3.2 Infrared spectrometers

Infrared spectrometers are passive instruments that measure the thermal radiation emitted by the atmosphere to retrieve information on its composition.

An example of this type of instrument is the Infrared Atmospheric Sounding Interferometer (IASI). It is a Fourier Transform Spectrometer based on a Michelson Interferometer coupled with an integrated imaging system that observes and measures infrared radiation in the spectral range of 3.62 to 15.5 µm. This enables the instrument to derive temperature and water vapor profiles for the troposphere and the lower stratosphere, as well as measure quantities of ozone, carbon monoxide, methane and other compounds such as SO2. Besides quantitative information, IASI also provides an estimate of the height of the SO2 plume.

SO2 observations with the IASI instrument are defined by the variation of brightness temperature measurements in the IR window region around 8.5 microns (Figure 24).

Figure 24: The Metop-C IASI image from 28 November 2022 shows the height of the SO2 plume at about 6 km in the immediate vicinity of the volcano (left). On 1 December, over the USA, the plume height was estimated to be at about 8-15 km (right). © ULB/ LATMOS


The Atmospheric InfraRed Sounder (AIRS) is an instrument on NASA's Aqua satellite and is complemented by the Advanced Microwave Sounding Unit (AMSU-A). An advanced, high-resolution spectrometer inside the instrument divides the infrared energy into discrete wavelengths.

AIRS measures incoming infrared brightness from the Earth's surface and the atmosphere. Atmospheric profiles of temperature, moisture and trace gases are derived from this data for climate and weather prediction applications. The sounder measures concentrations of trace greenhouse gases such as ozone, carbon monoxide, carbon dioxide, methane and SO2. It can follow giant plumes of these gases moving across the planet, allowing scientists to better monitor pollution transport patterns (Figure 25).

Figure 25: AIRS SO2 product from the Aqua satellite, Jebel at Tair eruption on 2 October 2007. © F. Prata.