Increasing ozone in marine boundary layer inflow at the west coasts of North America and Europe

摘要

An effective method is presented for determining the ozone (O) mixingratio in the onshore flow of marine air at the North American west coast. Bycombining the data available from all marine boundary layer (MBL) sites withsimultaneous wind data, decadal temporal trends of MBL O in allseasons are established with high precision. The average springtime temporaltrend over the past two decades is 0.46 ppbv/yr with a 95% confidencelimit of 0.13 ppbv/yr, and statistically significant trends are found forall seasons except autumn, which does have a significantly smaller trendthan other seasons. The average trend in mean annual ozone is 0.34±0.09 ppbv/yr.These decadal trends at the North American west coast presenta striking comparison and contrast with the trends reported for the Europeanwest coast at Mace Head, Ireland. The trends in the winter, spring andsummer seasons compare well at the two locations, while the Mace Head trendis significantly greater in autumn. Even though the trends are similar, theabsolute O mixing ratios differ markedly, with the marine air arrivingat Europe in all seasons containing 7±2 ppbv higher ozone than marineair arriving at North America. Further, the ozone mixing ratios at the NorthAmerican west coast show no indication of stabilizing as has been reportedfor Mace Head. In a larger historical context the background boundary layerO mixing ratios over the 130 years covered by available data haveincreased substantially (by a factor of two to three), and this increasecontinues at present, at least in the MBL of the Pacific coast region ofNorth America. The reproduction of the increasing trends in MBL O overthe past two decades, as well as the difference in the O mixing ratiosbetween the two coastal regions will present a significant challenge forglobal chemical transport models. Further, the ability of the models to atleast semi-quantitatively reproduce the longer-term, historical trends mayan even greater challenge.