Airshed

Part of speech: noun

Definitions

1. A defined geographic region, such as a basin or valley, whose air is monitored and regulated as a single unit for pollution control
2. An area of land and atmosphere treated as one zone so that its air quality can be measured and controlled together
3. A bounded region sharing one air mass or pollution profile, managed collectively by regulators to meet air quality standards

Etymology: The word "airshed" is a deliberate modern coinage, built from "air" and "shed" in the sense of a drainage area. The "shed" here is not the garden building but the older meaning of a watershed: the land whose rainfall drains into a particular river or basin. Environmental planners borrowed that picture and applied it to the atmosphere, treating a region's air as a bounded basin into which pollution flows and within which it can build up. The model is a clear one. A watershed collects water from everything that slopes toward a single outlet, and an airshed collects emissions from everything that sits under the same stretch of sky and gets trapped by the same weather and terrain. Valleys and basins with still air are the classic examples, because pollution released there cannot easily disperse. Naming such a zone after the watershed made the idea of regional, shared responsibility for air quality easy to grasp.