An airshed is a geographical area where local

topography Topography is the study of the forms and features of land surfaces. The topography of an area may refer to the landforms and features themselves, or a description or depiction in maps. Topography is a field of geoscience and planetary sci ...

and

meteorology Meteorology is the scientific study of the Earth's atmosphere and short-term atmospheric phenomena (i.e. weather), with a focus on weather forecasting. It has applications in the military, aviation, energy production, transport, agricultur ...

limit the dispersion of pollutants away from the area. They are formed by air masses moving across a landscape, thus influencing the

atmospheric An atmosphere () is a layer of gases that envelop an astronomical object, held in place by the gravity of the object. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A stellar atmosphere ...

composition of that area. Their boundaries are loosely defined, but can be quantified. Airborne chemicals disperse throughout an airshed and enter bodies of water in the area.

Defining airsheds

Airsheds are difficult to define precisely because they are spatially variable and can change over time. Their boundaries may change due to weather conditions and the sources of pollutants. Airsheds can be quantified for specific events, by season, or over a long period of time. Scientists model the movement of air masses over time to construct the dimensions of the upwind and downwind airsheds for a region.

Urban Airshed Model (UAM)

The Urban Airshed Model (UAM) is a three-dimensional grid model that is used to simulate chemical and physical atmospheric processes. The

United States Environmental Protection Agency The Environmental Protection Agency (EPA) is an independent agency of the United States government tasked with environmental protection matters. President Richard Nixon proposed the establishment of EPA on July 9, 1970; it began operation on De ...

uses this model to develop air quality plans for ozone in urban areas.

Uni-directional Impacts

The upwind airshed of a city represents the area where emissions and pollutants affecting that city originate. This includes compounds that form secondary pollutants such as fine particulate matter and ground level ozone. Downwind effects in airsheds are responsible for the spread of those pollutants outside of the city. The downwind airshed tends to affect a wider area than the upwind because they are a combination of the upwind airshed and pollutants produced within the city.

References

External links

Georgia Basin-Puget Sound International Airshed Strategy
{{Natural resources, state=collapsed Meteorological phenomena