in place, FT-IR systems can provide prolonged observations. FT-IR spectrometers offer many advantages over other systems, including having the ability to monitor several gases concurrently in real time and offering a spatial resolution that is highly suitable for comparisons.
FT-IR spectrometers have been used to assess gas concentrations in both the stratosphere and the troposphere. The presence of water vapor concentrations complicates any infrared spectroscopic analysis of the troposphere. Therefore, interference from water vapor is overcome by focusing on chemical species in thin bands of the infrared spectrum where water absorption is weak.
There is still considerable room for improvement in the atmospheric testing of pollution based on their spectra, as these processes are often complex, costly and requiring of highly-skilled personnel.
The primary constraint on absorption spectroscopy for air pollution testing has been the low amount of energy offered by conventional sources of radiation. For the investigation of trace levels of gas, an extended light path is generally necessary, and because radiation from light sources is divergent, a long path results in a weak signal at the receiver.
A laser-based IR spectroscopy system addresses the limitations of conventional light sources in many ways, thanks to small beam divergence, and researchers have been developing ways to take laser sensing out of the research phase. Using a properly-calibrated system, laser-based IR methods can check for all types of atmospheric pollution. Until recently, available lasers had not been advanced enough in terms of quality, stability, and durability to be useful in field spectroscopy.
Infrared absorption spectroscopy typically requires a continuum source; a source with all infrared wavelengths. The various absorption bands of a molecule act as a fingerprint to identify it. This information can be used to measure multiple pollutants.
If a laser produces one narrow line, it can be used in absorption measurements on multiple compounds, but a detection system requires a technique for changing the wavelength of the laser to match the absorption wavelengths of the target compounds. When coincidence of the laser and the absorption line is achieved, the thinness of the laser becomes an advantage, since it generates a high contrast in intensity when the absorber goes in and out of the beam.
During the 2008 Olympics, an open-path sensor system with a pulsed laser transmitter, a retro-mirror, and a receiver was used to identify trace levels of pollution in the air. The system laser’s wide tunability let the scientists target several chemical species, like ozone and carbon dioxide, with just one laser. When the study team contrasted their results with those from a conventional point sensor, both systems captured the comparable data. The team followed up on this work in 2013 and 2014 by taking a more thorough look at China’s air quality with their IR laser-based system.
Over the ensuing years, the same team also used their system to analyze the Arctic’s shifting amounts of methane, the amount of smoke released from burning wood in Ghana and the vehicle pollution found in areas of the United States.