Behavior of Primary and Secondary Pollutants in Ambient Air of Rome

Revista CENIC Ciencias Químicas, Vol. 36, No. Especial, 25 Behavior of Primary and Secondary Pollutants in Ambient Air of Rome Pasquale Avino 1 *, Mario Vincenzo Russo 2 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 167 184 Rome (Italy). Ph.: +39 64714242, Fax: +39 6474417; E-mail: pasquale.avino@uniroma1.it Via De Sanctis 861 Campobasso (Italy). Ph.: +39 874 44634; Fax: +39 874 44652; E-mail: mvrusso@unimol.it. 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 radiation 1,2. 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 production 5 at the ground level when the sunlight and nitrogen oxides are present. This

Revista CENIC Ciencias Químicas, Vol. 36, No. Especial, 25 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 Jenkin 6 were the first to determine that m-xylene, trimethylbenzenes and C 3 -C 4 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 SO 2, NO x, O 3, 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 (SM2, Opsis, Sweden). It should be emphased that the DOAS detector has acquired a strong interest, expecially for the measure of some pollutants (like SO 2, NO 2, 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 NO 2 and O 3, the equations describing their temporal evolution, are very complex. The parameter O x (sum of NO 2 and O 3 ) has been introduced 8 : it allows for interpretation of the temporal trends of NO 2 and O 3 and hence the photochemical pollution phenomenon. In very strong meteorological conditions (advective condition), Ox partial derivate vs. time could be considered constant and O x is constant. In fact, in stability conditions the radicalic processes are negligible and the reactions between NO 2 and O 3 are dominant and complementary. In atmospheric stability conditions, O x 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 relationship 1,2 : CR = α[ Φ R ] β( CR ) + 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[φ R ] (the term β(c R ) is negligible) while

Revista CENIC Ciencias Químicas, Vol. 36, No. Especial, 25 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 SO 2, NO 2, O 3, benzene, toluene, HCHO, HNO 2, 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 4 µg/m 3 during the wintertime, between 1 and 15 µg/m 3 during the summertime and between 2 and 75 µg/m 3 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. Benzene (ug/m3) 8 6 4 2 25 2 15 1 5 Toluene (ug/m3) 1/2 2/2 3/2 4/2 5/2 6/2 25 2 15 1 5 1/2 2/2 3/2 4/2 5/2 6/2 Figure 1. Typical benzene and toluene (µg/m 3 ) and beta radiation trends during a wintertime in downtown Rome.

Revista CENIC Ciencias Químicas, Vol. 36, No. Especial, 25 8 25 Benzene (ug/m3) 6 4 2 2 15 1 5 Toluene (ug/m3) 1/8 2/8 3/8 4/8 5/8 6/8 25 2 15 1 5 1/8 2/8 3/8 4/8 5/8 6/8 Figure 2. Typical benzene and toluene (µg/m 3 ) and beta radiation trends during summertime in downtown Rome. 8 25 Benzene (ug/m3) 6 4 2 2 15 1 5 Toluene (ug/m3) 1/11 2/11 3/11 4/11 5/11 6/11 25 2 15 1 5 1/11 2/11 3/11 4/11 5/11 6/11 Figure 3. Typical benzene and toluene (µg/m 3 ) and beta radiation trends during a fall time in downtown Rome.

Revista CENIC Ciencias Químicas, Vol. 36, No. Especial, 25 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 O 3, NO 2 and O 3 +NO 2 in a period of strong advection. The trends of O 3 and NO 2 have the characteristic to be complementary between them. The variable O x (sum of O 3 +NO 2 ) shows a light modulation to a value K equal to around 1 µg/m 3. Consequently, it results that the strong remixing conditions do not cause photochemical pollution: the temporal trends of O 3, NO 2 and O x can be used as indicative of oxidative episodes occurring in atmosphere. 16 O3, NO2 (ug/m3) 12 8 4 1/5 11/5 12/5 13/5 14/5 15/5 16/5 Figure 4. Typical O 3 (bold line) and NO 2 (µg/m 3 ) trends during a hot period in Rome. 3 12 Ox (ug/m3) 2 1 8 4 1/5 11/5 12/5 13/5 14/5 15/5 16/5 Figure 5. O x (µg/m 3 ) and beta radiation profiles corresponding to the same period of Fig. 4 measured in Rome.

Revista CENIC Ciencias Químicas, Vol. 36, No. Especial, 25 2 HCHO (ug/m3) 15 1 5 1/5 11/5 12/5 13/5 14/5 15/5 16/5 Figure 6. Typical HCHO (µg/m 3 ) trend during smog photochemical episodes occurring in Rome during a hot period. Analyzing the Figures 4 and 6 some typical smog photochemical episodes involving O 3, NO 2 and HCHO are represented: there is a 4-days period (1 th, 11 th, 113 th and 14 th 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 O 3 and HCHO (linear regression R>.7). Furthermore looking at the pollutant trends during the period from 11 th to 14 th of may, a rapid raising of formaldehyde, O 3 and NO 2 is observed. Such representation is typical in Rome atmosphere and is an index of a strong photochemical episode (O 3 reaches a maximum of 16 µg/m 3 ) during which the NO x photostationary cycle equilibrium breaks itself because of a strong presence of O 3 precursors, and there is a rapid O 3 level increase in consequence of a NO 2 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. 3 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 1 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). BIBLIOGRAPHY 1. Allegrini I., Febo A., Pasini A., Schirini S. Monitoring of the nocturnal mixed layer by means of particulate radon progeny measurements. J. Geophys. Res., 94, 765, 1994. 2. Avino P., Brocco D., Lepore L. and Pareti S. Interpretation of atmospheric pollution phenomena in relationship with the vertical atmospheric remixing by means of natural radioactivity measurements (radon) of particulate matter. Ann. Chim. (Rome), 93, 589, 23. 3. Hanson D.J. Toxics release inventory report shows chemical emissions continuing to fall. Chem. Engineering News, July, 29, 1996.

Revista CENIC Ciencias Químicas, Vol. 36, No. Especial, 25 4. Monod A., Sive B.C., Avino P., Chen T.Y., Blake D.R., Rowland F.S. Monoaromatic compounds in ambient air of various cities: a focus on correlations between the xylenes and ethylbenzene. Atmos. Environ., 35, 135, 21. 5. Carter W.P.L. Development of ozone reactivity scales for volatile organic compounds. J. Air Waste Manag. Ass., 44, 881, 1994. 6. Derwent R.G. and Jenkin M.E. Hydrocarbons and the long-range transport of ozone and PAN across Europe. Atmos. Environ., 25A, 1661, 1991. 7. Platt U., Perner D. Direct measurement of atmospheric CH 2 O, HNO 2, O 3 and SO 2 by Differential Optical Absorption. J. Geophys. Res., 85, 7435, 198. 8. Avino P., Brocco D. and Scalisi G. Criteria for evaluating the urban atmospheric pollution: the results of tenyear monitoring activity in Rome. Proceedings of the 16 th International Clean Air & Environment Conference, Christchurch (New Zealand), 19-22 August 22, 6-64.