3.1 Optical absorption spectrometers

Optical spectrometers use reflected short wave solar radiation to monitor the atmospheric composition. One instrument in this category is the Global Ozone Monitoring Experiment-2 (GOME-2) instrument. It is an optical absorption spectroscope onboard the MetOp satellite series that measures reflected and scattered sunlight from the atmosphere and the Earth's surface in visible and ultra-violet bands. Its main purpose is monitoring atmospheric ozone concentration, but it is also able to detect aerosols and atmospheric trace gases like SO2. The horizontal resolution of the instrument is 80 by 40 km, its vertical profiles are representative of the bottom 50 km of the atmosphere, and the instrument takes three days to cover the entire globe.

In the example below, the GOME-2 instrument detected a SO2 plume emitted from Raikoke volcano in June 2019. The AC SAF SO2 product (Figure 21) shows the very small expulsion of SO2 on 21 June 2019.

Figure 21: MetOp-A/B GOME-2 SO2 concentration, 21 June 2019. © AC SAF


The Ozone Monitoring Instrument (OMI) is part of the payload on NASA's Aura satellite, which has a polar sun-synchronous orbit. The instrument is a spectrometer that measures ultraviolet and visible radiation reflected by the atmosphere and the Earth's surface.

OMI distinguishes between aerosol types, such as smoke, dust, and sulfates (Figure 22), and measures cloud pressure and coverage.

Figure 22: OMI/Aura average SO2 column, Eruption of Mount Nyamuragira, 2006. © S. Carn, UC


Another instrument of a similar type is the TROPOMI instrument (TROPOspheric Monitoring Instrument) on the Sentinel-5P satellite. Compared to OMI, TROPOMI has extended scanning capabilities into the near-infrared part of the spectrum. The spectrometer monitors atmospheric trace gases such as ozone, methane and SO2.

As displayed in the image below (Figure 23), TROPOMI scanned the atmosphere next to Mount Sinabung on 19 February 2018 at 07:00 UTC, around five hours after its initial eruption. The volcanic plume drifted in multiple directions at lower levels, possibly due to inconsistent wind directions.

Figure 23: Sentinel-5P TROPOMI SO2 column density, Mount Sinabung eruption on 19 February 2018. © DLR/BIRA/ESA