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Atmospheric Environment 42 (2008) 4138–4148

www.elsevier.com/locate/atmosenv

Ten-year measurements of gaseous pollutants in urban airby an open-path analyzer

P. Avino�, M. Manigrasso

Laboratorio Inquinamento Chimico dell’Aria, DIPIA-ISPESL, via Urbana 167, 00184 Rome, Italy

Received 19 November 2007; received in revised form 14 January 2008; accepted 14 January 2008

Abstract

Object of this study is the versatility of a remote-sensing measurement system through experimental survey of the levels

of primary and secondary pollutants in urban areas. For the first time a long-range measurement of different pollutant for

a long period are reported in the urban area of Rome. Over 10 years of data collection, levels of NO2, O3, SO2, benzene and

toluene are measured and discussed by means of a differential optical absorption spectroscopy (DOAS) system installed at

ground level.

The period is interesting because it represents one of the most important life-changes in Europe and in Italy: in fact,

during the 1990s some significant decrees introduced the catalytic pots and limited the amount of benzene and sulfur

dioxide in the fuel. The paper will describe how the main primary and secondary pollutants are changed during all the

period: particularly, benzene and SO2 are not over a big social problem for the human health whereas the secondary

pollutants, ozone and nitrogen dioxide, have the same trends during the period, meaning a no contribution to their

evolution from the atmospheric processes.

The DOAS technique is resulted very useful and versatile: comparisons with other traditional analyzers (i.e., GC-PID

for benzene and toluene investigations) demonstrate how it allows to have information over all the territory saving the

qualitative trends and giving a similar quantitative result. In this respect the authors show that the difference is due to the

different sampling ways (the traditional analyzers are punctual whereas the DOAS is a remote-sensing technique covering a

wide area) and does not influence the atmospheric process interpretation.

Finally, it should be considered that the pollutant trends have been interpreted using the concentration measurements of

natural radioactivity as tracer of the dynamic properties of the atmospheric boundary layer.

r 2008 Elsevier Ltd. All rights reserved.

Keywords: DOAS; Air pollution; Primary pollutant; Radon; Photochemical smog

1. Introduction

In order to improve the air quality in Europe, theEuropean Commission issued some Directives with

e front matter r 2008 Elsevier Ltd. All rights reserved

mosenv.2008.01.024

ing author. Tel.: +39 06 4714242;

4017.

ess: pasquale.avino@ispesl.it (P. Avino).

regard to acceptable levels of a range of gaseouspollutants species as SO2, NO, NO2, O3 andbenzene (Council Directive 96/62/EC, 1996). Inparticular, measurements of ozone and benzenerepresent nowadays a very important target in theatmospheric studies, due to the different sourcesand health effects of both gaseous species. Benzenebeing recognized as carcinogenic for human (IARC

.

ARTICLE IN PRESSP. Avino, M. Manigrasso / Atmospheric Environment 42 (2008) 4138–4148 4139

Monograph, 1987) and due to its chemical stability,has constituted an element of constant hygienic-sanitary interest since many years.

Ozone has important adverse health (Mustafa,1990; Pryor et al., 1995; Cotgreave, 1996; Hazuchaaand Lefohn, 2007) and environmental effects suchas respiratory irritation: chemically, it is the primaryconstituent of smog, it is not emitted directly intothe air by specific sources but created by photo-chemical reactions involving sunlight, nitrogenoxides and volatile organic compounds (VOCs)(Morales et al., 2004).

Among the different methods used to analyzethese compounds, the techniques involving open-path spectroscopy are ideal for their advantagesover classical methods and point-source analyzers(Avino et al., 2004).

The differential optical absorption spectroscopy(DOAS) is one of the most versatile opticaltechniques for the determination of air pollutants.DOAS is a useful measuring technique for routinework to control air quality and pollution emissions,but also for applications in air pollution studies(Platt et al., 1979; Platt and Perner, 1980; Karlsson,1990).

The DOAS is a remote-sensing analytical techni-que finding strong consensus as demonstrated bynumerous papers published during these last years.The DOAS analytical method is based on the UVand vis light absorption (from 240 to 340 nm) ofspecies with fine vibrational structures like SO2,NO2, ozone, nitrous acid, formaldehyde, benzeneand toluene; the Lambert–Beer’s law regulates therelationship between the absorbed light intensityand the concentration of each species.

This technique furnishes the concentrations re-lated to a portion of atmospheric environmentvarying also up to some kilometers. In fact, these‘‘integrated’’ concentrations represent the averagearea pollution level better than the measurementsobtained by traditional analyzers, which are con-sidered ‘‘punctual’’.

The constituents investigated with DOAS techni-que include sulfur dioxide (SO2), nitrogen dioxide(NO2), nitrous acid (HNO2), formaldehyde(HCHO), ozone (O3), ammonia, mercury vapor(actually, these last two pollutants can be detectedusing a long optical path length) and aromatichydrocarbons (benzene and toluene).

This papers reports measurements of benzene,toluene, SO2, NO2 and O3 carried out in downtownRome in the decade from 1991 to 2000 by means of

a DOAS system installed at about 10m above theground level. All the measurements are interpretedusing the planetary boundary layer (PBL) evolutionassessed by means of a radon analyzer (Avino et al.,2003).

2. Experimental

The DOAS system (mod. AR 500, Opsis, Sweden)has been installed at the ISPESL monitoring stationlocated in downtown Rome (near St. MariaMaggiore Cathedral) at ground level in an urbanzone characterized by high density of anthropogenicactivities (autovehicular traffic, domestic heating,etc.). The DOAS system consists of an emitter (axenon lamp at high pressure), a receiver, a spectro-photometer equipped with an optical fiber and acomputer for the system management (data elabora-tion and data storage). The absorption spectra ofeach monitored chemical species are acquired attheir relative typical wavelength ranges; subse-quently, the interferences are eliminated by compar-ison with the reference spectra.

The distance between emitter and receiver isabout 280m: this parameter is important becauseit influences the sensitivity of the measures.

The absorbance of light from the emitter iscontinuously measured within the wavelength range240–350 nm to determine several compounds. Thearomatic hydrocarbons are detected in the wave-length range between 250 and 290 nm, where themajor interfering gases are oxygen, ozone and sulfurdioxide; around 100 spectra per second are collectedin this wavelength range and stored in a registerwith 1000 channels with a resolution of better than0.05 nm. The concentrations of air pollutants areautomatically calculated from the absorbance va-lues, in according with Lambert–Beer’s law (Plattet al., 1979; Platt and Perner, 1980; Brocco et al.,1992).

From the ambient air recording, a differentialabsorption spectrum for the atmosphere gases isproduced in two steps. First, the ambient airrecording is divided by a system reference spectrum,i.e. a spectrum which is pre-recorded in the totalabsence of gaseous absorption. The result is a totalabsorption spectrum of the ambient air. Second, afifth-order grade polynomial function is fitted to thespectrum to eliminate all broadband influences inthe atmosphere. From this quotient the logarithm iscalculated to obtain a differential absorptionspectrum. The concentrations are calculated by

ARTICLE IN PRESSP. Avino, M. Manigrasso / Atmospheric Environment 42 (2008) 4138–41484140

fitting a theoretical spectrum, based on the pre-stored differential cross section Da(i) onto themeasured spectrum A(i). The differential crosssections Da(i), i.e. the reference spectrum, areproduced in a laboratory with the same equipment,by using pure standard gas. The unknown concen-tration C is then varied until the equation

DaðiÞLC ¼ AðiÞ (1)

achieves the best solution. This is done by minimiz-ing the following expression, using least square fit:X

i

½AðiÞ � LCDaðiÞ�2, (2)

where L is the optical path length.The channels (i) used in the evaluation are chosen

for two criteria: a longer number of channel willimprove the statistical error and the intervalsshould be chosen in order to minimize influencesfrom interfering compounds, noise and otherdisturbance.

When several substances are interfering, pre-recorded differential cross section curves Daj(i) foreach one of the components are used in thedeconvolution of the measured spectrum. Expres-sions (1) and (2) above will be rewritten as

½Da1ðiÞC1 þ Da2ðiÞC2 þ Da3ðiÞC3 þ � � ��L ¼ AðiÞ

(3)

and

X

i

X

j

½AðiÞ � LCjDajðiÞ�2. (4)

A factor CjDaj is included in the analysis for eachone of the additional substances. For aromatichydrocarbon measurements only oxygen, ozoneand sulfur dioxide are taken into consideration(Hallastadius et al. 1991).

Spectra are required on an average time of 7minfor the system located in downtown Rome.

The specification provided by the manufacturersof the instrument used are:

�

Minimum detectable concentrations over280m ¼ 5 mgm�3. � Zero point stability ¼+10 mgm�3 per month. � Linearity ¼+10mgm�3 in the range 0–100mgm�3.

The cross sensitivities on benzene and toluenemeasurements, evaluated in accordance with the U.K.National Physical Laboratory methods (Partdridge

et al., 1995), are o2% of the concentration of eachinterfering gas (ozone, SO2 and NO2 were tested).

The Radon concentrations were measured bymeans of a SM200 Atmospheric Stability Monitor(Opsis, Sweden) (Avino et al., 2003).

3. Results and discussion

3.1. Planetary boundary layer evolution

The temporal trend of pollutant concentrationcan be interpreted using the measurements of theconcentrations of natural radioactivity as tracer ofthe dynamic properties of the atmospheric bound-ary layer (Avino et al., 2003). The atmosphericprimary pollution level is in fact determined by boththe intensity of its emissions and the dynamicproperties of the atmospheric boundary layer,causing pollutant dilution or accumulation. In thisrespect, radon and its short-lived decay productscan be considered as a tracer of the mixingproperties of the lower boundary layer, since radonis emitted at a rate that is spatially and temporallyconstant (Porstendorfer et al., 1991; Febo et al.,1996), consequently its concentration is mainlydetermined by the dilution properties of the atmo-spheric boundary layer.

The temporal evolution of Radon concentrationcan be described by the following equation:

qC

qt¼ aFR � bfCRg þAdv; (5)

where a is a parameter that links the mixingproperties of the boundary layer with the sourceintensity, FR is the emission rate of Radon, b{CR} isa term taking into account the vertical mixingproperties of the atmosphere, Adv is the advectiveterm.

Atmospheric stability conditions maximize aFR

(b{CR} is negligible) whereas atmospheric instabil-ity makes maximum the contribution of the termb{CR}.

3.2. Primary pollutants

The average concentrations of primary pollutantscan be regarded as closely related to the intensity oftheir emissions, so that their trends throughout awide temporal window of measurements directlyreflect important changes in the life style and the

ARTICLE IN PRESSP. Avino, M. Manigrasso / Atmospheric Environment 42 (2008) 4138–4148 4141

enactment of legislative decisions meant to intervenedirectly on the source of pollution.

Even if the gasoline consumption increased in thedecade 1990–2000 (De Lauretis et al., 2003), wehave assisted to a 60% reduction of benzeneemissions in Italy (De Lauretis et al., 2003) asshown in Fig. 1.

Such reduction was promoted by the growingpercentage of cars with catalytic converter and bythe Italian Law 413/97 (Law n, 1997) that fixed thelimit of 1% v/v of benzene in gasoline.

The same law prescribes that gasoline dispensingfacilities must be equipped with vapor recoveryequipment to control emissions of VOCs during thefilling of on-road motor vehicle fuel tanks. In anurban context dominated by the autovehiculartraffic pollution, as downtown Rome, a directconsequence of such legislative decisions was thesteady reduction of the benzene emissions. In thiscontext can be placed the sequence of 10 years ofmeasurements taken at the ISPESL monitoringstation and can be understood the overall declineof benzene annual average concentrations (Fig. 2).

Taking in account this consideration and theframe of measurements determined in Rome,it can be understood the overall decline of benzeneand toluene annual average concentrations (Figs. 2and 3).

Analyzing Figs. 2 and 3, we observe how thebenzene and toluene levels vary. In particular, theannual benzene trend, starting from 25 mgm�3 in1991, reaches 8 mgm�3 in 2000: maximum relativelevels of benzene are found during the last monthsof each year (from 30 mgm�3 in December 1991 to13 mgm�3 in December 2000) when meteorologicalconditions of high pressure are unfavorable for the

0

5

10

15

20

1990

Year

Gas

olin

e (M

t)

1991 1992 1993 1994 1995

Fig. 1. Gasoline consumption in Italy (bar) and relative benzen

pollutant dispersion. Further, it should be notedthat the drastic decreasing in concentrations levelsoccurred in 1994, 3 years before that the outcome ofthe law on the limit of the 1% v/v of benzene ingasoline.

The annual average toluene concentrations(Fig. 3) passed from 108 mgm�3 in 1991 to54 mgm�3 in 2000, following the same decreasingtrend as benzene one, with a toluene/benzene ratioof about 5, in agreement with the values of 3–5reported in literature for urban area pollution (Fieldet al., 1992). The two pollutants are well correlateddue to their common origin related to the auto-vehicular traffic, as shown in Fig. 4 (R2

¼ 0.984),reporting typical daily trends in the urban area ofRome during a cold period.

In particular, looking at the Fig. 4 it can bepossible to extrapolate some interesting considera-tions. First of all, the benzene and toluene concen-trations in urban area during a winter period areperfectly coherent stressing the same origin sourcesfor both pollutants. The high hourly levels reachedby benzene (up to 80mgm�3) during the evening ofthe 3rd depend strictly on the stability conditions inthat period. In fact, during the next days remixingconditions are present and the benzene levels godown to below 10mgm�3 until the afternoon of the7th when new stability conditions are present again.This boundary layer evolution is typical in aMediterranean area, especially in the urban area ofRome where cyclic stability/instability meteo condi-tions are present. Fig. 4 (middle) shows the trends ofbenzene calculated by two different systems: one bythe on-line DOAS and the second by a off-line GC-PID (Syntech-Spectra, The Netherlands) analyzersampling every 15min. It should be noted the

0

1

2

3

4

Ben

zene

(%

v/v

)

1996 1997 1998 1999 2000

e concentration allowed (line) (De Lauretis et al., 2003).

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35

40

01/91Year

Ben

zene

(μg

/m3 )

Annual average

01/92 01/93 01/94 01/95 01/96 01/97 01/98 01/99 01/00

Fig. 2. Monthly (bar) and annual (line) average levels of benzene concentrations (mgm�3) measured by a DOAS system in downtown

Rome.

0

20

40

60

80

100

120

140

160

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200

01/91

Year

Tolu

ene

(μg/

m3 )

Annual average

01/92 01/93 01/94 01/95 01/96 01/97 01/98 01/99 01/00

Fig. 3. Monthly (bar) and annual (line) average levels of toluene concentrations (mgm�3) measured by a DOAS system in downtown

Rome.

P. Avino, M. Manigrasso / Atmospheric Environment 42 (2008) 4138–41484142

behaviors not depending on the sampling or detec-tion systems: in fact, the trends appear similar even ifthe DOAS technique is a remote-sensing analysisinvolving a wide area portion whereas the GC-PIDsystem is a punctual analyzer determining thebenzene and toluene levels at the relative samplingpoint. According to these definitions we are expectedto find the same benzene modulation but lowervalues in the remote-sensing measurements as con-firmed in Fig. 4 (middle). Finally, Fig. 4 (below)reports the correlation curve between benzene datacollected by DOAS and GC-PID systems in the sameperiod: R2 is very good (0.984) and the relativeequation is y ¼ 0.427x�0.047 meaning the strictcoherence among the measurements performed bydifferent apparatus.

In the same decade, other important EuropeanDirectives, 93/12/EEC (Council Directive 93/12/EEC, 1993) and 98/70/EC (Council Directive 98/70/EC, 1998), have imposed and have scheduled for thenext years considerable reductions of the sulfurcontent in fuels (Fig. 5).

Consistently, the annual average concentrationsof SO2 measured at the ISPESL Pilot Stationare characterized by a clear decreasing trend(Fig. 6). In fact, by analyzing Fig. 6, we canunderstand that the annual levels vary from15 mgm�3 in 1991 to 4 mgm�3 in 2000. As in thecase of the previously analyzed pollutant, themaximum relative levels are found during thecoldest periods (every January from 36 mgm�3 in1992 to 10 mgm�3 in 2000).

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Ben

zene

(μg

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0

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300

0

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03/02date

Ben

zene

(μg

/m3 )

y = 0.427x - 0.047

0

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0

DOAS

BT

X

10 20 30 40 50 60 70 80

R2 = 0.984

04/02 05/02 06/02 07/02 08/02 09/02

03/02date

04/02 05/02 06/02 07/02 08/02 09/02

Tolu

ene

(μg/

m3 )

Fig. 4. (above) Typical winter (3–9 February 1997) trends of benzene and toluene in downtown Rome; (middle) comparison trends of

benzene determined by GC-spectra analyzer (line) and DOAS system (dot line); (below) correlation curve between GC-spectra and DOAS

data.

P. Avino, M. Manigrasso / Atmospheric Environment 42 (2008) 4138–4148 4143

This occurrence is due to the contribution ofdomestic heating, whereas the benzene, toluene andSO2 levels decrease clearly during summer perioddue to the meteorological conditions characterizedby long periods of solar radiation, favoring thepollutant dispersion due to convective mixing(Avino et al., 2003).

3.3. Secondary pollutants

For secondary pollutants, such as ozone andnitrogen dioxide, no clear trend can be recognized

looking at their average annual concentrations(Figs. 7 and 8), because they are not directly relatedto the pollution sources. Their pattern of variationis determined both by the atmospheric stabilityconditions and by the pool of reactions involvingO3, NOx, VOCs and UV radiation.

As regards the month average concentrations(Fig. 7), in summer ozone concentrations are higherthan in the rest of the year, due to the higherphotochemical activity in this period. Nitrogendioxide exhibits a complementary trend (Fig. 8) tothe O3 one. NO2 is produced through the oxidation

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500

150 50 10

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500350

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0

500

1000

1500

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2500

3000

3500

1990-1993

Years

Sulp

hur

cont

ent (

ppm

)

Gasoline

Diesel fuel

1994-1995 1996-1999 2000-2004 2005-2008 2009-

1000

Fig. 5. Sulphur content (ppm) allowed in gasoline and diesel fuels (European Directives).

0

5

10

15

20

25

30

35

40

45

01/91

Year

SO2

(μg/

m3 )

Annual average

01/92 01/93 01/94 01/95 01/96 01/97 01/98 01/99 01/00

Fig. 6. Monthly (bar) and annual (line) average levels (mgm�3) of SO2 concentrations measured by a DOAS system in downtown Rome.

P. Avino, M. Manigrasso / Atmospheric Environment 42 (2008) 4138–41484144

of NO by O3 and the radical RO2 and HO2. Ozoneis formed through the reaction of O2 with atomicoxygen deriving from the photolysis of NO2. No O3

accumulation would occur if the formation ofNO2 were due only to the reaction of O3 with NO.In this case NO2 and O3 show a complementarypattern of variation: maximum values of O3

corresponds to minimum values of NO2 and vice-versa. However, in the case of intense episodes ofpollution, in presence of reactive hydrocarbon andof OH radicals, the RO2 and HO2 radical that areformed oxidize NO and cause O3 accumulation(Avino, 2004). The occurrence of radical oxidativeprocesses in atmosphere can be represented through

the variable Ox, defined as the sum of O3 and NO2

concentrations.During periods of low photochemical activity and

of high advection, Ox is characterized by a trendfairly constant with oscillation around the back-ground ozone value. In this case, since ozone ismainly formed through the photolysis of NO2 anddecomposed by reaction with NO, the two pollu-tants show a complementary pattern of variation(Fig. 9). When the advective transport mechanism isnegligible and during atmospheric stability periods,radical oxidative processes may play an importantrole. Ox trend is very well structured (Fig. 10). Itdisplays minimum values at night, almost due to

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80

01/91

Year

Ozo

ne (

μg/m

3 )

Annual average

01/92 01/93 01/94 01/95 01/96 01/97 01/98 01/99 01/00

Fig. 7. Annual (line) and monthly (bar) average levels (mgm�3) of ozone concentrations measured by a DOAS system in downtown Rome.

0

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20

30

40

50

60

70

80

01/91

Year

NO

2 (μ

g/m

3 )

Annual average

01/92 01/93 01/94 01/95 01/96 01/97 01/98 01/99 01/00

Fig. 8. Annual (line) and monthly (bar) average levels (mgm�3) of NO2 concentrations measured by a DOAS system in downtown Rome.

P. Avino, M. Manigrasso / Atmospheric Environment 42 (2008) 4138–4148 4145

NO2, because the ozone concentrations are verylow. During the day, peak values above the back-ground value are indicative of radical oxidativeactivity. As a consequence, O3 and NO2 present ano more specular trend of variation.

The radon concentration trend (Fig. 10) displaysa peculiar modulation characterized by maximumvalues at night, due to nocturnal atmosphericstability, alternating with minimum values duringthe day, due to convective mixing. It is interesting tonote that such pattern is complementary to the Ox

one: minimum radon concentration value (highdegree of mixing of the low atmospheric layer)correspond to maximum Ox values (high solar

radiation and radical oxidative activity) and vice-versa.

During nighttime, ozone level within the mixingheight are relatively low due to the reaction of O3

with NO. However, since such mechanism ofdestruction of O3 operates in the presence ofemission sources of NOx, if transport of ozoneoccurs away from NOx sources, O3 decompositionis no more possible. During the daytime, when thesolar radiation is maximum and the convectivemixing mechanism is effective, ozone is homoge-neously distributed through the mixing layer. Atnight, due to the decline of the mixing height, NOx

emissions at the ground level can no more mix with

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5/11date

(μg/

m3 )

Ox

O3

NO2

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90

β co

unt (

a.u)

6/11 7/11 8/11 9/11 10/11 11/11

5/11date

6/11 7/11 8/11 9/11 10/11 11/11

Fig. 9. Typical daily trends of ozone, NO2 and Ox (above) and of radon concentration (below) during a cold period (2–11 November

1996).

P. Avino, M. Manigrasso / Atmospheric Environment 42 (2008) 4138–41484146

the layer above, consequently destruction of O3 inthe higher level is not possible and such level can actas a reservoir of ozone. As a consequence, ozonepeaks during nocturnal hours can be measured inthe presence of a vertical component of the wind.This is the case of Fig. 11 reporting the averagehourly concentrations of O3 in winter period nearan industrial site and displaying nighttimes ozonepeaks of the same order of magnitude of thosediurnal.

4. Conclusions

The DOAS technique is an advanced measure-ment system useful for integrating informationcoming from monitoring networks: furthermore, itallows investigating and understanding the complexmechanisms of formation and transformation ofatmospheric pollutants through the contemporary

measurements of gaseous primary and secondaryspecies. In this way the representation of the airpollution is more correct than using automaticanalyzers.

The results obtained take into account the spatial-temporal variations of the pollutant concentrationsdue to both the atmospheric dishomogeneity andthe contributions of sources not directly present inthe considered area.

Data referring to 10-year time span from1991–2000 in downtown Rome, have clearly showna steadily decreasing trend for some importantprimary pollutants, such as benzene and SO2. Thereduction of their atmospheric concentrations wasstrictly related to some normative decisions thatlimited the concentration of benzene and sulfurcontents in fuel. Non-clear temporal trends havebeen observed for the secondary pollutants, ozoneand dioxide nitrogen, in the investigated period.

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1/8 2/8 3/8 4/8 5/8

Date

Rad

on (

coun

ts)

0

2

4

6

8

10

12

14

1/8 2/8 4/8

date

Ox

(ppb

)

3/8

Fig. 10. Typical daily trends of the variable Ox (above) and of radon concentration (below) in a summer period (1–4 August 1997).

100

80

60

40

20

04 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29

Date

Ozo

ne (u

g/m

3 )

Fig. 11. Hourly average concentrations of ozone measured by a DOAS system in February 2000.

P. Avino, M. Manigrasso / Atmospheric Environment 42 (2008) 4138–4148 4147

Their evolution has been deeply studied trough theknowledge of the variable Ox (O3+NO2) asrepresentative of oxidative radicalic phenomenaoccurring in atmosphere. The radon short-lived

product measurements has provided a usefuldescription of the dynamic evolution of the lowboundary layer for interpreting the daily andseasonal trends of the pollutants investigated.

ARTICLE IN PRESSP. Avino, M. Manigrasso / Atmospheric Environment 42 (2008) 4138–41484148

Acknowledgments

This work was supported by ISPESL/DIPIA/P14Grant. The authors wish to thank Drs. L. Leporeand D. Brocco for your kind help in the resultdiscussion and the anonymous reviewers for havingimproved the paper.

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