MAY 2001: U.S. pollution influence on surface ozone at Bermuda in spring

Time series of surface ozone concentrations at Bermuda in March-May 1996. Measurements from S. Oltmans (green) are compared to a simulation with the GEOS-CHEM model. The model reproduces closely the observations including their day-to-day variability. High concentrations are associated with cold fronts sweeping from North America across the North Atlantic. Low concentrations are associated with tropical air. We isolated in this simulation the contributions to ozone from different source regions with a tagged tracer method [Wang et al., 1998]. We identify in the figure the contributions from ozone production in the lower troposphere over North America (LT; below 700 hPa), which is mostly from anthropogenic pollution, in the middle tropsophere (MT; 400-700 hPa), in the upper troposphere/lower stratosphere (UT/LS; above 400 hPa), and "other" (mostly LT over North Atlantic). We find that U.S. pollution dominates the supply of ozone over Bermuda in spring, in contrast to previous studies that had argued for a major stratospheric influence. Such a large pollution influence at a relatively remote site demonstrates the intercontinental transport of ozone pollution and the global influence of U.S. pollution on tropospheric ozone. This work was carried out by Qinbin Li and a preliminary account is presented in Li et al. [2001].