A previous post, EPA Makes Room for State Flexibility in Addressing “Interstate Transport” Under the Clean Air Act, discussed the evolving policy of the US Environmental Protection Agency (EPA) regarding approval of state plans—required under the “Good Neighbor Provision” of the federal Clean Air Act—addressing “interstate transport” of air pollution. That article reviewed a series of guidance documents EPA issued in 2018 to allow states flexibility in addressing wind-borne emissions that can contribute to ground-level ozone pollution in other states located downwind. At stake are not only downwind states’ air quality objectives but the prospect of expensive additional emission controls on upwind states’ manufacturing facilities and power plants.
One of EPA’s 2018 guidance documents addresses the seemingly technical question of what “contribution threshold” to apply. That term refers to the quantity—measured in parts per billion (ppb) of ozone in the air at ground level—below which an upwind state’s impact on a downwind state’s ozone concentrations is small enough that any contribution would be considered essentially de minimis. Generally, a state will want its emission contributions to be deemed low enough that it would be clear that its emission sources would not need new control requirements.