Ozone pollution in N. America linked to Asia

Ozone levels over the west coast of North America are rising because of air streams carrying the pollutant from Asia, an international team of researchers says.

The analysis of springtime ozone data collected since 1984 indicatesthe U.S. may be unable to reach its Clean Air Act goals because of ozone emissions from overseas.

Ozone is an essential component of the upper atmosphere, filtering ultraviolet light from the sun. At ground level, though, it is a pollutant created when chemicals released when fossil fuels that are burned react with sunlight.

Ozone is a component of smog andcan cause respiratory illnesses in people and can damage crops, as well as plants in natural ecosystems.

"In springtime, pollution from across the hemisphere, not nearby sources, contributes to the ozone increases above western North America," said Owen Cooper of the Co-operative Institute for Research in Environmental Sciences at the University of Colorado at Boulder.

The study, published this week in the journal Nature,used data collected in the spring because previous research found that airflow from Asia to North America is highest in the spring, meaning the pollution effects would be more pronounced and measurable.

High-altitude tests

The researchers used 100,000 ozone measurements collected from research balloons and aircraft, between three and eight kilometres above the ground below the ozone layer, but above ground-level pollution.

A large share of the data came from instruments installed on commercial aircraft, a program European scientists started in 1994.

Most of the data were collected between 1995 and 2008, but the scientists also used a large set of ozone measurements from 1984.

Nine research institutes, including the Meteorological Service of Canada, used global atmospheric wind records and computer modelling to match each ozone reading with data on air flow collected several days before the ozone recording.

In this way, the scientists tracked the ozone back to a broad region of origin.

They found that springtime ozone level increased 14 per cent from 1995 to 2008. When the data from 1984 were included, the results showed an increase of 29 per cent over those years.

"This study did not quantify how much of the ozone increase is solely due to Asia," Cooper said. "But we can say that the background ozone entering North America increased over the past 14 years and probably over the past 25 years."