Each year in January and February, satellites begin to detect waves of smoke and fire in Southeast Asia, particularly in highland forests in Cambodia, Laos, Myanmar, and Thailand. Fire activity continues to increase through March and April, reaches a peak during the height of the dry season, and then fades in May with the start of the rainy season.
Individual fires are usually small and short-lived. But they are often so numerous that smoke, along with air pollution from rural and urban areas, mixes to produce thick layers of haze that blanket the landscape. Such hazes contain mixtures of small airborne particles called aerosols and gaseous pollutants such as nitrogen dioxide, sulfur dioxide, carbon monoxide, and ozone that degrade air quality and have harmful health effects.
In spring 2024, an international field campaign—ASIA-AQ (Airborne and Satellite Investigation of Asian Air Quality)—headed to Thailand to take a close look at the haze. During the last two weeks of March 2024, NASA’s DC-8 and Gulfstream III aircraft flew several flights over Chiang Mai, Bangkok, and rural areas surrounding the cities to sample air quality with several sensors. At the same time, satellites observed the haze from above.
The natural-color satellite image above (left) shows a hazy day in Thailand during the region’s 2024 fire season. The image was captured by VIIRS (Visible Infrared Imaging Radiometer Suite) on the Suomi NPP (National Polar-orbiting Partnership) satellite on March 16. The map (right) shows the locations of fires detected that day by VIIRS, which identifies active fires by measuring infrared and visible light. The photographs below—showing a crop fire, haze over Bangkok, and scientists at work—were taken from the DC-8 during an ASIA-AQ flight on March 18, 2024.
“We’re collaborating closely with national and local government entities in Thailand,” said James Crawford, ASIA-AQ’s principal investigator. “The goal is to take as many simultaneous measurements of pollutants as possible from the ground, air, and space,” he said. The team will then use the data to refine models of how the region’s haze forms, evolves chemically over time, and moves throughout the region.
Fires, most of them lit intentionally, are a major contributor to the haze. One of the main reasons is agriculture, either to clear forests for planting or to burn post-harvest crop stubble. “Maize, which is primarily used for animal feeds, is the crop burned the most,” said Danny Marks, a Dublin City University geographer. It’s also common for people to start forest fires to aid in the collection of mushrooms and other forest products and to make it easier to hunt game. Subsistence farmers sometimes practice swidden agriculture, a technique that involves periodically cutting down trees and shrubs, letting the wood dry out for a few months, and then burning it to clear space for crops.
Skies have long been smoky in March. But Mary Mostafanezhad, a geographer at the University of Hawaii, says there has been heightened interest in the issue in recent years. “The haze has exposed complex rifts between rural populations in the highlands and urban populations, as well as between various economic interest groups and ethnic communities, about the activities that are most responsible for the haze, how dangerous it is, and what can be done about it,” said Mostafanezhad, who conducts research in Chiang Mai and has studied how the region’s seasonal fires are perceived.
Haze is known to have a considerable impact on people. Particles in smoke reach deep into the lungs, entering the bloodstream and contributing to respiratory and cardiovascular diseases. Exposure to outdoor air pollution may contribute to as many as 33,000 premature deaths and millions of illnesses in Thailand per year, according to researchers. Meanwhile, haze regularly roils transportation networks, shutters schools, and triggers burning bans.
But researchers say that the proportion of the haze that comes from urban sources, such as emissions from the transportation or industrial sectors, compared to fires or other rural sources of pollution, is unclear. And understanding the role of geographical features and meteorological factors, as well as the interactions between various pollutants, in controlling the distribution of haze remains a challenge. Chiang Mai, for instance, lies within a basin prone to temperature inversions that can prevent winds from dispersing air pollution.
NASA teams are also working with partners in the region and the USAID-NASA SERVIR program to develop tools designed to inform people about haze and smoke during the fire season. SERVIR Southeast Asia has worked with the Royal Thai Government’s Pollution Control Department (PCD) to develop the Mekong Air Quality Explorer tool. “This decision support tool combines satellite data from NASA with ground station data and machine-learning analytics to enable monitoring and forecasting of air quality in Thailand,” said Aekkapol Aekakkararungroj, a geospatial manager with SERVIR Southeast Asia.
Another tool called Smoke Watch, developed with assistance from the SERVIR program, can port timely VIIRS observations of fires into a real-time warning system that firefighters have used to pinpoint fires and fight them as quickly as possible, explained the application’s developer Nion Sirimongkollertkul, an environmental engineer at Rajamangala University of Technology Lanna Chiangrai.
There are some indications from satellite observations that there has been a slight downward trend in fire activity in Thailand in recent decades. “But it’s a modest decrease—not enough for most people to notice,” said Veerachai Tanpipat, a remote sensing expert with the Upper ASEAN Wildland Fire Special Research Unit at Kasetsart University who has written about monitoring fires in Thailand with satellites. “We still see many poor air quality days, and we don’t yet have a practical prescribed burning system in place. Meanwhile, we have seen increasing numbers of low-cost sensors in cities, so people in the area are more aware of the problem now than ever before.”
After scientists return from the ASIA-AQ campaign in April, they will continue analyzing data collected from sensors on the aircraft as well as from several satellite sensors including GEMS (Geostationary Environment Monitoring Spectrometer), TROPOMI (Tropospheric Monitoring Instrument), OMPS (Ozone Mapping and Profiler Suite), and VIIRS (Visible Infrared Imaging Radiometer Suite). Participating ground-based sensor networks include AERONET (Aerosol Robotic Network), MPLNET (Micro-Pulse Lidar Network), and the Pandora Project.
NASA Earth Observatory images by Wanmei Liang, using VIIRS data from NASA EOSDIS LANCE, GIBS/Worldview, the Suomi National Polar-orbiting Partnership, and Fire Information for Resource Management System (FIRMS). ASIA-AQ is a joint effort between NASA and Korea’s National Institute of Environmental Research and several international organizations including the Department of Environment and Natural Resources Philippines, the Universiti Kebangsaan Malaysia, the Geo-Informatics and Space Technology Development Agency Thailand, and the Ministry of Environment Taiwan. Photographs by Rafael Luis Méndez Peña (NASA Earth Science Project Office). Story by Adam Voiland.
