there is overwhelming evidence from animal 
 experimental models , 
 cell culture experiments , and cell free systems that exposure to particulate 
 matter ( pm ) causes oxidative stress ( os ) leading to 
 acute and chronic diseases . os results from excessive generation of reactive 
 oxygen species ( ros ) such as hydroxyl and superoxide radicals under physiological conditions . although a plethora of studies exist 
 on the mortality and morbidity of pm , the components that are responsible 
 for its toxicity and the observed adverse effects remain a quagmire . however , many researchers agree that the level of pm toxicity depends 
 on the chemical composition , particle 
 size , and shape . recently , we reported that the presence of epfrs on particulate 
 matter reduces molecular o2 to superoxide followed by dismutation 
 in aprotic media to form hydrogen peroxide and hydroxyl radicals . epfrs on pm are formed through interaction of transition metal oxides 
 ( such iron , copper , zinc , and nickel ) with aromatic compounds via 
 surface mediated processes . this results in the formation of surface - bound radical 
 species which are stable enough to persist in the atmospheric environment 
 for days , and which are also reactive in aquatic media to produce 
 ros . the concentration of metals in particulate matter may vary 
 greatly . 
 in fine particulate matter ( aerodynamic diameter < 2.5 m , 
 pm2.5 ) , 15  10 , 0.10.3 , 2.0 , 
 and 0.520  10 g / g of fe , the analysis and characterization 
 of transition metals from combustion systems and municipal incinerators 
 reveals 2.35% iron(iii ) oxide and 0.05% copper oxide . previous studies of epfrs that used the same concentration of metals 
 ( 5% by weight as oxide ) in particulates indicated that almost every 
 transition metal that was under study yielded epfrs on particle surfaces . the large distribution of metal concentration in particulate matter 
 raises the question of how the metal concentration affects yield , 
 lifetime , and chemical reactivity of the epfrs . one can anticipate 
 that changing the concentration of metal in particulates will affect 
 the metal oxide cluster size and its reactivity . in fact , the size 
 of metal / metal oxide clusters has been reported to be a pivotal property 
 in the catalytic activity . changing catalytic properties may affect the propensity of the metal 
 oxides to form epfrs , hence contributing to the different chemical 
 behavior of epfrs . in this study , we are attempting to answer the 
 above question by using different concentrations of copper oxide nanoclusters . silica ( cabosil ) based synthetic particulates containing varying concentrations 
 of cuo ( 0.255% by weight ) were tested for epfrs yield 
 and persistence . cabosil from cabot ( eh-5 , 99+% , 
 88 m / g bet surface area ) was impregnated with a 0.1 m 
 solution of cu(no3)22.5h2o 
 to obtain particles with different copper oxide concentrations : 0.25 , 
 0.5 , 0.75 , 1 , 2 , 3 , and 5 wt % . samples were left to adsorb copper 
 nitrate for 24 h at room temperature and then dried in the air at 
 120 c for 12 h before calcination in the air at 450 c 
 for 5 h to form cu(ii)o . the adsorbate chemicals , 
 phenol ( ph , 
 aldrich , 99+% ) , 2-monochlorophenol ( 2-mcp , aldrich , 99+% ) , and 1,2-dichlorobenzene 
 ( 1,2-dcbz , sigma - aldrich , 99% hplc grade ) were used as received without 
 further purification . epfrs were formed by exposing cuo / silica 
 particles to precursor vapors : phenol , 2-mcp , 1,2-dcbz . prior to the 
 precursor s exposure , the particles were heated in 
 situ in the air at 450 c for 1 h to remove organics 
 on the surface . in addition , these particles were exposed to the selected 
 precursor vapors at 230 c using a custom - made vacuum exposure 
 chamber for 5 min under vapor pressure conditions . after exposure , 
 samples were evacuated for 1 h to remove excess nonchemisorbed adsorbate 
 ( 20 ) at 10 torr . the dosed particles were cooled 
 under vacuum conditions to room temperature before epr spectra were 
 recorded . each experiment was repeated 3 , and the results were 
 reproducible within radical deviation of < 5% . epr spectra were recorded in a suprasil 
 epr tube at room temperature using a computer - controlled bruker emx 
 10/2.7 epr spectrometer . instrument parameters were as follows : center 
 field , 3470 g ; sweep width , 100 g ; microwave frequency , 9.7 ghz ; microwave 
 power , 2.0 mw ; modulation frequency , 4.0 g ; modulation amplitude , 
 4.0 g ; receiver gain , 3.54  10 ; time constant , 41.0 
 ms ; and three scans . radical concentration was calculated using the 
 dpph standard due to the similarity between the spectral profiles 
 of dpph and the radicals formed on cuo / silica . the kinetic studies were performed 
 to determine the persistency and stability of the radicals in the 
 air . the samples were exposed to ambient air and epr spectra obtained 
 regularly to determine the concentration of radicals as a function 
 of time until the sample had decayed and acquisition of the epr spectra 
 was at the noise level of the instrument . the 1/e lifetimes ( t1/e ) of 
 epfrs were evaluated using the following mathematical expression for 
 the first - order decay : rate constant k was found 
 from the slope of the correlation between logarithm of radical concentration 
 change ( r / r0 ) vs time , 
 and 1/e lifetime was calculated . adsorption of aromatic precursors on 
 cuo supported on silica resulted in the appearance of an epr signal 
 centered at 3400 g with a narrow line width hp - p  57 g for phenol 
 and 2-mcp , and a broader line width hp - p  816 g of 1,2-dcbz . these paramagnetic signals arise due to the interaction between the 
 hydroxyl- and chlorine - substituted molecules with a metal oxide surface . 
 the general mechanism of epfr formation has been established and confirmed 
 by our previous studies of copper , iron , nickel , zinc oxides , titania , 
 and alumina . the molecular precursors react with the 
 surface - hydroxyl groups and chemisorbs via elimination of h2o / hcl , resulting in a surface - bound epfr . this interaction result 
 in a 1-electron transfer to the metal cation center and a surface - stabilized 
 radical . the general scheme for the reaction is presented for 2-mcp 
 at cu sites on cu(ii)o / silica particle surface ( cf . scheme 1 ) . the precursor  metal oxide 
 interaction may result in more than one type of epfr depending on 
 the number and position of the substituent in the aromatic precursor with the overall epr spectrum being a superposition 
 of those species . deconvolution of the complex epfrs spectra facilitated the 
 identification of three paramagnetic species , namely , f - center , superimposed 
 at 1.99702.0020 , g2 ( attributed to phenoxyl radicals ) superimposed 
 at 2.00352.0040 , and g3 ( attributed to semiquinone radicals ) 
 superimposed at < 2.0050 . depending on the relative 
 concentration of g2 and g3 types of radicals , an overall shift of g value of the epr signal is observed . in the current study , the overall g values for the epfr species for phenol 
 and 2-mcp were similar ( 2.00372.0044 ) with a shift 
 toward the lower g values for phenol ( cf . the resultant radical 
 signals did not change either the position or the line width with 
 respect to concentration of copper oxide on silica ( cf . thus , it is evident that no change of radical speciation 
 occurred with decreasing cuo content . on the basis of the spectral 
 parameters from previous studies of silica supported with 5% cuo , 
 fe2o3 , and nio , together with the current results , 
 it can be concluded that a phenoxyl - type radical is formed on the 
 surface of all copper concentrations overall g value of radical signal with changing 
 cuo content in particles . change of hp - p of epr 
 spectra with changing cuo content . for the adsorption of 2-mcp , the overall g value 
 of spectra for all cuo concentrations changed only slightly with copper 
 content and is within the range of 2.00382.0044 . on the basis 
 of the above finding on the other 
 hand , the overall peak width , hp - p , was larger on average by 1 gauss for the radical species , resulting 
 from adsorption of 2-mcp compared with phenol adsorption ( cf . this indicates the 
 contribution of another radical species in the spectrum of 2-mcp exposed 
 samples , and it is believed to originate from semiquinone - type radicals 
 that are formed by the interaction between chlorine and hydroxyl groups 
 with the surface ( cf . interestingly , 
 unlike in the case of phenol , the peak width broadens with an increase 
 in g value ( vide infra ) . 1,2-dichlorobenzene 
 chemisorption on copper oxide - containing particles 
 resulted in highly asymmetric epr spectra and higher g values ( > 2.0048 ) and larger line width ( 7.716 
 g ) 
 than those resulting from the adsorption of phenol and 2-mcp ( cf . figures 1 and 2 and 
 table 1 ) . a high contribution of the o - semiquinone radicals to the overall spectrum is anticipated 
 for 1,2-dcbz as it had been observed in other metals . indeed , 1,2-dcbz 
 adsorption has been reported to proceed with simultaneous displacement 
 of both chlorine atoms , resulting in the formation of predominantly o - semiquinone radicals . however , 
 a distinct spectral change can be seen with decreasing copper content 
 ( cf . figure 3 ) when the overall g values of the spectra are decreasing ( cf . figure 1 ) . observed dramatic spectral changes result from changing 
 speciation of the formed radicals from predominantly semiquinone species 
 to a more balanced ratio of chlorophenoxyl to o - semiquinone 
 radicals or even predominantly chlorophenoxyl species for 0.25 and 
 0.5% cuo . adsorption of 
 1,2-dichlorobenzene produces a spectrum that resembles more the spectrum 
 that results from adsorption of 2-monochlorophenol than the one that 
 results from higher concentrations of cuo exposed to 1,2-dichlorobenzene 
 ( cf . epr spectra from the 
 adsorption of 1,2-dichlorobenzene and 2-monochlorophenol 
 on the particles containing different copper oxide concentrations . the surface concentration of 
 radicals formed upon the adsorption of precursors on cuo / silica samples 
 depends on the concentration of copper oxide ( cf . figures 4 and 5 ) . in general , the 
 adsorption of 2-mcp resulted in the highest concentration of radicals 
 on the surface , with the maximum yield at 1% cuo content . 
 above that concentration h 
 bond , which has a bond dissociation energy of 82 kcal / mol compared with 97 kcal / mol for the c since only copper sites are active in the formation of radicals , 
 a linear correlation between the radical yield and copper content 
 should be expected . however , as presented in figure 4 , this is not the case . data presented in figure 4 have been fitted to exponential decrease expression 
 ( solid lines ) . an exponential decrease in radical density per copper 
 atom was observed between 1% and 5% cuo content for 2-mcp adsorption , 
 with radical density drop - off below 1% . for copper oxide content below 
 1% , an exponential increase of radical concentration described the 
 observed trend the best , as marked by the dotted line in figure 4 . at the maximum observed radical concentration , 
 one radical is formed for every two copper atoms in the samples . one can imagine that once an adsorbed radical occupies a copper site , 
 the surface access to neighboring sites is hindered . thus , we can 
 assume a complete saturation of the surface sites with adsorbates 
 for 1% cuo . the 1:2 ratio of epfrs to copper ( for 1% cuo loading ) 
 is an indication that all copper atoms are surface available in this 
 case . up to 1% cuo content cuo clusters are two - dimensional with all 
 copper atoms to be surface available . increasing cuo content above 
 1% results in three - dimentional growths of the clusters and entrapment 
 of some of the copper atoms inside the clusters . we speculate that 
 this is the main reason for the radical yield drop for cuo content 
 above 1% , while the ratio of radicals to surface copper atoms ( radical 
 density ) remains unchanged . below 1% cuo content , a drop of radical 
 density from the 2-mcp precursor can be observed ( cf . we speculate that all copper sites are surface 
 available at the 1% cuo content , and a further decrease in copper 
 oxide concentration results in even smaller cuo clusters limiting 
 the number of adsorbed species due to steric effects . at 1% cuo loading , 
 for every 10 copper atoms , four are associated with radicals ( cf . figure 4 ) . assuming the decrease of cluster 
 size with decreasing cuo content , below 1% of cuo , the clusters become 
 smaller , limiting the number of adsorbed species ( for example , a nine 
 cu atom flat cuo cluster could fit only three radicals  a 0.3 
 coverage ratio etc . ) . at present , we consider steric effects as a 
 major cause of the radical yield decrease below 1% cuo content ; however , 
 electronic effects such as a lack of stabilization of reduced copper 
 can not be ruled out . dependence of epfr density with metal concentration dosed 
 with 
 1,2-dichlorobenzene , 2-monochlorophenol , and phenol on various cuo 
 loadings on silica . concentrations of organic 
 epfr adsorbed at 230 c for different 
 cuo contents based on a sample mass basis . phenol adsorption over cuo - containing particles resulted 
 in both 
 lower epfr yield and lower radical density , when compared with 2-monochlorophenol 
 ( cf . figures 4 and 5 ) . in this case , a maximum yield of radicals is detected between 
 2 and 3% cuo content . the differences in the radical yield of 2-monochlorophenol 
 and phenol result from the effect of the ortho group 
 substituent , ortho - substituted aromatics being 
 more reactive . an increase in radical density 
 below 1% copper oxide content indicates a higher reactivity of small 
 clusters . the same conclusion is supported by an exponential 
 decrease in 
 radical density with increasing copper oxide content after 1,2-dichlorobenzene 
 adsorption ( cf . figure 4 ) . as discussed earlier , 
 by decreasing the copper oxide content , radical species are gradually 
 changing from bidentate semiquinone to monodentate phenoxyl radicals . indeed , the adsorption of 1,2-dichlorobenzene is the only 
 case where a small linear increase of epfr yield is detected with 
 decreasing copper oxide content ( cf . one of the most important characteristics 
 of epfrs from an environmental perspective is their persistency in 
 the environment . epfrs have been reported to have a much longer lifetime 
 compared with other radicals such as acyl , hydroxyl , or superoxide , which makes them environmentally hazardous when emitted from combustion 
 sources . as previously mentioned , particulate matter may contain a 
 varying concentration of metal oxides ; therefore decay times have 
 to be evaluated relative to the metal concentration . the epfr 1/e lifetime in ambient air which is associated with cuo is 
 depicted in figure 6 . it is evident that by 
 decreasing the copper oxide content , epfrs 1/e lifetimes 
 increase to the maximum value at 0.50.75% cuo . interestingly , 
 for 1,2-dichlorobenzene and 2-monochlorophenol precursors , their observed 
 maximum 1/e lifetimes are very similar ( 2223 
 h ) , indicating again similar species dominating the surface at cuo 
 content  chlorophenoxyl species . at higher copper oxide contents , this is in line with the previous studies that demonstrated 
 longer lifetimes of phenoxyl compared with a semiquinone - type radical . it is also worth it to note that small clusters do not stabilize radicals 
 as efficiently as indicated by the drop in the lifetime of epfrs below 
 0.5% concentration . a word of caution is therefore necessary : 0.5% 
 cuo should not be taken as a limit for the highest stability of radicals 
 in environmental samples . in real life , a cluster size distribution 
 may not correlate with the concentration as in the case of those synthetic 
 samples . efprs 1/e lifetimes in ambient air at room temperature 
 formed on different cuo loadings on silica at 230 c . in summary , our results indicate that the reactivity 
 of aromatic 
 compounds on particulates is dependent on the concentrations of cuo . 
 the result of the interaction with the surfaces produces epfrs that 
 are stable and persist for hours . though we could not prove it directly , 
 it is inferred that the differences in reactivity result from different 
 cluster sizes of copper oxide affecting concentration , persistency , 
 and speciation of epfrs . the smaller the size of the nanoclusters , 
 the higher is the ability to catalyze epfrs formation / stabilization . 
 changing the persistency of epfrs with different cuo concentrations 
 is only partially correlated with changing radical speciation and 
 indicates the role of cluster size in radical stabilization . the consequence 
 of the higher stability of epfrs on smaller clusters is transport 
 to long distances from the source . another potential implication of 
 the presented studies is the potential of engineered nanomaterials 
 in consumer products to amplify epfrs production .
environmentally persistent free radicals 
 ( epfrs ) are formed by 
 the chemisorption of substituted aromatics on metal oxide surfaces 
 in both combustion sources and superfund sites . 
 the current study 
 reports the dependency of epfr yields and their persistency on metal 
 loading in particles ( 0.25 , 0.5 , 0.75 , 1 , 2 , and 5% cuo / silica ) . 
 the 
 epfrs were generated through exposure of particles to three adsorbate 
 vapors at 230 c : phenol , 2-monochlorophenol ( 2-mcp ) , and dichlorobenzene 
 ( dcbz ) . 
 adsorption resulted in the formation of surface - bound phenoxyl- 
 and semiquinoine - type radicals with characteristic epr spectra displaying 
 a g value ranging from 2.0037 to 2.006 . 
 the 
 highest epfr yield was observed for cuo concentrations between 1 and 
 3% in relation to mcp and phenol adsorption . 
 however , radical density , 
 which is expressed as the number of radicals per copper atom , was 
 highest at 0.751% cuo loading . for 1,2-dichlorobenzene adsorption , 
 radical concentration increased linearly with decreasing copper content . 
 at the same time , a qualitative change in the radicals formed was 
 observed  from semiquinone to chlorophenoxyl radicals . 
 the two 
 longest lifetimes , 25 and 23 h , were observed for phenoxyl - type radicals 
 on 0.5% cuo and chlorophenoxyl - type radicals on 0.75% cuo , respectively .