In a new study, scientists have pointed out that springtime ozone levels above western North America are increasing mainly due to air flowing eastward from the Pacific Ocean, especially when the air originates in Asia.
Such increases in ozone could make it more difficult for the United States to meet Clean Air Act standards for ozone pollution at ground level, according to the study.
"In springtime, pollution from across the hemisphere, not nearby sources, contributes to the ozone increases above western North America," said lead author Owen R. Cooper, of the NOAA-funded Cooperative Institute for Research in Environmental Sciences at the University of Colorado at Boulder. When air is transported from a broad region of south and east Asia, the trend is largest," he added.
The study focused on springtime ozone in a slice of the atmosphere from two to five miles above the surface of western North America, far below the protective ozone layer but above ozone-related, ground-level smog that is harmful to human health and crops.
Ozone in this intermediate region constitutes the northern hemisphere background, or baseline, level of ozone in the lower atmosphere.
The study was the first to pull together and analyze nearly 100,000 ozone observations gathered in separate studies by instruments on aircraft, balloons and other platforms.
Cooper and colleagues from NOAA's Earth System Research Laboratory in Boulder and eight other research institutes used historical data of global atmospheric wind records and sophisticated computer modeling to match each ozone measurement with air-flow patterns for several days before it was recorded.
This approach essentially let the scientists track ozone-producing emissions back to a broad region of origin.
When the dominant airflow came from south and east Asia, the scientists saw the largest increases in ozone measurements.
When airflow patterns were not directly from Asia, ozone still increased but at a lower rate, indicating the possibility that emissions from other places could be contributing to the ozone increases above North America.
The study used springtime ozone measurements because previous studies have shown that air transport from Asia to North America is strongest in spring, making it easier to discern possible effects of distant pollution on the North American ozone trends.