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Canadian wildfire smoke cooled New York by 3°C and trapped air toxicants, researchers find

When smoke from Canadian wildfires turned New York City skies apocalyptically orange in June 2023, it delivered catastrophic air quality and an unexpected side effect: cooling the New York City region by about 3 degrees Celsius.

This temperature drop, called "global dimming," which is the opposite of "global warming," trapped dangerous pollutants near the ground, according to Rutgers Health researchers who their findings in Communications Earth & Environment.

"With all you hear about the negative consequences of global warming, you might think the cooling would be good," said Philip Demokritou, senior study author and Henry Rutgers chair and professor of nanoscience and environmental bioengineering at the Rutgers School of Public Health and the School of Engineering. "But cooler temperatures are equally bad as global warming. This effect on microclimate can disrupt the hydrological cycle, trap other toxic air compounds and increase human exposure levels."

The researchers studied the physicochemical characteristics of particulate matter during the peak of the Canadian wildfire event, analyzing their light-absorbing and light-scattering properties. They found that the smoke particles, which traveled nearly 1,000 kilometers from Canadian wildfires, produced an unprecedented direct radiative forcing of—352.4 watts per square meter at ground level.

"The wildfire particulate matter that arrived in the New Jersey and New York City area on June 7 contained mostly brown carbon particles that had not been photobleached completely despite covering a distance of about 800 kilometers," said Georgios Kelesidis, an assistant professor at the Rutgers School of Public Health and lead author of the study.

Scientists determined that the cooling occurred because wildfire smoke contains primarily organic carbon particles that scatter sunlight away from the Earth's surface. Like a smoky umbrella, these particles blocked incoming solar radiation, cooling the areas below. However, this cooling came with dangerous consequences.

By reducing temperature, the smoke layer above the cities limited natural air circulation, preventing pollution dilution and vertical mixing. This mechanism trapped wildfire particles and other urban pollutants near ground level, potentially increasing human exposure to harmful airborne contaminants.

"This study documents for the first time the effect of wildfire nanoparticles on the microclimate of megacities," Demokritou said. "Such ambient air temperature reductions may exacerbate the urban heat island effect and limit the natural ventilation of megacities, exacerbating the exposure to air pollution."

During the event, New York City recorded an unprecedented daily average of particles less than 2.5 micrometers. The totals were more than three times higher than Environmental Protection Agency guidelines and eight times higher than World Health Organization recommendations.

The trapped pollutants likely contributed to health impacts throughout the region, Demokritou said, citing recent epidemiological studies in NYC that confirm increased emergency hospital visits and exacerbation of asthma and other respiratory health effects.

The radiative cooling study is part of a larger research effort examining the impacts of the 2023 Canadian wildfires on human health. A companion from the same Rutgers team, published in Environmental Science & Technology, revealed another concerning effect: A stunning 9.2 mg of smoke particles were deposited in the lungs of people in the area during the Canadian event and such particles significantly impaired human lung immune defenses.

That study found exposure to wildfire particulate matter reduced the viability of lung macrophages—immune cells that defend against inhaled pathogens—by about 20%. The particles also impaired these cells' ability to engulf foreign invaders by about 50%, potentially increasing vulnerability to respiratory infections.

Three separate epidemiological studies in New York City confirmed real-world health impacts during the event, with asthma-related emergency department visits increasing by 44% to 82% at the wildfire smoke's peak.

As climate change drives more frequent and intense wildfires, these findings highlight complex interactions between wildfire emissions, urban microclimates, and public health. The research demonstrates that beyond the immediate impacts of decreased visibility and breathing difficulties, wildfire smoke creates secondary effects that can amplify and prolong exposure to harmful pollutants.

"Wildfire events have increased in frequency and intensity dramatically due to extreme drought and heat and now affect even the Northeast U.S.," Demokritou said. "This was the first event of this scale in the region, but it probably won't be the last."