Weather Makes a Difference: 8-hour Ozone Trends for 1997-2005 State and Local Information for EPA Region 10


Weather Makes a Difference: 8-hour
Ozone Trends for 1997-2006

State and Local Information for EPA Region 10

Oregon
Washington

Composite trend for available sites in these states:

EPA Region 10 (selected sites)

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U.S. Environmental Protection Agency

Office of Air and Radiation

Office of Air Quality Planning and Standards

April 2007

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Oregon
Ozone

In Portland ozone declined 11 percent between 1997 and 2006. These improvements in
ozone are in response to both state and regional reductions in NOx and VOC emissions.
The level of ozone improvement varies from site to site.

Trends for 1997-2006 for rural sites and urban areas with complete ozone and
meteorology data are presented below. Ozone season (May 1 - September 30) averages
of daily maximum 8-hour ozone were adjusted to remove the influence of year-to-year
variability in weather conditions. The dotted line shows the trend in observed values at
monitoring sites, while the solid line illustrates the underlying ozone trend after removing
the effects of weather. The solid line serves as a more accurate ozone trend for assessing
changes in emissions. Typical weather conditions are determined by averaging
conditions (e.g., temperature, humidity, etc.) for the time period presented. The
information provided is useful for reviewing the weather influence for a particular ozone
season. The solid line represents ozone levels anticipated under typical weather
conditions.

Seasonal Average 8-hour Ozone Trends

Portland, OR

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Washington
Ozone

On average, ozone increased 20 percent between 1997 and 2006. There have been
improvements in Seattle in recent years. These improvements in ozone are in response to
both state and regional reductions in NOx and VOC emissions. Ozone trends vary from
site to site.

Trends for 1997-2006 for rural sites and urban areas with complete ozone and
meteorology data are presented below. Ozone season (May 1 - September 30) averages
of daily maximum 8-hour ozone were adjusted to remove the influence of year-to-year
variability in weather conditions. The dotted line shows the trend in observed values at
monitoring sites, while the solid line illustrates the underlying ozone trend after removing
the effects of weather. The solid line serves as a more accurate ozone trend for assessing
changes in emissions. Typical weather conditions are determined by averaging
conditions (e.g., temperature, humidity, etc.) for the time period presented. The
information provided is useful for reviewing the weather influence for a particular ozone
season. The solid line represents ozone levels anticipated under typical weather
conditions.

Seasonal Average 8-hour Ozone Trends

Seattle, WA

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Mount Rainier NP, WA

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North Cascades NP, WA

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