What is an Airshed?

An airshed can be compared to a watershed. When we talk of a watershed, we mean a geographic area where rivers, streams and run-off flow into a specific body of water. By comparison, an airshed is a geographic area where air pollutants from sources "upstream" or within the area flow and are present in the air.

What are the characteristics of our airshed?

Our airshed is greatly influenced by four factors: urban development, the Pacific Ocean, the mountains and the weather. Most urban development has taken place at elevations near sea level adjacent to the waters of the Puget Sound. The urban corridor extends from south of Tacoma, northward across the Canadian border to Vancouver B.C. Although it is not uniform in density, most of our air pollution comes from the cities and the network of highways along this north/south line.

The Puget Sound region has a modified marine climate. For most of the year our region's weather is dominated by influxes of clean, moist ocean air that penetrate at low elevations from the Chehalis gap to the south and the Strait of Juan de Fuca to the north. Temperatures are generally moderate with few extremely cold or hot days throughout the year. Wind-driven mixing regularly occurs which effectively disperses air pollutants.

During periods when our onshore air flow is interrupted, the combined effects of urban activities, the weather and topography lead to stagnation and rising air pollution. The Olympic mountains to the west and the Cascade mountain range to the east form the sides of a bowl when air pollution becomes trapped in the urban basin.