New York City’s air quality dropped to unhealthy levels Wednesday morning, but Canadian wildfires aren’t the main culprit this time.

The conditions are largely caused by typical summertime pollution from ground-level ozone, gasoline fumes and automobile exhaust getting trapped under a layer of calm air. Other major cities from Chicago to Washington will also start the day with polluted skies.

“Anywhere in the East, there doesn’t look like there is much contribution from the wildfires,” said Marc Chenard, a senior branch forecaster at the US Weather Prediction Center.

While ozone protects the Earth’s surface from the sun’s harmful rays, when it builds up close to the surface it can cause problems for the elderly and people with certain medical conditions. Air quality alerts have been posted from southern Connecticut to northern Virginia, as well as for Chicago and parts of Michigan.

Ozone is produced by the chemical reactions of hydrocarbons and other gases in the atmosphere, especially in urban areas, and can build up when winds are calm, according to the National Weather Service. That often happens in the morning and evening because the ground cools off faster than the atmosphere, creating an inversion that traps air near the surface.

As the surface heats up during the day, the pollution may thin out, Chenard said. The temperature is forecast to reach 90F (32C) in Manhattan on Wednesday.

Author: Brian K. Sullivan