What is an Airshed?
The first documented use of the term airshed was in Robert Avrill's Man and Environment (1) where he talked about the need for local, regional and national air quality management. It's analogous to the more widely used concept of the watershed(2). The term is not much used in the UK, but is the basis of regulatory approach in North America, Australia and New Zealand.
An air-shed is a part of the atmosphere that behaves in a coherent way with respect to the dispersion of emissions. Obviously the characteristics of a watershed are different from the atmosphere. The US EPA describes an airshed as the volume of air receiving emissions which predominantly affects a specific watershed or catchment. (3)
In most cases the impact of existing or planned emissions to the atmosphere can be predicted using appropriate dispersion models. In some cases simple screening models e.g. D1 or H1(4) can be used to determine if emissions are significant. These tools can be used to discount trivial processes without needing to conduct more rigorous assessments. Advanced dispersion models use mathematical equations to predict the transport and transformation of pollution by modelling the physical, chemical, and meteorological processes in air. At Airshed we specialise in the assessment of pollution within the micro and meso-scale - up to 10km from source, suitable for modelling releases from industry and transport. A different modelling approach is required for regional or trans-boundary pollutants such as acid rain and ozone.
The other work we do is all connected with atmospheric science in one way or another. In the case of noise, the propagation through the air. Sheds are also a good place for thinking and problem solving, (away from distractions) so airshed seems appropriate.