Why heat waves often bring air quality warnings to the Philadelphia area

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The heatwave that brought 100-degree temperatures to the Philadelphia region in June also brought warnings that the air quality was unhealthy for sensitive groups.

This was not a coincidence.

Heat and sunlight are key ingredients in the reaction that creates ground-level ozone, a common air pollutant that can cause coughing, wheezing and more frequent asthma attacks.

Ozone is what’s known as a secondary pollutant. It forms from other pollutants produced by sources such as cars, trucks and power plants.

In an urban area like Philadelphia, the air already contains plenty of these precursor pollutants, said Kabindra Shakya, a professor at Villanova University who studies air quality.

“They also need one other ingredient, which is the sunlight,” Shakya said. “When we see the high heat and the sun, then that actually drives more formation of the ozone.”

Increased reactivity of compounds in the air is not the only reason that air quality tends to be worse during heat waves, said Xiaomeng Jin, a professor at Rutgers University who also studies air quality. She said that hot days can also bring poor air quality because increased air conditioner use drives up emissions from power plants.

“Besides that, during these hot days, generally you’ll see the wind speed is low, which creates stagnant conditions that can somehow trap the air pollution in these areas, so there is not so much of the transport that can clean up the air,” she said.

In the northeastern U.S., rising temperatures due to climate change are expected to increase  the number of severe ozone days in the coming decades, according to the National Climate Assessment.