Air pollution from China and ozone coming down from the upper atmosphere have together offset actions taken in the western United States to reduce emissions, according to a new study.
The study done jointly by researchers affiliated with NASA and a Netherlands institute found that, between 2005 and 2010, emissions transported from China offset 43 percent of the reductions in ozone that were expected to occur in the atmosphere over the western United States. Ozone from the upper atmosphere offset the remaining 57 percent.
The authors said the study shows the need for a global approach to reducing air pollution.
"We conclude that global efforts may be required to address regional air quality and climate change," the authors wrote.
The study was published online yesterday in the journal Nature Geoscience. The California Institute of Technology’s Jet Propulsion Laboratory, which Caltech manages for NASA, collaborated with researchers from the Royal Netherlands Meteorological Institute.
Ground-level ozone is a key component of smog that’s been linked to negative health problems such as reduced lung function. It’s formed when nitrogen oxides and volatile organic compounds react in the presence of sunlight. Ozone can also make its way from the upper atmosphere to ground-level in high-elevation areas in the West (Greenwire, Nov. 17, 2014).
To study the levels of ozone in the mid-troposphere, about 10,000 to 30,000 feet above the ground, the research team at Caltech and the Netherlands used NASA satellite measurements and computer modeling.
They found that emissions of ozone precursor pollutants in the western United States dropped by 21 percent between 2005 and 2010. That reduction should have resulted in a drop of more than 2 percent in the mid-troposphere.
But during that time, emissions of ozone precursors in China increased and an unusually large amount of ozone from the upper atmosphere made its way to ground level. Over the western United States, there was no drop in the amount of ozone in the mid-troposphere despite the local actions taken to reduce emissions.
"The large contribution from the stratosphere is part of a natural up-and-down cycle of upper-atmosphere winds," Jet Propulsion Lab scientist and co-author Jessica Neu said in a statement. "We know pretty well what will happen to the stratospheric contribution in the next few decades; it will continue to go up and down every two years or so."
But she added: "On the other hand, the contribution from China increased steadily throughout the study, and we don’t know what will happen to it in the future because it depends on human rather than natural factors."
The results show that local and regional air quality and climate change policies could have a "limited impact if not considered in a global context," the authors wrote. It comes as U.S. EPA is considering reducing the national ambient air quality standard for ozone from 75 parts per billion to between 65 and 70 ppb.