A rigorous method is presented for determining the ozone concentration in the onshore flow of marine air at the North American west coast. By combining the data available from all marine boundary layer sites with simultaneous wind data, decadal temporal trends of MBL ozone concentrations in all seasons are established with high precision. 5 The average springtime temporal trend over the past two decades is 0.46 ppbv/yr with a 95% confidence limit of 0.13 ppbv/yr, and statistically significant

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... ends are found for all seasons except autumn, which does have a significantly smaller trend than other seasons. The average trend in mean annual ozone concentration is 0.34±0.09 ppbv/yr. These decadal trends at the North American west coast present a striking comparison 10 and contrast with the trends reported for the European west coast at Mace Head, Ireland. The trends in the winter, spring and summer seasons compare well at the two locations, while the Mace Head trend is significantly greater in autumn. Even though the trends are similar, the absolute ozone concentrations differ markedly, with the marine air arriving at Europe in all seasons containing 7±2 ppbv higher ozone concentra-15 tions than marine air arriving at North America. Further, the ozone concentrations at the North American west coast show no indication for stabilizing as has been reported for Mace Head. In a larger historical context the background boundary layer ozone concentrations over the 130 years covered by available data have increased substantially (by a factor of two to three), and this increase continues at present, at least in 20 the marine boundary layer (MBL) of the Pacific coast region of North America. The reproduction of the increasing trends in MBL ozone concentrations over the past two decades as well as the difference in the ozone concentrations between the two coastal regions will present a significant challenge for global chemical transport models. Further, the ability of the models to at least semi-quantitatively reproduce the longer-term, 25 historical trends may an even greater challenge. 13848 15