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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.

A tropospheric O3 pollution episode over the Atlantic coast of the Iberian Peninsula during 2–6 August 2018 has been analyzed. The episode was characterized by a permanent wind shear throughout the entire period, making the observed ozone surface distribution especially difficult to explain. A new methodology is described to analyze upper-level atmospheric parameters, such as temperature, wind direction, wind speed, and O3 concentrations, added to the traditional use of surface parameters, using the Weather Research and Forecasting (WRF) and Comprehensive Air Quality Model with Extensions (CAMx) models as well as available surface and upper-air observations. Results indicate that the episode was characterized by a first phase of a sudden increase in O3 concentrations produced by fumigation and inter-regional transport processes within the Iberian Peninsula, followed by continental O3 transport from Europe to the Atlantic coast. An Atlantic front produced the dissipation of the episode, generating an “ozone front” accompanying the cold front passage across the region.

Tropospheric O3 remains a major air-quality issue in the Mediterranean region. The combination of large anthropogenic emissions of precursors, transboundary contributions, a warm and dry aestival climate, and topographical features results in severe cases of photochemical pollution. Chemical transport models (CTMs) are essential tools for studying O3 dynamics and for assessing mitigation measures, but they need to be evaluated specifically for each air basin. In this study, we present an optimisation of the LOTOS-EUROS CTM for the Madrid air basin. Five configurations using different meteorological datasets (from the European Centre for Medium-Range Weather Forecast, ECMWF; and the Weather Research and Forecasting Model, WRF), horizontal resolution and number of vertical levels were compared for July 2016. LOTOS-EUROS responded satisfactorily in the five configurations reproducing observations of surface O3 with notable correlation and reduced bias and errors. However, the best-fit simulations for surface O3 were obtained by increasing spatial resolution and using a large number of vertical levels to reproduce vertical transport phenomena and the formation of reservoir layers. Using the optimal configuration obtained in the evaluation, three characteristic events have been described: recirculation (REC) episodes and northern and southern advection (NAD and SAD, respectively) events. REC events were found to produce the highest O3 due to the reduced ventilation associated with low wind speeds and the contribution of reservoir layers formed by vertical transport of O3 formed near the surface in the previous days of the event. NAD events, usually associated with higher wind speeds, present the lowest ground-level O3 concentrations in the region. During SAD episodes, external contributions along with low wind speeds allow O3 to increase considerably but not as much as in REC events because steady southerly winds disperse local emissions and hinder the formation of reservoir layers.

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.

Various studies have reported that the photochemical nucleation of new ultrafine particles (UFPs) in urban environments within high insolation regions occurs simultaneously with high ground ozone (O3) levels. In this work, we evaluate the atmospheric dynamics leading to summer O3 episodes in the Madrid air basin (central Iberia) by means of measuring a 3-D distribution of concentrations for both pollutants. To this end, we obtained vertical profiles (up to 1200 m above ground level) using tethered balloons and miniaturised instrumentation at a suburban site located to the SW of the Madrid Metropolitan Area (MMA), the Majadahonda site (MJDH), in July 2016. Simultaneously, measurements of an extensive number of air quality and meteorological parameters were carried out at three supersites across the MMA. Furthermore, data from O3 soundings and daily radio soundings were also used to interpret atmospheric dynamics.The results demonstrate the concatenation of venting and accumulation episodes, with relative lows (venting) and peaks (accumulation) in O3 surface levels. Regardless of the episode type, the fumigation of high-altitude O3 (arising from a variety of origins) contributes the major proportion of surface O3 concentrations. Accumulation episodes are characterised by a relatively thinner planetary boundary layer ( 2400 m a.s.l.). This orographic–meteorological setting causes the vertical recirculation of air masses and enrichment of O3 in the lower tropospheric layers. When the highly polluted urban plume from Madrid is affected by these dynamics, the highest Ox (O3+ NO2) concentrations are recorded in the MMA.Vertical O3 profiles during venting episodes, with strong synoptic winds and a deepening of the planetary boundary layer reaching > 2000 m a.s.l., were characterised by an upward gradient in O3 levels, whereas a reverse situation with O3 concentration maxima at lower levels was found during the accumulation episodes due to local and/or regional production. The two contributions to O3 surface levels (fumigation from high-altitude strata, a high O3 background, and/or regional production) require very different approaches for policy actions. In contrast to O3 vertical top-down transfer, UFPs are formed in the planetary boundary layer (PBL) and are transferred upwards progressively with the increase in PBL growth.

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.

Vertical profiles of CO2 concentration were collected during an intensive summer campaign in a coastal complex-terrain region within the frame of the European Project RECAB (Regional Assessment and Modelling of the Carbon Balance in Europe). The region presents marked diurnal mesoscale circulation patterns. These circulations result in a specific coupling between atmospherically transported CO2 and its surface fluxes. To understand the effects of this interaction on the spatial variability of the observed CO2 concentrations, we applied a high-resolution transport simulation to an idealized model of land biotic fluxes. The regional Net Ecosystem Exchange fluxes were extrapolated for different land-use classes by using a set of eddy-covariance measurements. The atmospheric transport model is a Lagrangian particle dispersion model, driven by a simulation of the RAMS mesoscale model. Our simulations were able to successfully reproduce some of the processes controlling the mesoscale transport of CO2. A semi-quantitative comparison between simulations and data allowed us to characterize how the coupling between mesoscale transport and surface fluxes produced CO2 spatial gradients in the domain. Temporal averages in the simulated CO2 field show a covariance between flux and transport consisting of: 1) horizontally, a CO2 deficit over land, mirrored by a CO2 excess over the sea and 2) vertically, the prevalence of a mean CO2 depletion between 500 and 2000 m, and a permanent build-up of CO2 in the lower levels.

We collected ground-based and aircraft vertical profile measurements of meteorological parameters during a 2-week intensive campaign over the Valencia basin, in order to understand how mesoscale circulations develop over complex terrain and affect the atmospheric transport of tracers. A high-resolution version of the RAMS model was run to simulate the campaign and characterize the diurnal patterns of the flow regime: night-time katabatic drainage, morning sea-breeze development and its subsequent coupling with mountain up-slopes, and evening flow-veering under larger-scale interactions. An application of this mesoscale model to the transport of CO2 is given in a companion paper. A careful evaluation of the model performances against diverse meteorological observations is carried out. Despite the complexity of the processes interacting with each other, and the uncertainties on modeled soil moisture boundary conditions and turbulence parameterizations, we show that it is possible to simulate faithfully the contrasted flow regimes during the course of one day, especially the inland progression and organization of the sea breeze. This gives confidence with respect to future applicability of mesoscale models to establish a reliable link between surface sources of tracers and their atmospheric concentration signals over complex terrain.

Two ozone episodes (occurring in June 2001 and June 2003) in the air quality monitoring network of the Basque Country (BC) are analyzed. The population information threshold was exceeded in many stations (urban, urban-background and rural). During this type of episodes, forced by a blocking anticyclone over the British Isles, ozone background concentrations over the area increase after the import of pollution from both, the continental Europe and the western Mediterranean areas (Gangoiti et al., 2002). For the present analysis, emphasis is made in the search for transport mechanisms, pathways and area sources contributing to the build-up of the episodes. Contributions from a selection of 17 urban and industrial conglomerates in the western European Atlantic (WEA) and the western Mediterranean (WM) are shown after the results of a coupled RAMS-HYPACT modelling system. Meteorological simulations are tested against both the high-resolution wind data recorded at the BC coastal area by a boundary layer wind-profiler radar (Alonso et al., 1998) and the wind soundings reported by the National Centres of Meteorology at a selection of European and north-African sites. Results show that during the accumulation phase of the episodes, background ozone concentrations increase in the whole territory as a consequence of transport from the Atlantic coast of France and the British Channel. For the peak phase, intrusions from new sources, located at the Western Mediterranean, Southern France, Ebro Valley, and, occasionally, the area of Madrid are added, resulting in a further increase in the ozone concentrations. Direct day and night transport within the north-easterly winds over the sea from the WEA source region, and night-time transport within the residual layer over continental areas (southern France, the Ebro Valley, and central Iberia) modulate the import sequence of pollutants and the local increase of ozone concentrations. The alternative direct use of low resolution meteorological data for the estimation of back-trajectories shows a more simple transport scheme with no contributions neither from the Western Mediterranean nor from the Madrid area.

The main characteristics, data availability and some case studies observed during the operation of the boundary layer profiler at Punta Galea (Bilbao, Spain) are described together with an intercomparison study with a radiosonde system. The profiler is owned by the Basque Service of Meteorology and it has been in operation sinceJanuary 1996. It is located at the entrance of the Port of Bilbao, on the right bank of the inlet and it fills the gap in the systematic upper air weather watch. In its present operational mode vertical profiles of horizontal and vertical winds are obtained up to 3000 m with a resolution of 100 and 200 m, together with virtual temperature profiles up to 700-1000 m, every 30 minutes when operating in the RASS (Radio Acoustic Sounding System) mode. The Environment Group at the School of Engineering in Bilbao was charged of the selection and setting up of the equipment and it carried out the experimental evaluation of the wind and virtual temperature records by comparison with those obtained by the radiosonde system. The results show a good agreement between the two measuring systems and the usefulness of the new equipment in, among other applications, the study of the diurnal and/or seasonal evolution of the mixing height, and therefore the analysis of the dispersion of pollutants in view of its capacity to detect vertical layering and document the ventilation conditions of the lower part of the troposphere.

This study aims to support the development of Spain's Ozone Mitigation Plan by evaluating the present-day spatial variation (2015-2019) and trends (2008-2019) for seven ground-level ozone (O 3 ) metrics relevant for human/ecosystems exposure and regulatory purposes. Results indicate that the spatial variation of O 3 depends on the part of the O 3 distribution being analyzed. Metrics associated with moderate O 3 concentrations depict an increasing O 3 gradient between the northern and Mediterranean coasts due to climatic factors, while for metrics considering the upper end of the O 3 distribution, this climatic gradient tends to attenuate in favor of hotspot regions pointing to relevant local/regional O 3 formation. A classification of atmospheric regions in Spain is proposed based on their O 3 pollution patterns, to identify priority areas (or O 3 hotspots) where local/regional precursor abatement might significantly reduce O 3 during pollution episodes. The trends assessment reveals a narrowing of the O 3 distribution at the national level, with metrics influenced by lower concentrations tending to increase over time, and those reflecting the higher end of the O 3 distribution tending to decrease. While most stations show no statistically significant variations, contrasting O 3 trends are evident among the O 3 hotspots. The Madrid area exhibits the majority of upward trends across all metrics, frequently with the highest increasing rates, implying increasing O 3 associated with both chronic and episodic exposure. The Valencian Community area exhibits a mixed variation pattern, with moderate to high O 3 metrics increasing and peak metrics decreasing, while O 3 in areas downwind of Barcelona, the Guadalquivir Valley and Puertollano shows no variations. Sevilla is the only large Spanish city with generalized O 3 decreasing trends. The different O 3 trends among hotspots highlight the need for mitigation measures to be designed at a local/regional scale to be effective. This approach may offer valuable insights for other countries developing O 3 mitigation plans., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 The Authors. Published by Elsevier Ltd.. All rights reserved.)

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 PM 10 and ten of PM 2.5 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 NO 2 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 PM 10 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 PM 2.5 (-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.

We offer an overview of the COVID-19 -driven air quality changes across 11 metropolises in Spain with the focus on lessons learned on how continuing abating pollution. Traffic flow decreased by up to 80% during the lockdown and remained relatively low during the full relaxation (June and July). After the lockdown a significant shift from public transport to private vehicles (+21% in Barcelona) persisted due to the pervasive fear that using public transport might increase the risk of SARS-CoV-2 infection, which need to be reverted as soon as possible. NO 2 levels fell below 50% of the WHO annual air quality guidelines (WHOAQGs), but those of PM 2.5 were reduced less than expected due to the lower contributions from traffic, increased contributions from agricultural and domestic biomass burning, or meteorological conditions favoring high secondary aerosol formation yields. Even during the lockdown, the annual PM 2.5 WHOAQG was exceeded in cities within the NE and E regions with high NH 3 emissions from farming and agriculture. Decreases in PM 10 levels were greater than in PM 2.5 due to reduced emissions from road dust, vehicle wear, and construction/demolition. Averaged O 3 daily 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 O 3 8hDM responses were heterogeneous, with increases or decreases depending on the period and location. Thus, after canceling out the effect of meteorology, 5 out of 11 cities experienced O 3 decreases during the lockdown, while the remaining 6 either did not experience relevant reductions or increased. During the relaxation period and coinciding with the growing O 3 season (June-July), most cities experienced decreases. However, the O 3 WHOAQG was still exceeded during the lockdown and full relaxation periods in several cities. For secondary pollutants, such as O 3 and PM 2.5 , further chemical and dispersion modeling along with source apportionment techniques to identify major precursor reduction targets are required to evaluate their abatement potential., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 The Authors. Published by Elsevier B.V. All rights reserved.)

The Guadalquivir Valley is one of three major O 3 hotspots in Spain. An airborne and surface measurement campaign was carried out from July 9th to 11th, 2019 to quantify the local/regional O 3 contributions using experimental approaches. Air quality and meteorology data from surface measurements, a microlight aircraft, a helium balloon, and remote sensing data (TROPOMI-NO2-ESA) were used to obtain the 3D distribution of O 3 and various tracer pollutants. O 3 accumulation over 2.5 days started with inputs from oceanic air masses transported inland by sea breezes, which drew O 3 and its precursors from a local/regional origin to the northeastern end of the basin. The orographic-meteorological setting of the valley caused vertical recirculation of the air masses inside the valley that caused the accumulation by increasing regional background O 3 concentration by 25-30 ppb. Furthermore, possible Mediterranean O 3 contributions and additional vertical recirculation through the entrainment zone of the convective boundary layer also contributed. Using particulate matter finer than 2.5 μm (PM 2.5 ), ultrafine particles (UFP), and black carbon (BC) as tracers of local sources, we calculated that local contributions increased regional O 3 levels by 20 ppb inside specific pollution plumes transported by the breeze into the valley, and by 10 ppb during midday when flying over an area with abundant agricultural burning during the morning. Air masses that crossed the southern boundaries of the Betic system at mid-altitude (400-1850 m a.s.l.) on July 10th and 11th may have provided additional O 3 . Meanwhile, a decreasing trend at high altitudes (3000-5000 m a.s.l.) was observed, signifying that the impact of stratospheric O 3 intrusion decreased during the campaign., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 The Authors. Published by Elsevier B.V. All rights reserved.)

It is well established that in Europe, high O 3 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 O 3 formation within each country relative to the imported (regional and hemispheric) O 3 is poorly quantified. This lack of quantitative knowledge prevents local authorities from effectively designing plans that reduce the exceedances of the O 3 target value set by the European air quality directive. O 3 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 O 3 and precursors from neighbouring regions, O 3 regional and hemispheric transport and stratospheric O 3 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 O 3 events in Spain relative to the contribution of imported (regional and hemispheric) O 3 . We also assess the potential of our source apportionment method to improve O 3 modelling. Our study applies and thoroughly evaluates a countrywide O 3 source apportionment method implemented in the CALIOPE air quality forecast system for Spain at high resolution (4 × 4 km 2 ) over a 10-day period characterized by typical summer conditions in the Iberian Peninsula (IP). The method tags both O 3 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 NO x local sources to high O 3 concentrations compared with the contribution of imported O 3 . We show, for the first time, that imported O 3 is the largest input to the ground-level O 3 concentration in the IP, accounting for 46 %-68 % of the daily mean O 3 concentration during exceedances of the European target value. The hourly imported O 3 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 O 3 peaks in areas downwind of the main urban and industrial regions (up to 40 % in hourly peaks). We also show that ground-level O 3 concentrations are strongly affected by vertical mixing of O 3 -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 O 3 at ground level in the IP. Our results demonstrate the need for detailed quantification of the local and remote contributions to high O 3 concentrations for local O 3 management, and show O 3 source apportionment to be an essential analysis prior to the design of O 3 mitigation plans in any non-attainment area. Achieving the European O 3 objectives in southern Europe requires not only ad hoc local actions but also decided national and European-wide strategies., Competing Interests: Competing interests. The authors declare that they have no conflict of interest.

On rural background areas atmospheric formaldehyde (HCHO) is important for its abundance and chemical reactivity, directly linked to the tropospheric ozone formation processes. HCHO is also toxic and carcinogenic to humans. Atmospheric HCHO was continuously measured in summer 2016 during 81 days (N = 6722, average: 1.42 ppbv) in a rural background area in Northern Spain, Valderejo Natural Park (VNP) using a Hantzsch fluorimetric system. To better characterize the photochemical processes the database was completed with hourly measurements of 63 Non-Methane Hydrocarbons (NMHC) performed by gas chromatography and other common atmospheric pollutants and meteorological parameters. HCHO mixing ratios were highly correlated with ozone and isoprene. Cloudy and rainy days, with low temperature and radiation, led to low HCHO mixing ratios, with maxima (<2 ppbv) registered around 14 UTC. On days with increased radiation and temperature HCHO maxima occurred slightly later (<6 ppbv, ≈16:00 UTC). During clear summer days with high temperature and radiation, two HCHO peaks were registered daily, one synchronized with the radiation maximum (≈3-4 ppbv, ≈13:00 UTC) and an absolute maximum (<10 ppbv, ≈18:00 UTC), associated with the addition of HCHO coming into VNP due to inbound transport of old polluted air masses. In the ozone episode studied, the processes of accumulation and recharge of ozone and of HCHO ran in parallel, leading to similar daily patterns of variation. Finally, HCHO mixing ratios measured in VNP were compared with other measurements at rural, forested, and remote sites all over the world, obtaining similar values., (Copyright © 2018 Elsevier B.V. All rights reserved.)

Odour complaints are frequent nowadays, particularly nearby industrial facilities emitting odorous compounds. Among all compounds susceptible of causing odour annoyance, reduced sulphur compounds (RSC) were studied, due to their unpleasant odour and low odour threshold. RSC ambient air mixing ratios, meteorological conditions and population complaints were analysed in an area of complex topography in order to identify their potential sources. Mixing ratios of three compounds, dimethyl sulphide (DMS), carbon disulphide (CDS) and dimethyl disulphide (DMDS), were continuously monitored using an online gas chromatograph coupled with a mass spectrometer detector (GC-MSD), which was placed in a mobile air quality monitoring unit. Measurement campaigns were performed during 2012 and 2013 for periods of 7-25 days in an urban area (four campaigns, N = 1368) and an urban area surrounded by industrial activities (three campaigns, N = 564). During such campaigns, RSC mixing ratios were frequently above their odour thresholds, which did not always involve citizen complaints. Average RSC ambient air mixing ratios tended to be lower in the urban area (DMS 0.06-0.33, CDS 0.05-0.10, DMDS 0.07-0.30 μg m -3 ) than in the industry surrounded one (DMS 0.30-2.39, 0.05-0.18, DMDS 0.09-0.62 μg m -3 ). The DMS/DMDS mixing ratio was frequently above 1, being a paper mill one of the main sources of RSC in the area. DMS/DMDS ratios below 1 were also recorded, suggesting a waste treatment plant as the RSC source or older air masses coming from the paper mill.

The 2000-2015 occurrences of the highest ozone (O 3 ) pollution episodes in Spain were evaluated to investigate their origin. To this end, data series available for urban and regional background (UB and RB), traffic (TR) and industrial (IN) sites were analysed separately and intercompared. Results evidenced that during these 16years mean O 3 levels in the RB sites did not change significantly, and remained constantly high. However, there is a clear increase at the TR and UB sites. Although sensitivity analysis is needed to interpret the cause of this increasing trend, this might be caused probably by the lower O 3 titration intensity due to the preferential abatement of NO vs NO 2 , as supported from the neutral trend of O X (NO 2 +O 3 ) at these sites. We found that the exceedances of the hourly information threshold for O 3 (>180μg/m 3 ) are recorded mostly at UB and IN sites located in seven areas of Spain (specific hotspots or at the tail end of large urban plumes), and that these increased during summer heatwaves (i.e. 2003 and 2015). Although the external contribution of regional-to-subcontinental transported O 3 might be relevant during the highest O 3 episodes in the Western Mediterranean, our results evidenced that in the above specific areas, regional-local O 3 production decisively contributes to the exceedances of the information threshold. Also that the human protection threshold and the AOT40 are more frequently exceeded in the Central, Southern and Mediterranean sides of the Iberian Peninsula. The design of effective episode abatement measures is quite complex in those conditions, due to both the nonlinearity of the chemical processes of O 3 formation and destruction, and to the interplay with the complex meteorological setting, causing frequent recirculation and in situ aging of air masses. However, the combination of meteorological forecasting of the main recirculation processes and sensitivity analysis of NO X /VOC emission abatement measures might be powerful tools to evaluate the effectiveness of potential O 3 mitigation strategies. Finally we would like to highlight that the current UB, RB, IN and TR classification (somewhat subjective) is not adequate to interpret the origin of O 3 exceedances in complex areas of Southern Europe. Thus, a UB station recording exceedances, and located in a small city in the tail end of an urban plume of a large city, receives not only the contribution from its own UB, but mainly from the specific high O 3 RB caused by the urban plume transport., (Copyright © 2016 Elsevier B.V. All rights reserved.)

A multidisciplinary study on aerosol characterization was performed at the regional background monitoring station of El Arenosillo, in SW Spain, between 28 June and 5 July 2006. The main aim of the Arenosillo aerosol measurement campaign 2006 was to compare the results of aerosol characterization obtained by different groups by measuring physical and chemical parameters using optical methods and in situ sampling. The campaign coincided with a long-range transport episode from Western Iberia, passing through the Gulf of Cadiz and the Straits of Gibraltar towards the study area. The results of the variability of PM levels and chemical composition of PM10, PM2.5 and PM1 at El Arenosillo and at three nearby regional and urban background sites were interpreted. Mean levels measured during the campaign reached 23, 15 and 12 microg m(-3) for PM10, PM2.5 and PM1, respectively, at El Arenosillo. PM during the Arenosillo campaign 2006 was dominated by the secondary inorganic aerosols (SIA, 24, 38 and 39% of PM10, PM2.5 and PM1 mass), carbonaceous aerosols (17, 21 and 23% of the mass), crustal material (13, 9 and 4%), and sea spray (10, 5 and 1%). These values are within the usual range of regional background sites of Southern Spain with the exception of the relatively low crustal load and the high SIA levels. Two major PM episodes were differentiated. The first one was characterized by high levels of Bi, As, Pb, Se, P and Zn, which are the main tracers of the industrial emissions near the town of Huelva. High concentrations of these elements were also recorded at the nearby sites. In the second episode, maximum levels of SO4(2-), V and coarse Cu as well as the bulk mass of PM1 were determined, tracing the polluted air mass transport from Western Iberia through the Straits of Gibraltar. These results underline the importance of the influence of long-range transport of pollutants on the levels and composition of regional background PM in SW Iberia, where local emissions may also play a role.

We present a very complete database of individual non-methane hydrocarbon (NMHC) measurements with high temporal resolution (hourly) in a rural background atmosphere. We show their use to characterize the biogenic NMHC as well as to identify the transport and impact of anthropogenic NMHC on rural areas. In January 2003 an automatic GC-FID analyzer of volatile organic compounds between 2 and 10 carbon atoms (C2-C10 VOCs) was placed in the centre of the Valderejo Natural Park in northern Iberia (42.87 degrees N, 3.22 degrees W), far away from important cities. The system operated continuously until December 2004. Data coverage was higher than 70% for a total of 59 VOC of both anthropogenic and biogenic origin, with detection limits in the range of pptv. Our results allow for the description of the behaviour of these compounds, in order to identify external impacts arriving to the sampling site which has been recognized to be highly representative of a rural background atmosphere. Biogenic VOC concentrations have been compared also with the calculated emissions, using Guenther's algorithm, and the discrepancies interpreted in terms of the different reactivity of such compounds.

This paper presents results of the processing and validation of data collected by an automatic gas chromatograph (AGC). This system was used to monitor 62 volatile organic compounds (VOCs) in urban air in the Basque Country, Spain. The nonpolar compounds (C2-C10) identified-paraffins, olefins, aromatics, and chlorinated compounds-accounted for 88% of the mass of total non-methane hydrocarbons (TNMHCs) in ambient air. The evaluation of linearity, precision, detection limits (DLs), and stability of retention times (RTs) indicates that the equipment is suitable for measuring ambient air automatically for prolonged periods (6 months). The calibration of the equipment using response factors calculated on the basis of the effective carbon number (ECN) showed variations of over 10% for acetylene, isoprene, and n-hexane. The results provided by the automatic chromatograph correlated significantly with simultaneous results from other widely used techniques for determining VOCs in ambient air: (1) portable GC, equipped with photoionization detector (PID), and (2) active adsorption on Tenax-TA followed by thermal desorption and chromatographic analysis.