Emissions of land transport and anthropogenic non-traffic emissions (e.g. industry, households and power generation) are significant sources of nitrogen oxides, carbon monoxide and volatile organic compounds (VOCs). These emissions are important precursors of tropospheric ozone and affect air quality. The contribution of the emission sectors to ozone cannot be measured directly but can only be calculated using sophisticated atmospheric chemistry models. For this study we apply the MECO(n) model system (MESSy-fied ECHAM and COSMO models nested n times) equipped with a source attribution method to investigate the contribution of various sources to ground-level ozone in Europe. Compared to previous source apportionment studies for Europe, for the first time we apply a combined NOx–VOC tagging implemented in an online nested global–regional chemistry–climate model to achieve a finer resolution over central Europe (12 km) but concurrently incorporating the effect of long-range transport. We distinguish 10 different source sectors and 4 geographical source regions, analysing especially the contribution from the land transport sector. Our analysis focuses on large ozone events during summer in four different regions, two major polluted regions (Po Valley and Benelux) and two more remote regions (Iberian Peninsula and Ireland). The analysis concentrates on results for summer 2017, during which measurement campaign EMeRGe took place. Measurement data from this campaign are used for model evaluation. Our analysis shows that European land transport emissions contribute largely (42 % and 44 %, respectively) to ground-level NOy mixing ratios over Benelux and the Po Valley. Due to the overall lower ozone production efficiency over Benelux compared to the Po Valley, however, the contributions to ground-level ozone are larger in the Po Valley (12 %) compared to Benelux (8 %). In line with previous publications using different source apportionment methods, our results underline the large importance of long-range transport of ozone, especially from North America (Benelux, Ireland), but also from Africa (Iberian Peninsula), and provide additional information about the sectoral contribution not available before. Our analysis shows that the contributions of European emissions from land transport and anthropogenic non-traffic sectors strongly increase with increasing values of MDA8 (daily maximum 8 h average) ozone over the Po Valley and in the Benelux region. Accordingly, these two sectors drive large MDA8 values in these regions. Inter-comparisons of results for 2018 and with a coarser model resolution (50 instead of 12 km) show that these results are robust with respect to inter-annual variability and model resolution. Comparing our results with results from other source attribution methods we find that the contributions to ozone from individual sectors, which have large NOx but rather low VOC emissions, are estimated to be lower, if their emissions of NOx and VOCs are regarded concurrently.@en

We quantify the contributions of emissions from the transport sector to tropospheric ozone and the hydroxyl radical (OH) by means of model simulations with a global chemistry-climate model equipped with a source attribution method. For the first time we applied a method which also allows for quantifying contributions to OH which is invariant upon disaggregation or recombination and additive. Based on these quantified contributions, we analyse the ozone radiative forcing (RF) and methane lifetime reductions attributable to emissions from the transport sectors. The contributions were analysed for each transport sector separately and for 2015 as well as for 2050 under the Shared Socioeconomic Pathways (SSPs) SSP1-1.9, SSP2-4.5, and SSP3-7.0. In line with previous publications using the source attribution approach, we quantify ozone RF attributable to emissions from land transport, shipping, and aviation for the year 2015 of 121, 60, and 31 mW m−2, respectively. At the same time, we diagnose a relative reduction in methane lifetime due to transport emissions of 14.3 % (land transport), 8.5 % (shipping), and 3.8 % (aviation). These reductions are significantly larger than reported by previous studies due to the application of the source attribution method. Compared to 2015, only SSP1-1.9 shows a strong decrease in ozone RF and methane lifetime reduction attributable to the entire transport sector in 2050. For the projections of SSP2-4.5, we find similar effects of the total transport sector as for 2015, while the effects in SSP3-7.0 increase compared to 2015. This small change in the effects for the two projections compared to 2015 is caused by two main factors. Firstly, aviation emissions are projected to increase in SSP2-4.5 (increase of 107 %) and SSP3-7.0 (+86 %) compared to 2015, resulting in projected ozone RF of 55 mW m−2 (+78 %) and 50 mW m−2 (+61 %) for the year 2050 from aviation emissions. Secondly, the non-linear effects of atmospheric chemistry in polluted regions such as Europe and North America lead to rather small reductions in ozone and OH in response to emission reductions, especially from land transport emissions. In addition, the increase in emissions from land transport in other parts of the world, particularly in South Asia, leads to an increased contribution of ozone and OH. In particular, ozone formed by land transport emissions from South Asia causes strong RF that partially offsets the reductions in Europe and North America. Moreover, our results show that besides the non-linear response, lack of international cooperation, as in the SSP3-7.0 projection, hinders mitigation of ground-level ozone.@en

We report on an inconsistency in the latitudinal distribution of aviation emissions between the data products of phases 5 and 6 of the Coupled Model Intercomparison Project (CMIP). Emissions in the CMIP6 data occur at higher latitudes than in the CMIP5 data for all scenarios, years, and emitted species. A comparative simulation with the chemistry-climate model ECHAM/MESSy Atmospheric Chemistry (EMAC) reveals that the difference in nitrogen oxide emission distribution leads to reduced overall ozone changes due to aviation in the CMIP6 scenarios because in those scenarios the distribution of emissions is partly shifted towards the chemically less active higher latitudes. The radiative forcing associated with aviation ozone is 7.6% higher, and the decrease in methane lifetime is 5.7% larger for the year 2015 when using the CMIP5 latitudinal distribution of emissions compared to when using the CMIP6 distribution. We do not find a statistically significant difference in the radiative forcing associated with aviation aerosol emissions. In total, future studies investigating the effects of aviation emissions on ozone and climate should consider the inconsistency reported here.

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Aviation produces a net climate warming contribution that comprises multiple forcing terms of mixed sign. Aircraft NOx emissions are associated with both warming and cooling terms, with the short-term increase in O3 induced by NOx emissions being the dominant warming effect. The uncertainty associated with the magnitude of this climate forcer is amongst the highest out of all contributors from aviation and is owed to the nonlinearity of the NOx-O3 chemistry and the large dependency of the response on space and time, i.e., on the meteorological condition and background atmospheric composition. This study addresses how transport patterns of emitted NOx and their climate effects vary with respect to regions (North America, South America, Africa, Eurasia and Australasia) and seasons (January-March and July-September in 2014) by employing global-scale simulations. We quantify the climate effects from NOx emissions released at a representative aircraft cruise altitude of 250 hPa (∼10400 m) in terms of radiative forcing resulting from their induced short-term contributions to O3. The emitted NOx is transported with Lagrangian air parcels within the ECHAM5/MESSy Atmospheric Chemistry (EMAC) model. To identify the main global transport patterns and associated climate impacts of the 14 000 simulated air parcel trajectories, the unsupervised QuickBundles clustering algorithm is adapted and applied. Results reveal a strong seasonal dependence of the contribution of NOx emissions to O3. For most regions, an inverse relationship is found between an air parcel's downward transport and its mean contribution to O3. NOx emitted in the northern regions (North America and Eurasia) experience the longest residence times in the upper midlatitudes (40 %-45 % of their lifetime), while those beginning in the south (South America, Africa and Australasia) remain mostly in the Tropics (45 %-50 % of their lifetime). Due to elevated O3 sensitivities, emissions in Australasia induce the highest overall radiative forcing, attaining values that are larger by factors of 2.7 and 1.2 relative to Eurasia during January and July, respectively. The location of the emissions does not necessarily correspond to the region that will be most affected - for instance, NOx over North America in July will induce the largest radiative forcing in Europe. Overall, this study highlights the spatially and temporally heterogeneous nature of the NOx-O3 chemistry from a global perspective, which needs to be accounted for in efforts to minimize aviation's climate impact, given the sector's resilient growth.

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Aviation contributes to 3.5% of anthropogenic climate change in terms of Effective Radiative Forcing (ERF) and 5% in terms of temperature change. Aviation climate impact is expected to increase rapidly due to the growth of air transport sector in most regions of the world and the effects of the COVID-19 pandemic are expected to only have a temporary effect on this growth. While efforts have been made to curb CO2 emissions, non-CO2 effects that are at least equally significant according to recent research, require more attention. The EU Horizon 2020 project ClimOp considers a comprehensive approach to tackling the climate impact of aviation using novel operational measures. One such measure is climate-optimised flight planning, where small deviations can be made in aircraft trajectories to minimise their overall climate impact. Algorithmic Climate Change Functions (aCCFs) are used to estimate the climate impact of local non-CO2 effects such as nitrogen oxide (NOx) emissions (via ozone (O3) formation and methane (CH4) depletion), aviation water vapour (H2O) and contrails using weather variables directly as inputs. By using these functions in an air traffic optimisation module, climate sensitive regions are detected and avoided leading to climate-optimised trajectories. Here, we focus specifically on evaluating the effectiveness of reducing the aviation NOx induced climate impact via O3 formation, using only O3 aCCFs for the optimisation strategy. This is achieved using the chemistry climate model EMAC (ECHAM5/MESSy) and various submodels. A summer and winter day, characterised by high spatial variability of O3 aCCFs are selected, following which, air traffic over the European airspace is optimised with respect to climate as well as operating cost. The air traffic is laterally and vertically optimised separately to enable an evaluation of the horizontal and vertical pattern of O3 aCCFs. It is shown that despite the significant impact of the synoptic situation on the transport of emitted NOx, the climate-optimised flights lead to lower O3 Radiative Forcing (RF) compared to the cost-optimised flights. The study finds that while O3 aCCFs can reduce the climate impact, there are certain discrepancies in the prediction of O3 impact from aviation NOx emissions, as seen for the selected summer day. Although the aCCFs concept is a rough simplification in predicting future pathways of emissions and subsequent climate impact, we could show that it enables a reasonable first estimate. Further research is required to better describe the aCCFs allowing an improved estimate in O3-RF reduction for optimisation approaches.@en

The resilient growth of air travel demands a comprehensive understanding of the climate effects from aviation emissions. The current level of knowledge of the environmental repercussions of CO2 emissions is considerably higher than that of non-CO2 emissions, which includes nitrogen oxides (NOx), sulfur oxides (SOx), other aerosols like black carbon (BC), water vapor and contrails. Aircraft NOx emissions not only possess a high degree of uncertainty because of the non-linearity of the NOx – O3 chemistry, but are also responsible for producing the second strongest net warming effect out of all non-CO2 climate forcers from aviation, right after contrails [1]. This study employs global-scale simulations to characterize the transport patterns of nitrogen oxides and assess their climate effects across several regions (North America, South America, Africa, Eurasia and Australasia) from January to March and July to September in 2014. Radiative forcing effects from the short-term increase in O3, which are triggered by NOx emissions, are estimated. These emissions, which are introduced at a typical cruising altitude, are modelled as Lagrangian air parcels that are transported within the ECHAM5/MESSy Atmospheric Chemistry (EMAC) model [2]. In order to summarize the dynamical and radiative forcing characteristics of more than 10,000 simulated trajectories, a clustering approach with an adapted distance metric is applied. The method itself is an unsupervised machine learning algorithm, called QuickBundles [3], that is most commonly used in the field of neuroscience. A strong seasonal dependence is found for the contribution of NOx emissions to O3. In terms of residence times, NOx emitted in Northern regions resides mainly in the upper mid-latitudes while those initiated in the South remain mostly in the Tropics. Due to pronounced zonal jets, the location of emission does not necessarily correspond to the region that will be most affected, i.e., an emission starting in N. America in July will induce the greatest warming in Europe. [1] Lee, D.S., Fahey, D.W., Skowron, A., Allen, M.R., Burkhardt, U., Chen, Q., Doherty, S.J., Freeman, S., Forster, P.M., Fuglestvedt, J., Gettelman, A., De León, R.R., Lim, L.L., Lund, M.T., Millar, R.J., Owen, B., Penner, J.E., Pitari, G., Prather, M.J., Sausen, R., Wilcox, L.J.: The contribution of global aviation to anthropogenic climate forcing for 2000 to 2018, Atmospheric Environment, Volume 244, 2021, 117834, ISSN 1352-2310, https://doi.org/10.1016/j.atmosenv.2020.117834. [2] Jöckel, P., Kerkweg, A., Pozzer, A., Sander, R., Tost, H., Riede, H., Baumgaertner, A., Gromov, S., Kern, B., Development cycle 2 of the Modular Earth Submodel System (MESSy2), Geoscientific Model Development, 3, 717-752, doi: 10.5194/gmd-3-717-2010, 2010. [3] Garyfallidis, E., Brett, M., Correia, M. M., Williams, G. B., Nimmo-Smith, I. QuickBundles, a Method for Tractography Simplification. Frontiers in neuroscience, 6, 175. https://doi.org/10.3389/fnins.2012.00175, 2012.@en

Hypersonic aircraft flying at Mach 5 to 8 are a means for traveling very long distances in extremely short times and are even significantly faster than supersonic transport (Mach 1.5 to 2.5). Fueled with liquid hydrogen (LH2), their emissions consist of water vapor (H2O), nitrogen oxides (NOx), and unburned hydrogen. If LH2 is produced in a climate- and carbon-neutral manner, carbon dioxide does not have to be included when calculating the climate footprint. H2O that is emitted near the surface has a very short residence time (hours) and thereby no considerable climate impact. Super- and hypersonic aviation emit at very high altitudes (15 to 35 km), and H2O residence times increase with altitude from months to several years, with large latitudinal variations. Therefore, emitted H2O has a substantial impact on climate via high altitude H2O changes. Since the (photo-)chemical lifetime of H2O largely decreases at altitudes above 30 km via the reaction with O(1D) and via photolysis, the question is whether the H2O climate impact from hypersonics flying above 30 km becomes smaller with higher cruise altitude. Here, we use two state-of-the-art chemistry-climate models and a climate response model to investigate atmospheric changes and respective climate impacts as a result of two potential hypersonic fleets flying at 26 and 35 km, respectively. We show, for the first time, that the (photo-)chemical H2O depletion of H2O emissions at these altitudes is overcompensated by a recombination of hydroxyl radicals to H2O and an enhanced methane and nitric acid depletion. These processes lead to an increase in H2O concentrations compared to a case with no emissions from hypersonic aircraft. This results in a steady increase with altitude of the H2O perturbation lifetime of up to 4.4±0.2 years at 35 km. We find a 18.2±2.8 and 36.9±3.4 mW m-2 increase in stratosphere-adjusted radiative forcing due to the two hypersonic fleets flying at 26 and 35 km, respectively. On average, ozone changes contribute 8 %-22 %, and water vapor changes contribute 78 %-92 % to the warming. Our calculations show that the climate impact, i.e., mean surface temperature change derived from the stratosphere-adjusted radiative forcing, of hypersonic transport is estimated to be roughly 8-20 times larger than a subsonic reference aircraft with the same transport volume (revenue passenger kilometers) and that the main contribution stems from H2O.

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One possibility to reduce the climate impact of aviation is the avoidance of climate-sensitive regions, which is synonymous with climate-optimised flight planning. Those regions can be identified by algorithmic Climate Change Functions (aCCFs) for nitrogen oxides (NOx), water vapour (H2O) as well as contrail cirrus, which provide a measure of climate effects associated with corresponding emissions. In this study, we evaluate the effectiveness of reducing the aviation-induced climate impact via ozone (O3) formation (resulting from NOx emissions), when solely using O3 aCCFs for the aircraft trajectory optimisation strategy. The effectiveness of such a strategy and the associated potential mitigation of climate effects is explored by using the chemistry–climate model EMAC (ECHAM5/MESSy) with various submodels. A summer and winter day, characterised by a large spatial variability of the O3 aCCFs, are selected. A one-day air traffic simulation is performed in the European airspace on those selected days to obtain both cost-optimised and climate-optimised aircraft trajectories, which more specifically minimised a NOx-induced climate effect of O3 (O3 aCCFs). The air traffic is laterally and vertically re-routed separately to enable an evaluation of the influences of the horizontal and vertical pattern of O3 aCCFs. The resulting aviation NOx emissions are then released in an atmospheric chemistry–climate simulation to simulate the contribution of these NOx emissions to atmospheric O3 and the resulting O3 change. Within this study, we use O3-RF as a proxy for climate impact. The results confirm that the climate-optimised flights lead to lower O3-RF compared to the cost-optimised flights, although the aCCFs cannot reproduce all aspects of the significant impact of the synoptic situation on the transport of emitted NOx. Overall, the climate impact is higher for the selected summer day than for the selected winter day. Lateral re-routing shows a greater potential to reduce climate impact compared to vertical re-routing for the chosen flight altitude. We find that while applying the O3 aCCFs in trajectory optimisation can reduce the climate impact, there are certain discrepancies in the prediction of O3 impact from aviation NOx emissions, as seen for the summer day. Although the O3 aCCFs concept is a rough simplification in estimating the climate impact of a local NOx emission, it enables a reasonable first estimate. Further research is required to better describe the O3 aCCFs allowing an improved estimate in the Average Temperature Response (ATR) of O3 from aviation NOx emissions. A general improvement in the scientific understanding of non-CO2 aviation effects could make climate-optimised flight planning practically feasible@en

The aviation industry is an essential contributor to total anthropogenic climate change, and the ever-growing demand for air transport requires serious attention. While efforts have been made to curb CO2 emissions, non-CO2 effects that are even more significant according to recent research have not been given enough attention. The EU Horizon 2020 project ClimOp steps in to follow a more holistic approach to tackling the climate impact of aviation using novel operational measures. One such measure is climate-optimised flight planning, where small deviations can be made in aircraft trajectories to minimise their overall climate impact. Algorithmic Climate Change Functions (aCCFs) are used to estimate the climate impact of local non-CO2 effects such as Nitrogen oxide (NOx) emissions (via Ozone formation and Methane depletion), aviation water vapour, and contrails by using meteorological inputs directly. By plugging these functions directly into an aircraft trajectory optimization module, climate sensitive regions are detected and avoided leading to climateoptimised trajectories. While a preliminary verification conducted specifically for NOx-ozone aCCFs showed positive signs, this research entails a more detailed verification procedure, which provides a deeper insight into its capability in predicting the impact of aviation NOx emissions on atmospheric changes in ozone. Air traffic simulation is performed concerning a subset of one-day European flights on the specifically chosen days characterized by high variability of NOx-ozone aCCFs. The air traffic on these days is optimized for cost and climate. A subsequent chemistry simulation captures the NOx effects from these re-routing procedures, and the climate impact of both scenarios can thereby be directly compared. It is expected that the results will confirm the effectiveness of NOx-ozone aCCFs in producing climate-friendly trajectories. @en

Emissions of aviation include CO2, H2O, NOx, sulfur oxides, and soot. Many studies have investigated the annual mean climate impact of aviation emissions. While CO2 has a long atmospheric residence time and is almost uniformly distributed in the atmosphere, non-CO2 gases and particles and their products have short atmospheric residence times and are heterogeneously distributed. The climate impact of non-CO2 aviation emissions is known to vary with different meteorological background situations. The aim of this study is to systematically investigate the influence of characteristic weather situations on aviation climate effects over the North Atlantic region, to identify the most sensitive areas, and to potentially detect systematic weather-related similarities. If aircraft were re-routed to avoid climate-sensitive regions, the overall aviation climate impact might be reduced. Hence, the sensitivity of the atmosphere to local emissions provides a basis for the assessment of weather-related, climate-optimized flight trajectory planning. To determine the climate change contribution of an individual emission as a function of location, time, and weather situation, the radiative impact of local emissions of NOx and H2O to changes in O3, CH4, H2O and contrail cirrus was computed by means of the ECHAM5/MESSy Atmospheric Chemistry model. From this, 4-dimensional climate change functions (CCFs) were derived. Typical weather situations in the North Atlantic region were considered for winter and summer. Weather-related differences in O3, CH4, H2O, and contrail cirrus CCFs were investigated. The following characteristics were identified: enhanced climate impact of contrail cirrus was detected for emissions in areas with large-scale lifting, whereas low climate impact of contrail cirrus was found in the area of the jet stream. Northwards of 60° N, contrails usually cause climate warming in winter, independent of the weather situation. NOx emissions cause a high positive climate impact if released in the area of the jet stream or in high-pressure ridges, which induces a south- and downward transport of the emitted species, whereas NOx emissions at, or transported towards, high latitudes cause low or even negative climate impact. Independent of the weather situation, total NOx effects show a minimum at ∼250g hPa, increasing towards higher and lower altitudes, with generally higher positive impact in summer than in winter. H2O emissions induce a high climate impact when released in regions with lower tropopause height, whereas low climate impact occurs for emissions in areas with higher tropopause height. H2O CCFs generally increase with height and are larger in winter than in summer. The CCFs of all individual species can be combined, facilitating the assessment of total climate impact of aircraft trajectories considering CO2 and spatially and temporally varying non-CO2 effects. Furthermore, they allow for the optimization of aircraft trajectories with reduced overall climate impact. This also facilitates a fair evaluation of trade-offs between individual species. In most regions, NOx and contrail cirrus dominate the sensitivity to local aviation emissions. The findings of this study recommend considering weather-related differences for flight trajectory optimization in favour of reducing total climate impact.

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Water vapor (H₂O) is the source of reactive hydrogen radicals in the middle atmosphere, whereas carbon monoxide (CO), being formed by CO₂photolysis, is suitable as a dynamical tracer. In the mesosphere, both H₂O and CO are sensitive to solar irradiance (SI) variability because of their destruction/production by solar radiation. This enables us to analyze the solar signal in both models and observed data. Here, we evaluate the mesospheric H₂O and CO response to solar irradiance variability using the Chemistry- Climate Model Initiative (CCMI-1) simulations and satellite observations. We analyzed the results of four CCMI models (CMAM, EMAC-L90MA, SOCOLv3, and CESM1- WACCM 3.5) operated in CCMI reference simulation REFC1SD in specified dynamics mode, covering the period from 1984-2017. Multiple linear regression analyses show a pronounced and statistically robust response of H₂O and CO to solar irradiance variability and to the annual and semiannual cycles. For periods with available satellite data, we compared the simulated solar signal against satellite observations, namely the GOZCARDS composite for 1992-2017 for H₂O and Aura/MLS measurements for 2005-2017 for CO. The model results generally agree with observations and reproduce an expected negative and positive correlation for H₂O and CO, respectively, with solar irradiance. However, the magnitude of the response and patterns of the solar signal varies among the considered models, indicating differences in the applied chemical reaction and dynamical schemes, including the representation of photolyzes. We suggest that there is no dominating thermospheric influence of solar irradiance in CO, as reported in previous studies, because the response to solar variability is comparable with observations in both low-top and high-top models. We stress the importance of this work for improving our understanding of the current ability and limitations of state-of-the-art models to simulate a solar signal in the chemistry and dynamics of the middle atmosphere.

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The aviation industry is an essential contributor to total anthropogenic climate change, and the ever-growing demand for air transport requires serious attention. While efforts have been made to curb CO2 emissions, non-CO2 effects that are even more significant according to recent research have not been given enough attention. The EU Horizon 2020 project ClimOp steps in to follow a more holistic approach to tackling the climate impact of aviation using novel operational measures. One such measure is climate-optimised flight planning, where small deviations can be made in aircraft trajectories to minimise their overall climate impact. In order to achieve this, algorithmic Climate Change Functions (aCCFs) are used to estimate the climate impact of local non-CO2 effects such as Nitrogen oxide (NOx) emissions (via Ozone formation and Methane depletion), aviation water vapour, and contrails by using meteorological inputs directly. By plugging these functions directly into an aircraft trajectory optimisation module, climate sensitive regions are detected and avoided leading to climate optimised trajectories. While a preliminary verification conducted specifically for NOx-ozone aCCFs showed positive signs, this research entails a more detailed verification procedure, which provides a deeper insight into its capability in predicting the impact of aviation NOx emissions on atmospheric changes in ozone. Air traffic simulation is performed concerning a subset of one-day European flights on the specifically chosen days characterised by high variability of NOx-ozone aCCFs. The air traffic on these days is optimised for cost and climate. A subsequent chemistry simulation captures the NOx effects from these re-routing procedures, and the climate impact of both scenarios can thereby be directly compared. It is expected that the results will confirm the effectiveness of NOx-ozone aCCFs in producing climate-friendly trajectories.@en

Climate-optimized routing is an operational measure to effectively reduce the climate impact of aviation with a slight increase in aircraft operating costs. This study examined variations in the flight characteristics among five aircraft routing strategies and discusses several characteristics of those routing strategies concerning typical weather conditions over the North Atlantic. The daily variability in the North Atlantic weather patterns was analyzed by using the European Center Hamburg general circulation model (ECHAM) and the Modular Earth Submodel System (MESSy) Atmospheric Chemistry (EMAC) model in the specified dynamics mode from December 2008 to August 2018. All days of the ten complete winters and summers in the simulations were classified into five weather types for winter and into three types for summer. The obtained frequency for each of the weather types was in good agreement with the literature data; and then representative days for each weather type were selected. Moreover, a total of 103 North Atlantic flights of an Airbus A330 aircraft were simulated with five aircraft routing strategies for each representative day by using the EMAC model with the air traffic simulation submodel AirTraf. For every weather type, climate-optimized routing shows the lowest climate impact, at which a trade-off exists between the operating costs and the climate impact. Cost-optimized routing lies between the time-and fuel-optimized routings and achieves the lowest operating costs by taking the best compromise between flight time and fuel use. The aircraft routing for contrail avoidance shows the second lowest climate impact; however, this routing causes extra operating costs. Our methodology could be extended to statistical analysis based on long-term simulations to clarify the relationship between the aircraft routing characteristics and weather conditions.

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The recent COVID-19 pandemic with its countermeasures, e.g. lock-downs, resulted in decreases in emissions of various trace gases. Here we investigate the changes of ozone over Europe associated with these emission reductions using a coupled global/regional chemistry climate model. We conducted and analysed a business as usual and a sensitivity (COVID19) simulation. A source apportionment (tagging) technique allows us to make a sector-wise attribution of these changes, e.g. to natural and anthropogenic sectors such as land transport. Our simulation results show a decrease of ozone of 8% over Europe in May 2020 due to the emission reductions. The simulated reductions are in line with observed changes in ground-level ozone. The source apportionment results show that this decrease is mainly due to the decreased ozone precursors from anthropogenic origin. Further, our results show that the ozone reduction is much smaller than the reduction of the total NO x emissions (around 20%), mainly caused by an increased ozone production efficiency. This means that more ozone is produced for each emitted NO x molecule. Hence, more ozone is formed from natural emissions and the ozone productivities of the remaining anthropogenic emissions increase. Our results show that politically induced emissions reductions cannot be transferred directly to ozone reductions, which needs to be considered when designing mitigation strategies.

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Land transport is an important emission source of nitrogen oxides, carbon monoxide, and volatile organic compounds. The emissions of nitrogen oxides affect air quality directly. Further, all of these emissions serve as a precursor for the formation of tropospheric ozone, thus leading to an indirect influence on air quality. In addition, ozone is radiatively active and its increase leads to a positive radiative forcing. Due to the strong non-linearity of the ozone chemistry, the contribution of emission sources to ozone cannot be calculated or measured directly. Instead, atmospheric chemistry models equipped with specific source attribution methods (e.g. tagging methods) are required. In this study we investigate the contribution of land transport emissions to ozone and ozone precursors using the MECO(n) model system (MESSy-fied ECHAM and COSMO models nested n times). This model system couples a global and a regional chemistry climate model and is equipped with a tagging diagnostic. We investigate the combined effect of long-range-transported ozone and ozone which is produced by European emissions by applying the tagging diagnostic simultaneously and consistently on the global and regional scale. We performed two simulations each covering 3 years with different anthropogenic emission inventories for Europe. We applied two regional refinements, i.e. one refinement covering Europe (50&thinsp;<span classCombining double low line"inline-formula">km</span> resolution) and one covering Germany (12&thinsp;<span classCombining double low line"inline-formula">km</span> resolution). The diagnosed absolute contributions of land transport emissions to reactive nitrogen (<span classCombining double low line"inline-formula">NOy</span>) near ground level are in the range of 5 to 10&thinsp;<span classCombining double low line"inline-formula">nmol mol-1</span>. This corresponds to relative contributions of 50&thinsp;% to 70&thinsp;%. The largest absolute contributions appear around Paris, southern England, Moscow, the Po Valley, and western Germany. The absolute contributions to carbon monoxide range from 30&thinsp;<span classCombining double low line"inline-formula">nmol mol-1</span> to more than 75&thinsp;<span classCombining double low line"inline-formula">nmol mol-1</span> near emission hot-spots such as Paris or Moscow. The ozone which is attributed to land transport emissions shows a strong seasonal cycle with absolute contributions of 3&thinsp;<span classCombining double low line"inline-formula">nmol mol-1</span> during winter and 5 to 10&thinsp;<span classCombining double low line"inline-formula">nmol mol-1</span> during summer. This corresponds to relative contributions of 8&thinsp;% to 10&thinsp;% during winter and up to 16&thinsp;% during summer. The largest values during summer are confined to the Po Valley, while the contributions in western Europe range from 12&thinsp;% to 14&thinsp;%. Only during summer are the ozone contributions slightly influenced by the anthropogenic emission inventory, but these differences are smaller than the range of the seasonal cycle of the contribution to land transport emissions. This cycle is caused by a complex interplay of seasonal cycles of other emissions (e.g. biogenic) and seasonal variations of the ozone regimes. In addition, our results suggest that during events with large ozone values the ozone contributions of land transport and biogenic emissions increase strongly. Here, the contribution of land transport emissions peaks up to 28&thinsp;%. Hence, our model results suggest that land transport emissions are an important contributor during periods with large ozone values.

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Supersonic transport was the subject of intense debate in the 1970s and commercial operation was eventually abandoned until recently due to economic and environmental concerns. Flight emissions at stratospheric altitude differ from tropospheric emissions mainly in terms of longevity. Long lifetimes of chemically reactive emissions, especially in the presence of the stratospheric ozone layer, require a detailed investigation of the long-term impact of emissions at this altitude.
Recent studies show a faster degradation of stratospheric water vapor with increasing altitude, driven by photolysis and chemical reaction with O1D. This is seen as an opportunity for civil hypersonic transport. However, the climate impact of hypersonic flight has not yet been investigated.
This is why our study focuses on the emissions of hydrogen-powered hypersonic aircraft fleets (H2O, NOx, H2) in the middle and upper stratosphere (27 and 36 km). Three different scenarios based on the HIKARI emission data allow an altitude dependent comparison of hypersonic emissions. The scenarios were simulated with ECHAM5/MESSy (v2.54.0), including a newly developed submodel H2OEMIS to integrate external water vapor emissions into the cycle of specific humidity.
Additional simulations using different models for comparison are planned with Didier Hauglustaine (LSCE) in the context of project 'Stratofly' funded by EU-Horizon 2020.
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A climate-optimized routing is expected as an operational measure to reduce the climate impact of aviation, whereas this routing causes extra aircraft operating costs. This study performs some air traffic simulations of nine aircraft routing strategies which include the climate-optimized routing, and examines characteristics of those routings. A total of 103 trans-Atlantic flights of an Airbus A330 is simulated for five weather types in winter and for three types in summer over the North Atlantic by using the chemistry-climate model EMAC with the air traffic simulation submodel AirTraf. For every weather type, the climate-optimized routing shows the minimum climate impact, whereas a trade-off exists between the costs and the climate impact. The cost-optimized routing lies between time- and fuel-optimized routings, and minimizes the costs. The aircraft routing for minimum contrail formation shows the second-lowest climate impact, whereas this routing also causes extra costs. @en

Anthropogenic and natural emissions influence the tropospheric ozone budget, thereby affecting air quality and climate. To study the influence of different emission sources on the ozone budget, often source apportionment studies with a tagged tracer approach are performed. Studies investigating air quality issues usually rely on regional models with a fine spatial resolution, while studies focusing on climaterelated questions often use coarsely resolved global models. It is well known that simulated ozone mixing ratios depend on the resolution of the model and the resolution of the emission inventory. Whether the contributions simulated using source apportionment approaches also depend on the model resolution, however, is still unclear. Therefore, this study attempts for the first time to analyse the impact of the model, the model resolution, and the emission inventory resolution on simulated ozone contributions using a diagnostic tagging method. The differences in the ozone contributions caused by these factors are compared with differences that arise from the usage of different emission inventories. To do so, we apply the MECO(n) (MESSy-fied ECHAM and COSMO models nested n times) model system which couples online a global chemistry-climate model with a regional chemistry-climate model equipped with a tagging scheme for source apportionment. The results of the global model (at 300 km horizontal resolution) are compared with the results of the regional model at 50 km (Europe) and 12 km (Germany) resolutions. Besides model-specific differences and biases that are discussed in detail, our results have important implications for other modelling studies and modellers applying source apportionment methods. First, contributions from anthropogenic emissions averaged over the continental scale are quite robust with respect to the model, model resolution, and emission inventory resolution. Second, differences on the regional scale caused by different models and model resolutions can be quite large, and regional models are indispensable for source apportionment studies on the subcontinental scale. Third, contributions from stratospheric ozone transported to the surface differ strongly between the models, mainly caused by differences in the efficiency of the vertical mixing. As stratospheric ozone plays an important role for ground level ozone, but the models show large differences in the amount of downward transported ozone, source apportionment methods should account for this source explicitly to better understand inter-model differences.

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Aviation contributes to climate change, and the climate impact of aviation is expected to increase further. Adaptations of aircraft routings in order to reduce the climate impact are an important climate change mitigation measure. The air traffic simulator AirTraf, as a submodel of the European Center HAMburg general circulation model (ECHAM) and Modular Earth Submodel System (MESSy) Atmospheric Chemistry (EMAC) model, enables the evaluation of such measures. For the first version of the submodel AirTraf, we concentrated on the general setup of the model, including departure and arrival, performance and emissions, and technical aspects such as the parallelization of the aircraft trajectory calculation with only a limited set of optimization possibilities (time and distance). Here, in the second version of AirTraf, we focus on enlarging the objective functions by seven new options to enable assessing operational improvements in many more aspects including economic costs, contrail occurrence, and climate impact. We verify that the AirTraf setup, e.g., in terms of number and choice of design variables for the genetic algorithm, allows us to find solutions even with highly structured fields such as contrail occurrence. This is shown by example simulations of the new routing options, including around 100 North Atlantic flights of an Airbus A330 aircraft for a typical winter day. The results clearly show that AirTraf 2.0 can find the different families of optimum flight trajectories (three-dimensional) for specific routing options; those trajectories minimize the corresponding objective functions successfully. The minimum cost option lies between the minimum time and the minimum fuel options. Thus, aircraft operating costs are minimized by taking the best compromise between flight time and fuel use. The aircraft routings for contrail avoidance and minimum climate impact reduce the potential climate impact which is estimated by using algorithmic climate change functions, whereas these two routings increase the aircraft operating costs. A trade-off between the aircraft operating costs and the climate impact is confirmed. The simulation results are compared with literature data, and the consistency of the submodel AirTraf 2.0 is verified.@en