Harmonisation and trends of 20-year tropical tropospheric ozone data
<p>Using a convective-cloud differential (CCD) method, developed in-house and applied to retrievals of total ozone and cloud data from three European satellite instruments (viz. GOME/ERS-2, 1995–2003; SCIAMACHY/Envisat, 2002–2012 and GOME-2/MetOp-A, 2007–2015),...
| Main Authors: | , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2018-07-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://www.atmos-chem-phys.net/18/9189/2018/acp-18-9189-2018.pdf |
| Summary: | <p>Using a convective-cloud differential (CCD) method, developed in-house and applied to retrievals of total ozone and cloud data from three
European satellite instruments (viz. GOME/ERS-2, 1995–2003;
SCIAMACHY/Envisat, 2002–2012 and GOME-2/MetOp-A, 2007–2015), monthly mean
tropical tropospheric columns of ozone (TTCO) have been retrieved, which are
in good agreement with ozonesondes (biases less than 6 DU). As small
differences in TTCO between the individual instruments were evident, it was
necessary to develop a scheme to harmonise the three datasets into one
consistent time series starting from 1996 until 2015. Correction offsets
(biases) between the instruments using SCIAMACHY as intermediate reference have
been calculated and six different harmonisation or merging scenarios have been
evaluated. Depending on the merging approach, the magnitude, pattern and
uncertainty in the trends strongly vary. The harmonisation or merging represents
an additional source of uncertainty in the trends (2 DU decade<sup>−1</sup> on
average, in most of the cases exceeding the uncertainty from the regression).
For studying further details on tropospheric ozone trends on various spatial
scales in the tropics, we stick with one preferred merged dataset that shows
best agreement with ozonesondes. In this merged dataset, no correction was
applied for GOME, and mean biases with respect to SCIAMACHY in the
overlapping period (2007–2012) were calculated and applied for GOME-2 in
each grid box (2.5° × 5°). In contrast with other
studies we found that the tropospheric trend averaged over the tropics
(−15° S to 15° N) is not statistically significant. The
mean tropospheric ozone trend equals
−0.2 ± 0.6 DU decade<sup>−1</sup> (2<i>σ</i>). Regionally,
tropospheric ozone has a statistically significant increase of
∼ 3 DU decade<sup>−1</sup> over southern Africa
( ∼ 1.5 % yr<sup>−1</sup>), the southern tropical Atlantic
( ∼ 1.5 % yr<sup>−1</sup>), southeastern tropical Pacific Ocean
( ∼ 1 % yr<sup>−1</sup>), and central Oceania
( ∼ 2 % yr<sup>−1</sup>) and by ∼ 2 DU decade<sup>−1</sup> over
central Africa (2–2.5 % yr<sup>−1</sup>) and south India
( ∼ 1.5 % yr<sup>−1</sup>). On the other hand, tropospheric O<sub>3</sub>
decreases by ∼ 3 DU decade<sup>−1</sup> over the Caribbean Sea and
parts of the North Pacific Ocean ( ∼ 2 % yr<sup>−1</sup>), and by less
than 2 DU decade<sup>−1</sup> over some regions of the southern Pacific
and Indian oceans ( ∼ 0.5–1 % yr<sup>−1</sup>).</p> |
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| ISSN: | 1680-7316 1680-7324 |