Revista CENIC Ciencias Químicas, Vol. 36, No. Especial, 2005
Behavior of Primary and Secondary Pollutants in
Ambient Air of Rome
Pasquale Avino1*, Mario Vincenzo Russo2
1
Laboratorio Inquinamento Chimico dell’Aria, Dipartimento Insediamenti Produttivi e Interazione con l’Ambiente
– Istituto Superiore per la Prevenzione E la Sicurezza sul Lavoro
2
Facoltà di Agraria (DISTAAM), Università del Molise,
Via Urbana 167 – 00184 Rome (Italy). Ph.: +39 064714242, Fax: +39 064744017; E-mail:
[email protected]
Via De Sanctis – 86100 Campobasso (Italy). Ph.: +39 0874 404634; Fax: +39 0874 404652; E-mail:
[email protected] .
ABSTRACT: The hydrocarbons are a significant component in urban air because of combustion, solvent and
fuel evaporation and tank leakage and most of aromatic compounds are considered as toxic air contaminants
(e.g. benzene) or potential toxic air contaminants (e.g. toluene, xylenes). The no-methane hydrocarbons are
considered as precursors for ozone production at the ground level when the sunlight and nitrogen oxides are
present. This ozone is usually considered as “bad ozone” which can seriously damage our environment and
human health. In fact, no-methane hydrocarbons and aromatic hydrocarbons participate in the formation of
urban and suburban photochemical smog with their concentrations influencing greatly the total ozone in different
percentage. In this paper, we report the results obtained during ten-years of measurements. The air quality
determinations were conducted by automatic analyzers and Differential Optical Absorption Spectrometry
investigating traditional atmospheric pollutants like ozone, nitrogen dioxide, nitrous acid, carbon monoxide,
formaldehyde, benzene and toluene at the ISPESL Pilot Station located in downtown Rome. The trends of
benzene, toluene, CO, NO and IPA are decreased, expecially because of introduction of both the green fuel and
the autovehicular catalytic pot. Even if the pollutant levels are decreasing, the sources are still the same and, in
particular, the emission from the incomplete combustion of LPG is the most important source of pollution in
Rome.
Keywords: Volatile Aromatic Compounds, Atmospheric pollution, Urban air pollution, Anthropogenic sources,
Remote-sensing methodology, Photochemical smog episodes.
INTRODUCTION
The study of the atmospheric pollution has always impassioned the scientific world for both the reasons linked to
the knowledge of the chemical reactions occurring in atmosphere up to the photochemical smog formation, and
because of the air contamination which is the main responsible factors of physical discomfort conditions. An
important part of this big “laboratory around the world” is the air chemical characterization and in particular the
identification and determination, at ppt levels, of those substances that could provoke harmful effects to the
human nature and the ecosystem as well. Furthermore, the interpretation of the atmospheric pollution
phenomena is really complex for the simultaneous presence of both emission processes and physical-chemical
transformation processes and formation of pollutants associated to meteorological conditions (diffusion and
transport).
Now, in order to control the emission of no-methane hydrocarbons (NMHCs) and to develop air quality criteria
like the recent Council Directive emitted by the European Union, it is important to understand their sources and
their profiles.
Recently, different researchers have introduced some variables able to describe in simple but precise way the
pollutant behavior and the dynamic properties of the boundary layer, e.g. the natural beta radiation1,2. In the
latter case, the meteorological measurements normally performed in the monitoring networks, do not furnish
sufficient information to describe the evolution of the atmospheric boundary layer remixing.
Among the various species present in atmosphere the hydrocarbons are a significant component in urban air
because of combustion, solvent and fuel evaporation and tank leakage and most of aromatic compounds are
considered as toxic air contaminants (e.g. benzene) or potential toxic air contaminants (e.g. toluene, xylenes)3,4.
Further, the NMHCs play a key role in the formation of photochemical air pollution. They are considered as
precursors for ozone production5 at the ground level when the sunlight and nitrogen oxides are present. This
Revista CENIC Ciencias Químicas, Vol. 36, No. Especial, 2005
ozone is usually considered as “bad ozone” which can seriously damage our environment and human health. In
fact, NMHCs and aromatic hydrocarbons participate in the formation of urban and suburban photochemical
smog with their concentrations influencing greatly the total ozone in different percentage. Derwent and Jenkin6
were the first to determine that m-xylene, trimethylbenzenes and C3-C4 alkenes produce more ozone than
ethylene. After, many authors have documented that many potential toxic and mutagenic compounds react in
atmosphere.4
In this paper are described some criteria for the air quality evaluation in an urban area, i.e. the study of the
primary and secondary pollutants, the physical-chemical parameters indicative of both the atmospheric activity
and the remixing of the low boundary layers, the behavior of the carbonaceous material as pollution index of
combustion processes. This investigation is carried out analyzing the data collected during an intensive
measurement campaign performed in downtown Rome during the 1999.
Experimental part
Sampling Site
The measurements were performed at ISPESL’ Pilot Station located in downtown Rome, in an area
characterized by high density of vehicular traffic.
Equipment
The air quality evaluation has been carried out by means of automatic sensors (traditional analyzers) and a
remote-sensing system, i.e. Differential Optical Absorption System (DOAS).7
The investigated pollutants are SO2, NOx, O3, CO, benzene, toluene, xylene, particulate matter, organic carbon
and elemental carbon (Rupprecht & Patasnick, NY, USA). The natural beta radiation has been measured by
means of a beta counter (SM200, Opsis, Sweden).
It should be emphased that the DOAS detector has acquired a strong interest, expecially for the measure of
some pollutants (like SO2, NO2, benzene, toluene, ozone, nitrous acid and formaldehyde) not subjects at
elevated spatial-temporal gradients and for the evaluation of some compounds not easy to determine.
RESULTS AND DISCUSSION
Primary and secondary pollutants
Primary pollutants are defined as compounds directly emitted from the emission sources (e.g., carbon monoxide,
nitrogen oxide, sulphur dioxide, benzene, volatile hydrocarbons, and metals). During their residence-time in
atmosphere they do not undergo physical-chemical transformations. The availability for measure the
atmospheric dispersion activity by means of the determination of the natural radioactivity, allows the identification
the concentration primary pollutant variations depending on the emission sources (vehicular traffic and/or relative
structural interventions).
Secondary pollutants are defined as the chemicals (e.g., ozone, nitrogen dioxide, nitric acid, nitrous acid,
nitrates, nitro-derivate, sulfates) derived from chemical and/or photochemical reactions occurring in atmosphere.
For the secondary pollutants at elevated reactivity, like NO2 and O3, the equations describing their temporal
evolution, are very complex. The parameter Ox (sum of NO2 and O3) has been introduced8: it allows for
interpretation of the temporal trends of NO2 and O3 and hence the photochemical pollution phenomenon.
In very strong meteorological conditions (advective condition), Ox partial derivate vs. time could be considered
constant and Ox is constant. In fact, in stability conditions the radicalic processes are negligible and the reactions
between NO2 and O3 are dominant and complementary. In atmospheric stability conditions, Ox shows a welldefined trend due to the presence of both the oxidative radicalic processes and the dynamic properties of the
boundary layer.
Interpretation of meteorological phenomena
A very important approach to describe the pollutant evolution is given by the radon concentration measurements.
The radon emission can be considered constant and spatially homogeneous for some kilometers. Therefore, the
temporal evolution of the radon concentration and of its partial derivate vs. the time depends on only the
dynamic of the boundary layer.1
Formally, the temporal radon evolution is defined by the following relationship1,2:
∂C R
= α[Φ R ] − β(C R ) + Adv
(1)
∂t
From the radon derivate trend it is possible to define both the stability and instability conditions. In fact, the high
stability conditions at ground level maximize the contribution of the term α[ΦR] (the term β(CR) is negligible) while
Revista CENIC Ciencias Químicas, Vol. 36, No. Especial, 2005
the transition to instability conditions maximizes the contribution of the term β(CR).
Concentration levels and temporal trends of the investigated pollutants
There are graphically reported typical concentration levels and trends measured in downtown Rome by means of
DOAS and traditional analyzers of SO2, NO2, O3, benzene, toluene, HCHO, HNO2, CO in 1999.
Typical episodes of primary and secondary pollution occurring in Rome
In Figure 1-3 are reported the typical trends of benzene and beta natural radiation in three different seasonal
periods measured in downtown Rome. Looking at the figures, it can be noted that the benzene ranges between
2 and 40 µg/m3 during the wintertime, between 1 and 15 µg/m3 during the summertime and between 2 and 75
µg/m3 during the fall time. These high value differences can be due to meteorological conditions present during
the three investigated periods (e.g., different atmospheric conditions, presence of wind and rain) and to the
different lifestyles (different emissions, e.g., absence of domestic heating and low autovehicular density during
the summertime). An other interesting thing to be noted is the toluene level: it is about 3-5 times than the
benzene values according to the literature where is reported that the toluene and benzene ratio is 3-5 times
when there are no typical emission sources of these two gaseous pollutants.
250
200
60
Toluene (ug/m3)
Benzene (ug/m3)
80
150
40
100
20
50
0
0
1/2
2/2
3/2
4/2
5/2
6/2
4/2
5/2
6/2
date
Beta radiation (a.u.)
2500
2000
1500
1000
500
0
1/2
2/2
3/2
date
Figure 1. Typical benzene and toluene (µg/m3) and beta radiation trends during a wintertime in downtown
Rome.
Revista CENIC Ciencias Químicas, Vol. 36, No. Especial, 2005
250
200
60
150
40
100
20
Toluene (ug/m3)
Benzene (ug/m3)
80
50
0
0
1/8
2/8
3/8
4/8
5/8
6/8
4/8
5/8
6/8
date
Beta radiation (a.u.)
2500
2000
1500
1000
500
0
1/8
2/8
3/8
date
Figure 2. Typical benzene and toluene (µg/m3) and beta radiation trends during summertime in downtown
Rome.
250
200
60
Toluene (ug/m3)
Benzene (ug/m3)
80
150
40
100
20
0
1/11
50
2/11
3/11
4/11
5/11
0
6/11
4/11
5/11
6/11
date
Beta radiation (a.u.)
2500
2000
1500
1000
500
0
1/11
2/11
3/11
date
Figure 3. Typical benzene and toluene (µg/m3) and beta radiation trends during a fall time in downtown Rome.
Revista CENIC Ciencias Químicas, Vol. 36, No. Especial, 2005
From a qualitative point of view the Figures 1-3 are very interesting because they show the typical
meteorological situation in Rome during the investigated seasons through the behavior of the beta radiation
parameter. Remembering the definition above reported on the beta radiation, an increase of the benzene
concentration values is evidenced with an increase of the beta radiation values and vice versa: this depends on
the different conditions of remixing of the atmospheric boundary layer. In fact, the beta radiation values increase
with the decreasing of atmospheric remixing: in this case the benzene dispersion is unfavored with a consequent
increase of the concentration levels. Increasing the remixing the opposite case occurs.
In the Figure 4 and 5 are reported the temporal trends of O3, NO2 and O3+NO2 in a period of strong advection.
The trends of O3 and NO2 have the characteristic to be complementary between them. The variable Ox (sum of
O3+NO2) shows a light modulation to a value K equal to around 100 µg/m3. Consequently, it results that the
strong remixing conditions do not cause photochemical pollution: the temporal trends of O3, NO2 and Ox can be
used as indicative of oxidative episodes occurring in atmosphere.
O3, NO2 (ug/m3)
160
120
80
40
0
10/5
11/5
12/5
13/5
14/5
15/5
16/5
date
300
1200
200
800
100
400
Beta radiation (a.u.)
Ox (ug/m3)
Figure 4. Typical O3 (bold line) and NO2 (µg/m3) trends during a hot period in Rome.
0
0
10/5
11/5
12/5
13/5
14/5
15/5
16/5
date
Figure 5. Ox (µg/m3) and beta radiation profiles corresponding to the same period of Fig. 4 measured in Rome.
Revista CENIC Ciencias Químicas, Vol. 36, No. Especial, 2005
HCHO (ug/m3)
20
15
10
5
0
10/5
11/5
12/5
13/5
14/5
15/5
16/5
date
3
Figure 6. Typical HCHO (µg/m ) trend during smog photochemical episodes occurring in Rome during a hot
period.
Analyzing the Figures 4 and 6 some typical smog photochemical episodes involving O3, NO2 and HCHO are
represented: there is a 4-days period (10th, 11th, 113th and 14th may) during which a smog photochemical
episode characterized by presence of HCHO coming from radical reactions (secondary origin exclusively)
occurs. Such phenomenon is confirmed by the strong correlation between O3 and HCHO (linear regression R>
0.7).
Furthermore looking at the pollutant trends during the period from 11th to 14th of may, a rapid raising of
formaldehyde, O3 and NO2 is observed. Such representation is typical in Rome atmosphere and is an index of a
strong photochemical episode (O3 reaches a maximum of 160 µg/m3) during which the NOx photostationary
cycle equilibrium breaks itself because of a strong presence of O3 precursors, and there is a rapid O3 level
increase in consequence of a NO2 formation due to radical reactions and rapid photolysis rection.
In the same way formaldehyde follows the ozone behavior, to forehead of a contribution of formation radicalica,
confirmed entirely by the Fig. 30 where the course of HCHO is coincident with the course of Ox, index of the
atmospheric reactivity.
CONCLUSION
The considerations reported in this paper show the influence of the meteorological conditions on the pollution
levels and trends in a megacity like Rome. The importance of a parameter such as the natural beta radiation is
represented and the formation of smog photochemical pollution is described. The air quality evaluation in a
megacity like Rome is very difficult for the presence of almost 1000 gaseous pollutants at ppt levels: no all these
species are important or toxicologically important but their synergic action is not deeply investigated and the
effects could also be dangerous for the humans.
Finally, it could be considered that the atmospheric pollution study here reported comes from anthropogenic
activities in urban area and is predominantly due to autovehicular traffic and domestic heating. However, the
problems are the same for big combustion plants but, of course, the differences rise from the process conditions,
that could vary the emission spectra, and in the atmospheric dilution properties (the meteorological conditions
affect the absolute compound concentration values while acting in the same way on the mechanisms of
reaction).
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