Increasing background ozone during spring on the west coast of North America

Using a 15‐year record of O 3 from Lassen Volcanic National Park, a rural elevated site in northern California, data from two aircraft campaigns conducted in 1984 and 2002 over the eastern North Pacific, and observations spanning 18 years from five U.S. west coast, marine boundary layer sites, we show that O 3 in air arriving from the Eastern Pacific in spring has increased by approximately 10 ppbv, i.e. 30% from the mid 1980s to the present. This positive trend in O 3 correlates with the increasing trend in global nitrogen oxide emissions, which is especially pronounced in Asia. As spring is the season of strongest transport of Asian emissions to the Pacific, we conclude that the emission trend is the most likely cause of the O 3 trend.