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In this work we present evidence that quasi-cyclical perturbations in total ozone (quasi-biennial oscillation - QBO, El Niño-Southern Oscillation - ENSO, and North Atlantic Oscillation - NAO) can be used as independent proxies in evaluating Global Ozone Monitoring Experiment (GOME) 2 aboard MetOp A (GOME-2A) satellite total ozone data, using ground-based (GB) measurements, other satellite data, and chemical transport model calculations. The analysis is performed in the frame of the validation strategy on longer time scales within the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT) Satellite Application Facility on Atmospheric Composition Monitoring (AC SAF) project, covering the period 2007-2016. Comparison of GOME-2A total ozone with ground observations shows mean differences of about -0.7±1.4 % in the tropics (0-30ĝ), about +0.1±2.1 % in the mid-latitudes (30-60ĝ), and about +2.5±3.2 % and 0.0±4.3 % over the northern and southern high latitudes (60-80ĝ), respectively. In general, we find that GOME-2A total ozone data depict the QBO-ENSO-NAO natural fluctuations in concurrence with the co-located solar backscatter ultraviolet radiometer (SBUV), GOME-type Total Ozone Essential Climate Variable (GTO-ECV; composed of total ozone observations from GOME, SCIAMACHY - SCanning Imaging Absorption SpectroMeter for Atmospheric CHartographY, GOME-2A, and OMI - ozone monitoring instrument, combined into one homogeneous time series), and ground-based observations. Total ozone from GOME-2A is well correlated with the QBO (highest correlation in the tropics of +0.8) in agreement with SBUV, GTO-ECV, and GB data which also give the highest correlation in the tropics. The differences between deseazonalized GOME-2A and GB total ozone in the tropics are within ±1 %. These differences were tested further as to their correlations with the QBO. The differences had practically no QBO signal, providing an independent test of the stability of the long-term variability of the satellite data. Correlations between GOME-2A total ozone and the Southern Oscillation Index (SOI) were studied over the tropical Pacific Ocean after removing seasonal, QBO, and solar-cycle-related variability. Correlations between ozone and the SOI are on the order of +0.5, consistent with SBUV and GB observations. Differences between GOME-2A and GB measurements at the station of Samoa (American Samoa; 14.25ĝ S, 170.6ĝ W) are within ±1.9 %. We also studied the impact of the NAO on total ozone in the northern mid-latitudes in winter. We find very good agreement between GOME-2A and GB observations over Canada and Europe as to their NAO-related variability, with mean differences reaching the ±1 % levels. The agreement and small differences which were found between the independently produced total ozone datasets as to the influence of the QBO, ENSO, and NAO show the importance of these climatological proxies as additional tool for monitoring the long-term stability of satellite-ground-truth biases. © 2019. This work is distributed under the Creative Commons Attribution 4.0 License.

This study provides a new look at the observed and calculated long-term temperature changes from the lower troposphere to the lower stratosphere since 1958 over the Northern Hemisphere. The data sets include the NCEP/NCAR reanalysis, the Free University of Berlin (FU-Berlin) and the RICH radiosonde data sets as well as historical simulations with the CESM1-WACCM global model participating in CMIP5. The analysis is mainly based on monthly layer mean temperatures derived from geopotential height thicknesses in order to take advantage of the use of the independent FU-Berlin stratospheric data set of geopotential height data since 1957. This approach was followed to extend the records for the investigation of the stratospheric temperature trends to the earliest possible time. After removing the natural variability with an autoregressive multiple regression model our analysis shows that the period 1958-2011 can be divided into two distinct sub-periods of long-term temperature variability and trends: before and after 1980. By calculating trends for the summer time to reduce interannual variability, the two periods are as follows. From 1958 until 1979, a non-significant trend (0.06 ± 0.06 °C decade-1 for NCEP) and slightly cooling trends (-0.12 ± 0.06 °C decade-1 for RICH) are found in the lower troposphere. The second period from 1980 to the end of the records shows significant warming (0.25 ± 0.05 °C decade-1 for both NCEP and RICH). Above the tropopause a significant cooling trend is clearly seen in the lower stratosphere both in the pre-1980 period (-0.58 ± 0.17 °C decade -1 for NCEP,-0.30 ± 0.16 °C decade-1 for RICH and-0.48 ± 0.20 °C decade-1 for FU-Berlin) and the post-1980 period (-0.79 ± 0.18 °C decade-1 for NCEP,-0.66 ± 0.16 °C decade-1 for RICH and-0.82 ± 0.19 °C decade-1 for FU-Berlin). The cooling in the lower stratosphere persists throughout the year from the tropics up to 60° N. At polar latitudes competing dynamical and radiative processes reduce the statistical significance of these trends. Model results are in line with reanalysis and the observations, indicating a persistent cooling (-0.33 °C decade-1) in the lower stratosphere during summer before and after 1980; a feature that is also seen throughout the year. However, the lower stratosphere CESM1-WACCM modelled trends are generally lower than reanalysis and the observations. The contrasting effects of ozone depletion at polar latitudes in winter/spring and the anticipated strengthening of the Brewer-Dobson circulation from man-made global warming at polar latitudes are discussed. Our results provide additional evidence for an early greenhouse cooling signal in the lower stratosphere before 1980, which appears well in advance relative to the tropospheric greenhouse warming signal. The suitability of early warning signals in the stratosphere relative to the troposphere is supported by the fact that the stratosphere is less sensitive to changes due to cloudiness, humidity and man-made aerosols. Our analysis also indicates that the relative contribution of the lower stratosphere versus the upper troposphere low-frequency variability is important for understanding the added value of the long-term tropopause variability related to human-induced global warming. © 2014 Author(s).

Ozone and methane are chemically active climate-forcing agents affected by climate-chemistry interactions in the atmosphere. Key chemical reactions and processes affecting ozone and methane are presented. It is shown that climate-chemistry interactions have a significant impact on the two compounds. Ozone, which is a secondary compound in the atmosphere, produced and broken down mainly in the troposphere and stratosphre through chemical reactions involving atomic oxygen (O), NOx compounds (NO, NO2), CO, hydrogen radicals (OH, HO2), volatile organic compounds (VOC) and chlorine (Cl, ClO) and bromine (Br, BrO). Ozone is broken down through changes in the atmospheric distribution of the afore mentioned compounds. Methane is a primary compound emitted from different sources (wetlands, rice production, livestock, mining, oil and gas production and landfills).Methane is broken down by the hydroxyl radical (OH). OH is significantly affected by methane emissions, defined by the feedback factor, currently estimated to be in the range 1.3 to 1.5, and increasing with increasing methane emission. Ozone and methane changes are affected by NOx emissions. While ozone in general increase with increases in NOx emission, methane is reduced, due to increases in OH. Several processes where current and future changes have implications for climate-chemistry interactions are identified. It is also shown that climatic changes through dynamic processes could have significant impact on the atmospheric chemical distribution of ozone and methane, as we can see through the impact of Quasi Biennial Oscillation (QBO). Modeling studies indicate that increases in ozone could be more pronounced toward the end of this century. Thawing permafrost could lead to important positive feedbacks in the climate system. Large amounts of organic material are stored in the upper layers of the permafrost in the yedoma deposits in Siberia, where 2 to 5% of the deposits could be organic material. During thawing of permafrost, parts of the organic material that is deposited could be converted to methane. Furthermore, methane stored in deposits under shallow waters in the Arctic have the potential to be released in a future warmer climate with enhanced climate impact on methane, ozone and stratospheric water vapor. Studies performed by several groups show that the transport sectors have the potential for significant impacts on climate-chemistry interactions. There are large uncertainties connected to ozone and methane changes from the transport sector, and to methane release and climate impact during permafrost thawing. © 2014 by the authors.

We summarise results from a workshop on "Model Benchmarking and Quality Assurance" of the EU-Network of Excellence ACCENT, including results from other activities (e.g. COST Action 732) and publications. A formalised evaluation protocol is presented, i.e. a generic formalism describing the procedure of how to perform a model evaluation. This includes eight steps and examples from global model applications which are given for illustration. The first and important step is concerning the purpose of the model application, i.e. the addressed underlying scientific or political question. We give examples to demonstrate that there is no model evaluation per se, i.e. without a focused purpose. Model evaluation is testing, whether a model is fit for its purpose. The following steps are deduced from the purpose and include model requirements, input data, key processes and quantities, benchmark data, quality indicators, sensitivities, as well as benchmarking and grading. We define "benchmarking" as the process of comparing the model output against either observational data or high fidelity model data, i.e. benchmark data. Special focus is given to the uncertainties, e.g. in observational data, which have the potential to lead to wrong conclusions in the model evaluation if not considered carefully.

The results of a comparison between total ozone amounts derived from solar backscatter ultraviolet (SBUV) satellite observations and those calculated from the chemical transport model Oslo CTM2 are presented for the period 2001-2007. Monthly mean total ozone amounts from improved model simulations were used to compute monthly, seasonal and annual zonal means over 10° latitude zones, and compared with respective satellite retrievals over the northern and southern hemispheres. The results show that the improved model simulations slightly underestimate total ozone over the northern hemisphere when compared with the satellites by 1.4% on average, and slightly overestimate total ozone over the southern extra-tropics, middle and high latitudes by 1.6% on average. The mean difference between the model- and satellite-derived total ozone columns from 75°S to 75°N is estimated to be about -0.3%. A linear regression analysis between the model- and satellite-derived total ozone data shows statistically significant correlations between the two data sets at all latitude zones (about+0.8 in the tropics and more than +0.9 over all other latitudes). The annual cycle of total ozone is shown to be well reproduced by the model at all latitudes. © 2011 Taylor & Francis.

International audience; European emission trends of nitrogen oxides since 1880 and up to present are presented here and are linked to the evolution of road transport emissions. Road transport has been the dominating source of NOx emissions since 1970, and contributes with 40% to the total emissions in 2005. Five trend regimes have been identified between 1880 and 2005. The first regime (1880?1950) is determined by a slow increase in fuel consumption all over Europe. The second regime (1950?1980) is characterized by a continued steep upward trend in liquid fuel use and by the introduction of the first regulations on road traffic emissions. Reduction in fuel consumption determines the emission trends in the third regime (1980?1990) that is also characterized by important differences between Eastern and Western Europe. Emissions from road traffic continue to grow in Western Europe in this period, and it is argued here that the reason for this continued NOx emission increase is related to early inefficient regulations for NOx in the transport sector. The fourth regime (1990?2000) involves a turning point for road traffic emissions, with a general decrease of emissions in Europe during that decade. It is in this period that we can identify the first emission reductions due to technological abatement in Western Europe. In the fifth regime (2000?2005), the economic recovery in Eastern Europe imposes increased emission from road traffic in this area. Western European emissions are on the other hand decoupled from the fuel consumption, and continue to decrease. The implementation of strict measures to control NOx emissions is demonstrated here to be a main reason for the continued Western European emission reductions. The results indicate that even though the effectiveness of European standards is hampered by a slow vehicle turnover, loopholes in the type-approval testing, and an increase in diesel consumption, the effect of such technical abatement measures is traceable in the evolution of European road traffic emissions over the last 15 years.

International audience; The global chemical transport model Oslo CTM2 has been extended to include the formation, transport and deposition of secondary organic aerosol (SOA). Precursor hydrocarbons which are oxidised to form condensible species include both biogenic species such as terpenes and isoprene, as well as species emitted predominantly by anthropogenic activities (toluene, m-xylene, methylbenzene and other aromatics). A model simulation for 2004 gives an annual global SOA production of approximately 55 Tg. Of this total, 2.5 Tg is found to consist of the oxidation products of anthropogenically emitted hydrocarbons, and about 15 Tg is formed by the oxidation products of isoprene. The global production of SOA is increased to about 69 Tg yr?1 by allowing semi-volatile species to partition to ammonium sulphate aerosol. This brings modelled organic aerosol values closer to those observed, however observations in Europe remain significantly underestimated. Allowing SOA to partition into ammonium sulphate aerosol increases the contribution of anthropogenic SOA from about 4.5% to 9.4% of the total production. Total modelled organic aerosol (OA) values are found to represent a lower fraction of the measured values in winter (when primary organic aerosol (POA) is the dominant OA component) than in summer, which may be an indication that estimates of POA emissions are too low. Additionally, for measurement stations where the summer OA values are higher than in winter, the model generally underestimates the increase in summertime OA. In order to correctly model the observed increase in OA in summer, additional SOA sources or formation mechanisms may be necessary. The importance of NO3 as an oxidant of SOA precursors is found to vary regionally, causing up to 50%?60% of the total amount of SOA near the surface in polluted regions and less than 25% in more remote areas, if the yield of condensible oxidation products for ?-pinene is used for NO3 oxidation of all terpenes. Reducing the yield for ?-pinene and limonene oxidation in line with recent measurements reduces the global fraction of SOA formed from NO3 oxidation products from 27% to about 21%. This study underscores the need for SOA to be represented in a more realistic way in global aerosol models in order to better reproduce observations of organic aerosol burdens in industrialised and biomass burning regions.

REanalysis of the TROpospheric chemical composition over thepast 40 years (RETRO)Objectives:• exploit (often under-utilised) existing data sets from ground based stations,aircraft, and satellite instruments, integrating these into common datasets,• develop tools for the analysis, interpretation and exploitation of the data,• formulate recommendations for future measurement strategies,• assess changes in trace compound emissions and their effect on troposphericchemical composition and aerosols, and the associated radiative forcing, overthe past 40 years,• provide an assessment of uncertainties caused by climate variability,• evaluate emission control strategies in Europe,• predict changes over the next 20 years in tropospheric composition, andradiative forcing through model studies using the emission scenarios definedfor the IPCC 2001 climate assessment,• analyze the magnitude of intercontinental pollutant transport.Scientific achievements:• first detailed, comprehensive and consistent data sets on global emissionsfrom fossil and biofuel combustion and from open vegetation burningcovering the time period 1960-2000; available as gridded data sets with0.5°×0.5° and monthly mean resolution,• first global long-term atmospheric chemistry integrations with several stateof-the-art models using the ERA-40 meteorological data, the RETROemissions and other constrains in a consistent and well-documented manner,• analysis of key parameters controlling the interannual and seasonalvariability and the longer-term trends in the tropospheric composition relatedto ozone and its precursors,• development of new software tools for the analysis of observational data andmodel results; standardisation of model output and data formats anddefinition of model evaluation metrics and skill scores,• development of a comprehensive data base for tropospheric compositionobservations with complete metadata definition and a user-friendly interfacefor data access,• multi-model analysis of specific scenarios related to power generation andthe traffic sector in OECD countries,• contributions to the IPCC 4th assessment report through participation in amulti-model assessment study coordinated by the ACCENT network ofexcellenceMain deliverables:• gridded data sets of global emissions from 1960 to 2000 (D1-4)• gridded data sets of simulated concentrations of ozone and ozone precursorsfrom 1960 to 2000 (D4-3)• data base of atmospheric observationsreport on the RETRO emission inventories (D1-6)o report on trend analysis based on observational data (D2-3)o reports on sensitivity studies (D3-2, D3-3 and D3-4)o report on the long-term reanalysis simulations (D4-4)o reports on changes in UV and radiative forcing (D5-1 and D5-2)o analysis of past policy measures including scenario studies (D5-5)All RETRO deliverables are available on the RETRO ftp serverftp://ftp.retro.enes.org/pub or via the RETRO web pages http://retro.enes.org.Socio-economic relevance and policy implications:Understanding past trends in large-scale air pollution is a key requirement toformulate future strategies to enhance air quality in Europe and elsewhere. Due tothe scarcity of observational data (particularly for years before 1990), numericalmodels are the only way to obtain comprehensive and consistent information onpast trends in tropospheric ozone and related substances. The model simulationsin RETRO also allow for evaluating the impact of important parameters on theinterannual variability of air pollution in Europe. From specific sensitivity studiesthe roles of meteorological variability, variability in anthropogenic emissions andvariability in wildland fire emissions could be determined. Scenario calculationson the impact of certain policy measures to reduce emissions from powergeneration or from the traffic sector provide a direct answer to the potentialconsequences of the introduction of more stringent air pollution abatementmeasures. Due to the effects of intercontinental transport of air pollution, localmeasures may not always achieve the expected results. These issues can only beaddressed with comprehensive, well-constrained global model simulations as theywere pioneered in the RETRO project. Several of the tools and methodologiesdeveloped in RETRO are now applied in ongoing projects with a more immediatepolicy link (especially the FP6 integrated project GEMS and the multi-modelassessment activity under the auspices of the task force on hemispheric transportof air pollution).Conclusions:• RETRO contributed significant developments to advance the state-of-the-artin global atmospheric chemistry modelling in terms of new long-termemissions data and the first comprehensive long-term multi-modelintegrations of atmospheric chemistry and transport,• The RETRO models generally show very good consistency in terms ofinterannual variability and trend patterns, but they occasionally differsignificantly in the absolute values of the simulated concentrations of ozoneand its precursors,• Comparison with observations shows that the RETRO models often capturethe atmospheric variability patterns rather well and that the mean modelgenerally provides a reasonable description of the chemical state of theatmosphere. Some exceptions are noted, in particular the absence of anincreasing ozone trend over central Europe in the 1990s which is clearly seenfrom measurements at several mountain stations,• According to the RETRO simulations, summertime boundary layerconcentrations of ozone over Europe increased between 5 and 15 ppb (10-20%) between 1960 and the year 2000. Further abatement measures in thetraffic sector (introduction of the EURO 5 standard in all OECD countries)could lead to a reduction of summertime ozone by 8-10%.• A renewed effort following on the RETRO activities would be highlydesirable in order to further expand the available emission data sets(inclusion of SO2, aerosol precursors and greenhouse gases and review ofnatural emissions), to further investigate the underlying causes fortropospheric composition change and to include the analysis of feedbacksbetween tropospheric ozone, climate change and aerosols.Dissemination of results:• more than 30 articles appeared in peer reviewed scientific journals with director indirect contributions from the RETRO project,• RETRO project results were presented in more than 100 conferencecontributions and presentations at international meetings,• a specific session on “tropospheric trends and variability” was inaugurated atthe EGU General Symposium 2004 and was repeated in 2006 and 2007,• RETRO emission data sets are being used in several ongoing researchprojects (e.g. GEMS and TFHTAP, GREENCYCLES, etc.),• All RETRO data sets and reports are available on the RETRO ftp serverftp://ftp.retro.enes.org/pub or via the RETRO web pages athttp://retro.enes.org.

Mean tropospheric hydroxyl radical (OH) abundance is often used as a measure of the oxidation capacity (or “self-cleansing”) of the atmosphere. The primary mechanism by which atmospheric pollutant gases are removed from the atmosphere is initiated by the reaction with OH. As a result, large interannual or decadal variations in OH concentrations, as suggested in recent reports, are of great concern. In addition, an important method for discerning tropospheric OH burdens and variability, the analysis of methyl chloroform (MCF) observations, will soon become less useful as the concentration of this industrial gas approaches zero. With these concerns in mind, a workshop focusing on global OH trends and variability was convened in Boulder, Colorado, on 28–30 November 2005. Although the concept of tropospheric mean OH does not do justice to regional OH differences, and ignores the less significant contributions by other oxidants, global OH changes integrate the response to large-scale atmospheric chemistry forcings. The latter include the tendencies of atmospheric water vapour, solar radiation and notably the human-induced emissions of NOx, CO, CH4 and other hydrocarbons. Analogously, the global mean surface temperature change is an integral climate response to natural and anthropogenic forcings by greenhouse gases, aerosols, etc. Furthermore, the variability of mean OH is an indicator of the sensitivity of atmospheric chemistry to global air pollution and natural events (e.g., large volcano eruptions, El Niño). A large response to a small forcing is typical for a system that is not well buffered and vice versa. The analysis of relatively long-lived gases for which emission magnitudes are well characterized can provide insight into the interannual and decadal variability of tropospheric OH. Analysis of MCF measurements, a tracer with a lifetime of about 5 yr owing to its removal by OH, points to a substantial OH growth in the 1980s, a decline in the 1990s and a recovery after 1998, indicating decadal OH changes of 10–15% (R.G. Prinn et al., 2005, Geophysical Research Letters 32, L07809, doi:10.1029/2004GL022228). MCF analysis furthermore suggests a large interannual OH variability of 8.5±1.0%; however, this may reflect uncertainties in the MCF emission inventory (P. Bousquet et al., 2005, Atmospheric Chemistry and Physics 5, 2635–2656). Using radiocarbon 14CO as a diagnostic for OH gives additional evidence of 10% variability in OH over timescales of less than a year, although the 14CO measurements are only representative of the extratropical southern hemisphere (M.R. Manning et al., 2005, Nature 436, 1001–1004). Even though chemistry–transport models fail to reproduce the large OH variability, many studies point to relatively large OH changes after the 1991 Mt. Pinatubo eruption and during the 1997/8 El Niño event. The likelihood of large OH variability is challenged by CH4 mass balance calculations based on the NOAA network measurements. Emissions of CH4 (E) can be derived from the measured global burden [CH4] and rate of CH4 increase, and an estimate of the CH4 lifetime: E=d[CH4]/dt+[CH4]/τ, where d[CH4]/dt is the observed rate of increase and τ the CH4 lifetime. Since τ is not constant in the real atmosphere, fixing it in the equation means that E includes variability of the sink, i.e. changes in OH. Calculation of E based on yields a mean source of 556±10 Tg CH4 yr−1 for 1984 to 2004, with a trend of 0.1±0.4 Tg yr−1. Maximum deviations from E are 18.4 Tg in 1991 and 27.0 Tg in 1998 (see Fig. 1). Assuming that emissions are uncorrelated with OH, these anomalies provide upper limits of the interannual variability of OH, e.g., 3–5% in 1991 and 1998. In other years the OH variability is typically less than 2%, in agreement with chemistry–transport models. Yet, there is little doubt that global OH decreased immediately after the Mt. Pinatubo eruption, as this is evident in both the CH4 and 14CO measurements. The large anomaly in CH4 in 1998 had contributions by increased emissions from wetlands and biomass burning (S. Morimoto et al., 2006, Geophysical Research Letters 33, L01807, doi:10.1029/2005GL024648), and decreased OH resulting from the Indonesian biomass burning emissions (T.M. Butler et al., 2005, Journal of Geophysical Research 110, D21310, doi:10.1029/2005JD006071).

International audience; Within the EU-project TRADEOFF, the impact of NOx (=NO+NO2) emissions from subsonic aviation upon the chemical composition of the atmosphere has been calculated with focus on changes in reactive nitrogen and ozone. We apply a 3-D chemical transport model that includes comprehensive chemistry for both the troposphere and the stratosphere and uses various aircraft emission scenarios developed during TRADEOFF for the year 2000. The environmental effects of enhanced air traffic along polar routes and of possible changes in cruising altitude are investigated, taking into account effects of flight route changes on fuel consumption and emissions. In a reference case including both civil and military aircraft the model predicts aircraft-induced maximum increases of zonal-mean NOy (=total reactive nitrogen) between 156 pptv (August) and 322 pptv (May) in the tropopause region of the Northern Hemisphere. Resulting maximum increases in zonal-mean ozone vary between 3.1 ppbv in September and 7.7 ppbv in June. Enhanced use of polar routes implies substantially larger zonal-mean ozone increases in high Northern latitudes during summer, while the effect is negligible in winter. Lowering the flight altitude leads to smaller ozone increases in the lower stratosphere and upper troposphere, and to larger ozone increases at altitudes below. Regarding total ozone change, the degree of cancellation between these two effects depends on latitude and season, but annually and globally averaged the contribution from higher altitudes dominates, mainly due to washout of NOy in the troposphere, which weakens the tropospheric increase. Raising flight altitudes increases the ozone burden both in the troposphere and the lower stratosphere, primarily due to a more efficient accumulation of pollutants in the stratosphere.

Model studies show that changes in photodissociation rates resulting from changes in ozone column densities induce changes in lower tropospheric ozone, which vary significantly with location and time of the year. The validity of the model results is tested against daily total ozone and ground level ozone at three selected stations (Samoa, Mauna Loa, and Hohenpeissenberg). Observational data for a period of more than 1 decade have been analyzed. Comparisons are made of model-simulated distribution of ozone and its precursors (NOx and CO) at the three stations. Further comparisons are made of observed and model-calculated sensitivity in surface ozone to reduction in ozone column densities. Calculations performed with a global-scale chemical transport model (CTM) with extensive ozone chemistry reproduce well the observed levels and seasonal distribution of NOx, CO, and ozone at remote background stations (Samoa and Mauna Loa) and at stations in more polluted regions (Hohenpeissenberg). A chemical box model is used to demonstrate the chemical link between surface ozone changes and changes in total ozone for different NOx levels. Model studies and analysis of the observational data show that ground level ozone at the remote, low-NOx stations of Mauna Loa and Samoa is correlated positively with total ozone, with an exception at Mauna Loa during winter months. A reduction in ozone column densities, which leads to enhanced photochemical activity, reduces ozone levels at ground level. The sensitivity of surface ozone to changes in total ozone is particularly large in the low-NOx regime at Samoa. An anticorrelation between ground level ozone and total ozone is found at the Hohenpeissenberg station both in the observational data and in the model results during wintertime with high NOx levels. Enhanced photochemical activity leads to enhanced ozone production. There is, however, a disagreement between the observed and CTM-modeled sensitivity in surface ozone to ozone column density during the summer months at Hohenpeissenberg. The strong anticorrelation found in the observations, giving increases in surface ozone at low ozone column densities, is not present in the CTM model studies. It is suggested that a correlation between low ozone column densities and stagnant high-pressure systems is an important cause for the observed anticorrelation. Copyright 2005 by the American Geophysical Union.

The international research project RECONCILE has addressed central questions regarding polar ozone depletion, with the objective to quantify some of the most relevant yet still uncertain physical and chemical processes and thereby improve prognostic modelling capabilities to realistically predict the response of the ozone layer to climate change. This overview paper outlines the scope and the general approach of RECONCILE, and it provides a summary of observations and modelling in 2010 and 2011 that have generated an in many respects unprecedented dataset to study processes in the Arctic winter stratosphere. Principally, it summarises important outcomes of RECONCILE including (i) better constraints and enhanced consistency on the set of parameters governing catalytic ozone destruction cycles, (ii) a better understanding of the role of cold binary aerosols in heterogeneous chlorine activation, (iii) an improved scheme of polar stratospheric cloud (PSC) processes that includes heterogeneous nucleation of nitric acid trihydrate (NAT) and ice on non-volatile background aerosol leading to better model parameterisations with respect to denitrification, and (iv) long transient simulations with a chemistry-climate model (CCM) updated based on the results of RECONCILE that better reproduce past ozone trends in Antarctica and are deemed to produce more reliable predictions of future ozone trends. The process studies and the global simulations conducted in RECONCILE show that in the Arctic, ozone depletion uncertainties in the chemical and microphysical processes are now clearly smaller than the sensitivity to dynamic variability., Atmospheric Chemistry and Physics, 13 (18), ISSN:1680-7375, ISSN:1680-7367