AUGUST 2002: Future Increases in the Global Ozone Background Could Thwart U.S. Efforts to Abate Ozone Pollution

The figure above shows the number of GEOS-CHEM model grid-square days per month in the United States with afternoon (1-5 p.m. local time) ozone concentrations in surface air in excess of a 70 ppbv threshold for a 1995 base case simulation (black bars) and a simulation of the 2030 atmosphere based upon emissions projections from the IPCC [2001] A1 scenario (red bars).  In the 2030 A1 simulation, global emissions of ozone precursors increase, but the distribution shifts from the developed world to the developing world.  In the United States, emissions of ozone precursors decline by 20-40% relative to 1995.  A 70 ppbv threshold is used here as a metric to gauge changes in the frequency of ozone pollution events in our simulations.  The 2030 A1 simulation reveals a longer ozone season, with increased exceedances of the 70 ppbv threshold as compared to 1995.  Results are similar for 60 and 80 ppbv thresholds. These changes are attributed to the rise in the global ozone background that occurs in the 2030 A1 simulation, in part due to substantial increases in methane emissions.  More details about this work, including the role of methane in linking ozone pollution and climate change mitigation objectives, are available in our recently submitted paper, Fiore et al. [2002].