Harmonisation and trends of 20-year tropical tropospheric ozone data

Abstract

Abstract. 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−1 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−1 (2σ). Regionally, tropospheric ozone has a statistically significant increase of ∼ 3 DU decade−1 over southern Africa ( ∼ 1.5 % yr−1), the southern tropical Atlantic ( ∼ 1.5 % yr−1), southeastern tropical Pacific Ocean ( ∼ 1 % yr−1), and central Oceania ( ∼ 2 % yr−1) and by ∼ 2 DU decade−1 over central Africa (2–2.5 % yr−1) and south India ( ∼ 1.5 % yr−1). On the other hand, tropospheric O3 decreases by ∼ 3 DU decade−1 over the Caribbean Sea and parts of the North Pacific Ocean ( ∼ 2 % yr−1), and by less than 2 DU decade−1 over some regions of the southern Pacific and Indian oceans ( ∼ 0.5–1 % yr−1).

Tropospheric Ozone Assessment Report

“…However, the products did not agree regarding short-term trends (2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016), and future research led by TOAR will explore the reasons for this discrepancy. Current research on long-term ozone trends has highlighted the power of combining satellite data sets to quantify ozone trends, including those since the late 1970s (Ziemke et al, 2019) and the mid-1990s (Heue et al, 2016;Leventidou et al, 2018). In the next decade, planned satellite measurements of ozone and ozone precursors will be acquired from both low earth orbit (LEO) and geostationary orbit (GEO).…”

TROPOMI tropospheric ozone column data: Geophysical assessment and comparison to ozonesondes, GOME-2B and OMI

“…Unlike other TROPOMI atmospheric data products, the tropospheric ozone column data are not retrieved directly from the radiance data using an inversion scheme. Rather, it is derived from total ozone column and cloud data using the Convective Cloud Differential approach (CCD), a technique that has been applied successfully to many other sensors such as TOMS, GOME, SCIAMACHY, OMI and GOME-2 (Ziemke et al, 1998;Valks et al, 2003Valks et al, , 2014Heue et al, 2016;Leventidou et al, 2016Leventidou et al, , 2018.…”

Trends in global tropospheric ozone inferred from a composite record of TOMS/OMI/MLS/OMPS satellite measurements and the MERRA-2 GMI simulation

“…Heue et al (2016) indicated that significant positive trends occurred over central and southern Africa that maximized during June-August, which represents the peak burning season for this region; they suggested that the trends in centralsouthern Africa are associated with an increase in biomass burning. Leventidou et al (2018), using similar (but processed differently) SCIAMACHY/GOME CCD TCO measurements for 1995-2015, found ∼ +3 DU decade −1 trend over southern Africa, but no statistical change in the tropics (15…”