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 2014 CLRP shows reductions of emissions of all three main pollutants between now and 2020, followed by a leveling off and then a slight increase between 2030 and 2040 for some mobile source emissions. The data show that estimated emissions are well within the mobile source emissions budget for each pollutant for 2017, 2020, 2030, and 2040. These results reflect the impact of better vehicle standards, cleaner fuels, and fleet turnover, as well as travel demand and operations management and transit investments. Absent any further improvements to the vehicle fleet, however, once the fleet has undergone a complete replacement, the amount of mobile source emissions will begin to rise due to overall increases in vehicle miles of travel (VMT).

The Air Quality Conformity Report is available by clicking here.





CO2 Emissions for 2013 CLRP and FY 2013-2018 TIP


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.

While some reduction in CO2 emissions by 2040 is currently forecast, total CO2 emissions are projected to increase between 2025 and 2040. The CO2 emissions, however, are rising at a slower rate than population and employment growth, resulting in forecast decreases in CO2 emissions per capita. Because CO2 emissions accumulate in the atmosphere over time, the failure to make improvements in total CO2 emissions now makes greenhouse gas emissions an even greater concern.