underground railway 
 systems are widely used mass transit systems 
 in many major cities , some carrying several million passengers per 
 day .
high mass concentrations of respirable 
 particulate matter ( pm ) with a mean aerodynamic diameter up to 10 
 m ( pm10 ; coarse ) , 2.5 m ( pm2.5 ; fine ) , or 0.1 m ( pm0.1 ; ultrafine ) have been 
 observed in many underground railway systems . in many cases , 

concentrations far exceed world health organization ( who ) recommended 
 limits for 24 h average particle exposure of 50 and 25 g / m for pm10 and pm2.5 , respectively , presenting 
 a potential risk for regular passengers and employees .
importantly , exposure to pm has been noted to be greater for underground 
 journeys than for equivalent journeys made by a variety of overground 
 modes of transport , and time spent in 
 underground railways has been suggested to be a better predictor of 
 metal exposure than duration of exposure to traffic - derived metal 
 pollutants .
there is evidence to 
 suggest that underground railway pm has high 
 concentrations of fe and other transition metals compared to ambient 
 pm .
transition metals are of interest as potential airborne toxicants 
 because of their ability to generate the reactive oxygen species ( ros ) 
 superoxide ( o2 ) , 
 hydrogen peroxide ( h2o2 ) and , via the fenton 
 reaction , hydroxyl radical ( oh ) via successive 
 single - electron reductions of molecular oxygen .
it is thought that many of the toxic effects of transition 
 metals arise from oxidative stress due to ros generation .
defined 
 as an excess of oxidative species that outweighs the antioxidant capacity 
 of a system , oxidative stress can result in oxidation and functional 
 modification of biomolecules such as lipids , proteins , and nucleic 
 acids and can result in inflammation and tissue injury .
however , transition metals , and also a variety 
 of other metals and metalloids such as lead and arsenic , can exert 
 toxic effects via mechanisms other than direct generation of ros ; 
 hence , study of concentrations of non - transition metals in airborne 
 pm is also warranted .
the composition of metal - rich pm from 
 a wide variety of sources 
 has previously been studied , including steel mills , smelting plants , 
 and welding fume . however
, underground pm studies generally focus on 
 coarse and fine fractions , without parallel analysis of ultrafine 
 pm composition .
although individual ultrafine 
 particles have a lower surface area than fine or coarse particles 
 ( 0.03 vs 19.6 and 314 m , respectively , for particles 
 of 0.1 , 2.5 , and 10 m diameter , assuming perfect sphericity ) , 
 ultrafine pm is often present in a much greater number concentration 
 than coarse or fine pm , and thus , their contribution to overall pm 
 surface area has the potential to be very important , possibly being 
 a key determinant of toxicity .
furthermore , 
 coarse and fine particles tend to accumulate in the ciliated airways 
 by impaction and are rapidly cleared by the mucociliary escalator , 
 whereas ultrafine particles predominantly settle by diffusion in the 
 alveoli , from where clearance is much slower .
ultrafine particles , unlike fine particles , are also able to translocate 
 from the airway lumen to the pulmonary interstitium and potentially 
 the systemic circulation , being detected in the liver , heart , kidneys , 
 and brain . because there is evidence 
 that underground pm is an important potential 
 toxicant , the aim of this study is to determine the concentration 
 of transition and nontransition metals in respirable , size - fractionated 
 pm collected at an underground railway station and to compare this 
 to pm collected from other process - specific sources , namely , a woodstove , 
 a roadwear generator , a road tunnel , and diesel exhaust
airborne pm was collected using a 
 versatile aerosol concentration and enrichment system ( vaces ) , with 
 aerodynamic diameter cutpoints of 102.5 m ( coarse ; 
 pm102.5 ) , 2.5 m ( fine / ultrafine with no 
 lower cutpoint , hereafter referred to as  fine  ;
pm was collected as a suspension 
 in ultrapure water from the following:(1)a woodstove , a portion of the exhaust 
 smoke being diluted with filtered air , fed into a sealed chamber containing 
 the vaces unit that sampled airborne pm at a concentration of 250 
 g / m.(2)a roadwear simulator , consisting of 
 a circular road surface on which four wheels with studded tires rotate . 
 prior to operation , the chamber was flushed with filtered air , and 
 the pm concentration was allowed to build up to a steady state of 
 5000 g / m.(3)a busy railway station located under 
 the main departures and arrivals terminal of a major european airport , 
 near the middle of a 5.1 km long tunnel , with further details published 
 previously .
the station is used by 60  000150  000 
 people per day , with three platform islands , each housing two platforms 
 400 m long .
2530 trains per hour pass through the 
 station during operating hours , all powered by overhead catenary . 
 during the night , there are occasional diesel - powered freight trains 
 passing through the station . the station is cleaned regularly during 
 daytime hours , principally using electrically powered ride - on machines 
 to clean the floor of dirt and dust .
there is no active air ventilation / conditioning 
 system in operation , with air exchange driven solely by
the vaces equipment was located 
 halfway along the central island platform , 3 m away from the 
 platform edge and 6 m from the centers of each pair of surrounding 
 tracks , with air intakes 3 m above the track level and 4 
 m below the overhead catenaries .
sampling was performed for 9 
 h between 08:30 and 17:30 on each of the three sampling days , all 
 of which were working weekdays in july 2010.(4)a heavily trafficked road tunnel in 
 the netherlands .
sampling was performed in a parking area immediately 
 adjacent to the tunnel exit , with sampling performed in midsummer .
a woodstove , a portion of the exhaust 
 smoke being diluted with filtered air , fed into a sealed chamber containing 
 the vaces unit that sampled airborne pm at a concentration of 250 
 g / m . a roadwear simulator , consisting of 
 a circular road surface on which four wheels with studded tires rotate . 
 prior to operation , the chamber was flushed with filtered air , and 
 the pm concentration
was allowed to build up to a steady state of 
 5000 g / m . a busy railway station located under 
 the main departures and arrivals terminal of a major european airport , 
 near the middle of a 5.1 km long tunnel , with further details published 
 previously .
the station is used by 60  000150  000 
 people per day , with three platform islands , each housing two platforms 
 400 m long .
2530 trains per hour pass through the 
 station during operating hours , all powered by overhead catenary . 
 during the night , there are occasional diesel - powered freight trains 
 passing through the station . the station is cleaned regularly during 
 daytime hours , principally using electrically powered ride - on machines 
 to clean the floor of dirt and dust .
there is no active air ventilation / conditioning 
 system in operation , with air exchange driven solely by
the vaces equipment was located 
 halfway along the central island platform , 3 m away from the 
 platform edge and 6 m from the centers of each pair of surrounding 
 tracks , with air intakes 3 m above the track level and 4 
 m below the overhead catenaries .
sampling was performed for 9 
 h between 08:30 and 17:30 on each of the three sampling days , all 
 of which were working weekdays in july 2010 . a heavily trafficked road tunnel in 
 the netherlands .
sampling was performed in a parking area immediately 
 adjacent to the tunnel exit , with sampling performed in midsummer .
in addition , diesel exhaust particulate 
 ( dep ) samples 
 from a diesel generator and exposure chamber were analyzed to provide 
 comparison as an expected metal - poor pm .
all analysis steps were 
 performed in a clean laboratory ( class 100 ) environment to minimize 
 possible contamination .
a 100 l aliquot of suspension was reserved for 
 anion analysis , and the remaining volume was recorded and transferred 
 to a teflon pot .
three overnight digestion steps were performed : respectively , 900 
 l of concentrated nitric acid ( 15 m , primar plus grade ; fisher 
 scientific , loughborough , uk ) with 100 l of concentrated hydrofluoric 
 acid ( 27 m , upa grade ; romil , cambridge , uk ) pressurized at 180 c , 
 1 ml of 6 m hydrochloric acid ( 12 m , primar plus grade ; fisher ) at 
 130 c , and 1 ml of 2% nitric acid spiked with be , in , and re 
 to monitor instrument drift .
hydrochloric and nitric acids were 
 sub - boiled prior to use . immediately prior to analysis , additional 
 2% nitric acid was added to produce a final mass of 3 g per 
 digest .
standards were prepared using a variety of commercially available 
 standard solutions to assess a range of metals ( table s1 , supporting information ) .
blanks were prepared 
 by performing acid digests in the absence of the pm suspension , to 
 monitor the contribution of any contamination during the digestion 
 process .
samples , standards , and blanks were analyzed by inductively 
 coupled plasma mass spectrometry ( icp - ms ) using a thermofisher xseries2 
 inductively coupled plasma mass spectrometer ( thermofisher scientific , 
 bremen , germany ) located in the isotope geochemistry instrument suite 
 at nocs , southampton .
concentrations 
 of the anions no3 , so4 , and cl in the particulate samples 
 were determined by ion chromatography .
a 100 l aliquot of each 
 particulate suspension was vortexed for 60 s and bath - sonicated for 
 60 min followed by centrifugation at 20  000 g for 10 min , and the supernatant was retained . 

supernatants were diluted in ultrapure water to a volume of 5 
 ml , before being analyzed on a dionex ics2500 ion chromatograph with 
 dionex chromeleon software ( dionex , sunnyvale , ca , usa ) .
standard 
 solutions for no3 , so4 , and cl were prepared from serial 
 dilutions of commercially available stock standards ( inorganic ventures , 
 christiansburg , va , usa ) , which were also used to monitor instrument 
 drift .
underground particle suspensions 
 were prepared for scanning electron microscopy ( sem ) analysis by evaporating 
 to dryness 100 l of pm suspension on an aluminum stub at 50 
 c overnight , followed by gold sputter coating ( hummer vi a sputter 
 coater , anatech , alexandria , va , usa ) to a thickness of 20 
 nm .
sem was performed using a leo 1450vp scanning electron microscope 
 ( carl zeiss nano technology systems , welwyn garden city , uk ) at 20 
 kv .
primary bronchial epithelial 
 cells ( pbec ) were obtained from healthy donors by fibreoptic bronchoscopy 
 as previously described .
cells were seeded 
 in collagen - coated 96-well plates at a density of 6000 cells per well 
 and cultured until 8090% confluent .
cells were then serum 
 starved ( 1.5 g / ml bovine serum albumin and 1x insulin / transferrin / sodium 
 selenite solution ( its ) both from sigma  aldrich , gillingham , 
 uk ) in bronchial epithelial basal medium ( bebm ; clonetics , san diego , 
 ca , usa ) overnight .
cells were washed once with hbss supplemented 
 with ca and mg ( hbsscamg ; invitrogen , carlsbad , ca , usa ) 
 before being loaded with 75 l of 10 m 27-dichlorofluorescein 
 diacetate ( h2dcf - da ; sigma 
cells were then washed twice with hbsscamg before application of 75 l of coarse , fine , or 
 ultrafine underground pm in supplement - free bebm at 6.3 or 12.5 g / cm ( equivalent to 25 and 50 g / ml , respectively ) .
dcf fluorescence was measured at 485 nm excitation and 
 530 nm emission 3 h postchallenge and calculated as the fold - change 
 in fluorescence compared to the control .
over the three sampling days 
 at the station , the mean ( 1 standard mean error ( se ) ) underground 
 pm10 mass concentration was 287  8 g / m , with coarse pm at 169  6 g / m , fine 
 at 75.3  5.9 g / m , and ultrafine at 37.7  
 4.5
chemical analysis was carried out on underground 
 pm as well as from the other three distinct sources and dep .
for woodstove , 
 roadwear , and underground pm sampling , individual daily samples were 
 analyzed separately .
road tunnel pm and dep were analyzed in the coarse 
 fraction only , as well - characterized references for comparison ( figure 1 and figure s1 , supporting information ) .
underground pm was the most transition - metal - rich pm , with fe 
 the most abundant element , comprising ( mean  1 se ) 407  
 43 , 404  70 , and 484  82 mg / g for coarse , fine , and ultrafine 
 fractions , respectively .
thus , fe comprises greater than 40% of the 
 total mass of the pm ( figure 1 ) .
conversely , 
 while all other pm samples contained detectable levels of fe , only 
 coarse and ultrafine roadwear pm ( 21.5  0.1 and 18.1  
 3.5 mg / g ) and road tunnel pm ( 27.7 mg / g ) contained fe levels greater 
 than 1% of the total mass of the pm .
cu was also elevated in underground 
 pm compared to pm from other sites , although the cu concentration 
 ( 21.9  2.5 , 20.7  3.9 , and 25.6  4.3 mg / g for coarse , 
 fine , and ultrafine fractions , respectively ) was lower than that of 
 fe . as with fe , these cu levels were considerably higher than those 
 seen in any other sampling locations , across all size fractions analyzed . 

other transition metals that were present at high concentration in 
 underground pm included mn , zr , mo , and sn .
in addition , levels of 
 v , cr , ni , nb , and hf were higher in underground pm than other sources , 
 although they were found at lower absolute concentrations . additionally , 
 ca was high in underground pm , particularly in the ultrafine fraction 
 ( 54  31 mg / g ) , and mg , ca , zn , ba , and sb were also found to 
 be elevated in underground pm , with relatively high levels of ga and 
 as also noted .
concentrations of fe , cu , cr , mn , zn , and k in pm of coarse 
 ( c ) , 
 fine ( f ) , and ultrafine ( uf ) fractions collected from a woodstove 
 ( ws ) , a roadwear generator ( rw ) , an underground station ( ug ) , and 
 a road tunnel ( rt ) .
values expressed as single values ( rt ) or mean 
  1 se of two ( ws , rw ) or three ( ug ) individual samples .
woodstove pm showed a marked enrichment 
 for k , while levels of 
 b and zn were similar to underground pm and in excess of levels in 
 other pm types .
furthermore , rb ( 144  3.2 g / g f , 192 
  1.8 g / g uf ) , cd ( 40.4  7.1 g / g f , 58.0 
  5.4 g / g uf ) , and pb ( 185  43 g / g f , 266 
  37 g / g uf ) were also relatively high in woodstove pm 
 compared to other pm sources , although they were low in terms of absolute 
 concentration .
roadwear pm possessed especially high concentrations 
 of al , with the three fractions showing concentrations of 71.0  
 3.1 , 27.3  1.8 , and 53.9  7.2 mg / g , respectively .
these concentrations were second only to underground 
 pm fe concentrations in terms of the most prevalent metals found at 
 any site .
roadwear pm also contained notably high levels of ti and 
 to a lesser extent sr , whereas levels of sc , la , and hg were found 
 to be greater than other pm types , albeit at trace levels .
interestingly , 
 roadwear pm showed generally lower metal concentrations in the fine 
 fraction compared to the coarse and ultrafine fractions .
road 
 tunnel coarse pm showed relatively high levels of li , b , and 
 na .
road tunnel pm also contained elevated levels of pb relative to 
 other pm samples at 516 g / g .
dep was also analyzed , as a source 
 of pm expected to be low in transition metals . as expected , the majority 
 of elements analyzed were present in lower concentrations in dep compared 
 to pm from the other sources tested , and many were not detected ( table 
 s2 , supporting information ) . statistical 
 analysis was performed to determine whether there was 
 any significant difference in the concentration of any element assayed 
 across each of the size fractions .
only b showed any pairwise difference , 
 the ultrafine fraction being high versus coarse ( p < 0.05 ) and fine ( p < 0.01 ) fractions .
the 
 data were further analyzed to test for correlations between fe and 
 other elements across the underground pm samples , testing with spearman s 
 rank correlation coefficient .
sr was the element most strongly correlated 
 with fe ( r = 1.00 ; p < 0.0001 ) . 
 however , this may be of limited importance due to the low overall 
 concentration of sr , generally below 100 g / g . indeed , 32 of 
 the 40 elements showed concentrations correlated with those of fe 
 ( p < 0.05 ) .
the strongest correlations among the 
 abundant metals were observed for mn , ni , and cu , while v was also 
 strongly correlated with fe ( figure s2 , supporting 
 information ) .
although no negative correlations were found , 
 the crustal elements na ( r = 0.517 ) and k ( r = 0.433 ) along with b ( r = 0.467 ) showed 
 the weakest correlation with fe .
ion chromatography analyses 
 show that road tunnel pm possessed the highest concentrations of so4 , cl , and no3 ( figure 2 ) .
roadwear pm 
 generally showed the lowest so4 , cl , and no3 concentrations 
 of any of the pm tested , suggesting that these anions are derived 
 from fuel combustion rather than road and mechanical sources in the 
 road tunnel pm .
when the concentrations of each species were compared 
 between the three size fractions of underground pm , coarse and fine 
 fractions showed similar levels of the three anions , but ultrafine 
 underground pm showed enrichments of so4 , cl , and no3 , and 
 for so4 , this difference was of statistical 
 significance versus coarse and fine fractions .
each anion showed only 
 weak positive correlation with fe concentration ( r = 0.583 , 0.483 , and 0.367 for so4 , cl , and no3 , respectively ) .
concentrations 
 of so4 ( left panel ) , 
 cl ( center panel ) , and no3 ( right panel ) in coarse ( c ) , fine ( f ) , and ultrafine ( uf ) fractions 
 of pm collected from a woodstove ( ws ) , a roadwear generator ( rw ) , 
 an underground station ( ug ) , and a road tunnel ( rt ) . values expressed 
 as single values ( rt ) or mean  1 se of two ( ws , rw ) or three 
 ( ug ) individual samples .
( * ) p < 0.05 and ( * * ) p < 0.01 , analyzed by one - way repeated measures anova
. examination of the coarse fraction 
 of underground pm revealed that most particles were well below the 
 10 m diameter used as a cutpoint for this fraction , with a 
 smallest dimension of 23 m or less .
however , there 
 was a clear population of particles with sizes close to the 10 m 
 cutpoint .
the smaller particles had a nonuniform , irregular granular 
 morphology , while the larger particles had a flakelike appearance 
 with jagged edges ( figure 3 ) .
when examined 
 at a greater magnification , the flat surfaces of these larger , flakelike 
 particles were often marked by ridges or indentations .
the fine fraction 
 of underground pm contained a similar set of small particles with 
 a largest dimension of 2 m .
significantly , there was 
 a paucity of flakelike particles compared to the coarse fraction . 

all fractions contained some particles 
 of a larger size than should theoretically have been permitted by 
 the cutpoint of the virtual impactor .
these larger particles were 
 commonly composed of 12 m particles agglomerated to 
 form a single mass , suggesting agglomeration post - collection .
sem micrographs 
 showing morphology of coarse ( c ; 5000 ) , fine 
 ( f ; 5000 ) , and ultrafine ( uf ; 30  000 ) underground 
 pm .
scale bars represent 10 m ( c and f ) or 2 m 
 ( uf ) . in order to measure 
 the ros - generating capacity of underground pm
, pbec monolayers were 
 loaded with the oxidant sensitive dye h2dcf - da , which is 
 fluorescent only after oxidation .
following 3 h exposure to underground 
 pm , fluorescence was seen to increase in a concentration- and size - fraction - dependent 
 manner ( figure 4 ) , with ultrafine and fine 
 fractions generally inducing greater fluorescence than the coarse 
 fraction ( 11.5  0.8 , 9.3  0.4 , and 7.7  0.9 mean 
 fold increase  1 se , respectively ) .
dcf fluorescence induced 
 by 3 h incubation of pbecs with coarse , 
 fine , or ultrafine underground pm .
( * * * ) p < 
 0.001 vs control ; ( # ) p < 0.05 for fine or ultrafine 
 vs respective concentration of coarse pm .
0.01 and ( # # # ) p < 0.001 , analyzed by one - way 
 repeated measures anova .
this study examined the levels of transition 
 and nontransition 
 metals and selected metalloids in size - fractionated underground railway 
 pm , with woodstove , roadwear , and road tunnel pm used as comparators . 

icp - ms analyses of a large range of metals , especially in the rarely 
 studied ultrafine fraction , provides new information on the chemistry 
 of underground airborne pm .
the underground railway station on which 
 this study focused forms part of a mainline international railway . 

this is important because ( 1 ) the studied railway draws power from 
 an overhead catenary , as opposed to an electrified rail , and ( 2 ) trains 
 running on this line are larger than would generally be found on urban 
 underground railway networks .
this observation is in agreement with other studies , which 
 have found high fe content in underground pm ( e.g. , 4059% 
 in stockholm underground pm10 , 61% 
 and 42% from paris rer and metro pm10 , respectively ) .
overall , the results from underground stations 
 are in clear contrast to samples from road traffic predominant areas , 
 which tend to be rich in elemental carbon .
unlike the vast majority of studies of underground pm , the 
 present 
 study also analyzed the elemental composition of ultrafine pm ( pm0.1 ) , which is rarely studied .
in urban environments , ultrafine pm is generally metal - poor and mainly composed of elemental c along with the products of secondary 
 reactions between gaseous pollutants that condense to form pm . in terms of their chemistry , these secondary ultrafine particles 
 are thought to be of little toxicological significance .
however , some processes involving high temperatures 
 and resultant vaporization or combustion of substrate material have 
 the potential to generate metal - rich ultrafine pm .
indeed , most metals analyzed in this study were found to have higher 
 concentrations in the ultrafine fraction compared to the coarse and 
 fine fractions , although the differences were not statistically significant 
 for any metal .
furthermore , our study may underestimate the relatively 
 metal - rich nature of the ultrafine fraction compared to the fine fraction 
 as the vaces equipment includes some ultrafine pm in the pm2.5 fraction . the majority of particles 
 in underground pm samples are thought to be derived from interaction 
 between wheels , rails , and brakes , generating 
 airborne particles that consist mainly of fe but also contain among 
 others mn , cr , v , zn , and as .
although 
 abrasive forces between wheels , rails , and brakes can clearly generate 
 coarse and fine pm due to shearing , there is evidence to suggest that 
 ultrafine pm can be generated via the high temperatures of friction 
 at interfaces between these components , with subsequent vaporization 
 of the substrate .
there is also likely to be a 
 contribution from arcing of the electrical current from the source 
 to the contact point on the train , through which electrical current 
 is drawn to power the train .
crucially , however , 
 unlike most urban underground systems that draw electric current through 
 a third rail running parallel to the other rails , the railway in the 
 present study is powered by an overhead catenary with the current 
 drawn through a pantograph . contact wires , which run above the railway 
 line , are generally composed of cu , alloyed with 0.10.5% ag , 
 sn , mg , or cd .
the precise composition of these 
 wires depends on the speed reached by trains drawing current .
similarly 
 cu or cu  pb  sn alloys form the contact material of the 
 pantograph , although in this case , it is generally as a component 
 of a metallized carbon contact strip
. this can be 
 contrasted with the third - rail system of power , where similar materials 
 are used on the current - collecting component ( in this case a third - rail 
  shoe  ) , but the third rail itself is an al  stainless 
 steel composite . as such , overhead contact 
 wire - powered underground railways
may be expected to show increased 
 airborne cu levels compared to electrified rail - powered systems . by 
 using the previously stated underground airborne pm mass concentrations 
 for each size fraction described above , in conjunction with the concentration 
 of mn in each fraction
, the mean (  1 se ) airborne mn concentration 
 over the three days is calculated as 1010  93 ng / m well in excess of the who recommended limit of 150 ng / m annual average and would still exceed this limit after allowing 
 for working a 35 h exposure period per week , with zero mn exposure 
 outside of working hours . because mn 
 overexposure in welders and miners has been linked to symptoms resembling 
 those of parkinsonism
, further study is needed of the potential effects 
 of chronic exposure to underground dust . as the antithesis of an element found to be relatively enriched 
 in underground pm , k was particularly high in woodstove pm .
wood combustion 
 is a significant contributor to airborne k , so it is unsurprising 
 that k was found at high levels in wood burner emissions .
interestingly , 
 woodstove pm displayed levels of rb , cd , and pb that were markedly 
 enriched relative to other pm .
cd is a characteristic waste product 
 of many industrial processes and is toxic to multiple organs , and 
 it may be significant that the trees used to fuel the woodstove in 
 this study were grown in the vicinity of a waste incinerator .
waste incineration plants have been shown to 
 release cd as part of the incineration process , and pb was also enriched in woodstove pm compared to all other pm 
 with the exception of road tunnel pm .
it is well documented that the 
 alkali metals rb and cs ( which was also elevated in woodstove pm compared 
 to other pm , although only at trace levels ) act as analogues for k , 
 also an alkali metal , in plant cation uptake , explaining their accumulation 
 in plant material .
both roadwear 
 pm and road tunnel pm have slightly raised levels 
 of ba but well below those seen in underground pm .
ba is also found 
 in the brake shoes of trains ; hence , brake wear is a possible source 
 of the high ba concentration in underground pm .
roadwear and road tunnel pm samples also have high concentrations 
 of na , while roadwear pm was relatively rich in al , k , ti , and sr 
 and road tunnel pm was rich in b and pb
. these differences may reflect 
 generation : roadwear pm is from an artificial roadwear generator while 
 road tunnel pm is from an operational road tunnel .
ti and k are both 
 found in brake pads , while road dust 
 contains aluminosilicates .
elevated na 
 levels in road tunnel pm likely derive from the nearby ( 30 
 km ) north sea coast or from the addition of road salt .
the elevated 
 level of pb in road tunnel pm is noteworthy as pb is not currently 
 used in dutch petrol .
however , pb has been detected in road dust samples 
 in other studies , and probably reflects the greater 
 volatility of pb compared to other anthropogenically enriched toxic 
 metals .
the analyses presented 
 here only yield the concentrations of the 
 various elements under consideration .
while this information is important , 
 more detailed assessment of the biochemical impacts of these elevated 
 metal levels would require information concerning their oxidation 
 states .
for example , distinction between fe(ii ) and fe(iii ) is important 
 in discussing fenton reactions and radical formation , which influences 
 interaction with biomolecules .
in addition , there is a need for study 
 of the nature of the metal compounds in the pm .
for example , environmental 
 fe is often found in an insoluble oxide form , whereas metal chlorides are generally soluble .
indeed , more than 
 one form of iron oxide has been observed in airborne pm , with urban 
 pm fe being mainly in the form of the hematite ( fe2o3 ) , while the predominant species of underground iron has been 
 reported for different systems as being magnetite ( fe3o4 ) or metallic iron , with minor hematite levels .
improved identification of particular compounds could , for example , 
 be attained by use of x - ray diffraction .
although metal speciation was not explicitly determined in 
 this 
 study , ros generation by underground pm was investigated . not only 

is ros generation dependent upon the metal oxidation state , but it 
 is also an intermediate step in transition - metal toxicity . in the 
 present study
, each fraction of underground pm was found to result 
 in increased ros generation as measured by dcf fluorescence .
furthermore , 
 this ros generation increased with pm concentration and was also greater 
 for fine and ultrafine fractions compared to the coarse fraction . 

considering the similarities in the metal composition of the different 
 fractions , this effect is most likely due to the increased surface 
 area / volume ratio as the pm size decreases .
previous studies have 
 suggested that transition metals in lower oxidation states are better 
 able to exert oxidative effects than those in the higher oxidation 
 states , which suggests that a toxicologically significant 
 proportion of underground pm is either in a submaximal oxidation state 
 or is able to be reduced in vitro to a state where it can further 
 catalyze ros generation .
analysis of metal concentrations in 
 all samples showed a strong 
 correlation between different elements .
notably , fe was positively 
 correlated with 32 of the elements , including several of the most 
 abundant transition metals , such as v , cr , mn , co , ni , cu , and zn . 

it is likely that these correlations are partly due to elements coming 
 from the same source , such as fe being alloyed with other elements 
 to modify the properties of steel , but also partly due to the level 
 of general mechanical activity contributing to pm load .
ion 
 chromatography analyses showed that road tunnel pm contained 
 the highest concentrations of so4 , 
 cl , and no3 .
this 
 finding is unsurprising , given that much of the pm from a road tunnel 
 is likely to be derived from fuel combustion and thus be more representative 
 of urban pm that is known to contain high concentrations of these 
 anions .
there is also likely 
 to be a contribution from aged pm originating from outside the tunnel , 
 which has accumulated these anions during transport to the sampling 
 site .
nonetheless , the lack of these anions in roadwear pm compared 
 to other particles analyzed also accords with their predominance in 
 environments where fuel combustion is taking place and also explains 
 their high levels in woodstove pm .
however , it may be noteworthy that , 
 of all the pm sources , with the exception of road tunnel , underground 
 pm showed the greatest concentration of anions . weak correlations 
 of so4 , cl , and 
 no3 concentrations with fe concentration 
 indicates that they are unlikely to be derived from mechanical wear . 

the airport is one 
 of the busiest airports in the world , by passenger number , and there 
 is a considerable amount of motor vehicle usage in the vicinity of 
 the airport .
the railway station lies beneath a complex of car parks 
 and passenger drop - off / pick - up points .
thus , it is likely that pm 
 from car exhaust is drawn into the station by the  piston action  
 of train movement
, although the extent of this input is a matter of 
 debate . additionally , contributions from aircraft particulate 
 emissions can not be excluded .
another 
 potential source is the diesel - powered goods trains that pass occasionally 
 through the station at night .
although no such trains passed through 
 the station during sampling periods , particles deposited by diesel 
 locomotives could be re - entrained by trains passing during the day 
 or by cleaning vehicles that are in regular use . in support of this 
 hypothesis
, the reduced levels of cl and no3 compared to levels of so4 in underground pm , especially the ultrafine fraction , 
 suggest that the contribution of secondary species from outside is 
 minor .
as this study focused on metals in pm , there remains 
 a proportion 
 of the mass of underground pm that was not identified in this study 
 ( 45% , 44% , and 19% by mass for coarse , fine , and ultrafine fractions , 
 respectively ) .
si was not quantified in this study as the hydrofluoric 
 acid digestion technique precludes accurate quantification .
however , 
 si has been found in underground pm by other groups and has been ascribed 
 to either brake blocks or the dumping of sand to improve wheel traction 
 under braking .
because underground pm contains substantial 
 levels of iron oxide , it is likely that oxygen makes up a significant 
 proportion of the unidentified pm mass .
a 
 wide range of organic compounds have been found in underground pm , and while these may be derived from diesel train 
 passage , it has also been noted that pm in areas located immediately 
 below ground level , as with the station in this study , may be more 
 influenced by above - ground sources than would be the case for deeper 
 environments .
this is particularly pertinent 
 here as the underground station lies directly beneath a large multistory 
 car park . indeed
, underground pm is likely to contain toxicants such 
 as polyaromatic hydrocarbons and redox - active quinones , although the source of these is harder to verify . 

they are likely to be derived from above - ground traffic sources , and 
 their concentrations may vary depending on the underground system 
 ( e.g. , ventilation controls ) , above - ground urban pollution levels , 
 and weather conditions .
this study also examined the morphology 
 of underground pm because 
 morphology can often serve as an indicator of the source of the particulate 
 or at least the processes involved in its creation . in terms of particle numbers ,
most pm of all size fractions 
 had a granular appearance , with rough , uneven faces .
however , coarse pm contained a considerable number 
 of particles of a flakelike , angular appearance , characteristic of 
 particles created by abrasion and shearing .
such flakes may have considerably 
 lower aerodynamic diameters than geometric diameters , resulting in 
 an increased likelihood of deposition in the respiratory tract .
very few flakelike 
 particles were observed in the fine fraction , while none were seen 
 in the ultrafine pm . whether particulate angular shape affects uptake 
 by cells or particle  particle interactions is not known .
however , 
 the angular nature of these particles may allow them to impinge upon 
 the structure of the cell .
particle 
 agglomerates were seen in all fractions , smaller structures 
 comprising fewer than 10 individual particles , while larger structures 
 were also observed , in excess of the respective vaces cutpoint , suggesting 
 that the agglomerate had formed after collection .
it was also observed 
 that some particles particularly in the ultrafine ( pm0.18 ) fraction were larger than the stated cutpoint , although this may 
 be reconciled by understanding that the diameter relates to the aerodynamic 
 behavior equivalent to a sphere of unit - density of a set diameter .
therefore , a particle may have an aerodynamic 
 diameter lower than suggested by consideration of only its largest 
 dimension .
furthermore , the stated vaces 
 cutpoint is not an absolute value but a 50% elimination value , meaning 
 that , although 50% of particles larger than the cutpoint of 0.18 m 
 are eliminated , some larger particles , including up to 5% of those 
 above 0.5 m , may remain
. there 
 also exists the possibility for smaller particles to enter larger - cutpoint 
 fractions by adhering to larger particles . in conclusion , this study has characterized and compared size - fractionated 
 mainline underground pm .
the results show that underground pm contains 
 a high concentration of fe , correlated with levels of other transition 
 metals , notably , mn , ni , cu , and v , which are significantly elevated 
 compared to pm from other sources .
crucially , ultrafine underground 
 dust was at least as rich in metals as coarse and fine underground 
 pm , which may have important implications for potential hazards posed 
 by underground pm , and warrants further study of the hitherto neglected 
 ultrafine fraction in particular .
 underground railway 
 stations are known to have elevated particulate 
 matter ( pm ) loads compared to ambient air . as these particles 
 are 
 derived from metal - rich sources and transition metals may pose a risk 
 to health by virtue of their ability to catalyze generation of reactive 
 oxygen species ( ros ) , their potential enrichment in underground environments 
 is a source of concern . 
 compared to coarse ( pm10 ) and fine 
 ( pm2.5 ) particulate fractions of underground railway airborne 
 pm , little is known about the chemistry of the ultrafine ( pm0.1 ) fraction that may contribute significantly to particulate number 
 and surface area concentrations . 
 this study uses inductively coupled 
 plasma mass spectrometry and ion chromatography to compare the elemental 
 composition of size - fractionated underground pm with woodstove , roadwear 
 generator , and road tunnel pm . 
 underground pm is notably rich in fe , 
 accounting for greater than 40% by mass of each fraction , and several 
 other transition metals ( cu , cr , mn , and zn ) compared to pm from other 
 sources . 
 importantly , ultrafine underground pm shows similar metal - rich 
 concentrations as the coarse and fine fractions . 
 scanning electron 
 microscopy revealed that a component of the coarse fraction of underground 
 pm has a morphology indicative of generation by abrasion , absent for 
 fine and ultrafine particulates , which may be derived from high - temperature 
 processes . 
 furthermore , underground pm generated ros in a concentration- 
 and size - dependent manner . 
 this study suggests that the potential 
 health effects of exposure to the ultrafine fraction of underground 
 pm warrant further investigation as a consequence of its greater surface 
 area / volume ratio and high metal content . 
Introduction
Methods
Results
Discussion