Nashville, TN Air Quality Index (AQI) Today

Middle Tennessee’s Central Basin is a closed depression and Nashville occupies its floor. The Highland Rim rises 300 to 400 feet above that floor and runs the whole way around it. The bowl began as an uplift, the Nashville Dome, whose arched center then eroded faster than the ground around it until what had been high ended up low. The Cumberland River drains through the middle of it. The shape is shallow but continuous, and on a still autumn or winter night cold air settles onto the floor with a warmer layer sitting above it.

That is when PM2.5 climbs. I-40, I-24 and I-65 all converge inside Davidson County, and a morning commute released under a lid a few hundred feet deep concentrates in a way the same traffic would not on open ground. The lid usually breaks by late morning and the reading follows it down.

Summer swaps the problem to ozone, which is why Nashville’s air pollution control program forecasts it from March 1 through October 31 rather than only across the hottest weeks. Ozone is built in place out of exhaust, heat and sunlight, and it accumulates through the afternoon, well after the night’s lid has already lifted. The two problems here run on opposite halves of the day, off the same traffic.