TPB News

Concerted Action Leads to Major Improvements in Region's Air Quality

Oct 13, 2014

Ground-level ozone concentrations in the Washington region exceeded healthy levels on just four days each of the last two summers, down from 43 such "exceedance" days as recently as the summer of 2007, and 67 during the summer of 1998.

That's according to data released recently by the Metropolitan Washington Council of Governments following the end of this year's official "ozone season," which ran April 1 to September 30. Each year, 13 on-the-ground monitoring devices around the region measure ozone concentrations to determine whether they exceed national standards set by a team of scientists and doctors.


Major improvements in ozone concentrations have come as a result of concerted action by federal, state, and local regulators, and by businesses and individuals, to reduce emissions of the two main ingredients in the formation of ozone: nitrogen oxides (NOx) and volatile organic compounds (VOCs).

Many sources are responsible for NOx and VOC emissions, including power-generation plants, factories, construction and landscaping equipment, and the millions of cars, trucks, buses, and other vehicles that travel on the region's roadways each day.

Within transportation, reductions in emissions of NOx and VOCs have resulted mostly from federal requirements for cleaner, more fuel-efficient vehicles and for cleaner-burning fuels. Efforts to reduce roadway congestion and to encourage less driving have also contributed.

In all, the Transportation Planning Board estimates that total NOx emissions from the region's transportation sector have fallen more than 70% since 1990, and that VOC emissions have fallen more than 80%. These declines have come even as population has swelled some 40% and as total driving, measured in vehicle-miles traveled (VMT), has grown by a similar margin.

Looking to the future, the TPB anticipates further declines.

In its most recent Air Quality Conformity Analysis, carried out in conjunction with the 2014 update of the region's Constrained Long-Range Transportation Plan (CLRP), the TPB forecasts a drop in VOC emissions of 32% compared to today, and a drop in NOx emissions of 54% -- all while the region's population grows an additional 25%.

These declines will be driven mostly by the slow but continual process of replacing older, less efficient vehicles with newer ones that meet the latest efficiency standards. Continued efforts to reduce congestion and encourage less driving, both through transportation investments and changes in land-use patterns, are also expected to contribute to further reductions.

Air quality in the Washington region has improved significantly over the last couple of decades. The improvements have come largely as a result of concerted action by federal, state, and local regulators, and by businesses and individuals, to reduce emissions of ozone-forming pollutants across all sectors. Further future reductions, which may be needed if federal standards are toughened, are likely to be achieved through the same kind of broad, comprehensive approach that the region has taken over the last couple of decades.

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For more information on the 2014 CLRP update, including TPB Weekly Report coverage of the update's final stages over the last month, please visit or follow #CLRP2014 on Twitter.

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