Air pollution from Asia wafting to USA

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 finds.

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.

Scientists measured ozone levels recorded at springtime for the past 25 years in 16 national parks in the western U. S., including Yellowstone, Yosemite and Grand Canyon.

The parks’ locations farther away from cities, where smog is typically expected, made them ideal spots for the study.

The team looked at levels in the spring when wind and weather patterns push Asian pollution across the Pacific Ocean, said Meiyun Lin, a scientist at the National Oceanic and Atmospheric Administration who led the study. In the summer, when those weather patterns subside, ozone levels in the national parks remained well above normal.

Since 1992, Asia has tripled its emissions of smog- forming chemicals such as nitrogen oxides. Though China and India are the worst offenders, North and South Korea and Japan also contribute, Lin said.

Asian pollution only slightly contributes to smog in the eastern U. S., the study found. Levels there typically spike during intense summer heat waves.

The study was published Wednesday in the journal Atmospheric Chemistry and Physics.