Interannual Variability in Baseline Ozone and Its Relationship to Surface Ozone in the Western U.S.

Abstract

Baseline ozone refers to observed concentrations of tropospheric ozone at sites that have a negligible influence from local emissions. The Mount Bachelor Observatory (MBO) was established in 2004 to examine baseline air masses as they arrive to North America from the west. In May 2012, we observed an O₃ increase of 2.0–8.5 ppbv in monthly average maximum daily 8-hour average O₃ mixing ratio (MDA8 O₃) at MBO and numerous other sites in the western U.S. compared to previous years. This shift in the O₃ distribution had an impact on the number of exceedance days. We also observed a good correlation between daily MDA8 variations at MBO and at downwind sites. This suggests that under specific meteorological conditions, synoptic variation in O₃ at MBO can be observed at other surface sites in the western U.S. At MBO, the elevated O₃ concentrations in May 2012 are associated with low CO values and low water vapor values, consistent with transport from the upper troposphere/lower stratosphere (UT/LS). Furthermore, the Real-time Air Quality Modeling System (RAQMS) analyses indicate that a large flux of O₃ from the UT/LS in May 2012 contributed to the observed enhanced O₃ across the western U.S. Our results suggest that a network of mountaintop observations, LiDAR and satellite observations of O₃ could provide key data on daily and interannual variations in baseline O₃