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The MODIS instruments capture images of nearly all of Earth’s surface every day. With their mid-infrared and thermal bands, these sensors can detect hot spots caused by active fires. In 2015, the two sensors detected more than 120,000 hot spots in Indonesia—more than they did during any other year over the past decade.
“Active fire counts from MODIS have provided an excellent way of systematically tracking fire locations in Indonesia,” said Robert Field, a climatologist at NASA’s Goddard Institute for Space Studies. “Ground and aircraft-based reporting, like we have in North America, is still limited in Indonesia.”
But detecting fire from space with imagers is not always easy. Smoke or clouds can impair an instrument’s ability to sense fires on the surface. And low-temperature or smoldering, underground fires, which are common in Indonesia, sometimes cannot be detected at all.
“Getting the right sensors on the ground is also critical in this part of the world,” said Ellsworth Welton, the principal investigator for NASA’s Micro Pulse Lidar Netork, a global network of ground-based cloud and aerosol sensors. “Clouds over Southeast Asia are quite dense, so even with multiple satellite platforms, the weather can still pose a real obstacle.”
Other types of sensors can help flesh out the story of fire, smoke, and their impacts. For instance, the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) on the CALIPSO satellite and the Multi-angle Imaging SpectroRadiometer (MISR) can measure the height of smoke plumes. CALIOP does this by reflecting laser pulses off the land surface and determining how long it takes for reflected and back-scattered light to return to the sensor. MISR does so by imaging the smoke with nine cameras pointed at different angles.
Knowing the height of a smoke plume is critical for atmospheric scientists and meteorologists as they develop models to predict where smoke will blow, explained Michael Tosca, an atmospheric scientist at NASA’s Jet Propulsion Laboratory. In many parts of the world, fires can loft smoke 5 or more kilometers (3 miles) into the air, and occasionally as high as 10 kilometers. But Tosca has found that smoke from Indonesian fires often remains relatively low in the atmosphere—between the surface and 3 kilometers.
When Tosca reviewed MISR and CALIPSO data from September and October 2015, he found that the recent plumes in Indonesia fit this pattern. CALIPSO showed that a large smoke plume over central Kalimantan on October 4, 2015, reached 2 kilometers. During some overpasses, MISR observed thick smoke layers even closer to the surface. (Note, however, that MISR makes measurements over Indonesia in the morning, before sunlight has had much time to heat the ground and drive convection and updrafts.)
Although smoke from Indonesian fires does tend to remain fairly low, some smoke does get caught up with faster-moving, upper-level winds capable of transporting it across Southeast Asia. “These winds tend to push the smoke in a northwesterly direction toward Malaysia, Singapore, southern Thailand, southern Cambodia, and Vietnam, where the impacts on air quality can be severe,” Tosca said. In 2015, the influx of smoke elevated particulate levels to dangerous levels in Malaysia and Singapore, prompting authorities to close schools on multiple occasions.
Satellites also can reveal key characteristics of smoke. The same multi-angle views that allow MISR to estimate plume height also allow it to observe the size, shape and, in some cases, the brightness of the tiny particles in that smoke. Most wildfire smoke tends to be dominated by small, spherical, and dark-colored particles. Atmospheric scientists generally call this “black carbon” because of how it readily absorbs light. However, the smoke cloud over Indonesia in 2015 had particles that were relatively large. In a tropical environment like Indonesia, smoke particles encounter high humidity even in the dry season, explained Ralph Kahn, a senior research scientist at NASA’s Goddard Space Flight Center. “When they do, the particles absorb the water and expand.”
Other sensors can study gases that are invisible to human eyes. The Measurement of Pollution in the Troposphere (MOPITT) sensor on Terra can detect carbon monoxide, an odorless, colorless, and poisonous gas that is abundant in peat fires. When inhaled, carbon monoxide reduces the amount of oxygen that blood can deliver to the body’s organs, including the heart and brain.
“The 2015 Indonesian fires produced some of the highest concentrations of carbon monoxide that we have ever seen with MOPITT,” said Helen Worden, a scientist at the National Center for Atmospheric Research. Average carbon monoxide concentrations over Indonesia are usually about 100 parts per billion. In some parts of Borneo in 2015, MOPITT measured carbon monoxide concentrations at the surface up to nearly 1,300 parts per billion.
All of that carbon monoxide did not stay at Earth’s surface. The Atmospheric Infrared Sounder (AIRS) observed significant concentrations of carbon monoxide about 5 kilometers above the surface. Another sensor, the Microwave Limb Sounder (MLS), detected elevated levels of carbon monoxide at upwards of 9 kilometers in late October. The gas was likely lofted up by convection from incoming monsoon storms and rain.