Air pollution from China, India and several other Asian countries has wafted across the Pacific Ocean over the past 25 years, increasing levels of smog in the western U.S., a study found.
Smog, also known as ground-level ozone, is harmful to human health, because it can exacerbate asthma attacks and cause difficulty breathing. It also harms sensitive trees and crops. It's different than the 'good' ozone up in the stratosphere, which protects life on Earth from the sun's harmful ultraviolet rays.
The team looked at levels in the spring when wind and weather patterns push Asian pollution across the Pacific Ocean, said Meiyun Lin, lead scientist. In the summer, when those weather patterns subside, ozone levels in the national parks remained well above normal.
Asian air pollution was, by far, the biggest contributor to smog in the West, the researchers found. The team also looked at other factors, such as wildfires and methane from livestock. Asian air pollution contributed as much as 65% of the western U.S. ozone increase, while wildfire emissions supplied less than 10% and methane about 15%.
Since 1992, Asia has tripled its emissions of smog-forming chemicals such as nitrogen oxides. Though China and India are the worst offenders, South Korea and Japan also contribute, said Lin, who is also a research scholar at Princeton University. The smog levels in the western U.S. have increased each year despite a 50% reduction in U.S. emissions of smog-forming pollutants.
"Twenty years ago, scientists first speculated that rising Asian emissions would one day offset some of the United States' domestic ozone reductions," said Owen Cooper, a senior research scientist.
 Lin et al: US surface ozone trends and extremes from 1980 to 2014: quantifying the roles of rising Asian emissions, domestic controls, wildfires, and climate in Atmospheric Chemistry and Physics – 2017