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.