NEW DELHI, June 9, 2023 A new study published in the journal Geophysical Research Letters finds that dense smog associated with winter haze and late-autumn crop burning in northern India has gotten worse over the past four decades. The smog, which causes hazardous air pollution and reduces visibility, is impacting a region of more than 600 million people in the country’s Gangetic Plains, considered a hotspot for air pollution.
“Smog in this region is getting more intense due to a combination of factors,” said Ritesh Gautam, Lead Senior Scientist at Environmental Defense Fund Inc., and the study’s lead author. “Tiny aerosol particles from emissions warm the atmosphere while cooling the ground, then interact with meteorology in a feedback loop over urban and rural areas, fueling this intensity, especially in the late autumn and early winter.”
Meteorological feedbacks intensify smog
The authors studied the links between the intensification of smog and long-term changes in the lower atmosphere, by combining two decades of multi-satellite observations, 40 years of meteorological observations and atmospheric modeling datasets from 1980 until 2019.
They found that aerosol particles such as soot, which can influence how much energy from the sun reaches the earth’s surface, warmed the lower atmosphere while cooling the planet’s surface. Temperatures in the lower troposphere—the lowest layer of the atmosphere—should generally decrease with height, but here, the opposite occurred. This situation is responsible for what is known as “inversion” events when the earth’s surface is cooler than the air above it. A growing number of observed inversion days, coupled with increasing relative humidity and a lowering of boundary layer height helped lead to more intense smog in this area.
Furthermore, the authors found a buildup of aerosol pollution across the region, not just in large, urban areas, where it receives the most attention.
“These aerosol-radiative-meteorological feedbacks, coupled with emissions are driving the long-term smog intensification across the entire region,” Gautam said.
Regional landscape adds to challenges
This area of India lies in a valley-type terrain to the south of the Himalayas and is vulnerable to buildup of excess pollution, due in part to geography. And while emissions alone may not be fully responsible for increasingly poor air quality, they combine with the meteorological feedbacks to drive smog’s intensity in the region.
“The buildup of pollution from crop burning and the winter haze in the shallow boundary layer of the atmosphere, combined with the weather is really a double whammy of factors that feed on one another which has intensified the severity of smog,” Gautam said.
Potential for substantial mitigation in semi-urban, rural areas
The findings pointing to smog from rural areas and small towns suggest that monitoring and mitigation efforts could deliver more substantial and measurable gains if implemented across the region, rather than just major cities. “These findings suggest we may need more robust or different types of mitigation measures to control air pollution in the area.” says Parthaa Bosu, who leads EDF’s air quality work in India.
“We need to reduce pollution levels systematically across urban, semi-urban and rural areas along the Gangetic Plains, from Punjab to West Bengal,” Bosu says. “This has the potential to reduce smog substantially from November through January in northern India.
“Smog is a shared problem across the Gangetic plains,” Bosu said. To create awareness and develop lasting solutions, he suggests developing analytic metrics like a smog-severity index, which could illustrate the challenges in both rural and urban communities, across the region’s various landscapes. “These findings support the need for more vigorous, region-wide air quality monitoring, enforcement and improvement efforts,” Bosu said.
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