Air pollution in China. A new study links the increase in ozone precursor emissions in Asia to increased levels of ozone over the US's West Coast.

In the study, published Monday, a team of six researchers from US and Dutch universities found that ozone concentrations over China increased by about 7% between 2005 and 2010 and that ozone traveling in the air from China has reached the western part of the US, challenging the reduction of ozone levels there.

China's meandering pollution likely offset the 2005-10 reduction in ozone that had been expected following US policies aimed at reducing emissions, by roughly 43%, the researchers found.

Over that period, the US government put in place emission-reducing measures and curbed the production of ozone-forming nitrogen oxides by 20% on the West Coast, according to Wageningen University. Yet that did not improve the quality of the air especially in terms of ozone reduction.

And the increased air pollution in Asia might be at least partly to blame.

Lead researcher Willem Verstraeten of Wageningen University in the Netherlands said in statement that the "dominant westerly winds blew this air pollution straight across to the United States."

He added: "As a manner of speaking, China is exporting its air pollution to the West Coast of America."

Smog over LA. Flickr/Al Pavangkanan

The researchers determined what was happening by using satellite measurements of nitrogen oxide and ozone and combining it with a chemistry transport model to identify "the causes of increasing ozone levels and analyze intercontinental transport of ozone pollution for the first time ever," according to Verstraeten.

When levels of ozone are high in the lower atmosphere (this type of ozone is referred to as "bad ozone," or ground-level ozone) they have dangerous effects on human health and are a main component of smog.

High levels of ground-level ozone act as a greenhouse gas, contributing to pollution and climate change. ("Good ozone," on the other hand, is a natural component of the Earth's upper atmosphere and helps protect us from the sun's harmful UV rays.)

The researchers say this shows that the fight against increasing ozone levels and climate change must be global. "Local measures to improve air quality certainly help, but the real solution lies in a global strategy," Verstraeten said.