Abstract

Conflicting interpretations of the spring ozone maximum observed at Bermuda (32^oN, 65^oW) 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 ⁷Be and ²¹⁰Pb at Bermuda; the correlation with ⁷Be 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 ⁷Be-²¹⁰Pb correlation. This mixing appears to have been an obfuscating factor in past interpretations of subsiding backtrajectories 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 O3-⁷Be and negative O3-²¹⁰Pb correlations observed at Tenerife (28^oN, 16^oW, 2.4 km) in summer are reproduced by the model and are consistent with a middletropospheric 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.