Seminar

Abstract

The influence of stratosphere-to-troposphere transport (STT) on surface ozone (O₃) has a long and somewhat controversial history. Prior to the 1980s, it was generally believed that most tropospheric O₃ outside of urban areas originated from the stratosphere. This view began to change in the late 1970s and it is now generally accepted that tropospheric O₃ is maintained in steady state by photochemical production and destruction, with a smaller influx from the stratosphere. The estimated magnitude of the stratospheric influx has gradually declined since the 1980s, and the GEOS-Chem modelling studies (Fiore et al., 2003) used by the EPA to estimate background ozone in the U.S. imply that the stratospheric contribution to surface ozone is negligible, a complete reversal from the earlier paradigm.

Observations suggest a situation somewhere between these two extremes. Measurements from remote high elevation surface sites show that episodic influxes of stratospheric ozone do affect surface O₃, and recently published measurements from the Denver-Boulder area show evidence of a stratospheric intrusion directly contributing to an exceedance of the National Ambient Air Quality Standard (NAAQS) for O₃. In this talk, I will describe these earlier measurements, and use surface, aircraft, and ozonesonde measurements, along with FLEXPART trajectories, to show an example of STT affecting surface O3 in the Los Angeles area that occurred during the CalNex and IONS-2010 campaigns. This intrusion led to a peak 1-h O3 concentration of 88 ppbv at Joshua Tree National Monument on May 28, 2010 with widespread entrainment of stratospheric air into the boundary layer that increased the local background O₃ to ~55 ppbv on May 29-30. This background was 10-15 ppbv higher than the baseline O3 in air transported ashore from the Pacific Ocean, and when combined with locally produced O3 led to several exceedances of the current National Ambient Air Quality Standard (NAAQS). These findings show that the contribution of STT to surface O₃ must be carefully considered in the ongoing discussion of whether or not to lower the current NAAQS.