Summer is more than half over, and thus far it has been a very good one for air quality in theWashington, D.C. metropolitan area. By many measures, June and July were among the cleanest on record. Of course, Washington's relative hiatus from summer smog may or may not last through August, but long-term trends indicate improvement in years ahead.
On only two days since June 1 did any of the area's 19 air pollution monitors record a violation of the federal smog standard in place since 1979. A newer, more stringent standard has been violated five times. Both numbers are well below the average for the first two months of summer in Washington. On most other days, Washingtonians were breathing air as clean as that in remote regions. Not bad for a metropolitan area with more than 4 million residents and chronic traffic congestion.
Luck has had much to do with it. Summer smog is created by two pollutants, nitrogen oxides (NOx) and volatile organic compounds (VOCs), in the presence of sunlight.
Conditions conducive to smog formation—hot, cloudless, stagnant summer days that can really cook ambient NOx and VOC molecules into smog—have been unusually rare for the Middle Atlantic region in the last two months. What has been bad news for beachgoers and air conditioner sellers has been good news for air quality.
But it isn't all luck. NOx and VOCs emissions have declined substantially in recent decades, and continue to go down each year. Motor vehicles are the single largest source of these smog precursors, and they have gotten significantly cleaner since the 1970s. Strict federal emission limits for cars and trucks have more than compensated for the area's population growth and increases in number of vehicles and miles traveled.
This trend will continue for the long-term. New federal regulations, which will be phased in over the next few years, demand even greater reductions in smog-forming compounds from cars, SUVs, and trucks. The Metropolitan Washington Council of Governments, which coordinates air pollution control efforts by the District of Columbia and the Maryland and Virginia suburbs, estimates further declines of 66.4 percent for NOx and 50.6 percent for VOCs from next year to 2015.
No additional measures are needed to achieve these improvements, just the inevitable turnover from older and dirtier vehicles to cleaner new ones. Over time, we will see less and less summer smog, regardless of the weather.
A hot, humid, and smoggy August may yet await us, but the unusually clean air of June and July will become more typical of future Washington summers.