Stratospheric contribution to high surface ozone in Colorado during springtime

The contribution of stratosphere‐to‐troposphere transport to high surface ozone (O 3 ) along the Colorado Front Range during spring of 1999 is examined using lidar and surface measurements. A deep tropopause fold brought ∼215 ppbv of O 3 to within 1 km of the highest peaks in the Rocky Mountains on 6 May 1999. One‐minute average O 3 mixing ratios exceeding 100 ppbv were subsequently measured at a surface site in Boulder, and daily maximum 8‐hour O 3 concentrations greater or equal to the 2008 NAAQS O 3 standard of 0.075 ppmv were recorded at 3 of 9 Front Range monitoring stations. Other springtime peaks in surface O 3 are also shown to coincide with passage of upper level troughs and dry stable layers aloft. These results show that the stratospheric contribution to surface ozone is significant, and can lead to exceedance of the 2008 NAAQS O 3 standards in a major U.S. metropolitan area.