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An atmospheric stability index based on radon progeny measurements for the evaluation of primary urban pollution
Atmospheric Environment 35 (2001) 5235–5244
An atmospheric stability index based on radon progeny measurements for the evaluation of primary urban pollution Cinzia Perrino*, Adriana Pietrodangelo, Antonio Febo C.N.R. Institute of Atmospheric Pollution, Via Salaria km 29,300, C.P. 10, 00016 Monterotondo Stazione (Rome), Italy Received 5 April 2001; received in revised form 16 June 2001; accepted 22 June 2001
Abstract An atmospheric stability index for the evaluation of urban primary pollution, based on the elaboration of natural radioactivity data yielded by a stability monitor, has been developed. The instrument determines the atmospheric concentration of the short-lived decay products of radon, whose emanation rate can be assumed to be constant in the time and space scale of observation. The index gives information about the dilution properties of the lower boundary layer and allows to highlight the relevant role of the dilution factor in determining primary pollution events. The atmospheric stability indices have been calculated during a 1-yr study carried out in the urban area of Rome (October 1999–September 2000). On the basis of the index, every day of the period has been classified in terms of intensity of a potential primary pollution event. The comparison between this classification and the real concentration value of primary pollutants, measured in the background urban station of Rome, yielded very good results. This shows that the index constitutes a powerful and valuable tool for describing primary pollution events in urban areas and confirms that the role played by the mixing properties of the lower boundary layer is essential in determining primary pollution in urban areas. r 2001 Elsevier Science Ltd. All rights reserved. Keywords: Radon progeny; Natural radioactivity; Mixing height; Atmospheric boundary layer; Primary pollution
1. Introduction It is well known that air pollution over urban areas is the result of a complex interaction between chemistry and meteorology and that the atmospheric concentration of pollutants depends on their emission, transformation and deposition rate as well as on their dilution in the planetary boundary layer. The knowledge of the dilution properties of the lower air layers is, thus, an essential tool for understanding the accumulation of pollutants and, in general, the time evolution of all pollution processes. We could obtain useful information about the dilution potential of the planetary boundary layer, which is not directly measured by any standard meteorological procedure, by monitoring a ground*Corresponding author. Tel.: +39-06-9067-2263; fax: +3906-9067-2660. E-mail address: [email protected] (C. Perrino).
emitted and chemically stable compound whose emission rate can be considered to be constant in the space and time scale of our observations Radon gas, which is produced in the soil by the radioactive decay of 226Ra, a member of the 238U series, is released from the soil into the surface layer of the atmosphere, where it is dispersed mainly by turbulent diffusion (Jacobi and Andr"e, 1963; Porstendorfer et al., 1991). The atmospheric concentration of radon and of its short-lived decay products (radon progeny), which are fixed on atmospheric aerosol particles, is governed by the source term and the dilution factor. Radon emanation rate varies from one place to another according to soil composition, moisture content, porosity and permeability, but the variations can be considered to be negligible in a time scale of some days and a space scale of some kilometres (Pearson and Jones, 1965; Shweikani et al., 1995). It follows that radon concentration in the air mainly depends on the dilution
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factor and that radon progeny can be considered as a good natural tracer of the mixing properties of the lower boundary layer (Hsu et al., 1980; Leach and Chandler, 1992; Allegrini et al., 1994; Duenas et al., 1996; Febo et al., 1997). This paper reports the results of a research project aimed to describe the mixing properties of the lower boundary layer in the urban area of the city of Rome by monitoring the air concentration of radon progeny during a 1-yr period. On the basis of radon progeny measurements, an atmospheric stability index able to classify each day of the period under study in terms of pollutants dispersionFand thus in terms of intensity of a potential primary pollution eventFhas been defined. The index allows us to uncouple the two contributions that determine the atmospheric concentration of a primary non-reactive pollutant, that is the emission factor and the dilution factor.
2. Experimental Natural radioactivity has been measured by means of a stability monitor (SM200, OPSIS AB, Furulund, Sweden) that consists of a particulate matter sampler equipped with a Geiger–Muller counter for determining the total beta activity of the short-lived radon progeny. The instrument is automatic and operates on two filters at the same time: sampling is performed on the first filter for a 2-h sampling duration, then this filter undergoes the beta measurement phase while a second filter undergoes the sampling phase (residual radioactivity is taken into account by a software procedure). This instrumental feature assures that the short-lived beta activity of the particles is determined continuously over an integration time of 2 h and that the beta measurement period is long enough to guarantee a good accuracy of the results. The accuracy is also assured by the many automatic quality control procedures, among which are the subtraction of background radiation (cosmic rays) and the continuous monitoring of the stability of the high voltage to the Geiger detector and normalisation of this value to a reference value (Perrino et al., 2000). Measurements have been carried out in Rome, at the urban background measuring station of Villa Ada, sited inside a park and not directly influenced by emission sources. Benzene and carbon monoxide measurements, carried out by continuous analysers, have been provided by the local monitoring network, as well as traffic flow monitoring. Measurement period was from 1 October 1999 to 30 September 2000. Since the atmospheric mixing is strongly dependent on the duration and strength of the sunlight (convective mixing), the year has been divided into three different periods, according to solar radiation intensity: winter period (from October to February),
summer period (from May to July), intermediate period (March, April, August and September), and the atmospheric mixing index has been calculated accordingly.
3. Results and discussion 3.1. Interpretation of natural radioactivity trends Since radon flux emanating from the ground can be considered, for practical purposes, to be horizontally uniform and constant in time, the time variations of natural radioactivity constitute a good tracer of the mixing properties of the lower atmosphere. In particular, in case of advection or convective mixing of the atmosphere the air concentration of radon has no possibility to build up, while in case of stability radon accumulates into the lower layers of the atmosphere and its concentration increases. The temporal trends of natural radioactivity during the months of August and December are compared in Fig. 1. During August and during all warm months, which in Italy are generally characterised by large-scale high-pressure systems, natural radioactivity shows a well-defined and modulated temporal trend and all days are very similar. The trend shows minimum values during daytime hours (convective mixing of the lower atmosphere that dissipates the inversion layer) that alternates to maximum values during the night (nocturnal atmospheric stability); the mixing period starts very early in the morning and lasts until the late evening. During December and during all cold months, instead, long high-pressure periods are sporadic, while advection often occurs and long atmospheric stability, also persisting during daytime hours, is sometimes observed. During advection, natural radioactivity always shows very low values (e.g. 5–6, 15, 21–22, 28 and 31 December); during prolonged stability, instead, the nocturnal inversion only slackens during the day hours and natural radioactivity exhibits high values also during the day (e.g. 2, 8, 19, 24 December). During the winter period, the diurnal mixing is not only weak but also of limited duration, since it generally stays for a few hours, from the late morning to the early afternoon. The very different duration of the atmospheric mixing phase along the year has an important consequence for atmospheric primary pollution. By monitoring the traffic flows in Rome, it can been shown that during both the summer and the winter, the traffic increases between 7 and 8 a.m., keeps a high value during the whole day and decreases between 8 and 10 p.m. The trend of natural radioactivity reveals that, during the summer, the lower atmosphere is already well mixed when the traffic flow increases and that the night time stability occurs when the traffic flow has already distinctly decreased (Fig. 2a). During the winter period, instead,
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Fig. 1. Temporal trend of natural radioactivity at the urban background station of Villa Ada, Rome, during August 2000 and December 1999.
the morning increase of the traffic flow corresponds to a still undeveloped mixed layer and the evening stability occurs when the traffic flow is still high (Fig. 2b). In the latter conditions, the usual concentration trend of primary non-reactive pollutants shows a first peak in the morning, when traffic emissions accumulate into the stable layer, a decrease during the warm hours, when traffic emissions can dilute, and a second stronger peak in
the evening, when the emission flux superimposes again on atmospheric stability. However, when atmospheric stability persists also during daytime, the decrease of primary pollutant concentration during warm hours is less distinct and a quite high primary pollution level can be detected also during the central part of the day. In Fig. 3, the temporal trends of natural radioactivity and of carbon monoxide concentration in the urban
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Fig. 2. Overlapping of traffic flow and atmospheric mixing (estimated from natural radioactivity values) during a typical summer day (a) and winter day (b).
background station of Villa Ada during the period 1–5 December are compared. These five days are a good example of the different meteorological situations commonly occurring during the winter. On 1 December, natural radioactivity indicated that nocturnal stability and daytime mixing of the lower atmosphere occurred; as a consequence, CO concentration trend showed the typical shape, with a first peak during the morning and a second peak during the evening. On 2 December,
instead, daytime mixing was very weak (slight and brief decrease of natural radioactivity during the middle of the day) and CO concentration kept values higher than 1.5 mg/m3 from 8 a.m. to the next morning. 3 December and the morning of 4 December showed intermediate conditions, followed by an advection period which began during the afternoon of 4th and lasted until the end of 5th (natural radioactivity was constantly low). As expected, during this advection period, CO concentra-
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Fig. 3. Temporal trend of natural radioactivity and of carbon monoxide concentration at the urban background station of Villa Ada during the first five days of December 1999.
tion kept very low values (about 0.5 mg/m3), and only a little increase during the morning and the evening was observed. This close link between the temporal trend of natural radioactivity and primary pollutants concentration gives a first confirmation of the relevant role played by meteorology in determining primary pollution events. 3.2. Atmospheric stability index In order to make the information contained in the trends of natural radioactivity more easily perceived and
interpreted, we have developed an atmospheric stability index (ASI) able to characterise each day in terms of meteorological predisposition to the occurrence of a primary pollution event. The index is made of two scalars, one referred to morning hours, from 6 a.m. to 6 p.m. (morning index) and the other one referred to evening hours, from 6 p.m. to 6 a.m. of the next day (evening index). The calculation of the two scalars has been performed on the basis of a multiple regression analysis against benzene concentration values at the background urban station. The regression analysis was
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carried out on a data set containing both natural radioactivity values and their time derivativesFfor example, the evening index for the winter period was calculated from the radioactivity value in the 2-h period starting at 18:00 and two time derivatives (periods 18:00–20:00 p.m. and 20:00–22:00 p.m.). Time derivatives of natural radioactivity during two subsequent periods have shown to be particularly suitable for describing the mixing properties of the lower boundary layer; a high and positive value of the time derivative indicates a rapid stabilisation of the atmosphere, while a negative derivative indicates an increase of the mixing height. The most significant time derivatives for the evening index are those calculated between 18:00 and 20:00 and between 20:00 and 22:00 for the winter period, between 20:00 and 22:00 and between 22:00 and 24:00 for the summer period. As far as the morning index is concerned, the most significant parameters are the derivatives calculated between 6:00 and 8:00 and between 8:00 and 10:00 for the winter period, between 8:00 and 10:00 and between 10:00 and 12:00 a.m. for the summer period. The scalars, expressed as arbitrary units, have been normalised to the seasonal average benzene value measured in the background urban station (winter, summer and intermediate months). In the ASI graph, days identified by a high value of the y-axis (evening index) and a high diameter of the symbol (morning index) are meteorologically suitable for a primary pollution event. Referring to the example reported in Fig. 4a, during 4–6, 8–10, 21–22, 28 and 31 January, a primary pollution event was very probable, while during 1, 13, 17, 19 and 23–26 January, primary pollution events were not favoured. The comparison with Fig. 4b, reporting the average daily concentration of benzene in the urban background measuring station of Villa Ada, shows that primary pollution events were closely dependent upon the mixing conditions of the lower atmospheric layers, well described by the ASI. Particularly interesting is the case of 5 January, when a daily-average benzene concentration value of about 12 mg/m3 was recorded in the urban background station. This day was characterised by meteorological conditions favouring primary pollution episodes, but the concentration recorded on 5 January was distinctly higher than that predictable on the only basis of the mixing properties of the lower atmosphere. This indicates a traffic flow higher than usual, and this hypothesis is consistent with the noticeable increase of the evening traffic intensity generally recorded in Rome, the evening before the Epiphany holiday. The comparison of the evening index with the average benzene value during evening hours gives a correlation coefficient of 0.80, 0.74 and 0.72 for the winter period, summer period and intermediate period, respectively;
the correlation coefficients for the morning index are 0.66, 0.73 and 0.65 for the same three periods. The satisfactory values of the correlation coefficient indicate that the ASI is a good tool for interpretation of primary pollution events. A better correlation between ASI and pollution values is, nevertheless, not expected, since the ASI takes into account only one of the two driving forces that determine pollutants concentration, that is the meteorological factor, while it does not take into account the day-to-day variations in the emission fluxes. In other words, the two data sets should coincide only if the emission fluxes were constant in time. The study of the differences between the two data sets, anyway, can be useful to identify situations when atmospheric pollution was heavier than predictable on the only basis of the meteorological situation (e.g. in case of heavy traffic episodes) or lighter (e.g. in case of traffic restriction measures). For example, in the case of the scatter plot between morning ASI and benzene concentration values during morning hours of the summer period, it is perceptible that the data groups into three different parts of the planeFweekdays in the higher part, Saturdays in the middle and Sundays in the lower section (Fig. 5a). This indicates that, assuming the same ASI value (that is, the same meteorological situation), on Saturday, the benzene concentration is about 70% of the weekday concentration, and on Sunday it is about 50%. This is in agreement with the lower traffic flow observed, on average, during Saturday (about 15% lower) and Sunday (about 40% lower), as shown in Fig. 5b. 3.3. Combined atmospheric stability index Combining linearly the two parameters that constitute the ASI, we obtain a combined index (cASI) that allows a more easy classification of the days of a given period according to the atmospheric mixing. cASI was derived as follows: cASI=0.33 e.i.+0.68 m.i., where e.i. and m.i. are the evening and the morning index, respectively. The comparison between the cASI and the average benzene concentration during the whole period from October 1999 to September 2000 is reported in Fig. 6a, and the scatter plot in Fig. 6b. The data show that the mixing of the lower atmospheric layer, well described by the cASI, is of primary relevance in determining the average concentration of non-reactive primary pollutants in the urban area of Rome. The correlation between the index and the average benzene concentration is very good both during winter months, characterised by urban background daily average benzene concentrations up to 10–12 mg/m3, and during summer months, when the benzene concentration rarely exceeded the value of 4 mg/m3. A lower correlation, as expected, is only observed during August, when the emission flux was distinctly lower than during the rest of
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Fig. 4. Atmospheric stability index (a) and benzene daily average concentration at the urban background station of Villa Ada (b) during January 2000.
the year due to summer holidays; as a consequence, during this month, primary pollution was lower than that predictable on the only basis of the dispersion properties of the atmosphere. 3.4. Air quality classification On the basis of the cASI value, the days of the period under study have been grouped into five classes, ranging
from ‘‘very good’’ to ‘‘very bad’’ air quality in relation to primary pollution. The cASI range and the benzene concentration range that is expected for each class at the urban background station are reported in Table 1 (since the ASI has been normalised to the seasonal average benzene value, the ASI values also express numerically the predicted benzene background concentration). This classification has been verified on the basis of the real daily benzene values measured in the urban background
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Fig. 5. Difference between weekdays, Saturday and Sunday in the scatter plot of the morning atmospheric stability index and the benzene morning average concentration (a) and the daily time-trend of the average traffic flow (b) during the summer period (May–July 2000). The grey area refers to the period covered by the morning atmospheric stability index.
station during 349 d (benzene data referring to the remaining 17 d of the year did not pass the quality control). The data reported in Table 1 show that in 96% of the examined cases, the classification according to the cISA resulted to be verified, that is, on the basis of the benzene value, the days belonged to the same class; for the rest of the cases, the days belonged to a class close to the one identified on the cASI basis. For example, according to the cASI, 80 d were classified as characterised by an ‘‘acceptable’’ air quality (third class). According to the
measured benzene values, 77 d really belonged to the third class, and 3 belonged to a near class. It is interesting to note that 9 out of the 13 d when the cASI classification was not verified were underestimation, and 6 of these cases were holidays (1 November, 8 December and four Sundays), when a lower primary pollutant emission is expected. All these findings indicate that the role played by the mixing properties of the lower boundary layer is predominant with respect to traffic emissions in determining primary pollution levels in urban areas and that
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Fig. 6. Comparison between the combined atmospheric stability index and the average benzene concentration during the whole period from October 1999 to September 2000Ftemporal trend (a) and scatter plot (b).
the atmospheric stability index is a powerful and reliable tool for the interpretation of primary pollution events.
4. Conclusion The atmospheric stability index, calculated on the basis of natural radioactivity data, allows the characterisation
of the period under study in terms of meteorological predisposition of each day to the occurrence of a primary pollution event. The ASI allows the uncoupling of the two main factors determining primary pollution eventsFthe dilution properties of the lower atmosphere and the emission flows. This study has shown that the mixing properties of the boundary layer play a main role in the occurrence of primary pollution events and that the ASI constitutes a valuable and reliable tool for the
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Table 1 Air quality classification according to the combined atmospheric stability index and the benzene concentration measured at a background urban site cASI value
o3 3–4 4–6 6–8 >8
Air quality (primary pollution)
Very good Good Acceptable Bad Very bad
Expected C6H6 concentration at the urban background station (mg/m3) o371 371–471 471–671 671–871 >871
characterisation, comparison and comprehension of primary pollution events in urban areas.
Acknowledgements The authors are indebted to S. Pareti, whose reliable technical assistance made this study possible. Special thanks are also extended to M. Giusto and M. Montagnoli for their support. This study (Progetto AUGUSTO) was financed by the Environmental and Agricultural Policy Department of the Town Council of Rome. The Department is also gratefully acknowledged for having provided traffic flow and primary pollution data.
References Allegrini, I., Febo, A., Pasini, A., Schiarini, S., 1994. Monitoring of the nocturnal mixed layer by means of particulate radon progeny measurement. Journal of Geophysical Research 99, 18765–18777. Duenas, C., Perez, M., Fernandez, M.C., Carretero, J., 1996. Radon concentrations in the surface air and vertical atmospheric stability in the lower atmosphere. Journal of Environmental Radioactivity 31, 87–102.
Classification according to cASI values (d)
163 72 80 27 7
Classification according to C6H6 concentration
Same class (d)
Adjacent class (d)
163 67 77 23 6
5 3 4 1
Febo, A., Perrino, C., Giliberti, C., 1997. A method for the interpretation of ground level ozone measurements in air quality networks. In: Schaug, J., Uhse, K. (Eds.), EMEP/ CCC-Report 10/97, pp. 165–171. Hsu, S.A., Larson, R.E., Bressan, D.J., 1980. Diurnal variation of radon and mixing heights along a coast: a case study. Journal of Geophysical Research 85, 4107–4110. Jacobi, W., Andr"e, K., 1963. The vertical distribution of radon 222, radon 220 and their decay products in the atmosphere. Journal of Geophysical Research 68, 3799–3814. Leach, V.A., Chandler, W.P., 1992. Atmospheric dispersion of radon gas and its decay products under stable conditions in arid regions of Australia. Environmental Monitoring and Assessment 20, 1–17. Pearson, J.E., Jones, G.E., 1965. Emanation of radon 222 from soils and its use as a tracer. Journal of Geophysical Research 70, 5279–5285. Perrino, C., Febo, A., Allegrini, I., 2000. A new beta gauge monitor for the measurement of PM10 air concentration. In: Hanssen, J.E., Ballaman, R., Gehrig, R. (Ed.), EMEP/ CCC-Report 9/2000, pp. 147–152. Porstendorfer, J., Butterweck, G., Reineking, A., 1991. Diurnal variation of the concentrations of radon and its short-lived daughters in the atmosphere near the ground. Atmospheric Environment 25, 709–713. Shweikani, R., Giaddut, T.G., Durrani, S.A., 1995. The effect of soil parameters on the radon concentration values in the environment. Radiation Measurements 25, 581–584.
An atmospheric stability index based on radon progeny measurements for the evaluation of primary urban pollution Cinzia Perrino*, Adriana Pietrodangelo, Antonio Febo C.N.R. Institute of Atmospheric Pollution, Via Salaria km 29,300, C.P. 10, 00016 Monterotondo Stazione (Rome), Italy Received 5 April 2001; received in revised form 16 June 2001; accepted 22 June 2001
Abstract An atmospheric stability index for the evaluation of urban primary pollution, based on the elaboration of natural radioactivity data yielded by a stability monitor, has been developed. The instrument determines the atmospheric concentration of the short-lived decay products of radon, whose emanation rate can be assumed to be constant in the time and space scale of observation. The index gives information about the dilution properties of the lower boundary layer and allows to highlight the relevant role of the dilution factor in determining primary pollution events. The atmospheric stability indices have been calculated during a 1-yr study carried out in the urban area of Rome (October 1999–September 2000). On the basis of the index, every day of the period has been classified in terms of intensity of a potential primary pollution event. The comparison between this classification and the real concentration value of primary pollutants, measured in the background urban station of Rome, yielded very good results. This shows that the index constitutes a powerful and valuable tool for describing primary pollution events in urban areas and confirms that the role played by the mixing properties of the lower boundary layer is essential in determining primary pollution in urban areas. r 2001 Elsevier Science Ltd. All rights reserved. Keywords: Radon progeny; Natural radioactivity; Mixing height; Atmospheric boundary layer; Primary pollution
1. Introduction It is well known that air pollution over urban areas is the result of a complex interaction between chemistry and meteorology and that the atmospheric concentration of pollutants depends on their emission, transformation and deposition rate as well as on their dilution in the planetary boundary layer. The knowledge of the dilution properties of the lower air layers is, thus, an essential tool for understanding the accumulation of pollutants and, in general, the time evolution of all pollution processes. We could obtain useful information about the dilution potential of the planetary boundary layer, which is not directly measured by any standard meteorological procedure, by monitoring a ground*Corresponding author. Tel.: +39-06-9067-2263; fax: +3906-9067-2660. E-mail address: [email protected] (C. Perrino).
emitted and chemically stable compound whose emission rate can be considered to be constant in the space and time scale of our observations Radon gas, which is produced in the soil by the radioactive decay of 226Ra, a member of the 238U series, is released from the soil into the surface layer of the atmosphere, where it is dispersed mainly by turbulent diffusion (Jacobi and Andr"e, 1963; Porstendorfer et al., 1991). The atmospheric concentration of radon and of its short-lived decay products (radon progeny), which are fixed on atmospheric aerosol particles, is governed by the source term and the dilution factor. Radon emanation rate varies from one place to another according to soil composition, moisture content, porosity and permeability, but the variations can be considered to be negligible in a time scale of some days and a space scale of some kilometres (Pearson and Jones, 1965; Shweikani et al., 1995). It follows that radon concentration in the air mainly depends on the dilution
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factor and that radon progeny can be considered as a good natural tracer of the mixing properties of the lower boundary layer (Hsu et al., 1980; Leach and Chandler, 1992; Allegrini et al., 1994; Duenas et al., 1996; Febo et al., 1997). This paper reports the results of a research project aimed to describe the mixing properties of the lower boundary layer in the urban area of the city of Rome by monitoring the air concentration of radon progeny during a 1-yr period. On the basis of radon progeny measurements, an atmospheric stability index able to classify each day of the period under study in terms of pollutants dispersionFand thus in terms of intensity of a potential primary pollution eventFhas been defined. The index allows us to uncouple the two contributions that determine the atmospheric concentration of a primary non-reactive pollutant, that is the emission factor and the dilution factor.
2. Experimental Natural radioactivity has been measured by means of a stability monitor (SM200, OPSIS AB, Furulund, Sweden) that consists of a particulate matter sampler equipped with a Geiger–Muller counter for determining the total beta activity of the short-lived radon progeny. The instrument is automatic and operates on two filters at the same time: sampling is performed on the first filter for a 2-h sampling duration, then this filter undergoes the beta measurement phase while a second filter undergoes the sampling phase (residual radioactivity is taken into account by a software procedure). This instrumental feature assures that the short-lived beta activity of the particles is determined continuously over an integration time of 2 h and that the beta measurement period is long enough to guarantee a good accuracy of the results. The accuracy is also assured by the many automatic quality control procedures, among which are the subtraction of background radiation (cosmic rays) and the continuous monitoring of the stability of the high voltage to the Geiger detector and normalisation of this value to a reference value (Perrino et al., 2000). Measurements have been carried out in Rome, at the urban background measuring station of Villa Ada, sited inside a park and not directly influenced by emission sources. Benzene and carbon monoxide measurements, carried out by continuous analysers, have been provided by the local monitoring network, as well as traffic flow monitoring. Measurement period was from 1 October 1999 to 30 September 2000. Since the atmospheric mixing is strongly dependent on the duration and strength of the sunlight (convective mixing), the year has been divided into three different periods, according to solar radiation intensity: winter period (from October to February),
summer period (from May to July), intermediate period (March, April, August and September), and the atmospheric mixing index has been calculated accordingly.
3. Results and discussion 3.1. Interpretation of natural radioactivity trends Since radon flux emanating from the ground can be considered, for practical purposes, to be horizontally uniform and constant in time, the time variations of natural radioactivity constitute a good tracer of the mixing properties of the lower atmosphere. In particular, in case of advection or convective mixing of the atmosphere the air concentration of radon has no possibility to build up, while in case of stability radon accumulates into the lower layers of the atmosphere and its concentration increases. The temporal trends of natural radioactivity during the months of August and December are compared in Fig. 1. During August and during all warm months, which in Italy are generally characterised by large-scale high-pressure systems, natural radioactivity shows a well-defined and modulated temporal trend and all days are very similar. The trend shows minimum values during daytime hours (convective mixing of the lower atmosphere that dissipates the inversion layer) that alternates to maximum values during the night (nocturnal atmospheric stability); the mixing period starts very early in the morning and lasts until the late evening. During December and during all cold months, instead, long high-pressure periods are sporadic, while advection often occurs and long atmospheric stability, also persisting during daytime hours, is sometimes observed. During advection, natural radioactivity always shows very low values (e.g. 5–6, 15, 21–22, 28 and 31 December); during prolonged stability, instead, the nocturnal inversion only slackens during the day hours and natural radioactivity exhibits high values also during the day (e.g. 2, 8, 19, 24 December). During the winter period, the diurnal mixing is not only weak but also of limited duration, since it generally stays for a few hours, from the late morning to the early afternoon. The very different duration of the atmospheric mixing phase along the year has an important consequence for atmospheric primary pollution. By monitoring the traffic flows in Rome, it can been shown that during both the summer and the winter, the traffic increases between 7 and 8 a.m., keeps a high value during the whole day and decreases between 8 and 10 p.m. The trend of natural radioactivity reveals that, during the summer, the lower atmosphere is already well mixed when the traffic flow increases and that the night time stability occurs when the traffic flow has already distinctly decreased (Fig. 2a). During the winter period, instead,
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Fig. 1. Temporal trend of natural radioactivity at the urban background station of Villa Ada, Rome, during August 2000 and December 1999.
the morning increase of the traffic flow corresponds to a still undeveloped mixed layer and the evening stability occurs when the traffic flow is still high (Fig. 2b). In the latter conditions, the usual concentration trend of primary non-reactive pollutants shows a first peak in the morning, when traffic emissions accumulate into the stable layer, a decrease during the warm hours, when traffic emissions can dilute, and a second stronger peak in
the evening, when the emission flux superimposes again on atmospheric stability. However, when atmospheric stability persists also during daytime, the decrease of primary pollutant concentration during warm hours is less distinct and a quite high primary pollution level can be detected also during the central part of the day. In Fig. 3, the temporal trends of natural radioactivity and of carbon monoxide concentration in the urban
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Fig. 2. Overlapping of traffic flow and atmospheric mixing (estimated from natural radioactivity values) during a typical summer day (a) and winter day (b).
background station of Villa Ada during the period 1–5 December are compared. These five days are a good example of the different meteorological situations commonly occurring during the winter. On 1 December, natural radioactivity indicated that nocturnal stability and daytime mixing of the lower atmosphere occurred; as a consequence, CO concentration trend showed the typical shape, with a first peak during the morning and a second peak during the evening. On 2 December,
instead, daytime mixing was very weak (slight and brief decrease of natural radioactivity during the middle of the day) and CO concentration kept values higher than 1.5 mg/m3 from 8 a.m. to the next morning. 3 December and the morning of 4 December showed intermediate conditions, followed by an advection period which began during the afternoon of 4th and lasted until the end of 5th (natural radioactivity was constantly low). As expected, during this advection period, CO concentra-
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Fig. 3. Temporal trend of natural radioactivity and of carbon monoxide concentration at the urban background station of Villa Ada during the first five days of December 1999.
tion kept very low values (about 0.5 mg/m3), and only a little increase during the morning and the evening was observed. This close link between the temporal trend of natural radioactivity and primary pollutants concentration gives a first confirmation of the relevant role played by meteorology in determining primary pollution events. 3.2. Atmospheric stability index In order to make the information contained in the trends of natural radioactivity more easily perceived and
interpreted, we have developed an atmospheric stability index (ASI) able to characterise each day in terms of meteorological predisposition to the occurrence of a primary pollution event. The index is made of two scalars, one referred to morning hours, from 6 a.m. to 6 p.m. (morning index) and the other one referred to evening hours, from 6 p.m. to 6 a.m. of the next day (evening index). The calculation of the two scalars has been performed on the basis of a multiple regression analysis against benzene concentration values at the background urban station. The regression analysis was
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carried out on a data set containing both natural radioactivity values and their time derivativesFfor example, the evening index for the winter period was calculated from the radioactivity value in the 2-h period starting at 18:00 and two time derivatives (periods 18:00–20:00 p.m. and 20:00–22:00 p.m.). Time derivatives of natural radioactivity during two subsequent periods have shown to be particularly suitable for describing the mixing properties of the lower boundary layer; a high and positive value of the time derivative indicates a rapid stabilisation of the atmosphere, while a negative derivative indicates an increase of the mixing height. The most significant time derivatives for the evening index are those calculated between 18:00 and 20:00 and between 20:00 and 22:00 for the winter period, between 20:00 and 22:00 and between 22:00 and 24:00 for the summer period. As far as the morning index is concerned, the most significant parameters are the derivatives calculated between 6:00 and 8:00 and between 8:00 and 10:00 for the winter period, between 8:00 and 10:00 and between 10:00 and 12:00 a.m. for the summer period. The scalars, expressed as arbitrary units, have been normalised to the seasonal average benzene value measured in the background urban station (winter, summer and intermediate months). In the ASI graph, days identified by a high value of the y-axis (evening index) and a high diameter of the symbol (morning index) are meteorologically suitable for a primary pollution event. Referring to the example reported in Fig. 4a, during 4–6, 8–10, 21–22, 28 and 31 January, a primary pollution event was very probable, while during 1, 13, 17, 19 and 23–26 January, primary pollution events were not favoured. The comparison with Fig. 4b, reporting the average daily concentration of benzene in the urban background measuring station of Villa Ada, shows that primary pollution events were closely dependent upon the mixing conditions of the lower atmospheric layers, well described by the ASI. Particularly interesting is the case of 5 January, when a daily-average benzene concentration value of about 12 mg/m3 was recorded in the urban background station. This day was characterised by meteorological conditions favouring primary pollution episodes, but the concentration recorded on 5 January was distinctly higher than that predictable on the only basis of the mixing properties of the lower atmosphere. This indicates a traffic flow higher than usual, and this hypothesis is consistent with the noticeable increase of the evening traffic intensity generally recorded in Rome, the evening before the Epiphany holiday. The comparison of the evening index with the average benzene value during evening hours gives a correlation coefficient of 0.80, 0.74 and 0.72 for the winter period, summer period and intermediate period, respectively;
the correlation coefficients for the morning index are 0.66, 0.73 and 0.65 for the same three periods. The satisfactory values of the correlation coefficient indicate that the ASI is a good tool for interpretation of primary pollution events. A better correlation between ASI and pollution values is, nevertheless, not expected, since the ASI takes into account only one of the two driving forces that determine pollutants concentration, that is the meteorological factor, while it does not take into account the day-to-day variations in the emission fluxes. In other words, the two data sets should coincide only if the emission fluxes were constant in time. The study of the differences between the two data sets, anyway, can be useful to identify situations when atmospheric pollution was heavier than predictable on the only basis of the meteorological situation (e.g. in case of heavy traffic episodes) or lighter (e.g. in case of traffic restriction measures). For example, in the case of the scatter plot between morning ASI and benzene concentration values during morning hours of the summer period, it is perceptible that the data groups into three different parts of the planeFweekdays in the higher part, Saturdays in the middle and Sundays in the lower section (Fig. 5a). This indicates that, assuming the same ASI value (that is, the same meteorological situation), on Saturday, the benzene concentration is about 70% of the weekday concentration, and on Sunday it is about 50%. This is in agreement with the lower traffic flow observed, on average, during Saturday (about 15% lower) and Sunday (about 40% lower), as shown in Fig. 5b. 3.3. Combined atmospheric stability index Combining linearly the two parameters that constitute the ASI, we obtain a combined index (cASI) that allows a more easy classification of the days of a given period according to the atmospheric mixing. cASI was derived as follows: cASI=0.33 e.i.+0.68 m.i., where e.i. and m.i. are the evening and the morning index, respectively. The comparison between the cASI and the average benzene concentration during the whole period from October 1999 to September 2000 is reported in Fig. 6a, and the scatter plot in Fig. 6b. The data show that the mixing of the lower atmospheric layer, well described by the cASI, is of primary relevance in determining the average concentration of non-reactive primary pollutants in the urban area of Rome. The correlation between the index and the average benzene concentration is very good both during winter months, characterised by urban background daily average benzene concentrations up to 10–12 mg/m3, and during summer months, when the benzene concentration rarely exceeded the value of 4 mg/m3. A lower correlation, as expected, is only observed during August, when the emission flux was distinctly lower than during the rest of
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Fig. 4. Atmospheric stability index (a) and benzene daily average concentration at the urban background station of Villa Ada (b) during January 2000.
the year due to summer holidays; as a consequence, during this month, primary pollution was lower than that predictable on the only basis of the dispersion properties of the atmosphere. 3.4. Air quality classification On the basis of the cASI value, the days of the period under study have been grouped into five classes, ranging
from ‘‘very good’’ to ‘‘very bad’’ air quality in relation to primary pollution. The cASI range and the benzene concentration range that is expected for each class at the urban background station are reported in Table 1 (since the ASI has been normalised to the seasonal average benzene value, the ASI values also express numerically the predicted benzene background concentration). This classification has been verified on the basis of the real daily benzene values measured in the urban background
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Fig. 5. Difference between weekdays, Saturday and Sunday in the scatter plot of the morning atmospheric stability index and the benzene morning average concentration (a) and the daily time-trend of the average traffic flow (b) during the summer period (May–July 2000). The grey area refers to the period covered by the morning atmospheric stability index.
station during 349 d (benzene data referring to the remaining 17 d of the year did not pass the quality control). The data reported in Table 1 show that in 96% of the examined cases, the classification according to the cISA resulted to be verified, that is, on the basis of the benzene value, the days belonged to the same class; for the rest of the cases, the days belonged to a class close to the one identified on the cASI basis. For example, according to the cASI, 80 d were classified as characterised by an ‘‘acceptable’’ air quality (third class). According to the
measured benzene values, 77 d really belonged to the third class, and 3 belonged to a near class. It is interesting to note that 9 out of the 13 d when the cASI classification was not verified were underestimation, and 6 of these cases were holidays (1 November, 8 December and four Sundays), when a lower primary pollutant emission is expected. All these findings indicate that the role played by the mixing properties of the lower boundary layer is predominant with respect to traffic emissions in determining primary pollution levels in urban areas and that
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Fig. 6. Comparison between the combined atmospheric stability index and the average benzene concentration during the whole period from October 1999 to September 2000Ftemporal trend (a) and scatter plot (b).
the atmospheric stability index is a powerful and reliable tool for the interpretation of primary pollution events.
4. Conclusion The atmospheric stability index, calculated on the basis of natural radioactivity data, allows the characterisation
of the period under study in terms of meteorological predisposition of each day to the occurrence of a primary pollution event. The ASI allows the uncoupling of the two main factors determining primary pollution eventsFthe dilution properties of the lower atmosphere and the emission flows. This study has shown that the mixing properties of the boundary layer play a main role in the occurrence of primary pollution events and that the ASI constitutes a valuable and reliable tool for the
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Table 1 Air quality classification according to the combined atmospheric stability index and the benzene concentration measured at a background urban site cASI value
o3 3–4 4–6 6–8 >8
Air quality (primary pollution)
Very good Good Acceptable Bad Very bad
Expected C6H6 concentration at the urban background station (mg/m3) o371 371–471 471–671 671–871 >871
characterisation, comparison and comprehension of primary pollution events in urban areas.
Acknowledgements The authors are indebted to S. Pareti, whose reliable technical assistance made this study possible. Special thanks are also extended to M. Giusto and M. Montagnoli for their support. This study (Progetto AUGUSTO) was financed by the Environmental and Agricultural Policy Department of the Town Council of Rome. The Department is also gratefully acknowledged for having provided traffic flow and primary pollution data.
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Classification according to cASI values (d)
163 72 80 27 7
Classification according to C6H6 concentration
Same class (d)
Adjacent class (d)
163 67 77 23 6
5 3 4 1
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