Air Quality

Under the federal Clean Air Act, the CLRP is required to conform to regional air quality improvement goals. Before the CLRP can be approved, the TPB must approve a “conformity determination” showing that anticipated vehicle emissions will conform to emissions ceilings (called “mobile emissions budgets”) contained in the region’s air quality improvement plan. The Metropolitan Washington Air Quality Committee (MWAQC) is the body responsible for developing the regional air quality plan in close coordination with development of the CLRP.

MWAQC and the TPB are concerned with emissions of smog-producing Volatile Organic Compounds (VOCs) and Nitrogen Oxides (NOx). These pollutants combine in sunlight on hot summer days to form ground-level ozone. Motor vehicles are responsible for a large portion of VOC and NOx emissions in the region, but so are non-mobile sources like power plants.

In addition to NOx and VOCs, the plan also tracks and estimates emissions of particulate matter of less than 2.5 micrometers in diameter (PM2.5). PM2.5 is of special concern because these ultra-fine particles can easily lodge in the lungs of humans and cause health problems. Since concern about PM2.5 has developed relatively recently, PM2.5 was not tracked or estimated in 1990.

Analysis of the 2015 Amendment to the CLRP shows reductions of emissions of all three main pollutants between now and 2020. The data show that estimated emissions are well within the mobile source emissions budget for each pollutant for 2017, 2025, 2030, and 2040. These results reflect the impact of better vehicle standards, cleaner fuels, and fleet turnover, as well as changes in development patterns, investment in transit and other travel options, and improved operational efficiency or area roadways.

The Air Quality Conformity Report is available by clicking here.





CO2 Emissions for 2015 CLRP Amendment

Over the past decade, concerns have emerged about global climate change and greenhouse gases like carbon dioxide (CO2). Based on climate science and consideration of policies of jurisdictions in the region, the COG Climate Change Report of November 2008 set a goal of reducing the region’s CO2 output to 80% below 2005 levels by 2050. Applying this goal to transportation would require reducing the region’s transportation-related CO2 emissions by 60% compared to 2005 levels by 2040, the horizon year for the CLRP.

Total CO2e emissions under the 2015 CLRP Amendment are forecast to drop by 22% by 2040, while CO2e emissions per capita are expected to drop by 44%. A significan amount of the greenhouse gas reductions are due to new tougher federal fuel efficiency standards. In addition, changes in development patterns and investments and other travel options will contribute to reductions.


Currently, no federal standards exist for greenhouse gas emissions. These emissions are not a required part of the transportation Air Quality Conformity Analysis.