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Atmospheric Environment 42 (2008) 7034–7042

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Atmospheric Environment

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Using 137Cs and 40K to identify natural Saharan dust contributions toPM10 concentrations and air quality impairment in the Canary Islands

L. Karlsson a, F. Hernandez a,*, S. Rodrıguez b,d, M. Lopez-Perez a, J. Hernandez-Armas a,S. Alonso-Perez b,c, E. Cuevas b

a Medical Physics and Environmental Radioactivity Laboratory, Department of Medical Physics and Pharmacology, University of La Laguna, 38320 La Laguna,Tenerife, Spainb Izana Atmospheric Research Centre, AEMet, Associated Unit to CSIC ‘‘Studies on Atmospheric Pollution’’, C/ La Marina, 20, 6a Planta, E38001, Santa Cruz deTenerife, Canary, Islands, Spainc Institute of Earth Science ‘‘Jaume Almera’’, CSIC, Lluis Sole i Sabaris S/N, E08028, Barcelona, Spaind Department of Geology, University of Huelva, Associated Unit to CSIC on ‘‘Air Pollution’’, Campus El Carmen, E21071, Huelva, Spain

a r t i c l e i n f o

Article history:Received 14 January 2008Received in revised form 8 June 2008Accepted 12 June 2008

Keywords:RadiotracersCaesium-137Potassium-40PM10Saharan dustAir qualityAtmospheric aerosolsBack-trajectoriesDust intrusions

* Corresponding author. Tel.: þ34 922 319422; faE-mail address: underzog@yahoo.com (F. Hernan

1352-2310/$ – see front matter � 2008 Elsevier Ltddoi:10.1016/j.atmosenv.2008.06.016

a b s t r a c t

Tenerife (Canary Islands) is often affected by Saharan mineral dust outbreaks. These eventsresult in high PM10 concentrations in ambient air, well above the limiting values adopted by theEuropean Union (Directive EU/1999/30). To comply with the EU Air Quality Directive, a quan-titative proxy for the characterisation of high PM10 events is required. Furthermore, this proxymust be capable of differentiating natural sources (as defined by the Directive), such as Saharanmineral dust outbreaks, from other anthropogenic sources. In this study, we have analysed thetime series of 137Cs, 40K and PM10 recorded at the island of Tenerife (2000–2006) in the MarineBoundary Layer (MBL), to test the possible usefulness of the two mentioned radiotracers asmarkers of Saharan mineral dust events. The results of the analysis showed that this is indeedthe case for 137Cs and that there are, at least, two different sources of 40K concentrations in thecollected atmospheric aerosol filters. While the detection of 137Cs in atmospheric aerosol filterscould be directly associated with the arrival of low altitude (<2000 m.a.s.l.) dust intrusions tothe study site, 40K concentrations could be associated, in most cases, with either low or highaltitude (>2000 m.a.s.l) dust intrusions. Moreover, two different mechanisms of aerosolloading in the MBL were, also, identified with the mentioned radiotracers.Good correlations (R2 > 0.6) were found between the three parameters when all thecompiled data was considered. The correlation values increased to nearly 0.8 when onlythose samples that had 137Cs above detection limits were considered. The highestconcentrations of 137Cs, 40K and PM10 were recorded in March 2004 in connection witha very intense low altitude Saharan dust intrusion. The results of this study, also, indicatedthat the 137Cs/40K ratio could be potentially useful to differentiate low from high altitudedust intrusions at this site.

� 2008 Elsevier Ltd. All rights reserved.

1. Introduction

According to the European Commission Air QualityDirective EU/1999/30, any given place in the EU may exceed

x: þ34 922 643165.dez).

. All rights reserved.

the daily PM10 (mass concentration of particles with anaerodynamic diameter smaller than 10 mm) limit of 50 mgm�3 during a maximum number of 35 days per year.However, PM10 limiting values are not to be applied tothose atmospheric events which are defined as ‘‘natural’’ bythe Directive. These natural events are, for example,particle inputs from desert areas, volcanic eruptions or wildfires. The implementation of the Directive, therefore,

L. Karlsson et al. / Atmospheric Environment 42 (2008) 7034–7042 7035

requires the identification and distinction of natural andanthropogenic sources of atmospheric particulate matter.Moreover, Article 5, Section 4 of the mentioned law indi-cates that Member States must provide to the Commissionthe necessary justification to demonstrate which exceedingdays are produced by natural events.

Northern Africa and the Arabian Peninsula are wellknown sources of mineral dust particles, which, undercertain meteorological conditions, may be transported overlong distances (e.g. Goudie and Middleton, 2001).The island of Tenerife (Canary Islands, Spain) is located inthe North Atlantic Ocean, approximately 210 km west ofthe coast of Morocco. Although in general, 74% of the airmasses in the marine boundary layer (MBL) at this sitecome from the North Atlantic, 16% from Europe and only10% from North Africa (Viana et al., 2002), the long-rangetransport of suspended dust from northern Africa oftenproduces the arrival of large amounts of atmosphericaerosols. As a consequence of these atmospheric events,PM10 levels, recorded at the MBL at this site, increasesignificantly reaching levels way above regulation limits(e.g. Rodrıguez et al., 2001; Perez-Marrero et al., 2002;Viana et al., 2002, Alastauey et al., 2005; Hernandez et al.,2005b). African dust inputs are, in fact, the main source ofnatural atmospheric events at this site (Viana et al., 2002)and the main reason why PM10 limiting values are sur-passed throughout the year. As stated above, these daysshould not be considered in the 35-day limitation. A proxythat could quantitatively identify these events would bevery useful to calculate the proper number of days whenthe PM10 levels are surpassed due to anthropogenicsources.

Chemical speciation analysis of aerosol filters has beenone of the most common approaches described in recentliterature to differentiate natural from anthropogenicsources of aerosols (e.g. Viana et al., 2006a, b, 2007; Wilsonet al., 2002; Dordevic et al., 2004). However, these analysesare costly and time consuming. More recently, Escuderoet al. (2007) suggested the utilisation of backgroundstations to determine the contribution of natural dust toPM10 concentrations. However, more than one measuringstation would be required for this purpose. The utilisationof a methodology that allows the differentiation of varioussources of particulate matter in a single direct measure-ment would be more advantageous. Hernandez et al.(2005b) identified a connection between the increment oftwo radiotracers, 137Cs and 40K, and the arrival of twointense (PM10 >150 mg m�3) African dust intrusions toTenerife. However, it was not concluded, in that study,whether the detection of these radiotracers could also beassociated with less intense African dust inputs.

A systematic 6-year analysis (September 2000–December 2006) of the concentrations of radiotracers inaerosol filters, back-trajectories and atmospheric modellingresults (aerosol loading and deposition) has been per-formed here to test the possible utility of 137Cs and 40K astracers of mineral dust inputs of African origin to Tenerife.Attention was focused on weeks that had relatively highconcentrations (with respect to the annual averageconcentration) of either 137Cs or 40K and on the identifi-cation of their origins. Statistical analysis of the data, back

trajectory analysis and Dust Regional Atmospheric Model(DREAM) results were used to identify the diverse types ofAfrican dust outbreaks, differing in seasonal occurrence,source origin and impact on the PM10 concentrations.

1.1. The study area

Detailed description of the study area can be found inHernandez et al. (2005a,b). In summary, Tenerife (28�180–28�350 N and 16�070 – 16�490 W) is the largest and highestisland of the Canary Islands with a surface area of2034 km2. The low troposphere is strongly stratified asresult of the confluence of a subsidence regime in the upperlevels with the North Atlantic trade winds blowing (NNE) inthe marine boundary layer (MBL). Because of this,a temperature inversion is present throughout the yearbelow 2 km above sea level (Bustos et al., 1998, Torres et al.,2001; see details in Rodrıguez et al., 2004).

Predominant NNE trade winds in the MBL are driventhroughout the year by the Atlantic wind regime, which isinterrupted for short periods (3–8 days duration) by low-level African intrusions, mainly from January to March.During these African intrusions a strong increase (>2 timesthe average concentration) in urban pollution and naturaldesert mineral dust can be observed (Goudie and Mid-dleton, 2001; Rodrıguez et al., 2004; Viana et al. 2002).Generally, during winter intrusions, Saharan dust reachesthe island of Tenerife at low altitudes (MBL), while duringsummer, the dust reaches the island at higher altitudes,>2000 m.a.s.l (Viana et al., 2002).

2. Material and methods

The aerosol sampling and measuring procedure utilisedin this study is described in detail elsewhere (Hernandezet al. 2005a,b). In summary, three aerosol monitoringstations, located within the Santa Cruz de Tenerife metro-politan area in Tenerife, Canary Islands), were used forsampling.

The sampling station used to collect aerosol samples forradiometric measurements was a PTI ASS-500, TSP (totalsuspended particulate) air pump. The air flow rate of thispump was, on average, 8500 l min�1. The pump was fittedwith square polypropylene filters (440 � 440 mm2) thatwere changed weekly. On a few occasions, the samplingweek was either shorter or longer than 7 days due topersonnel availability. The location and positioning of thisaerosol pump was chosen to minimise the contaminationof the filters by re-suspension of local soil particles.

All the radiometric measurements were performed bylow-level gamma spectrometry with coaxial-type germa-nium detectors (Canberra Industries Inc., USA). Thesedetectors were calibrated using certified reference gamma-ray cocktails purchased from Amersham Industries(Amersham plc. UK), and known concentrations of chemi-cally pure potassium chloride (Merck, E., Germany). Eachfilter was measured for 4 days and the same time-framewas used for background measurements. Spectra analysesand background subtraction were performed with theGenie2K spectrometry software version 2.0 (CanberraIndustries Inc., USA). The characteristic gamma peaks

L. Karlsson et al. / Atmospheric Environment 42 (2008) 7034–70427036

selected for the determination of the different radionu-clides were 661 keV for the 137Cs and 1460 keV for the 40K.

Considering the amount of data collected, fromSeptember 2000 to December 2006 (328 weeks), and inorder to simplify the study, only those weeks when theconcentrations of the radiotracers exceeded a certainactivity threshold (chosen arbitrarily) were considered inthe analysis. The thresholds were set in this case to 1 mBqm�3 for the 137Cs and 50 mBq m�3 for the 40K. This resultedin the consideration of 21 events regarding the detection of137Cs and 26 regarding 40K, herein including 12 eventswhich fulfilled both requirements.

The two different sampling stations used for themeasurement of PM10 were ‘‘Los Gladiolos’’ (LGO;28�2703700 N; 16� 1600200 W, 92 m.a.s.l.) and Santa CruzObservatory (SCO; 28�28021.100 N; 16�14050.500 W,52 m.a.s.l.). PM10 concentrations (uncorrected) weremeasured every hour at LGO station by the Beta attenua-tion method. Generally, PM10 data obtained with betainstruments needs to be corrected because of the volatili-sation of ammonium-nitrate and some organic compo-nents, mainly in winter due to the heating of the sample inthe inlet (typically at 50 �C) to avoid water condensation.However, such semi-volatile compounds are not present inthe particle phase in the Canary Islands, mainly becausetemperatures are high throughout the year (between 20and 35�C). The SCO station is a complementary urbanstation managed by the same research group that managesthe Izana Global Atmospheric Watch (GAW) Observatory(located at 2400 m.a.s.l., but not included in this study).PM10 concentrations were derived at SCO by convertingthe aerosol volume concentrations (monitored with anoptical particle counter GRIMM-1108) to PM10 massconcentrations by using experimentally determinedvolume-to-mass conversion factors. These factors wereobtained by collecting simultaneous PM10 samples on filtertwice per week. In both stations PM10 data was averagedweekly to fit the radiotracer sampling period.

Back-trajectories at 12 UTC were calculated daily withthe Hybrid Single-Particle Lagrangian Integrated Trajectory(HYSPLIT) 4.0 dispersion model (Draxler and Rolph, 2003;Rolph, 2003, http://www.arl.noaa.gov/ss/models/hysplit.html). Five-day back-trajectories at six different altitudes(300, 1000, 1500, 2000, 3000, and 4000 m.a.s.l.), wereobtained for each day of the studied events. In addition,0–72-h forecasts of dust loading in the atmosphere anddeposition were obtained with the Dust Regional Atmo-spheric Model (DREAM) (http://www.bsc.es/projects/earthscience/DREAM/index.psp).

2.1. Calculation of monthly and average indexes

Due to the large variations observed in the annualaverages of 137Cs and 40K activities as well as PM10concentrations, an approach using indexes was used toease the comparison of the available data. The approachused in this study has been described and used by otherauthors (e.g. Ferris, 1998; Todorovic et al., 1999; Kulan,2006). This method, when applied to the data, enhancesthe strength of seasonal variations in comparison to annualaverages. The monthly indexes, expressed in percent (%),

were calculated as the ratio of the monthly averages andthe corresponding annual averages. Following this, theaverage indexes were calculated as the average of themonthly indexes of all of the six considered years.

3. Results and discussion

The samples considered in this study were collectedbetween September 2000 and December 2006(328 weeks). During this period, 1% of the radiometric dataand 10% of the PM10 data was lost due to measuring and/orsampling failures. 137Cs was measured above detectionlimit (LID) in 54 of the 328 weeks, while 40K was measuredabove LID on 254 occasions. The 137Cs, 40K and PM10 datacollected during this period is shown in Fig. 1a,b. Fig. 1b,also, displays the calculated 137Cs/40K ratios that will bediscussed below. A series of samples (weeks) from this datawere selected for further analysis and are indicated in bothfigures. No uncertainties were plotted in Fig. 1a,b tosimplify these plots. A summary of the one-sigma uncer-tainties and other basic statistical parameters (average,maximum, minimum concentration, etc.) has beencompiled in Table 1. The highest 137Cs, 40K and PM10weekly concentrations, measured at this site, were11.7 � 0.6 mBq m�3, 340 � 14 mBq m�3 and 380 � 20 mgm�3, respectively. These concentrations were recorded inconnection with a very intense low altitude dust intrusionthat occurred in March 2004 (Hernandez et al., 2005b).

In order to highlight unusual increments in thecompiled time series, baseline values were calculated forthe 137Cs, 40K and PM10 concentrations for the consideredtime period. For this purpose, the data collected for twoindependent periods of consecutive weeks (92 weeks intotal) with relatively low concentrations of the threementioned parameters were chosen (from 2001-03-05 to2001-12-24 and from 2005-03-14 to 2006-02-27 [yyyy-mm-dd]). The results of these baseline values are shown inTable 2. The annual average PM10 concentration for theurban environment in Tenerife was estimated to bebetween 30 and 45 mg m�3 by Viana et al. (2002). The PM10baseline value calculated in this study was 29 � 10 mg m�3,which is within the range obtained by Viana et al. (2002).

Fig. 2a,b,c displays the average indexes calculated forthe 137Cs, 40K and PM10 data. All three parameters showhigh index values during winter, especially during theJanuary–March period. This result agrees with previousfindings regarding the concentrations of PM10 in theatmosphere (e.g. Viana et al., 2002; Alonso-Perez et al.,2007), that indicated that from January to March, theSaharan dust events occurred at low altitude. The 40Kaverage index figure (Fig. 2c) shows a second maximumduring the summer months, June–August period. Thisresult seems to indicate that 40K, unlike 137Cs, hasa secondary source or a different mechanism of reachingthe lower atmosphere at Tenerife. This issue is discussed inmore detail later on.

3.1. Concerning the origin of 137Cs

A previous study, by Hernandez et al. (2005b), identifiedan increase in 137Cs concentrations in aerosol filters during

Fig. 1. (a) 137Cs and 40K concentrations (in mBq m�3) measured in aerosol filters in the period 2000–2006. (b) PM10 concentrations (in mg m�3) and 137Cs/40Kratios calculated for the same time period. Sampling dates, understood as the day when sampling was concluded, are given in a [yyyy-mm-dd] format. Samplesthat had 40K > 50 mBq m�3 are indicated in the figure with black triangles., samples that had 137Cs > 1 mBq m�3 are indicated with black circles, and samples thatfulfilled both conditions are indicated with black rhombuses. More than one symbol is shown in a single place, for clarity, when consecutive weeks fulfilled thedescribed conditions.

L. Karlsson et al. / Atmospheric Environment 42 (2008) 7034–7042 7037

the arrival of two intensive African dust intrusions to Ten-erife. However, it was not concluded in that publication ifthe detection of these radiotracers could also be associatedwith less intense African dust events (with weekly meanPM10 concentrations < 150 mg m�3). In order to establish,in a more general way, the origin of the 137Cs concentra-tions in the collected aerosol filters, 21 weeks (highlightedin Fig. 1a) with concentrations above 1 mBq m�3 wereconsidered. The limit of 1 mBq m�3 was selected arbitrarily(above the calculated baseline value) to simplify theanalysis.

All of the selected 21 weeks had 40K concentrationsabove its baseline value of 15 mBq m�3 and a 137Cs/40K ratiohigher than 0.019. Only during two weeks (2004-01-05 –2004-01-12 and 2006-03-06 – 2006-03-13), were thePM10 concentrations below 40 mg m�3. In both cases, thePM10 concentrations were approximately 37 mg m�3.

Back-trajectory analysis of these 21 weeks indicatedthat, in 19 of them, air masses, that travelled at low–midaltitudes (<2000 m.a.s.l.) over the North African desert,arrived at Tenerife at 300 m.a.s.l. The back-trajectoriesshowed that these air masses crossed mostly over Morocco,

Table 1Summary of statistical parameters calculated from the available data(2000-09-11 to 2007-01-02)

Average�standarddeviation

Maximumvalue

Minimumvalue oraverage* LID

Average 1suncertainties(%)

40K (mBq m�3) 22 � 34 342 13* 22137 Cs (mBq m�3) 0.25 � 0.92 11.7 0.7* 25PM10 (mg m�3) 35.6 � 31.7 380 8 8Sampled

volume (m3)80000 �20000

158400 23530 7

LID, detection limit.

West Sahara, Algeria, Mali and Mauritania. In only twocases, 2002-06-30 – 2002-10-07 and 2005-07-18 – 2005-07-25, the 300 and 1000-m.a.s.l. back-trajectories did notshow African but Atlantic origins (during each day of thesame sampling week). In these two cases, the back-trajec-tories at 1500 m did show North African origins andtransport at low levels somewhere in Algeria. These twoweeks had the lowest 137Cs concentrations of the 21considered weeks.

Atmospheric dust loading results, obtained with theDREAM model for the same days when the back-trajecto-ries had African origins, showed levels of varying intensity(>0.06 g m�2) at Tenerife (except for two weeks whenDREAM results were not available). Fig. 3 shows, as anexample, both back-trajectory and DREAM model predic-tion results for a whole sampling week (2001-02-05 –2001-02-14). The 137Cs concentration for this week was2.4 � 0.2 mBq m�3, more than double the calculated base-line value for this radionuclide (0.6 mBq m�3). It can beobserved in this figure that high dust concentrations in theatmosphere below 3000 m.a.s.l. at Tenerife (calculatedwith the DREAM model) were observed for all the dayswhen air masses came from the African continent.

Table 2Calculated baseline values for the 137Cs, 40K and PM10 data collected from2001-03-05 to 2001-12-24 and from 2005-03-14 to 2006-02-27(92 weeks in total)

Average � standarddeviation

Maximumvalue

Minimumvalue

40K (mBq m�3) 15 � 6 36 3137 Cs (mBq m�3) 0.6 � 0.4 1.0 0.3PM10 (mg m�3) 29 � 10 59 8

Fig. 2. Average indexes calculated for the: (a) 137Cs, (b) 40K and (c) PM10 data.

L. Karlsson et al. / Atmospheric Environment 42 (2008) 7034–70427038

Eighteen of the 21 considered weeks took placebetween January and April and only three during thesummer period (May – September). This result is, again, inaccordance with previous studies showing that low

Fig. 3. Back-trajectories (top row) and DREAM model results obtained for some oaltitude (MBL) dust intrusion occurred. The position of the island of Tenerife is ind

altitude (MBL) dust intrusions take place, mainly, duringwinter, while high altitude dust storms occur duringsummer (e.g. Alonso-Perez et al., 2007).

The presented results indicate that high concentrationsof 137Cs (>1 mBq m�3) in aerosol filters collected at the MBLat Tenerife can be, mostly (90% of the cases), associatedwith low altitude (<2000 m.a.s.l.) African dust intrusionsand are, often, also characterised by high 40K and PM10concentrations. Thus, 137Cs concentrations in aerosol filterscould, in principle, be used as qualitative markers of naturalatmospheric events (low altitude Saharan dust intrusions),as defined by the EU Air Quality Directive, at this site.

3.2. Concerning the origin of 40K

A similar approach as for the 137Cs was utilised toinvestigate the origin of 40K in aerosol samples and toevaluate its utility as a marker of natural events. Onceagain, an arbitrary concentration was selected, 50 mBq m�3

(chosen well above the calculated baseline value of 15 mBqm�3). This resulted in a total of 26 weeks. Twelve of theseweeks (group 1) had 137Cs concentrations over 1 mBq m�3.Two additional weeks of the remaining 14 had unusuallyhigh detection limits for the 137Cs (>2 mBq m�3), due tomeasuring problems. Back-trajectories and DREAM modelresults indicated that low altitude Saharan dust intrusionsoccurred during those two weeks. It is, therefore, verylikely that longer measuring times would have allowed thedetermination of 137Cs concentrations in those sampleswith values above 1 mBq m�3.

f the days within a sampling week (2001-02-05 – 2001-02-14) when a lowicated with a black dot or TFE label.

Fig. 4. Back-trajectories (300, 1000 and 1500 m.a.s.l. on the top row; 2000, 3000 and 4000 m.a.s.l. on the middle row) and DREAM model results obtained forsome days within a sampling week (2003-06-23–2003-06-30) when a high altitude (>3000 m.a.s.l.) dust intrusion occurred. The position of the island of Tenerifeis indicated with a black dot or TFE label.

L. Karlsson et al. / Atmospheric Environment 42 (2008) 7034–7042 7039

Concerning the 12 remaining weeks (group 2) whichhad concentrations of 137Cs below 1 mBq/m3, back-trajec-tories indicated that during 10 of those weeks, the airmasses that reached Tenerife had travelled over Africa.Wind speeds at 3000 m.a.s.l. were comparably low incomparison to previously discussed weeks and the back-trajectories indicated that those air masses arrived atrelatively high altitude (3000 m.a.s.l.). Lower altitude airmasses (300 and 1000 m.a.s.l.) during those weeks camefrom the Atlantic Ocean. Two weeks could not be associatedto African dust intrusions and could, therefore, haveanother origin such as firework displays, sea spray orbiomass burning.

3.2.1. Weeks belonging to group 1These weeks were, as previously discussed in Section

3.1, associated with low altitude Saharan dust storms. Mostof these weeks were in winter time (December – March).

3.2.2. Weeks belonging to group 2Most of the weeks in group 2 were in summer 2003. It

has been reported that, during that summer, there weremany intense high altitude African dust intrusions (Alonso-Perez et al., 2007). The average PM10 concentration

calculated for the weeks from group 2, 34 � 13 mg m�3, waslower than that for the weeks in group 1, 107 � 75 mg m�3.In a few cases 137Cs was detected in these weeks, but theconcentrations were quite low (<1 mBq m�3). In all thesecases, the 137Cs/40K ratio was below 0.014. Fig. 4 displaysthe back-trajectories and modelling results calculated fora week (2003-06-23 – 2003-06-30) when a high altitudeSaharan dust intrusion occurred. It can be observed in thisfigure that dust loading and concentrations were smallerthan those shown in Fig. 3. It can, also, be appreciated thatthe air masses (back-trajectories) arriving at 3000 m.a.s.l.travelled at low altitude over the African continent 3 daysprior to their arrival in Tenerife.

The fact that K is normally associated in atmosphericaerosols with somewhat larger size particles in comparisonto 137Cs (Viana, 2003), may explain why 137Cs concentra-tions were lower in weeks from group 2 (when high alti-tude dust intrusions occurred). The reason for this might bethat, in high altitude dust intrusions with low wind speed,larger size particles deposit gravimetrically at a higher ratethan smaller ones (shorter aerosol residence times). This is,also, supported by the results obtained for the averagemonthly indexes (Fig. 2a,c). The period January–March,dominated by low altitude dust intrusions, showed the

Table 3Summary of the calculated correlation coefficients (R2) for the available data (11-09-00 to 02-01-07)

Considering all theavailabledata 2000–2006

Considering onlycases when 137Cs wasdetected (>LID)

Considering onlycases when 137Cs wasbelow LID

Considering onlythose cases whenPM10 � 40 mg m�3

Considering onlythose cases whenPM10 < 40 mg m�3

40K 137Cs PM10 40K 137Cs PM10 40K 137Cs PM10 40K 137Cs PM10 40K 137Cs PM1040K 1.00 1.00 1.00 1.00 1.00137 Cs 0.60 1.00 0.79 1.00 – – 0.78 1.00 0.06 1.00PM10 0.57 0.69 1.00 0.65 0.78 1.00 0.11 – 1.00 0.63 0.74 1.00 0.04 0.02 1.00

LID, detection limit.

Fig. 5. 137Cs vs PM10 correlation studies when only samples with137Cs > detection limits (LID) were considered.

L. Karlsson et al. / Atmospheric Environment 42 (2008) 7034–70427040

highest average indexes for 137Cs, 40K and PM10. The June–August period, dominated by high altitude dust intrusions,showed an additional 40K peak. 137Cs and PM10 concen-trations only showed a smaller increase during the summerperiod.

3.3. Correlation studies

Cross correlation analysis was performed on all the137Cs, 40K and PM10 data compiled between 11-09-00 and02-01-07. The results are shown in Table 3. Differentgroupings of the data were considered in this analysis.Linear correlations were found between 137Cs, 40K andPM10 concentrations when all the available data pointswere considered. The correlation coefficients for theseparameters became higher when: (1) only those eventsthat had 137Cs concentrations over detection limits wereconsidered, and (2) when PM10 concentrations were above40 mg m�3. No correlation was found between 40K andPM10 concentrations for those weeks when 137Cs concen-trations were below the detection limit, nor was anycorrelation observed between any of the parameters whenPM10 concentrations were below 40 mg m�3. These results,most likely, show that when 137Cs was detected, the majorcontributing source of 137Cs, 40K and PM10 to the atmo-spheric aerosols was the same, namely African dust intru-sions. The lack of correlation found between 40K and PM10,when PM10 concentrations were below 40 mg m�3, seemsto indicate that there was at least one additional source of40K other than the African dust intrusions and/or a differenttransport mechanism to the MBL at Tenerife.

The good correlation observed between the 137Cs andPM10 concentrations (Fig. 5), R2 ¼ 0.78 indicates that thisradionuclide may be used as a quantitative marker of the‘‘strength’’ of the dust intrusions to Tenerife without theneed for chemical speciation. Careful calibration of thisapproach could allow differentiation of the local contribu-tions of particles to the total PM10 levels with a singlemeasurement since, at this site, there are no local sources(traffic or industrial) of 137Cs. However, more powerfulaerosol collectors than the one used here would be neededto allow daily determinations of 137Cs in aerosol filters atthis site.

In many ways, 40K would be a more advantageous tracerto be used to identify the dust intrusions, especially since itis present at higher concentrations (i.e. less measuringuncertainty and detected on more occasions) and its lifetime ensures that this radionuclide will be present for yearsto come. However, there seem to be several sources of this

nuclide in the aerosol filters collected at this site. Thismakes its use as a tracer of atmospheric dust intrusions atthe Canary Islands more difficult. The use of a combinationof several natural radionuclides, also present in the aerosolfilters (such as 7Be, 210-212-214Pb and 208Tl) may allow theuse of 40K as a tracer for dust intrusions in the future.Further analysis is, however, required in this respect.Nevertheless this radionuclide may be useful, in combina-tion with the 137Cs data (via their ratio, shown in Fig. 1b), todifferentiate the type of dust intrusion in cases when themeteorological conditions are unknown, i.e. high altitudeintrusion – high 137Cs/40K ratio, or low altitude intrusion –low 137Cs/40K ratio.

3.4. Number of days with PM10 concentrations exceeding thedaily limiting value

In order to assess the utility of 137Cs as a tracer of naturaldust events, the number of days on which the daily PM10limit value of 50 mg/m3 (established by EU Directive 1999/30/EC) was exceeded for every sampling week was calcu-lated for the period May 2002 – December 2006. Duringthis period (232 weeks), weekly mean 137Cs concentrationshigher than 0.4 mBq m�3 were detected in 35 weeks. Ofthese 35 weeks, 28 had days with average concentrationsthat exceeded the PM10 daily limit (which accounts for 80%of the 35 weeks). During the remaining 7 weeks, incre-ments in PM10 concentration were recorded, but did notexceed the limiting values. During the 28 weeks when thePM10 daily limit was exceeded, the number of days/weekgreater than the limiting value ranged from 1 to 6 days/

Fig. 6. (A) Number of days per week in which PM10 concentrations excee-ded the PM10 daily limiting value and that had weekly mean concentrationsof 137Cs above 0.4 mBq m�3; (B) distribution of the daily mean PM10 valuesduring the 28 weeks with 137Cs > 0.4 mBq m�3 and with PM10 concentra-tions exceeding the PM10 daily limiting value.

L. Karlsson et al. / Atmospheric Environment 42 (2008) 7034–7042 7041

week (see Fig. 6), and daily mean values as high as 2052 mgm�3 (4th of March 2004) were recorded. During the weekswhen 137Cs concentrations were lower than 0.4 mBq m�3

(including those days when 137Cs was not detected), noneof the days exceeded the daily limiting values. Thus, theseresults show how weekly mean concentrations of137Cs > 0.4 mBq m�3 can be used as proxy (high probability,80%) of days exceeding the daily limiting values for PM10.

4. Conclusions

The results of this study indicate that 137Cs concentra-tions measured in atmospheric aerosol filters can be usedto identify, qualitatively and quantitatively, natural atmo-spheric events (low altitude dust intrusions) at Tenerife.This approach has the advantage of not needing costlychemical speciation studies and could, in principle, beapplied at any other sites, such as Southern Europe,Northern Africa, Eastern Asia and Australia. Furtherimprovements in the collection and measuring systemsmay also allow the use of this radiotracer to identify highaltitude dust intrusions at this site. Twenty-four-hoursampling would be required to use this tool for assessingthe natural contributions of dust to PM10 levels in fullcompliance with the requirements of the EU Air qualityDirectives.

Further studies are required to allow the use of 40K,instead of 137Cs, as a tracer of African dust intrusions. This isdue to the fact that more than one source of 40K to theaerosol filters were observed. However, on some occasions,this radionuclide could be used to characterise high alti-tude dust intrusions, something which cannot currently be

achieved by the 137Cs method due to sampling andmeasuring limitations.

Most low altitude dust intrusions were recorded, at thissite (data available between 2000 and 2006), in the winter,January to March, period. This result agrees with previouslyreported findings for Tenerife. The highest 137Cs, 40K andPM10 concentrations measured in the 2000–2006 periodwere 11.7 � 0.6 mBq m�3, 340 � 14 mBq m�3 and380 � 20 mg m�3, respectively. These concentrations wererecorded in connection with a very intense low altitudedust intrusion that occurred in March 2004.

The three parameters considered in this study, 137Cs, 40Kand PM10, correlated well in most circumstances. Theseresults show the quantitative usefulness of radiotracerssuch as 137Cs and 40K in atmospheric studies. It was onlywhen PM10 concentrations were low in the atmosphere,<40 mg m�3, that no correlations were found between anyof the parameters or when 137Cs was not detected.

Acknowledgements

We wish to express our gratitude to the Consejo deSeguridad Nuclear (CSN), Spain, for their support of theEnvironmental Radioactivity Laboratory of the Universityof La Laguna. Silvia Alonso-Perez enjoys a grant founded byan agreement between the Environment Ministry of Spainand the Consejo Superior de Investigaciones Cientıficas(CSIC) to perform studies of atmospheric pollution byparticulate matter in Spain. The authors gratefullyacknowledge the NOAA Air Resources Laboratory (ARL) forthe provision of the HYSPLIT transport and dispersionmodel and/or READY website (http://www.arl.noaa.gov/ready.html) used in this publication. We would, also, liketo acknowledge the Earth Sciences Division of the Barce-lona Supercomputing Center (Centro Nacional de Super-computacion) for providing DREAM dust model forecasts.More detailed information can be found at the followingsite: http://www.bsc.es/projects/earthscien/DREAM/. Thisstudy has been partially supported by the projects GRACCIE(CONSOLIDER-INGENIO; Ministry of Science and Innova-tion) and CEPAUR (336/PC08/3-10.1; Ministry of environ-ment and rural and marina affairs).

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