You’ve already heard this: People who live in cities pollute less and waste less than people who live in the suburbs. Not because they’re better people, necessarily, but because their surroundings just allow them to be more efficient.

But how do we know, really? It’s reasonably easy to track things like energy, water, and garbage—-all those things come from a single geographical point, after all. Measuring stuff in the air, however, becomes a bit more complicated.

Take fine particulates, for example—-the soot, dirt, dust, and smoke known in the business as PM2.5. That can come from four different kinds of sources: Point (fixed emittors like smokestacks), Area (smaller point sources like cooking and dry cleaning), Non-road (construction, logging, boating, etc) and On-road (your trucks and passenger vehicles). 

The last one is the one under examination right now by the Metropolitan Washington Council of Governments, because the Environmental Protection Agency requires regions—-delineated as metropolitan planning organizations, or MPOs—-to put together plans for reducing emissions of fine particulates, as well as nitrogen oxide (NOx), in order to get federal funds for transportation projects. Overall, emissions have declined over the last decade because of more efficient cars, and the trend is forecasted to continue under package of transportation projects that Maryland, D.C., and Virginia have put together—-but not evenly.

Although it’s very difficult to determine just how much of a region’s pollution can be traced to one jurisdiction, fancy models do as good a job as possible. Here’s what they found: On-road NOx emissions will keep declining in suburban Maryland, due to the institution of the California Clean Car Program, but will increase in Northern Virginia. On-road PM2.5 is projected to increase after 2025 in suburban Maryland and Virginia because of increasing vehicle use, and decrease in D.C., despite a population on the rise. (The graph above shows the breakdown of emissions by jurisdiction in 2007, compared to population).

What does this mean? Suburban governments that zone for single family homes far away from transit will generate more car trips, while continued investment in dense, mixed-use neighborhoods will generate fewer. And yet, for the purposes of federal funding for transportation, we’re all lumped in together—-Virginia can lag while the District excels.

Some people in the regional planning scene—-on the D.C. side, at least—-think it makes sense for the MPO to be broken down into its component parts, which would prod local governments towards better land use policies in order to stay in compliance. It’ll probably never happen. But maybe they could at least give out gold stars for exemplary performance, or something.