For much of November 2023, NASA satellites observed heavy smoke and haze darkening skies in northern India. The poor air quality is a seasonal occurrence—the product of smoke from crop fires mixing with urban pollution from traffic and industry, emissions from cooking and heating fires, windblown dust, and a range of other sources. The timing is also important: all this particle pollution enters the atmosphere when seasonal weather patterns tend to trap air pollution near the ground.
The MODIS (Moderate Resolution Imaging Spectroradiometer) on NASA’s Aqua satellite captured this image of haze blanketing the region on November 20, 2023. Ground-based air quality monitors, managed by India's Central Pollution Control Board, the Delhi Pollution Control Committee, the U.S. State Department, and Nepal’s Department of Environment, reported unhealthy to hazardous levels of airborne particulate matter that day. From above, satellites detected widespread crop fires in northern Pakistan and the Indian states of Punjab, Haryana, and Uttar Pradesh on November 20.
The World Health Organization considers 15 micrograms per cubic meter of fine particulate matter (PM2.5) to be a safe limit. But ground-based air quality monitors routinely measured levels that exceeded 300 and, at times, 500 micrograms per cubic meter in November. As air quality deteriorated, doctors urged people to limit outdoor activity and reported increases in respiratory problems. The haze also led to pauses in construction, restrictions on vehicle use, and school closures.
Hiren Jethva, a Morgan State University researcher based at NASA’s Goddard Space Flight Center, uses satellites to observe vegetation and track fire activity in the region each fall. “In terms of satellite-detected fire counts, the fires in Punjab and Haryana have been less severe this year—about 40 percent less than the five-year average detected by Aqua’s MODIS—but they’re still numerous enough to cause poor air quality in Delhi,” he said. One analysis of the contribution of crop fires to the overall air pollution problem conducted by India’s Ministry of the Environment attributed about a fifth of Delhi’s PM2.5 to fires in early November.
Ritesh Gautam, previously an atmospheric scientist at NASA and now with the Environmental Defense Fund, has been taking a longer-term look at trends in fire activity, haze, and atmospheric stability in the region. Gautam, Jethva, and several other colleagues recently found that increasing numbers of fires since the early 2000s have coincided with a 90 percent increase in the amount of aerosol-laden haze that satellites observe in November and increased stability of the lower troposphere. Their analysis of the radiative effects of aerosol particles found that as smoke from crop fires builds up, it likely warms the atmosphere and cools the ground. This increases the likelihood of temperature inversions—a meteorological condition that traps pollution near the surface, further intensifying the haze.
“It’s a double whammy. It’s not just the smoke itself, but likely the way that the increasing smoke is changing the meteorology that makes this such an intense problem,” Gautam said. “On the other hand, our results suggest that reducing late autumn burning and other anthropogenic emissions during the winter might have an even greater effect in reducing the haze than previously thought.”
NASA Earth Observatory image by Michala Garrison, using MODIS data from NASA EOSDIS LANCE and GIBS/Worldview. Story by Adam Voiland.