Ozone Recovery from Merged Observational Data and Model
Analysis (OREGANO)
Project highlights
Near-global total ozone recovery is ongoing
Figure 1/10: Trends were analysed by applying a multiple linear regression
(MLR) to the median of five merged total ozone datasets
(WOUDC, SBUV MOD, SBUV COH, GSG, GTO) updated to the end of 2023.
Compared to the results from the last WMO/UNEP Ozone Assessment 2022
using data until 2020, the near-global ozone recovery after 1996,
the time when the cumulative amount of ozone-depleting substances in
the stratosphere (ODS) reached maximum, remains unchanged at a rate
of about +0.5%/decade. Updated from Weber et al.
(doi:10.5194/acp-22-6843-2022).
Total ozone and stratospheric ozone column trends
Figure 2/10. There is currently a debate about whether total column trends
are consistent with stratospheric column trends. The tropospheric column makes
a small contribution to the total column, and trends in tropospheric ozone could,
therefore, lead to differences in the total and stratospheric column trends.
This figure shows zonal man stratospheric column trends and uncertainties derived
from four merged limb datasets (GOZCARDS, SWOOSH, SAGE-SCIA-OMPS, and SAGE-CCI-OMPS,
red lines) as a function of latitude. They are compared with the trends of the median
of five merged total ozone datasets. From this figure, it is evident that the
contribution of zonal mean tropospheric ozone trends is within uncertainties near zero.
Requirements of ozone satellite observation for trend monitoring
Figure 3/10. For observing decadal trends on the order of a few percent,
high stability in the instrument performance is required. The stability is generally
expressed in units of %/decade, which is also termed instrument drift. Since the
typical lifetime of a satellite is seven years, trend estimates are nowadays based
upon merged datasets that extend over longer time scales. In a Monte Carlo
simulation, we estimated the trend uncertainty depending on the length of the merged
data record, the typical lifetime of individual instruments of which the long-term
dataset is composed, and (residual) bias between successive instruments. The three
panels, a, b, and c, show the results assuming different stability requirements
(0., 1., and 1,5 %/decade). For a given record length (x-axis), retrieved trends
that are below the shown trend uncertainties (y-axis) are not different from
zero with confidence. Longer lifetimes lead to larger trend uncertainties (more
persistent drift), but in the case of larger bias uncertainties, the opposite is
often true. A more effective way of reducing trend uncertainties is to have multiple
parallel missions. A cluster of three (possibly small) instruments can reduce the
trend uncertainties by about half. From Weber (doi:10.5194/amt-17-3597-2024).
Climate Data Record of Stratospheric Aerosols (CREST)
Figure 4/10. Climate-related studies need information about the distribution
of stratospheric aerosols, which influence the energy balance of the Earth’s atmosphere.
A recent paper by Sofieva et al. (10.5194/essd-16-5227-2024) presents a merged dataset
of vertically resolved stratospheric aerosol extinction coefficients, which is derived
using data from six limb and occultation satellite instruments: SAGE II on ERBS, GOMOS
and SCIAMACHY on Envisat, OSIRIS on Odin, OMPS-LP on Suomi NPP, and SAGE III on the
International Space Station. The merged time series of vertically resolved monthly
mean aerosol extinction coefficients at 750 nm is provided in 10° latitudinal bins
from 90° S to 90° N, ranging from 8.5 to 39.5 km altitude. The time series of the
stratospheric aerosol optical depth (SAOD) is created via the integration of aerosol
extinction profiles from the tropopause to 39.5 km; it is also provided as monthly
mean data in 10° latitudinal bins. The created aerosol climate record covers the period
from October 1984 until December 2023, and it is intended to be extended in the future.
It can be used in various climate-related studies. The merged CREST aerosol dataset (v2)
is available at https://doi.org/10.57707/fmi-b2share.dfe14351fd8548bcaca3c2956b17f665 and is
used as one of the proxies in ozone trend regression.
Zonal asymmetry in Arctic ozone trends
Figure 5/10. Asymmetries in ozone trends at northern high latitudes were
investigated over the 2004-2022 period by using satellite observations (the merged
SCIA+OMPS data set) and the TOMCAT chemistry transport model. This figure shows
seasonal ozone trends for SCIA+OMPS (top row) and for a full-chemistry TOMCAT run
(bottom row) at 32 km altitude for spring (MA), summer (JJA) and autumn (SO). Only
2 months are used in spring and autumn to obtain a better coverage of the polar
regions. The TOMCAT time series was masked to mirror the availability of satellite
data. During summer, the trend fields are fairly homogeneous over longitude,
displaying significant positive values of about 1% per decade for SCIA+OMPS and close
to zero for TOMCAT. In contrast, during spring and fall, the asymmetry is well
pronounced. In particular, we note a strong zonal asymmetry in the spring‐time trends
in SCIA+OMPS that is captured very well by TOMCAT, with the positive maximum located
over the North Atlantic sector. The negative values between Scandinavia and Siberia
are also statistically significant (at 2σ level) for both observations and the model.
A similar bi‐polar pattern is also found in SO, but more confined to polar latitudes
and shifted in longitude. The agreement between TOMCAT and observations is also held
in this case.
Impact of polar vortex position on Arctic ozone trends
Figure 6/10. We investigated the asymmetry in ozone trends at northern
high latitudes in connection with the position and strength of the polar vortex.
In this figure (top panels), we compare the trends of the modified potential
vorticity (PV) in the middle stratosphere (700 K) over the two periods 1980–2004
and 2000–2022. We clearly see a reversal of the pattern over the polar regions,
with panel (a) showing a shift of the polar vortex to Eurasia and panel (b)
indicating a successive shift of its mean position again toward North America
over the last 20 years. The bottom panels of the figure display the ozone trends
on the 700 K isentropic surface from the TOMCAT dataset over the same periods.
Also, for ozone changes, we note a reversal of the pattern: the negative values
were largest over the Atlantic/Scandinavian sector during the first period, and
positive values were largest in the same region during the second period. This
analysis of the polar vortex position and of the trends in potential vorticity
in the middle stratosphere suggests the relationship between the shift in the
mean polar vortex position and the ozone trend asymmetry. The overall pattern
underwent decadal changes over the last 40 years, with the most recent two decades
seeing a probable strengthening of the vortex and a shift toward North America.
From Arosio et al. 2024 (doi:10.1029/2023JD040353)
Inter-comparison of Limb-Nadir combined tropospheric ozone
Figure 7/10. We performed an inter-comparison of satellite-based
tropospheric ozone column (TrOC) datasets derived with the so-called “residual
technique”. This consists of removing the stratospheric column (SOC) from limb
observations from total column (TOC) nadir measurements to obtain TrOC as a
residual. The figure displays the annual mean climatology for seven selected
data sets. In the title of each subplot, we report the mean global TrOC values
and their standard deviations. We notice an overall bias present between the maps,
with global average ozone values differing by up to 8-10 DU (from 23.9 DU for
GTO-LIMB to 32.5 for S5P-BASCOE, with high TrOC values, especially in the northern
hemisphere). These biases have several possible reasons: e.g., the TOC and SOC
biases between different satellites, the tropopause definition adopted to construct
the residual product, the climatology used to fill stratospheric profile gaps, or
the criteria used for the subtraction between TOC and SOC. Some common features are
visible for all datasets, in particular, the wave-1 pattern in the tropics. This is
a clear zonal asymmetry in the TrOC distribution, with higher ozone concentrations
over the Atlantic and African regions and lower concentrations over the Pacific and
Indian Oceans. The main reasons for this pattern are related to the large biomass
burning taking place in the African and South American regions, but also the weaker
intensity of deep convection in the Atlantic sector. Regional hot spots of TrOC are
visible over polluted areas where precursor emissions are high, such as over China,
Southern Europe, between the Arabian Sea and India, and the west coast of the US.
Low ozone concentrations are typical over the oceans and unpolluted regions due to
limited precursor availability. From Arosio et al. (doi:10.5194/egusphere-2024-3737).
Regional tropospheric ozone trends
Figure 8/10. We used three TrOC data sets (OMI-LIMB, SCIA+OMPS and OMI-MLS)
with a coverage of the last two decades to investigate trends in specific regions of interest
due to human-related activities and their changes over the last decades. From the time series
regression model, which includes three proxies: the first two principal components of the QBO
and the Multivariate El Nino Southern Oscillation (ENSO) Index (MEI). The figure shows the
time series and the respective trends in five defined regions for the three datasets from
2004-2022. The subplots show thicker lines for the 13-month running mean of the dataset time
series averaged within each designated region. The respective MLR trend values and uncertainties
are also reported. The only region with a clear and significant positive trend in all datasets
is Southeast Asia, with positive trends up to 1.5 DU per decade for OMI-MLS but closer to zero
for OMI-LIMB. The positive trend from SCIA+OMPS in the Amazon is likely related to artefacts in
the datasets at the very beginning and end of the time period. Close to zero trends are found
in the West Africa region and the US. Summer-time trends over the US are also close to zero.
The Mediterranean region shows significant negative trends, possibly related to the EU policies
introduced to improve air quality. From Arosio et al. (doi:10.5194/egusphere-2024-3737).
Hemispheric asymmetry in HCl and feedback on ozone recovery
Figure 9/10. We assessed the chlorine-driven contribution to observed and
modeled ozone trends over two different periods (2004–2018 and 2004–2021). The motivation
behind this analysis stems from the need to quantify the impact of declining halogen loading
on ozone recovery, particularly in light of interhemispheric differences in observed
stratospheric trends. The figure employs the TOMCAT model-derived ratio of ozone-to-HCl trends,
which are applied separately to observational data from the ACE-FTS instrument and model
simulations. Panels (a) and (b) display the chlorine-induced ozone linear trends for 2004–2018,
calculated using the TOMCAT-derived ratio in combination with HCl trends from ACE-FTS and
TOMCAT CTL simulations, respectively. These panels help delineate the role of halogen chemistry
in ozone trends, particularly highlighting interhemispheric differences. The trends in panels
(c) and (d) extend the analysis to 2004–2021, using the respective TOMCAT ratio for the same
period. Regions, where the ACE-FTS or TOMCAT CTL ozone trends are positive (negative), are
indicated by solid (dashed) black contours, with the zero line represented by the thickest
contour. The results confirm that chlorine-driven ozone recovery is more pronounced in the
Southern Hemisphere mid-latitudes, whereas in the Northern Hemisphere, dynamical variability
appears to exert a stronger influence. The extension to 2021 suggests a reduced chemical
contribution to ozone changes, indicating that other factors, such as stratospheric transport,
may play an increasing role in observed ozone trends. Adapted from Chrysanthou et al.
(doi:10.1029/2024JD042161).
Antarctic ozone loss due to Hunga Tonga-Hunga Ha'apai (HTHH) eruption
Figure 10/10. The underwater Hunga Tonga-Hunga Ha'apai (HTHH) eruption
lifted huge amount of water vapor into the stratosphere increasing the stratospheric
water burden by 10%. Modeled and observed stratospheric ozone changes in 2022 and
2023 over the Antarctic after the HTHH eruption. Time series of the mean total column
ozone (DU) at SH high-latitudes (65°S–90°S) are shown comparing a model run without
excess water vapor (control) in 2022 (bold blue), 2023 (dashed gray), and years 1980–2021
(gray) with model runs including excess water vapor (HT) in 2022 (thin navy) and 2023
(solid red), and with Ozone Monitoring Instrument (triangle), and Infrared Atmospheric
Sounding Interferometer (cross) satellite observations in 2022 and 2023. Bottome panel
shows modeled monthly mean difference in column ozone (DU) between runs HT and Control
for August and September in 2022 and 2023. In 2022 no additional ozone loss was observed
due to HTHH, but small additional ozone losses of up to 10 DU were observed at the edge
of the polar vortex (the boundary of the ozone hole).
From Zhou et al. (doi:10.1029/2023GL107630).
