Abstract
Abstract. Increases in free-tropospheric (FT) ozone based on ozonesonde records from the early 1990s through 2008 over two subtropical stations, Irene (near Pretoria, South Africa) and Réunion (21° S, 55° E; ~2800 km NE of Irene in the Indian Ocean), have been reported. Over Irene a large increase in the urban-influenced boundary layer (BL, 1.5–4 km) was also observed during the 18-year period, equivalent to 30% decade−1. Here we show that the Irene BL trend is at least partly due to a gradual change in the sonde launch times from early morning to the midday period. The FT ozone profiles over Irene in 1990–2007 are re-examined, filling in a 1995–1999 gap with ozone profiles taken during the Measurements of Ozone by Airbus In-service Aircraft (MOZAIC) project over nearby Johannesburg. A multivariate regression model that accounts for the annual ozone cycle, El Niño–Southern Oscillation (ENSO) and possible tropopause changes was applied to monthly averaged Irene data from 4 to 11 km and to 1992–2011 Réunion sonde data from 4 to 15 km. Statistically significant trends appear predominantly in the middle and upper troposphere (UT; 4–11 km over Irene, 4–15 km over Réunion) in winter (June–August), with increases ~1 ppbv yr−1 over Irene and ~2 ppbv yr−1 over Réunion. These changes are equivalent to ~25 and 35–45% decade−1, respectively. Both stations also display smaller positive trends in summer, with a 45% decade−1 ozone increase near the tropopause over Réunion in December. To explain the ozone increases, we investigated a time series of dynamical markers, e.g., potential vorticity (PV) at 330–350 K. PV affects UT ozone over Irene in November–December but displays little relationship with ozone over Réunion. A more likely reason for wintertime FT ozone increases over Irene and Réunion appears to be long-range transport of growing pollution in the Southern Hemisphere. The ozone increases are consistent with trajectory origins of air parcels sampled by the sondes and with recent NOx emissions trends estimated for Africa, South America and Madagascar. For Réunion trajectories also point to sources from the eastern Indian Ocean and Asia.
Highlights
We are motivated to assess tropospheric ozone trends for two reasons: (1) air quality, where surface data are typically used, and (2) climate perturbations, where free-tropospheric ozone exercises a positive radiative forcing (Shindell et al, 2006)
Thompson et al.: Tropospheric ozone increases over the southern Africa region ozone precursors, NOx, carbon monoxide (CO) and volatile organic compounds (VOCs) that originate from a range of anthropogenic and natural processes (Oltmans et al, 2013)
Due to the strength of upper troposphere (UT) ozone increases over Irene and Réunion, we look for evidence of dynamical changes, using potential vorticity as a proxy for stratospheric influence
Summary
We are motivated to assess tropospheric ozone trends for two reasons: (1) air quality, where surface data are typically used, and (2) climate perturbations, where free-tropospheric ozone exercises a positive radiative forcing (Shindell et al, 2006). In order to examine possible ozone trends in the FT in the J-P region, Clain et al (2009) used sonde data from the SHADOZ (Southern Hemisphere Additional Ozonesondes; Thompson et al, 2003, 2012) station at Irene (25.9◦ S, 28.2◦ E). Clain et al (2009) hypothesized that the Irene and Réunion ozone growth in the lower and middle troposphere could be associated with increases in industrialization and biomass burning. They pointed out that the Réunion UT ozone increases occur when STE processes are most prevalent.
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