Figure 2. Ozonesondes in support of SEACIONS were launched at
the Saint Louis University ozonesonde station located at the James S.
McDonnell Planetarium in Forest Park (90.27˚W, 38.63˚N, 181 m asl), 5 km
west of downtown St. Louis. Vertical tropospheric profiles at
~18:30 UTC (1:30 p.m. Local time) ozone profiles are
averaged vertically every 500 m and shown below the thermal tropopause
level (~15 km). Cases of ozone enhancements above the
background (~55 ppbv) are shown by source origin: the
gray box for Stratospheric-Tropospheric Transport (STT), the solid black
line boxes for biomass burning (BB), and the dashed line black boxes for
combined STT and BB.
The vertically averaged near-daily profiles of ozone are shown in Figure
2. The profiles are averaged every 500 m vertically from the surface to
the tropopause (~15 km) with annotations for enhanced
ozone rich plumes, above the tropospheric column mean (55.7 ppbv), from
biomass burning and stratospheric intrusions. Ozone gradually increases
with altitude in the lower troposphere (below about 3.5 km). Average
ozone is about 55 ppbv near the surface but decreases to 45 ppbv near
3.5 km. Above 3.5 km, ozone increases slowly throughout the entire
troposphere a rate of 3 ppbv km-1. In the middle
troposphere (4 to 10 km, height), average ozone is highest in August and
lowest in September. While above 10 km, ozone is similar for both
months. August profiles averaged 10 to 15 ppbv higher than September.
Ozone is lowest (~40 ppbv) during the month of September
near the surface and remains low below 3.5 km. However, on 5 September
the second highest recorded surface ozone day occurred at the surface
(60 ppbv ozonesonde, 95 ppbv nearby surface monitor later in the day).
The maximum measured ozone from a sonde at the surface occurred 30
August at 65 ppbv. Other notable enhancements (NAAQS exceedances) of
O3 (> 70 ppb near the surface) occurred on
23 August, and 4 to 6 and 10 to 11 September. A mean PBL of 2.4 km and a
mean thermal tropopause at 14.9 km could be clearly identified in the
ozonesonde profiles. Note that these figures for biomass burning
contributions from boreal fire contributions to ozone downwind are
somewhat larger than the 5 to 15% estimates of this quantity by
Thompson et al. (2011; see Figure 13) or Moeini et al. (2020) that are
based on Canadian soundings. These last two studies attribute 20 to 25%
of the ozone column amounts to stratospheric origins.
4.2. Stratospheric-tropospheric transport tracer impacts
Table 2 lists the GEOS-5 determined stratospheric instructions events
that impacted St. Louis during August and September 2013, expressed as
deep or shallow with the pressure level 600 hPa being the threshold. The
44 day study period consisted of 13 events. The intrusions recorded
resulted from either a shortwave trough (6 days: 7, 10, 23, 28, 30
August and 6 September), frontal passage (7 days: 3, 12 August and 3,
12, 15, 22 September), or cut-off low (1 day: 14 August). Three frontal
pass intrusions reached the lower troposphere and penetrated the
boundary layer. About 40% of the intrusions reached the middle
troposphere (~500 hPa). Stratospheric intrusion
generally lasted 1 to 3 days occurring either early morning before 3:00
(UTC – 5 h) or around noon local time. The intrusions where found to
contribute on average 10 to 25 ppbv to tropospheric ozone columns which
equates to around 5 to 15% of the total tropospheric ozone column.