the long - range atmospheric transport of 
 particulate matter ( pm)-bound 
 polycyclic aromatic hydrocarbons ( pahs ) and nitro - pahs ( npahs ) to 
 remote sites , including mountains in france , norway , sweden , czech republic , and the canadian arctic , as well as trans - pacific transport to the olympic 
 peninsula of washington and to oregon has 
 been documented . once emitted from combustion sources , some pahs undergo 
 reaction with oh radicals , no3 radicals , n2o5 , and o3 , converting the parent pahs into more 
 polar species , including npahs . the transformation of pahs presumably 
 can occur locally ( i.e. , near emission sources ) and/or enroute to 
 downwind receptor sites . since some pah derivatives exhibit higher 
 direct acting mutagenicity than their parent pahs , human 
 exposure to pah derivatives , including npahs , is of interest and these 
 pah derivatives have been detected at many sites throughout the world . some npahs are classified as  probable or possible human 
 carcinogens  and have been identified 
 as major contributors to the overall direct - acting mutagenicity of 
 ambient pm despite being present at much lower concentrations than 
 those of their parent pahs . the 
 reactivity of pm - bound pahs varies , to some extent , with the 
 composition and the microenvironment of the particles . the mineral content , organic and black carbon concentrations , water 
 content , physical state of the organic layer surrounding the core 
 of the particles , and surface coverage of parent pah may all influence 
 the reactivity of pm - bound pahs . various artificial substrates , including silica , graphite , diesel 
 soot , fly ash , wood smoke , and kerosene soot , have been used in laboratory 
 experiments to simulate pm - bound pah reactions and derive heterogeneous 
 rate coefficients . we recently reported that pahs with more than 4 rings sorbed to 
 quartz fiber filters are transformed to npahs by reaction with no3/n2o5 . however , 
 few laboratory studies have been conducted on the transformation of 
 pahs and npahs on ambient pm , which can provide important 
 information needed in extrapolating model studies to ambient conditions . we concluded from our recent study of the formation of npah from 
 reactions of ambient particles with no3/n2o5 that pah in pm quickly become  deactivated  
 or unavailable for nitration . the evidence 
 included little formation of 1-nitropyrene ( 1-np ) from no3/n2o5 exposures of daytime ambient pm samples , 
 compared to nighttime samples with similar pyrene concentration , and 
 relatively little formation of 1-np in both daytime and nighttime 
 samples from downwind sites ( riverside and banning , ca ) upon exposure 
 to no3/n2o5 , relative to exposure 
 of a downtown los angeles , ca , nighttime pm sample . we speculated that daytime gas - phase reactions of volatile 
 organic compounds ( vocs ) with oh radicals and/or o3 producing 
 products , which then adsorbed onto the pm , were responsible for the 
 apparent  deactivation  of the pm - bound pahs . in addition , 
 2-nitrofluoranthene ( 2-nf ) , the most abundant npah in the unexposed 
 ambient pm samples , was not formed from the heterogeneous exposure 
 of the pm to no3/n2o5 . the lack of 
 heterogeneous formation of 2-nf is consistent with 2-nf being only 
 formed during gas - phase radical - initiated reactions of fluoranthene 
 and confirms its usefulness as a marker of atmospheric  aging  
 ( ref ( 33 ) and references 
 therein ) . in this earlier work , ambient 24 h samples from beijing , 
 china , 
 and a los angeles nighttime sample were the most reactive samples 
 in terms of 1-np formation . the objectives of our present research 
 were to ( 1 ) confirm the reactivity differences for no3/n2o5 exposure between samples collected at an urban 
 site , namely , beijing , china , and the downwind site of riverside , 
 ca . beijing pm has very high pah and npah concentrations resulting 
 from strong primary emissions , while 
 riverside can be strongly influenced by chemically aged pm transported 
 from upwind areas in the los angeles air basin , in addition to local 
 primary emissions ; ( 2 ) extend the study of the reactivity 
 of pm - bound pahs and npahs to include heterogeneous exposures to oh 
 radicals and o3 ; and ( 3 ) examine differences in bacterial 
 mutagenicity of the pm extracts from beijing prior to and after heterogeneous 
 exposures . the pah and npah measurements were carried out using gc / ms , 
 and the salmonella mutagenicity assay was conducted with and without 
 microsomal activation . to simulate long - range atmospheric transport 
 conditions in our environmental chamber , the average concentrations 
 of no3 radicals , oh radicals , and o3 over the 
 8 h chamber exposure periods were 420 ppt , 0.8 
 ppt , and 800 ppb , respectively . these concentrations were 
 equivalent to 7 days exposure to ambient concentrations of 
 no3 radicals , oh radicals , and o3 ( see supporting information for further discussion ) , 
 noting that the n2o5 concentrations in the chamber 
 no3/n2o5 exposures were at least 
 2 orders of magnitude higher than average ambient concentration . the 
 23 parent pahs and 20 npahs measured 
 ( and their abbreviations ) are listed in table 1 . deuterium - labeled pahs and npahs were purchased from cdn isotopes 
 ( point - claire , quebec , canada ) and cambridge isotope laboratories 
 ( andover , ma ) . the isotopically labeled pah and npah surrogates used 
 as recovery controls included fluorene - d10 , phenanthrene - d10 , pyrene - d10 , triphenylene - d12 , benzo[a]pyrene - d12 , benzo[ghi]perylene - d12 , 1-nitronaphthalene - d7 , 5-nitroacenaphthene - d9 , 9-nitroanthracene - d9 , 3-nitrofluoranthene - d9 , 1-nitropyrene - d9 , and 6-nitrochrysene - d11 . the labeled pah and npah internal standards included acenaphthene - d10 , fluoranthene - d10 , benzo[k]fluoranthene - d12 , 2-nitrobiphenyl - d9 , and 2-nitrofluorene - d9 . purchased from 
 accustandard ( new 
 haven , ct ) and chem service ( west chester , pa ) . purchased from chiron as ( norway ) , accustandard ( new haven , ct ) , chem service ( west chester , pa ) , and 
 sigma - aldrich corp . the beijing sampling site was located 
 on the roof of the 7-story ( about 25 m above ground ) geology building 
 on the peking university campus ( pku ) . this site 
 is located in northwestern beijing and is primarily a residential 
 and commercial area . dominant pah emission sources near the site include 
 vehicular traffic and fuel combustion for cooking . pm2.5 and pm10 were collected on prebaked ( 350 c ) quartz 
 fiber filters ( no.1851 - 865 , tisch environmental , cleves , oh ) using 
 a high volume cascade impactor ( series 230 , tisch environmental , cleves , 
 oh ) . pm was collected continuously over 24 h periods , with the sampler 
 being changed over in the late morning . pm10 and 
 pm2.5 samples were collected from may 2009 to february 
 2010 and in april 2011 , respectively ( table si.1 , supporting information ) . briefly , sampling in riverside 
 was conducted at a site in the agricultural operations area at the 
 university of california , riverside , campus , approximately 90 km downwind 
 of los angeles . pm2.5 samples were 
 collected during october 1997 using an ultrahigh volume particulate 
 sampler , containing four teflon - impregnated glass fiber ( tigf ) filters 
 ( each 40.6 cm  50.8 cm ) . after collection , the filters were 
 stored at 20 c . for this study , three 20.3 cm  
 25.4 cm portions were cut out from 40.6 cm  50.8 cm filters 
 ( table si.1 , supporting information ) . the average flow rate for each cut - out filter portion was 1 
 m min . beijing pm pah and 
 npah concentrations vary significantly from day - to - day . in order to measure changes in the pah and npah 
 concentrations with and without exposure in the chamber for the chemical 
 study , single 24 h 20.3 cm  25.4 cm filter samples were cut 
 into six equal portions of 8.5 cm  10.2 cm ( figure si.1 , supporting information ) . three 8.5 cm  
 10.2 cm portions were exposed in the chamber , and the remaining three 
 8.5 cm  10.2 cm portions of single 24 h filter samples were 
 used as unexposed controls . in order to measure changes in the mutagenicity 
 of the pm with and without exposure in the chamber for the mutagenicity 
 study , single 24 h 20.3 cm  25.4 cm filter samples were cut 
 into four equal portions of 10.2 cm  12.7 cm because the salmonella 
 assay did not adequately measure the mutagenicity of the 8.5 cm  
 10.2 cm portions ( figure si.1 , supporting information ) . two 10.2 cm  12.7 cm portions of the single 24 h filter 
 samples were exposed in the chamber , and the remaining two 10.2 cm 
  12.7 cm portions were used as unexposed controls . the pah and 
 npah concentrations of the exposed and unexposed 10.2 cm  12.7 
 cm portions used for the mutagenicity study were also measured and 
 directly compared to the results of the salmonella assay . overall , 
 six pku and three riverside 20.3 cm  25.4 cm filter samples 
 were tested in each exposure . the pm2.5 and pm10 filters were exposed to no3/n2o5 , oh radicals , and o3 in a 7000 
 l indoor collapsible 
 teflon film chamber equipped with two parallel banks of blacklamps 
 ( used for the oh radical exposures ) and a teflon - coated fan at room 
 temperature ( 296 k ) and 735 torr pressure . the filters were placed within the teflon chamber as shown , for 
 example , in figure si.2 , supporting information . for all exposure experiments , blank , clean 
 filters were also placed in the teflon chamber to test for background 
 contamination in the chemistry analyses and mutagenicity assays . the 
 details of the no3/n2o5 , oh radical , 
 and o3 exposures are given in the supporting information and have been previously reported . details of the sample 
 extraction and analysis have been previously described and are given 
 in the supporting information . the 
 basic methodology 
 followed that reported by maron and ames and used salmonella typhimurium strain ta98 . the positive control doses were 2 g of 2-aminoanthracene 
 ( 2-aa ) and 20 g of 4-nitro-1,2-phenylenediamine ( npd ) for assays 
 with and without metabolic activation ( rat s9 mix ) , respectively . all filter extracts 
 were tested in triplicate . on the basis of preliminary studies 
 and the limit of detection in the salmonella assay , only the beijing 
 pm samples were tested for mutagenic activity ( table si.1 , supporting information ) . positive controls 
 of npd and 2-aa gave mean revertant counts of 3500/plate and 
 1000/plate , respectively . the revertant counts 
 for the control blanks were comparable to the background revertant 
 count , indicating no interference from the purified air in the chamber . it should be noted that different sets of filters were used for the 
 mutagenicity and chemical studies ( figure si.1 , supporting information ) and that the pah and npah concentrations 
 of the pm samples used for the mutagenicity testing were measured 
 and directly compared to the results from the salmonella assay . the masses of individual pahs and 
 npahs on each 24 h filter sample cut - out ( shown in figure si.1 , supporting information ) were measured for the 
 exposed filters ( pahexposed and npahexposed ) 
 and unexposed filters ( pahunexposed and npahunexposed ) . the amount of individual pah or npah degraded ( or formed ) after 
 exposure to no3/n2o5 , oh radicals , 
 or o3 was calculated by dividing the exposed mass by the 
 unexposed mass ( a ratio close to 1.0 indicates no net degradation 
 or formation of a given individual pah or npah after exposure to the 
 various oxidants ) . the pahexposed / pahunexposed ratios and the npahexposed / npahunexposed ratios 
 for the beijing pm after exposure to no3/n2o5 , oh radicals , and o3 are shown in figure 1a , b , respectively . an asterisk indicates a statistically 
 significant difference in mass after exposure to the various oxidants 
 ( p - value < 0.05 , according to the paired student s t test ) . the means and standard errors of pah and npah masses , 
 measured in the beijing filters before and after exposure to the various 
 oxidants , are given in tables si.2si.4 , supporting information . the 3- to 4-ring pahs exist in both 
 the gas and particulate phases in the atmosphere at ambient temperatures . while it is possible that a portion of the pah 
 initially sorbed to the pm desorbed from the pm into the gas phase 
 of the chamber during the experiments , it is expected that pah available 
 for volatilization would have done so in the atmosphere and/or during 
 collection of the pm . therefore , we believe that volatilization of 
 these compounds from the pm , during the course of the experiments , 
 was minimal . the temperature in the chamber was maintained at room 
 temperature ( 296 k ) and held constant to 2 k ; no formation 
 of npahs due to gas - phase reactions was observed ( see below ) , and 
 the blank filters installed in the reactor at the same time as the 
 pm filter samples had pah concentrations below the detection limit . ( a ) pahexposed / pahunexposed and ( b ) npahexposed / npahunexposed of beijing pm filters ( n = 9 for no3/n2o5 and 
 oh radical exposures , and n = 8 for o3 exposure ) used for the chemical study . an asterisk denotes the statistically 
 significant difference between the unexposed and exposed masses ( n.d . 
 = as shown in figure 1a and in tables si.2si.4 , supporting information , all of the pahs underwent 
 some degradation upon exposure to no3/n2o5 , oh radicals , and o3 , apart from chr + tri exposed 
 to oh radicals . the data denoted by asterisks in figure 1a identify those pahs which had statistically significant 
 losses . the highest percent removal was 57% for ant exposed to no3/n2o5 , and a number of other pahs ( including 
 pyr , bcflu , baa , and bap ) had up to 50% removal from one or 
 more of the exposures ( figure 1a ) . noteworthy 
 is the observation that none of the pahs were completely 
 removed from exposures to no3/n2o5 , oh radicals , or o3 . our data from the o3 exposure 
 are consistent with a previous study where ambient pm was exposed 
 to 4 ppm of o3 for 60 min , with the pyr and baa concentrations being observed to plateau at 
 70% of their initial concentrations following exposure to 
 o3 . in contrast , a study examining 
 what can be considered  fully available  pyr ( coated 
 on silica particles ) found that the pyr reacted completely , within 
 10 min , after exposure to 1.3 ppm of o3 in a flow 
 reactor . there have now been a number 
 of studies that indicate that the reactivity of particle - bound pahs is substrate - specific 
 and can be inhibited by the decreased availability of these pahs in 
 the ambient pm . of the pahs containing more than 4 rings measured 
 on the beijing 
 pm , bap exhibited the greatest relative degradation upon exposure 
 to no3/n2o5 , oh radicals , and o3 ( figure 1a ) . this is consistent with 
 the high previously reported photochemical reactivity of bap and a study of ambient pm collected in paris , france , which identified 
 bap as the most reactive pah toward o3 , oh radicals , and 
 no2/o3 . the 
 means and standard errors of npah masses measured on the beijing pm 
 samples , before and after exposure to the various oxidants , are given 
 in tables si.2si.4 , supporting information . 2-nf and 3-nf were chromatographically separated and quantified 
 using a 30 m db-17 gc column in all beijing pm samples that were exposed 
 to no3/n2o5 , as well as in one - third 
 of the beijing pm samples that were exposed to oh radicals and o3 . the 3-nf concentrations in the beijing pm samples , both 
 before and after exposure to oh radicals and o3 , were below 
 the detection limit , suggesting no marked increase in concentration 
 upon exposure . in all of the beijing pm exposures , there was no significant 
 increase in the masses of 2-nf and 2-np , which are products of gas - phase 
 radical - initiated reactions . this provides 
 further evidence that there was no significant volatilization of 4-ring 
 pahs from the ambient pm into the gas phase in the chamber and implies 
 that the increase in the mass of other nitro - pahs during the exposure 
 experiments was likely due to heterogeneous formation . a number 
 of the npahs were formed to a statistically significant 
 degree during the no3/n2o5 exposure , 
 with the npahexposed / npahunexposed ratios reaching 
 49 for 6-nch ( figure 1b and table si.2 , supporting information ) . while 1-np and 6-nbap 
 were formed from both the no3/n2o5 and oh radical exposures , 9-nph , 3-nf , 7-nbaa , 1-ntr , 6-nch , 2-ntr , 
 and 1,8-dnp were only formed during the no3/n2o5 exposure ( figure 1b ) . our measured 
 formation of 3-nf and 1-np is consistent with results from previous 
 studies of the reaction of fla and pyr adsorbed on tigf filters with 
 gaseous n2o5 . these previous 
 studies also showed that the major npah isomers formed from the heterogeneous 
 reactions of fla and pyr with n2o5 ( 3- , 8- , 
 7- , and 1-nf and 1-np ) are distinct from the 2-nf and 2-np formed 
 from gas - phase reactions of no3 radicals with fla and pyr . 
 while 2-np is formed in ambient air from the oh radical - initiated 
 reaction of pyr , it is only formed from the no3 radical - initiated 
 reaction at no2 concentrations much higher than those observed 
 in ambient air . the major npah isomers formed 
 by heterogeneous nitration have the no2 group added to 
 the highest electron density position . the radical - initiated reactions of fla and pyr are expected to follow 
 a different mechanism where the oh or no3 radical attacks 
 fla or pyr at the highest electron density position , followed by no2 addition at the ortho position and subsequent 
 loss of h2o or hno3 , yielding 2-nf and 2-np 
 as the major npah products . in addition , the n2o5 concentration and the n2o5/no2 ratio in the photochemical chamber during our no3/n2o5 exposure experiments were at least 2 
 orders of magnitude higher than the average ambient concentration , suggesting a higher potential for heterogeneous 
 reactions with n2o5 . the heterogeneous 
 and gas - phase reactions of tri were previously 
 reported to both form 1- and 2-ntr , and 
 in this study , 1-ntr and 2-ntr were formed in equal amounts upon exposure 
 to no3/n2o5 ( figure 1b ) . our measured formation of 7-nbaa , 6-nch , and 6-nbap ( figure 1b ) was in agreement with previous results showing 
 their formation when pahs associated with diesel soot were reacted 
 with n2o5 . ratios of 1-npexposed /1-npunexposed ( mean 
 of 4.8 ) and 6-nbapexposed /6-nbapunexposed ( mean 
 of 6.5 ) were measured when the beijing pm was exposed to oh radicals 
 ( figure 1b ) . nitration of pyr by the radical - initiated 
 reaction discussed above would be expected to lead to 2-np . therefore , 
 the formation of 1-np and 6-nbap is attributed to heterogeneous nitration 
 of parent pahs by hno3/no2 . during the oh radical exposure experiments , no2 was formed from the reaction of ho2 and organic peroxy 
 radicals with no after the photolysis of methyl nitrite ( the oh radical 
 precursor ) and reaction of no2 with oh radicals leads to 
 formation of hno3 . in contrast to 1-np and 6-nbap , 3-nph 
 and 2 + 3-nf , and especially 9-nan and 7-nbaa , were degraded on the 
 beijing pm during the oh radical exposure ( figure 1b ) . because these same npahs were not as significantly degraded 
 during the dark exposure with o3 ( figure 1b ) , it is possible that the degradation observed during the 
 oh radical exposure could be due , in part , to direct photolysis . previously , 
 pitts et al . observed the direct photolysis 
 of 9-nan adsorbed on silica gel and identified quinones as degradation 
 products . in another study , the direct photolysis of 7-nbaa resulted 
 in increased direct - acting mutagenicity to ta98 in the salmonella 
 assay with increasing irradiation time . the orientation of the nitro group has been related to the photochemical 
 stability of npah , and both 9-nan and 
 7-nbaa have nitro group orientations that are out of the aromatic 
 plane , reducing the steric effects exerted by two peri - hydrogens . this structure makes these pahs less photochemically stable and may 
 explain their significant degradation during the oh radical exposure 
 ( figure 1b ) . although 
 6-nbap also has a structure where the nitro group is out of the aromatic 
 plane and is therefore expected to be photolabile , there was still 
 a net formation after exposure of beijing pm to oh radicals ( mean 
 nbapexposed / nbapunexposed = 6.5 ) ( figure 1b ) . as expected , exposure of the beijing pm to o3 did not lead to significant npah formation ( figure 1b and table si.4 , supporting 
 information ) and , except for the degradation of 7-nbaa , the 
 npahs sorbed to beijing pm were not significantly degraded during 
 o3 exposure ( figure 1b ) . the pahexposed / pahunexposed ratios and the npahexposed / npahunexposed ratios 
 for the riverside pm samples after exposure to no3/n2o5 , oh radicals , and o3 are shown 
 in figures si.3a and si.3b , respectively , supporting 
 information . the means and standard errors of pah and npah 
 masses measured in the riverside filters , before and after exposure 
 to the various oxidants , are given in tables si.5si.7 , supporting information . compared to the beijing 
 pm samples , a smaller number of individual pah and npah in the riverside 
 pm samples was present at concentrations above their detection limits . pahexposed / pahunexposed ratios slightly above 
 1.0 and statistically significant ( p - value < 0.05 ) 
 were measured for 2-mphe , 1-mphe , fla , pyr , and chr + tri upon exposure 
 of riverside pm to oh radicals and similarly for phe upon exposure 
 to no3/n2o5 ( ratio = 2.9 ) and oh 
 radicals ( ratio = 3.3 ) ( figure si.3a , supporting 
 information ) . however , these small changes in mass ( tables 
 si.5 and si.6 , supporting information ) are attributed to experimental uncertainty ( for example , possible 
 inhomogeneity in the particles across the filter ) , since pah formation 
 in these experiments was not possible . compared to the beijing pm 
 samples , the higher - ring pahs in the riverside pm samples were more 
 resistant to degradation during the no3/n2o5 and oh radical exposures ( figures 1a and si.3a , supporting information ) . exposure of the riverside pm samples to o3 did not result 
 in significant degradation of pahs , including the higher - ring pahs 
 ( figure si.3a , supporting information ) . consistent with the absence of significant pah degradation on 
 the riverside pm , there was no significant formation of npahs in any 
 of the riverside pm exposures , including the no3/n2o5 exposure ( figure si.3b , supporting information ) . a small , but statistically significant 
 ( p - value < 0.05 ) , reduction in mass was measured 
 for 2 + 3-nf after exposure to oh radicals and for 3-nph , 2 + 3-nf , 
 and 1-np after exposure to o3 ( figure si.3b , supporting information ) . although artifacts 
 during filter collection may occur , it has previously 
 been shown that 2-nf is not formed during collection and the artifactual 
 formation of 1-np is minimal ; therefore , 
 heterogeneous formation of these npah can be examined using pm - bound 
 pahs on filters . furthermore , our recent study utilizing 16 samples 
 collected during a 1997 photochemical pollution episode showed an 
 excellent correlation between the naphthalene and other volatile pahs 
 analyzed immediately after collection and pah measured on the archived 
 filters from this study . for example 
 ( see figure s10 in ref ( 33 ) ) , the highest naphthalene , bghip , and bep concentrations occurred 
 in the morning ( 612 h ) and evening ( 1806 h ) los angeles 
 samples and the concentrations decreased monotonically at the increasingly 
 downwind sites of azusa , riverside , and banning . because 1-np has been 
 consistently reported to form from heterogeneous reactions of pyr 
 adsorbed to a surface or particles , we used 1-np 
 formation as a measure of the reactivity of pm to npah formation . the percent reactivity of each of the beijing pm samples was calculated 
 as1where [1-np ] = ( [ 1-np]exposed  [ 1-np]unexposed ) and [ pyr]0 = [ pyr]unexposed . we also calculated 
 the [ 2-nf]/[bep ] ratio of the unexposed pm samples to indicate the 
 degree to which the pm had undergone atmospheric processing ( or aging ) 
 in the atmosphere prior to collection . 2-nf is formed from the gas - phase 
 reaction of fla with oh and no3 radicals in the presence 
 of no2 , while bep is a relatively stable 
 pah . a larger [ 2-nf]/[bep ] ratio therefore 
 indicates a greater contribution of photochemical atmospheric aging 
 in comparison to direct emissions . the relationship between the reactivity 
 toward nitration upon no3/n2o5 exposure 
 and the degree of aging has been previously discussed in zimmermann 
 et al . a plot of the reactivity 
 versus [ 2-nf]/[bep ] is shown in figure si.4 , supporting information , for the present study , and the reactivity 
 decreases monotonically as the atmospheric aging of the pm increases , 
 as measured by the [ 2-nf]/[bep ] ratio . beijing pm samples with lower 
 reactivity had higher [ 2-nf]/[bep ] ratios , suggesting that the pahs 
 sorbed to beijing pm that had undergone more aging in the atmosphere 
 were less available for reaction with no3/n2o5 . the lack of reactivity of the riverside pm samples 
 has been explained by a higher extent of aging in the atmosphere prior 
 to sample collection . although 
 the riverside samples were archived filters , we found 
 a similar lack of reactivity to no3/n2o5 for riverside samples collected in 2012 with minimal storage 
 times ( < 4 days ) , and we also encountered a reactive riverside nighttime 
 sample ( presumably with local pah emissions and little aging , based 
 on its 2-nf / bep ratio ) . additionally , 
 a set of 16 archived daytime / nighttime pm samples , collected from 
 four sites along the los angeles air basin during a photochemical 
 pollution episode , showed a range of reactivities , with the nighttime 
 urban la sample having the highest reactivity . therefore , we have no evidence that cold - storage of the 
 pm on filters affects the pah reactivity toward heterogeneous nitration . most 
 npahs are known to 
 be direct - acting mutagens , requiring no exogenous bioactivation to 
 convert them into the active form . the 
 direct - acting mutagenic activity ( without addition of s9 ) of the paired 
 beijing pm samples ( with and without exposure to no3/n2o5 , oh radical , and o3 ) is shown in 
 figure 2 . figure si.5 , supporting information , shows the pahexposed / pahunexposed ratios and the npahexposed / npahunexposed ratios for the beijing pm used in the mutagenicity assay after exposure 
 to no3/n2o5 , oh radicals , and o3 . in general , the results were comparable to the beijing pm 
 used in the chemical study in that there was significant formation 
 of npahs upon exposure of the beijing pm - bound pahs to no3/n2o5 ( figures 1b and 
 si.5b , supporting information ) . direct - acting 
 and indirect - acting mutagen densities ( rev / m ) of exposed 
 and unexposed beijing pm extracts and the associated 
 total pah and npah concentrations ( pg / m ) . lower direct - acting mutagenic activity was measured 
 in the two 
 unexposed beijing extracts with lower npah concentrations ( pku-3 and 
 pku-4 ) , compared to the unexposed beijing pm extract with higher npah 
 concentrations ( pku-5 ) ( figure 2 and table 
 si.8 , supporting information ) . this is 
 consistent with a previous study from our laboratory that showed significant 
 daily variation in the direct - acting mutagenicity , and in the corresponding 
 npah concentrations , of beijing pm . after 
 no3/n2o5 exposure , the direct - acting 
 mutagenicity of the beijing pm increased 2- to 26-fold ( figure 2 ) . of the beijing pm samples exposed to no3/n2o5 , the largest increase in direct - acting 
 mutagenicity ( 26-fold ) was measured in sample pku-4 , which also had 
 the largest increases in 1,3- , 1,6- , and 1,8-dnp masses after exposure 
 to no3/n2o5 ( table si.8 , supporting information ) . among these dnps , the greatest increase in mass was for 1,8-dnp ( 10.2 ng ) ( table si.8 , supporting information ) . the 1,8-dnp concentrations 
 measured in the other two mutagenicity study samples exposed to no3/n2o5 ( pku-3 and pku-5 ) were either 
 below the detection limit ( pku-3 ) or three times less than that measured 
 in the pku-4 sample ( pku-5 ) . in previous mutagenicity assay studies , dnps , especially 1,8-dnp , were found to be powerful direct - acting 
 mutagens . small amounts of dnps have 
 been shown to contribute significantly to the total direct - acting 
 mutagenicity of diesel particles . for the oh radical exposure in the direct - acting mutagenicity study , 
 two of the beijing pm samples had no 1-np formation ( and hence zero 
 reactivity according to eq 1 ) ( pku-8 and pku-9 ) 
 and one sample had only 4.1% reactivity ( pku-7 ) ( figure 2 and table si.9 , supporting information ) . overall , the increase in direct - acting mutagenicity after oh radical 
 exposure ( mean of 1.7-fold ) was lower than the increase in direct - acting 
 mutagenicity after no3/n2o5 exposure 
 ( mean of 11-fold ) . the npahexposed /npahunexposed profiles , from both the mutagenicity and chemical studies , showed 
 the formation of 1-np and 6-nbap from oh radical exposure ( figures 1b and si.5b , supporting information ) . given the significant 6-nbap formation in all of the beijing pm 
 samples from exposure to oh radicals , resulting from the formation 
 of hno3 in the chamber ( figures 1b and si.5b , supporting information ) , 
 and the minimal corresponding increase in direct - acting mutagenicity 
 of these same samples , 6-nbap does not appear to contribute significantly 
 to the overall direct - acting mutagenicity of the beijing pm . this 
 is consistent with the structure of 6-nbap which has a no2 group nearly perpendicular to the aromatic ring that does not allow 
 for favorable nitro - reduction . however , 
 an increase in direct - acting mutagenicity and a decrease in total 
 npah mass , as well as a decrease in mutagenic 1-np mass , was observed 
 after pku-8 and pku-9 were exposed to oh radicals ( figure 2 ) . this suggests that other mutagenic degradation 
 products , not measured in this study or below the detection limit 
 of our analytical method , may have contributed to the enhanced direct - acting 
 mutagenicity of the beijing pm after exposure to oh radicals . for the beijing pm samples , figure si.5b , supporting 
 information , shows that 9-nan , 3-nph , 2 + 3-nf , 
 1-np , and 2-np were degraded significantly during the o3 exposure in the mutagenicity study , while only 7-nbaa was significantly 
 degraded during the o3 exposure in the chemical study ( figures 1b and si.5b , supporting information ) . only pku-14 showed a significant decrease in direct - acting mutagenicity 
 of the beijing pm after exposure to o3 , which is consistent 
 with the corresponding decrease in the total npah mass ( figure 2 and table si.10 , supporting 
 information ) . some 
 parent pahs and npahs , 
 including 6-nbap and 1-nitrocoronene , contribute to the indirect - acting 
 mutagenicity of pm . on average , there 
 was a 7-fold and 1.4-fold increase in the indirect - acting 
 mutagenicity of the beijing pm after exposure to no3/n2o5 and oh radicals , respectively ( figure 2 ) . because most parent pahs were degraded after 
 exposure to no3/n2o5 and oh radicals 
 ( figure 2 and tables si.8 and si.9 , supporting information ) , it is possible that 
 the npahs formed contributed to the increase in indirect - acting mutagenicity . 
 increased indirect - acting mutagenicity after parent pah exposure to 
 no3/n2o5 and oh radicals was observed 
 in our previous studies . . found that the exposure of wood soot to no2 and o3 resulted in an increase in both direct- 
 and indirect - acting mutagenicity of the npah fraction . in the same 
 study , the most polar fraction made the largest contribution to the 
 total indirect - acting mutagenicity . this suggests that other , more 
 polar , transformation products may contribute significantly to the 
 indirect - acting mutagenicity of the extracts . moreover , the high molecular 
 weight pahs ( mw 302 ) , including dibenzo[a , l]pyrene which is 30 times 
 more toxic than bap , may also play a 
 significant role in the indirect - acting mutagenicity and were shown 
 to be a significant contributor to the inhalation cancer risk in beijing 
 air . the results of our previous study 
 on the heterogeneous nitration of dibenzo[a , l]pyrene adsorbed on filters 
 showed that the indirect - acting mutagenicity increased 2.5-fold after 
 the no3/n2o5 exposure and that 6-nitrodibenzo[a , l]pyrene 
 was the only nitro product identified . for the o3 exposure , the pahexposed / pahunexposed profiles of beijing pm samples were comparable in 
 the mutagenicity and chemical studies ( figures 1a and si.5a , supporting information ) . 
 the reduction in the mutagenicity ( 33% ) of the two beijing samples 
 exposed to o3 ( pku-12 and pku-14 ) may be associated with 
 the degradation of the total parent pah and npah masses ( figure 2 ) . however , there was no significant change in the 
 indirect - acting mutagenicity in pku-13 when total pah and npah masses 
 decreased ( figure 2 ) . our present and previous data suggest that the riverside pm had undergone 
 more aging prior to collection and was less reactive than the beijing 
 pm . the decreased reactivity of the riverside pm , compared to the 
 beijing pm , may be because the riverside sampling site is located 
 downwind of los angeles and receives photochemically  aged moreover , we previously observed that a night - time pm sample 
 collected from downtown los angeles had a similar reactivity and [ 2-nf]/[bep ] 
 ratio as some of the beijing pm samples . in contrast to riverside ( but like downtown los angeles at night ) , 
 the beijing sampling site is located within a major urban source region 
 and appears to receive air masses that are not as photochemically 
 aged and more influenced by direct emissions . one can therefore conclude 
 that aged particles exhibit decreased reactivity , most probably due 
 to a lower availability of the pahs for reaction and due at least 
 in part to the accumulation of atmospheric reaction products on the 
 surface of ambient pm . on the basis of the observed decrease in reactivity 
 of pm collected along an upwind  downwind trajectory in the 
 los angeles air basin , it appears that , 
 at least in that air basin at that time , photochemical aging occurred 
 rapidly , on a time - scale of a few hours during daytime , and that little 
 or no additional heterogeneous reactions would occur for additional 
 atmospheric transport times . the extent of pah transformation 
 observed in this study may be limited by the multilayer coverage of 
 the ambient pm on the filters . in reality , the transformation of pahs 
 on pm may be more significant in the atmosphere , where the pahs are 
 present on individual particles in the atmosphere rather than on pm 
 layered onto filters . however , the resistance of the riverside pm 
 samples to chemical reaction suggests that secondary pollutant formation 
 plays an important role in making pm - bound pahs less available for 
 chemical reactions . overall , our present and previous results suggest that heterogeneous pah degradation and 
 concurrent npah formation on ambient pm occurs mainly , or only , for 
 particles with minimal photochemical aging and that photochemically 
 aged particles are nonreactive toward heterogeneous reactions of pahs .