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2020-02-07

During the previous months, the exceptionally large wildfires in Australia were frequently mentioned in the news. As Royal Belgian Institute for Space Aeronomy, we are involved in space-borne and ground-based instruments that can detect such fires from space, in an indirect way. But what does this mean exactly, and how are these observations meaningful?

What kind of eyes can detect fires from space?

BIRA-IASB is strongly involved in the processing and validation of data collected by TROPOMI, the scientific instrument onboard the Sentinel-5P satellite. This satellite orbits Earth at approximately 800km altitude and crosses the north and south pole about sixteen times per day. During each orbit the instrument captures solar light that is reflected on the Earth’s surface over a 2600 km wide area and with an individual pixel size down to 3.5x5km2 (an incredibly sharp resolution considering how far away the satellite is). In this configuration it scans almost every location on Earth on a daily basis and provides us a highly detailed view on the state of our entire atmosphere.

What have we seen?

The TROPOMI-instrument records a very broad range of wavelengths, from ultraviolet to infrared. Apart from seeing an increased presence of smoke and dust particles that are also visible to the naked eye, the satellite can detect the presence of fires with light that is invisible to our eyes. In the case of these Australian fires, it can detect them through the strongly increased presence of atmospheric gases that are produced during the massive combustion of plant life: carbon monoxide (CO), formaldehyde (HCHO), methane (CH4) and nitrogen oxides (NOx), among others. Each of these gases influences the reflected solar light captured by TROPOMI in its own way, which allows us to deduce the concentration of each gas separately. Furthermore, TROPOMI does not measure concentrations at the Earth’s surface (where we walk around), but provides us with vertically averaged quantities of atmospheric constituents. This means that the data represents the total gas amount between the surface and the top of the atmosphere, as if you would squeeze a fancy multi-layered cake to a flat pancake.

TROPOMI detected very large amounts of carbon monoxide (CO) above southeastern Australia (see Figure 1) which are then transported by winds across the Pacific Ocean towards South America. Carbon monoxide is produced during incomplete combustion (when there is insufficient oxygen present) and is a toxic gas. The values measured by the satellite represent vertically averaged quantities and should not be compared directly to the possibly toxic concentrations measured near the surface. Figure 1 shows the CO plumes generated by the fire, and how these plumes are transported over long distances before they are dissolved in the atmosphere.

Australia Wildfires
Figure 1: Evolution of carbon monoxide emissions from the Australian wildfires. Very large plumes of CO can be seen in the first half of January, moving across the Pacific Ocean and reaching South America.

Another gas produced by forest fires is formaldehyde and is also observed by TROPOMI. Similar to CO, the large quantities of formaldehyde originating from the Australian fires are clearly visible in the image of January 1, 2020 (see Figure 2). In comparison to CO, formaldehyde dissolves quickly in the atmosphere through interaction with other gases and it is rare that formaldehyde is transported over such large distances.

The formaldehyde plume was also witnessed by a ground station in Lauder, New Zealand (see Figure 4). A limited number of locations on Earth are equipped with highly specialized instruments that can measure similar data as TROPOMI: vertically averaged measurements of atmospheric gases. These ground-based measurements are used to assess the quality of satellite data. In this particular case both measurements are in good agreement, so we can be sure it reflects reality. BIRA-IASB is strongly involved in the networks that provide such ground-based data, for instance the “Network for the Detection of Atmospheric Composition Change” and the “Total Carbon Column Observing Network”.

Lauder
Figure 4: Ground observations of formaldehyde in Lauder, New Zealand, for the year 2019, from Dan Smale (NIWA- National Institute of Water and Atmospheric Research). A peak in the data can be seen around December 31, 2019, as the plume of emissions from the Australian wildfires passes over the country. This confirms the TROPOMI observations shown in Figure 2 and 3, where the formaldehyde is clearly seen passing over Southern New Zealand, where Lauder is located.

What is TROPOMI’s data used for?

The Sentinel-5P satellite is part of the space component of the Copernicus programme, which is coordinated by the European Commission. The programme uses a set of dedicated satellites (the Sentinel family – See Figure 5 for the family picture) and in situ (local) observations to help us better understand the complex system of the planet we inhabit. We strongly need the information provided by the Copernicus services, because it impacts many domains in our day-to-day life, from human health to marine monitoring, from climate change to disaster management, and many more.

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Figure 2: A plume of formaldehyde gas is seen travelling southeast over New Zealand.
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Figure 3: Average concentration of formaldehyde (HCHO) over the period of January 1 to January 5, 2020. Averaged data of plumes of HCHO appear more diffuse, partly due to the gas' short lifetime, but is still indicative of very large-scale emissions.
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Figure 5: A family of Sentinels dedicated to the European Copernicus programme for Earth observation. Credit: ESA.