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.