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Using a transient simulation for the period 1960–2100 with the state-of-the-art ECHAM5/MESSy Atmospheric Chemistry (EMAC) global model and a tropopause fold identification algorithm, we explore the future projected changes in tropopause folds, stratosphere-to-troposphere transport (STT) of ozone, and tropospheric ozone under the RCP6.0 scenario. Statistically significant changes in tropopause fold frequencies from 1970–1999 to 2070–2099 are identified in both hemispheres, regionally exceeding 3 %, and are associated with the projected changes in the position and intensity of the subtropical jet streams. A strengthening of ozone STT is projected for the future in both hemispheres, with an induced increase in transported stratospheric ozone tracer throughout the whole troposphere, reaching up to 10 nmol mol−1 in the upper troposphere, 8 nmol mol−1 in the middle troposphere, and 3 nmol mol−1 near the surface. Notably, the regions exhibiting the largest changes of ozone STT at 400 hPa coincide with those with the highest fold frequency changes, highlighting the role of the tropopause folding mechanism in STT processes under a changing climate. For both the eastern Mediterranean and Middle East (EMME) and Afghanistan (AFG) regions, which are known as hotspots of fold activity and ozone STT during the summer period, the year-to-year variability of middle-tropospheric ozone with stratospheric origin is largely explained by the short-term variations in ozone at 150 hPa and tropopause fold frequency. Finally, ozone in the lower troposphere is projected to decrease under the RCP6.0 scenario during MAM (March, April, and May) and JJA (June, July, and August) in the Northern Hemisphere and during DJF (December, January, and February) in the Southern Hemisphere, due to the decline of ozone precursor emissions and the enhanced ozone loss from higher water vapour abundances, while in the rest of the troposphere ozone shows a remarkable increase owing mainly to the STT strengthening and the stratospheric ozone recovery.

We present here the first cloud condensation nuclei (CCN) concentration profiles derived from measurements with the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) aboard the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO), for different aerosol types at a supersaturation of 0.15%. CCN concentrations, along with the corresponding uncertainties, were inferred for a nighttime CALIPSO overpass on 9 September 2011, with coincident observations with the Facility for Airborne Atmospheric Measurements (FAAM) BAe-146 research aircraft, within the framework of the Evaluation of CALIPSO’s Aerosol Classification scheme over Eastern Mediterranean (ACEMED) research campaign over Thessaloniki, Greece. The CALIPSO aerosol typing is evaluated, based on data from the Copernicus Atmosphere Monitoring Service (CAMS) reanalysis. Backward trajectories and satellite-based fire counts are used to examine the origin of air masses on that day. Our CCN retrievals are evaluated against particle number concentration retrievals at different height levels, based on the ACEMED airborne measurements and compared against CCN-related retrievals from the Moderate Resolution Imaging Spectroradiometer (MODIS) sensors aboard Terra and Aqua product over Thessaloniki showing that it is feasible to obtain CCN concentrations from CALIPSO, with an uncertainty of a factor of two to three.

Within the framework of the Copernicus Atmosphere Monitoring Service (CAMS) element CAMS-84 (Global and regional a posteriori evaluation and quality assurance), we analyze and evaluate the performance of CAMS forecast systems during the passage of ex-hurricane Ophelia in mid-October 2017, carrying Saharan dust and Iberian fire smoke over several Western European regions. To this end, day-1 forecasts from CAMS-global (ECMWF Integrated Forecast System; IFS) and CAMS-regional (ensemble of seven regional air quality models) products are compared against satellite retrievals (MODIS/Terra and Aqua, CALIPSO) and ground-based measurements. The analysis indicates that dust and smoke are injected into the warm sector of Ophelia, lying in the vicinity of the warm and cold front, respectively, gradually affecting the air quality and atmospheric composition over France, the Netherlands and Great Britain. The distinct pattern of enhanced aerosol optical depth (AOD) over Western coastal Europe seen in satellite retrievals is well reproduced by the CAMS near-real time forecast. The observed implications for air quality (PM10 and PM2.5) are satisfactorily forecasted in qualitative terms by both CAMS-global and CAMS-regional systems, while in quantitative terms, the CAMS-regional system exhibits a better performance in predicting surface PM concentrations (higher correlation and lower bias) compared to the global.

We present an analysis of the fast responses on pre-industrial climate due to present-day aerosols in a multi-model study based on Coupled Model Intercomparison Project Phase 6 (CMIP6) simulations from 10 Earth System Models (ESMs) and General Circulation Models (GCMs). The aforementioned simulations were implemented within the framework of the Aerosol Chemistry Model Intercomparison Project (AerChemMIP). All models carried out two sets of simulations; a control experiment with all forcings set to the year 1850 and a perturbation experiment with all forcings identical to the control, except for aerosols with precursor emissions set to the year 2014. The perturbation by the present-day aerosols indicates negative top of the atmosphere (TOA) effective radiative forcing (ERF) values around the globe, especially over continental regions of the Northern Hemisphere in summer, with the largest negative values appearing over East Asia. Simulations in 3 models (CNRM-ESM2-1, MRI-ESM2-0 and NorESM2-LM) with individual perturbation experiments using present day SO2, BC and OC emissions show the dominating role of sulfates in all-aerosols ERF. In response to the pattern of all aerosols ERF, the fast temperature responses are characterised by cooling over the continental areas, especially in the Northern Hemisphere, with the largest cooling over East Asia and India and sulfate being the dominant aerosol surface temperature driver for present-day emissions. The largest fast precipitation responses are seen in the tropical belt regions, generally characterized by a reduction over continental regions and a southward shift of the tropical rain belt. This is a characteristic and robust feature among most models in this study, associated with a southward shift of the Intertropical convergence zone (ITCZ) and a weakening of the monsoon systems around the globe (Asia, Africa and America) in response to hemispherically asymmetric cooling from a Northern Hemisphere aerosol perturbation. An interesting feature in aerosol induced circulation changes is a characteristic dipole pattern with intensification of the Icelandic Low and an anticyclonic anomaly over Southeastern Europe, inducing warm air advection towards the northern polar latitudes in winter.This research was funded by the project "PANhellenic infrastructure for Atmospheric Composition and climatE change" (MIS 5021516) which is implemented under the Action "Reinforcement of the Research and Innovation Infrastructure", funded by the Operational Programme "Competitiveness, Entrepreneurship and Innovation" (NSRF 2014-2020) and co-financed by Greece and the European Union (European Regional Development Fund).

Following the rapid spread of COVID-19, a lockdown was imposed in Thessaloniki, Greece, resulting in an abrupt reduction of human activities. To unravel the impact of restrictions on the urban air quality of Thessaloniki, NO2 and O3 observations are compared against the business-as-usual (BAU) concentrations for the lockdown period. BAU conditions are modeled, applying the XGBoost (eXtreme Gradient Boosting) machine learning algorithm on air quality and meteorological surface measurements, and reanalysis data. A reduction in NO2 concentrations is found during the lockdown period due to the restriction policies at both AGSOFIA and EGNATIA stations of −24.9 [−26.6, −23.2]% and −18.4 [−19.6, −17.1]%, respectively. A reverse effect is revealed for O3 concentrations at AGSOFIA with an increase of 12.7 [10.8, 14.8]%, reflecting the reduced O3 titration by NOx. The implications of COVID-19 lockdowns in the urban air quality of Thessaloniki are in line with the results of several recent studies for other urban areas around the world, highlighting the necessity of more sophisticated emission control strategies for urban air quality management.

In this work, we use Coupled Model Intercomparison Project Phase 6 (CMIP6) simulations from 10 Earth system models (ESMs) and general circulation models (GCMs) to study the fast climate responses on pre-industrial climate, due to present-day aerosols. All models carried out two sets of simulations: a control experiment with all forcings set to the year 1850 and a perturbation experiment with all forcings identical to the control, except for aerosols with precursor emissions set to the year 2014. In response to the pattern of all aerosols effective radiative forcing (ERF), the fast temperature responses are characterized by cooling over the continental areas, especially in the Northern Hemisphere, with the largest cooling over East Asia and India, sulfate being the dominant aerosol surface temperature driver for present-day emissions. In the Arctic there is a warming signal for winter in the ensemble mean of fast temperature responses, but the model-to-model variability is large, and it is presumably linked to aerosol-induced circulation changes. The largest fast precipitation responses are seen in the tropical belt regions, generally characterized by a reduction over continental regions and presumably a southward shift of the tropical rain belt. This is a characteristic and robust feature among most models in this study, associated with weakening of the monsoon systems around the globe (Asia, Africa and America) in response to hemispherically asymmetric cooling from a Northern Hemisphere aerosol perturbation, forcing possibly the Intertropical Convergence Zone (ITCZ) and tropical precipitation to shift away from the cooled hemisphere despite that aerosols' effects on temperature and precipitation are only partly realized in these simulations as the sea surface temperatures are kept fixed. An interesting feature in aerosol-induced circulation changes is a characteristic dipole pattern with intensification of the Icelandic Low and an anticyclonic anomaly over southeastern Europe, inducing warm air advection towards the northern polar latitudes in winter.

Climate change projections predict the increase of no-rain periods and storm intensity resulting in high hydrologic alteration of the Mediterranean rivers. Intermittent flow Rivers and Ephemeral Streams (IRES) are particularly vulnerable to spatiotemporal variation of climate variables, land use changes and other anthropogenic factors. In this work, the impact of climate change on the aquatic state of IRES is assessed by the combination of the hydrological model Soil and Water Assessment Tool (SWAT) and the Temporary Rivers Ecological and Hydrological Status (TREHS) tool under two different Representative Concentration Pathways (RCP 4.5 and RCP 8.5) using CORDEX model simulations. A significant decrease of 20–40% of the annual flow of the examined river (Tsiknias River, Greece) is predicted during the next 100 years with an increase in the frequency of extreme flood events as captured with almost all Regional Climate Models (RCMs) simulations. The occurrence patterns of hyporheic and edaphic aquatic states show a temporal extension of these states through the whole year due to the elongation of the dry period. A shift to the Intermittent-Pools regime type shows dominance according to numerous climate change scenarios, harming, as a consequence, both the ecological system and the social-economic one.

The summertime Eastern Mediterranean experiences among the highest worldwide near-surface ozone values often exceeding the EU air-quality standard for human health protection. Downward transport of stratospheric ozone-rich air masses through tropopause folds has been shown to significantly contribute to these high tropospheric ozone levels. The summer circulation over the Eastern Mediterranean is dominated by large-scale subsidence and persistent northerly winds, the Etesians, as well as tropopause fold activity affecting tropospheric ozone levels and variability. This is the first study that directly investigates the connections of the Etesians with tropopause folds and tropospheric ozone over the Eastern Mediterranean region using the state-of-the-art ECHAM5/MESSy Atmospheric Chemistry (EMAC) climate model and a tropopause fold identification algorithm for the period June‐August 1979 to 2013. Model validation against reanalysis and observational measurements indicates that EMAC simulates well the sea level pressure gradient over the Aegean Sea reproducing the occurrence frequency of the Etesians. The shared variance between the simulated and the observed year-to-year frequency of the Etesian winds is higher than 80%. The year-to-year variability of summertime tropopause folds frequency is significantly correlated with the Etesian winds with a correlation coefficient close to 0.7. In addition, a significant increase of the fold activity and the ozone concentrations are observed during intense Etesians. Our findings may have implications for air quality studies and could support human health policy, as the Etesians coincide with favorable conditions for the formation of tropopause folds and stratosphere-to-troposphere transport and therefore, with enhanced ozone levels over the Eastern Mediterranean.

Stratosphere-to-troposphere transport (STT) is an important natural source of tropospheric ozone, which can occasionally influence ground-level ozone concentrations relevant for air quality. Here, we analyse and evaluate the Copernicus Atmosphere Monitoring Service (CAMS) global and regional forecast systems during a deep STT event over Europe for the time period from 4 to 9 January 2017. The predominant synoptic condition is described by a deep upper level trough over eastern and central Europe, favouring the formation of tropopause folding events along the jet stream axis and therefore the intrusion of stratospheric ozone into the troposphere. Both global and regional CAMS forecast products reproduce the “hook-shaped” streamer of ozone-rich and dry air in the middle troposphere depicted from the observed satellite images of water vapour. The CAMS global model successfully reproduces the folding of the tropopause at various European sites, such as Trapani (Italy), where a deep folding down to 550 hPa is seen. The stratospheric ozone intrusions into the troposphere observed by WOUDC ozonesonde and IAGOS aircraft measurements are satisfactorily forecasted up to 3 days in advance by the CAMS global model in terms of both temporal and vertical features of ozone. The fractional gross error (FGE) of CAMS ozone day 1 forecast between 300 and 500 hPa is 0.13 over Prague, while over Frankfurt it is 0.04 and 0.19, highlighting the contribution of data assimilation, which in most cases improves the model performance. Finally, the meteorological and chemical forcing of CAMS global forecast system in the CAMS regional forecast systems is found to be beneficial for predicting the enhanced ozone concentrations in the middle troposphere during a deep STT event.

The RegCM3/CAMx modeling system is applied to estimate near surface ozone trends over Europe for the time period 1996–2006. In order to assess the impact of changing anthropogenic emissions and meteorology on modeled ozone trends two simulations were performed: The first simulation (CONEM) was forced from constant anthropogenic emissions based on the EMEP emissions of the year 1996, while the second simulation (VAREM) was forced from year to year varying anthropogenic emissions based on the EMEP emissions of the years 1996–2006. Near surface ozone measurements from the EMEP database are used for assessing the skill of the modeling system to reproduce the observed ozone trends. Concerning VAREM simulation the main pattern is a significant positive annual trend over southern UK and Benelux which is also detected in the observations, attributed to the reduction of ozone titration by NO as a result of NOx emissions control policies. Significant negative ozone trends are found over the Mediterranean due to reduction of ozone precursors emissions over continental Europe. As far as it concerns CONEM simulation a significant increase of ozone is found during summer associated with the heatwave event of 2003. Overall, the observed ozone trends are captured fairly well by the RegCM3/CAMx modeling system over the highly polluted areas of central and western Europe but only partially reproduced or not captured at all at the rest of Europe, probably due to limitations of the model setup to capture background ozone variability related to processes such as intercontinental transport and stratosphere–troposphere exchange.

A new global reanalysis data set of atmospheric composition (AC) for the period 2003–2015 has been produced by the Copernicus Atmosphere Monitoring Service (CAMS). Satellite observations of total column (TC) carbon monoxide (CO) and aerosol optical depth (AOD), as well as several TC and profile observations of ozone, have been assimilated with the Integrated Forecasting System for Composition (C-IFS) of the European Centre for Medium-Range Weather Forecasting. Compared to the previous Monitoring Atmospheric Composition and Climate (MACC) reanalysis (MACCRA), the new CAMS interim reanalysis (CAMSiRA) is of a coarser horizontal resolution of about 110 km, compared to 80 km, but covers a longer period with the intent to be continued to present day. This paper compares CAMSiRA with MACCRA and a control run experiment (CR) without assimilation of AC retrievals. CAMSiRA has smaller biases than the CR with respect to independent observations of CO, AOD and stratospheric ozone. However, ozone at the surface could not be improved by the assimilation because of the strong impact of surface processes such as dry deposition and titration with nitrogen monoxide (NO), which were both unchanged by the assimilation. The assimilation of AOD led to a global reduction of sea salt and desert dust as well as an exaggerated increase in sulfate. Compared to MACCRA, CAMSiRA had smaller biases for AOD, surface CO and TC ozone as well as for upper stratospheric and tropospheric ozone. Finally, the temporal consistency of CAMSiRA was better than the one of MACCRA. This was achieved by using a revised emission data set as well as by applying careful selection and bias correction to the assimilated retrievals. CAMSiRA is therefore better suited than MACCRA for the study of interannual variability, as demonstrated for trends in surface CO.

A 35 year climatological analysis (1979–2013) of tropopause fold frequency has been performed over the eastern Mediterranean and the Middle East, focusing on the implications for tropospheric ozone. The analysis is based on simulations with the EMAC (ECHAM5-MESSy) atmospheric chemistry climate model using a T42L90MA resolution, nudged tropospheric meteorology towards ERA-Interim reanalysis data and a tracer for stratospheric ozone (O3s). We implement a 3-D labeling algorithm, in order to detect tropopause folding events in EMAC simulations as areas of multiple crossings of the dynamical tropopause. A clear hot spot of tropopause folds occurs over the broader eastern Mediterranean and Middle East region during the summer season. A significant contribution of tropopause folds to the summertime pool of high tropospheric ozone over the eastern Mediterranean is identified during the selected fold events. Finally, a year-to-year analysis indicates a significant positive correlation between the observed near surface ozone and EMAC simulated middle/lower tropospheric ozone over the eastern Mediterranean.

This case study investigates a stratospheric intrusion event down to the earth’s surface (near sea-level pressure) of the greater area of Athens (23.43°E 37.58°N), which occurred on 9 October 2003 and caused a remarkable increase in surface ozone concentrations not related to photochemical production. This event is among the rare case studies investigating, on the one hand, a deep stratospheric intrusion down to the earth’s surface at near sea-level pressure and, on the other, an event affecting the near surface ozone of a megacity such as Athens. The synoptic situation is described by a deep upper lever trough at 300 and 500 hPa extending over Greece, which is related to a deep tropopause fold as revealed by vertical cross sections of potential vorticity, relative humidity, divergence and vertical velocity. The analysis of potential vorticity at several isentropic levels indicates a hook-shaped streamer of high PV values (greater than 4 pvu at the 315 K isentropic level) over southeast Europe, which coincides with a streamer of dry air as observed from satellite images of water vapor. The aforementioned structure characterizes a textbook case study of stratosphere-to-troposphere transport. The Lagrangian particle dispersion model FLEXPART was used to calculate the trajectories of air particles reaching the receptor site and the fraction of particles with stratospheric origin. It reveals an important direct stratospheric impact within 1 day related to the tropopause fold described in this study with the fraction of stratospheric particles reaching maximum values of 1.9 and 4.5% for threshold values of the dynamical tropopause 2 and 1.5 pvu, respectively. Furthermore, a larger indirect aged stratospheric contribution is also revealed 4 to 5 days prior to the release, related to stratospheric intrusion events at the western Atlantic Ocean, reaching maximum values of 2.5 and 6.9% of particles crossing the 2 and 1.5 pvu potential vorticity surfaces, respectively.

A modeling system based on the air quality model CAMx driven off-line by the regional climate model RegCM3 is used for assessing the impact of lateral boundary conditions and anthropogenic emissions on tropospheric ozone over Europe for the period 1996–2006. The RegCM3 and CAMx simulations were performed on a 50 km × 50 km grid over Europe with RegCM3 driven by NCEP reanalysis fields. Average monthly concentration values obtained from the global chemistry climate model ECHAM5-MOZ were used as chemical boundary conditions for the CAMx simulations. The present period (1996–2006) was simulated four times. The first run was forced with time and space invariable lateral chemical boundary conditions and EMEP emissions based on the year 1996. The second decadal simulation was based on ECHAM5-MOZ chemical boundary conditions and emissions both fixed for the year 1996. The third decadal simulation was based on ECHAM5-MOZ chemical boundary conditions with interannual variation but fixed emissions from the year 1996. Finally, the fourth decadal simulation was based on ECHAM5-MOZ chemical boundary conditions and emissions, both having interannual variation. Simulated ozone concentrations are compared against measurements from the EMEP network in order to evaluate the modeling system.