Wiktionary
airshed
n. (alternative spelling of air shed English)
Wikipedia
Airshed
An airshed is a part of the atmosphere that behaves in a coherent way with respect to the dispersion of emissions. It typically forms an analytical or management unit. Also: A geographic boundary for air quality standards.
Alternatively - An airshed is a geographical area where local topography and meteorology limit the dispersion of pollutants away from the area.