MARCH 2002: Stratospheric Versus Pollution Influences on Ozone at Bermuda

Figure 1: Daily mean surface ozone concentrations at Bermuda in March-June 1996. The contributions from different source regions are shown using the tagged ozone tracers. ``LT N. America'' refers to ozone produced in the lower troposphere (700 hPa-surface) over North America and the western North Atlantic (25-75°N, 50-160°W). ``MT'' refers to ozone produced in the middle troposphere (400-700 hPa). ``UT/ST'' refers to ozone produced in the upper troposphere (400hPa-tropopause) and the stratosphere. ``Other'' refers to ozone produced in the lower troposphere outside of North America and the western North Atlantic (25-75°N and 50-160°W).

Figure 2: Isentropic back-trajectory computed from the GEOS meteorological fields at 290 K potential temperature for air arriving at Bermuda at 0000 UTC, March 16, 1996. Shown is the average trajectory for 25 parcels ending within the area of 30-34°N and 64-68°W. The parcels are initialized at a horizontal resolution of 1° × 1°. The error bars show the standard deviations for longitude, latitude, and pressure at each 24-hour interval. The color contours show the GEOS-CHEM daily mean net ozone production (ppbv day^-1) sampled every 6 hours along the trajectory.

Conflicting interpretations of the spring ozone maximum observed at Bermuda (32°N, 65°W) have fueled the debate on stratospheric influence versus tropospheric production as sources of tropospheric ozone. We use the GEOS-CHEM global 3-D model to reconcile these past interpretations. The model captures the springtime day-to-day variability driven by high-ozone events (Figure 1) and reproduces the observed seasonal cycle. We find that boundary layer 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 observed positive ozone correlation with ⁷Be reflects the strong subsidence behind cold fronts, and the positive ⁷Be:²¹⁰Pb correlation indicates the mixing of middle-tropospheric air with continental boundary layer outflow. 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 the GEOS-CHEM model reproduce the previously reported subsidence associated with with high-ozone events (Figure 2). Even in the free troposphere we find that the stratosphere contributes less than 5 ppbv (< 10%) to spring ozone over Bermuda. 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. This work was led by Qinbin Li and a full account is given in Li et al. [2002].