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Trends and sources for heavy metals in urban atmosphere
Nuclear Instruments and Methods in Physics Research B 189 (2002) 227–232 www.elsevier.com/locate/nimb
Trends and sources for heavy metals in urban atmosphere K are Kemp
*
National Environmental Research Institute, P.O. Box 358, Frederiksborgvei 399, DK-4000 Roskilde, Denmark
Abstract The concentrations of a number of heavy metals in the air in three Danish cities have been measured by means of PIXE for more than two decades. The well-known capability of PIXE for fast and efficient analysis of aerosol samples has been employed for analysis of daily samples from several sites during the whole period. The main sources are traffic, domestic heating and long-range transport. Source apportionment and trends for single metals are assessed by means of simple statistical methods. The most striking change has occurred for the Pb concentration, which is reduced by almost a factor of 100 following the reduction of the Pb content in petrol. The main source of Cu, Cr and Zn is the traffic. The concentrations of these elements have been slightly increasing. The concentrations for most of the other heavy metals, which originate mainly from sources outside the cities, have been decreasing. 2002 Elsevier Science B.V. All rights reserved. PACS: 32.30.Rj; 42.68.Kh; 82.80.Ej; 92.60.Sz Keywords: Air quality; Heavy metals; PIXE; Particulate matter
1. Introduction Heavy metals (HMs) in the atmosphere may cause health problems. The WHO gives guidelines for several of the heavy elements [1]. Some HMs are found in carcinogenic compounds. Apart from Hg all of these metals can be determined by routine PIXE analysis. In the Danish air quality networks the PIXE method has been used for elemental analysis since around 1980. The large database which has been collected enables an assessment of the occurrence of the heavy metals in the atmosphere based on a simple statistical treatment. In the following, measurements at sites
close to roads with dense traffic in cities are described. In general the air quality has been improved during the last 20 years due to removal of sulfur compounds from fossil fuel, three way catalysts on petrol driven cars, and better control of combustion and emission. All of the heavy metals listed by WHO can be determined by PIXE except for Hg, which – due to the volatile character of many of its compounds – needs special sampling and analysis techniques. However, the ambient concentrations of many of the heavy metals are nowadays close to the detection limits for the PIXE method. 2. Measurements and techniques
*
Tel.: +45-4630-1842; fax: +45-4630-1214. E-mail address: [email protected] (K. Kemp).
The two main air quality programs in Denmark are managed by the National Environmental
0168-583X/02/$ - see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 1 ) 0 1 0 4 7 - 3
228
K. Kemp / Nucl. Instr. and Meth. in Phys. Res. B 189 (2002) 227–232
Research Institute. (The map in Fig. 1 show the locations of the sampling sites): • The Urban Program (LMP) [2] was started in 1982. The program is designed to assess the exposure of the population in the cities to air pollution. The program has been revised several times in order to focus on the current air quality issues. The sulfur compounds coming from industries and domestic heating were a main subject during the first years, but now the emphasis is shifted towards nitrogen compounds and volatile organic compounds coming from the traffic. Measurements are at present performed in three Danish cities. • The Background Monitoring Program [3] was started in 1978. The main topics have been to describe the long-range air pollution transport and to assess the deposition of acidifying and eutrophying species to farmland, forest and
sea. As for the LMP program, the focus has shifted from sulfur compounds towards nitrogen compounds and organic compounds (POPs). Measurements are at present performed at six places representative for the different biotopes. Particles are in both programs collected with socalled filterpack samplers. A particle filter with an open downward pointed suction is followed by one to three impregnated filters for collection of i.a. SO2 , HNO3 and NH3 . Mixed cellulose esters membrane filters are used for collection of the particles. All particle sampling is performed as continuous sampling in 24-h periods. Since the start of the programs all particle filters have been analysed by PIXE. The PIXE analysis gives a positive determination of 10–20 different elements in each sample. Among these are the HMs V, Cr, Mn, Fe, Ni, Cu, Zn, As, Se, Mo, Cd, Sn, Sb and Pb. The filters from the urban network
Fig. 1. Cities and sites in the air quality networks. Results from cities and site marked with name are included in this work.
K. Kemp / Nucl. Instr. and Meth. in Phys. Res. B 189 (2002) 227–232
are bombarded with 20 lC of 2.5 MeV protons. The detection limits for the mentioned HMs are between 2 and 5 ng/m3 . The filters from the background network are bombarded first with 7.5 lC of 2 MeV protons and then with 150 lC of 3 MeV protons in order to obtain better detection limits especially for the high-Z elements. Detection limits at or below 1 ng/m3 are obtained in most cases. The average concentration of several of these metals is however close to or slightly below the detection limit with the analysis parameters used. The calculation of the average may be doubtful because systematic analytic errors, e.g. in the spectrum background subtraction routines, often will be of same order as the detection limit. A fit to a log–normal distribution can be used in order to get a better representation of the average value. The distribution of most species approximates a log–normal distribution quite well. The technique is illustrated in Fig. 2 for a zinc time series consisting of the results all above the detection limit. The ‘‘real’’ average value is compared to the average value for a log–normal distributions calculated based on the highest 50% and 10% of the results. It is seen that even in the latter case the average value is close to the real value. In the following the log–normal fit is used in cases where less than 90% of the results are above the detection limit.
3. Average values and trends The average values for the first 5-year period (1982–86) and the last 5-year period (1995–1999) are shown in Table 1 for two street stations. Most concentrations have decreased from the first to the second period. Most elements at the city site are apparently from sources within the city, with As, Se, Cd and Sb as exceptions. For V, Ni and Pb the differences between city and background have been reduced substantially in the second period. Some trends to illustrate this claim are shown in Fig. 3. A maximum around 1999 is observed for the background trends for most of the heavy metals as a result of the economic changes in
229
Fig. 2. Fit to log–normal distributions for a one-year time series for Zn measured at a background site in 1998. The concentrations are represented by the curly full-drawn curve in the ‘‘cumulative log–normal probability’’ plot. A log–normal distribution will be represented as a straight line.
Table 1 Average values of HM concentration (ng/m3 ) for five-year periods at a city and background site Element
V Cr Mn Fe Ni Cu Zn As Se Mo Cd Sn Sb Pb
Odense (city street)
Tange (background)
1982–1986
1995–1999
1982–1986
1995–1999
18.7 2.6 20.7 760.0 7.6 20.8 99.7
5.0 4.4 19.8 640.0 2.7 20.4 47.4 1.2 0.7 1.4 0.6 1.4 5.6 15.7
7.0 2.0 10.3 268.0 3.6 2.8 32.7 2.0 0.6 0.3 0.5 1.4 1.1 38.9
2.8 0.5 5.4 140.0 1.2 1.8 16.0 0.9 0.5 0.2 0.3 0.7 0.7 7.6
0.4 1.1 0.6 2.4 3.2 572.0
230
K. Kemp / Nucl. Instr. and Meth. in Phys. Res. B 189 (2002) 227–232
eastern Europe. The trend for Pb at the city sites is the most striking. The removal of Pb from gasoline has reduced the concentration by almost two orders of magnitude.
4. Wind direction dependency
Fig. 3. Bi-annual averages for measurements at two city stations and one background station. All concentrations are in ng/m3 .
Sorting the concentrations according to the wind direction gives also information regarding the source contributions. The traffic is a main source at the street station. A similarity between species, which are known to come from the traffic and another species, is an indication of the traffic as the main source for this species as well. Fig. 4 shows almost the same behavior for NOx (NO þ NO2 ) and Cu, indicating that the main source for Cu as well as for NOx is the traffic. The distributions are typical for a ‘‘street canyon’’. The wind direction at street level is opposite to the direction above roof level, due to the turbulence created in the canyon.
Fig. 4. Wind direction distributions for nitrogen oxide and Cu. The radii of the circle sections are proportional to the average concentrations when the wind is coming from the direction the section points toward. The measurements are from a street site in Copenhagen performed from 1995 to 1999. The traffic density at the street along the station is 25,000 vehicles per day.
K. Kemp / Nucl. Instr. and Meth. in Phys. Res. B 189 (2002) 227–232
231
Monotone trends will show up as positive correlation coefficients for all time intervals. Fig. 6 shows the autocorrelation for V, Fe and Pb. The autocorrelation for V and Pb is positive even up to 3 years separation, which indicates a trend, which in this case is decreasing. V has a pronounced yearly variation, while Fe has both a weekly and a half-yearly variation. Fe arises to a large extent from road dust re-suspended by traffic and wind. The relatively dry condition during spring and the windy conditions explain the halfyearly cycle, while the traffic causes the weekly cycles.
Fig. 5. Regression plot between NOx and Cu. The measurements are divided in summer () and winter () results. The full drawn lines are orthogonal regression lines. The upper is for summer results and the lower for winter results. The results are from a street station in Copenhagen from 1995 to 1999.
5. Correlation A regression between NOx and Cu has been made in order to inquire a little more deeply into the origin of Cu (Fig. 5). There is a rather good correlation between the two species but a significant difference between summer and winter is observed. The explanation may be that the NOx is emitted from the exhaust pipe, while Cu is from the brake pads. Within the last decade the asbestos has been replaced with fiber material containing i.a. Cu.
6. Autocorrelation Information regarding cyclic variations and trends can be obtained by employing the correlation of measurements with equidistant time intervals. If there is a seasonal variation in the concentration of a specific element the correlation between measurements a year apart will be relatively high.
Fig. 6. Autocorrelation for measurements at a street station in Aalborg from 1982 to 2000. The left curves are expanded views for three weeks of the right curves, which cover three years.
232
K. Kemp / Nucl. Instr. and Meth. in Phys. Res. B 189 (2002) 227–232
7. Discussion
References
Based on a large volume of statistical material much information regarding the origin and behavior of heavy metals in the atmosphere may be achieved using simple statistical methods and by combining different techniques and employing the knowledge of the sites. The PIXE method enables the analysis of samples which are collected in networks that are set up to comply with various legally implemented air quality standards.
[1] Air Quality Guidelines for Europe, second ed., European Series no. 98, WHO Regional Publications, Copenhagen, 2000. [2] K. Kemp, F. Palmgren, Danish Air Quality Monitoring Program, Annual Data Report 1998, NERI Technical Report no. 296, National Environmental Research Institute, Roskilde, Denmark, 1999. [3] T. Ellermann, O. Hertel, K. Kemp, C.A. Skjøth, Atmosfærisk deposition af kvælstof, Vandmiljøplanens Overv agningsprogram, 1999, Atmospheric deposition of nitrogen (in Danish), Faglig rapport fra DMU, no. 332, National Environmental Research Institute, Roskilde, Denmark, 2000.
Trends and sources for heavy metals in urban atmosphere K are Kemp
*
National Environmental Research Institute, P.O. Box 358, Frederiksborgvei 399, DK-4000 Roskilde, Denmark
Abstract The concentrations of a number of heavy metals in the air in three Danish cities have been measured by means of PIXE for more than two decades. The well-known capability of PIXE for fast and efficient analysis of aerosol samples has been employed for analysis of daily samples from several sites during the whole period. The main sources are traffic, domestic heating and long-range transport. Source apportionment and trends for single metals are assessed by means of simple statistical methods. The most striking change has occurred for the Pb concentration, which is reduced by almost a factor of 100 following the reduction of the Pb content in petrol. The main source of Cu, Cr and Zn is the traffic. The concentrations of these elements have been slightly increasing. The concentrations for most of the other heavy metals, which originate mainly from sources outside the cities, have been decreasing. 2002 Elsevier Science B.V. All rights reserved. PACS: 32.30.Rj; 42.68.Kh; 82.80.Ej; 92.60.Sz Keywords: Air quality; Heavy metals; PIXE; Particulate matter
1. Introduction Heavy metals (HMs) in the atmosphere may cause health problems. The WHO gives guidelines for several of the heavy elements [1]. Some HMs are found in carcinogenic compounds. Apart from Hg all of these metals can be determined by routine PIXE analysis. In the Danish air quality networks the PIXE method has been used for elemental analysis since around 1980. The large database which has been collected enables an assessment of the occurrence of the heavy metals in the atmosphere based on a simple statistical treatment. In the following, measurements at sites
close to roads with dense traffic in cities are described. In general the air quality has been improved during the last 20 years due to removal of sulfur compounds from fossil fuel, three way catalysts on petrol driven cars, and better control of combustion and emission. All of the heavy metals listed by WHO can be determined by PIXE except for Hg, which – due to the volatile character of many of its compounds – needs special sampling and analysis techniques. However, the ambient concentrations of many of the heavy metals are nowadays close to the detection limits for the PIXE method. 2. Measurements and techniques
*
Tel.: +45-4630-1842; fax: +45-4630-1214. E-mail address: [email protected] (K. Kemp).
The two main air quality programs in Denmark are managed by the National Environmental
0168-583X/02/$ - see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 1 ) 0 1 0 4 7 - 3
228
K. Kemp / Nucl. Instr. and Meth. in Phys. Res. B 189 (2002) 227–232
Research Institute. (The map in Fig. 1 show the locations of the sampling sites): • The Urban Program (LMP) [2] was started in 1982. The program is designed to assess the exposure of the population in the cities to air pollution. The program has been revised several times in order to focus on the current air quality issues. The sulfur compounds coming from industries and domestic heating were a main subject during the first years, but now the emphasis is shifted towards nitrogen compounds and volatile organic compounds coming from the traffic. Measurements are at present performed in three Danish cities. • The Background Monitoring Program [3] was started in 1978. The main topics have been to describe the long-range air pollution transport and to assess the deposition of acidifying and eutrophying species to farmland, forest and
sea. As for the LMP program, the focus has shifted from sulfur compounds towards nitrogen compounds and organic compounds (POPs). Measurements are at present performed at six places representative for the different biotopes. Particles are in both programs collected with socalled filterpack samplers. A particle filter with an open downward pointed suction is followed by one to three impregnated filters for collection of i.a. SO2 , HNO3 and NH3 . Mixed cellulose esters membrane filters are used for collection of the particles. All particle sampling is performed as continuous sampling in 24-h periods. Since the start of the programs all particle filters have been analysed by PIXE. The PIXE analysis gives a positive determination of 10–20 different elements in each sample. Among these are the HMs V, Cr, Mn, Fe, Ni, Cu, Zn, As, Se, Mo, Cd, Sn, Sb and Pb. The filters from the urban network
Fig. 1. Cities and sites in the air quality networks. Results from cities and site marked with name are included in this work.
K. Kemp / Nucl. Instr. and Meth. in Phys. Res. B 189 (2002) 227–232
are bombarded with 20 lC of 2.5 MeV protons. The detection limits for the mentioned HMs are between 2 and 5 ng/m3 . The filters from the background network are bombarded first with 7.5 lC of 2 MeV protons and then with 150 lC of 3 MeV protons in order to obtain better detection limits especially for the high-Z elements. Detection limits at or below 1 ng/m3 are obtained in most cases. The average concentration of several of these metals is however close to or slightly below the detection limit with the analysis parameters used. The calculation of the average may be doubtful because systematic analytic errors, e.g. in the spectrum background subtraction routines, often will be of same order as the detection limit. A fit to a log–normal distribution can be used in order to get a better representation of the average value. The distribution of most species approximates a log–normal distribution quite well. The technique is illustrated in Fig. 2 for a zinc time series consisting of the results all above the detection limit. The ‘‘real’’ average value is compared to the average value for a log–normal distributions calculated based on the highest 50% and 10% of the results. It is seen that even in the latter case the average value is close to the real value. In the following the log–normal fit is used in cases where less than 90% of the results are above the detection limit.
3. Average values and trends The average values for the first 5-year period (1982–86) and the last 5-year period (1995–1999) are shown in Table 1 for two street stations. Most concentrations have decreased from the first to the second period. Most elements at the city site are apparently from sources within the city, with As, Se, Cd and Sb as exceptions. For V, Ni and Pb the differences between city and background have been reduced substantially in the second period. Some trends to illustrate this claim are shown in Fig. 3. A maximum around 1999 is observed for the background trends for most of the heavy metals as a result of the economic changes in
229
Fig. 2. Fit to log–normal distributions for a one-year time series for Zn measured at a background site in 1998. The concentrations are represented by the curly full-drawn curve in the ‘‘cumulative log–normal probability’’ plot. A log–normal distribution will be represented as a straight line.
Table 1 Average values of HM concentration (ng/m3 ) for five-year periods at a city and background site Element
V Cr Mn Fe Ni Cu Zn As Se Mo Cd Sn Sb Pb
Odense (city street)
Tange (background)
1982–1986
1995–1999
1982–1986
1995–1999
18.7 2.6 20.7 760.0 7.6 20.8 99.7
5.0 4.4 19.8 640.0 2.7 20.4 47.4 1.2 0.7 1.4 0.6 1.4 5.6 15.7
7.0 2.0 10.3 268.0 3.6 2.8 32.7 2.0 0.6 0.3 0.5 1.4 1.1 38.9
2.8 0.5 5.4 140.0 1.2 1.8 16.0 0.9 0.5 0.2 0.3 0.7 0.7 7.6
0.4 1.1 0.6 2.4 3.2 572.0
230
K. Kemp / Nucl. Instr. and Meth. in Phys. Res. B 189 (2002) 227–232
eastern Europe. The trend for Pb at the city sites is the most striking. The removal of Pb from gasoline has reduced the concentration by almost two orders of magnitude.
4. Wind direction dependency
Fig. 3. Bi-annual averages for measurements at two city stations and one background station. All concentrations are in ng/m3 .
Sorting the concentrations according to the wind direction gives also information regarding the source contributions. The traffic is a main source at the street station. A similarity between species, which are known to come from the traffic and another species, is an indication of the traffic as the main source for this species as well. Fig. 4 shows almost the same behavior for NOx (NO þ NO2 ) and Cu, indicating that the main source for Cu as well as for NOx is the traffic. The distributions are typical for a ‘‘street canyon’’. The wind direction at street level is opposite to the direction above roof level, due to the turbulence created in the canyon.
Fig. 4. Wind direction distributions for nitrogen oxide and Cu. The radii of the circle sections are proportional to the average concentrations when the wind is coming from the direction the section points toward. The measurements are from a street site in Copenhagen performed from 1995 to 1999. The traffic density at the street along the station is 25,000 vehicles per day.
K. Kemp / Nucl. Instr. and Meth. in Phys. Res. B 189 (2002) 227–232
231
Monotone trends will show up as positive correlation coefficients for all time intervals. Fig. 6 shows the autocorrelation for V, Fe and Pb. The autocorrelation for V and Pb is positive even up to 3 years separation, which indicates a trend, which in this case is decreasing. V has a pronounced yearly variation, while Fe has both a weekly and a half-yearly variation. Fe arises to a large extent from road dust re-suspended by traffic and wind. The relatively dry condition during spring and the windy conditions explain the halfyearly cycle, while the traffic causes the weekly cycles.
Fig. 5. Regression plot between NOx and Cu. The measurements are divided in summer () and winter () results. The full drawn lines are orthogonal regression lines. The upper is for summer results and the lower for winter results. The results are from a street station in Copenhagen from 1995 to 1999.
5. Correlation A regression between NOx and Cu has been made in order to inquire a little more deeply into the origin of Cu (Fig. 5). There is a rather good correlation between the two species but a significant difference between summer and winter is observed. The explanation may be that the NOx is emitted from the exhaust pipe, while Cu is from the brake pads. Within the last decade the asbestos has been replaced with fiber material containing i.a. Cu.
6. Autocorrelation Information regarding cyclic variations and trends can be obtained by employing the correlation of measurements with equidistant time intervals. If there is a seasonal variation in the concentration of a specific element the correlation between measurements a year apart will be relatively high.
Fig. 6. Autocorrelation for measurements at a street station in Aalborg from 1982 to 2000. The left curves are expanded views for three weeks of the right curves, which cover three years.
232
K. Kemp / Nucl. Instr. and Meth. in Phys. Res. B 189 (2002) 227–232
7. Discussion
References
Based on a large volume of statistical material much information regarding the origin and behavior of heavy metals in the atmosphere may be achieved using simple statistical methods and by combining different techniques and employing the knowledge of the sites. The PIXE method enables the analysis of samples which are collected in networks that are set up to comply with various legally implemented air quality standards.
[1] Air Quality Guidelines for Europe, second ed., European Series no. 98, WHO Regional Publications, Copenhagen, 2000. [2] K. Kemp, F. Palmgren, Danish Air Quality Monitoring Program, Annual Data Report 1998, NERI Technical Report no. 296, National Environmental Research Institute, Roskilde, Denmark, 1999. [3] T. Ellermann, O. Hertel, K. Kemp, C.A. Skjøth, Atmosfærisk deposition af kvælstof, Vandmiljøplanens Overv agningsprogram, 1999, Atmospheric deposition of nitrogen (in Danish), Faglig rapport fra DMU, no. 332, National Environmental Research Institute, Roskilde, Denmark, 2000.