Observations of stratospheric O3 intrusions in air quality monitoring data in Ontario, Canada

Intrusions of stratospheric ozone (O3) to the troposphere at tropopause fold events are one mechanism of stratosphere–troposphere exchange, which is known to be a significant component of the tropospheric O3 budget. Although these intrusions rarely extend downward to the surface and are not observed directly, air quality measurements of ground-level O3 in Ontario have shown brief anomalous spikes in O3 concentrations coinciding with the weather conditions associated with tropopause fold events. Several events are documented in this article that occurred over southern Ontario during December 28, 2008; February 11–12, 2009; January 20, 2013; and January 31, 2013. Each case presents several examples of sudden increases in O3 concentrations (generally about 10–20 ppb above a background of about 30–40 ppb) along with the related synoptic weather factors associated with tropopause folds and stratospheric O3 intrusions.