A Shot of Smoke for Delhi
Every November, satellites detect large plumes of smoke and heightened fire activity in northwestern India as farmers burn off excess paddy straw after the rice harvest. Many farmers, particularly in the states of Punjab and Haryana, use fire as a fast, cheap way to clean up and fertilize fields before planting winter wheat crops. However, the surge of fires in the heart of the densely populated Indo-Gangetic Plain often contributes to a sharp deterioration of air quality in November and December.
Though lingering monsoon rains this year kept fire activity at low levels for a few weeks longer than usual, satellites observed elevated fire activity in November as the pace of burning accelerated. On November 11, 2021, the Visible Infrared Imaging Radiometer Suite (VIIRS) on the Suomi NPP satellite acquired this natural-color image of a river of smoke streaming from fires in Punjab and Haryana toward Delhi, one of India’s most populous cities. Use the image comparison slider to see the locations of hotspots observed by VIIRS that afternoon. Fires in northern Pakistan likely contributed some of the smoke as well.
“Looking at the size of the plume on November 11 and the population density in this area, I would say that a conservative estimate is that at least 22 million people were affected by smoke on this one day,” said Pawan Gupta, a Universities Space Research Association (USRA) scientist at NASA’s Marshall Space Flight Center.
As in years past, sensors in Delhi and elsewhere in northern India have recorded soaring levels of air pollution. Sensors in the capital area—including one at the U.S. Embassy—recorded concentrations of fine particulate matter (PM2.5) and coarse particulate matter (PM10) well above 400 micrograms per cubic meter on several occasions in November. Since particulate matter is linked to a range of respiratory, cardiovascular, and other health problems, World Health Organization guidelines recommend that 24-hour mean PM2.5 concentrations be kept below 15 micrograms per cubic meter. The high pollution levels led to partial lockdowns, school closures, and halts in construction in Delhi and other cities.
Smoke from crop fires is not the only contributor to the hazy skies. Influxes of dust sometimes arrive from the Thar Desert to the west—as they did on November 12, 2021 (see image below). An array of other human-caused sources of air pollution in cities, including motor vehicle fumes, industrial and construction activity, fireworks, and fires for heating and cooking also produces particulate matter and other pollutants.
Geography and weather also exacerbate the region’s air quality problems. Temperature inversions are common in November and December as air rolls off the Tibetan Plateau and mixes with smoky air from the Indo-Gangetic plain. An inversion can function like a lid, with the warm air trapping pollutants near the surface and helping hem pollutants in between the Himalayan Mountains to the north and the Vindhya Mountains to the south.
Hiren Jethva, a Universities Space Research Association (USRA) scientist based at NASA’s Goddard Space Flight Center, uses measures of the “greenness,” or Normalized Difference Vegetation Index (NDVI) to anticipate fire activity each year. The data he uses comes from the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Aqua satellite.
“Earlier in the summer, we saw one of the largest NDVI values in the 20-plus year record. Based on that, I predicted this would be one of the most active fire seasons on record, and that is exactly what we have seen,” said Jethva. “We still have a few weeks of burning to go, but already Aqua MODIS has detected more than 17,000 hotspots in Punjab and Haryana—making this the most active fire season on record.” Aqua MODIS began collecting data in 2002.
The Visible Infrared Imaging Radiometer Suite (VIIRS) sensor makes similar measurements, but it can detect many small and low-temperature hotspots that MODIS misses. As of November 16, VIIRS had detected more than 74,000 hotspots in Punjab. “That’s approaching the nearly 85,000 the sensor detected in 2016, the most active year in the VIIRS record,” said Gupta.
While total fire counts have remained consistently high in Punjab, satellite data indicate that campaigns to get farmers to clear fields without using fire have proven more successful in Haryana. “Over nine years of VIIRS observation, we don’t see much of a trend in Punjab. However, in Haryana, we saw a 45 percent decrease in the total number of fires in 2020 compared to the 2012-2019 average,” Gupta added. “But fire counts seem to be on the higher end in Haryana again this year.”
NASA Earth Observatory images by Lauren Dauphin, using MODIS data from NASA EOSDIS LANCE and GIBS/Worldview. Story by Adam Voiland.