Stratospheric versus pollution influences on ozone at Bermuda: Reconciling past analyses

Conflicting interpretations of the spring ozone maximum observed at Bermuda (32degreesN, 65degreesW) have fueled the debate on stratospheric influence versus tropospheric production as sources of tropospheric ozone. We use a global three-dimensional (3-D) model of tropospheric ozone-NOx-hydrocarbon chemistry driven by assimilated meteorological observations to reconcile these past interpretations. The model reproduces the observed seasonal cycle of surface ozone at Bermuda and captures the springtime day-to-day variability (r=0. 82, n=122, p<0.001) driven by high-ozone events. We find that transport of North American pollution behind cold fronts is the principal contributor to springtime surface ozone at Bermuda and is responsible for all the high-ozone events. The model reproduces the observed positive correlations of surface ozone with Be-7 and Pb-210 at Bermuda; the correlation with Be-7 reflects the strong subsidence behind cold fronts, resulting in the mixing of middle-tropospheric air with continental outflow in the air arriving at Bermuda, as indicated by the positive Be-7-Pb-210 correlation. This mixing appears to have been an obfuscating factor in past interpretations of subsiding back-trajectories at Bermuda as evidence for a stratospheric or upper tropospheric origin for ozone. Isentropic back-trajectories computed in our model reproduce the previously reported subsidence associated with high-ozone events. Even in the free troposphere, we find that the stratosphere contributes less than 5 ppbv (<10%) to spring ozone over Bermuda. Positive O-3-Be-7 and negative O-3-Pb-210 correlations observed at Tenerife (28degreesN, 16degreesW, 2.4 km) in summer are reproduced by the model and are consistent with a middle-ropospheric source of ozone, not an upper tropospheric or stratospheric source as previously suggested. A regional budget for the North Atlantic in spring indicates that the stratosphere contributes less than 10 ppbv ozone (<5%) below 500 hPa, while the lower troposphere contributes 20-40 ppbv ozone throughout the troposphere.