In a series of 10-day campaigns in Ontario and Quebec, Canada, between 2005 and 2007,
ozonesondes were launched twice daily in conjunction with continuous high-resolution
wind-profiling radar measurements. Windprofilers can measure rapid changes in the
height of the tropopause, and in some cases follow stratospheric intrusions. Observed
stratospheric intrusions were studied with the aid of a Lagrangian particle dispersion
model and the Canadian operational weather forecast system. Definite stratosphere-troposphere
transport (STT) events occurred approximately every 2–3 days during the spring and
summer campaigns, whereas during autumn and winter, the frequency was reduced to every
4–5 days. Although most events reached the lower troposphere, only three events appear
to have significantly contributed to ozone amounts in the surface boundary layer.
Detailed calculations find that STT, while highly variable, is responsible for an
average, over the seven campaigns, of 3.1% of boundary layer ozone (1.2 ppb), but
13% (5.4 ppb) in the lower troposphere and 34% (22 ppb) in the middle and upper troposphere,
where these layers are defined as 0–1 km, 1–3 km, and 3–8 km respectively. Estimates
based on counting laminae in ozonesonde profiles, with judicious choices of ozone
and relative humidity thresholds, compare moderately well, on average, with these
values. The lamina detection algorithm is then applied to a large dataset from four
summer ozonesonde campaigns at 18 North American sites between 2006 and 2011. The
results show some site-to-site and year-to-year variability, but stratospheric ozone
contributions average 4.6% (boundary layer), 15% (lower troposphere) and 26% (middle/upper
troposphere). Calculations were also performed based on the TOST global 3D trajectory-mapped
ozone data product. Maps of STT in the same three layers of the troposphere suggest
that the STT ozone flux is greater over the North American continent than Europe,
and much greater in winter and spring than in summer or fall. When averaged over all
seasons, magnitudes over North America show similar ratios between levels to the previous
calculations, but are overall 3–4 times smaller. This may be because of limitations
(trajectory length and vertical resolution) to the current TOST-based calculation.