Climate Chnage: China Asian ozone pollution covers Pacific and US West Coast

China Asian pollution drifts east toward North America in 2010. 

In this picture Hawaii is denoted by the star.

Image: Nature Geoscience.

Air pollution from China and Asia has been rising for several decades but Hawaii had seemed to escape the ozone pollution that drifts east with the springtime winds.

Now a team of researchers has found that shifts in atmospheric circulation explain the trends in Hawaiian ozone pollution.

The researchers found that since the mid-1990s, these shifts in atmospheric circulation have caused China and Asian ozone pollution reaching Hawaii to be relatively low in spring but rise significantly in autumn.

The study, led by Meiyun Lin, an associate research scholar in the Program in Atmospheric and Oceanic Sciences (AOS) at Princeton University and a scientist at the National Oceanic and Atmospheric Administration's (NOAA) Geophysical Fluid Dynamics Laboratory, was published in Nature Geoscience.

"The findings indicate that decade-long variability in climate must be taken into account when attributing U.S. surface ozone trends to rising China and Asian emissions," Lin said.

Although protective at high altitudes, ozone near the Earth's surface is a greenhouse gas and a health-damaging air pollutant. The longest record of ozone measurements in the U.S. dates back to 1974 in Hawaii.

Over the past few decades, emissions of ozone precursors in China and Asia has tripled, yet the 40-year Hawaiian record revealed little change in ozone levels during spring, but a surprising rise in autumn.

Through their research, Lin and her colleagues solved the puzzle. "We found that changing wind patterns 'hide' the increase in China and Asian pollution reaching Hawaii in the spring, but amplify the change in the autumn," Lin said.

Using chemistry-climate models and observations, Lin and her colleagues uncovered the different mechanisms driving spring versus autumn changes in atmospheric circulation patterns.

The stronger transport of China and Asian pollution to Hawaii during autumn since the mid-1990s corresponds to a positive pattern of atmospheric circulation variability known as the Pacific-North American pattern (PNA).

"This study not only solves the mystery of Hawaiian ozone changes since 1974, but it also has broad implications for interpreting trends in surface ozone levels globally," Lin said.

"Characterizing shifts in atmospheric circulation is of paramount importance for understanding the response of surface ozone levels to a changing climate and evolving global emissions of ozone precursors," she said.

More information: Tropospheric ozone trends at Mauna Loa Observatory tied to decadal climate variability; Nature Geoscience (2014) doi:10.1038/ngeo2066