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. So, an airshed by comparison is a geographic area where air pollutants from sources "upstream" or within the area flow and present in the air.
We live in a unique airshed that is heavily influenced by three natural factors: the mountains, the Pacific Ocean and the weather. The region is bounded by the Olympic mountains to the west and the Cascade mountain range to the east. These mountain ranges play a major role for weather patterns and air pollution levels.
The Puget Sound region can be described as a modified marine climate. This means that 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.
Health Insurance Quotes 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.
During periods when our onshore air flow is interrupted, the combined effects of the meteorological pattern, geography and urban features result in stagnation and trapping of air pollutants generated by sources within the airshed.
A picture of our airshed is below. The arrows illustrate the on-shore marine air flows that help keep Puget Sound area air clean. (This picture is 355k and may take a few minutes to download.)
The term temperature inversion literally refers to a reversal of normal temperature patterns seen in our lower atmosphere. Temperature inversions are relatively common in the Puget Sound area. In wintertime, a temperature inversion occurs when cold air close to the ground is trapped by a layer of warmer air. As the inversion continues, air becomes stagnant and pollution becomes trapped close to the ground. Since our area lies in a basin, it takes an extra push of marine air to flush the pollution out of our area. Inversions also occur during the summer months, but are a product of even hotter upper air trapping warm air close to the ground. The result is the same: we are unable to rid ourselves of the everyday pollution that we create. Air pollution will continue to accumulate until the weather pattern changes. Just how much air quality degrades depends on the actions of people who are using many of the things that pollute the air like cars, trucks, lawnmowers and wood stoves.
The following two graphs show the difference between a normal weather day and a temperature inversion day. The vertical axel on the graph shows distance or height from the ground's surface up into the lower atmosphere. The horizontal axel shows temperature. The colored lines show air characteristics at different times of the morning hours. The black line is a measurement at 6 a.m.
On a normal day, the temperature is warmest at the ground's surface and the air gets cooler at increasing altitudes. The above graph shows that the temperature is about 7 degrees celsius (or about 40 degrees farenheit) at ground level. The temperature decreases the farther up we measure. This means that air is mixing and that air pollution is able to rise and be dispersed. In this situation, our air pollution has a lot of room in which to disperse upward into the lower atmosphere.
The graph below depicts the opposite scenario.
Warm air has formed a layer on top of cooler surface air. This layer is called the temperature inversion. This means that temperatures actually rise as we measure farther up into the air. Little or no mixing occurs and concentrations of atmospheric particulate pollution and vapors are trapped below the level where temperatures again decrease with altitude. This point is referred to as the top of the inversion. Inversions can occur at the surface or aloft in the atmosphere. Those close to the surface have the greatest influence on air quality because air pollution has little room to be dispersed.
In this graph, the temperature initially is increasing with altitude. At about the 300 meter mark, the top of the inversion, the temperature pattern is beginning to revert to normal. This inversion means that instead of having virtually endless amounts of air in which to dissipate, our daily air pollution might be confined to a space only 1500 feet above the ground's surface. For more information about temperature inversions, check out weather web sites hosted many universities and the National Oceanic and Atmospheric Administration (NOAA).
Updated/reviewed May 1998
