Metal Emissions from Brake Linings and Tires: Case Studies of Stockholm, Sweden 1995/1998 and 2005

Transcription

1 Environ. Sci. Technol. 2007, 41, Metl Emissions from Brke Linings nd Tires: Cse Studies of Stockholm, Sweden 1995/1998 nd 2005 DAVID S. T. HJORTENKRANS,* BO G. BERGBÄCK, AND AGNETA V. HÄGGERUD School of Pure nd Applied Nturl Sciences, University of Klmr, Sweden Rod trffic hs been highlighted s mjor source of metl emissions in urbn res. Brke linings nd tires re known emission sources of prticulte mtter to ir; the im of the current study ws to follow the development of metl emissions from these sources over the period 1995/ , nd to compre the emitted metl quntities to other metl emission sources. Stockholm, Sweden ws chosen s study site. The clcultions were bsed on mteril metl concentrtions, trffic volume, prticle emission fctors, nd vehicle sles figures. The results for metl emissions from brke linings/tire tred rubber in 2005 were s follows: Cd 0.061/0.47 kg/yer, Cu 3800/5.3 kg/yer, Pb 35/3.7 kg/yer, Sb 710/0.54 kg/yer, nd Zn 1000/4200 kg/ yer. The clculted Cu nd Zn emissions from brke linings were unchnged in 2005 compred to 1998, indicting tht brke linings still remin one of the min emission sources for these metls. Further, brke linings re source of ntimony. In contrst, Pb nd Cd emissions hve decresed to one tenth compred to The results lso showed tht tires still re one of the min sources of Zn nd Cd emissions in the city. Introduction Globlly, metls re ccumulted in the technosphere, especilly in urbn res where the in-use stock is incresing, e.g., for copper (1) nd zinc (2). From the stock, metl emissions to the biosphere my be significnt, e.g., refs 3 nd 4. Severl studies hve stressed rod trffic s source of metl emissions, prticulrly in urbn res (e.g., refs 5 nd 6), nd brke linings s well s tires re known emission sources of prticulte mtter to the surrounding environment (7-10). Asbestos ws used in brke linings until the 1980s when it ws bnned in Sweden to improve both working environments nd the outdoor ir qulity of cities. The replcement mteril ws complex mixture of vrious substnces: reinforcement fibers of glss, steel, nd plstic; friction modifiers ; fillers in the form of ntimony compounds nd brss chips; nd iron filings nd steel wool s het-conducting mterils (11). The mterils used in brke linings re of environmentl relevnce s greter prt of the mteril is dispersed directly into the environment when used. Westerlund (12) mesured metl concentrtions in brke linings * Corresponding uthor phone: ; fx: ; e-mil: dvid.hjortenkrns@hik.se. nd clculted the metl emissions in Stockholm for 1998 for number of metls including cdmium, copper, led, nd zinc. It hs clerly been shown tht brke linings re mjor source of metl emissions in urbn res (e.g., refs 13 nd 14). As there re only few similr studies, EMEP (the Co-opertive Progrm for Monitoring nd Evlution of the Long-Rnge Trnsmission of Air Pollutnts in Europe) hs bsed its clcultion model for brke lining emissions minly on dt from the Westerlund study(14). With n incresing willingness to phse out number of metls in the pst decde, s well s the introduction of led-free brke linings (11), there is need to updte Westerlund s study to enble the chnges in mteril nd emissions to be monitored. Further, studies hve shown tht lrge mounts of ntimony might be emitted from brke linings, s ntimony (Sb 2S 3)is used by some mnufcturers s filler nd lubricnt in brke linings (e.g., refs 5, 15-17). Further, Uxeküll et l. (17) emphsize tht Sb in friction mteril might pose humn cncer risk nd its use should, therefore, be discourged. The totl Zn emissions from tires in Sweden hve been clculted t 150 tonnes/yer, which is significnt figure when compred to other sources (18). The metl emissions in Stockholm hve been briefly quntified erlier (1995) (19). Furthermore, Sörme nd Lgerkvist (20) clculted metl emissions from tires vi stormwter to the dringe re for one of Stockholm s wstewter tretment plnts. The clculted emissions in their study re bsed on the metl concentrtions in tires given by Legrett nd Pgotto (21). This is one of the few studies to nlyze metl concentrtions in the prt of the tire tht wers while driving. Most other studies hve nlyzed the totl concentrtion in tire chips, s they focused on emissions from tires s combustion fuel. This method might result in overestimtion s the tire body often contins metl reinforcements. An updted clcultion of Cd emissions from tires in Stockholm cn be found in Hjortenkrns (22), who concludes tht there is lck of relevnt metl nlysis studies of tire tred rubber. According to the directive of the Europen Prliment nd Council (23), mterils nd components in vehicles produced fter July 2003 should not contin led, mercury, cdmium, or hexvlent chromium. Brke linings were one of the components dded in June 2002 to Appendix II of the directive s n exception to these restrictions (24). This ppendix sttes tht the use of copper contining more thn 0.5 weight percentge led in brke linings is llowed for vehicle models pproved before July 2003, including mintennce of these vehicles until July After tht time, concentrtion of up to 0.4 weight percentge led in copper in brke linings is permissible until July 2007, provided tht it is not intentionlly dded. In Sweden, the directive hs only been implemented for pick-up trucks nd privte crs tht re not pproved EU models (25). In prctice this mens tht Swedish EU-pproved models of privte crs re llowed to use replcement brke linings contining led. The im of the current study ws to follow the development of metl emissions from brke linings nd tire tred rubber over the period 1995/ , nd to put the emitted metl quntities in reltion to other metl emission sources in n urbn re. The focus ws on Cd, Cu, Pb, Sb, nd Zn, nd for tires, chromium nd nickel. Stockholm, the cpitl of Sweden, ws chosen s study site. The city is reltively densely populted with pproximtely inhbitnts in n re of 190 km ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 41, NO. 15, /es070198o CCC: $ Americn Chemicl Society Published on Web 06/22/2007

2 Mterils nd Methods Smpling. Westerlund s (12) study of metl emissions from brke lining wer ws bsed on mnul vehicle count, with brke linings of the most common 63% of vehicles nlyzed for metls. As vehicle counts re rre, the current study uses vehicle sles figures for 2004 (Supporting Informtion Tble S1), reflecting the vehicle pool tht will contribute to metl emissions from brke linings for severl yers to come. In ccordnce with Westerlund, brke linings from the two most common lorry nd bus models were lso exmined. Some of the brke linings were of the sme kind for different models. A totl of 42 vehicle replcement brke linings were smpled. As not ll owners of privte crs use brnded replcement brke linings, brke lining smples from independent suppliers were needed. The independent sector hs seen lrge increse in mrket shre since new EU regultions were introduced in 2003 (26). The new regultions llow independent grges to service nd repir new crs, s long s the vehicle mnufcturer s requirements re met. Accordingly, the proportion of repirs crried out by independent grges is expected to continue to increse. Tody, the independent sector represents pproximtely hlf of the ftersles mrket for spre prts. To nlyze the metl concentrtions in replcement brke linings for privte crs from independent suppliers, the two lrgest suppliers in Sweden were selected, nmely Mekonomen nd Mec (26). Bsed on brke linings sles figures (27-29), five cr models were chosen nd brke linings for front nd rer wheels smpled. A totl of 20 brke linings from independent suppliers were nlyzed. Titnium-covered drills where used to obtin smples from the brke linings, s titnium ws not included in the nlysis. The composition of tire rubber vries by yer of mnufcture, mke nd model, fctory nd rubber mss btch. For economic, prcticl, nd environmentl resons, discrded tires were smpled. The im ws to tke tire tred rubber smples from mnufcturers corresponding to 75% of the totl nnul turnover of tires in 2005 (Supporting Informtion Tble S2). Mke, model, nd yer were noted, nd totl of 52 tires were smpled. Rubber smples from the outer 5 mm of the tred were removed with knife. The rubber chips were frgmented with scissors nd then wshed repetedly in 18.2 MΩ/cm 2 Milli-Q wter. Anlyticl Methods. Smples were dried to constnt weight t 60 C. About g of smple ws digested in 3 ml concentrted HNO 3 nd 3 ml concentrted HCl in closed vessels in microwve oven (Perkin-Elmer). The progrm used ws 400 W for 6 min, 900 W for 10 min, nd cooling for 15 min. The vessels were not opened until the temperture of the solution hd decresed to below 30 C to void evportion of voltile metl compounds (such s ntimony chlorides). The smples were finlly diluted to 100 ml with 18.2 MΩ/cm 2 Milli-Q wter. The metl concentrtions were nlyzed using n tomic bsorption spectrophotometer (AAS) nd grphite furnce tomic bsorption spectrophotometer (GFAAS) (Perkin-Elmer Anlyst 800). Detils of the instrumentl prmeters re given in Tbles S3 S4 in the Supporting Informtion. Vrition nd Qulity Control. To nlyze the vrition of metl concentrtions within one unit, six replictes of two brke linings nd two to six replictes from 10 different tires were mde. To control the precision of the nlysis method, six brke lining nd six tire tred smples were sent to n externl ccredited lbortory for control (Anlytic, Luleå Sweden). Anlytic is not ccredited for metl nlyses of brke lining nd tire rubber composition; insted they used two different ccredited digestion methods for plstic. Those methods differed slightly from those used in our study, s qu regi (1:3) ws used for brke linings nd concentrted HNO 3 nd H 2O 2 for tires. Both digestion methods used closed vessels in microwve oven. Anlytic s results were compred with our nlyses using one-smple t test for ll metls. The precision, ccurcy, nd reproducibility were checked with control blnks nd qulity control solutions (SPS-SW1, SPS-WW1, Spectrpure Stndrds AS, Oslo, Norwy nd Certified Reference Mteril TM-15, Environment Cnd, Ntionl Wter Reserch Institute, Burlington, Ontrio, Cnd). Clultion of Metl Emissions. The mount of prticulte mtter from brke linings ws clculted differently depending on the type of vehicle. Clcultions were bsed on the wer of brke linings before replcement, the weight of the brke linings, the distnce driven before replcement, nd the totl distnce driven per yer for ech vehicle type (for detils see Westerlund, ref 12). The mount of prticulte mtter from ech xle ws corrected for updted trffic volumes. The ssumption tht brke linings from brnded nd independed suppliers hve the sme lifetime hs been mde. Westerlund ssigned 40% of trffic volume to new vehicles (less thn 4 yers old) nd 60% to older vehicles, mking the ssumption tht ll new vehicles were using brnded brke linings nd ll old vehicles were using unbrnded brke linings. For the ske of comprbility, the sme ssumptions were mde in this study. The clcultions of metl emissions from brke linings were mde ccording to eq 1. (P front C front ) + (P rer C rer ) 1000 ] (1) ME ) v [ where ME is the mount of metl emitted per yer (kg/yer), v is the type nd mke of vehicle (type: privte cr, pick-up, bus, or lorry), P is the the totl mount prticulte mtter emitted from brke linings from ech xle per yer (tonnes/ yer) nd includes prticulte mtter emitted per kilometer nd distnce driven by specific type nd mke of vehicle, nd C is the men metl concentrtion in brke linings for ech xle (mg/kg). The tire emission clcultions were bsed on trffic volume for Stockholm nd Sweden, respectively (Supporting Informtion Tble S5), sle figures for tires (where the nnul percentges sles of ech brnd re ssumed to reflect the distnce driven with tht brnd), prticle emission fctors for tire wer, nd nlyzed metl concentrtions in tires. The clcultions of metl emissions from tires were mde ccording to eq 2. ME ) v [ P C TV F 1000 ] (2) where TV is trffic volume (Mvkm/yer) nd F is the proportion driven with ech type of vehicle nd brnd, expressed s nnul sles figures. A more strightforwrd clcultion excluding sles figures ws lso performed. Prticle emission fctors for tires hve been compiled by the EEA (14) nd Gustvsson (30). The fctors vry by 1 order of mgnitude depending on which references re used. The emission fctors for privte crs rnge from 24 to 360 mg/ vkm (vehicle kilometers) (men ( stndrd devition 122 ( 96 mg/vkm), for pick-ups mg/vkm (102 ( 16 mg/ vkm), nd for hevy vehicles mg/vkm (628 ( 450 mg/vkm). A men metl concentrtion of brnd regrdless of model hs been used in the clcultions. Clcultions for pick-ups follow the mrket shres for privte crs. As no newer sttistics for hevy vehicles were known to us, we followed the Swedish mrket shre dt of Ahlbom nd Duus (9) (Supporting Informtion Tble S6). VOL. 41, NO. 15, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

3 FIGURE Men metl concentrtions (Cd, Cu, Pb, Sb, nd Zn) in privte cr brke linings from brnded nd independent suppliers in 2005 in Sweden (mg/kg) (s ntimony nd copper concentrtions were low in brke linings from independent suppliers, the columns re not shown). Estimtes of trffic volumes ntionlly re mde every 4 yers. The most recent clcultions re from However, trffic volume hs incresed t rte of 3.5% from 2002 to 2005, nd thus the vlues used here re figures for 2002 djusted by 3.5% (31). For Stockholm the increse for the period is estimted to be 2.5% (32). Trffic volumes were divided ccording to type of vehicle, s shown in Tble S5 in the Supporting Informtion. The clculted emissions for Stockholm re presented s mens ( reltive stndrd devition (rsd) s percentge, together with n intervl with minimum nd mximum vlues bsed on the reltive stndrd devition. Clultions of Uncertinties. As the clcultions re multiplictive expressions, the uncertinties hve been clculted ccording to eq 3. r.s.d. ) σ ME ME ( ) σ P P ) 2 + ( σ C C ) 2 + ( σ TV) 2 + ( σ F F ) 2 (3) where r. s. d. is the reltive stndrd devition (%), σ ME is the stndrd devition for mount of metl emitted per yer, ME is the mount of metl emitted per yer, σ P is the stndrd devition for emitted mount of brke lining or tire compound, P is the men for emitted mount of brke lining or tire compound, σ C is the stndrd devition for metl concentrtion in brke lining or tire compound, C is the men for metl concentrtion in brke lining or tire compound, σ TV is the stndrd devition for trffic volume, TV is the men for trffic volume, σ F is the stndrd devition shre of trffic volume, expressed s nnul sles figures, for tires only, nd F is the men for shre of trffic volume, expressed s nnul sles figures, for tires only Results nd Discussion Metl Concentrtions. The men metl concentrtions in nlyzed brnded brke linings for privte crs from 2005 were for front/rer brke linings 1.2/4.0 ppm Cd, / ppm Cu, 120/2900 ppm Pb, /9500 ppm Sb, nd / ppm Zn (Figure 1, for detils see Supporting Informtion Tble S7). Here, Cu nd Zn concentrtions re high in both front nd rer brke linings nd of the sme mgnitude s in Antimony lso shows high concentrtion, especilly in front brke linings (ntimony ws not nlyzed in 1998). The men Pb concentrtion in brnded brke linings from 2005 is much lower thn the corresponding vlues from In 1998 the Cd concentrtions were between <0.974 nd 19.9 ppm, most round 10 ppm. No Cd men ws clculted, however, s dt were often ner or below the detection limit. For 2005 the Cd men ws 1-4 ppm, i.e., most likely decrese from The metl concentrtions in privte cr replcement brke linings from independent suppliers re considerbly lower for 2005 thn for The men metl concentrtions for front/rer brke linings were 0.5/0.39 ppm Cd, 200/110 ppm Cu, 100/290 ppm Pb, 29/10 ppm Sb, nd 5000/4400 ppm Zn. (Figure 1, for detils see Supporting Informtion Tble S8). The fct tht brke linings from different independent suppliers hve been nlyzed hinders comprison between the yers. However, the Cu nd Pb concentrtions in 2005 re only frction of the concentrtions in The differences in metl concentrtions in brke linings for privte crs from brnded nd independent suppliers for 2005 re shown in Figure 1. There is cler difference between metl concentrtions in brnded nd independent brke linings, with the ltter hving the lower concentrtions. If consumers wish to limit their contribution of Cu nd Sb emissions to the trffic environment, brke linings from independent suppliers re to be preferred. There re lso lrge differences within brnded brke linings; for exmple, one supplier hs brke linings with Pb concentrtions between 14 nd ppm. The concentrtions of Pb nd Cu were below the stipulted 0.4% in 41 of 48 brke linings from brnded suppliers; however, ll of the brke linings from independent suppliers exceeded this level (Supporting Informtion Tble S9). The choice of Pb/Cu quot in the regultory document mkes it difficult to ssess the environmentl relevnce of the directive. Despite the fct tht brke linings from independent suppliers hve lower metl concentrtions thn brnded brke linings, the ltter ones re those tht will pss the EU ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 41, NO. 15, 2007

4 TABLE 1. Clculted Totl Metl Emissions from Rod Trffic from Brke Linings in Stockholm (kg/yer) for 1998 nd Cd Cu Pb Sb Zn Cd Cu Pb Sb Zn privte crs b lorries b buses b totl b From ref 12. b Could not be clculted due to metl concentrtions ner or below the detection limit. TABLE 2. Clculted Metl Emissions (Cd, Cr, Cu, Ni, Pb, Sb, nd Zn) from Tire Tred Rubber in Stockholm (kg/yer) for 2005 Cd Cr Cu Ni Pb Sb Zn privte crs pick-ups lorries totl Clcultions re bsed on trffic volume for Stockholm, tire sles figures, prticle emission fctors for tire wer, nd metl concentrtions in tire tred rubber. TABLE 3. Clculted Metl Emissions (Cd, Cr, Cu, Ni, Pb, Sb, nd Zn) From Tire Tred Rubber in Stockholm (kg/yer) Using Simplified Clcultion, Where the Men of All Metl Concentrtions Ws Used Insted of Sles Figures Cd Cr Cu Ni Pb Sb Zn privte crs pick-ups lorries totl regultions. This indictes the problem with hving quot rther then n bsolute vlue s regultory limit. Cu concentrtions incresed for both lorries nd buses between 1998 nd 2005, while the other metls remined pproximtely the sme (Supporting Informtion Tbles S10 nd S11). Cu, Pb, nd Sb hve higher concentrtions in disk brke thn drum brke linings. This grees with the findings of Uexküll et l., (17) s do the metl concentrtion levels. The men metl concentrtions in nlyzed retred tire tred rubber were 0.86 ppm Cd, 1.3 ppm Cr, 7.4 ppm Cu, 2.9 ppm Ni, 9.5 ppm Pb, 1.1 ppm Sb och, nd ppm Zn. In nlyzed nonretred tire tred rubber the men metl concentrtions were 1.1 ppm Cd, 1.7 ppm Cr, 8.6 ppm Cu, 3.2 ppm Ni, 9.4 ppm Pb, 1.0 ppm Sb och, nd 9400 ppm Zn (for detils see Supporting Informtion Tbles S12 nd S13). The vlues re in the sme order of mgnitude s those of Legret nd Pgottos (17) (i.e., 2.6 ppm Cd, 1.8 ppm Cu, 6.3 ppm Pb, nd ppm Zn). The dte of production for the nlyzed tires vried between 1985 nd Thirty-one of the tires were from (Tbles S12 nd S13). Metl concentrtions in retred nd nonretred tires showed significnt differences for Zn only (Mnn-Whitney U test, p < 0.05). The Zn concentrtion ws highest for retred tires. Metl Emissions in Stockholm. The clculted totl metl emissions from brke linings in Stockholm from rod trffic (eq 1) re presented in Tble 1. There re no notble chnges in Cu nd Zn emissions between 1998 nd 2005, wheres Pb emissions decresed significntly. The low metl concentrtions in brke linings from independent suppliers (clcultion bsed on 60% usge) influence the results significntly. An lterntive clcultion ws mde with n ssumption tht hlf of the worn brke linings were replced by brnded brke linings (30% usge of brke linings from independent suppliers). The results of these clcultions gve 35-50% higher emissions: kg TABLE 4. Totl Clculted Metl Emissions from Rod Trffic from Brke Linings in Sweden 2005, (kg/yer) 2005 Cd Cu Pb Sb Zn privte crs lorries buses totl Cd/yer, 5700 kg Cu/yer, 48 kg Pb/yer, 970 kg Sb/yer, nd 1400 kg Zn/yer. The clculted totl metl emissions from tire tred in Stockholm, bsed on mrket shre (eq 2), re presented in Tble 2. The emissions were clculted for ech vehicle type nd then dded together. The clculted totl metl emissions from tire tred in Stockholm in 2005 were s follows: 0.47 kg Cd/yer, 0.76 kg Cr/yer, 5.3 kg Cu/yer, 1.4 kg Ni/yer, 3.7 kg Pb/yer, 0.54 kg Sb/yer, nd 4200 kg Zn/yer. A simplified clcultion, where only men of ll metl concentrtions ws used insted of sles figures, gve similr results (Tble 3). These simplified clcultions gve -4.2% for Cd, +12% for Cr, -19% for Cu, +14% for Ni, +16% for Pb, +11% for Sb, nd +12% for Zn. Metl Emissions in Sweden. Corresponding clculted totl metl emissions from brke linings for Sweden (bsed on 60% usge of brke linings from independent suppliers) re shown in Tble 4. The trffic volume in Stockholm is 6.3% of the totl ntionl trffic volume, wheres the metl emissions represent bout 5%. The differences result from different proportions of hevy trffic in Stockholm nd Sweden. The corresponding clculted totl metl emissions from tire tred for Sweden, bsed on mrket shre, resulted in 8.8 VOL. 41, NO. 15, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

5 TABLE 5. Clculted Metl Emissions (Cd, Cr, Cu, Ni, Pb, Sb, nd Zn) from Tire Tred Rubber in Sweden (kg/yer) in 2005 Cd Cr Cu Ni Pb Sb Zn privte crs lorries totl Clcultions re bsed on ntionl trffic volume, sles figures for tires, prticle emission fctors for tire wer nd metl concentrtions in tire tred rubber. TABLE 6. Clculted Metl Emissions (Cd, Cu, Pb, Sb, nd Zn) in Stockholm from Brke Linings, 1998 nd 2005 metl emissions 2005 (kg/yer) metl metl emissions 1998 (kg/yer) men ( r.s.d. (%) intervl Cd not clculted ( 57% Cu ( 57% Pb ( 62% Sb not clculted 710 ( 69% Zn ( 54% Westerlund, K.-G. Metl emissions from Stockholm trffic - Wer of brke linings. Reports from SLB-nlys, 2:2001, Environment nd Helth Protection Administrtion Stockholm, kg Cd/yer, 15 kg Cr/yer, 99 kg Cu/yer, 25 kg Ni/yer, 70 kg Pb/yer, 10 kg Sb/yer, nd kg Zn/yer (Tble 5). Uncertnties. The men coefficient of vrition of metl concentrtions within one brke lining ws 5.3% for Cd, 2.7% for Cu, 19% for Pb, 27% for Sb, nd 1.5% for Zn. This indictes tht the friction mteril in brke linings is reltively homogeneous. The externl control of the precision of the metl nlyses showed tht there were differences between our nd Anlytic s nlyses. These were 21% for Cd, 20% for Cu, 26% for Pb, 35% for Sb, nd 7.5% for Zn. None of the lbortories ws ccredited for metl nlysis on friction mterils, nd different strengths of cids were used. The differences between the methods hve been used s possible uncertinty in the nlyticl method. The totl uncertinties of metl concentrtions in the friction mteril re bsed on both the vrition within one brke lining s result of inhomogeneous mteril nd the uncertinty of the nlyticl method. In this cse, it hs been clculted ccording to (menerror(%)brkelining)2 + (menerror(%)nlyticlmethod) 2, giving (21% for Cd, (20% for Cu, (32% for Pb, (44% for Sb, nd (7.7% for Zn. The uncertinty of the wer of the friction mteril is estimted by Hedbrnt nd Sörme (33) tobe(33%. The EEA s (14) compiltion of friction mteril wer gives n emission of 18.8 ( 42% mg friction mterils per privte cr kilometer. Westerlund (12) nd the current study (for the ske of comprbility) use 17 mg/vkm. The uncertinty used ws (42% even if the recommendtions re lower. The vrition coefficients of metl concentrtions within single tire were in the following rnge: % for Cd, % for Cr, % for Cu, % for Ni, % for Pb, % for Sb, nd % for Zn. In the externl control of the nlyticl precision, it ws uncler whether there ws significnt difference between our nd Anlytic s nlyses (Supporting Informtion Tble S14). However, the control showed tht significnt results lso included higher exchnge in our method. As our method used stronger cids, this ws to be expected. The differences in outcome between the two methods re used s potentil nlyticl uncertinty. The totl uncertinties of metl concentrtions in tire tred rubber re bsed on both the vrition within single tire due to inhomogeneities of the mteril nd the uncertinty of the nlyticl method. In this cse it hs been clculted ccording to (menerror(%)tiretredrubber)2 + (menerror(%)nlyticlmethod) 2, giving (35% for Cd, (19% for Cr, (18% for Cu, (39% for Ni, (15% for Pb, (25% for Sb, nd (3.7% for Zn. The uncertinty of the prticulte wer of the tire tred rubber ws clculted t (79% for privte crs, (16% for pick-ups, nd (72% for lorries. As pick-ups hve only limited contribution to emissions, we used (75% in the clcultions. The uncertinty of the trffic volume figures re difficult to estimte. The Swedish Rod Administrtion cnnot stte the uncertinty of its figures, s these re prtly modeled. Here Hedbrnt nd Sörme s (33) recommendtion of (33% for officil sttistics t regionl/ntionl level hs been used. The uncertinty of the distnce driven (km) with ech tire brnd, expressed s percentge of the nnul sles figures, could be mtter of debte. The differences (4-19%) between emissions clculted with or without sles figures were used s the potentil uncertinty even though this will overestimte the error term. The reltive stndrd devition R.S.D. for metl emissions from brke linings (totl uncertinty for clculted mount of metl emission) is, ccording to eq 3, (57% for Cd nd Cu, (62% for Pb, (69% for Sb, nd (54% for Zn. For tires, the reltive stndrd devition R.S.D. is (89% for Cd, (85% for Cr, (86% for Cu, (92% for Ni, (85% for Pb, (86% for Sb, nd (83% for Zn. The gretest prt of the uncertinty derives from the prticulte emissions of tire tred rubber. The lrge vrition in emission fctors is probbly correct, s different tires (e.g., soft/hrd rubber) with different compositions result in different mounts of wer per kilometer. However, the men vlues of the wer probbly reflect the sitution on the rod. Comprison of Metl Emissions Between Yers. This study llows us to follow the development of clculted metl emissions from brke linings in Stockholm for the period Depending on the proportion of brnded nd nonbrnded brke linings used s replcements, the clculted metl emissions for 2005 differ. Considering this, the most plusible true vlue for 2005 will be found in the upper prt of the intervls presented here (Tble 6). The clculted Cu nd Zn emissions for 2005 re on the sme order of mgnitude s for Led nd probbly Cd emissions hve decresed to tenth of 1998 levels ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 41, NO. 15, 2007

6 TABLE 7. Metl Emissions (Cd, Cr, Cu, Ni, Pb, Sb, nd Zn) in Stockholm from Tire Tred Rubber Wer for 1995, 2002, nd 2005 metl metl emissions 1995 (kg/yer) b metl emissions 2002 (kg/yer) c metl emissions 2005 (kg/yer) men (kg/yer) ( r.s.d. (%) intervl Cd ( 89% Cr 200 NC 0.76 ( 85% Cu ( 86% Ni 200 NC 1.4 ( 92% Pb ( 85% Sb N.C. NC 0.54 ( 86% Zn ( 83% NC, not nlculted. b From ref 19. c From ref 20. Corrected to include ll emissions from tire tred wer in Stockholm rther thn only emissions ending up in stormwter ( 2.7). Brke linings re lso source of Sb emissions. Two studies hve found the verge Cu:Sb rtio in rod tunnel erosols to be 5.6:1 (34) nd 4.4:1 (15). As our verge Cu:Sb rtio is 5.3:1, brke linings must lso be considered mjor rod trffic source of Sb. Mnufcturers seem to hve tken seriously their responsibility for phsing out Cd nd in prticulr Pb. The concentrtions in the friction mteril hve been reduced, resulting in lower emissions. The independent mnufcturers hve shown tht it is possible to produce brke linings with lower mounts of both Cu nd Sb while mintining qulity. The clculted metl emissions (2005) from tire tred rubber re lower for ll metls thn corresponding figures for 1995 (Tble 7). This is probbly not result of decresing metl concentrtions in the tire treds, rther lck of dt in the 1990s when the only relible dt were for Cd (35). The Zn concentrtions were estimted from the mount ZnO dded in production nd were, therefore, only pproximte. The other metls were estimted from few vilble references. The clculted metl emissions of Sörme nd Lgerkvist (20) from 2002 re pproximtely the sme s in the current study. For Zn, n optimized consumption of ZnO in the production stge could reduce the emissions from tires. Cd is lso probbly derived from ZnO s contminnt. If ll mnufcturers could deliver tires with the lowest mesured Cd concentrtion, emissions should decrese to tenth of their current levels. Metl Emissions from Brke Linings nd Tires Compred to Other Sources of Concern. For 1995, the totl metl emissions (Cd, Cr, Cu, Ni, Pb, nd Zn) from the Stockholm technosphere were clculted, e.g., from vehicles, buildings, infrstructure, nd households. Some mjor identified emission sources were rtists pint (Cd), tp wter system/ roofs (Cu), mmunition/sinkers (Pb), nd roofs/tp wter system/ glvnized goods (Zn). However, emissions relted to the rod trffic sector were dominnt (pproximtely 50% or more of totl emissions depending on metl) for Cd (cr wshes), Cr (rod pvements, tires), Cu (brkes, rod pvement, tires), Ni (rod pvements, tires), Pb (brkes, tires, mmunition/sinkers excluded), nd Zn (tires, brkes, rod pvements) (13). From 1998 to 2005 no mjor chnges in emissions from tp wter system, roofs, nd glvnized goods re to be expected, but the results for 2005 show tht tires cn be excluded s significnt source of Cr, Cu, Ni, Pb, nd (probbly) Sb compred to other sources of concern in Stockholm. However, tires remin one of the lrgest sources of Zn nd significnt source of Cd. Further, brke linings must still be considered s mjor source of emissions of Cu nd Zn (nd probbly Sb) in Stockholm. As Stockholm represents rther verge city in most respects, the results from this study my be relevnt for mny other urbn res. Acknowledgments Finncil support for this project ws received from the City of Stockholm Environment nd Helth Administrtion nd the Fculty of Nturl Sciences, University of Klmr. Supporting Informtion Avilble Detils of the nlyticl instrumentl prmeters nd detiled tbles on specil brnds nd models of brke linings nd tires. This mteril is vilble free of chrge vi the Internet t Literture Cited (1) Gredel, T. E.; Bertrm, M.; Kpur, A.; Beck, B.; Sptri, S. Explortory dt nlysis of the multilevel nthropogenic copper cycle. Environ. Sci. Technol. 2004, 38, (2) Gredel, T. E.; vn Beers, D.; Bertrm, M.; Fuse, K.; Gordon, R. B.; Gritsinin, A.; Hrper, E. M.; Kpur, A.; Klee, R. 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