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Atmospheric Environment 43 (2009) 3044–3051

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Atmospheric Environment

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Influence of transport and trends in atmospheric CO2 at Lampedusa

F. Artuso a,*, P. Chamard a, S. Piacentino b, D.M. Sferlazzo c, L. De Silvestri a, A. di Sarra a,D. Meloni a, F. Monteleone b

a ENEA, ACS-CLIMOSS, Via Anguillarese 301, 00123 S. Maria di Galeria, Rome, Italyb ENEA, ACS-CLIMOSS, Via Catania 2, 90141 Palermo, Italyc ENEA, ACS-CLIMOSS, Capo Grecale, 92010 Lampedusa, Italy

a r t i c l e i n f o

Article history:Received 16 October 2008Received in revised form9 March 2009Accepted 13 March 2009

Keywords:Atmospheric CO2

Interannual variabilitySeasonal cycle amplitudeBackward trajectory

* Corresponding author. Tel.: þ39 0630483232; faxE-mail address: florinda.artuso@enea.it (F. Artuso)

1352-2310/$ – see front matter � 2009 Elsevier Ltd.doi:10.1016/j.atmosenv.2009.03.027

a b s t r a c t

The study of the CO2 15-year records at Lampedusa (35� 310N, 12� 370E) is presented in this work. Short-and long-term CO2 variability has been investigated. No significant diurnal variations are observable thusremarking the background character and representativeness of the observation site. The CO2 long-termtrend shows a mean linear growth rate (GR) of 1.9 ppm yr�1. The periodic behaviour of the time serieshas been analysed and the mean seasonal cycle amplitude has been found to be 8.72 ppm. The seasonalcycle amplitude shows a marked interannual variability. The lowest value of the seasonal cycle amplitudehas been detected in 2003, in concomitance with the strong anomalous heat wave recorded in Europe.CO2 GR behaviour has been related to global processes such as El Nino Southern Oscillation (ENSO) andglobal temperature (Tg). The influence of ENSO event on GR is remarkable only during 1998. CO2 GRcurve shows peaks in the periods 1995, 2001 and 2005 (1.9, 3.7, 3.2 ppm yr�1 respectively) that arecharacterized by high Tg values and by intense biomass burning events. The anomalous decrease in theGR during the warm 2003 has been attributed to changes in the atmospheric circulation regime.Evaluation of the influence of transport on CO2 variability has been carried out using backward air-masstrajectory analysis and highlights the effect of the regional distribution of sources and sinks. Theindustrial activities and forests located in the Eastern European and Russian sector strongly affect the CO2

mixing ratio. The CO2 content of air-masses originating from this region is influenced in summertime bythe high efficiency of the vegetation sink while in the winter period prevails the effect of industrialemissions.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

The key role played by the increasing level of the anthropogenicgreenhouse gases (GHGs) on the global climate warming has beenclearly remarked and assessed in the recent Fourth Assessment Reportof the Intergovernmental Panel on Climate Change (IPCC, 2007).

Among the long-lived GHGs, carbon dioxide (CO2) is the mostabundant and exerts a substantial warming effect on climate. Itsconcentration increase in the troposphere from 1750 to 2005 hasled to a radiative forcing (RF) ofþ1.66 W m�2, that is more than 60%of the total RF due to anthropogenic GHGs. The increase of thecurrent CO2 RF compared to the one calculated in the previous IPCCreport (IPCC, 2001) results from the recent changes of its atmo-spheric level. Global mean concentration has reached today a valueof 379 ppm.

: þ39 0630486678..

All rights reserved.

Studies on the chemical composition of bubble air trapped in icecores from Antarctica have contributed to the reconstruction of CO2

variations in the past and revealed that the current level has neverbeen experienced in the past 800,000 years (Luthi et al., 2008; Petitet al., 1999). Vostok and EPICA Dome C ice core records indicatedthat CO2 mixing ratio in the pre-industrial period has neverexceeded 300 ppm. Since 1750, the date recognized worldwide asthe beginning of the industrial revolution, a rapid rise of CO2 hasbeen observed. In order to define the global atmospheric CO2

budget and to constrain its sources and sinks, accurate long-termmeasurements of atmospheric concentration have been started atMauna Loa, Hawai, about 50 years ago (Keeling et al., 1976; Keeling,2008). Since then a globally distributed network of GHGmeasurement stations has been developed all around the worldunder the Global Atmosphere Watch (GAW) program that has beenestablished by the World Meteorological Organization (WMO,2005). Since the late 1970s in USA, the National Oceanic andAtmospheric Administration (NOAA) launched a global mea-surement network of CO2 and other gases relevant for climate

F. Artuso et al. / Atmospheric Environment 43 (2009) 3044–3051 3045

(Conway et al., 1994; Tans et al., 1989). Global CO2 records are usedto understand the processes that rule the CO2 cycle that areessential for predicting its future trend and for planning interna-tional policy strategy for its reduction.

The long-term behaviour of CO2 is the combination of a gradualincreasing trend inferred by anthropogenic emissions, overlapped toa large interannual variability related to climate-driven changes insource and sink strengths (Rayner and Law,1999; Keeling et al.,1995).

Human activities contributed to the noteworthy increase ofatmospheric CO2. Fossil fuel emissions, cement production andland use changes are the main responsible. Changes in land use,determine a considerable reduction of CO2 uptake capacity byplants, thus amplifying the general global increasing trend(Canadell et al., 2007).

Variability of CO2 is the result of different superimposed naturaland anthropogenic mechanisms operating at different spatial andtemporal scales. Interannual fluctuations are mainly due to naturalvariations in source and sink efficiency of terrestrial and oceanicsystems induced by climate changes. It has been demonstrated thatnatural global-scale phenomena, like ENSO, have a remarkablecorrelation with CO2 year-to-year variations (Zeng et al., 2005a;Feely et al., 1999; Rayner et al., 1999; Bacastow, 1976). ENSO may inturn alter processes, such as precipitation, temperature and oceanupwelling, that regulate the atmosphere–land–ocean carbonfluxes. The CO2 behaviour exhibits also a short term variability thatreflects the influence of the interplay between local or regionalsource and sink distribution and air-mass transport.

Global CO2 observations combined with atmospheric transportmodels are used for deriving latitudinal and seasonal patterns ofsources and sinks and for quantitatively estimating their magni-tude (Tans et al., 1989; Rodenbeck et al., 2003; Gurney et al.,2002). Such information is crucial for the prediction of future CO2

trends and for quantification of the human contribution to theobserved build-up.

At present a number of sites globally distributed are part of theGAW observation network on CO2 but observations are still lackingin some areas of the world. Focusing on the Mediterranean basin,the network is biased toward continental sites. This has promptedthe need to identify a suitable remote site for CO2 monitoring in thisarea. A long-term GHG monitoring program has been started ona weekly basis by ENEA (the Italian National Agency for Newtechnologies, Energy and Environment) in May 1992, for back-ground CO2 measurements at the remote island of Lampedusa(Chamard et al., 2003). At present Lampedusa represents the onlymaritime site of GAW and NOAA networks dedicated to CO2

observations in the Mediterranean. The island has been selected forits geographical position that is particularly suitable for climatestudies. Surrounded by the sea and scarcely inhabited, it is almostunperturbed by human activities. Moreover the poor vegetation ofthe island has only a limited influence on the observed CO2 mixingratio. For this reason the values measured at Lampedusa may beconsidered representative of a large area of the Mediterranean.

Here we present more than 15 years of accurate CO2 measure-ments at our station. The aim of this work is to analyse the CO2

variability on different temporal and spatial scales in order toevaluate possible mechanisms influencing the Mediterraneancarbon budget. A detailed characterization of CO2 records has beencarried out through the investigation of the short and long-termvariability. Analysis of the diurnal and seasonal cycles permits toevaluate temporal and spatial variation of regional sources andsinks. Growth rate interannual variability has been related to long-range transport through a backward trajectory approach andcorrelation with large-scale phenomena has been carried out. Abrief comparison with records of other Italian stations has also beenincluded in this study.

2. Sampling site and methods

2.1. Site description

Geographical characteristics of Lampedusa and the ENEA GHGprogram history are described previously (e.g., Artuso et al., 2007;Chamard et al., 2003). For its strategic position Lampedusa is a fav-oured observation site of climatic processes. Besides studies on GHGevolution, aerosol properties and radiative budget, this site is partic-ularly suitable for investigations on the influence of atmosphericcirculation because it is possible to observe alternatively air-massesoriginating from Europe, where most of sources and sinks are sited,and less polluted African air-masses. The stationproduces high qualityreference data and offers scientific support to national and interna-tional organizations. It is part of the GHG monitoring station networkof GAW and has been included in the Cooperative Global Air SamplingNetwork of NOAA since October 2006. Lampedusa is also part of theItalian national network for background CO2 measurements togetherwith Monte Cimone (44� 110N,10� 420E, 2165 m a.s.l.) and Plateau Rosa(45� 460N, 7� 420E, 3480 m a.s.l.) stations (Apadula et al., 2005).

Complementary measurements of the main meteorologicalparameters, such as temperature, pressure, humidity, wind speedand direction, are also performed and used as support to GHG datainterpretation. Local meteorological conditions are described else-where (Chamard et al., 2003; Artuso et al., 2007).

2.2. CO2 measurements

CO2 monitoring has been run on a weekly basis and withcontinuous measurements. CO2 in situ continuous measurementshave been performed since 1998 with an interruption from 2003 to2005. The weekly air samples have been measured since May 1992,using a Siemens Ultramat 5E non-dispersive infrared analyser(NDIR). Sampling and measurement procedures were describedelsewhere (Chamard et al., 2003).

Duplicate samples are collected and measured. The data repor-ted in this work are the mean value of each pair. Flask measure-ments are missing during most of August 2002. Since the annualminimum occurs in August, the derived seasonal amplitude for year2002 must be taken with caution. The CO2 flask measurement ofAugust 2, 2002 is relatively high (374.6 ppm); an additional valuebased on continuous measurements made in the early part of themonth, was included in the dataset.

Determination of CO2 mixing ratio on flask samples is obtainedwith a standard deviation of about 0.04 ppm. To ensure accuratemeasurements, eight CO2 mixtures certified by NOAA ClimateMonitoring and Diagnostic Laboratory (CMDL), are used as primarystandards. Two working standards provided by the nationalcompany SIAD, are used to perform the calibration every 3 h duringcontinuous measurements. These working standards are calibratedtoward the primary mixtures every 15 days. Since 1992 our labo-ratory participated to numerous intercomparison experimentsorganized by the Scripps Institute of Oceanography (SIO), by NOAAand also in the frame of the European Carboeurope Project. Theresults of the 4th WMO Round Robin Test show that the meandifference between the ENEA and NOAA standard measurementdata was around �0.12 ppm. Results of Cucumber Carboeuropeintercomparison experiment give a mean deviation of �0.04.

3. Results and discussion

3.1. Diurnal variability: short term changes

Diurnal changes in CO2 mixing ratio reflect the combined effectof regional sources/sinks and short/medium range transport. The

F. Artuso et al. / Atmospheric Environment 43 (2009) 3044–30513046

boundary layer typical of marine sites like Lampedusa is supposedto be quite stable during the day and hence not perturbing diurnalvariations. Fig. 1 shows the monthly mean diurnal cycle duringJanuary and July 2007. Diurnal cycles have been derived fromcontinuous measurements using hourly mean data for calculations.A 2s criterion has been used for data selection, where s indicatesthe standard deviation of the data recorded in 1 h. Diurnal cyclesshow a seasonal pattern. At wintertime the CO2 level remains quitestable and no cycle is observable during the day. The summerseason displays a very small diurnal CO2 cycle thus reflectinga possible influence of photosynthetic activity. The daily peakoccurs at about 9 am, and the minimum at about 6 pm. Day-to-dayvariability seems to be slightly higher in summertime than duringthe winter (1s is �2 ppm in July and �1.5 ppm in January). Incomparison with results recorded at mountain sites (Necki et al.,2003), no significant seasonal differences of diurnal cycle ampli-tude have been found. This result remarks the limited influence oflocal sources and sinks at Lampedusa.

376

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0 10 15 20 25time (hour)

July

383

384

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386

387

388

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390

CO

2 (p

pm

)

January

5

Fig. 1. Mean CO2 diurnal cycle recorded at Lampedusa in January and July 2007. Errorbars indicate standard deviations at a given hour.

3.2. Curve fitting: CO2 long-term trend and seasonalcycle amplitude

The weekly CO2 mixing ratio recorded at Lampedusa in theperiod May 1992–September 2007 is shown in Fig. 2. It exhibitsa linear growth rate of þ1.9 ppm yr�1 and significant year-to-yearvariability. CO2 record is a combination of a long-term trend signaloverlapped to a sinusoidal seasonal cycle. In order to extractinformation on the mean periodic behaviour and growth of the 15-year record, a function consisting of an exponential term and twoharmonics has been fitted to the available data according to theFourier analysis described elsewhere (see Chamard et al., 2003).The fitting function used for the calculations is:

y¼ c1þc2eamþAsin½2pðmþf1Þ=T1�þBsin½2pðmþf2Þ=T2� (1)

where y is the weekly CO2 mixing ratio, m is the week number, a, c1

and c2 are constants. The semi-annual and annual periods are T1

and T2, while f1 and f1, A and B, are the semi-annual and annualphases and semi-amplitudes respectively. The second harmonic,with a period of 52 weeks, fits the seasonal variation due to naturalphenomena as photosynthesis and ecosystem respiration, whilethe first harmonic (T1¼ 26 weeks) with smaller amplitude is linkedto seasonal variations of energy consumption due to summer airconditioning and winter heating (Erickson et al., 2008). Periodsshorter than 6 months are neglected. The least-squares fit resultsare plotted in Fig. 2 and the calculated parameters are listed inTable 1.

The observed increasing long-term trend is determined by theglobal CO2 released by anthropogenic activities. The annual cyclehas mean amplitude of 8.72 ppm, as may be derived from param-eter B of Table 1. The periodic behaviour recorded at Lampedusareveals a typical minimum in August due to the elevated strength ofthe vegetation sink. When the plant growing season ceases, the CO2

level starts to rise up reaching a peak during the winter. The CO2

build-up ends in the middle of March with the reactivation of thephotosynthetic sink. This seasonal behaviour is characteristic ofmidlatitude sites in the northern hemisphere.

The year-to-year variability of the annual cycle has been alsoanalysed using the fitting curve defined by the sinusoidal part ofthe expression (1). The fit has been applied year-to-year to thedetrended mixing ratio values smoothed over 13 weeks. In this waythe annual semi-amplitude B has been derived for each year andthe resulting seasonal cycle amplitude has been calculated (Fig. 3).

Fig. 3 displays the seasonal cycle amplitude variation recordedat Lampedusa. Low seasonal cycle amplitudes occurred in 1995 andacross the period 1998–2003. Anomalies recorded in 1995 reflect

340

350

360

370

380

390

400

1992 1994 1996 1998 2000 2002 2004 2006 2008

CO

2 (p

pm

)

time (year)

Fig. 2. Record of CO2 mixing ratios at Lampedusa (open circles) and fit (black curve)calculated from expression (1). Grey curve represents the exponential portion of fit.

Table 1Fit parameters calculated by expression (1).

Parameter Value

A 0.95 ppmB 4.36 ppma 3.97 � 10�4 wk�1

f1 5.18 wkf2 23.06 wk

3

4

5a Lampedusa (35°31'N, 12°37'E)

F. Artuso et al. / Atmospheric Environment 43 (2009) 3044–3051 3047

the influence of large biomass burning events in tropical and borealregions as described by Langenfelds et al. (2002).

Annual cycle changes are the signatures of many superimposedprocesses. Seasonal behaviour is mainly driven by terrestrialbiosphere processes such as photosynthesis and plant and soilrespiration. However also seasonal variation of anthropogenicemissions, ocean uptake (Ramonet and Monfray, 1996) and biomassburning events (Wittenberg et al., 1998) may alter significantly theCO2 seasonal signal.

Droughts experienced from 1998 to 2003 in the northernhemisphere midlatitude regions caused less precipitation, a reduc-tion of vegetation growth and an increase in soil respiration. Allthose phenomena induce a significant reduction of the CO2

seasonal signal (Zeng et al., 2005b). In particular the intensedrought occurred in 1998 has been a consequence of the large ElNino event. The lowest value of the seasonal cycle amplitude hasbeen observed in 2003, in concomitance with the strong anoma-lous heat wave recorded in Europe that have caused enormouscarbon losses due to a reduction in net primary productivity (Ciaiset al., 2005).

0

1

2

b

c

0

1

2

3

4

CO

2 G

R (p

pm

yr-1) Pt.Rosa (45°46'N, 7°42'E)

1

2

3

4Mt.Cimone (44°11'N, 10°42'E)

3.3. Growth rate and interannual variability: comparisonwith SOI, Tg and SST

Phenomena occurring on large scales directly affect the rate ofCO2 mixing ratio change. With the intention to probe the causes ofthose changes in this study we compared parameters representa-tive of some global processes with the derivative of the 13-monthmoving average curve of CO2 (GR).

The smoothed 13-month GR curve, shown in Fig. 4a, is charac-terized by four peaks occurring in the periods 1995, 1998, 2001 and2005 respectively. A similar behaviour but with different intensitieshas been observed at the Italian stations of Plateau Rosa (Fig. 4b)and Monte Cimone (Fig. 4c). All stations experienced an anomaloushigh growth rate in 1998 that is very likely linked to the 1997/98ENSO event. The 1998 correlation between GR curve and theSouthern Oscillation Index (SOI), the parameter representative ofthe ENSO, has been already discussed by Chamard et al. (2003). The

0

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12

1993

1994

1995

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1997

cycle a

mp

litu

de (p

pm

)

1998

1999

2000

2001

2002

2003

2004

2005

2006

time (year)

Fig. 3. Seasonal cycle amplitude versus time.

1998 peak grows with the decrease of the station altitude thusremarking the influence of transport dynamics.

The interannual CO2 variability due to ENSO is based on oceanicand terrestrial flux changes. In contrast with the old literaturetheories (Bacastow, 1976) which emphasize the role of the oceanson the CO2 atmosphere accumulation during warm ENSO periods,Zeng et al. (2005a) have recently demonstrated that most of theinterannual CO2 GR variability is driven by land–atmosphere fluxvariations. In particular climate change anomalies induced by ENSOin the tropical regions, such as warmer temperature and lessrainfall, cause a reduction in net primary production (NPP) and anincrease in heterotrophic respiration. Both phenomena promotethe enhancement of carbon loss from terrestrial biosphere to theatmosphere. Northern hemisphere regions give a smaller contri-bution to the land carbon flux. However ENSO signature isobservable also at northern latitudes with a delay of a few months.At Lampedusa the ENSO response on CO2 GR appears during 1998with a time lag of 9 months. Apart from the period 1997/1998,a correlation between SOI and CO2 GR time series is not evident.Changes in the atmospheric CO2 seem to be better related withglobal air temperature (Fig. 4d). Recent reconstructions of the pastclimate and CO2 airborne content (Came et al., 2007) support theidea that increase in CO2 mixing ratios is linked to a rise of globaltemperature and vice versa. Mean surface temperature perturbs

d

0

00.10.20.30.40.50.60.7

1992 1994 1996 1998 2000 2002 2004 2006 2008

T an

om

aly (°C

)

time (year)

Fig. 4. Annual average CO2 growth rates recorded at stations of (a) Lampedusa,(b) Plateau Rosa, and (c) Monte Cimone. (d): Annual average evolution of globaltemperature anomaly.

Fig. 5. Map of four geographical sectors used for backward air-mass tarjectoryanalysis.

Table 2Seasonal distribution of number of trajectories (n.), average values of d and standarddeviation of average sd satisfying 50% permanence criterion for each geographicalsector (DJF ¼ winter; MAM ¼ spring, JJA ¼ summer, SON ¼ autumn).

SEA AFR

season DJF MAM JJA SON DJF MAM JJA SONd 0.08 0.20 �0.19 �0.46 �1.00 �0.83 5.18 �0.79sd 0.22 0.14 0.17 0.19 0.36 0.41 0.25n. 73 130 154 120 38 10 1 30

EUE EUW

season DJF MAM JJA SON DJF MAM JJA SONd 1.16 1.92 �2.96 0.06 1.31 0.45 �0.52 0.70sd 0.44 0.47 0.42 0.53 0.48 0.36 0.40 0.75n. 23 12 4 19 15 19 29 16

F. Artuso et al. / Atmospheric Environment 43 (2009) 3044–30513048

oceanic and land biosphere carbon sinks and sources inducinga variation of the CO2 growth. Carbon dioxide accumulated in theatmosphere may in turn amplify increased global temperatures.

The complex interplay between the global air temperature andthe year-to-year variability of CO2 GR may be attributed toa number of natural processes, while the GR long-term trenddepends on the emissions from anthropogenic activities. Naturalcontribution substantially involves ocean and land emission anduptake processes. Climate affects terrestrial ecosystem carbonfluxes through the modification of the biosphere respiration andphotosynthesis mechanisms. Global warming leads to a net posi-tive feedback to the carbon balance. Tropical contributionpredominates because of two main factors (Zeng et al., 2005a). Hightemperatures reduce precipitations and consequently plantproductivity, and also enhance soil respiration. Even if the terres-trial source accounts for most of the interannual variability of CO2,oceans play also a significant role. The efficiency of ocean sinks ismuch sensitive to climate. Reactions implied in the sea–atmo-sphere carbon cycle, such as CO2 dissolution in sea water andcalcification, are strongly temperature dependent. A reduction ofthe Southern Ocean sink activity due to wind modifications relatedto climate changes has been remarked by Le Quere et al. (2007).

Dry regimes bring about biomass burning events that couldamplify the effect due to climate anomalies on carbon fluxes(Simmonds et al., 2005). The increase of CO2 growth rate detectablein 1995 is supposed to have a correlation with the numerous fireevents recorded in that period and triggered by high temperatures(Langenfelds et al., 2002).

Global average surface temperature and CO2 GR curves havebeen cross-correlated. They show a positive correlation coefficientof 0.59. A higher correlation has been found (þ0.81) with PlateauRosa data. Both curves are almost always in phase with each other.All peaks are well correlated except during the heat wave of 2003.At that time the GR and Tg time series are out of phase because theincrease of temperature corresponds to a minimum in the rate ofCO2 mixing ratio changes. Such behaviour is unexpected because ithas been demonstrated that 2003 droughts have induced signifi-cant carbon release from the biosphere in Europe (Ciais et al.,2005). A correlation between CO2 GR and mean surface tempera-tures of macro-regions around Lampedusa (North Africa, Europeand North Atlantic) has first been attempted. The averagetemperature in the selected macro-regions do not display ananomalous behaviour in 2003, significantly different from theglobal mean, that may be singles out to justify the GR evolution.

An interpretation of the GR anomalous behaviour during 2003with respect to changes in the atmospheric circulation regime willbe given in the next section.

3.4. Backward air-mass trajectory analysis

Long and short range transport influence the CO2 behaviourrecorded at remote sites on different time scales. The alternation ofair-masses originating from Africa or from the European continentmay modulate CO2 mixing ratio recorded at Lampedusa, ona timescale of a few days (see e.g., di Sarra et al., 2005).

Evaluation of the influence of transport on CO2 variability hasbeen carried out using the backward air-mass trajectory analysis.Trajectories have been calculated by using the HYSPLIT (HYbridSingle Particle Lagrangian Integrated Trajectory) dispersion model(Draxler and Rolph, 2003) provided by National Oceanic andAtmospheric Administration/Air resources Laboratory. HYSPLITcalculations have been computed using National Centre for Envi-ronmental Prediction/National Centre for Atmospheric Research(NCEP/NCAR) reanalysis data archives. This model has been used tocalculate 5-day backward trajectories ending at Lampedusa at the

sampling time (usually 7 UT) and at the ground level of 45 m asl forthe 15-year-period 1992–2007. Trajectories have been classified onthe basis of regional sector patterns defined in relation to thedistribution of CO2 sources and sinks, as already done in Artusoet al. (2007), for CH4 measurements. Four geographical sectors (seeFig. 5) in the area surrounding Lampedusa have been taken intoaccount: the Western Europe (EUW) has been distinguished fromEastern Europe (EUE) because of the different contribution to CO2

emissions and uptake. A SEA region, comprehending the AtlanticOcean and the Mediterranean sea, has been considered separatelyin order to evaluate the contribution of the sea to the CO2 fluxes.The north African area (AFR), where predominates the presence ofthe Sahara desert poor of CO2 sources and sinks, is expected toinduce a smaller variability on the CO2 behaviour. Trajectories havebeen selected on the basis of the residence time spent in eachsector (50% permanence criterion) and then associated to therespective CO2 value recorded. Influence of the seasonal variabilityand interannual variability in the long-term trend has been sub-tracted by associating each trajectory to the corresponding devia-tion of the weekly value from the 13-week smoothed curve (d). Asin the case of CO2 the natural source strength is strongly seasondependent because of the influence of the photosynthesis andrespiration of land plants, deviations have been also classified byseasonal periods (see Table 2).

F. Artuso et al. / Atmospheric Environment 43 (2009) 3044–3051 3049

The results highlight the effect of the latitudinal distribution ofsources and sinks. The high density of industrialized regions andforests located in the European sectors (EUW and EUE) hasa substantial impact on the CO2 air-mass content arriving atLampedusa. Focusing the attention on the winter period (DJF),when the European vegetation sink is less effective, air-massesoriginating from Western Europe have the highest CO2 content.Influence of the fossil fuel consumption in the industrializedregions is more evident during the winter, when the vegetationsink is less efficient. During the spring highest values of CO2 areobserved at Lampedusa when the air-masses pass mainly overEastern Europe and Russia. This trend is reversed during thesummer. Forests densely populating the North Eastern continentinduce a CO2 depletion. For this reason higher values of d corre-spond to trajectories of African or SEA origin.

Using the same trajectories classification, the anomalous GRbehaviour during 2003 has been analysed and related to changes inair-mass transport. Since SEA trajectories are dominant during thewhole period of analysis, the percentage of SEA trajectories for eachyear has been evaluated. The GR is a derivative of the 13-monthmoving average curve of CO2 and for this reason its behaviour issensitive to year-to-year variations. Differences between theoccurrence of SEA trajectories in one year with respect to theprevious one show a singularity in 2003: the number of trajectoriesoriginating from the SEA sector in 2003 is much higher then during2004 (77% against 57%). Fig. 6a and b display the bar diagramrepresenting the yearly fractions of the number of trajectories pergeographical area classified by season (winter ¼ SONDJF andsummer ¼ MAMJJA). Results, compared with Table 1 data, revealthat the decreasing trend of GR observed in 2003 may be due to thefollowing factors. 1) In general during 2003 air-masses of marineorigin are more frequent than during 2004, thus generating a netnegative CO2 mixing ratio. 2) During winter the strongest CO2

source is Western Europe. In this season it is possible to observethat the occurrence of EUW trajectories in 2003 is only 5%, while it

Fig. 6. Bar diagrams representing the yearly percentage of trajectories per geographi

is 13% in 2004. 3) In summertime Eastern Europe is the more effi-cient sink area. In this period almost 9% of the trajectories comingfrom this region arrive to Lampedusa during 2003 while during2004 EUE trajectories are completely missing in summer. Thesefindings corroborate the hypothesis that during 2003, in whichintense climatology anomalies occurred, the GR has been perturbedby shifts in atmospheric circulation regime.

The influence of air-mass origin appears also remarkable whenwe analyze the changes in the seasonal cycle amplitude. All datahave been reported as originating from each geographical sector byadding the d of the sector considered to the mean value recorded.The semi-amplitudes of each series of data have been evaluatedusing the parameter B of expression (1). Results are shown inTable 3 and highlight the influence of sources and sinks located inthe Eastern Europe on the CO2 periodic behaviour.

4. Conclusions

The monitoring program of atmospheric CO2 mixing ratio hasbeen started at Lampedusa since May 1992. The time series hasbeen analysed in order to study the short- and long-term variabilityand to evaluate the possible mechanisms influencing the Medi-terranean carbon budget. A small diurnal cycle is observable duringthe summer while no significant diurnal variations are detectedduring wintertime. This result remarks the limited influence oflocal sinks and sources. The annual and interannual variability ofthe atmospheric CO2 has been investigated by using a fittingfunction. A linear increasing trend of þ1.9 ppm yr�1 is recorded inthe period of investigation. The seasonal cycle with mean ampli-tude of about 8.72 ppm dominates the periodic behaviour. Theseasonal cycle amplitude displays a marked interannual variability.The small seasonal cycle occurring in 1995 reflects the influence oflarge biomass burning events. The reduction of the CO2 seasonalsignal experienced from 1998 to 2003 is linked to high tempera-tures which inhibit the vegetation growth and induce an increase in

cal area classified by season ((a) winter ¼ SONDJF and (b) summer ¼ MAMJJA).

Table 3Values of parameter B (annual semi-amplitude) calculated by expression (1) on datareported to each geographical sector.

EUE EUW SEA AFR

B (ppm) 5.86 4.38 4.26 2.79

F. Artuso et al. / Atmospheric Environment 43 (2009) 3044–30513050

soil respiration. The lowest value of the seasonal cycle amplitudehas been recorded in 2003 when a strong anomalous heat waveoccurred in Europe.

Changes of the rate of CO2 mixing ratio have been first relatedwith SOI and Tg. Apart from the period 1997/1998, during whichanomalies are related to changes in carbon fluxes induced by ENSOevent, a correlation between SOI and CO2 GR time series is notpresent.

GR peaks are also observed in the periods 1995, 2001 and 2005(1.9, 3.7, 3.2 ppm yr�1 respectively). These years are hallmarked byhigh Tg values and by intense biomass burning events. Anomalousanti-correlation between CO2 GR and Tg has been observed duringthe heat wave of 2003. Such behaviour has been related to changesin atmospheric circulation regime.

With in mind the aim to study the influence of transport on CO2

variability a backward air-mass trajectory analysis has been per-formed. The results emphasize the significance of the influence ofregional source and sink distribution. Fossil fuel consumption,industrial activities and forests located in the Eastern Europe andRussia strongly perturb the CO2 mixing ratio. During the winter air-masses transported from this region load high level of CO2 due tothe large industrial emissions. In summertime prevails the effect ofthe vegetation sink and CO2 depleted air-masses originate from theNorth Eastern continent. Back-trajectories analysis shows that thedecreasing trend of CO2 GR observed during 2003 may be due tochanges in the atmospheric circulation.

Acknowledgements

We gratefully thank CESI RICERCA (Dr. F. Apadula) and theItalian Air Force meteorological service (Dr. R. Santaguida) formaking available data from Plateau Rosa and Monte Cimonerespectively. We acknowledge the NOAA Air Resources Laboratory(ARL) for provision of the HYSPLIT transport and dispersion model,and of the READY website (http://www.arl.noaa.gov/ready.html),whose results are used in this publication. Thanks are due toL. Ciattaglia, who contributed to the measurement program, and totwo anonymous reviewers for their precious suggestions andcomments.

The measurements at Lampedusa have been supported by theItalian Ministry for the University and Research, and Ministry forthe Environment. This work has been partially funded by the Car-boeurope IP research program of the European Union.

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