There are two ways biomass can burn: it can flame, or it can smoulder. Flaming biomass is a more efficient way of burning, and happens at a fairly high temperature. Smouldering is less efficient, and worse for the environment since it emits typically three times as much smoke – that is, particulate matter – for every kilo of biomass. For that reason, scientists are keen to understand when biomass can be expected to smoulder rather than burn.

Until now, however, there has been no easy way to detect remotely, via satellite, whether an area of peatland is flaming or smouldering. The two types of burning occur at different temperatures, but existing methods of satellite fire-detection do not provide temperature data. For that reason, Christopher Elvidge of the US National Oceanic and Atmospheric Administration (NOAA) together with colleagues from other US and Indonesian institutions turned to Landsat 8, a satellite run by NASA and the US Geological Survey. Since Landsat 8 is a multispectral satellite, it’s possible to calculate temperature by fitting a standard "black-body" curve to its infrared wavelength data at night-time.

Landsat 8 collects night-time data in short-wave infrared (SWIR) and long-wave infrared (LWIR) bands. Elvidge and colleagues found that flaming peat generated a signal on the SWIR band, while smouldering peat generated a signal on the LWIR band.

"Validation data were collected in the field, including site visits within 24 hours of the Landsat data collections, to confirm the presence of smouldering peat fires," said Elvidge.

Elvidge and colleagues’ detection method is sensitive enough to spot fires larger than 40–90 square metres in area. The researchers believe it will have benefits for emissions modelling, as well as for those who have to manage peatland fires.

The problem of smouldering biomass is particularly prevalent in Indonesia. Here, burning peatland is common, especially in years affected by the El Niño cycle when the dry season sees widespread drought. Sometimes fires are started deliberately by farmers in order to clear, for agriculture, a region that was previously logged. Once the remnants of heavier leftover wood ignite, they can in turn ignite peat soils, which smoulder.

The smoke generated by large areas of smouldering peat can spread into populated regions, creating a health and safety hazard. In the past, airports in Singapore and Kuala Lumpur have been forced to close as a result.

Now the scientists want to find out if they can repeat their fire analysis success in a global study using NOAA’s Visible Infrared Imaging Radiometer Suite. "We will try to duplicate the results with night-time data collected globally, at coarser spatial resolution, on a nightly basis," said Elvidge.

The results are published in Environmental Research Letters (ERL) in the ERL Focus on Land Cover, Land Use Changes and Air Pollution in Asia.

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