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Continuous measurement of gaseous pollutants in Buenos Aires city
Atmospheric Environment 33 (1999) 2587}2598
Continuous measurement of gaseous pollutants in Buenos Aires city Horacio Bogo, R. MartmH n Negri, Enrique San RomaH n* INQUIMAE and Departamento de Qun& mica Inorga& nica, Analn& tica y Qun& mica Fn& sica, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Ciudad Universitaria, Pabello& n II, 1428 Buenos Aires, Argentina
Abstract Data on CO, NO, NO and O concentrations measured in Buenos Aires city using a continuous monitoring station are reported. This is the "rst systematic study of this kind carried out in the city, which is, together with its surroundings, the third more populated in Latin America. Measurements were performed during 12 months in one of the principal avenues near downtown. Results indicate that vehicular tra$c is the principal source of CO and NO . The concentration V of O is generally quite low and results from the mixing of clean air masses with exhaust gases containing high amounts of NO. The monthly averages of CO and NO decrease from Winter to Summer in correlation with the increase of the mean wind speed and average temperature. These results are compared with previous measurements on the spatial distribution of NO in the whole city using passive di!usion tubes and with the concentration of CO, which is being continuously registered since several years in the downtown area. Measurements performed at a green, windy, low tra$c area beneath the La Plata river are also shown. 1999 Elsevier Science Ltd. All rights reserved. Keywords: Urban pollution; Mobile sources; Carbon monoxide; Nitrogen oxides; Ozone
1. Introduction Buenos Aires city and surroundings occupy roughly a semicircle of 50 km radius centered at 34334 S and 58325 W at an altitude of ca. 20 m with an approximate population of 13.8 million inhabitants. It is one of the three megalopolises in Latin America, including Mexico city and Sa o Paulo in Brazil. In contrast to these cities, Buenos Aires lies on a vast plain area and is limited by the La Plata river, whose width near the city is around 50 km. The federal district is mainly an urban area of
* Corresponding author. [email protected].
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near 15 km diameter (see Fig. 1) populated by 2.9 million people. The mean population density reaches 14 000 inh km\ in the federal district and decreases to near 2000 inh km\ in the surrounding area. Thermal power stations are located at the side of the river. Very few industries are located within the city, but industries of several kinds including leather, oil, chemicals, textile, etc. can be found in the surrounding area. The climate is temperate with annual mean temperatures of 183C in the city and 153C in the surroundings (average values for the period 1981}1991 measured by the National Meteorological Service). The precipitation regime is 900} 1600 mm yr\, in#uenced by wet winds from the Atlantic Ocean. The federal district is circulated mainly by motor cars and public transport buses, which are generally not equipped with exhaust catalyzers. Only in this area the
1352-2310/99/$ - see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 9 8 ) 0 0 2 7 0 - 2
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city. Previous monitoring of NO and SO (AramendmH a et al., 1995) shows nearly constant average concentrations within the whole city with the exception of the downtown area, where averages are comparatively higher. Therefore, most of the data were collected during 12 months at one of the principal avenues near downtown representative of this general behavior. Previously, some data were obtained at the University Campus, a green, windy, low tra$c area at the side of the La Plata river. The main purpose of this "rst e!ort was to obtain information on primarily emitted pollutants (although ozone was also measured) and not on their fate, as could be the case of studies performed in other cities of the world where primary emissions are well characterized (Seinfeld, 1995; Sillman, 1995). As a result, correlations between measured pollutant concentrations, meteorological parameters of relevance and tra$c pro"les were obtained and will be discussed in detail.
2. Experimental
Fig. 1. Buenos Aires city and surroundings, showing the position of the University Campus (1), station at Belgrano Avenue (2), the meteorological station of Villa OrtuH zar (3) and downtown area (4).
number of registered vehicles is about 1.5 millions. A small area inside downtown is restricted only to public transport during most of the daily hours. Tra$c in and out from the city circulates through moderately wide avenues and small streets. Rush hours are around 8}10 a.m. and 5}8 p.m. During rush hours a signi"cant lowering of mean speed is observed and, though automatic signaling is in general well regulated, frequent stops take place. Aside from some earlier measurements (MaH rsico, 1974), a screening of NO and SO concentrations deter mined using passive tubes (AramendmH a et al., 1995) and data on CO concentrations regularly measured by a private foundation at one location inside downtown (results are published currently in local newspapers but no comprehensive reports are available) there is no systematic control of the various atmospheric pollutants in the city of Buenos Aires. This situation is re#ected in a recent publication of local circulation (Moretton, 1996). Accordingly, the aim of the present work was to perform a systematic monitoring of several atmospheric pollutants (CO, NO, NO and O ) measured simulta neously and continuously at selected locations within the
A fully automatic station composed of three independent instruments for the determination of CO, NO}NO and O was used. CO was measured by nondispersive infrared photometry using a gas "lter correlation instrument (Monitor Labs, ML 9830). NO and NO were measured separately by gas-phase chemiluminescence detection (Monitor Labs, ML 9841A). O was measured by UV absorption at 254 nm (Monitor Labs, ML 9812). Calibration was performed following US-EPA (Environmental Protection Agency, USA) and Monitor Labs Inc. standard procedures and recommendations (see Code of Federal Regulations, US-EPA, Subchapter C: Air Programs (1993), Part 50, pp. 697}777 and Monitor Labs User Manuals, Parts C98300068, C98412026 and C98120004, respectively). For the calibration of the ML 9830 and ML 9841A instruments, Scott CO (10 ppmV$ 2%) and NO (10 ppmV$5%) calibration gases were used. The NO measuring system was calibrated by gas phase titration of NO with O generated by a Hg low pressure UV lamp. The ozone measuring instrument was calibrated by biamperometric titration of O following the generation of I from I\ in the presence of S O\ at pH"7. Ozone was generated using the same mercury lamp and the #ow of O into the biamperometrical cell was measured simultaneously with the I determination. Data were recorded primarily as one-minute averages. A personal computer served simultaneously to perform instrumental control and data storage. The system was initialized daily at 12.00 a.m. and, while measuring near downtown, a control worksheet was sent automatically to the University Campus. Data from the various instruments were sequentially recorded and stored. Periodically, data were downloaded, averaged and analyzed.
H. Bogo et al. / Atmospheric Environment 33 (1999) 2587}2598
The measurement station was placed from July 1996, beginning of the winter season, to June 1997, in a building at Belgrano Avenue, one of the principal streets of Buenos Aires, in a populated region near downtown (Fig. 1). This avenue has synchronized tra$c lights and the circulation of vehicles is normally fast. Measurements were performed at a horizontal distance of approximately 4 m from the tra$c line. A previous series of measurements were performed from October to December 1995 at the Campus of the University of Buenos Aires, away from the more populated region (Fig. 1). At both measurement sites air sampling was performed at a height of 10 m from the ground. Tra$c circulation was measured as hourly averages at the monitoring station in Belgrano Avenue during the second and third weeks of September 1996. Measurements were performed by the Buenos Aires Urban Transportation Project of the National Transport Secretary using an automatic recording system. With the exception of January and February, tra$c circulation can be regarded as constant throughout the year both in regard to the number and distribution of vehicles. During Summer the number of cars is signi"cantly lower, but quantitative data are not available at present. Relevant meteorological parameters, namely hourly values of temperature and wind velocity and direction, were taken into account on analyzing chemical data. Two meteorological stations are managed within the city by the National Meteorological Service, one located in Villa OrtuH zar (Fig. 1), well within the urban area, and the other at Aeroparque, the local airport nearby the La Plata River. Data from the "rst station was used for the analysis at Belgrano Avenue. Although it is possible that some local e!ects could not be well represented, it has been tested that general trends, such as seasonal variation of the monthly averaged wind speed or the calculated atmospheric boundary layer altitude, are similar when data from the Villa OrtuH zar or the Aeroparque meteorological stations are taken into account (Mazzeo and Gassmann, 1990). For the analysis of the results obtained at the University Campus, meteorological data were locally measured by the Department of Atmospheric Sciences of the School of Sciences. Di!usion tube results on the concentration of NO and SO were published elsewhere (AramendmH a et al., 1995) and are reproduced in Fig. 2. Plastic cylinders (length: 71 mm; i.d. 10 mm) open to the atmosphere in one extreme and "tted with a "lter with suitable reagents at the other extreme were used (Heal and Cape, 1997; Campbell et al., 1997; Ferm, 1991). These tubes were placed in 21 sites at an average height of 10 m. Except for some locations as the University Campus, the altitude of 10 m is well below the mean altitude of most apartment buildings within the city.
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Fig. 2. Map of Buenos Aires city showing the location of the di!usion pipes. Concentration of NO (grey bars) and SO (black bars) are represented by the height of the corresponding stabs; maximum concentrations are 46.6 ppbV for NO and 7 ppmV for SO . Measurements were performed as monthly averages from May to June 1994.
3. Results and discussion 3.1. University Campus Monthly averaged concentrations of O , CO, NO and NO measured during Spring at the University Campus are shown in Table 1. The very low amounts found for CO, NO and NO can be explained by the proximity of the La Plata river together with the existence of prevailing North and East winds blowing from cleaner areas. A detailed analysis of hourly values shows O peaks reaching 0.030}0.050 ppmV for most of the days, within the limits typical for a clean atmosphere (Seinfeld, 1986). During the measurement period the maximum peak concentration of O was around 0.060 ppmV near noon and occurred only once. This observation was coincident with a NO /NO ratio greater than unity, low values of NO and northeast wind. In Fig. 3 it is shown that V NO /NO'1 for North and northeast winds. NO con centrations increase when the wind speed decreases or
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when the wind blows from the city area, yielding NO /NO(1. The resulting monthly average values of NO and NO are very similar, as shown in Table 1. The features shown in Fig. 3. are typical of clean air incidence together with local perturbations. The O con centrations are relatively high during the night and until noon when wind blows from the North, because the incoming air is cleaner. At evening the wind speed de-
Table 1 Monthly average concentrations at the University Campus Month
October 1995 November 1995 December 1995
O (ppmV)
CO (ppmV)
NO (ppmV)
NO (ppmV)
0.013 0.013 0.018
0.67 0.44 0.29
0.014 0.005 0.003
0.010 0.004 0.004
creases and the NO concentration increases while the O concentration falls down due to titration. The same e!ect is observed when wind carries polluted air from the city area, causing an increase of NO and CO. In this case values of O below the instrumental sensitivity (0.002 ppmV) are obtained during workdays at rush hours. As a result, the average concentration of O is lower than the typical values for a clean atmosphere. Though the existing data does not allow to establish a general trend, Table 1 shows decreasing values for the average concentration of primary pollutants and NO and an increasing concentration of O on going from October to December, in agreement with the seasonal increase of wind speed (see later). 3.2. Belgrano Avenue Monthly averaged values of O , CO, NO and NO registered at Belgrano Avenue are shown in Table 2. Comparison of data obtained at this site with data meas-
Fig. 3. Concentrations of O (䊊), CO (䊐), NO (䉲) and NO (䉬), represented as hourly averages, measured at the University Campus on Friday 20 October 1995. Wind direction and speed, measured at the University Campus, are also shown.
H. Bogo et al. / Atmospheric Environment 33 (1999) 2587}2598 Table 2 Monthly average concentrations at Belgrano Avenue Month
O (ppmV)
CO (ppmV)
NO (ppmV)
NO (ppmV)
July 1996 August 1996 September 1996 October 1996 November 1996 December 1996 January 1997 February 1997 March 1997 April 1997 May 1997 June 1997
0.002 0.003 0.004 0.004 0.005 0.005 0.004 0.004 0.003 0.002 0.001 0.002
2.92 2.69 2.24 2.21 1.93 1.93 1.65 1.45 2.35 2.41 2.54 2.46
0.150 0.124 0.110 0.101 0.072 0.074 0.069 0.076 0.109 0.111 0.133 0.143
0.040 0.041 0.035 0.036 0.029 0.033 0.029 0.026 0.034 0.036 0.037 0.033
ured at the University Campus is not straightforward as measuring periods do not overlap. Nevertheless, Table 2 shows a regular trend characterized by a decrease of average concentrations of CO, NO and NO from July 1996 to January or February followed by an increase until average concentrations found in June 1997 approximately match those found at the beginning. Comparison of data obtained during Spring shows CO values three to six times larger and NO and NO values up to one order of magnitude larger at the Belgrano Avenue. The concentration of O at the last site is one order of magnitude smaller, normally below the instrumental sensitivity. As tra$c circulation does not change systematically between June and December, the lowering of the concentration of primary pollutants may be attributed to an increased air mixing capacity due to a higher wind speed and to the increase of the inversion layer altitude. The monthly vectorially averaged wind velocity calculated from data recorded at the Villa OrtuH zar observatory increased steadily from July 1996 to December 1996 from 0.2 to 2.2 knots (0.37}4.11 km h\) and the same holds normally for the calculated monthly average values of the atmospheric boundary layer altitude (Ulke and Mazzeo, 1998; Mazzeo and Gassmann, 1990; Fochessato, 1997; Lavorato et al., 1996a and b). Additionally, the emission factors for CO in non-catalytic light duty gasoline powered vehicles decrease signi"cantly with temperature (Economopoulos, 1993). In general, the pro"les of CO, NO and NO found between 7 a.m. and 8 p.m. di!er substantially between workdays and weekends. During weekends the concentration of these gases is much lower and the time distribution is much more di!use. On the other hand, the concentration of O do not vary signi"cantly between workdays and weekends and is always less than 0.015 ppmV, a very low value compared to clean air
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concentrations. Fig. 4a and b show the concentrations of CO, NO, NO and O at Belgrano Avenue in a weekend day and in a normal workday during Winter, respectively. A moderate breeze was present on both days. During the night, the concentrations of all gases except O drop to very low values, showing a rapid removal of pollutants. Only in a few days of low wind speed the 8 h average concentration of CO was higher than 9 ppmV, the "rst alarm value of US-EPA. High concentrations of NO were found also during those days. In particular, 17 July and 18 July represent the only case of two consecutive days with almost no breeze in the reported period. The concentrations of CO, NO, NO and O for July 18 are shown in Fig. 4c. It may be noticed that the memory of the last day is retained in this case, leading to a cummulative e!ect in the concentration of all pollutants. In the general case, however, the concentrations of CO and NO are very low at night and start to increase at V about 7 a.m. During workdays a maximum is observed in the morning, followed by a decrease toward the afternoon. A new increase is then observed, reaching a second maximum during the evening. The pattern described for Belgrano Avenue is almost coincident with the tra$c pro"le modulated by the wind speed (see Fig. 5). The two peaks obtained in the CO and NO daily pro"les during V workdays correspond to rush hours. This trend was also observed in previous CO measurements performed downtown (AramendmH a et al., 1995) although in that case much higher values were obtained because the tra$c level was also much higher and the streets narrower at that site. Data corresponding to 23 September 1996 are shown in Fig. 5. Southeast winds prevailed during the whole day, blowing clean air masses from the river into the city. As it is seen from the "gure, there is a strong correlation between the wind speed and the departure between the concentration of CO and the tra$c curve. High wind speeds cause the CO curve to lay below the tra$c curve and viceversa (notice that the area under both curves is approximately equal). This strongly suggests that CO levels are due principally to vehicular emissions. The concentrations of CO and NO correlate strongly V during the whole measurement period. As an example, Fig. 6 represents the correlation obtained during July 1996. A linear scatter diagram with zero intercept is obtained, which is interpreted considering that vehicular emissions are the only source of NO in this location and V that fresh pollution with little or no photochemical conversion is measured (Goldan et al., 1995). The measured values of the CO/NO ratio are in the expected range for V vehicular emissions (see CETESB, 1994 and SWEPA, 1990). As emissions are highly dependent on engine type, tra$c conditions and temperature (Bower et al., 1994), it is very di$cult to make a right prediction of the CO/NO V ratio in Buenos Aires because of the lack of information,
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Fig. 4. Concentrations of O (䊊), CO (䊐), NO (䉲) and NO (䉬) measured at the station of Belgrano Avenue, represented as hourly averages, for: (a) a weekend with moderated breeze (Sunday 21 July 1996). Average wind intensity about 4 knot. (b) a workday with moderated breeze (Friday 5 July 1996). Average wind intensity about 5 knot. (c) a workday with almost no breeze at all (Thursday 18 July 1996). Average wind intensity about 1 knot.
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Fig. 4. (continued)
statistics and car testings. In particular, measurements on the CO/NO ratio at the exhaust of vehicles have not V been performed under typical driving conditions in Buenos Aires city. Monthly average concentrations of CO from July 1996 to July 1997 are plotted in Fig. 7 as a function of time, showing the annual cyclic behavior. In the same "gure the wind average speed values and the averaged values of the calculated atmospheric boundary layer altitude are presented (Mazzeo and Gassmann, 1990). It is observed that the monthly averaged values of CO are well correlated with the atmospheric boundary layer altitude and less strongly with wind average speeds. From July to December the wind speed and the calculated atmospheric boundary layer altitude increase almost monotonically. Then, a statistical study of the daily time dependence of CO was performed against the actual wind velocities in the following way. Days from 18 June 1996 to 15 December 1996 were classi"ed in four categories according to the value of the average wind speed (< ), taken between 7 a.m. and 11 p.m. where the concentration of CO is relatively important. Thus, a value of < is assigned to each day. The categories are the following: I: < (5.5 km h\; II: 5.5)< ( 11.5 km h\; III: 11.5)< (19.5 km h\; IV: < *
19.5 km h\. This classi"cation is based on the Beaufort meteorological scale, although a coarser description considering only four category groups was used in order to facilitate the calculations, to improve statistics and to make clear the analysis and discussion. Days were classi"ed into workdays, Saturdays and Sundays. The average values of < that were obtained for the grouped categories are shown in Table 3 (these values are referred as a double average, 1< 2). The distributions of the hourly values of CO are very di!erent for workdays, Saturdays and Sundays. Most Saturdays and Sundays show not well de"ned peaks, re#ecting a more or less random tra$c circulation. On Sundays, concentrations are much lower than on Saturdays and the hourly distribution is even more di!use. Hence, the following study was performed only for workdays, for which the data of each category group (I, II, III and IV) were analyzed. The hourly averaged values of CO for each category group were "tted by the sum of two Gaussian functions: [CO](t)"[CO] A (t!k ) G . G # exp ! 2p 2p (n/2 G G G
(1)
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Fig. 5. Tra$c (䊉) and concentration of carbon monoxide (䊐) for 23 September 1996, measured at the station of Belgrano Avenue. The wind speed (䉬), measured at Villa Ortuzar, is also shown.
Workday data belonging to category groups II and III are shown in Figs. 8a and b, respectively. In practice periodical behavior was assumed and four Gaussians were used to "t a 48 h period. Two of the Gaussians were forced to have the same parameters as the other two. Values of k , p and A were obtained for each Gaussian G G G function and shown in Table 4. It is essential to analyze the hypotheses used in the foregoing analysis. First, we have considered only the wind speed as the meteorological parameter of relevance. Second, the variation of wind speed along the day was not considered. Taking into account these approximations, the "ts shown in Fig. 8a and b may be used as predictive tools for daily averaged CO concentration values for a workday of a given class. The "ts are statistically representative of the time variation of CO concentration for those days. The two values of k for each category are coincident G with rush hours. Non-zero values of [CO] were re
covered only for category I, which is associated to the persistence of CO emitted the day before when the wind speed is low. The recovered values of p are almost coincident for the four categories. p is always larger than p and although its values are alike for the di!erent categories, a slight increase with the wind speed was noticed. The ratio A /A follows the ratio p /p as the peak values of both Gaussians are very similar. The concentration of O measured at Belgrano Av enue is typically less than 0.010 ppmV, always very low compared to clean air concentrations due to titration of O by NO. Because of the very low values obtained for O , it is ventured to establish correlations between O and NO or CO. Average concentrations of NO and SO taken over 40 days were reported for the sites shown in Fig. 2 during May } June 1994 (AramendmH a et al., 1995). Values ranged between 0.027 and 0.047 ppmV for NO and 0.002 and
H. Bogo et al. / Atmospheric Environment 33 (1999) 2587}2598
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Fig. 6. Concentration of CO versus NO for July 1996 measured at Belgrano Avenue. V
0.007 ppmV for SO . The very low values registered for SO are in agreement with the low level of sulfur found in local gasolines. The values reported for NO are in excel lent agreement with the monthly averages obtained at Belgrano Avenue. The highest values correspond to the pipes located downtown in areas with the highest tra$c levels. However, even for the areas with the lowest tra$c, these values are not far from those found downtown. These results are consistent with the presence of changing winds causing a continuous mixing of air masses.
E E
4. Conclusions The reported data on CO, NO, NO and O concen trations constitute the "rst systematic study of atmospheric pollutants in the city of Buenos Aires, obtained continuously with a fully automatic station. On the basis of the results described in the previous section, the next main conclusions emerge: E The concentration values of all the species considered are a result of the mixing of vehicular emissions with clean air masses. Vehicular emissions are undoubtedly the main source of CO and NO . The relatively high V
E
E
wind speed and the #at geography of the city do not allow the accumulation of pollutants during most days. The situation can be of concern for the case of consecutive days without breeze, as it happened once in Winter, where 8 h average concentration of CO higher than 9 ppmV were observed at an altitude of 10 m over a relatively wide avenue. The atmosphere is cleaned up during the night, when the tra$c stops and, as a general rule, every morning a renewed atmosphere is observed. The spatial distribution of the average concentrations of SO and NO measured using di!usion tubes shows no substantial variations over the city. This fact may be interpreted in terms of the high mixing capacity due to moderate winds blowing in all directions. Photochemical processes bear a minor role at the sites of Belgrano Avenue and the University Campus and, expectedly, in the whole urban area. The concentrations of O were always low, even in Summer when high radiation values are expected. The low levels of O are due to the high concentration of NO prevailing during the day. A highly repetitive daily CO (and accordingly NO ) V pro"le is obtained for periods where the tra$c distribution remains almost constant. This feature allowed
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Fig. 7. Monthly average values of CO concentration measured in Belgrano Avenue between July 1996 and June 1997. The monthly average values of wind speed considering the period 1981}1990 (measured at Villa OrtuH zar) are shown in the same "gure. The calculated monthly averaged atmospheric boundary layer altitude (Mazzeo and Gassmann) is also represented.
Table 3 Values of the CO average concentrations (1[CO]2), average wind speed (<) for each category group (I, II, II and IV, see text for details), and number of days (N)
Workdays
Saturdays
Sundays
1[CO]2 (ppmV) 1< 2 (km/h) N 1[CO]2 (ppmV) 1< 2 (km/h) N 1[CO]2 (ppmV) 1< 2 (km/h) N
I
II
III
IV
4.91 3.5 5
2.92 8.1 31 2.69 9.1 7 1.59 8.7 9
2.38 14.3 71 1.74 14.3 10 1.19 14.3 7
1.82 20.7 6
Data collected in Belgrano Avenue between 18th June and 15th December 1996.
to perform a statistic analysis of the CO daily pro"le, classifying the days according to the associated value of the average wind velocity, < . This analysis renders the possibility of predicting CO hourly concentrations for a given day, by giving only its < value.
Acknowledgements This work was sponsored by the Gesellshaft fuK r Technische Zusammenarbeit (Germany). We wish to thank the collaboration of the National Meteorological Service,
H. Bogo et al. / Atmospheric Environment 33 (1999) 2587}2598
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Fig. 8. Fits of CO concentration for workdays of categories II (a) and III (b), using two Gaussian functions ("tting parameters are shown in Table 4). The error bars represent the standard deviation of the experimental data for each hour. Data were collected in Belgrano Avenue between 18 June and 15 December 1996. Table 4 CO concentration "tting parameters based on Eq. (1) for each category group (I, II, II and IV, see text for details)
[CO] (ppmV) k (h) p (h) A (ppmV h) k (h) p (h) A (ppmV h)
I 1.7$0.3 9.6$0.2 2.4$0.2 34 $4 19.1$0.2 2.9$0.3 45 $5
II 0
III 0
IV 0
9.9$0.2 2.4$0.2 27 $2 19.0$0.2 3.5$0.2 44 $2
9.7$0.2 2.1$0.1 18 $1 18.5$0.2 3.8$0.2 39 $2
9.7$0.2 1.8$0.4 11 $2 17.1$1 4.4$0.4 31 $3
Data collected in Belgrano Avenue between 18th June and 15th December 1996 (only workdays are considered).
in particular to Dr E. Piacentini; to Dr A. Flores from the Department of Atmospheric Sciences, School of Sciences, University of Buenos Aires; and to Ing. HeH ctor Collado of the National Transport Secretary. We wish to thank also to Dr R.J. FernaH ndez Prini for his constant support and collaboration while preparing this manuscript and to Dr P. F. AramendmH a and Dr G. Gordillo for discussions on earlier measurements. E.S.R. and R.M.N. are members of the Consejo Nacional de Investigaciones CientmH "cas y TeH cnicas.
References AramendmH a, P.F., FernaH ndez Prini, R.J., Gordillo, G., 1995. Buenos Aires en Buenos Aires? Ciencia Hoy (Argentina) 6, 55}64.
Bower, J.S., Broughton, G.F.J., Stedman, J.R., 1994. A winter NO smog episode in UK. Atmospheric Environment 28, 461}475. Campbell, G.W., Stedman, J.R., Stevenson, K., 1994. A survey of nitrogen dioxide concentrations in the United Kingdom using di!usion tubes, July}December 1991. Atmospheric Environment 28, 477}486. CETESB, 1994. Relatorio de Qualidade do Aire no Estado de Sa o Paulo (CETESB) Companhia de Tecnologia de Saneamento Ambiental, Secretaria do Meio Ambiente, Brazil. Economopoulos, A.P., 1993. A Guide to rapid source inventory techniques and their use in formulating environmental control strategies. Environmental Technology Series Assesment of Sources of Air, Water and Land Pollution. World Health Organization (WHO/PEP/GETNET/93.1-A and 93.1-B). Ferm, M., 1991. A sensitive di!usional sampler. Report of the Swedish Environmental Research Institute, GoK teborg.
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Fochesatto, J., 1997. CEILAP, Argentina, private communication. Goldan, P.D., Trainer, M., Kuster, W.C., Parrish, D.D., Carpenter, J., Roberts, J.M., Yee, J.E., Fehsenfeld., 1995. Measurements of hydrocarbons, oxygenated hydrocarbons, carbon monoxide, and nitrogen oxides in an urban basin in Colorado: implications for emission inventories. Journal of Geophysics Research 100, 22 771}22 783. Heal, M.R., Cape, J.N., 1997. A numerical evaluation of chemical interferences in the measurement of ambient nitrogen dioxide by passive di!usion samplers. Atmospheric Environment 31, 1911}1923. Lavorato, M., Fochesatto, J., Quel, E., Flamant, P.H., Pelon, J., 1996a. Monitoring of cirrus, clouds and planetary layer in Southern Hemisphere at Buenos Aires for climate application in: Asmann, A., Neuber, R., Rairoux, P., Wandinger, U. (Eds.), Advances in Atmospheric Remote Sensing with Lidar. Selected papers of the 18th International Laser Radar Conference (ILRC), Berlin. Lavorato, M., Fochesatto, J., Quel, E., 1996. Preliminary backscatter lidar measurements of atmospheric boundary layer and cirrus clouds in Buenos Aires. Proc. of I.G.A.R.S.S. 1, 4}6.
MaH rsico, A.D., 1974. Estudio de las condiciones de higiene del aire de la ciudad de Buenos Aires. Editorial de la Universidad de Buenos Aires (Eudeba). Argentina. Mazzeo, N.A., Gassmann, M.I., 1990. Mixing heights and wind direction analysis of urban and suburban areas of Buenos Aires city. Energy and Buildings 15}16, 333}337. Moretton, J., 1996. ContaminacioH n del aire en la Argentina. Editorial Universitaria. Argentina. Seinfeld, J.H.., 1986. Atmospheric Chemistry and Physics of Air Pollution. Wiley, New York. Seinfeld, J.H., 1995. Chemistry of ozone in the urban and regional atmosphere. In: Barker, J.R., (Ed.), Progress and Problems in Atmospheric Chemistry. Advances Series in Physical Chemistry, vol. 3. World Scienti"c, Singapore. Sillman, S., 1995. New developments in understanding the relation between ozone, NOx and hydrocarbons in urban atmospheres. In: Barker J.R., (Ed.), Progress and Problems in Atmospheric Chemistry. Advances Series in Physical Chemistry, vol.3, World Scienti"c, Singapore. SWEPA, 1990. Light Vehicles and Cleaner Air. More Stringent Emission Requirements, Report 3750, Sweden. Ulke, A.G., Mazzeo, N.A., 1998. Climatological aspects of the daytime mixing height in Buenos Aires city, Argentina. Atmospheric Environment 32, 1615}1622.
Continuous measurement of gaseous pollutants in Buenos Aires city Horacio Bogo, R. MartmH n Negri, Enrique San RomaH n* INQUIMAE and Departamento de Qun& mica Inorga& nica, Analn& tica y Qun& mica Fn& sica, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Ciudad Universitaria, Pabello& n II, 1428 Buenos Aires, Argentina
Abstract Data on CO, NO, NO and O concentrations measured in Buenos Aires city using a continuous monitoring station are reported. This is the "rst systematic study of this kind carried out in the city, which is, together with its surroundings, the third more populated in Latin America. Measurements were performed during 12 months in one of the principal avenues near downtown. Results indicate that vehicular tra$c is the principal source of CO and NO . The concentration V of O is generally quite low and results from the mixing of clean air masses with exhaust gases containing high amounts of NO. The monthly averages of CO and NO decrease from Winter to Summer in correlation with the increase of the mean wind speed and average temperature. These results are compared with previous measurements on the spatial distribution of NO in the whole city using passive di!usion tubes and with the concentration of CO, which is being continuously registered since several years in the downtown area. Measurements performed at a green, windy, low tra$c area beneath the La Plata river are also shown. 1999 Elsevier Science Ltd. All rights reserved. Keywords: Urban pollution; Mobile sources; Carbon monoxide; Nitrogen oxides; Ozone
1. Introduction Buenos Aires city and surroundings occupy roughly a semicircle of 50 km radius centered at 34334 S and 58325 W at an altitude of ca. 20 m with an approximate population of 13.8 million inhabitants. It is one of the three megalopolises in Latin America, including Mexico city and Sa o Paulo in Brazil. In contrast to these cities, Buenos Aires lies on a vast plain area and is limited by the La Plata river, whose width near the city is around 50 km. The federal district is mainly an urban area of
* Corresponding author. [email protected].
Fax: #54 1 576 3341;
e-mail:
near 15 km diameter (see Fig. 1) populated by 2.9 million people. The mean population density reaches 14 000 inh km\ in the federal district and decreases to near 2000 inh km\ in the surrounding area. Thermal power stations are located at the side of the river. Very few industries are located within the city, but industries of several kinds including leather, oil, chemicals, textile, etc. can be found in the surrounding area. The climate is temperate with annual mean temperatures of 183C in the city and 153C in the surroundings (average values for the period 1981}1991 measured by the National Meteorological Service). The precipitation regime is 900} 1600 mm yr\, in#uenced by wet winds from the Atlantic Ocean. The federal district is circulated mainly by motor cars and public transport buses, which are generally not equipped with exhaust catalyzers. Only in this area the
1352-2310/99/$ - see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 9 8 ) 0 0 2 7 0 - 2
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city. Previous monitoring of NO and SO (AramendmH a et al., 1995) shows nearly constant average concentrations within the whole city with the exception of the downtown area, where averages are comparatively higher. Therefore, most of the data were collected during 12 months at one of the principal avenues near downtown representative of this general behavior. Previously, some data were obtained at the University Campus, a green, windy, low tra$c area at the side of the La Plata river. The main purpose of this "rst e!ort was to obtain information on primarily emitted pollutants (although ozone was also measured) and not on their fate, as could be the case of studies performed in other cities of the world where primary emissions are well characterized (Seinfeld, 1995; Sillman, 1995). As a result, correlations between measured pollutant concentrations, meteorological parameters of relevance and tra$c pro"les were obtained and will be discussed in detail.
2. Experimental
Fig. 1. Buenos Aires city and surroundings, showing the position of the University Campus (1), station at Belgrano Avenue (2), the meteorological station of Villa OrtuH zar (3) and downtown area (4).
number of registered vehicles is about 1.5 millions. A small area inside downtown is restricted only to public transport during most of the daily hours. Tra$c in and out from the city circulates through moderately wide avenues and small streets. Rush hours are around 8}10 a.m. and 5}8 p.m. During rush hours a signi"cant lowering of mean speed is observed and, though automatic signaling is in general well regulated, frequent stops take place. Aside from some earlier measurements (MaH rsico, 1974), a screening of NO and SO concentrations deter mined using passive tubes (AramendmH a et al., 1995) and data on CO concentrations regularly measured by a private foundation at one location inside downtown (results are published currently in local newspapers but no comprehensive reports are available) there is no systematic control of the various atmospheric pollutants in the city of Buenos Aires. This situation is re#ected in a recent publication of local circulation (Moretton, 1996). Accordingly, the aim of the present work was to perform a systematic monitoring of several atmospheric pollutants (CO, NO, NO and O ) measured simulta neously and continuously at selected locations within the
A fully automatic station composed of three independent instruments for the determination of CO, NO}NO and O was used. CO was measured by nondispersive infrared photometry using a gas "lter correlation instrument (Monitor Labs, ML 9830). NO and NO were measured separately by gas-phase chemiluminescence detection (Monitor Labs, ML 9841A). O was measured by UV absorption at 254 nm (Monitor Labs, ML 9812). Calibration was performed following US-EPA (Environmental Protection Agency, USA) and Monitor Labs Inc. standard procedures and recommendations (see Code of Federal Regulations, US-EPA, Subchapter C: Air Programs (1993), Part 50, pp. 697}777 and Monitor Labs User Manuals, Parts C98300068, C98412026 and C98120004, respectively). For the calibration of the ML 9830 and ML 9841A instruments, Scott CO (10 ppmV$ 2%) and NO (10 ppmV$5%) calibration gases were used. The NO measuring system was calibrated by gas phase titration of NO with O generated by a Hg low pressure UV lamp. The ozone measuring instrument was calibrated by biamperometric titration of O following the generation of I from I\ in the presence of S O\ at pH"7. Ozone was generated using the same mercury lamp and the #ow of O into the biamperometrical cell was measured simultaneously with the I determination. Data were recorded primarily as one-minute averages. A personal computer served simultaneously to perform instrumental control and data storage. The system was initialized daily at 12.00 a.m. and, while measuring near downtown, a control worksheet was sent automatically to the University Campus. Data from the various instruments were sequentially recorded and stored. Periodically, data were downloaded, averaged and analyzed.
H. Bogo et al. / Atmospheric Environment 33 (1999) 2587}2598
The measurement station was placed from July 1996, beginning of the winter season, to June 1997, in a building at Belgrano Avenue, one of the principal streets of Buenos Aires, in a populated region near downtown (Fig. 1). This avenue has synchronized tra$c lights and the circulation of vehicles is normally fast. Measurements were performed at a horizontal distance of approximately 4 m from the tra$c line. A previous series of measurements were performed from October to December 1995 at the Campus of the University of Buenos Aires, away from the more populated region (Fig. 1). At both measurement sites air sampling was performed at a height of 10 m from the ground. Tra$c circulation was measured as hourly averages at the monitoring station in Belgrano Avenue during the second and third weeks of September 1996. Measurements were performed by the Buenos Aires Urban Transportation Project of the National Transport Secretary using an automatic recording system. With the exception of January and February, tra$c circulation can be regarded as constant throughout the year both in regard to the number and distribution of vehicles. During Summer the number of cars is signi"cantly lower, but quantitative data are not available at present. Relevant meteorological parameters, namely hourly values of temperature and wind velocity and direction, were taken into account on analyzing chemical data. Two meteorological stations are managed within the city by the National Meteorological Service, one located in Villa OrtuH zar (Fig. 1), well within the urban area, and the other at Aeroparque, the local airport nearby the La Plata River. Data from the "rst station was used for the analysis at Belgrano Avenue. Although it is possible that some local e!ects could not be well represented, it has been tested that general trends, such as seasonal variation of the monthly averaged wind speed or the calculated atmospheric boundary layer altitude, are similar when data from the Villa OrtuH zar or the Aeroparque meteorological stations are taken into account (Mazzeo and Gassmann, 1990). For the analysis of the results obtained at the University Campus, meteorological data were locally measured by the Department of Atmospheric Sciences of the School of Sciences. Di!usion tube results on the concentration of NO and SO were published elsewhere (AramendmH a et al., 1995) and are reproduced in Fig. 2. Plastic cylinders (length: 71 mm; i.d. 10 mm) open to the atmosphere in one extreme and "tted with a "lter with suitable reagents at the other extreme were used (Heal and Cape, 1997; Campbell et al., 1997; Ferm, 1991). These tubes were placed in 21 sites at an average height of 10 m. Except for some locations as the University Campus, the altitude of 10 m is well below the mean altitude of most apartment buildings within the city.
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Fig. 2. Map of Buenos Aires city showing the location of the di!usion pipes. Concentration of NO (grey bars) and SO (black bars) are represented by the height of the corresponding stabs; maximum concentrations are 46.6 ppbV for NO and 7 ppmV for SO . Measurements were performed as monthly averages from May to June 1994.
3. Results and discussion 3.1. University Campus Monthly averaged concentrations of O , CO, NO and NO measured during Spring at the University Campus are shown in Table 1. The very low amounts found for CO, NO and NO can be explained by the proximity of the La Plata river together with the existence of prevailing North and East winds blowing from cleaner areas. A detailed analysis of hourly values shows O peaks reaching 0.030}0.050 ppmV for most of the days, within the limits typical for a clean atmosphere (Seinfeld, 1986). During the measurement period the maximum peak concentration of O was around 0.060 ppmV near noon and occurred only once. This observation was coincident with a NO /NO ratio greater than unity, low values of NO and northeast wind. In Fig. 3 it is shown that V NO /NO'1 for North and northeast winds. NO con centrations increase when the wind speed decreases or
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when the wind blows from the city area, yielding NO /NO(1. The resulting monthly average values of NO and NO are very similar, as shown in Table 1. The features shown in Fig. 3. are typical of clean air incidence together with local perturbations. The O con centrations are relatively high during the night and until noon when wind blows from the North, because the incoming air is cleaner. At evening the wind speed de-
Table 1 Monthly average concentrations at the University Campus Month
October 1995 November 1995 December 1995
O (ppmV)
CO (ppmV)
NO (ppmV)
NO (ppmV)
0.013 0.013 0.018
0.67 0.44 0.29
0.014 0.005 0.003
0.010 0.004 0.004
creases and the NO concentration increases while the O concentration falls down due to titration. The same e!ect is observed when wind carries polluted air from the city area, causing an increase of NO and CO. In this case values of O below the instrumental sensitivity (0.002 ppmV) are obtained during workdays at rush hours. As a result, the average concentration of O is lower than the typical values for a clean atmosphere. Though the existing data does not allow to establish a general trend, Table 1 shows decreasing values for the average concentration of primary pollutants and NO and an increasing concentration of O on going from October to December, in agreement with the seasonal increase of wind speed (see later). 3.2. Belgrano Avenue Monthly averaged values of O , CO, NO and NO registered at Belgrano Avenue are shown in Table 2. Comparison of data obtained at this site with data meas-
Fig. 3. Concentrations of O (䊊), CO (䊐), NO (䉲) and NO (䉬), represented as hourly averages, measured at the University Campus on Friday 20 October 1995. Wind direction and speed, measured at the University Campus, are also shown.
H. Bogo et al. / Atmospheric Environment 33 (1999) 2587}2598 Table 2 Monthly average concentrations at Belgrano Avenue Month
O (ppmV)
CO (ppmV)
NO (ppmV)
NO (ppmV)
July 1996 August 1996 September 1996 October 1996 November 1996 December 1996 January 1997 February 1997 March 1997 April 1997 May 1997 June 1997
0.002 0.003 0.004 0.004 0.005 0.005 0.004 0.004 0.003 0.002 0.001 0.002
2.92 2.69 2.24 2.21 1.93 1.93 1.65 1.45 2.35 2.41 2.54 2.46
0.150 0.124 0.110 0.101 0.072 0.074 0.069 0.076 0.109 0.111 0.133 0.143
0.040 0.041 0.035 0.036 0.029 0.033 0.029 0.026 0.034 0.036 0.037 0.033
ured at the University Campus is not straightforward as measuring periods do not overlap. Nevertheless, Table 2 shows a regular trend characterized by a decrease of average concentrations of CO, NO and NO from July 1996 to January or February followed by an increase until average concentrations found in June 1997 approximately match those found at the beginning. Comparison of data obtained during Spring shows CO values three to six times larger and NO and NO values up to one order of magnitude larger at the Belgrano Avenue. The concentration of O at the last site is one order of magnitude smaller, normally below the instrumental sensitivity. As tra$c circulation does not change systematically between June and December, the lowering of the concentration of primary pollutants may be attributed to an increased air mixing capacity due to a higher wind speed and to the increase of the inversion layer altitude. The monthly vectorially averaged wind velocity calculated from data recorded at the Villa OrtuH zar observatory increased steadily from July 1996 to December 1996 from 0.2 to 2.2 knots (0.37}4.11 km h\) and the same holds normally for the calculated monthly average values of the atmospheric boundary layer altitude (Ulke and Mazzeo, 1998; Mazzeo and Gassmann, 1990; Fochessato, 1997; Lavorato et al., 1996a and b). Additionally, the emission factors for CO in non-catalytic light duty gasoline powered vehicles decrease signi"cantly with temperature (Economopoulos, 1993). In general, the pro"les of CO, NO and NO found between 7 a.m. and 8 p.m. di!er substantially between workdays and weekends. During weekends the concentration of these gases is much lower and the time distribution is much more di!use. On the other hand, the concentration of O do not vary signi"cantly between workdays and weekends and is always less than 0.015 ppmV, a very low value compared to clean air
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concentrations. Fig. 4a and b show the concentrations of CO, NO, NO and O at Belgrano Avenue in a weekend day and in a normal workday during Winter, respectively. A moderate breeze was present on both days. During the night, the concentrations of all gases except O drop to very low values, showing a rapid removal of pollutants. Only in a few days of low wind speed the 8 h average concentration of CO was higher than 9 ppmV, the "rst alarm value of US-EPA. High concentrations of NO were found also during those days. In particular, 17 July and 18 July represent the only case of two consecutive days with almost no breeze in the reported period. The concentrations of CO, NO, NO and O for July 18 are shown in Fig. 4c. It may be noticed that the memory of the last day is retained in this case, leading to a cummulative e!ect in the concentration of all pollutants. In the general case, however, the concentrations of CO and NO are very low at night and start to increase at V about 7 a.m. During workdays a maximum is observed in the morning, followed by a decrease toward the afternoon. A new increase is then observed, reaching a second maximum during the evening. The pattern described for Belgrano Avenue is almost coincident with the tra$c pro"le modulated by the wind speed (see Fig. 5). The two peaks obtained in the CO and NO daily pro"les during V workdays correspond to rush hours. This trend was also observed in previous CO measurements performed downtown (AramendmH a et al., 1995) although in that case much higher values were obtained because the tra$c level was also much higher and the streets narrower at that site. Data corresponding to 23 September 1996 are shown in Fig. 5. Southeast winds prevailed during the whole day, blowing clean air masses from the river into the city. As it is seen from the "gure, there is a strong correlation between the wind speed and the departure between the concentration of CO and the tra$c curve. High wind speeds cause the CO curve to lay below the tra$c curve and viceversa (notice that the area under both curves is approximately equal). This strongly suggests that CO levels are due principally to vehicular emissions. The concentrations of CO and NO correlate strongly V during the whole measurement period. As an example, Fig. 6 represents the correlation obtained during July 1996. A linear scatter diagram with zero intercept is obtained, which is interpreted considering that vehicular emissions are the only source of NO in this location and V that fresh pollution with little or no photochemical conversion is measured (Goldan et al., 1995). The measured values of the CO/NO ratio are in the expected range for V vehicular emissions (see CETESB, 1994 and SWEPA, 1990). As emissions are highly dependent on engine type, tra$c conditions and temperature (Bower et al., 1994), it is very di$cult to make a right prediction of the CO/NO V ratio in Buenos Aires because of the lack of information,
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Fig. 4. Concentrations of O (䊊), CO (䊐), NO (䉲) and NO (䉬) measured at the station of Belgrano Avenue, represented as hourly averages, for: (a) a weekend with moderated breeze (Sunday 21 July 1996). Average wind intensity about 4 knot. (b) a workday with moderated breeze (Friday 5 July 1996). Average wind intensity about 5 knot. (c) a workday with almost no breeze at all (Thursday 18 July 1996). Average wind intensity about 1 knot.
H. Bogo et al. / Atmospheric Environment 33 (1999) 2587}2598
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Fig. 4. (continued)
statistics and car testings. In particular, measurements on the CO/NO ratio at the exhaust of vehicles have not V been performed under typical driving conditions in Buenos Aires city. Monthly average concentrations of CO from July 1996 to July 1997 are plotted in Fig. 7 as a function of time, showing the annual cyclic behavior. In the same "gure the wind average speed values and the averaged values of the calculated atmospheric boundary layer altitude are presented (Mazzeo and Gassmann, 1990). It is observed that the monthly averaged values of CO are well correlated with the atmospheric boundary layer altitude and less strongly with wind average speeds. From July to December the wind speed and the calculated atmospheric boundary layer altitude increase almost monotonically. Then, a statistical study of the daily time dependence of CO was performed against the actual wind velocities in the following way. Days from 18 June 1996 to 15 December 1996 were classi"ed in four categories according to the value of the average wind speed (< ), taken between 7 a.m. and 11 p.m. where the concentration of CO is relatively important. Thus, a value of < is assigned to each day. The categories are the following: I: < (5.5 km h\; II: 5.5)< ( 11.5 km h\; III: 11.5)< (19.5 km h\; IV: < *
19.5 km h\. This classi"cation is based on the Beaufort meteorological scale, although a coarser description considering only four category groups was used in order to facilitate the calculations, to improve statistics and to make clear the analysis and discussion. Days were classi"ed into workdays, Saturdays and Sundays. The average values of < that were obtained for the grouped categories are shown in Table 3 (these values are referred as a double average, 1< 2). The distributions of the hourly values of CO are very di!erent for workdays, Saturdays and Sundays. Most Saturdays and Sundays show not well de"ned peaks, re#ecting a more or less random tra$c circulation. On Sundays, concentrations are much lower than on Saturdays and the hourly distribution is even more di!use. Hence, the following study was performed only for workdays, for which the data of each category group (I, II, III and IV) were analyzed. The hourly averaged values of CO for each category group were "tted by the sum of two Gaussian functions: [CO](t)"[CO] A (t!k ) G . G # exp ! 2p 2p (n/2 G G G
(1)
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Fig. 5. Tra$c (䊉) and concentration of carbon monoxide (䊐) for 23 September 1996, measured at the station of Belgrano Avenue. The wind speed (䉬), measured at Villa Ortuzar, is also shown.
Workday data belonging to category groups II and III are shown in Figs. 8a and b, respectively. In practice periodical behavior was assumed and four Gaussians were used to "t a 48 h period. Two of the Gaussians were forced to have the same parameters as the other two. Values of k , p and A were obtained for each Gaussian G G G function and shown in Table 4. It is essential to analyze the hypotheses used in the foregoing analysis. First, we have considered only the wind speed as the meteorological parameter of relevance. Second, the variation of wind speed along the day was not considered. Taking into account these approximations, the "ts shown in Fig. 8a and b may be used as predictive tools for daily averaged CO concentration values for a workday of a given class. The "ts are statistically representative of the time variation of CO concentration for those days. The two values of k for each category are coincident G with rush hours. Non-zero values of [CO] were re
covered only for category I, which is associated to the persistence of CO emitted the day before when the wind speed is low. The recovered values of p are almost coincident for the four categories. p is always larger than p and although its values are alike for the di!erent categories, a slight increase with the wind speed was noticed. The ratio A /A follows the ratio p /p as the peak values of both Gaussians are very similar. The concentration of O measured at Belgrano Av enue is typically less than 0.010 ppmV, always very low compared to clean air concentrations due to titration of O by NO. Because of the very low values obtained for O , it is ventured to establish correlations between O and NO or CO. Average concentrations of NO and SO taken over 40 days were reported for the sites shown in Fig. 2 during May } June 1994 (AramendmH a et al., 1995). Values ranged between 0.027 and 0.047 ppmV for NO and 0.002 and
H. Bogo et al. / Atmospheric Environment 33 (1999) 2587}2598
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Fig. 6. Concentration of CO versus NO for July 1996 measured at Belgrano Avenue. V
0.007 ppmV for SO . The very low values registered for SO are in agreement with the low level of sulfur found in local gasolines. The values reported for NO are in excel lent agreement with the monthly averages obtained at Belgrano Avenue. The highest values correspond to the pipes located downtown in areas with the highest tra$c levels. However, even for the areas with the lowest tra$c, these values are not far from those found downtown. These results are consistent with the presence of changing winds causing a continuous mixing of air masses.
E E
4. Conclusions The reported data on CO, NO, NO and O concen trations constitute the "rst systematic study of atmospheric pollutants in the city of Buenos Aires, obtained continuously with a fully automatic station. On the basis of the results described in the previous section, the next main conclusions emerge: E The concentration values of all the species considered are a result of the mixing of vehicular emissions with clean air masses. Vehicular emissions are undoubtedly the main source of CO and NO . The relatively high V
E
E
wind speed and the #at geography of the city do not allow the accumulation of pollutants during most days. The situation can be of concern for the case of consecutive days without breeze, as it happened once in Winter, where 8 h average concentration of CO higher than 9 ppmV were observed at an altitude of 10 m over a relatively wide avenue. The atmosphere is cleaned up during the night, when the tra$c stops and, as a general rule, every morning a renewed atmosphere is observed. The spatial distribution of the average concentrations of SO and NO measured using di!usion tubes shows no substantial variations over the city. This fact may be interpreted in terms of the high mixing capacity due to moderate winds blowing in all directions. Photochemical processes bear a minor role at the sites of Belgrano Avenue and the University Campus and, expectedly, in the whole urban area. The concentrations of O were always low, even in Summer when high radiation values are expected. The low levels of O are due to the high concentration of NO prevailing during the day. A highly repetitive daily CO (and accordingly NO ) V pro"le is obtained for periods where the tra$c distribution remains almost constant. This feature allowed
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Fig. 7. Monthly average values of CO concentration measured in Belgrano Avenue between July 1996 and June 1997. The monthly average values of wind speed considering the period 1981}1990 (measured at Villa OrtuH zar) are shown in the same "gure. The calculated monthly averaged atmospheric boundary layer altitude (Mazzeo and Gassmann) is also represented.
Table 3 Values of the CO average concentrations (1[CO]2), average wind speed (<) for each category group (I, II, II and IV, see text for details), and number of days (N)
Workdays
Saturdays
Sundays
1[CO]2 (ppmV) 1< 2 (km/h) N 1[CO]2 (ppmV) 1< 2 (km/h) N 1[CO]2 (ppmV) 1< 2 (km/h) N
I
II
III
IV
4.91 3.5 5
2.92 8.1 31 2.69 9.1 7 1.59 8.7 9
2.38 14.3 71 1.74 14.3 10 1.19 14.3 7
1.82 20.7 6
Data collected in Belgrano Avenue between 18th June and 15th December 1996.
to perform a statistic analysis of the CO daily pro"le, classifying the days according to the associated value of the average wind velocity, < . This analysis renders the possibility of predicting CO hourly concentrations for a given day, by giving only its < value.
Acknowledgements This work was sponsored by the Gesellshaft fuK r Technische Zusammenarbeit (Germany). We wish to thank the collaboration of the National Meteorological Service,
H. Bogo et al. / Atmospheric Environment 33 (1999) 2587}2598
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Fig. 8. Fits of CO concentration for workdays of categories II (a) and III (b), using two Gaussian functions ("tting parameters are shown in Table 4). The error bars represent the standard deviation of the experimental data for each hour. Data were collected in Belgrano Avenue between 18 June and 15 December 1996. Table 4 CO concentration "tting parameters based on Eq. (1) for each category group (I, II, II and IV, see text for details)
[CO] (ppmV) k (h) p (h) A (ppmV h) k (h) p (h) A (ppmV h)
I 1.7$0.3 9.6$0.2 2.4$0.2 34 $4 19.1$0.2 2.9$0.3 45 $5
II 0
III 0
IV 0
9.9$0.2 2.4$0.2 27 $2 19.0$0.2 3.5$0.2 44 $2
9.7$0.2 2.1$0.1 18 $1 18.5$0.2 3.8$0.2 39 $2
9.7$0.2 1.8$0.4 11 $2 17.1$1 4.4$0.4 31 $3
Data collected in Belgrano Avenue between 18th June and 15th December 1996 (only workdays are considered).
in particular to Dr E. Piacentini; to Dr A. Flores from the Department of Atmospheric Sciences, School of Sciences, University of Buenos Aires; and to Ing. HeH ctor Collado of the National Transport Secretary. We wish to thank also to Dr R.J. FernaH ndez Prini for his constant support and collaboration while preparing this manuscript and to Dr P. F. AramendmH a and Dr G. Gordillo for discussions on earlier measurements. E.S.R. and R.M.N. are members of the Consejo Nacional de Investigaciones CientmH "cas y TeH cnicas.
References AramendmH a, P.F., FernaH ndez Prini, R.J., Gordillo, G., 1995. Buenos Aires en Buenos Aires? Ciencia Hoy (Argentina) 6, 55}64.
Bower, J.S., Broughton, G.F.J., Stedman, J.R., 1994. A winter NO smog episode in UK. Atmospheric Environment 28, 461}475. Campbell, G.W., Stedman, J.R., Stevenson, K., 1994. A survey of nitrogen dioxide concentrations in the United Kingdom using di!usion tubes, July}December 1991. Atmospheric Environment 28, 477}486. CETESB, 1994. Relatorio de Qualidade do Aire no Estado de Sa o Paulo (CETESB) Companhia de Tecnologia de Saneamento Ambiental, Secretaria do Meio Ambiente, Brazil. Economopoulos, A.P., 1993. A Guide to rapid source inventory techniques and their use in formulating environmental control strategies. Environmental Technology Series Assesment of Sources of Air, Water and Land Pollution. World Health Organization (WHO/PEP/GETNET/93.1-A and 93.1-B). Ferm, M., 1991. A sensitive di!usional sampler. Report of the Swedish Environmental Research Institute, GoK teborg.
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H. Bogo et al. / Atmospheric Environment 33 (1999) 2587}2598
Fochesatto, J., 1997. CEILAP, Argentina, private communication. Goldan, P.D., Trainer, M., Kuster, W.C., Parrish, D.D., Carpenter, J., Roberts, J.M., Yee, J.E., Fehsenfeld., 1995. Measurements of hydrocarbons, oxygenated hydrocarbons, carbon monoxide, and nitrogen oxides in an urban basin in Colorado: implications for emission inventories. Journal of Geophysics Research 100, 22 771}22 783. Heal, M.R., Cape, J.N., 1997. A numerical evaluation of chemical interferences in the measurement of ambient nitrogen dioxide by passive di!usion samplers. Atmospheric Environment 31, 1911}1923. Lavorato, M., Fochesatto, J., Quel, E., Flamant, P.H., Pelon, J., 1996a. Monitoring of cirrus, clouds and planetary layer in Southern Hemisphere at Buenos Aires for climate application in: Asmann, A., Neuber, R., Rairoux, P., Wandinger, U. (Eds.), Advances in Atmospheric Remote Sensing with Lidar. Selected papers of the 18th International Laser Radar Conference (ILRC), Berlin. Lavorato, M., Fochesatto, J., Quel, E., 1996. Preliminary backscatter lidar measurements of atmospheric boundary layer and cirrus clouds in Buenos Aires. Proc. of I.G.A.R.S.S. 1, 4}6.
MaH rsico, A.D., 1974. Estudio de las condiciones de higiene del aire de la ciudad de Buenos Aires. Editorial de la Universidad de Buenos Aires (Eudeba). Argentina. Mazzeo, N.A., Gassmann, M.I., 1990. Mixing heights and wind direction analysis of urban and suburban areas of Buenos Aires city. Energy and Buildings 15}16, 333}337. Moretton, J., 1996. ContaminacioH n del aire en la Argentina. Editorial Universitaria. Argentina. Seinfeld, J.H.., 1986. Atmospheric Chemistry and Physics of Air Pollution. Wiley, New York. Seinfeld, J.H., 1995. Chemistry of ozone in the urban and regional atmosphere. In: Barker, J.R., (Ed.), Progress and Problems in Atmospheric Chemistry. Advances Series in Physical Chemistry, vol. 3. World Scienti"c, Singapore. Sillman, S., 1995. New developments in understanding the relation between ozone, NOx and hydrocarbons in urban atmospheres. In: Barker J.R., (Ed.), Progress and Problems in Atmospheric Chemistry. Advances Series in Physical Chemistry, vol.3, World Scienti"c, Singapore. SWEPA, 1990. Light Vehicles and Cleaner Air. More Stringent Emission Requirements, Report 3750, Sweden. Ulke, A.G., Mazzeo, N.A., 1998. Climatological aspects of the daytime mixing height in Buenos Aires city, Argentina. Atmospheric Environment 32, 1615}1622.