Tropospheric ozone increases over the southern Africa region: bellwether for rapid growth in Southern Hemisphere pollution?
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 urba...
| Main Authors: | , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2014-09-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | http://www.atmos-chem-phys.net/14/9855/2014/acp-14-9855-2014.pdf |
| Summary: | 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<sup>−1</sup>. 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<sup>−1</sup> over Irene and
~2 ppbv yr<sup>−1</sup> over Réunion. These changes are equivalent to
~25 and 35–45% decade<sup>−1</sup>, respectively. Both stations also
display smaller positive trends in summer, with a 45% decade<sup>−1</sup>
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
NO<sub>x</sub> emissions trends estimated for Africa, South America and
Madagascar. For Réunion trajectories also point to sources from the
eastern Indian Ocean and Asia. |
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| ISSN: | 1680-7316 1680-7324 |