The influence of stratosphere-to-troposphere transport (STT) on surface ozone (O3) concentrations in the greater Los Angeles area during the CalNex and IONS-2010 measurement campaigns has been investigated. Principal component analysis (PCA) of surface O3measurements from 41 sampling stations indicates that ∼13% of the variance in the maximum daily 8-h average (MDA8) O3between May 10 and June 19, 2010 was associated with changes of 2–3 day duration linked to the passage of upper-level troughs. Ozonesondes launched from Joshua Tree National Park and airborne lidar measurements show that these changes coincided with the appearance of stratospheric intrusions in the lower troposphere above southern California. The Lagrangian particle dispersion model FLEXPART reproduces most of these intrusions, and supports the conclusion from the PCA that significant transport of stratospheric air to the surface occurred on May 28–30. This intrusion led to a peak 1-h O3 concentration of 88 ppbv at Joshua Tree National Monument near the ozonesonde launch site on May 28, and widespread entrainment of stratospheric air into the boundary layer increased the local background O3 over the entire area 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) on the following day.