Ozone source apportionment during peak summer events over southwestern Europe

It is well established that in Europe, high O3 concentrations are most pronounced in southern/Mediterranean countries due to the more favourable climatological conditions for its formation. However, the contribution of the different sources of precursors to O3 formation within each country relative to the imported (regional and hemispheric) O3 is poorly quantified. This lack of quantitative knowledge prevents local authorities from effectively designing plans that reduce the exceedances of the O3 target value set by the European air quality directive. O3 source attribution is a challenge because the concentration at each location and time results not only from local biogenic and anthropogenic precursors, but also from the transport of O3 and precursors from neighbouring regions, O3 regional and hemispheric transport and stratospheric O3 injections. The main goal of this study is to provide a first quantitative estimation of the contribution of the main anthropogenic activity sectors to peak O3 events in Spain relative to the contribution of imported (regional and hemispheric) O3. We also assess the potential of our source apportionment method to improve O3 modelling. Our study applies and thoroughly evaluates a countrywide O3 source apportionment method implemented in the CALIOPE air quality forecast system for Spain at high resolution (4 × 4 km2) over a 10-day period characterized by typical summer conditions in the Iberian Peninsula (IP). The method tags both O3 and its gas precursor emissions from source sectors within one simulation, and each tagged species is subject to the typical physico-chemical processes (advection, vertical mixing, deposition, emission and chemistry) as the actual conditions remain unperturbed. We quantify the individual contributions of the largest NOx local sources to high O3 concentrations compared with the contribution of imported O3. We show, for the first time, that imported O3 is the largest input to the ground-level O3 concentration in the IP, accounting for 46 %–68 % of the daily mean O3 concentration during exceedances of the European target value. The hourly imported O3 increases during typical northwestern advections (70 %–90 %, 60–80 µg m−3), and decreases during typical stagnant conditions (30 %–40 %, 30–60 µg m−3) due to the local NO titration. During stagnant conditions, the local anthropogenic precursors control the O3 peaks in areas downwind of the main urban and industrial regions (up to 40 % in hourly peaks). We also show that ground-level O3 concentrations are strongly affected by vertical mixing of O3-rich layers present in the free troposphere, which result from local/regional layering and accumulation, and continental/hemispheric transport. Indeed, vertical mixing largely explains the presence of imported O3 at ground level in the IP. Our results demonstrate the need for detailed quantification of the local and remote contributions to high O3 concentrations for local O3 management, and show O3 source apportionment to be an essential analysis prior to the design of O3 mitigation plans in any non-attainment area. Achieving the European O3 objectives in southern Europe requires not only ad hoc local actions but also decided national and European-wide strategies.This study has been supported by the Spanish Ministry of Economy and Competitiveness and FEDER funds under the PAISA (CGL2016-75725-R) project. This work was granted access to the high performance computer resources of the “Red Española de Supercomputación” (AECT-2017-1-0008). The views expressed in this article are those of the authors and do not necessarily represent the views or policies of the US Environmental Protection Agency. Carlos Pérez García-Pando acknowledges longterm support from the AXA Research Fund, as well as the support received through the Ramón y Cajal programme (grant no. RYC-2015-18690) of the Spanish Ministry of Economy and Competitiveness.Peer ReviewedPostprint (published version

Extreme ozone episodes in a major Mediterranean urban area

This study analyses three extreme ozone (O3) episodes that occurred in Barcelona (NE Spain) during the summers of 2015, 2018, and 2019. These episodes exceeded the EU's hourly information threshold (180 µg m−3) for the first time since at least the year 2000, raising concerns due to Barcelona's large population. By employing experimental data and various modelling tools, our main objective is to elucidate the underlying phenomena of these recent O3 episodes and improve predictive capabilities. The findings indicate that the factors contributing to these occurrences are largely consistent across episodes. These factors, with estimated O3 contributions specified for particular instances, comprise (i) initial O3 accumulation in surrounding coastal areas; (ii) weekend occurrence, accompanied by the corresponding weekend effect (+15 µg m−3); and (iii) the prevalence of Tramontana meteorological conditions during above-normal temperatures, which (iv) force the convergence of multiregional polluted air masses to the city (+45–65 µg m−3). Major source areas include regions of southern France through the Gulf of Lion, the interior of the Mediterranean, and eastern Spanish coastal regions, including Barcelona's pollution plume. Some of these factors, which may manifest in the days preceding the episodes, are observable or can be anticipated. This study enhances understanding of the mechanisms driving extreme O3 episodes recently observed in Barcelona and provides valuable insights for prediction

Analisis y diagnóstico de episodios de meteorología severa en el País Vasco

Informe de las Galernas del 25 Jul 1995 y 30 May 1996The atmospheric processes prior to and during the outbreak of 2 galernas (July 25, 1995 and May 30, 1996) are analyzed. Data from surface stations, meteorological satellites, the Punta Galea wind profiler radar and numerical simulations with the mesoscalar model RAMS are used. It is concluded that the origin of the classic galerna of the warm season is associated with the irruption of a cold front on the N coast of the Iberian Peninsula. This front, which in most cases is not recorded on the synoptic weather charts, is associated with the presence of a low-pressure N-to-S trough between a European anticyclone and the Azores anticyclone. The surface front is preceded by an intense Foehn on the north coast, which is clearly visible in the satellite images, and which causes high temperatures on the Basque coast during the morning. The frontal advection over the land mass of the northwestern half of the peninsula can leave precipitation on the W-coast and southern slopes of the mountains in Galicia (galerna of July 1995). However, over the sea, a relative cold marine boundary (MBL) layer is transported parallel to the coast from Galicia (with colder water) to the Basque coast (with warmer sea surface temperatures). It runs parallel to the coast and remains uncoupled from the south-westerly winds, which cross over the Cantabrian Mountains and blow above the MBL. The decoupling is caused by the temperature inversion associated with the cold advection of the MBL and the presence of the Cantabrian mountain range. On the Asturian coast, protected by the highest mountains, the cold coastal advection and solar heating of the slopes, cause sea-land and up-slope breezes (Avilés weather station). An intense convergence occurs at the top of the Cantabrian mountain range: SW winds that blow on its S slope and those from the NW associated with the combined sea and up-slope breezes. Data from the wind profiler radar at Punta Galea (Getxo) documented a galerna depth of 1,500 meters and intense winds from the SW blowing above, confirmed by numerical simulations with the RAMS mesoscale model (3 km horizontal resolution and 1 hour resolution).Direccion de Meteorología y Climatología del Gobierno Vasco, 200

Impact of the COVID-19 Lockdown in a European Regional Monitoring Network (Spain): Are We Free from Pollution Episodes?

The impact of the lockdown, during the period from March to June in 2020, upon the air quality of the Basque Country in northern Spain is analyzed. The evaluation accounts for the meteorology of the period. Daily and sub-daily analysis of aerosol and ozone records show that the territory was repeatedly affected by episodes of pollutants from outer regions. Three episodes of PM10 and ten of PM2.5 were caused by transported anthropogenic European sulfates, African dust, and wildland fires. The region, with a varied orographic climatology, shows high and diverse industrial activity. Urban and interurban road traffic of the region decreased by 49% and 53%, respectively, whereas industrial activity showed a lower reduction of 20%. Consequently, the average concentrations of NO2 in the cities during the period fell to 12.4 µg·m−3 (−45%). Ozone showed up to five exceedances of the WHOAQG for the daily maximum 8-h average in both rural and urban sites, associated with transport through France and the Bay of Biscay, under periods of European blocking anticyclones. However, averages showed a moderate decrease (−11%) in rural environments, in line with the precursor reductions, and disparate changes in the cities, which reproduced the weekend effect of their historical records. The PM10 decreased less than expected (−10% and −21%, in the urban and rural environments, respectively), probably caused by the modest decrease of industrial activity around urban sites and favorable meteorology for secondary aerosol formation, which could also influence the lower changes observed in the PM2.5 (−1% and +3% at the urban and rural sites, respectively). Consequently, in a future low NOx traffic emission scenario, the inter-regional PM and ozone control will require actions across various sectors, including the industry and common pollution control strategies.This research was funded by the Basque Government and the University of the Basque Country (GIC15/152 and GIU13/03) and by the Environment Vice-Department of the Basque Government for the measurement of biogenic volatile organic compounds in Valderejo Natural Park

Galernas: A history of coastally trapped disturbances (2003−2020) with hidden frontogenesis in the Bay of Biscay

Galerna is the term accepted for an abrupt westerly change that affects the north coast of Spain. The wind surge travels from the mid-north coast of Spain to France, generally reaching their maximum intensity at the Basque Coast, and cuts off a period of hot weather, clear skies, and calm conditions at sea. The galernas have a large history of shipwrecks and fishermen deaths. They have been characterized as coastally trapped disturbances (CTD) and their propagation, enhanced with the local formation of a micro-front, was documented to behave like a density current. Alternatively, synoptic fronts have also been reported to cause galernas, considered to be more intense than those generated by a local micro-front. In this article we have generated the first climatology (2003–2020) of these events based on an objective identification methodology. The developed Event Identification Software (EIS), based on both 10-min surface station data and hourly ERA5 reanalysis fields, together with a new Front Identification Scheme (FIS) have enabled a deeper study into the origin and development of these micro-fronts, and a more comprehensive exploration of the interaction of the oceanic fronts entering the Bay of Biscay. Our results show that the area receives an average of four to five relatively intense galernas (Vmax > 50 km h−1) per year. Their number shows a great interannual variability (from one to seven) and a marked seasonality: May and June concentrate the largest fraction (almost one episode each year) and practically no episodes in winter. They occur more frequently between noon and the late afternoon, where the most intense wind records concentrate. Very strong galernas (Vmax > 72 km h−1) have occurred in all 18 years, can happen in any month from February to November, and their monthly distribution does not show the mentioned seasonality. On the contrary, the highest rates of temperature decrease across the galerna front in the coastal stations (−∆T/0.5 h > 4 °C) do have a stronger seasonality, with May and June concentrating a relatively large number of cases with a more abrupt temperature drop. The FIS shows that most of the galernas (83.5%) have a local origin inside the Bay of Biscay, and only a few ones (16.5%) are caused by oceanic fronts initiated out of the region. The local frontogenesis is more frequently initiated by the relatively cold marine southwesterly pre-frontals preceding a parent oceanic front and blowing against the warm continentals inside the Bay of Biscay, after being ducted along the north and northwestern coast of Spain. This hidden local frontogenesis, first revealed by the FIS, seems to be enhanced by the observed lee troughing, which could have both a thermal and dynamic origin, acting simultaneously after the intense Foehn at the coastal strip, preceding the formation of the galerna front. The local front enhancement appears to be the reason for the apparent jump of the primary front, which may eventually weaken, and even disappear, as the galerna front sharpens. Even during the more occasional frontal galernas, directly caused by the westerlies or north-westerlies behind the oceanic front, their eastward propagation is more rapid over the coastal area. The front deforms in shape and may cause its characteristic unexpected/abrupt irruption. All the EIS detected galernas, even the frontal ones, are wind reversals caused by a coastally trapped marine boundary layer. The upper-level ridge over Europe, observed in all of them, seems to be a synoptic ingredient for their development, preventing the eastward propagation of Atlantic depressions and enhancing at the same time the temperature and pressure gradients between the marine and continental air masses.The authors wish to thank the Basque Government and the University of the Basque Country UPV/EHU as the source of our main financial support: GIA consolidated Research Groups (https://www.ehu.eus/es/web/gia) IT1057-16 (GIC15/152) and GIU13/03. These financing bodies have played an exclusively economic role in the study

Lessons from the COVID-19 air pollution decrease in Spain: Now what?

We offer an overview of the COVID-19 -driven air quality changes across 11 metropolises in Spain with the focuson lessons learned on how continuing abating pollution. Trafficflow decreased by up to 80% during the lockdownand remained relatively low during the full relaxation (June and July). After the lockdown a significant shift frompublic transport to private vehicles (+21% in Barcelona) persisted due to the pervasive fear that using publictransport might increase the risk of SARS-CoV-2 infection, which need to be reverted as soon as possible. NO2levels fell below 50% of the WHO annual air quality guidelines (WHOAQGs), but those of PM2.5were reducedless than expected due to the lower contributions from traffic, increased contributions from agricultural and do-mestic biomass burning, or meteorological conditions favoring high secondary aerosol formation yields. Evenduring the lockdown, the annual PM2.5WHOAQG was exceeded in cities within the NE and E regions withhigh NH3emissions from farming and agriculture. Decreases in PM10levels were greater than in PM2.5due to reduced emissions from road dust, vehicle wear, and construction/demolition. Averaged O3daily maximum 8-h(8hDM) experienced a generalized decrease in the rural receptor sites in the relaxation (June–July) with−20%reduced mobility. For urban areas O38hDM responses were heterogeneous, with increases or decreases depend-ing on the period and location. Thus, after canceling out the effect of meteorology, 5 out of 11 cities experiencedO3decreases during the lockdown, while the remaining 6 either did not experience relevant reductions or in-creased. During the relaxation period and coinciding with the growing O3season (June–July), most cities expe-rienced decreases. However, the O3WHOAQG was still exceeded during the lockdown and full relaxationperiods in several cities. For secondary pollutants, such as O3and PM2.5, further chemical and dispersion model-ing along with source apportionment techniques to identify major precursor reduction targets are required toevaluate their abatement potential

Phenomenology of high-ozone episodes in NE Spain

Ground-level and vertical measurements (performed using tethered and non-tethered balloons), coupled with modelling, of ozone (O3), other gaseous pollutants (NO, NO2, CO, SO2) and aerosols were carried out in the plains (Vic Plain) and valleys of the northern region of the Barcelona metropolitan area (BMA) in July 2015, an area typically recording the highest O3 episodes in Spain. Our results suggest that these very high O3 episodes were originated by three main contributions: (i) the surface fumigation from high O3 reservoir layers located at 1500-3000 m a.g.l. (according to modelling and non-tethered balloon measurements), and originated during the previous day(s) injections of polluted air masses at high altitude; (ii) local/regional photochemical production and transport (at lower heights) from the BMA and the surrounding coastal settlements, into the inland valleys; and (iii) external (to the study area) contributions of both O3 and precursors. These processes gave rise to maximal O3 levels in the inland plains and valleys northwards from the BMA when compared to the higher mountain sites. Thus, a maximum O3 concentration was observed within the lower tropospheric layer, characterised by an upward increase of O3 and black carbon (BC) up to around 100-200 m a.g.l. (reaching up to 300 µg m−3 of O3 as a 10 s average), followed by a decrease of both pollutants at higher altitudes, where BC and O3 concentrations alternate in layers with parallel variations, probably as a consequence of the atmospheric transport from the BMA and the return flows (to the sea) of strata injected at certain heights the previous day(s). At the highest altitudes reached in this study with the tethered balloons (900-1000 m a.g.l.) during the campaign, BC and O3 were often anti-correlated or unrelated, possibly due to a prevailing regional or even hemispheric contribution of O3 at those altitudes. In the central hours of the days a homogeneous O3 distribution was evidenced for the lowest 1 km of the atmosphere, although probably important variations could be expected at higher levels, where the high O3 return strata are injected according to the modelling results and non-tethered balloon data. Relatively low concentrations of ultrafine particles (UFPs) were found during the study, and nucleation episodes were only detected in the boundary layer. Two types of O3 episodes were identified: type A with major exceedances of the O3 information threshold (180 µg m−3 on an hourly basis) caused by a clear daily concatenation of local/regional production with accumulation (at upper levels), fumigation and direct transport from the BMA (closed circulation); and type B with regional O3 production without major recirculation (or fumigation) of the polluted BMA/regional air masses (open circulation), and relatively lower O3 levels, but still exceeding the 8 h averaged health target. To implement potential O3 control and abatement strategies two major key tasks are proposed: (i) meteorological forecasting, from June to August, to predict recirculation episodes so that NOx and VOC abatement measures can be applied before these episodes start; (ii) sensitivity analysis with high-resolution modelling to evaluate the effectiveness of these potential abatement measures of precursors for O3 reduction

Identification of "Galerna" events in the regional meteorological network of the Basque Country: a selection of 7 events in 2002

The analysis of the generation and development of a series of galernas has shown the presence of intense winds from the SW at relatively low altitudes, some hours before the triggering of a typical summer galerna. It is now also known that the mechanism of its generation and development includes other factors, such as the advection of a cold air mass in the marine boundary layer, which runs parallel to the coast from W to E, and a significant foehn on the coast of the Basque Country before noon. These results are contained in the report "Analysis and diagnosis of severe weather episodes in the Basque Country" (http://hdl.handle.net/10810/55032). Here, it is intended to use the meteorological data from the coastal meteorological surface stations of the region and the profiler radar of Punta Galea, located on the Basque shoreline, to establish a scheme for evaluating the capacity of a micro-front to deepen and generate a violent galerna in the Basque coast. At a synoptic level, all the galerna events in this report present a cold front over the Galician coast at noon, in addition to a characteristic configuration of the pressure centers: a depression over the west coast of Ireland and high pressures over Western Europe and Western Mediterranean, that block westerly flow over the continent (becomes more S by the western end of the anticyclone) causing larger E-W temperature differences. However, it has been proven that this configuration of the synoptic pressure centers and the position of the fronts is not sufficient for a correct assessment of galerna risk. It is also necessary to know the near frontal activity (either from a synoptic or from any other sub-synoptic front). This activity could be assessed at the local level, using the Punta Galea radar wind profiler output during a time window of several hours before the solar noon: intense SW winds (greater than 10-12 ms-1) at all the levels in the surveillance time interval are indicators of the approach of a cold front with sufficient activity to cause a galerna event. This characteristic, together with an appreciable solar activity, capable of making the boundary layer over the Basque coastal land mass sufficiently unstable (more likely between April and September) and the presence of a mass of cold air trapped to the north of the Cantabrian Mountain Range, over the sea, running parallel to the coast from W to E, are sufficient ingredients for the generation of a galerna. In this regard, it is important to highlight the fact that for the three frontal galernas, out of the seven possible analyzed events, the galerna fronts that penetrates into the Basque Country are always located ahead of the synoptic front, and the latter is not always detected in the surface stations (May 20). The temperature differences between the south-westerlies blowing ahead of the galerna front and the westerlies at the rear easily exceeds 10°C in the warm season, but the W-E thermal contrasts cannot be used only by themselves to make an adequate assessment of galerna risk: the May 24, June 16 and August 17 events are clear examples. Only the combination of the factors described and their proper monitoring could lead to a valid operational prediction.Dirección de Meteorología y Climatología del Gobierno Vasco, 200

Validation of optical remote sensing measurement strategies applied to industrial gas emissions

In May 2004 a field-campaign was conducted at a power plant in Spain, aiming at validating the use of a miniaturized, fibre-optic, ultraviolet, differential optical absorption spectrometer (mini-DOAS) instrument for sulphur dioxide flux (SO2) quantification. Emissions were determined by integrating the total number of molecules in a vertical cross-section of the gas plume, and multiplying them by the wind component at plume height. Calculated wind data was compared with balloon soundings. Plume height, computed from a tomographic reconstruction of the plume was compared with stereo-photogrammetric methods and a plume rise model. Finally, SO2 fluxes were compared to emissions reported by the power station. Although the meteorological conditions during the field-campaign were adverse, with large fluctuations in wind-speed and wind-direction, the optical measurements performed proved to be practically feasible and compared well with the techniques used for the validation. SO2 fluxes derived by mini-DOAS were within 7% of the ones reported by the power station

Drivers of divergent trends in tropospheric ozone hotspots in Spain, 2008–2019

This study aimed to investigate the causes of contrasting ozone (O3) trends in Spanish O3 hotspots between 2008 and 2019, as documented in recent studies. The analysis involved data on key O3 precursors, such as nitrogen oxides (NOx) and volatile organic compounds (VOCs), among other species, along with meteorological parameters associated with O3. The dataset comprised ground-level and satellite observations, emissions inventory estimates, and meteorological reanalysis.The results suggest that the increasing O3 trends observed in the Madrid area were mostly due to major decreases in NOx emissions from the road transport sector in this urban VOC-limited environment, as well as variations in meteorological parameters conducive to O3 production. Conversely, the decreasing O3 trends in the Sevilla area likely resulted from a decrease in NOx emissions in a peculiar urban NOx-limited regime caused by substantial VOC contributions from a large upwind petrochemical area. Unchanged O3 concentrations in other NOx-limited hotspots may be attributed to the stagnation of emissions from sectors other than road transport, coupled with increased emissions from certain sectors, likely due to the economic recovery from the 2008 financial crisis, and the absence of meteorological variations favorable to O3 production.In this study, the parameters influencing O3 varied distinctively across the different hotspots, emphasizing the significance of adopting an independent regional/local approach for O3 mitigation planning. Overall, our findings provide valuable insights into the causes of contrasting O3 trends in different regions of Spain, which can be used as a basis for guiding future measures to mitigate O3 levels