Reactive Trace Gases 2

Stratospheric concentrations of ozone depleting substances (ODSs) have slowly decreased since the mid-1990s due to the successful implementation of the Montreal Protocol (1987) and its subsequent enforcements. Comprehensive analysis of profile ozone changes is vital to documenting the beneficial effect of the Montreal Protocol on the ozone layer, particularly since ozone recovery is expected to first become visible in the upper stratosphere where ODSs are photolyzed and affect ozone most strongly. The global picture needs to be based on merged satellite ozone series since no single satellite instrument covers the entire period from the 1970s, when ODSs started affecting stratospheric ozone, until the present.

Seven merged satellite data sets were analysed by applying a multiple linear regression model describing the annual cycle and including proxies for the QBO (Quasi-Biennial Oscillation), solar cycle, ENSO (El Niño Southern Oscillation), and volcanic aerosols (see Tummon et al., Atmos. Chem. Phys. Discuss., 14, 25687–25745, 2014). Long-term ozone trends were calculated for the period 1984-2011 using a piece-wise linear regression, with a change in trend prescribed at the end of 1997. The best agreement amongst data sets is seen in the mid-latitude lower and middle stratosphere, with larger differences in the equatorial lower stratosphere and the upper stratosphere. In most cases, differences in the choice of underlying instrument records that were merged produced larger differences between data sets than the use of different merging techniques. For the 1984–1997 period, (downward) trends tend to be most similar between data sets (with largest negative trends ranging from -4 to -8%/decade in the mid-latitude upper stratosphere). This is largely due to the fact that most data sets are predominantly (or only) based on SAGE-II for this period. Trends in the middle and lower stratosphere are much smaller, and particularly for the lower stratosphere, large uncertainties remain.

For the recovery period (1998–2011), the picture is less conclusive: trends varied from approximately -1 to +5%/decade in the mid-latitude upper stratosphere. Again, middle and lower stratospheric trends were smaller and for most data sets not statistically significant. These differences might be explicable for several reasons: (i) from 2005 onwards, when the SAGE-II instrument was switched off, observations from several different instruments are used to complete the time series; (ii) ODS concentrations are decreasing much more slowly than they increased in the first period (i.e. leading to smaller trends).

Overall, however, there is a clear shift from mostly negative to mostly positive trends between the two periods over much of the profile. The data sets and trends derived from them provide important information for model validation. Given the effects of changing climate on ozone and its recovery, it is vital that measurements of the ozone profile from space continue so that we can document ozone profile changes in future.

The work presented here results from the SI2N activity, which is an initiative supported by SPARC, the International Ozone Commission (IO3C), IGACO O3/UV of GAW (Global Atmosphere Watch), and NDACC (Network for the Detection of Atmospheric Composition Changes).

Atmospheric composition is rapidly changing in response to changes in industrialization, land-use, and climate. Tropospheric ozone is at the nexus of atmospheric chemistry, air-quality, and climate as it is not only the third most important greenhouse gas and a primary air pollutant, but also affects carbon dioxide by damaging plants and the lifetime of atmospheric methane by influencing the oxidative capacity of the atmosphere.
 
Observed trends in free-tropospheric ozone as observed by ozone-sondes and more recently by satellite measurements from the Aura TES and IASI instruments do not agree with models that are driven by observed changes in ozone pre-cursor emissions. As a consequence, estimates of ozone radiative forcing and the future trajectory of tropospheric ozone concentrations are highly uncertain. In this study, we explore the use of satellite observations of ozone and its pre-cursors for constraining the sensitivity of Northern hemispheric tropospheric ozone to anthropogenic emissions. New measurements of peroxyacetyl nitrate (PAN) from the Aura TES instrument suggest that one explanation for the model/data mismatch in trends is reduced, modeled ventilation of reactive nitrogen into the free-troposphere over Asia. Ultimately, continued well validated observation of ozone and its pre-cursors from IASI, AIRS, CRIS, and Trop-OMI will be needed to solve this critical scientific question.

We present the results of a technique that uses information from the Chappuis ozone bands in the visible part of the spectrum to improve the near-surface sensitivity of UV-retrieved ozone profiles from GOME-2 aboard MetOp-A and B.  The RAL UV ozone profile algorithm, which contributes to the ESA Ozone ECV, has already been optimised to use the temperature dependence of ozone absorption in the Huggins bands to extend profile information to the lower troposphere.   By virtue of the higher land surface reflectivity and lower Rayliegh scattering at visible wavelengths, the Chappuis bands can, in principle, increase sensitivity to near-surface ozone. However, in practice, there are several major challenges; not least those associated with the characterisation of surface reflectance on similar spectral scales to ozone absorption, other interfering gases and instrumental features that need to be accounted for.  We will show that it is possible to identify ozone enhancement in the boundary layer using the combination of UV ad visible spectral information which UV measurements alone cannot detect.  We compare our results to both chemical transport models and ozonesondes.

Ozone is the third most important greenhouse gas in terms of radiative forcing (0.4 W/m²) as a result of increases in ozone precursor emissions since pre-industrial times. Consequently, tropospheric ozone is recognized as one of the key atmospheric Essential Climate Variables by GCOS and WMO. Until recently, the ozone radiative forcing calculations were entirely model based, which led to significant discrepancies due to different model characteristics, as shown in the IPCC AR5. Satellite sounders operating in the infrared now offer the possibility to infer directly the Longwave Radiative Effect (LWRE) of ozone in the 9.6 micron band, with respect to its vertical distribution, allowing to better constrain model estimates of the ozone radiative forcing and its future predictions. Measurements of the ozone LWRE from the NASA Aura TES sounder have for instance been used to reduce intermodel RF uncertainty in the IPCC AR5.

In this presentation we calculate the ozone LWRE by exploiting the measurements of IASI on MetOp, which are performed at unprecedented temporal and spatial sampling and therefore allow investigating ozone radiative impacts on various scales. The calculation method, which we show to be more precise than previous methods based on an anisotropy correction for the angular integration, is very briefly presented. We then focus the presentation on the first daily global distribution of the LWRE from IASI and show also the first seasonal variations. The global variability of the LWRE and more local processes, such as stratospheric intrusions, are analyzed, considering also the impacts of surface temperature and humidity changes. Finally we present preliminary results of the comparison between IASI and TES/Aura. We show that IASI LWRE measurements continue and extend those of TES and that together they represent a robust dataset to benchmark future chemistry-climate model assessments, such as the Chemistry-Climate Model Initiative (CCMI).

Air quality monitoring from space gives a helpful complement to in situ measurements and regional chemical transport models (rCTM) in order to draw a more comprehensive picture of pollution processes. In the case of tropospheric ozone important progresses in the field of atmospheric sounding from space have been accomplished during the last decade. The lower troposphere is now available from IASI (Infrared Atmospheric Sounding Interferometer) with a maximum of sensitivity between 3 and 4 km. We use satellite observations from IASI on board the MetOp-A satellite to evaluate the springtime daily variability of lower tropospheric ozone over East Asia. The availability of semi-independent columns of ozone from the surface up to 12 km simultaneously with CO columns from IASI provide a powerful observational dataset to identify the processes controlling tropospheric ozone enhancement at synoptic scales. In addition, we combine IASI observations with meteorological reanalyses from ERA-Interim in order to investigate in more details the processes that control the spatial and temporal distribution of lower tropospheric ozone. The succession of low- and high-pressure systems drives the day-to-day variability of lower tropospheric ozone over North East Asia. A case study analysis in May 2008 shows that reversible subsiding and ascending ozone transfers in the UTLS region occurring in the vicinity of low-pressure systems and associated with tropopause perturbations affect the free and lower tropospheric ozone over large regions, especially north to 40°N and largely explain the ozone enhancement observed with IASI in the North Asian troposphere. Irreversible downward transport of ozone-rich air masses from the UTLS to the lower troposphere occurs more locally. Its contribution to the lower tropospheric ozone column is difficult to dissociate from the tropopause perturbations induced by the weather systems. Over Chinese highly polluted regions, the analysis of ozone observations in correlation with CO and NO₂ observations reveals a more complex situation where the photochemical production of ozone often plays a concomitant role to explain ozone enhancements in the lower troposphere.

We provide a new perspective on the current state of the ozone layer using a comprehensive long-term total ozone data record, which has been recently released as an Essential Climate Variable (ECV) within the framework of the European Space Agency's Climate Change Initiative (ESA-CCI). The data record has been compiled from European satellite sensors GOME/ERS-2, SCIAMACHY/ENIVSAT, and GOME-2/MetOp, which provide global observations for the last 20 years.

A key issue is the detection of the expected onset of ozone recovery and its spatial fingerprint as a consequence of the 1987 Montreal Protocol and its subsequent amendments. The protocol controls and regulates the production and release of Ozone Depleting Substances (ODSs) and measurements indicate that their stratospheric concentrations peaked and have begun to decrease since the turn of the century.

We use the GOME-type Total Ozone ECV (GTO-ECV) data record in order to disentangle the various aspects of ozone change and variability at global and regional scales using a multivariate regression analysis taking into account several explanatory variables describing natural and/or anthropogenic forcings. It turned out that given dominant natural variabilitiy due to complex feedback mechanisms between chemical and dynamical atmospheric processes the expected midlatitude onset of ozone recovery is still not significant and would need additional years of observations. A regional increase in the tropical Pacific is a likely manifestation of a long-term change in El Nino - Southern Oscillation intensity over the last two decades induced by strong El Nino in 1997/98 and strong La Nina in 2010/11.

The results are compared with appropriate ground-based measurements at various locations and two Chemistry-Climate Model simulations. Furthermore we will provide an outlook on the second phase of the ESA ozone CCI project. The climate data record will be further extended with GOME-2/MetOp-B and OMI/Aura data and we will address the spatial and temporal sampling inhomogeneities among the different satellite instruments which may have systematic effects and may therefore lead to erroneous average estimates.