Saharan dust intrusions in the Iberian Peninsula: Predominant synoptic conditions

Science of the Total Environment 717 (2020) 137041

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Saharan dust intrusions in the Iberian Peninsula: Predominant synoptic conditions A. Russo a,⁎, P.M. Sousa a, R.M. Durão b,c, A.M. Ramos a, P. Salvador d, C. Linares e, J. Díaz e, R.M. Trigo a,f a

Instituto Dom Luíz, Faculdade de Ciências da Universidade de Lisboa, Campo Grande, Edifício C8, Piso 3, 1749-016 Lisboa, Portugal IPMA-Instituto Português do Mar e Atmosfera, Lisboa, Portugal Centro de Recursos Naturais e Ambiente, Departamento de Engenharia Civil, Arquitectura e Georrecursos, Instituto Superior Técnico, Universidade de Lisboa, Portugal d Environmental Department of the Research Center for Energy, Environment and Technology (CIEMAT), Madrid, Spain e Department of Epidemiology and Biostatistic, National School of Public Health, Carlos III National Institute of Health, Madrid, Spain f Departamento de Meteorologia, Universidade Federal do Rio de Janeiro, 21941-916, Rio de Janeiro, Brazil b c

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• Characterization of the preferential weather patterns associated to dust events in Iberia. • Intrusion episodes occur more frequently at central and southern Iberia. • Saharan dust intrusions are more frequent in summer months. • Saharan dust intrusions are more probable to occur under southerly component regimes. • Circulation patterns and backtrajectories for two events found a clear path originating from northern Africa.

a r t i c l e

i n f o

Article history: Received 25 November 2019 Received in revised form 15 January 2020 Accepted 30 January 2020 Available online 31 January 2020 Editor: Pavlos Kassomenos Keywords: Particulate matter (PM) Circulation weather types (CWT) Seasonal analysis Pollution events Circulation-to-environmental approach

a b s t r a c t The Iberian Peninsula (IP) is recurrently affected by dust transport from the Sahara Desert and from the semi-arid Sahel regions. African dust is one of the most important sources of particulate matter in the southern Mediterranean. Therefore, it is vital to understand the underlying processes that lead to episodes of air pollution associated to the occurrence of dust intrusions. This work proposes to make an extended characterization of the preferential circulation weather patterns associated to the onset of dust events affecting the IP between 2006 and 2016. Saharan dust intrusions were analysed and an automatic objective classification procedure was used to classify circulation weather patterns associated to dust events. The spatial distribution of intrusion episodes is not homogeneous throughout the IP, occurring less frequently at northern and northwestern locations than at central and southern sites. Moreover, days with Saharan dust intrusions were more frequent in summer months, and more probable to occur under regimes with a southerly component. Finally, two extreme events with high concentration of particulate matter were analysed relatively to their life-cycle and particle trajectories. The distinct extreme episodes can be associated to different synoptic situations. However, and despite different large-scale configurations, a south or south-easterly component over the region is responsible for the establishment of a dust transport from the Saharan region towards Iberia, and thus leading to the intrusion onset. These results were supported by the calculation of back-trajectories which allowed to source apportioning the particles' origin, through a clear trajectory of air parcels originating from northern Africa in both events. The proposed framework

⁎ Corresponding author at: Instituto Dom Luíz, Faculdade de Ciências da Universidade de Lisboa, Campo Grande, Edifício C1, Piso 1, Sala 1.1.6, 1749-016 Lisboa, Portugal. E-mail address: [email protected] (A. Russo).

https://doi.org/10.1016/j.scitotenv.2020.137041 0048-9697/© 2020 Elsevier B.V. All rights reserved.

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A. Russo et al. / Science of the Total Environment 717 (2020) 137041

can be useful to the prediction of dust and air pollution events based on the forecast of circulation weather patterns, as the results show that these events across the IP are mainly induced by specific patterns. © 2020 Elsevier B.V. All rights reserved.

1. Introduction Atmospheric particulate matter (PM) is a combination of solid and liquid particles with different sizes, which result both from natural phenomenon and anthropogenic activities (Kamani et al., 2014). PM concentrations are usually characterized by high values of spatial and temporal variability (Gupta et al., 2006), mainly due to different atmospheric dispersive conditions and to a variety of natural and human emissions (Neff et al., 2013). Due to its characteristics, PM is a significant constituent of the Earth‘s climatic system, as PM causes direct radiative effects of scattering and absorption (Hu et al., 2017), semi-directly by changing the atmospheric cloud cover through evaporation of cloud droplet, and through indirect radiative effects via their influence on clouds microphysics (Bangert et al., 2012; Choobari et al., 2014). PM also affects atmospheric radiative balance and its chemical composition (Choobari et al., 2014); reduces biological diversity (Hartono et al., 2017; Wu and Zhang, 2018); affects crops development (Middleton and Kang, 2017); contributes to water salinization and contamination (Middleton and Kang, 2017). Some of the more short-term effects from PM include adverse impacts on air quality (Querol et al., 2009; Jiménez et al., 2010), human health (Pérez et al., 2012; Reyes et al., 2014; Reddington et al., 2015; WHO, 2016; Newell et al., 2017) and visibility (Dayan and Levy, 2005; Singh and Dey, 2012). Dust storms are natural events with high atmospheric PM concentrations, usually formed in dry and low vegetated cover areas such as arid, semi-arid, or desert areas (Jaafari et al., 2018), where precipitation is virtually absent (Goudie and Middelton, 2006) and sediments accumulate (Choobari et al., 2014). Thus, several areas in the globe constitute prevailing sources of dust aerosols, namely the Sahara and Sahel deserts in Northern Africa, the Arabic Peninsula desert in Middle East, and Southwest and East Asia (Prospero et al., 2002; Jiménez et al., 2010; Choobari et al., 2014). Moreover, dust storms are essentially a surface phenomenon which propagates throughout the troposphere, with particles being transported by strong turbulent winds over great distances (Tanaka et al., 2005; Touré et al., 2012; Middleton and Kang, 2017; Bodenheimer et al., 2019), Quite often PM are able to cross the large sea areas and oceans (Prospero et al., 2002; Engelstaedter and Washington, 2007; Varga et al., 2014; Díaz et al., 2017). The Iberian Peninsula (IP) is on one of the main routes of desert dust transport (Middleton and Kang, 2017), being repeatedly affected by dust storms from the Sahara Desert and from the semi-arid Sahel regions (Jiménez et al., 2010; Pey et al., 2013). The long-range transport from northern-Africa through the IP and the Atlantic is sustained by the occurrence of powerful winds (Prospero et al., 2002; Rodríguez et al., 2015) and characterized by a marked seasonal cycle. The seasonality is nevertheless different for eastern and western Mediterranean, as clear summer prevalence has been detected in the western side (Sicard et al., 2016) and higher contributions of desert dust have been commonly produced in spring-early summer in the eastern side (Nastos, 2012; Varga et al., 2014). During summer months dust events in the IP are favored by stable weather conditions and the absence of precipitation, and consequently, of wet deposition (Nastos, 2012). Moreover, the transport from Northern Africa to Europe, and particularly to Iberia, is dominated by mechanisms which are usually driven by more complex wind fields than in the case of the Atlantic transport and often include cyclonic activity inside and around the Mediterranean basin (Moulin et al., 1998; Ginoux et al., 2012). In addition, Sousa et al. (2019), shown the role of Saharan dust intrusions in the early August 2018 and late June 2019 heatwaves in the Iberian Peninsula.

Numerous research articles have addressed the driving mechanisms of dust transport from Northern Africa towards higher latitudes, focusing on wind and cyclonic activity inside and around the Mediterranean basin (Alpert et al., 1990; Moulin et al., 1998). More recently, several authors identified that certain preferential weather configurations (Circulation Weather Types, CWTs) favor the transport of dust from North Africa towards northern latitudes (Barkan et al., 2005; Nastos, 2012; Pey et al., 2013; Russo et al., 2014; Díaz et al., 2017). CWT are a wellestablished methodology on regional climate impact studies which have been frequently applied to the identification of predominant weather patterns associated to the occurrence of extreme events (e.g. Ramos et al., 2014; Salvador et al., 2014; Russo et al., 2014, 2015, 2016). CWTs are identified using different approaches and the last decades have witnessed a growing interest on the development of automated classifications of regional CWTs. These have the particularity of being region-specific, as they result from the examination of synoptic weather data (e.g. sea level pressure (SLP) or geopotential height at 500 hPa) (Ramos et al., 2014). Some classification procedures are based on the application of statistical selection rules (e.g. cluster analysis, principal component analyses and regression trees) but can also be based on the determination of physical parameters related with the prevailing atmospheric circulation pattern. Previously, Díaz et al. (2017) performed a thorough study on Saharan dust intrusions in Spain, focusing on its impacts on health and the most significant weather patterns associated to dust events from a 6 yr database (2004–2009). Furthermore, the referred study was relatively limited in its scope, focusing on the identification of the days with intrusions and the calculation of composite maps of several meteorological variables. The aim of this work is to characterize the persistent circulation weather patterns which are associated to the onset and maintenance of dust events reaching different IP sectors, with special interest in their seasonal patterns. Moreover, in the present work, a longer database of Saharan dust intrusions (11 yr, 2006–2016) covering all the IP will be analysed. This aim is met through the following specific objectives: (1) to identify regions of similar seasonal behaviour through the study of historical dust events; (2) to identify the main CWTs associated to dust events based on an automatic classification procedure of the CWTs; and finally (3) to analyse two extreme events affecting the IP combining a synoptic and trajectories approach. 2. Data and methods 2.1. Days characterized by Saharan dust intrusions For the classification of days with Saharan dust intrusions in the period 2006–2016, information supplied by two national authorities, namely the Spanish Ministry for the Ecological Transition (Ministerio para la Transición Ecológica - MITECO) for Spain and by the Agência Portuguesa do Ambiente and UNINOVA - Instituto de Desenvolvimento de Novas Tecnologias for Portugal was obtained. The data for the Spanish and Portuguese regions was organized into 9 and 5 main areas, respectively (Fig. 1). The selection of the regions follows the geographical criteria determined by the Spanish MITECO and by the Portuguese Environmental Agency (Agência Portuguesa do Ambiente, APA) and FCT-NOVA from Portugal in order to comply with European directives (Querol et al., 2013). Currently, both entities adopted a common approach following one of the official methods recommended by the European Commission for evaluating the occurrence of African dust intrusions and quantifying its contributions (Commission Staff

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Fig. 1. Geographical location of the 14 main areas in the Iberian Peninsula.

Working Paper, 2011). The data is extracted for each one of the 14 regions in the IP after being processed following a standard procedure implemented by both countries, which follows:

regions. It has been demonstrated that the contribution of mineral dust clearly increased the regional PM10 background levels during Saharan dust episodic days in the IP (Escudero et al., 2007).

1) The data is assembled using a robust method based on the daily interpretation of air mass back-trajectories and meteorological products. First, back-trajectories of air masses (HYSPLIT: https://www. arl.noaa.gov/hysplit/hysplit/) are computed and interpreted to account for the transport of air masses from African deserts. Namely, the HYSPLIT model (Hybrid Single-Particle Lagrangian Integrated Trajectory) (http://ready.arl.noaa.gov/HYSPLIT.php) is a complete simulation system which is able to determine air parcel trajectories, calculate single scattering processes and perform complex simulations of deposition. HYSPLIT has the advantage of allowing the user to choose the meteorological data used as input, but also the chosen method for pollutants' dispersion (either puff or particle dispersion) (Stein et al., 2015; Rolph, 2013). HYSPLIT has been applied to the analysis of Saharan desert dust (Querol et al., 2009; Negral et al., 2012), but also to other pollutants (Saavedra et al., 2012) and extreme events (Adame et al., 2012). 2) Numerical models available on-line generate daily forecast aerosol maps showing estimations of the dust concentration over geographical locations (BSC-DREAM8b: https://ess.bsc.es/bsc-dust-dailyforecast; NAAPS-NRL: https://www.nrlmry.navy.mil/aerosol/; SKIRON: http://www.forecast.uoa.gr). These maps are daily analysed to identify desert dust plumes moving towards and reaching any of the 14 regions in the IP. The evolution of the dust plumes could be frequently tracked with satellite imagery (SeaWiFS: http://oceancolor.gsfc.nasa.gov/SeaWiFS/ ; MODIS: http:// modis.gsfc.nasa.gov/). It usually results in the consideration of additional episodic days impacted by dust. Next, for specific cases meteorological maps are calculated (NCEP/NCAR: https://www.esrl.noaa. gov/psd/data/composites/hour/) to verify the existence of favorable scenarios for the transport of dust. This is a qualitative way to detect the appearance of Saharan dust plumes over each region of the IP and ensures the identification of almost all the African dust episodes, independently of their intensity (Pey et al., 2013). 3) Finally, the real impact of the Saharan dust transported by the air masses is later evaluated on surface levels of PM10 recorded at rural background monitoring sites located at any of the 14 study

This methodology represents the Spanish and Portuguese reference method to identify Saharan dust contributions to PM10 levels since 2004, allowing for a common analysis to the whole Peninsula. Apart from its capabilities on identifying intrusions, it is also used to quantify the dust contribution to the PM10 daily records during each potential dusty day by means of a statistical analysis of the time series of PM10 values registered at regional background monitoring sites (Escudero et al., 2007; Viana et al., 2010). This methodology has been recurrently applied by several authors, particularly in Southern Europe (e.g. Pey et al., 2013; Salvador et al., 2014). 2.2. Synoptic classification The most favorable synoptic conditions for intrusions to take place were identified based on daily mean sea level pressure (MSLP) and geopotential height at 500 hPa obtained from the European Centre for Medium-Range Weather Forecasts (ECMWF) Era-Interim reanalyzes (Dee et al., 2011) across the period 2006–2016. We have applied the Circulation Weather Type classification methodology adopted by Trigo and DaCamara, (2000) for the Iberia Peninsula, which accounts for physical and geometrical considerations of the geostrophic flow airflow like the direction, strength and the degree of cyclonicity. This approach is based on the corresponding objective classification defined for the British Isles (Jenkinson and Collison, 1977; Jones et al., 1993) and describes the regional atmospheric circulation in terms of a small set of relatively simple circulation parameters (Ramos et al., 2014). The general idea of applying CWT classification procedures is to agglomerate multivariate information to a univariate time series with attributed classes of behaviour (Philipp et al., 2010). The daily CWTs for the period 2006–2016 were computed by means of the daily SLP on a 0.75° latitude-longitude grid retrieved from the ERA-Interim Reanalyses. The circulation conditions were determined using the geostrophic wind approximation and adopting physical or geometrical parameters which are as follows: southerly flow (SF); westerly flow (WF); total flow (F); southerly shear vorticity (ZS); westerly shear vorticity (ZW); and total shear vorticity (Z). All these indices

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were computed using MSLP values obtained for the 16 grid points centred over the Iberian Peninsula. The conditions established to define different types of circulation are the same as those considered in Trigo and DaCamara, (2000), and thus the same set of rules was adopted: (a) Direction of flow was given by tan−1 (WF/SF), 180° being added if WF was positive. The appropriate direction was computed using an eight-point compass, allowing 45° per sector. (b) If |Z| b F, the flow is essentially straight and was considered to be of a pure directional type (eight different cases, according to the directions of the compass). (c) If |Z| N 2F, the pattern was considered to be of a pure cyclonic type if Z N 0, or of a pure anticyclonic type if Z b 0. (d) If F b |Z| b 2F, the flow was considered to be of a hybrid type and was therefore characterized by both direction and circulation (8 × 2 different types). Taking into account this set of rules, a total of 26 CWTs were defined: 10 pure types (Fig. 2: NE, E, SE, S, SW, W, NW, N, C, and A), and 16 hybrid types (8 for each C or A hybrid). To provide a more straightforward analysis, statistics are shown for the 10 pure types, being the frequencies for hybrid types days equally distributed into the 10 pure types, and where 8 pure types associated with a specific wind direction are called Directional types. For a more comprehensive description of the methodology please refer to Ramos et al. (2014). Fig. 2 shows the average distribution of the Mean Sea Level Pressure for each CWT. It is quite clear that anticyclonic days (A) are the most frequent throughout the year in Iberia (~1/3 of total days). 2.3. Intrusion episodes The occurrence of intrusion episodes can be explained by the association of several factors, including the prevalence of winds favouring the advection of air masses from arid or desert regions (Russo et al., 2014, 2015, 2016; Salvador et al., 2014). This section focuses on the analysis of two specific historical extreme episodes (5–9 April 2011 and 21 February 2016) that have occurred in Iberia. The goal is to gain a clearer insight on the preferential paths visited by the air masses when transporting air pollutants and mineral dust to the Iberian Peninsula. In addition to the CWT classification method described previously, there are several other methods that allows classifying the motion of air masses according to clusters of proveniences. These two events were chosen by their characteristics, with the first one being reported to affect a very large area in Europe reaching Scandinavia between 5 and 11 April 2011 (Preißler et al., 2011; Huneeus et al., 2016), and the second one corresponding to a strong winter event which severely affected air quality in Portugal and Spain (Titos et al., 2017; Oduber

et al., 2019). The 2011 episode was reported to have maximum aerosol optical depths of approximately 2 in Portugal as a consequence of the extreme dust intrusion episode (Preißler et al., 2011). These values are similar to those registered during 20 to 23 February 2017, which is considered to be record-breaking event over the IP since July 2012 (Fernández et al., 2019), being the 2017 event however not included in the studied period. Usually during winter months this kind of events is not very frequent and does not often reach the northwest of the Peninsula (Sorribas et al., 2017; Oduber et al., 2019). Nevertheless, several extreme events have been reported to affect the IP. Namely, the February 2016 unusual event was also studied by Titos et al. (2017) and Oduber et al. (2019) in Northeastern and Northwest Spain, respectively. Titos et al. (2017) reported an exceedance of PM10 across Iberia and Balearic Islands and found that 80% of the PM10 mass was from mineral origin. Oduber et al. (2019) identified in Northwest Spain a progressive increase in the PM10 concentration, reaching the maximum hourly value of 113 μg m−3. The two events have been simulated and compared against surface background measurements, to check if the HYSPLIT model is capable to reproduce the occurrence of these dust events, as observed at the monitoring stations. The chosen monitoring stations are classified as regional background. PM10 daily mean data registered at two air quality monitoring stations in Spain (Viznar and Campisábalos), and one in Portugal (Terena) were also collected. The Spanish stations are members of EMEP (Co-operative Programme for Monitoring and Evaluation of the Long-Range Transmission of Air Pollutants in Europe) while the Portuguese station is part of the Portuguese Air quality Agency and all three stations are classified as regional background stations. In this kind of monitoring sites, the contributions of PM10 produced by long-range transport of air mass of external origin are more evident than in urban or industrial stations, which are influenced by the emissions of pollutants from the main local sources. The meteorological dataset used to run HYSPLIT model is from the Global Data Assimilation System (GDAS) model with a horizontal resolution of 0.5° (from NOAA). To identify dust sources backwardtrajectories were computed for 72-hours periods, fixing the arrival times at 12:00 UTC, at three different altitudes (100 m, 500 m and 1000 m) above the ground level (AGL). Backward-trajectories starting heights/altitudes were chosen in order to ensure that trajectories start in the atmospheric boundary layer and reduce topographic effects on HYSPLIT simulations. 3. Results and discussion 3.1. Saharan dust intrusions During the period 2006–2016, 2301 and 1069 episodic days were identified in Spain (SP) and Portugal (PT), respectively. These values

Fig. 2. Average distribution of the Mean Sea Level Pressure for each CWT.

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Fig. 3. Total number of days with aerosols intrusions between 2006 and 2016, by country and by month.

account for each intrusion reported in one or more regions. The highest number of episodic days in one year was recorded in 2016 (154 days in the Canary Islands, SP) and the lowest in 2013 (2 days in the Northwest region, SP). In order to perform an inter- and intra-annual analysis of the intrusions (Fig. 1), we selected the days with intrusions and grouped the results by month (Fig. 3) and by year (Fig. 4) for both Spain and Portugal. The episodic days occurred less frequently at northern and northwestern locations, than at central and southern locations of the area of study (Fig. 3). These results are in accordance with the expected, as a higher frequency of episodic days is anticipated in southernmost locations, due to their proximity to the African continent. In general, days with Saharan dust intrusions are more frequent in summer months (Figs. 3–4), except in the Canary Islands, which presents also a high number of intrusions during winter. Thus, most of the regions reveal a marked seasonal cycle, with clear maxima (minima) during summer (winter) months, particularly for both Spanish

and Portuguese southern and central sectors (Southeast, Southwest, Centre, Northeast, East, Balearic Islands, Alentejo and Algarve). Less clear differences between months can be observed on the northern sectors (Northwest, North, North PT, Center PT).

Fig. 4. Annual frequency of days with intrusions in (a) Spain and (b) Portugal.

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In order to compare the annual behaviour for Portugal and Spain, their inter-annual variability was analysed through a box-plot representation (Fig. 4), where a distinct behaviour is found for Spain (SP) and Portugal (PT). Box-plots are a standardized way of displaying the distribution of data based on a five-number summary, i.e., lowest and highest observation, median, lower and upper quartile, which is especially useful for indicating whether a distribution is skewed and whether there are potential unusual observations (outliers) in the data set. On each box of the box-plots, the central mark indicates the median, and the bottom and top edges of the thick lines indicate the 25th and 75th percentiles, respectively. The narrow whiskers extend to the most extreme data points not considered outliers. The annual frequency in SP regions presents higher variability than the PT data as a result of the distinct behaviour among a larger set of regions. Additionally, the concordance between the years in Portugal and Spain with the higher annual frequency is only partial, with the years 2007 and 2011 being identified in both countries. In terms of duration, most of the considered regions were affected by intrusion events which took between 1 day and 10 consecutive days (Fig. 5). Although events longer than 10 days also occur, their frequency is low. Some of the shorter episodes only transport dust to one of the regions. On the contrary, during episodes with longer duration, dust could be transported to more than one region. 3.2. Circulation weather types The relative frequency of each CWT over the period 2006–2016 on annual and seasonal basis was analysed (Table 1). The anticyclonic (A) type is the most frequent circulation pattern throughout the year. Nevertheless, during summer months, when intrusion days occur more often, northeast (NE) and east (E) CWTs also present high frequencies. The predominance of the A type is strongly related to the migration of the Azores anticyclone towards the IP (Russo et al., 2014). On the other hand, the occurrence of the NE and E types is predominantly associated to the presence of a thermal low over northwestern Africa (sometimes extending over the IP) during the summer (Hoinka and Castro, 2003). Moreover, the meridional circulation types north (N) and south (S) present very distinct behaviour: while S is rather infrequent throughout the year (but especially during summer), N is one of the predominant CWT during the warmer months. The directional CWTs with a significant southern component (i.e. S, SE, SW) are

Fig. 5. Number of events (y-axis) with consecutive days of intrusions (x-axis) between 2006 and 2016.

Table 1 Circulation weather type (CWT) frequency during the 2006–2016 period for the Iberian Peninsula on a seasonal basis. Values highlighted in bold represent the most significant CWTs.

Annual DJF MMA JJA SON

A

C

NE

E

SE

S

SW

W

NW

N

Total

33.3 37.7 28.3 36.8 30.5

10.1 8.7 14.3 5.9 11.6

8.6 5.5 8.3 13.9 6.3

13.4 7.6 11.8 19.2 14.7

6.2 5.8 7.9 3.9 7.0

2.3 2.5 2.1 0.8 3.8

4.4 6.1 3.3 0.6 7.5

7.2 12.0 6.9 2.4 7.6

7.6 9.0 8.7 6.5 6.0

7.0 5.1 8.3 9.9 4.8

100 100 100 100 100

the least frequent throughout the year. Nevertheless, on a seasonal basis, the SW regime increases its frequency during autumn and the SE regime during autumn and spring. Throughout the year, the relative frequency of cyclonic (C) situations has two relative peaks, one in the spring and another during autumn months. 3.3. Circulation-to-environmental approach Previous works focusing on the relation between intrusions and pollution events concluded that days classified as intrusion days were in fact characterized by statistically significant higher values of PM10 when compared to non-intrusion days (e.g. Díaz et al., 2017; Querol et al., 2019; Salvador et al., 2019). Moreover, some studies have been conducted associating PM dispersion in southwestern Europe (e.g. Russo et al. (2014) in Portugal; Pey et al. (2013), Gaetani and Pasqui (2014) and Salvador et al. (2014) for PM in the Mediterranean) and a certain type of circulation pattern to the long-range transport. They link certain air dispersion conditions to the mesoscale meteorological behaviour that controls the regional transport of air pollution (Russo et al., 2014). Beforehand to the application of the methodology exposed in Sections 2, the data was aggregated in order to analyse regions with similar seasonal characteristics in terms of intrusions days. Therefore, two distinct large regions and the Canary Islands were identified (Fig. 6, top panel). The Canary Islands will be excluded from this synoptic analysis since the region is in the transition between tropical and extra-tropical atmospheric circulation, and therefore influenced by

Fig. 6. Probability of occurrence of intrusion according to each circulation weather type (CWT) during the 2006–2016 period for the two clusters on a seasonal basis.

A. Russo et al. / Science of the Total Environment 717 (2020) 137041 Table 2 Total number of days of intrusions by CWT during the 2006–2016 period for the Iberian Peninsula on a seasonal basis. Cluster 1

A C NE E SE S SW W NW N

Cluster 2

DJF

MMA

JJA

SON

DJF

MMA

JJA

SON

71.5 10 2.5 18 32.5 15.5 17.5 6.5 3 2

63.5 40 19 66 63 11.5 10 7.5 2.5 9

90.5 39 45 125 34.5 8 6 6.5 13.5 21

93 29 14.5 56 44 22 21 3.5 4 4

86 22.5 4.5 17 33 16 28 16 3 7

103.5 86.5 31.5 74.5 64.5 15 22 22 13 29.5

240.5 53.5 105.5 144 33 8 6.5 15.5 42.5 70

122 65.5 26.5 63.5 45.5 29 48 22 13 12

Total number of days above 100 are highlighted in bold.

ratter different meteorological conditions than those related to events in the peninsula (Alonso-Pérez et al., 2011). Moreover, from a synoptic circulation perspective it is important to separate intrusions occurring during different seasons as the climatological circulation over northern Africa and Iberia varies considerably throughout the year (Russo et al., 2014).

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The probability of occurrence of an intrusion associated to each CWT is shown in Fig. 6, and was calculated as the ratio between the number of intrusion days in each CWT and the total amount of days of each CWT. This assessment was obtained on a seasonal basis for the period spanning 2006–2016 and for the two large spatial clusters (Fig. 6, top panel). This ratio highlights the contribution of certain CWT which present low absolute frequencies of occurrence (e.g. SW, S, SE, Table 2) but that are associated to a higher probability for an intrusion to occur over the Iberian Peninsula. The anticyclonic (A) type is the circulation pattern which is more frequent in the IP during summer months, followed by the Eastern (E) (Table 1). However, the A CWT is not the CWT with the higher probabilities of occurrence of intrusions. In fact, and except for summer days over Cluster 2, only a relatively small fraction of days characterized by the A CWT are associated to intrusions. This is particularly notable for Cluster 1, where the probability of an intrusion being detected during an anticyclonic day is always below 40%, being this regime one of the least efficient for the occurrence of intrusions. Actually, intrusion events are much more likely to occur under other circulation types, depending on the season. During autumn (SON), winter (DJF), and spring (MAM) southerly CWTs (S, SW, SE) are the ones which present a higher probability for the occurrence of intrusions. Although relatively rare, intrusions were detected in N50% (up to 80% in some seasons) of the days

Fig. 7. Daily maps of MSLP (colored solid contours) and Z500 (grey dotted contours) during the lifecycle of two selected extreme events.

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characterized by these regimes. For example, during summer these values are close to 100%, meaning that in the case of a day characterized by a southerly regime the concomitant occurrence of an intrusion is highly expectable. However, it must be noted that these regimes correspond to around 6% of Cyclonic days, which are also quite efficient for the occurrence of intrusions, in particular during summer (N60%). This is coherent with the typical MSLP pattern associated to this CWT (see Fig. 2), also favouring a southerly advection, particularly over eastern Iberia. Interestingly, CWTs with a more oceanic component (W, NW and N) show the largest differences between the probability of occurrence of intrusions over the two clusters (around 3 times larger for Cluster 2). During most of the year none of these probabilities are high, which is not surprising accounting for the relatively strong zonal flow associated to these regimes. However, during summer, when zonal transport is at its lowest over the Iberian Peninsula, intrusions are detected over the Cluster 2 in N50% of days with W, NW and N flows. This notable increasing west-east propensity for the occurrence of Saharan intrusions has already been noted by Sousa et al. (2019). 3.4. Extreme events In the IP, PM10 episodes are often related to the intrusion of air masses containing African dust (Querol et al., 2019). However, the impact of forests fire (in summer), the sea spray, or even the recirculation of aged air masses may have a high impact on PM levels (Querol et al., 2009; Carvalho et al., 2011), as well as other factors (Querol et al., 2019). Nevertheless, African dust emerged as the largest PM10 source in regional background southern sites of the Mediterranean (Querol et al., 2019). Therefore, it is vital to understand the underlying lifecycle of dust intrusions. This section focuses on the analysis of two specific historical extreme episodes that have occurred in the IP (Figs. 7–8). The first event (2011 Event) affected a very large area in Europe reaching the Scandinavia (Figs. 7–8, left panel) between 5 and 11 April 2011. The second event (2016 Event) was a strong winter event with severe implications on the IP's air quality (Figs. 7–8, right panel). These figures are useful to provide a clearer insight into the meteorological factors which contributed to both events. Fig. 8 shows that in the 2011 and 2016 events the WHO limit value for human health protection (50 μg m−3; WHO, 2008) was exceeded in both countries, particularly in Spain where it reaches values higher than 3 times the allowed values. These results indicate that PM10 values generated by dust intrusion from the Sahara put an additional weight to the air pollution levels produced by local emissions. This intensification, produced by cumulative local and long-range transport, which has been observed during the 2011 and 2016 events, is similar to those found in other studies undertaken in the Mediterranean Basin (e.g. Querol et al., 2009; Díaz et al., 2017). The composites of MSLP (hPa) and geopotential height at 500 hPa synoptic fields for these two events are shown in Fig. 7. The associated CWTs, which are representative of the prevailing synoptic situation associated to each event's life-cycle, are quite different. During the 2011 event, there is a predominance of a strong SE component is fostered by a strong pressure dipole (anticyclone over France, and low pressure southwest of Iberia), enabling a strong advection originating from northern Africa arid areas, only replaced by a NW flow during its decay phase, when the dusty air mass is cleared from the peninsula. The SE CWT is one of the least frequent CWTs throughout the year (see Table 1), accounting for only 3.9% (JJA) to 7.9% (MMA) of the totality of days. However, and as shown in Fig. 6, it is one of the most efficient CWTs for the occurrence of intrusions in the study area, as it enables a strong advection originating from northern Africa arid areas. During the February 2016 event, the onset is associated to a more intense anticyclone located west of the IP (and a less pronounced thermal low over northwestern Africa). Its extension towards the Gulf

Fig. 8. (Top) Location of the three monitoring stations; (Middle) PM10 at the monitoring sites for the month of the events; (Bottom) HYSPLIT model results for the two episodes.

of Biscay leads once again to a pattern favouring a southeasterly advection, and leading to the intrusion, although in this event with a less marked southerly component. These results are consistent with previous results, i.e., PM10 extreme events occur predominantly under easterly and/or southerly weather patterns, which are associated to an advection of very dry and warm air masses (Querol et al., 2009; Russo et al., 2014). The NOAA HYSPLIT back trajectories allow consubstantiating this synoptic analysis. Based on HYSPLIT outputs for the 2011 and 2016 EVENTS it is possible to source apportioning clearly the particles' origin, through a clear trajectory of air parcels originating from northern Africa in both events (Fig. 8). During the 2011 event, trajectories cross the IP and reach northern Europe (not shown), carried in the westerly flank of the anticyclone which was located over France/UK (as shown in Fig. 7). A peak in PM10 concentration around April 8 is followed by a substantial decrease when the flow turns to a NW component (April 9–10). During the February 2016 event, the location and extension of the anticyclonic ridge limits/holds the dust intrusion to lower latitudes, thus particularly affecting the IP, as the SE flow is associated to a tilted trajectory towards Iberia. This strong advection reached its peak February 21, prompted by a thermal low over Morocco (see Fig. 7), leading to the outstanding PM10 values registered over IP stations (Fig. 8). 4. Conclusions In this work, dust events in all the IP are analysed on a regional basis. This is the first time, to the best of our knowledge, that a similar analysis is made for the whole IP using a long dataset (11 yr), which makes it especially relevant for the Iberian and western Europe research community. Previous studies mostly analysed dust events affecting different, sometimes aggregated, regions of the Mediterranean basin (Pey et al., 2013; Gaetani and Pasqui, 2014; Querol et al., 2019), using smaller

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datasets (Díaz et al., 2017; Oduber et al., 2019) or mainly focusing on the identification of the events and on the transport of dust (Dayan et al., 1991; Fernández et al., 2019). Thus, the first novelty of the present work is related with the a much higher spatial resolution improving what had been performed in previous efforts developed for the IP. This study relies on two dust intrusions' datasets which were aggregated for both IP countries following a similar approach as in previous studies for Spain (e.g. Pey et al., 2013; Salvador et al., 2014). The two datasets were aggregated and afterwards weather-dust interactions were analysed based on a circulation-to-environmental approach, which constitutes the second novelty of the present work. This approach identifies circulation weather patterns associated to the occurrence of dust events based on an automatic and objective method for the identification of CWT. Prevailing circulation weather patterns affecting the Iberian Peninsula were determined using a widely employed classification method developed for Iberia (Trigo and DaCamara, 2000), which is simple, easy to implement for different areas, and computationally inexpensive. This approach can easily be applied to the output of different climate models and datasets or different spatial domains, particularly at mid and high latitudes of both hemispheres. Based on this approach, the interannual variability of the CWTs affecting the IP was determined for the period 2006–2016 on an annual and seasonal basis. The association, performed at the daily scale, between each CWT and dust intrusion days was studied for two large areas of the IP. The higher probabilities of occurrence of intrusions are mostly associated with situations characterized by CWTs veering from easterly (E) to southern (S). This circulation-to-environmental approach can easily be transferred to other geographical areas, and therefore constitutes a promising tool in discriminating atmospheric conditions leading to the occurrence of dust intrusions and air pollutants. Moreover, the proposed objective automated approach does not include any a-priori assumption on the determination of the CWT as other methods in the literature (e.g. Salvador et al., 2014). Within the context of air quality and public health issues, besides particulate matter directly related to dust intrusion events, most Mediterranean countries endure a supplementary burden during summer months associated with forest fires (Pereira et al., 2011), which may contribute significantly to the observed PM values. Therefore, two PM10 extreme episodes, occurring outside the typical summer fire season, were evaluated in order to obtain a better insight into the lifecycle of dust intrusion events. The two episodes analysed (April 2011 and February 2016) correspond to two outstanding situations, not only in terms of hourly PM10 concentrations but also in what concerns the spatial range of influence. The analysis of the temporal evolution of synoptic conditions and back trajectories of air particles during these two episodes enables the characterization of PM transport pathways and sources, showing clearly the northward dust transport process from northern Africa towards the IP and linked to relatively high concentrations of PM10 observed at background monitoring stations over the peninsula. This combined approach of CWT and backtrajectories allows for a clear characterization of the northward dust transport from the northern Africa to the IP. Also, it depicts how different locations of high-pressure systems control the northward extension of these dust intrusions, how thermal lows foster strong advection processes over the IP, and how a shift to a westerly/northerly component relates to the decay of dust intrusions (particularly over western IP). The results obtained in the present study show that dust events across the IP were induced by different CWTs, conditioning the air quality. These results are also important as an aid to air quality forecasting since the high concentrations of atmospheric pollution and intrusion events are associated to less frequent CWTs and therefore the occurrence of these CWTs can be used as a warning signal for the occurrence of extreme events (Messeri et al., 2018). Thus, this approach is highly valuable and can be easily used as a complementary tool for forecasts, distinguishing between air masses coming from different areas of the African continent with different circulation characteristics. This is

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therefore an advantage on impact studies which intend to assess the effects of dust on human health, ecosystems or rain composition. Declaration of competing interest The authors declare no conflict of interests. Acknowledgements Publication supported by Fundação para a Ciência e a Tecnologia (FCT) - project UIDB/50019/2020 - Instituto Dom Luiz. A.M. Ramos was supported by the Scientific Employment Stimulus 2017 from Fundação para a Ciência e a Tecnologia (FCT, CEECIND/00027/2017). The authors wish to thank the Spanish Ministry for the Ecological Transition (MITECO) for providing the time series of PM10 daily concentrations from the Spanish regional background monitoring stations. The authors wish to thank the Portuguese Agency for the Environment (APA) for providing the time series of PM10 daily concentrations from the Portuguese regional background monitoring station. The authors wish to thank the NOAA Air Resources Laboratory (ARL) for the provision of the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model.The HYSPLIT model is available at http:// ready.arl.noaa.gov/HYSPLIT.php. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.scitotenv.2020.137041. References Adame, J.A., Hernández-Ceballos, M.A., Bolívar, J.P., De la Morena, B., 2012. Assessment of an air pollution event in the southwestern Iberian Peninsula. Atmos. Environ. 55, 245–256. https://doi.org/10.1016/j.atmosenv.2012.03.010. Alonso-Pérez, S., Cuevas, E., Querol, X., 2011. Objective identification of synoptic meteorological patterns favouring African dust intrusions into the Marine Boundary Layer of the Subtropical Eastern North Atlantic Region. Meteorog. Atmos. Phys. 113, 109–124 (2011). Alpert, P., Neeman, B.U., Shay-El, Y., 1990. Climatological analysis of Mediterranean ciclones using ECMWF data. Tellus 42A, 65–67. Bangert, M., Nenes, A., Vogel, B., Vogel, H., Barahona, D., Karydis, V.A., ... Blahak, U., 2012. Saharan dust event impacts on cloud formation and radiation over Western Europe. Atmospheric Chemistry and Physics 12 (9), 4045–4063. https://doi.org/10.5194/acp12-4045-2012. Barkan, J., Alpert, P., Kutiel, H., Kishcha, P., 2005. Synoptics of dust transportation days from Africa toward Italy and central Europe. J. Geophys. Res. 110 (7), 1–14 Article ID D07208. Bodenheimer, S., Lensky, I.M., Dayan, U., 2019. Characterization of Eastern Mediterranean dust storms by area of origin; North Africa vs. Arabian Peninsula. Atmos. Environ., 158–165 https://doi.org/10.1016/j.atmosenv.2018.10.034. Carvalho, A., Monteiro, A., Flannigan, M., Solman, S., Miranda, A.I., Borrego, C., 2011. Forest fires in a changing climate and their impacts on air quality. Atmos. Environ. 45, 5545–5553. Choobari, O., Zawar-Reza, P., Sturman, A., 2014. The global distribution of mineral dust and its impacts on the climate system: a review. Atmos. Res. 138, 152–165. https:// doi.org/10.1016/J.ATMOSRES.2013.11.007. Commission Staff Working Paper, 2011. Establishing Guidelines for Demonstration and Subtraction of Exceedances Attributable to Natural Sources under the Directive 2008/50/EC on Ambient Air Quality and Cleaner Air for Europe, Brussels, 15.02.2011. SEC (2011) 208 Final. , p. 37. http://ec.europa.eu/environment/air/quality/legislation/pdf/sec_2011_0208.pdf (Last Access: 07 January 2020). Dayan, U., Levy, I., 2005. The influence of meteorological conditions and atmospheric circulation types on PM 10 and visibility in Tel Aviv. J. Appl. Meteorol. 44 (5), 606–619. https://doi.org/10.1175/JAM2232.1. Dayan, U., Heffter, J., Miller, J., Gutman, G., 1991. Dust intrusion events into the Mediterranean Basin. Journal of Applied Meteorology (1988–2005) 30 (8), 1185–1199 (August 1991). Dee, D.P., Uppala, S.M., Simmons, A.J., Berrisford, P., Poli, P., Kobayashi, S., Andrae, U., Balmaseda, M.A., Balsamo, G., Bauer, P., Bechtold, P., Beljaars, A.C.M., 2011. The ERA‐Interim reanalysis: configuration and performance of the data assimilation system. Q.J.R. Meteorol. Soc. 137 (597), 553. https://doi.org/10.1002/qj.828. Díaz, J., Linares, C., Carmona, R., Russo, A., Ortiz, C., Salvador, P., Trigo, R.M., 2017. Saharan dust intrusions in Spain: health impacts and associated synoptic conditions. Environ. Res. 156, 455–467. Engelstaedter, S., Washington, R., 2007. Atmospheric controls on the annual cycle of North African dust. J. Geophys. Res. 112, D03103. https://doi.org/10.1029/ 2006JD007195.

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