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Int. J. Environment and Pollution, Vol. 36, Nos. 1/2/3, 2009 287

Copyright 2009 Inderscience Enterprises Ltd.

The impact of landfills on the air quality of towns:a simple heuristic model for the city of Palermo

Salvatore Barbaro*Department of Energy and Environmental Researches,University of Palermo,Viale delle Scienze, Palermo 90128, ItalyE-mail: [email protected]*Corresponding author

Angelo BonannoItalian National Research Council (IAMC CNR),Mazara del Vallo TP 91026, ItalyE-mail: [email protected]

Maria Letizia BosciaSoil Unit of the Agency for the Environment Protection of theProvince of Trento,Piazza V. Veneto 4,Torbole Sul Garda (Trento), Italy

E-mail: [email protected]

Gianfranco Rizzo and Salvatore AronicaDepartment of Energy and Environmental Researches,University of Palermo,Viale delle Scienze, Palermo 90128, ItalyE-mail: [email protected]: [email protected]

Abstract: In this study, the landfill of Palermo, is investigated as a potentialsource of the unusual methane concentrations found in the urban context. Thesource for these pollution episodes is identified by means of a simple heuristic

method. A cross-correlation analysis between wind data and methaneconcentration levels is also used to confirm the hypotheses formulated. DopplerSound Detection And Ranging (SODAR) measurements are used to investigatethe air masses dynamics at the landfill, in order to better support theadopted assumptions. This interpretative method can be adopted in the firstassessment stages of the environmental site performance in order to single outthe candidate pollution sources in urban areas, before any analyticalcomputation takes place. In this sense, the method is intended as a viable toolin decision-making processes where in local administrations are currentlyinvolved in the selection of sustainable policies for the correct managementof the territory.


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288 S. Barbaro et al.

Keywords: air pollution; landfill; urban solid waste; methane emission; urban

pollutants concentration; heuristic models.

Reference to this paper should be made as follows: Barbaro, S., Bonanno, A.,Boscia, M.L., Rizzo, G. and Aronica, S. (2009) The impact of landfills on theair quality of towns: a simple heuristic model for the city of Palermo,

Int. J. Environment and Pollution , Vol. 36, Nos. 1/2/3, pp.287304.

Biographical notes: Salvatore Barbaro graduated in Mechanical Engineeringat the University of Palermo in 1974. Since 1981, he was a first Researcher,then Professor and, since 2003, he has been a full-time Professor of Environmental Technical Physics at the University of Palermo. He isalso interested in problems of atmospheric and acoustic environmentalimpact valuation in urbanised areas, actively cooperating in studies andconventions, both in national and international contexts. He developedhis scientific activity essentially by being interested in three principalresearch fields: thermal techniques and heat transmission, solar energyand climatology, measurement, valuation and control of the environmentalquality. He is the author of various scientific publications in national andinternational reviews.

Angelo Bonanno is an Electronic Engineer; since 27 July 2001, he has been aresearcher at the Istituto per lAmbiente Marino Costiero of the ItalianNational Research Council (IAMC CNR) in Mazzara del Vallo (TP Italy).He has been working since 1992 with the Interdisciplinary Group of Oceanography of IAMC CNR, dealing with analysis of acoustic signalscollected during surveys carried out onboard research vessels. He got theDoctorate in Environmental Technical Physics at the University of Palermo.He is involved in many research projects funded by the European Community,in collaboration with researchers of the Acoustics Laboratory of VNIRO inMoscow, of the Instituto Espanol de Oceanografia di Fuengirola (Spain) and of the University of Malaga (Spain).

Maria Letizia Boscia is an Environmental Engineer in the field of theenvironmental assessment of waste and energy systems. She was awardedin 2005 a national Italian award for her degree thesis concerning thepollutant emissions of the landfill of the town of Palermo. Currently, she isworking with the Agency for the Environmental Protection of the Provinceof Trento (Italy).

Gianfranco Rizzo has been a Full-time Professor of EnvironmentalTechnical Physics at the University of Palermo since 1994. He is the authorof about 200 scientific publications in journals, conferences and researchreports. He is the Chairman of the Degree Course in Environment andLand Engineering at the University of Palermo; the Chairman of the PhDcourse on Environmental Technical Physics; responsible for severalresearch projects concerning indoor climate control, energy and environmentand a member of the Work Group UNI in the sector Microclimate.He has teaching experience in the University of Luanda (Angola) andresearch experience at Lawrence Berkeley Laboratory (Berkeley, CA) and isa referee of international scientific journals. His research interests includeindoor thermal comfort in residential and museum buildings, environmentalimpact of building materials, pollution from the transportation sector, energyand environmental sustainability of the built environment and indoorenvironment engineering.


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The impact of landfills on the air quality of towns 289

Salvatore Aronica is an Engineer in Telecommunications Electronics presently

involved in a SODAR system project devoted to the analysis of the ABL(Atmospheric Boundary Layer), in collaboration with the IAMC Institute of theItalian Research National Council in Mazara del Vallo (Sicily), where he alsoattended a Masters Course in marine and coastal-marine environmentmonitoring. In particular, he has concentrated his attention to studying ABL inmarine environment, by installing the SODAR system on the Research VesselUrania, during some research cruises. In 2003, he participated in the study of the ABL at the landfill site of Palermo. Currently, he is attending an EnergeticPhD course at the University of Palermo.

1 Introduction

Human societies are characterised by a linear way of production, which managesresources almost without feedbacks, from the cradle to the grave. Consequently,waste represents the major environmental problem to deal with in urban contexts.In fact, this way of producing freights and services only considers once the energyand material contribution of resources. In other words, provided that a resourcehas offered its contribution for only one goal, it is simply considered as waste tobe disposed.

As a result, our towns are surrounded by disquieting locations, dedicated to thestorage of a huge amount of materials that are assumed to have reached the end of theirlife cycle.

Apart from other kinds of pollution related to waste management (Campbell, 1996;Christensen et al., 2001; Cossu et al., 1996; Eikmann, 1996; Frechen, 1996; Gregoryet al., 1997), in the last few decades, pollution created by biogas emissions has becomean increasingly relevant environmental problem: on a small scale, this is a problem astraces of undesired substances present in the air create pollution; on a global scale, this isa problem as growing concentrations of methane have been detected and are presumed toaffect climate changes.

Since this situation, obviously, assigns to landfills a preeminent role in themanagement of waste produced by municipalities, local administrations are called toadopt proper actions with the aim of correctly managing the problem and of mitigatingits undesired effects on the environment and on human health. In orderto achieve these goals, administrators should have at their disposal easy and viabletools for the evaluation of pollutant effects produced by waste managementfacilities and, in general, for the environmental ranking of pollution sources in urban

contexts.With this aim, this work can be considered as a contribution to the assessment of a

simple methodology allowing the singling out of the most responsible source formethane concentration in urban areas. Such a heuristic method is essentially based on aphenomenological approach that tries to put in the correct relationship among themethane emission sites, the orography of the zone and the local climatic characteristics of the site.

It is well known that the term heuristics denotes a strategy adopted for solving anissue. The heuristic approach is typical for an expert who builds a mental model of hisown and rapidly finds the best solution.


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In the technical and scientific literature, a number of applications of the heuristic ormeta-heuristic methods are reported for example, interpreting molecular spectra, settingup and assessing plans within the social sciences, conceiving an architectural work,studying the behaviour of reinforced concrete buildings and so on.

A wide list of applications also exists concerning the diagnosis, prognosis orprevention of air, water and soil pollution as well as environmental policies andsustainable developments (Du, 2001; Husted, 2004; Meyers, 2003; Pennington, 1999,2001; Veiga and Meech, 1994).

The approach here is applied to the emissions released by the landfill of the town of Palermo, in Sicily. A cross-correlation procedure between wind data gathered at the tipand the urban concentration values is applied in order to justify the adopted empiricalassumptions. Moreover, experimental data from SODAR equipment are utilised forconfirming the hypotheses assumed with the profile of the nocturnal Stable Boundary

Layer (SBL) at the landfill site.

2 The Palermo landfill

The landfill of Palermo, in the territory named Bellolampo was begun in the 1960s as asite in which the inhabitants of the outer city would discharge their waste without anycontrol or limitation. Only since the 1980s this process has been regulated by realising acontrolled refuse tip. In 2003, about 615 thousands tons of waste ware deposited in thelandfill, with a yearly increase of nearly 4 % in weight.

Presently, the collected biogas from Bellolampo tip is used to produce electricalenergy through a valorisation plant which has five 1 MW generation modules. Table 1

summarises the monthly averages of biogas volume and flow, which have been treatedby the biogas valorisation plant and the average percentages of methane and oxygendetected in each volume for the year 2003.

Table 1 Relevant parameters referring to the collected biogas (year 2003) in thePalermo landfill

Symbol Parameters Unit Monthly average

V Treated biogas volume mc 909.141

P Biogas hourly average m 3 /h 1.244

E Total electric energy produced kWh 2.015.466

CH 4 Average percentage of methane volume % 52

O2 Average percentage of oxygen volume % 1

A fraction of the biogas produced in Bellolampo tip, which is generated by processes of biological degradation, flows undetected through the protection systems that shield thetip and is dispersed in the atmosphere.

In order to highlight which effects could be linked to this diffusive phenomenon,methane concentration data detected by Belgio and Boccadifalco stations, belongingto the communal network for the detection of atmospheric pollution (see Figure 1) and


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located within the urban layout, was examined. In particular, from the time series (withhourly intervals) for 2002 and 2003, we chose the episodes in which there were peaks of CH 4 compared to normal data.

The data analysis was done through the event-based model described below.

Figure 1 Position of the meteorological stations Belgio and Boccadifalco , with respects to Bellolampo landfill

3 The proposed heuristic method

As it has been previously pointed out, a heuristic model will be used to determine thereasons for the methane concentrations detected at the measurement stations in the twourban sites.

The heuristic approach utilised in this work for the problem of singling out the mainresponsible source of methane concentrations is a logical IF-THEN rule-based scheme.The model is able to easily handle uncertain or vague data, like those referring topollution of urban areas. This procedure can reduce the need for sophisticated analyticalapproaches and can provide useful information to non-technical people.

The logical scheme of this model is shown in Figure 2.An explication of the meaning of each box of the above-described logical scheme is

provided in the following pages.


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Figure 2 Logical scheme of the proposed heuristic model

3.1 Singling out the candidate sources of methane emissions

The potential methane sources, that are relevant to this study in the urban area of Palermo, are preliminarily investigated. Among these sources, it will be singled out, bymeans of a heuristic approach, the one that is mainly responsible of the methane

concentrations in the town.The first candidate methane source is the methane compression and supply facilityfor auto-traction: AMG (the Municipal Enterprise for Gas) realised this facility in 1999within a deposit. Apart from refueling of buses, this complex is also used by localgovernment vehicles and special businesses, although in the future it will be open tothe public.

Another possible source is the methane urban distribution network: in fact, themethanisation works, which began in 1994, are nearing their completion. Furthermore,AMG promoted a campaign for the diffusion of methane-fuelled central heating incondominiums and for the transformation of the old gasoline boilers in methane ones inpublic schools.

Finally, Bellolampo refuse tip must be considered among the most candidate sourcesof the methane that reaches the measurement stations present in the town. In the landfill,in fact, among other biogases, methane has a presence of 53 % within the whole gasvolume: this gas is produced and not intercepted by the extraction facilities alreadypresent on the site.

3.2 Analysis of the gathered data describing concentrations of CO,C 6 H 6 and CH 4

The data analysed in this work are available at the AMIA (Municipal Agency forEnvironmental Hygiene) of Palermo. In particular, CO, C 6H6 and CH 4 concentrations,wind speed and wind direction are routinely collected by the Agency in some different


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sites of the city. In a generic station of this network, a multiprobe system acquires, withhigh frequency measurements, environmental and pollution parameters from relatedsensors. AMIA provides hourly averages of each variable for public use and scientificpurposes.

In this work, measurements for wind speed and direction were collected in theweather station located in Bellolampo landfill site (Figure 1); CO, and CH 4 concentrations were acquired both at Belgio and Boccadifalco stations (see Figure 1),while C 6H6 concentrations were acquired only at Boccadifalco station. COConcentrations were estimated by using a carbon monoxide analyser, with a samplingperiod of 10 s; CH 4 concentrations were measured by means of a gas chromatographicanalyser, with a sampling period of 3 min. Concentrations of C 6H6 were estimated byusing passive samplers exposed for one hour to the external environment; a gaschromatographic analyser was then used in the laboratory to evaluate benzene

concentrations. Wind speed measurements have been collected by means of ananemometer showing a sampling period of 1 sec.The analysis of the hourly methane concentrations refer to data gathered continually

from 1 January 2002 to 31 December 2003 in Belgio and Boccadifalco stations. Someuseful considerations can be deduced from these data.

First of all, during the entire observation time, excluding the days in which somesporadic peak episodes took place, the average hourly value for methane concentrationvaried between 1.7 ppm and 2.2 ppm (see Figure 3). As far as methane is concerned, thecommon and national laws do not specify levels for air quality, therefore, a thresholdvalue of 2.5 ppm (closely corresponding to the peak found out in the measurements) waschosen here for the following studies.

Figure 3 Typical behaviour of methane concentrations at Belgio (left) and Boccadifalco (right) stations

The episodes in which the methane concentration trespassed the above-mentionedthreshold were recorded only during night time and, typically, between 2.00 and6.00 am.

At Belgio station in the year 2002, as far as 64 occurrences of methane concentrationpeaks (that is trespassing a concentration of 2.5 ppm) were observed. In 2003, 72 weredetected. In Boccadifalco station, in the years 2002 and 2003, respectively 31 and 44peaks were detected (Table 2).


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Table 2 Number of peak events of CH 4 concentration in the years 2002 and 2003

Number of peak events of CH 4 concentration

Belgio Boccadifalco

2002

Winter 29 7

Spring 17 16

Summer 7 5

Autumn 11 3

Total 64 31

2003

Winter 14 4

Spring 23 21

Summer 18 13

Autumn 17 6

Total 72 44

When these peaks occurred, the concentration data of other pollutants was also taken intoconsideration. In particular, as the peaks of CH 4 took place during the night, thosepollutants originating from secondary photochemical reactions occurring in theatmosphere when UV radiation is present, were purposely overlooked. At Boccadifalcostation, we therefore analysed the benzene and carbon monoxide concentrations.At Belgio station, we only analysed the carbon monoxide, as benzene is not detected by

the measuring facility. By analysing the benzene concentration paths, we deduced thatthere is no time correspondence between the methane peak events and those of C 6H6 andCO; in fact, the daily-recorded concentration peak of carbon monoxide between 8.00 and9.00 am is due to car traffic (see Figure 4).

Finally, it must be noted that the above-defined CH 4 peaks are not linked toatmospheric pressure as they were indifferently gathered in conditions both of high andlow pressure.

Figure 4 Contemporary presence of carbon monoxide and benzene with the methaneconcentrations at Boccadifalco station


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3.3 Correlation between peak concentrations and wind direction analysis

The preliminary results shown in Section 3.2 must be integrated with the otherconsiderations based on wind study that could provide useful suggestions in order toproperly interpret the methane concentrations in the urban context.

First of all, it is important to remark that the CH 4 peaks were recorded at wind speedscorresponding to less than 1 m/s (wind calmness).

In Figure 5, a typical path of mean hourly methane concentration values gathered atBelgio station is shown (left), along with the corresponding occurrence of weak windsobserved at the landfill site (right).

Figure 5 Mean hourly methane concentration values gathered at Belgio station (left) andcorresponding occurrence of weak winds observed at the landfill site (right)

The analysis of such behavioural correlations, as shown in Figure 5, allowed us to verify

that while the presence of peaks of CH 4 concentration was recorded at Belgio station,wind direction observed at the landfill site fell between the 11th and 15th sector of thewind rose. Having this in mind, it is relevant to notice that the azimuthal direction of Belgio station in respect to Bellolampo landfill is approximately 80 (see Figure 1).

In correspondence to the peaks recorded at Boccadifalco station, we noticed that thewind direction fell between the 13th and 1st sector of the wind rose. Having this in mind,it is relevant to notice that the azimuthal direction of Boccadifalco station in respect toBellolampo landfill is approximately 150 (see Figure 1).

3.4 Exclusion of the sources not responsible for CH 4 concentrationsin the urban stations

Based on what has just been asserted, it is possible to deduce that the AMG facility forthe compression and distribution of methane for auto transportation is not a possiblecause for the peak episodes. In fact, in order to detect a CH 4 concentration increase atBoccadifalco station, which was hypothetically released by such facility, the wind shouldblow from North-North East (NNE), in other words in a perpendicular direction to theone actually observed in correspondence to the peak events.

To further confirm the exclusion of this source as a cause for the observed peaks, wealso considered the time series gathered at Boccadifalco in periods earlier than theinstallation of this facility. In fact, in the year 1998, similar amplitude peaks to the onesgathered in 2002 and 2003 had been recorded, both in Belgio and Boccadifalco stations.


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On the other hand, methane leakage originating from the gas distribution network isalso to be excluded as a potential cause for the detected peaks. In fact, if thegas distribution network was a cause, it would not be possible to explain why such peakstake place only between 2.00 and 6.00 am and there are no high methane valuesappearing during the whole day.

As observed earlier, if the wind blows with intensity below 1 m/s and towards the 3rdand 4th quadrant of the wind rose, the biogas (carried according to wind direction) willbe of interest to the above-mentioned sites. On the other hand, if greater wind intensitiesare present, there will be a corresponding remixing of the lower atmosphere layer and aconsequent absence of methane peaks.

3.5 The influence of night-time SBL

It is known that in high pressure conditions the Atmospheric Boundary Layer (ABL)evolves according to a well defined daily cycle within which it is possible to identify theprincipal components of its structure: the Mixed Layer (ML), the Residual Layer (RL)and the nocturnal SBL (Oke, 1991; Stull, 1988).

It is also known that during the night, the lower part of the RL transforms into thenocturnal stable layer as a consequence of contact with the earth surface. Such layer ischaracterised by statically stable air with sporadic turbulences. In fact, the night winds inthis layer are generally weak at earth surface level even when very high velocity windsare present higher above.

Clearly, the landfill site of Palermo can be thought of as a source of pollutant insidethe SBL.

The vertical structure of lower atmosphere in Bellolampo landfill was investigated byusing a Doppler Sound Detection And Ranging (SODAR), designed and realised in theframework of a project funded by the Italian Ministry of University and ScientificResearch. This system is a remote sensing technique suitable, among other things, formaking Doppler measurements of wind velocity using turbulent scattering regions astracers. This acoustic technique, started by McAllister et al. (1969), has been rapidlyestablished as an important tool for probing the Planetary Boundary Layer (PBL) invarious parts of the world.

The SODAR facility, installed in order to study the dynamics of the loweratmosphere layers which are in contact with the surface of Bellolampo tip, ischaracterised by a monostatic triaxial configuration (Figure 6); such instrument allowedthe monitoring of the ABL evolution up to a 1 Km altitude and to estimate the windvelocity behaviours at a very high time resolution.

Figure 6 The SODAR Doppler along with the meteorological station at the landfill site


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3.6 Individuation of the principal emission source

The above-mentioned consideration led to assemble a probable hypothesis as to what justifies the presence of the CH 4 peaks identified in these nocturnal times. The sourceseems to be the Urban Solid Refusals (RSU) Bellolampo tip, as only such a methanesource could increase local methane concentrations.

As 53 % of the biogas (generated and not collected by the specific extraction systemsin Bellolampo tip) is composed by methane, it is believed that only an atmospherictransportation phenomenon could justify the high CH 4 concentration peaks detected inthe urban area close to the tip.

The achieved results allowed to point out a near cause-effect relationship that can besummarised as such during night hours, if the wind speed slows down to less than1 m/s and its direction lies approximately along the line connecting the tip with one of

the two methane measurement stations (that is Belgio or Boccadifalco), then such stationwill record a greater methane concentration ( > 2.5 ppm) compared to the typical levelrecorded in the atmosphere (between 1.7 and 2.0 ppm).

Vice versa, if such situation does not take place or if the wind velocity is greater thanthe discovered threshold, then at that same station the typical base values known in theliterature will be observed.

4 Analytical-experimental verification of the interpretativemodel results

Following the previous steps, an interpretative model of the methane emission dynamicsin the urban area of Palermo has been assessed. It can be summarised by means of thelogical scheme shown in Figure 7.

Figure 7 Summarising scheme of the wind-pollutant relationship

This easy way of approaching the pollution problems is clearly very suitable for

non-technical people, as local administrators and stakeholders usually are. Furthermore,we should aim at establishing their validity in other contexts which differ from the one inwhich they were applied.

However, although the results deriving from the application of this simple heuristicmethodology sounds very reasonable, they must be verified on the basis of someobjective interpretations. Particularly, the relationships between wind regimes andmethane concentrations should be deeply investigated.

Following, a cross-correlation method is applied for the above purpose. Further,some results of the SODAR equipment will be presented, in order to explain the role of the ABL.


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4.1 Confirming the proposed hypotheses through a cross-correlation of thewind data gathered at the tip and the urban concentrations values

A cross-correlation analysis (Bendat and Piersol, 1971) was carried out, on the basis of hourly data, in order to describe the general dependence between sets of wind data andurban concentration data. Among the typical applications of such procedure, thecross-correlation analysis here is particularly aimed at evaluating the time delay betweenthe two data sets, considered as input and output of a system. Of course, the limited timebase of the present available data sets could affect this approach; anyway the continuousactivity of gathering data, undertaken by the municipal administration, could rapidlyimprove this situation.

With this goal in mind, a data cross-correlation study was carried out. The wind dataat Bellolampo landfill and the methane concentration data at the two stationswere cross-correlated. To perform such analysis, we chose to consider the wind vectorcomponent along the direction that connects the refuse tip with each measurement stationrather than the general direction of the wind (see Figure 1).

From now on, we shall indicate as V 1 the wind component measured at Bellolampolandfill site and directed along Bellolampo-Belgio line and as V 2 the wind componentmeasured at landfill site and directed along Bellolampo-Boccadifalco line.

The adopted procedure is composed of the following steps:

1 estimate of the V 1 and V 2 wind velocity components at the landfill site

2 extract the CH 4 time series, with V 1 and V 2 in the periods with methane peak episodes (we chose not to consider the entire biennial time series in order tofocus our attention on the peak situations)

3 perform a cross-correlation analysis between CH 4 and V 1 (this is done with thestatistical package Systat , release 5.0)

4 perform a cross-correlation analysis between CH 4 and V 2 and

5 repeat the points III and IV while considering the delays in time series, byconsidering time lags of 1, 2, 3 and 4 hr.

For each of the analyses described in the steps 3, 4 and 5, plots were produced describingthe frequency distributions of the different cross-correlation values.

The analysis results are shown in Figures 8 and 9.At Boccadifalco station (see Figure 8), the different frequency distributions show

higher cross-correlation values in correspondence to the lag factors of 0, 1 and 2. Whenthe time lag is increased the correlation values decrease. This consideration allows us topoint out a delay of about 1 hr between the wind reduction at Bellolampo and the highmethane observations at Boccadifalco station.

As far as Belgio station is concerned, Figure 9 shows a much more complexsituation, where the high correlation values (rather frequent) were estimated at intervalsthat lie between lag factors 1 and 3.

The results that are concerned with the 2002 data, both in correspondence toBoccadifalco and Belgio stations, show a frequency distribution not unlike what wasobserved in each station in the year 2003.

The different time delay recorded in the two urban sites (that is Boccadifalco andBelgio) is principally linkable to the different distance between the tip and the twomeasurement stations. At the same time, however, it should be emphasised that while


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Boccadifalco station, which lies in a low building-density area, could be consideredinfluenced only by the orography of the territory, Belgio station is substantially insertedin Palermos urban texture and is thus influenced by the complex city traffic flow.

Figure 8 The analysis results of cross-correlation between the wind and the CH 4 concentrationsin the peak periods for the year 2003 (Boccadifalco station)


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Figure 9 The analysis results of cross-correlation between the wind and the CH 4 concentration in

the peak periods for the year 2003 (Belgio station)


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The main conclusion of this analysis is that the methane emission episodes at the landfillsite do certainly affect the methane concentrations of the urban measurement stations,while the relative values are influenced by the distance from the landfill, by theorography of the territory and by the buildings urban layout. Anyway, the landfill site isconfirmed here as the main source of the methane concentrations in the urban context.

4.2 Further verification by using Doppler SODAR measurement

Alongside the typical ABL behaviour in high-pressure conditions that takes place in thelandfill site during day hours, gas mixing phenomena in correspondence to the layer thatis closer to the ground were also observed in the night hours (see Figure 10). Theextension of this mixed superficial layer undergoes variations that are related toatmospheric pressure. The typical nocturnal residual stratification was recorded abovethe ML.

Figure 10 Facsimile of eco-signal intensity acquired by SODAR system in the nightbetween 29 and 30 March 2003

During the survey period, the SODAR system estimated vertical profiles of the windspeed up to 1000 m, with a repetition rate of 6 sec. Facsimile plots (greyscale pictures of acquired signals) also permitted to qualitatively analyse the vertical structure of the loweratmosphere.

In this study, it was discovered that the methane peaks (above 2.5 ppm) wereappearing in the nighttime period (between 2 and 5 am) and in correspondence withdecreased values of horizontal wind velocity (smaller than 1 m/s).

This leads to the hypothesis that the biogas produced by the tip, in suchcircumstances, is confined to the first atmospheric layer, within which a high gasemission is recorded.

The methane that is produced at ground level within this ML remains confined to itand moves according to the horizontal wind component acting on such layer. In such atrend, the gas shows a relatively little dispersion in the vertical direction, whereas itspreads out much faster in what is known as a fan fashion, in the horizontal direction.


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What has just been described can be deduced from Figure 10 (SODAR facsimilerepresentation) where: starting from 20:38 the RLs are visible as parallel lines among them the lower part of atmosphere is characterised by vertical movements of air

masses (nocturnal mixing layer) after 1:00 am the maximum altitude of the ML is around 400 m below such stratification an unusual vertical dynamics is present and biogas inputted in the lower atmospheric layer is then transported towards the

city by a weak wind.

Thus, it is possible to affirm that the biogas, transported by the weak wind, moves withinthe first ABL layer, reaching one of the two (or both) monitoring stations.

In synthesis, this instrumental analysis also confirms that the landfill is responsiblefor the methane concentration behaviour found in Palermos urban context.

5 Conclusions

A typical heuristic approach has been adopted here to investigate the pollutantcharacteristics of the urban area of the town of Palermo, focusing on methaneconcentrations recovered in some measurement stations of the municipal network. Theprimary purpose of this study was to identify the main cause of such methane emissions.Methane in fact can be assumed as a tracer of several pollution phenomena and, byanalysing its dynamics, it is possible to assess properly the facilities and infrastructures,with the aim of limiting the pressure exerted by human activities on the environment inthe urban development process.

This simple phenomenological approach, that essentially proceeded by means of anexclusion technique, allowed to point out a near cause-effect relationship that can besummarised as such: during night hours, if the wind speed slows down to less than 1 m/sand its direction lies approximately along the line connecting the tip with one of the twoCH 4 measurement urban stations (Belgio or Boccadifalco), then such a station will recorda greater methane concentration ( > 2.5 ppm) compared to the typical level recorded fromthe atmosphere (between 1.7 and 2 ppm). The delay with which the CH 4 peaks show atBelgio station is almost 2 hr, whereas Boccadifalco station will record themapproximately after one hour.

On the other hand, if the wind direction does not correspond to the directionsconnecting the stations (i.e. Bellolampo-Belgio or Bellolampo-Boccadifalco), or if the

wind speed is greater than the threshold here discovered, the stations will yield thetypical base levels known in the literature.

In a qualitative fashion, this study takes a step in understanding and analysing theeffects of the principal processes responsible for biogas emissions of Bellolampo landfillas far as the air quality of Palermo, assuming that CH 4 is the tracer of the emissions in theair that are generated by the tip. In particular, the attention was focused on concentrationpeaks (above 2.5 ppm) detected in two urban-network meteorological stations.

The results have been verified on the basis of a statistical analysis, by means of across-correlation method, between the concentrations of methane recorded at themeasurement stations in the urban area and the wind characteristics in the whole area,


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including the landfill site. Moreover, the assumptions concerning the movement of themethane released by the landfill within the ABL, have been confirmed by means of measurements taken with a SODAR instrumentation installed in the landfill.

The investigation should certainly be carried further, by undertaking measurementcampaigns with greater space-time resolution. These measurements should also be aimedat the acquisition of this data and integrated with simulation models of theemissions-system and of the diffusion mechanism. This kind of further research could beused to predict main causes of pollutant concentrations in any part of the territory inorder to eventually confirm the empirical assumptions taken here on the basis of heuristics approaches.

Thus, the undertaken research can be considered a first investigation that should bekept in consideration in future studies that aim at the installation of industrial facilities inthe waste site like, for example, waste thermo-valorisation plants.

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