Abstract
Abstract. An effective method is presented for determining the ozone (O3) mixing ratio in the onshore flow of marine air at the North American west coast. By combining the data available from all marine boundary layer (MBL) sites with simultaneous wind data, decadal temporal trends of MBL O3 in all seasons are established with high precision. 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 trends are found for all seasons except autumn, which does have a significantly smaller trend than 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 present a striking comparison 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 O3 mixing ratios differ markedly, with the marine air arriving at Europe in all seasons containing 7±2 ppbv higher ozone than marine air arriving at North America. Further, the ozone mixing ratios at the North American west coast show no indication of stabilizing as has been reported for Mace Head. In a larger historical context the background boundary layer O3 mixing ratios 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 the MBL of the Pacific coast region of North America. The reproduction of the increasing trends in MBL O3 over the past two decades, as well as the difference in the O3 mixing ratios 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, historical trends may an even greater challenge.
Highlights
Establishing the temporal trends of background O3 in the northern mid latitudes is very important to our understanding of the budget of tropospheric O3
In an effort to reach a resolution of this disagreement, the first goal of this paper is to comprehensively review the available observations and analyses that provide the basis for determining the O3 temporal trend at the North American west coast, and to define that temporal trend as accurately and precisely as possible
The ability to discern temporal trends in marine boundary layer (MBL) O3 from the relatively short, decadal time scale data records that are available is limited by two factors: first, our ability to accurately and precisely determine the average O3 mixing ration in each year, and second, the interannual variability in the average O3 between years that systematically differs from the assumed functional form of the temporal trend
Summary
Establishing the temporal trends of background O3 in the northern mid latitudes is very important to our understanding of the budget of tropospheric O3. Measurements within the marine air inflow provide the potential to sample O3 in an environment without dominant confounding continental influences such as in situ O3 production from locally emitted precursors, transport of air masses with widely varying histories, surface deposition, reaction of O3 with local emissions, etc Temporal changes in these influences undoubtedly contribute to the significant regional differences found in O3 trends at northern mid latitudes (Oltmans et al, 2006; Jaffe and Ray, 2007). If trends in background O3 can be established for the downwind sides of the two large oceans at northern mid latitudes, they are likely of more fundamental importance to our understanding of the background tropospheric O3 budget than are the variable regional trends over the continents To characterize these marine trends from land surface based measurements requires careful discrimination against continental effects in the analysis of the available data sets.
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