Tennessee air quality today

The white haze over the Great Smokies is not the same thing as the blue one. The blue tint comes from vegetation releasing volatile organic compounds and is as old as the forest. The white version is sulfate particles, formed from the sulfur released by burning fossil fuels, and the National Park Service names sulfates as the primary cause of visibility degradation in the park. Humidity worsens the effect mechanically: the particles take on water and grow, scattering more light at the same mass. Ozone behaves oddly here as well. The Park Service reports ridgetop levels running up to twice those measured in nearby cities, and at elevation ozone persists overnight instead of falling away as it does at street level after dark.

Most of the sulfate was emitted somewhere other than Tennessee, across a power grid covering several states, and it arrives as a regional background rather than as a plume from a nearby stack. That background has thinned as coal generation in the Tennessee Valley has been retired or fitted with controls — a decade-scale change that a daily index, which describes the last hour at one monitor, is not built to show.

Regulation is split five ways. Four counties — Davidson, Hamilton, Knox and Shelby — run their own air pollution control programs, covering Nashville, Chattanooga, Knoxville and Memphis. The remaining ninety-one counties fall to the Division of Air Pollution Control at the Tennessee Department of Environment and Conservation. Chattanooga’s long history with trapped air comes out of the same ridge-and-valley terrain: the city sits in the river gap between Lookout Mountain and Missionary Ridge, which gives cold air somewhere to settle.