ABSTRACT
We apply a spectral un-mixing procedure to uniquely identify low-temperature (320–500 K) peatland smoldering in Indonesia using nighttime data collected by the Visible Infrared Imaging Radiometer Suite (VIIRS). The analysis begins with the detection of subpixel infrared (IR) emitters in six spectral bands spanning the near-infrared (NIR), shortwave infrared (SWIR), and midwave infrared (MWIR). With sunlight eliminated, the NIR and SWIR radiances can be fully attributed to the IR emitter. The presence of a smoldering phase is tested for in pixels having detections in two SWIR and two MWIR channels. For this pixel set, the flaming-phase Planck curve is modeled using the NIR and SWIR radiances, yielding estimates of the flaming-phase radiances for subtraction from the MWIR and long-wave infrared (LWIR) data. The residual MWIR and LWIR radiances are then analyzed with dual Planck curve fitting for the background and a possible low-temperature combustion phase. The result is a set of pixels with two temperatures, high and low, each with an independent source area estimate. The dual IR emitter pixels are tested for spurious results, in which case they revert back to the standard VIIRS nightfire (VNF) single IR emitter analysis. Spurious results are indicated by extremely low or high source areas for the low-temperature-phase IR emitter. The prototyping for smoldering peatland combustion was conducted with a single VIIRS orbital segment covering Sumatra and parts of Borneo collected on September 27, 2014. IR emitters were identified in four temperature ranges: natural gas flaring (1400–2000 K), flaming (750–1400 K), glowing embers (600–750), and smoldering peat soils (320–500 K). Overlay of the low-temperature smoldering pixels with a soil map and daytime Landsat acquired the same day confirms that these were on peatland soils and were the source for a massive smoke plume. The results indicate that it is possible to analyze nighttime VIIRS data to detect and characterize combustion ranging from high-temperature natural gas flaring to three types of biomass burning: flaming, glowing embers, and low-temperature smoldering soils.
