Harmonisation and trends of 20-years tropical tropospheric ozone data

Abstract

Using the convective clouds differential (CCD) method on total ozone and cloud data from three European satellite instruments GOME/ERS-2 (1995–2003), SCIAMACHY/Envisat (2002–2012), and GOME-2/MetOp-A (2007–2015) it is possible to retrieve tropical tropospheric columns of ozone (TTCO) which are in good agreement with in-situ measurements biases less than 6.4 DU and the root mean square (RMS) deviation less than 13 DU. Small differences in TTCO between the individual instruments are evident and therefore the individual datasets retrieved are harmonised into one consistent time-series starting from 1996 until 2015. Correction offsets (bias) between the instruments using SCIAMACHY as intermediate reference have been calculated and six different harmonisation scenarios have been tested. Finally, the datasets have been harmonised applying no correction to GOME data while GOME-2 has been corrected using for each grid-box the mean bias with respect SCIAMACHY for the years of common operation (2007–2012). Depending on the choice of harmonisation, the magnitude, pattern, and uncertainty of the trend can strongly vary. The harmonisation represents an additional source of uncertainty in the merged dataset and derived trend estimates In contrast with other studies we found that tropospheric ozone does not display a statistically significant trend in global tropics (20o S-20o N). The trend of the tropically averaged tropospheric ozone is equal to 0±0.64 DU decade−1 (2σ). Regionally, tropospheric ozone has a statistically significant increase by ∼3 DU decade−1 over southern Africa (∼1.5 % year−1 ), the southern tropical Atlantic (∼1.5 % year−1 ), southeastern tropical Pacific Ocean (∼1 % year−1 ), and central Oceania (∼2 % year−1 ) and by ∼2 DU decade−1 over central Africa (2–2.5 % year−1 ) and south India (∼1.5 % year−1 ). These regional positive tropospheric ozone trends maybe linked to anthropogenic activities such as emissions in mega cities or biomass burning in combination with changes in meteorology or/and long range transport of precursor emissions. On the other hand, tropospheric O3 decreases by ∼-3 DU decade−1 over the Caribbean sea and parts of the North Pacific Ocean (∼ -2 % year−1), and by less than -2 DU decade−1 over some regions of the southern Pacific and Indian Ocean (∼ -0.5 – -1 % year−1 ). The comparison of the calculated trends from the current study with tropospheric ozone trends from Heue et al. (2016) and Ebojie et al. (2016) in ten selected mega-cities showed that they agree within 2σ of the trend uncertainty.