Air pollution levels are falling in the US 鈥� but not as rapidly as the US Environmental Protection Agency thinks they are.
The US has been (NOx) and carbon monoxide for about 50 years. The EPA keeps tabs on the progress, in part by calculating how technological improvements should change the emissions from vehicles and factories.
But those calculations seem to be overestimating the progress being made, according to at the University of Science and Technology of China in Hefei. While at the National Center for Atmospheric Research in Boulder, Colorado, Jiang and his colleagues examined pollution data from satellites and ground-based sensors.
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The team found that NOx concentrations in the air dropped by 7 per cent each year between 2005 and 2009 鈥� but by only 1.7 per cent each year between 2011 and 2015. That鈥檚 a 76 per cent slowdown. The EPA鈥檚 figures estimate the slowdown should have been just 16 per cent.
Unwelcome slowdown
鈥淥ur analysis suggests the EPA overestimated the effect of regulations for heavy duty diesel trucks, which will result in an underestimation of NOx emissions,鈥� says Jiang. The discrepancy might also be due to a relative increase in contributions from off-road sources that are less strictly monitored, like farm equipment and lawn mowers.
Last year, showed the decline in roadside NO2 emissions since 2010 was larger than expected from government policy projections. Jiang says it鈥檚 not clear why certain emissions cuts have been underestimated in Europe and overestimated in the US.
One thing that probably isn鈥檛 a factor in the US is the diesel emissions scandal that engulfed car manufacturer Volkswagen. The firm was found to have used software in its cars to detect when it was undergoing an NOx emissions test and adjust engine performance to ensure they passed. 鈥淭he number of Volkswagen diesel vehicles in US is much smaller than in Europe,鈥� says Jiang.
The US government could soon make the situation worse. The Trump administration is reportedly considering , rather than progressively tightening them as the Obama administration planned. Forcing vehicles to become more fuel-efficient would reduce pollutant emissions, as well as cutting the greenhouse gas emissions that cause climate change.
PNAS
Read more: Cutting through the smog on air pollution
