Northwest International Air Quality Environmental Science and Technology Consortium


Criteria pollutant design values


The prevailing concentration of criteria pollutants is often needed in areas without adequate monitoring in order to understand and maintain air quality. For example, permitting of stationary sources requires the ambient concentrations prior to construction of the source. These ambient concentrations are referred to as "background concentrations", and are typically measured at a neighborhood or urban scale representing an area of up to a few dozen kilometers. Concentration gradients are usually relatively low. These background concentrations are the result of emissions from all sources other than the new source, and include, but are not limited to:

  • Domestic emissions
  • All forms of transportation, including road, rail, air or shipping
  • Industrial and commercial activities
  • Natural sources

An air quality analysis of impacts from the new source will include dispersion modeling of the new source and other nearby emissions sources producing large concentration gradients near the new source, together with a representative background for the area. The design value has been chosen as the appropriate background concentration.

The scarcity of monitors in some areas often leaves modelers and permit engineers little choice but to use background concentrations measured in distant urban locations. These are usually higher than rural backgrounds and could result in inaccurate modeling analyses and overly restrictive permit conditions.

To address this concern, Washington, Oregon and Idaho have developed a model-monitor interpolated product which provides more realistic background design values at a 12km grid resolution. Archived CMAQ model data from the 3-state daily air quality forecast model (AIRPACT3: see for details), and all available monitoring data are used for this purpose. Model and monitoring data from 2009 through 2011 were used to compute 2011 design values.

This website serves background concentrations of criteria pollutants by converting user-supplied coordinates to their appropriate AIRPACT3 grid-cell, and looking up the model-monitor interpolated value at that grid-cell. The results from this tool may not be representative of background concentrations in every area. The use of these values in air quality permit modeling should be discussed with the relevant permitting authorities in advance.

Civil & Environmental Engineering, PO Box 642910, Washington State University, Pullman WA 99164-2910, 509-335-2576