Emissions from land-based transport, aviation, and shipping contribute significantly to climate change. Besides CO₂, these emissions include short-lived compounds that affect air quality but are also climatically relevant. We use a global chemistry-climate model to show that the climate effects of these non-CO₂ emissions are substantial across all transport sectors both now and in the future. In sum, the non-CO₂ impacts result in a cooling, which offsets the positive climate forcing from transport-induced CO₂ by around 80% at present and between 25 and 60% in different scenarios for 2050. The trade-off that air pollutants mitigate global warming is strongly reduced in a future scenario with low anthropogenic emissions, where even small remaining amounts of non-CO₂ compounds cause significant cooling as they are released in a very clean atmosphere. Our findings emphasize the need to take non-CO₂ effects into account when assessing climate protection strategies for the transport sectors.

The photolysis of nitrous acid (HONO) produces hydroxyl radicals (OH), the most important cleaning agent of the troposphere. For decades, HONO has been measured in concentrations which exceed the photo-stationary concentration arising from its gas phase formation via the reaction NO + OH and destruction by photolysis. Several photochemical and heterogeneous formation mechanisms, including the photolysis of nitrate have been proposed which may explain this excess HONO. This study reports on airborne remote sensing measurements of the mini-DOAS instrument over continental Europe, Southeast Asia, and the tropical Atlantic. The observations form a C-shaped profile in the troposphere with maximum volume mixing ratios of approximately 150 ppt in the planetary boundary layer, about 10 ppt in the free troposphere and up to 100 ppt in the tropical upper troposphere. These measurements of HONO throughout the troposphere exceed model predictions by up to an order of magnitude. Together with a host of other measured species and parameters, various formation mechanisms are explored to investigate in situ HONO sources. Although a precise formation mechanism in the polluted boundary layer remains elusive, the photolysis of particulate nitrate may explain excess HONO in the marine boundary layer. The excess HONO observed in the upper troposphere requires a gas phase source with a formation rate of up to 300 ppth^-1. The possible role of peroxynitrous acid (HOONO), formed by the reactions NO + HO₂ + M and NO₂ + OH + M, and further oxidation by reactions with NO or O₃, is explored.

The chemical sink of atmospheric methane (CH₄) depends on the temperature and on the chemical composition. Here, we assess the feedback on atmospheric CH₄ induced by changes in the chemical sink in a warming climate using a CH₄-emission-flux-driven set-up of the chemistry-climate model EMAC (ECHAM/MESSy Atmospheric Chemistry), in which the chemical feedback of CH₄ mixing ratios can evolve explicitly. We perform idealized perturbation simulations driven either by increased carbon dioxide (CO₂) mixing ratios or by increased CH₄ emission fluxes. The CH₄ emission flux perturbation leads to a large increase of CH₄ mixing ratios. Remarkably, the factor by which the CH₄ mixing ratio increases is larger than the increase factor of the emission flux, because the atmospheric lifetime of CH₄ is extended. In contrast, the individual effect of the global surface air temperature (GSAT) increase is to shorten the CH₄ lifetime, which results in a significant reduction of CH₄ mixing ratios in our set-up. The corresponding radiative feedback is estimated at -0.041 and -0.089 W m^-2 K^-1 for the CO₂ and CH₄ perturbation, respectively. The explicit response of CH₄ mixing ratios leads to secondary feedbacks on the hydroxyl radical (OH) and ozone (O₃). Firstly, the OH response includes the CH₄-OH feedback, which enhances the CH₄ lifetime change, and, secondly, the formation of tropospheric O₃ is reduced.

This study assesses the influence of anthropogenic emission inventories of ozone (O3) precursor species (i.e. NOx, CO, and non-methane hydrocarbons (NMHCs)) prescribed in the simulations of the two phases of the Chemistry-Climate Model Initiative (CCMI) on tropospheric O3, hydroxyl radical (OH), and the methane (CH4) lifetime. We performed two transient simulations for the period 2000-2010 with the chemistry-climate model EMAC, one prescribing the emission inventory of CCMI-1 and the other that of CCMI-2022. Using the tagging approach, we attribute the differences of O3, OH, and the tropospheric CH4 lifetime to individual emission sectors. It is, to our knowledge, the first application of the tagging approach to attribute changes of the simulated CH4 lifetime to individual emission sectors. The emission inventory used for CCMI-2022 leads to a 3.7 % larger tropospheric O3 column and to a 3.2 % shorter tropospheric CH4 lifetime compared to CCMI-1 in the Northern Hemisphere. In the Southern Hemisphere, the tropospheric O3 column is 4.5 % larger, and the tropospheric CH4 lifetime 4.3 % shorter. Differences in tropospheric O3 are largely driven by changes of emissions from the anthropogenic non-traffic and land transport sectors in the Northern Hemisphere. In the Southern Hemisphere, the primary contributors are emissions from anthropogenic non-traffic, biomass burning, and shipping. These sectors also play a significant role in reducing the simulated tropospheric CH4 lifetime. However, the contribution of a particular sector to changes in O3 does not necessarily align with its impact on the CH4 lifetime.

Flight altitude is relevant to the climate effects resulting from aircraft emissions. Other research has shown that flying higher within the troposphere leads to larger warming from O ₃ production. Aircraft NO _x emissions are of particular interest, as they lead to warming via the short-term production of O ₃, but also to reduced warming via processes like CH ₄ depletion. We focus on short-term O ₃ production, as it constitutes one of aviation’s largest warming components. Understanding how O ₃ formation varies altitudinally throughout the upper troposphere/lower stratosphere is essential for designing climate-compatible aircraft and routing. We quantify this variation by performing simulations with a global atmospheric chemistry model for three representative cruise altitudes, five regions and two seasons using three methods: Eulerian tagging, perturbation and Lagrangian tagging. This multi-method, regional approach overcomes limitations of previous studies that utilize only one of these methods and apply global emission inventories biased towards present-day flight distributions, thus limiting their applicability to future aviation scenarios. Our results highlight that underrepresenting emissions in areas with growing flight activity (e.g. Asia Pacific) may lead to significant, regional underestimations of the altitudinal sensitivity of short-term NO _x -related O ₃ warming effects in certain cases. We find that emitting in Southern regions, like Australasia, leads to warming larger by a factor of two when compared to global averages. Our findings also suggest that flying lower translates to lower warming from short-term O ₃ production and that this effect is strongest during the local summer. We estimate differences ranging from a factor of 1.2-2.6 between tagging and perturbation results that are attributable to non-linearities of NO _x -O ₃ chemistry, and derived regional correction factors for a widely-used sub-model. Overall, we stress that a combination of all three methods is necessary for a robust assessment of aviation climate effects as they address fundamentally different questions.

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.

During spring 2020, the COVID-19 pandemic caused massive reductions in emissions from industry and ground and airborne transportation. To explore the resulting atmospheric composition changes, we conducted the BLUESKY campaign with two research aircraft and measured trace gases, aerosols, and cloud properties from the boundary layer to the lower stratosphere. From 16 May to 9 June 2020, we performed 20 flights in the early COVID-19 lockdown phase over Europe and the Atlantic Ocean. We found up to 50% reductions in boundary layer nitrogen dioxide concentrations in urban areas from GOME-2B satellite data, along with carbon monoxide reductions in the pollution hot spots. We measured 20%-70% reductions in total reactive nitrogen, carbon monoxide, and fine mode aerosol concentration in profiles over German cities compared to a 10-yr dataset from passenger aircraft. The total aerosol mass was significantly reduced below 5 km altitude, and the organic aerosol fraction also aloft, indicative of decreased organic precursor gas emissions. The reduced aerosol optical thickness caused a perceptible shift in sky color toward the blue part of the spectrum (hence BLUESKY) and increased shortwave radiation at the surface. We find that the 80% decline in air traffic led to substantial reductions in nitrogen oxides at cruise altitudes, in contrail cover, and in resulting radiative forcing. The light extinction and depolarization by cirrus were also reduced in regions with substantially decreased air traffic. General circulation-chemistry model simulations indicate good agreement with the measurements when applying a reduced emission scenario. The comprehensive BLUESKY dataset documents the major impact of anthropogenic emissions on the atmospheric composition.

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.

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.

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.

We quantify the contribution of land transport and shipping emissions to tropospheric ozone for the first time with a chemistry-climate model including an advanced tagging method (also known as source apportionment), which considers not only the emissions of nitrogen oxides (NOx, NO, and NO2), carbon monoxide (CO), and volatile organic compounds (VOC) separately, but also their non-linear interaction in producing ozone. For summer conditions a contribution of land transport emissions to ground-level ozone of up to 18% in North America and Southern Europe is estimated, which corresponds to 12 and 10nmolĝ€†mol-1, respectively. The simulation results indicate a contribution of shipping emissions to ground-level ozone during summer on the order of up to 30% in the North Pacific Ocean (up to 12nmolĝ€†mol-1) and 20% in the North Atlantic Ocean (12nmolĝ€†mol-1). With respect to the contribution to the tropospheric ozone burden, we quantified values of 8 and 6% for land transport and shipping emissions, respectively. Overall, the emissions from land transport contribute around 20% to the net ozone production near the source regions, while shipping emissions contribute up to 52% to the net ozone production in the North Pacific Ocean. To put these estimates in the context of literature values, we review previous studies. Most of them used the perturbation approach, in which the results for two simulations, one with all emissions and one with changed emissions for the source of interest, are compared. For a better comparability with these studies, we also performed additional perturbation simulations, which allow for a consistent comparison of results using the perturbation and the tagging approach. The comparison shows that the results strongly depend on the chosen methodology (tagging or perturbation approach) and on the strength of the perturbation. A more in-depth analysis for the land transport emissions reveals that the two approaches give different results, particularly in regions with large emissions (up to a factor of 4 for Europe). Our estimates of the ozone radiative forcing due to land transport and shipping emissions are, based on the tagging method, 92 and 62mWĝ€†m-2, respectively. Compared to our best estimates, previously reported values using the perturbation approach are almost a factor of 2 lower, while previous estimates using NOx-only tagging are almost a factor of 2 larger. Overall our results highlight the importance of differentiating between the perturbation and the tagging approach, as they answer two different questions. In line with previous studies, we argue that only the tagging approach (or source apportionment approaches in general) can estimate the contribution of emissions, which is important to attribute emission sources to climate change and/or extreme ozone events. The perturbation approach, however, is important to investigate the effect of an emission change. To effectively assess mitigation options, both approaches should be combined. This combination allows us to track changes in the ozone production efficiency of emissions from sources which are not mitigated and shows how the ozone share caused by these unmitigated emission sources subsequently increases.

To mitigate the human impact on climate change, it is essential to determine the contribution of emissions to the concentration of trace gases. In particular, the source attribution of short-lived species such as OH and HO₂ is important as they play a crucial role for atmospheric chemistry. This study presents an advanced version of a tagging method for OH and HO₂ (HO_x) which attributes HO_x concentrations to emissions. While the former version (V1.0) only considered 12 reactions in the troposphere, the new version (V1.1), presented here, takes 19 reactions in the troposphere into account. For the first time, the main chemical reactions for the HO_x chemistry in the stratosphere are also regarded (in total 27 reactions). To fully take into account the main HO₂ source by the reaction of H and O₂, the tagging of the H radical is introduced. In order to ensure the steady-state assumption, we introduce rest terms which balance the deviation of HO_x production and loss. This closes the budget between the sum of all contributions and the total concentration. The contributions to OH and HO₂ obtained by the advanced tagging method V1.1 deviate from V1.0 in certain source categories. For OH, major changes are found in the categories biomass burning, biogenic emissions and methane decomposition. For HO₂, the contributions differ strongly in the categories biogenic emissions and methane decomposition. As HO_x reacts with ozone (O₃), carbon monoxide (CO), reactive nitrogen compounds (NO_y), non-methane hydrocarbons (NMHCs) and peroxyacyl nitrates (PAN), the contributions to these species are also modified by the advanced HO_x tagging method V1.1. The contributions to NO_y, NMHC and PAN show only little change, whereas O₃ from biogenic emissions and methane decomposition increases in the tropical troposphere. Variations for CO from biogenic emissions and biomass burning are only found in the Southern Hemisphere.