Elevated PM10 concentrations and high PM2.5/PM10 ratio in the Brussels Urban Area during the 2006 car-free Sunday

Transcription

1 264 Int. J. Environment and Waste Management, Vol. 6, Nos. 3/4, 2010 Elevated PM10 concentrations and high PM2.5/PM10 ratio in the Brussels Urban Area during the 2006 car-free Sunday P. Vanderstraeten*, M. Forton, Y. Lénelle and A. Meurrens IBGE-BIM. Brussels Institute for Environmental Management, Laboratory for Environmental Research, Gulledelle 100, B-1200 Brussels, Belgium Fax: pvd@ibgebim.be mfo@ibgebim.be yle@ibgebim.be ame@ibgebim.be *Corresponding author D. Carati and L. Brenig ULB. Université Libre de Bruxelles, ULB- Campus Plaine, CP.231, Boulevard du Triomphe, B-1050 Brussels, Belgium Fax: dcarati@ulb.ac.be lbrenig@ulb.ac.be Z.Y. Offer BGU. Ben Gurion University, Sede Boker Campus, 84990, Israel Fax: offer@bgu.ac.il E. Zaady Gilat Research Center Department of Natural Resources, Mobile Post Negev 85280, Israel zaadye@volcani.agri.gov.il Abstract: The results of the Car-Free Sundays in Brussels demonstrate the close relationship between the traffic and the presence of the gaseous pollutants such as NO, NO 2, CO, CO 2 and O 3 in a traffic environment. The relationship with PM10 and PM2.5 seems to be much more complex. In fact, the PM10 and Copyright 2010 Inderscience Enterprises Ltd.

2 Elevated PM10 concentrations and high PM2.5/PM10 ratio 265 PM2.5 concentrations on the Car-Free Sunday 2006 were amongst the highest that year and they were three times higher than on an average Sunday or working day. Analysis of the data indicate that direct particle emissions from traffic only have a limited contribution to the overall PM concentration. Keywords: car-free day; airborne particulates; PM10; PM2.5; secondary aerosol; ratio PM2.5/PM10; air quality and traffic. Reference to this paper should be made as follows: Vanderstraeten, P., Forton, M., Lénelle, Y., Meurrens, A., Carati, D., Brenig, L., Offer, Z.Y. and Zaady, E. (2010) Elevated PM10 concentrations and high PM2.5/PM10 ratio in the Brussels Urban Area during the 2006 car-free Sunday, Int. J. Environment and Waste Management, Vol. 6, Nos. 3/4, pp Biographical notes: Peter Vanderstraeten has a University Degree in Chemical Engineering, University of Ghent, in 1975, and has more than 30 years experience with air-quality monitoring and air quality data analysis. His main interests are the relation between air quality and traffic. He is senior scientist at the IBGE-BIM and responsible for the air quality network in the Brussels Capital Region. He is the author of about 100 reports on air quality and about 35 scientific contributions at national and international conferences and in scientific journals. He works under the supervision of Y. Lénelle and A. Meurrens. Michael Forton has a University Degree in Geological Sciences, University of Brussels, in 1998, and a postgraduate in the Physical and Chemical Sciences of the Atmosphere in He is a Junior Scientist at the Laboratory for Environmental Research of the IBGE-BIM and assistant to Peter Vanderstraeten. His main interests are the relation between air quality and traffic, data analysis and air-quality monitoring. Yves Lénelle has a University Degree in Chemical Sciences, University of Liège, in He is the Head of the Department Laboratory at the IBGE-BIM and responsible for Quality Control. He has more than 30 years experience with different analytical techniques in the field of Air Pollution Control. His main points of interest are the presence of PAH and trace metals in particulates and VOC in indoor air pollution. Annick Meurrens has a University Degree in Pharmacological Sciences, University of Brussels, in 1973 and a postgraduate in Environmental Sciences in She is the Director of the Research Division at the IBGE-BIM. Her main points of interest are scientific support for decision-makers and environmental governance. Daniele Carati has a PhD in Sciences, University of Brussels (université libre de Bruxelles), in He is the Head of the Statistical and Plasma Physics Research Unit at ULB. His main points of interest are the study of turbulence and the prediction of transport phenomena in both fluid and plasmas using theoretical as well as numerical approaches. Leon Brenig is Professor at the Physics Department of ULB (Université Libre de Bruxelles) located in Brussels, Belgium. He presented a PhD in Physics in the field of Statistical Mechanics and Fluid Mechanics in His main current research subject concerns the fundamental physics of granular flows

3 266 P. Vanderstraeten et al. with application to the transportation of solid particles in the atmosphere. He is also currently working on a weather modification technique based on the heat island effect that is intended for inducing cloud formation and convective rainfalls in arid and semi-arid regions. Zvi. Y. Offer is Professor at the Ben-Gurion University of the Negev. His scientific interest is related to determine the physical, mechanical and chemical composition of dust particles, the study of dust erosion, transport and deposition, theoretical modelling and wind-tunnel simulations. Practical applications in agriculture, solar systems efficiency, road transport and preserving the environment are studied. Eli Zaady is a Senior Scientist at the Agriculture Research Organisation, Volcani Institute, Israel. He has a PhD in Soil Ecology and open spaces with ten years experience on airborne particles distribution. 1 Summary Road traffic is often indicated as an important contributor to the PM10 concentration, especially in a city environment. Although it may be one of the most important sources for the health-related components, e.g., diesel soot and carbon-based particulates, there are several indications that traffic-related particle emissions are not a dominant source in the total PM10 mass concentration. Relatively, high PM10 concentrations have been observed at some of the official holidays, as was the case for the carnival period and Eastern Monday in the year The car-free day of 2003, on a dry and hot Sunday of September, showed PM10 levels exceeding those of an average Sunday during the dry and hot summer period of At that car-free Sunday, during the traffic ban hours no significant decrease in PM10 concentration was observed as it was the case for traffic-related pollutants such as NO, NO 2, CO and CO 2. The car-free Sunday of 2006 showed even more spectacular results. The PM10 and PM2.5 concentration were about three times higher than on an average Sunday or average working day during the period May September 2006, and no sharp decline in the PM concentration was observed during the traffic ban hours. Moreover, in all measuring stations of the Brussels Capital Region, the daily average PM10 and PM2.5 values of this car-free Sunday did exceed the 50 µg/m 3 EU limit value set for the daily PM10 concentration. This happened despite the absence of traffic during part of the day, only minor contributions of domestic heating (ambient temperature ~20 C), restricted industrial emissions (Sunday) and meteorological conditions (wind coming from the West) that led to concentrations lower as usual for NO, NO 2 and CO. About 80 90% of the PM10 mass concentration consisted of PM2.5 and secondary aerosol accounted for 35 40% of the total PM mass concentration. To filter out the influence of the meteorological conditions at one particular day, the average level of the five car-free Sundays organised so far in Brussels ( ) was compared with the situation of the average Sunday and average working day during the different periods May September of the years 2002 till For traffic-related pollutants such as NO, NO 2, CO and CO 2, a decrease in concentration is clearly observed during the traffic ban hours. For NO 2, this effect can even be seen in all 11 measuring stations, including the city suburban and

4 Elevated PM10 concentrations and high PM2.5/PM10 ratio 267 background stations. For PM10, however, no substantial decrease in concentration is seen during the traffic ban hours and the average concentration of all car-free Sundays is at the same level or still slightly higher than on the average Sunday or working day. 2 General situation and methodology The Brussels Capital Region has a population of about 1 million and a surface of about 160 km 2. Compliance with the EU air quality objectives is still problematic for the NO 2 limit value of 40 µg/m 3 as the annual average concentration to be met by 2010 and for the 50 µg/m 3 daily PM10 limit value, not to be exceeded more than 35 times per year from 2005 onwards. Regarding the average weekend concentration for NO 2 and the number of PM10 exceeding concentrations on weekend days, reducing every day emissions to the average weekend level will not be sufficient to respect the stringent air quality objectives in due time in all Brussels measuring sites. The Brussels telemetric network for controlling the air quality has 11 fixed stations for controlling the ambient air quality and two road tunnel stations. The PM10 concentration is monitored at six different sites. Over the past years, the PM10 annual average at the different sites in the Brussels area ranged from about µg/m 3 at a city background and suburban site to hardly less than 40 µg/m 3 in the city centre and up to µg/m 3 at an industrial site [Report IBGE-BIM]. There was no clear tendency observed over the past years and the highest annual average concentration was observed for the year 2003, with its dry and hot summer period. Although the EU limit value for the annual average was respected in all Brussels sites in the years 2005 and 2006, the limit value for the daily average concentration was not respected in two sites, situated along the industrial and commercial axis of the city. Measurements to obtain PM10 concentrations by means of R&P TEOM 1400Ab continuous instruments have been started up during the period at six different sites in the Brussels Capital Region. Three of the sites are representative of the general activities in the city (traffic, domestic heating, business and commercial activities), a fourth one is situated in an industrial area (city naval port) and two of the sites are more typical for a city residential environment. In the period up to 2003, the temperature in the TEOM instruments was maintained at 50 C. To obtain PM10 data comparable with the EU gravimetric reference method, the raw measured data from TEOM-analysers in Belgium for that period were multiplied by a factor (VMM-study on the comparability of PM10-methods 2003). From 2004 onwards, all Brussels TEOM PM10 analysers are equipped with a FDMS module (Filter Dynamics Measurement System) and they operate at 30 C. The PM10-FDMS results obtained are, by measurement, more directly comparable with the gravimetric reference method. A recent field study in Belgium showed a ratio close to unity between PM10-FDMS and the reference method. From January 2006 onwards, PM2.5-FDMS is measured along with PM10-FDMS in four sites. Results for ammonium, nitrate and sulphate in the particulate fraction at a residential site are available since May 2006.

5 268 P. Vanderstraeten et al. 3 Traffic-related pollutant levels during car-free Sundays On Sunday 17 September 2006 and already for the fifth time since 2002, a car-free Sunday was organised by the Brussels Authorities. From 7:00 till 17:00 h UT (9 19 h local time), nearly all motorised traffic was banned from the entire surface of the Brussels Capital Region. The use of Public transport was free and besides the busses from the Brussels Public transport company exceptions were given to a limited number of taxis, to emergency services and, on request, to a few thousand individuals. During the car-free hours of the day, the speed limit was set at 30 km/h. The day was characterised by mild meteorological conditions: no temperature inversion close to the surface neither in the morning nor in the evening, a relatively high but decreasing humidity (90 70%), a medium wind velocity (2 3 m/s) and a temperature rising from 19 C in the morning to about 22 C in the afternoon, winds coming from the West (North Sea) and a cloud cover during the whole day. With an exception for PM, the concentration levels of different pollutants were low in the early morning. The results of the traffic ban can be read easily from the graphs representing the concentration evolution on the car-free Sunday, an average Sunday and an average working day during the period May September The concentration decrease is best seen in a road tunnel where the concentration levels are much higher than in the ambient air and where the influence of the meteorological conditions on the concentration is less important. The graph in Figure 1 represents the concentration of NO in a road tunnel leading to the centre of the city. Figure 1 Road tunnel NO concentration car-free Sunday, average Sunday and average working day during May September 2006 (see online version for colours) Before and after the traffic ban hours, the concentrations on the car-free Sunday (in front of the graph) follow the concentrations of the average Sunday. A sharp and sudden decrease in the concentrations can be observed at 6:00 h UT, even before the official start of the traffic ban period. Blocking of the main access roads to the city centre may have started earlier in the morning. By the end of that period (17:00 h UT), when the traffic returns, a sudden increase in the concentrations can be seen. Similar pictures are

6 Elevated PM10 concentrations and high PM2.5/PM10 ratio 269 also obtained for the NO 2 and CO concentration in the tunnel, for any of the car-free Sundays and for the average of all five car-free Sundays so far organised in Brussels. Similar, but less spectacular observations can be made for the traffic-related pollutants at the traffic-oriented measuring stations at the surface. Since the ambient concentrations at one particular day can be strongly dependent on the meteorological conditions of the moment, it is more appropriate to represent the average situation over all five car-free Sundays and compare it with the average situation of all Sundays and all working days during the periods May September of the years 2002 till Figures 2 and 3 are representing the results for the NO and NO 2 concentration in a traffic-oriented measuring site. For this type of site, similar evolutions are obtained for CO and CO 2. Unlike the case for the tunnel stations, the concentration levels in ambient air strongly depend on the meteorological conditions of the moment, which leads to a greater spread on the concentration levels of the individual car-free Sundays. However, for the traffic-related gaseous pollutants and for any of the car-free Sundays, the concentration was always found to be lower during the traffic ban period compared with the same period on an average Sunday or average working day. Furthermore, a sudden concentration change appears always at the beginning and at the end of the car-free period. During the traffic ban hours of the car-free Sundays, a decrease in the NO 2 concentration is observed in all 11 Brussels measuring sites, in traffic-oriented sites as well as in the urban background and suburban sites. This observation makes clear that the NO 2 problem with respect to EU limit values can probably be solved at the condition that traffic NO X emissions can be reduced permanently by a similar amount but on a much larger spatial scale. However, such drastic emission reductions are not a realistic option for the short term. Figure 4 represents the evolution of the NO 2 concentration as an average of five car-free Sundays ( ) computed at four different sites: urban background, suburban, industrial and traffic stations. The concentration decrease starts about 2 h later in the urban background station compared with the traffic street station. Figure 2 NO at a traffic-oriented site average of all five car-free Sundays ( ), average Sunday and average working day during May September (see online version for colours)

7 270 P. Vanderstraeten et al. Figure 3 NO 2 at a traffic-oriented site average of all five car free Sundays ( ), average Sunday and average working day during May September (see online version for colours) Figure 4 NO 2 average of all five car-free Sundays ( ) at four different sites (see online version for colours) Complementary to this NO 2 decrease, an increase in the ozone concentration in all sites has been observed. Moreover, owing to the lack of nitrogen monoxide, the ozone concentration became quite uniformly distributed over the entire Brussels Region. 4 PM10 and PM2.5 levels during car-free Sundays The PM10 and PM2.5 concentrations on the car-free Sunday of 17 September 2006 were about three times higher than on an average Sunday or average working day during the period May September This can be read from Figures 5 and 6 representing for the Molenbeek site, the concentration evolution for, respectively, PM10 and PM2.5 on an average Sunday, an average working day and the car-free Sunday. Because of the high concentration levels, the presentation of the results of the car-free Sunday was moved to

8 Elevated PM10 concentrations and high PM2.5/PM10 ratio 271 the back of the graph. Similar results were obtained in all Brussels sites measuring PM10 or PM2.5. Daily PM10 values ranged between 75 µg/m 3 and 92 µg/m 3 and daily PM2.5 values between 70 µg/m 3 and 78 µg/m 3. Over the entire year 2006, the car-free Sunday signed for one of the highest PM10 and PM2.5 daily concentrations. Figure 5 PM10 at Molenbeek average Sunday and working day during the period May September 2006 and on the car-free Sunday (see online version for colours) Figure 6 PM2.5 at Molenbeek average Sunday and working day during the period May September 2006 and on the car-free Sunday (see online version for colours) The EU limit value of 50 µg/m 3 for the daily PM10 concentration was exceeded in all Brussels measuring sites. The daily average PM2.5 value also exceeded 50 µg/m 3 in all sites. At the beginning and at the end of the car-free period, one cannot observe a sharp or sudden concentration change as it is the case for traffic-related pollutants such as NO, NO 2, CO and CO 2. These very high concentrations were obtained despite the absence of motorised traffic during part of the day, only minor contributions of domestic heating (19 22 C ambient temperature), restricted industrial and commercial activities (Sunday) and meteorological conditions (wind coming from the West North Sea) that led to concentrations lower as usual for the gaseous pollutants. During the car-free hours, a 10-ppm CO 2 ( 18.3 mg/m 3 ) concentration drop was observed in a traffic-oriented site. Based on this concentration drop, on an average CO 2

9 272 P. Vanderstraeten et al. emission of nearly 160 g/km/car and a 60% share of diesel in traffic CO 2 emissions, one can estimate that the decrease in diesel particulate emissions from g/km/car to g/km/car (80% reduction from Euro IV to Euro V) will lead to a PM concentration drop in the order of 1 2 µg/m 3. The elimination of soot and carbon-based particles may be very important for health reasons but the limited effect of this emission reduction on the ambient PM10 concentration will not enable compliance with the EU air quality objective. The graphs in Figure 7 represent, for the period September 2006, the dynamic evolution of the hourly values for PM10 and PM2.5 measured at the Molenbeek site, close to the city centre (habitation, traffic) and situated along the commercial and industrial axis of the Brussels Capital Region. It can be seen that nearly 80 90% of the total PM10 mass concentration consist in fact of PM2.5. A concentration build-up has taken place from Friday 15 September in the afternoon and it continued on Saturday 16 during the whole day. Although PM concentrations decreased during the car-free day as a result of the general meteorological conditions (wind speed), they remained at a relatively high level (above 60 µg/m 3 ) during the afternoon. The daily average PM10 and PM2.5 concentrations are represented as a histogram. The horizontal line right across the graph represents the EU limit value of 50 µg/m 3 for the daily PM10 concentration. Figure 7 Molenbeek dynamic evolution for PM10 and PM2.5 concentration as well as the loss of volatile fraction (VO10 and VO2.5) during the period September 2006 (see online version for colours) The TEOM-FDMS mass system registers also the loss of mass owing to the presence of volatile or dissociating components (VO10 and VO2.5). The volatile fraction seems to belong entirely to the PM2.5 fraction. Results of the additionally taken filter revealed that ammonium, nitrate and sulphate accounted for about 35 40% of the total PM mass concentration. Under conditions of mild temperature and high humidity range, the dissociation of the ammonium salts is not significant [Stelson] and their hygroscopic properties may even have increased their contribution to the PM mass concentration. The evolution of the PM10 and PM2.5 concentration seems not to have much in common with the dynamic evolution of the NO and NO 2 concentration (Figure 8). The graph represents the concentration evolution in two traffic-oriented sites: Ixelles (R002) and Molenbeek (R001) during the period September Comparison of the dynamic evolution of the PM10 and PM2.5 concentration, on the one hand, and of the NO and NO 2 concentration, on the other hand, indicates that the particulate

10 Elevated PM10 concentrations and high PM2.5/PM10 ratio 273 concentration build-up is from a different nature. Furthermore, the quite similar concentration evolution of PM10-FDMS in the Flanders Region, north of Brussels, reveals that the spatial distribution of this phenomenon is much wider than the Brussels Capital Region. Figure 8 Traffic-oriented sites Molenbeek (R001) and Ixelles (R002) dynamic evolution for NO and NO 2 during the period September 2006 (see online version for colours) High PM10 values and a high PM2.5/PM10 ratio have already been observed several times under comparable conditions, on Sundays or official holidays with far less traffic as usual, a limited contribution of domestic heating and industrial activity and in the presence of low concentrations for the gaseous pollutants. Common factors seem to be a mild temperature (8 20 C) and a high humidity range (75 90%). The average concentration computed for all five car-free Sundays ( ) organised so far is of the same order or slightly higher than the average concentration on all Sundays or all working days. Figure 9 represents the evolution of the average PM10 concentration for all the car-free Sundays, the average Sundays and the average working days in the different periods May September One cannot observe a sharp or sudden change in the PM10 concentration neither at the beginning nor at the end of the traffic ban period. Figure 9 PM10 at Molenbeek average Sunday, average of all car-free Sundays ( ) and average working day during May September (see online version for colours)

11 274 P. Vanderstraeten et al. 5 Respect of EU limit values for PM10 The EU limit values for PM10 must be respected from 2005 onwards. In 2005 and 2006, the limit value of 40 µg/m 3 for the annual average PM10 concentration is respected in all PM10 measuring sites in Brussels. The second limit value, 50 µg/m 3 as the PM10 daily value, not to be exceeded more than 35 times per year, is not respected in two of the Brussels PM10 measuring sites. The PM10 annual average concentrations in different measuring sites in Brussels for the year 2005 and 2006 are given in the upper part of Table 1. The values between brackets refer to the average concentration on weekend days. With the exception of the industrial site of Haren, the average weekend concentration is only slightly lower or slightly higher (0 10%) than the real annual average. Table 1 PM10 annual average concentration and number of days with daily value exceeding 50 µg/m 3 real date without brackets for the calendar year 2005 and 2006 All days [Weekend] Year Molenbeek Berchem Uccle Haren Woluwe Average PM [28] 26 [24] 27 [26] 36 [28] 28 [25] Concentration [µg/m 3 ] [30] 23 [24] 29 [30] 34 [30] 27 [27] Number of days [45] 17 [17] 23 [28] 66 [24] 24 [28] Exceeding 50 µg/m [45] 17 [28] 25 [42] 56 [52] 29 [45] Values between brackets refer to the average weekend concentration and to the number of weekend exceeding days extrapolated for a year period with permanent weekend regime. The lower part of the table represents the number of days exceeding the limit value of 50 µg/m 3. The values between brackets refer to the number of weekend days exceeding the limit value, but extrapolated for a complete year period with permanent weekend regime, e.g., eight weekend days exceeding the limit value in 2005 at the Uccle site lead to an annual value of 28 days (=8 365/104). With the exception for the Haren site, a permanent weekend regime would have led in the years 2005 and 2006 to a comparable number of days exceeding the limit value as was the case for the real calendar years. In 2006, Berchem, Uccle and Woluwe have a substantial higher number of exceeding days during weekend regime than during the calendar year. Reducing the emission activities permanently to the actual average weekend level (drastic measures) will, therefore, not be sufficient to respect the stringent EU PM10 limit value in all the measuring sites of the Brussels Air pollution network. Days with dry weather conditions and prevailing winds coming from the continent (large sector East) are well represented amongst the days with PM10 levels exceeding the limit value of 50 µg/m 3. Over the past years ( ), the large sector East (North East to South East) represents about 28 30% of the time, but it accounts for 45 50% of the exceeding periods. Under these conditions, the higher concentrations are mainly observed in two measuring sites (Molenbeek and Haren), with increased differences between the PM10 and PM2.5 concentration levels. The local activities seem to be responsible for the (re)suspension of the coarser fraction (particles from 2.5 µm to 10 µm). On average, periods with a relative humidity beneath 60% show a PM10 concentration that is 20 µg/m 3 higher than for periods with a humidity above 80%.

12 Elevated PM10 concentrations and high PM2.5/PM10 ratio 275 On the other hand, some very high PM10 results, well above the limit value, have been obtained on all sites, under conditions with a relative high humidity range (80% or higher) and at moderate temperature (10 20 C). In these cases, practically 80 90% of the PM10 mass concentration consists of PM2.5. The TEOM-FDMS mass detection systems reveal the presence of volatile mass, mainly within the PM2.5 fraction. For about 50% of the exceeding days, the contribution of volatile mass is substantial and in about 10% it is comparable or even higher than on the car-free Sunday. In many of the more recent cases (since May 2006), the presence of ammonium salts has been confirmed. 6 Ratio PM2.5/PM10 and average weekly pattern During the calendar year 2006, PM10 was measured in five instead of six measuring sites. During the year 2006, we had simultaneously FDMS systems for PM10 and PM2.5 at three sites: Molenbeek (habitation, traffic and commercial), Uccle (residential) and Haren (industrial). Table 2 represents the average concentration for PM10 and PM2.5 in these sites for different selections of days: all days of the year, the days with PM10 value exceeding the limit value of 50 µg/m 3, weekend days exceeding the limit value and the car-free Sunday. Table 2 Average PM10 and PM2.5 concentration in the year 2006 and results for the PM2.5 to PM10 ratio for different selections: all days, all days exceeding limit value and weekend days exceeding limit value of 50 µg/m 3 PM10 Year 2006 Molenbeek Uccle Haren Concentration [µg/m 3 ] PM10 PM2.5 PM10 PM2.5 PM10 PM2.5 All days All exceeding days Weekend exceeding days Car-free Sunday 17/ Ratio PM2.5/PM10 Molenbeek (%) Uccle (%) Haren (%) All days All exceeding days Weekend exceeding days Car-free Sunday 17/ The second part of Table 2 represents the computed PM2.5/PM10 ratio (% mass/mass). For all three sites, one can observe a higher PM2.5 to PM10 ratio for the exceeding days and a still higher ratio for the weekend exceeding days. This seems to indicate that the formation of secondary aerosol and therefore the PM2.5 fraction is a non-negligible factor for explaining the many exceeding days in the Brussels Capital Region. The average weekly pattern for PM10 and PM2.5 does not follow the one for nitrogen monoxide (NO), still one of the most specific traffic-related pollutants. The graph in Figure 10 represents the normalised average weekly evolution for the concentration of NO, NO 2, PM10 and PM2.5 in a Brussels city environment (traffic, habitation,

13 276 P. Vanderstraeten et al. commercial activity) during the calendar year For each day of the week, Monday to Sunday, the average concentration is computed and divided by the average concentration on working days. In this way, a value close to unity is obtained for working days and the decrease in the concentration during the weekend in terms of percentage can easily be read from the graph. Figure 10 Normalised average weekly pattern for NO, NO 2, PM10 and PM2.5 in a city environment (Molenbeek-Woluwe) with traffic, habitation and commercial/industrial activity (see online version for colours) Compared with the situation of working days, the NO concentration falls by 40% (Saturday) and 60% (Sunday) during the weekend, in agreement with the order of magnitude of the decrease in traffic intensity. The NO 2 concentration falls only by 20 30% and the PM10 and PM2.5 concentration stays nearly constant during the whole week. During a previous period ( ), the concentration decrease for PM10 was about 10 15% during the weekend [Vanderstraeten et alii]. 7 Micromorphology The analysis of particle micromorphology is of special importance since particle shape distribution is expected to directly affect transport and the deposition-accumulation processes as well as the impact with the ground surface. For the study of the micromorphology, filters were sampled at three different measuring sites: the Brussels Environmental Institute (IBGE-BIM), the Belgian Meteorological Institute (Uccle) and the Brussels University (ULB). The micromorphology of the airborne particles has been analysed by computerised SEM (type JSM 5410 JEOL) investigation using a Soft Imaging System (SIS digital solutions for imaging and microscopy). Analysis was made at various resolutions, especially for the small particulates. On each filter, between 100 and 200 particles were selected in function of the density of the particles on the same portion of the filters. Our estimation of the particle size is obtained by a projection of its shape on a plane. It is defined as the area (in µm 2 ) enclosed in the projected perimeter of the particle s border (in µm). It should be stressed that the above definition differs from the classical size parameter,

14 Elevated PM10 concentrations and high PM2.5/PM10 ratio 277 that is the diameter (D) of the smallest circle enclosing the whole plane projection of the particle (Alshibli et al., 2004). Using a SEM instrument, a series of parameters were measured on a large number of particles (see below): the projected surface (A, in µm 2 ), the projected perimeter (P, in µm) and the projected long and short axis (L and l, in µm). From these values, two parameters, R 1 and R 2, were computed for a large number of particles. The first parameter, R 1 [in µm], is defined as R 1 = A/P. This parameter gives an estimation of the average diameter (divided by 4) of the projected surface of the particle, this is easily checked by computing R 1 for a circle. The second parameter, R 2, refers to the elongation of the particle and corresponds to the ratio of the projected major axis, L, divided by the minor axis, l, of the smallest ellipse enclosing the planar projection of the particle. The average area value for the station IBGE-BIM increases from 2.8 µm 2 in 16 September to 4.9 µm 2 in 17 September (57%) and decreases to 3.6 µm 2 in 18 (36%). Similar phenomenon was shown in the Uccle station (increase of 23% from 16th to 17th and decreased by 26% in the 18 of September). In the ULB station, the trend was different. The average area size distribution decreases from 4.5 µm 2 in 16 September to 1.2 µm 2 in 17 September (73%) and increases to 4.6 µm 2 in the 18th (74%). The values for the parameter R 1 showed a similar pattern for IBGE-BIM and Uccle stations, i.e., a noticeable increase for the car-free day while for the ULB station a neat decrease was observed. The elongation ratios R 2 showed that most of the airborne particles that were collected during the car-free day were more elongated for the stations IBGE-BIM (a 85% increase from the 16th to 17th and a 29% decrease from the 17th to the 18th) and Uccle (a 50% increase from the 16th to 17th and a 48% decrease from the 17th to the 18th). As for the other parameters, an opposite trend was found at the ULB station (31% decrease from the 16th to 17th and a 14% increase from the 17th to the 18th). For the micromorphology, one could summarise that the characteristics of the particles measured on the car-free day were significantly different from the values of the normal traffic days before and after. On the car-free day, higher values were observed for the area, the average diameter and the elongation of the particles at the IBGE-BIM and Uccle stations, while an important decrease was observed at the ULB station. The presence of some research laboratories on the university campus may eventually be the cause of the opposite evolution observed at the ULB station. 8 Discussion and conclusion For the organisation of the different car-free Sundays, the Brussels Authorities achieved the maximum possible for banning the motorised traffic out of the entire Brussels Capital Region. During the interdiction hours, the concentration of traffic-related pollutants (NO, NO 2, CO, CO 2 ) is decreasing significantly in road tunnels and in traffic-oriented measuring stations at the surface. Sharp and sudden concentration changes are observed at the beginning and at the end of the traffic ban hours. Similar observations can be made for any of the car-free Sundays organised so far, under different meteorological conditions.

15 278 P. Vanderstraeten et al. On all car-free Sundays, the NO 2 concentration decreased at all 11 measuring sites of the Brussels Capital Region, in the traffic stations as well as in the suburban and background stations. This important observation points out that drastic emission reductions in the future will enable compliance with the EU limit value for the annual average NO 2 concentration (40 µg/m 3 to be met by 2010). At the car-free Sunday of 17 September 2006, very high PM10 and PM2.5 concentrations have been observed in the Brussels Region. The concentrations were three times higher than those of an average Sunday or average working day. The absence of a sharp and sudden concentration change at the beginning and at the end of the traffic interdiction period seems to indicate that direct particle emissions from traffic contribute only little to the total PM10 or PM2.5 mass concentration. The presence of high PM concentrations on this car-free day, with practically no traffic and no domestic heating and with only a limited contribution of the commercial and industrial activities, do accentuate the complexity of the PM problematic. Compared with the dynamic evolution of traffic-related pollutants (NO, NO 2, CO, CO 2 ), the PM10 and PM2.5 concentration build-up seemed to be the result of a totally different phenomenon. The much larger spatial scale and the presence of ammonium salts reflect the importance of the formation process of secondary aerosol. The PM10 and PM2.5 daily concentrations on this car-free Sunday were amongst the highest values of the entire year The daily values for PM10 and PM2.5 in all Brussels stations did exceed the 50 µg/m 3 level of the EU limit value for PM10 and the PM2.5/PM10 ratio ranged as high as 85 93%. Over the past few years, daily PM10 concentrations exceeding the EU limit value have been observed on several occasions under similar conditions: far less traffic as usual, restricted contributions from domestic heating and industrial activity, and in the presence of low concentrations for traffic-related gaseous pollutants. A common factor seems to be a mild temperature (10 20 C) and a high humidity range (>80%). In these cases, high values are obtained for the PM2.5/PM10 ratio and the continuous mass detection system (TEOM-FDMS) detected a loss of volatile material and more recently the presence of ammonium salts has been confirmed. Over the entire year 2006 and with exception of the industrial site, the average PM10 concentration on weekend days is only slightly lower than the real annual average. The normalised weekly pattern for PM10 and PM2.5 does not follow that for NO or NO 2. During the weekend, no sharp decline of the PM concentration is detected. The number of weekend days with PM10 values above the limit value indicate that compliance with the EU limit is probably not guaranteed in the case of a permanent weekend emission regime. All these observations lead to the conclusion that the direct particulate traffic emissions are not the main contributor to the total PM10 mass concentration. In fact, other processes with a much greater impact are involved and therefore high PM10 concentrations can be observed under quite contradictory conditions. Reference Alshibli, K.A., Asce, M., Alsaleh, M.I. and Asce, A.M. (2004) Characterizing surface roughness and shape of sands using digital microscopy, Journal of Computing in Civil Engineering, Vol. 18, pp

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