secondary organic aerosol 
 ( soa ) , formed in situ from the conversion 
 of gas - phase volatile organic compounds ( vocs ) to oxidized products 
 that partition preferentially to the particle phase , is an important 
 source of aerosol mass both regionally and globally .
it is estimated 
 that approximately 70% of total global organic aerosol production 
 is secondary ( 150 tgc / yr ) , but 
 formation mechanisms remain uncertain . a large fraction of total 
 organic aerosol is thought to originate 
 from the oxidation products of biogenic voc emissions .
these naturally emitted compounds constitute the majority of voc 
 emissions to the atmosphere globally , with a diverse and regionally 
 varied mixture of isoprenoid chemical structures , depending on regional 
 plant species distributions .
the gas - phase oxidation of vocs to condensable 
 products is initiated by reaction with o3 or the oh or 
 no3 radicals .
the relative importance of each competing 
 oxidant depends on the relevant rate constants ( which depends on the 
 voc molecular structure ) and ambient concentrations .
for example , 
 no3-initiated reactions with biogenic alkenes have large 
 rate constants , and will be most important 
 at night ( when photolysis and no reaction sinks for no3 radical are minimized ) , and in regions that contain both high biogenic 
 emissions and elevated nox such as downwind 
 of urban areas or industrial point sources .
soa yields from oh and 
 o3 oxidation of bvoc have been studied extensively , but less is known about reactions involving no3-induced oxidation . because partitioning to the aerosol 
 phase depends on molecular 
 weight and polarity , the precursor voc 
 generally needs to contain a minimum number of carbon atoms ( or undergo 
 condensed phase oligomerization reactions ) to produce condensable 
 oxidation products and thus form soa .
isoprene ( c5h8 ) is the globally dominant bvoc , comprising 50% of 
 total global nonmethane voc emissions by mass with total emissions estimated at approximately 500 tg yr .
however , isoprene and its no3-induced oxidation products 
 are quite volatile , and thus have relatively low no3 soa 
 yields of 424% . the larger monoterpenes 
 ( c10h16 ) and sesquiterpenes ( c15h24 ) , which are much more prevalent in coniferous forests , are 
 more efficient soa precursors via their production of extremely low - volatility 
 organic compounds .
the no3 oxidation of some terpenes has been studied 
 in laboratory experiments , summarized in table 1 .
wngberg et al . , hallquist 
 et al . , and spittler et al .
all observed substantially higher ketone yields 
 and lower organonitrate yields for -pinene than for the other 
 terpenes ; where soa yield measurements are available they roughly 
 correlate with organonitrate yields , showing lower yield for -pinene . 

we note that these chamber experiments may preclude further oxidation 
 of initial products , which could produce additional soa in the real 
 atmosphere ( e.g. , photo - oxidation of pinonaldehyde ) . in hallquist et al . , these ketone yields 
 refer to the dominant single 
 ketone product , e.g. in the case of -pinene , pinonaldehyde .
a systematic controlled chamber 
 study of soa production from six 
 mono- and sesquiterpenes is the focus of the work presented here . 

we measured the soa yield from no3 reacting with the individual 
 monoterpenes -pinene , -pinene , -3-carene , limonene , 
 sabinene , and with the sesquiterpene -caryophyllene ( structures 
 shown in supporting information ( si ) figure 
 s6 and in the abstract ) . since the nitrato - peroxy radicals produced 
 from these terpenes are all tertiary , we expect no ho2 production 
 in the chamber .
hence , these experiments explore ro2ro2 and ro2no3 chemistry only , 
 omitting ro2ho2 chemistry that would 
 contribute in the real nighttime atmosphere .
these experiments were 
 performed in continuous - flow mode in a dark chamber at the atmospheric 
 chemistry division of the national center for atmospheric research 
 ( ncar ) in fall 2011 .
we monitored particle size distributions and 
 total alkyl nitrate concentrations to enable determination of soa 
 mass yields and alkyl nitrate yields .
a detailed 
 description of the 
 ncar chamber facility instrumentation and cleaning and operating procedures 
 for the experiments described herein are provided in the si . figure si shows 
 the chamber diagram , and table si the instrumentation 
 employed in these experiments .
briefly , the 10 m chamber was run with all reagents input continuously under dark conditions , 
 with an average chamber residence time of 4 h. the no3 and 
 n2o5 concentrations were alternately measured 
 either before entering the chamber , or from the chamber itself .
flows 
 were adjusted to give initial [ n2o5 + no3 ] and [ bvoc ] of 10100 ppb in dry zero air .
soa experiments 
 were initiated by injecting bvoc ( 50 ppm in n2 ) 
 into the n2o5 + no3 filled chamber , 
 delivered from the standard cylinders using flow restrictors and added 
 to the zero air flow to obtain the final desired mixing ratios ( table 2 ) .
a series 
 of 11 experiments were conducted with the six bvoc , in most cases 
 with two levels of oxidant and bvoc concentrations : a nominally  high  
 ( 50 ppb ) and a  low 
( 10 ppb ) concentration 
 of each ( see table 2 ) . for the high concentration 
 experiments
only ( expts . 1 , 2 , 4 , 6 , and 9 ) , organic nitrate measurements 
 are available .
in these experiments , total alkyl + peroxy nitrate 
 mixing ratios ( ans + pns ) , subsequently referred to collectively 
 as organonitrates , are determined in the gas and aerosol phases by 
 sampling directly or through a charcoal denuder . except 
 -pinene
experiments , the ratio of n2o5 to bvoc in the 
 inlet line was approximately 1:1 ; for the  high  experiments , 
 the oxidant was generally in slight excess ( with the exception of 
 expt # 9 ) . the highest purity sabinene source available was 80% , and 
 contained 20% -pinene .
the following measurements were made 
 during each experiment : ( 1 ) no3 and n2o5 concentrations on both inlet and outlet flows by cavity ringdown 
 spectroscopy ( crds , wagner et al . ; dube 
 et al . ) , and on the outlet side only : 
 ( 2 ) no and no2 by chemiluminescence , ( 3 ) o3 by 
 absorption spectroscopy , ( 4 ) particle number and size distribution 
 by a scanning mobility particle sizer ( smps ) , and ( 5 ) organonitrates 
 by thermal dissociation  laser - induced fluorescence ( td - lif , 
 rollins et al . ) . the specifications of 
 each instrument used in these experiments are shown in table s1 in 
 the si . in 
 order to interpret
observed aerosol 
 formation , it is essential to understand the effect of chamber wall 
 losses on measured aerosol size distributions . following the methods 
 of van reken et al . , we measured the 
 size - dependent wall loss of ammonium sulfate aerosol at steady - state 
 flow . results and additional analysis details are shown in the si ( figure s2 ) .
average input and output size 
 distributions were analyzed to derive the first - order size - dependent 
 wall loss coefficient , (dp ) , with the chamber considered 
 to behave as a continuously stirred tank reactor:1 the losses are applied cumulatively , 
 so that corrected aerosol mass timeseries include all particles lost 
 since the start of the experiment , following the method of mcmurry 
 and grosjean , and we find values consistent 
 with previous chamber studies .
recent work has shown that vapor - phase wall losses may also substantially 
 contribute to underestimation of chamber soa yields , suggesting that the yields reported here may be lower limits .
the 
 implications of this unconstrained process for observed relative organonitrate 
 yields is discussed in below .
particle 
 formation and growth was observed for all bvocs except -pinene , 
 although with widely varying yields .
the observed time evolution of 
 the particle size distribution function of a representative experiment 
 ( experiment 3 , -pinene at low concentration ) is shown in figure 1 , illustrating the new particle formation and growth 
 that occurred in each experiment ( except -pinene ) .
wall loss corrected 
 soa growth curve for experiment 3 ( -pinene 
 low concentration ) .
figure 2 shows the time evolution of aerosol 
 number ( ntot , figure 2a ) and volume concentration ( vtot , figure 2b , c ) .
these plots show the comparison 
 of soa growth from different bvoc precursors , including both concentration 
 conditions for each precursor ( nominally 50 and 10 ppb for both no3 and bvoc ) . generally , we observe the largest peaks in ntot from the oxidation of the bvocs with two 
 double bonds , -caryophyllene and limonene , regardless of low 
 or high concentration . hence
, for example , the peak ntot from limonene at 10 ppb initial concentration is larger than the peak ntot for 
 all the monoterpenes with a single double bond , even at the higher 
 concentration initial conditions .
in contrast , the vtot curves grow fastest for the higher concentration experiments 
 of all terpene precursors . in the case of -caryophyllene
, the 
 faster volume growth at low loading may be due to differing chemical 
 regime ; higher no3/bvoc ratio could lead to more second - generation 
 products .
time series of total wall - loss corrected aerosol ( a ) particle number 
 concentration , and ( b ) volume and mass concentration , from all experiments 
 producing soa .
panel ( c ) repeats ( b ) , zoomed in on the lowest 30 g 
 m of the y axis to show that 
 the same pattern in volume growth is observed for the low - concentration 
 series as for the high .
low and high indicate the nominal concentrations 
 of bvoc and oxidant in each experiment ; bpin denotes -pinene ; 
 dcar , -3-carene , limo , limonene ; bcary , -caryophyllene ; 
 and sabi , sabinene .
we determined the unitless 
 time - dependent soa mass yields over the course of the experiments , 
 defined as y = m/voc , 
 based on wall - loss corrected change in aerosol mass m ( g m ) divided by amount of 
 voc reacted voc ( g m ) , referenced 
 to the beginning of the experiment when both aerosol mass ( m ) and voc were zero .
since each experiment was initiated 
 with voc injection into a chamber already filled with oxidant , the 
 voc began to react immediately , preventing measurement of its concentration 
 during initial reaction .
since there are uncertainties in both the reaction 
 kinetics and chamber mixing , we employ two separate strategies to 
 bracket the possible range of voc : ( 1 ) a simple observationally 
 constrained method that more accurately captures mixing dynamics due 
 to the high precision and time resolution measurement of [ no3 ] , and ( 2 ) a free - running kinetics box model assuming a well - mixed 
 chamber , that may more accurately capture subsequent ro2 chemistry , but is subject to large uncertainties in ro2 rate constants for these systems and can not capture mixing . for the first method
, we use the known amount of voc injected and 
 the observed [ no3 ] in each time step ( t ) to determine the amount of voc consumed via eq 2:2where rate constants k are taken from calvert et al .
( 2000 ) , [ no3 ] is the instantaneous observed concentration 
 ( and hence includes no3 produced from n2o5 added at each time step ) , and [ voc ] is the sum of concentration 
 added to the chamber and the unreacted concentration in the chamber 
 from the previous time step .
if this calculation of voc results 
 in a reacted amount less than the voc injected during that time step , 
 then the residual voc builds up in the chamber and contributes to 
 the instantaneous rate calculation for the next timsestep . in this 
 way , a voc timeseries is derived that 
 depends only on observed no3 consumption , assuming a one - to - one 
 no3 to voc stoichiometry .
this method ignores any reactions 
 of no3 with first - generation oxidation products ( no3 + ro2 reactions ) , thus rendering it an upper limit 
 on the amount of voc reacted , which leads to lower limit soa yields . 
 however , it has the advantage of allowing our most precise , highest 
 time - resolution measurement to directly incorporate the time scale 
 of chamber mixing into the calculation of voc .
thus , for each 
 experiment we determine a complete voc time series that is 
 constrained until no3 concentration decreases to zero , 
 as shown in the si ( figure s3 ) .
often , 
 the usable smps data is truncated before this occurs , because the 
 aerosol size distribution grows to diameters beyond the measurement 
 range ( 350 nm ) . in the second method of voc calculation
, 
 we employ a free - running 
 kinetics box model constrained only by the known addition rates of 
 n2o5 and voc .
the cumulative voc reacted over 
 time is determined , by iterative solution of coupled kinetic equations 
 for all x molecules:3where [ x]in is the concentration of x in the inlet 
 flow ( only n2o5 and voc are added , at known 
 concentrations shown in table 2 ) , [ x]ch is the concentration in the chamber , q is the flow rate through the chamber ( 40 lpm ) , and v the chamber volume ( 10  000 l ) , y are all chemical 
 losses ( for no3 , reactions with voc , ro2 , and 
 no2 ; for voc , only reaction with no3 ) , z are 
 all chemical sources ( n2o5 decomposition for 
 no3 ; none for voc ) , and kwall , x is the rate of loss of x to chamber walls .
the complete 
 set of reactions and rate constants used are described in the si figure s4 and table s2 .
this method is superior in its chemical complexity , explicitly 
 including ro2 + no3 , ro2 + ro2 , and ro2 + no2 reactions after the 
 initial no3 + voc  ro2 .
however , because 
 these reaction rates are poorly known for monoterpene nitrato - ro2 , this introduces some uncertainty .
further , since we are 
 no longer constrained by no3 data , we must invoke wall 
 losses for no3 and n2o5 . in order 
 to run this model consistently across experiments and with the minimum 
 number of arbitrarily tunable variables , we  spun up  
 the model with continuous n2o5 addition to the 
 initial conditions of each chamber experiment , determining no3 and n2o5 wall loss rate constants that 
 best fit all available data .
each experiment then proceeds by continuous 
 addition of voc and n2o5 at the known injection 
 rates , which we model free - running , with no tuning to match observed 
 no3 decay .
this model assumes a well - mixed chamber at all 
 times , so we interpret discrepancies with observed no3 decay 
 to be due to mixing inhomogeneity or uncertainties in ro2 rate constants ( see si figure s5 for 
 an exemplary model fit of no3 decay ) . because this method 
 incorporates additional losses of no3 , resulting voc consumption 
 is slower , leading to higher apparent soa yields .
both methods 
 of voc determination ignore reaction with the 
 second double bonds in limonene or -caryophyllene , which would 
 deplete no3 further and enhance soa yields .
the rate of no3 reaction at the second , 
 exocyclic double bond is predicted to be 30-fold slower in the case 
 of limonene , so this is not expected to be a large effect , although it may be more competitive in the low 
 -caryophyllene experiment , where n2o5 concentration greatly exceeded bvoc .  
each wall - loss corrected 
 smps size distribution in the time series 
 was converted to m as follows .
first , we iteratively 
 applied the size - dependent wall loss corrections to the full smps 
 time series to derive a complete time series of the size distributions 
 that would have been observed had no particles been lost the walls . 

then , number concentration data from each size bin were converted 
 to soa mass concentration in g m using 
 the mean bin diameter and assumed organic aerosol density of 1.4 g 
 cm .
since no seed 
 particles were used , all observed aerosol is organic , and the wall - loss 
 corrected aerosol mass gives cumulative m . 

the ratio of this m to cumulative voc consumed 
 is the unitless mass yield , ( y = m/voc ) .
we observe differences between high and low concentration mass 
 yield curves ( figure 3 ) that vary with bvoc , 
 suggesting that the experiments differ in more than simply the total 
 aerosol mass loading .
we note from the kinetics modeling that in all 
 experiments , the ratio of ro2no3 to 
 ro2ro2 rates is highest initially and 
 decreases , in the case of the low concentration experiments crossing 
 over so that after several hours of reaction , ro2ro2 reactions are predicted to be dominant , while in the high 
 concentration experiments , ro2no3 reactions 
 dominate throughout . because these rate constants are quite uncertain 
 and not known for specific bvoc precursors ,
we simply conclude that 
 some of the explanation of the higher yields for the high concentration 
 experiments may lie in the differences in ro2 radical fate . 

we note here again an important difference between these experiments 
 and what would be observed in the atmosphere : ho2 radicals 
 were nonexistent in these chamber experiments , but are expected to 
 be a significant reaction partner for ro2 in the real atmosphere .
yield 
 curves for high and low no3 + bvoc soa experiments , 
 using ( a ) observed no3 decay , or ( b ) complete kinetics 
 model to determine reacted bvoc .
bvoc labeling is as in figure 2 . in all low concentration 
 experiments , due to the slower reaction , 

the voc is better constrained for longer periods , and the mixing 
 time scale is faster relative to reactions , resulting in yield curves 
 with less uncertainty but with the same inter - bvoc trend observed 
 at both concentration conditions . due to these differences ,
subsequent 
 discussion will focus only on these low concentration experiments . 
 as is apparent in figure 3 , the high concentration 
 experiments ( and early stages of some low concentration experiments ) 
 produce unrealistically high yields , especially in the full kinetics 
 model , suggesting that in these cases , reactions were faster than 
 chamber mixing and that this box model may be inaccurate . according 
 to absorptive partitioning theory , yields should increase with aerosol 
 mass loading , assuming a constant product distribution over time . 
 if , however , the product distribution is changing over time as ro2 reactive fate changes , then mass yields could appear to decrease 
 as an artifact of a shift from early high molecular weight products 
 to later lower molecular weight products . as a result of these uncertainties 
 at early times
, we do not fit the mass - dependent yield for these experiments , 
 but rather report yields at 10 g m for 
 each low concentration experiment ( table 3 ) . 

we choose 10 g m because it is sufficiently 
 late in the low concentration experiments ( generally 1.5 h ) 
 that irregularities in the yield curve have subsided .
method 1 represents a lower limit 
 yield , since no other ro2 reactions are included , while 
 method 2 s uncertainty lies in the uncertainty of rate constants 
 ( see si discussion around figures s3s6 ) . for -caryophyllene
, 
 extremely 
 rapid modeled ro2 + no3 reactions result in 
 predicted very slow bvoc consumption and thus unreasonably high yields . 

this may be indicative of the rate constants being different for sequiterpene 
 ro2 . because the predicted relative excess of ro2 + no3 reactions is greatest for -caryophyllene , 
 it is likely that the reported yield from the first method ( which 
 neglects these pathways entirely ) is an underestimate .
although we observe different yields for 
 the high and low concentration 
 experiments for each bvoc ( figure 3 ) , the inter - bvoc 
 trend of volume growth observed at the higher oxidant + bvoc concentrations 
 follows a similar pattern seen in the lower concentration experiments 
 ( figure 2 ) .
this suggests that the mechanism 
 responsible for this behavior corresponds to the chemical nature of 
 each individual bvoc . among the monoterpenes with one double bond , 
 -3-carene s initial volume growth ( figure 2b )
exceeds both -pinene and sabinene for both high 
 and low concentration experiments , not matching the trend in reaction 
 rate constants ( sabinene s rate of reaction with no3 is faster than -3-carene s ) .
sabinene was included 
 in these experiments to assess whether the instability of the three - membered 
 ring was responsible for -3-carene s high soa yield ; 
 sabinene s relatively low
a possible mechanism for the observed 
 soa formation trends emerges when examining the structural differences 
 of these monoterpenes .
the no3 radical preferentially adds 
 to the double bond in such a way as to make the more substituted alkyl 
 radical , which will quickly react with o2 to become an 
 alkyl peroxy radical , and then upon reaction with another no3 or ro2 , will produce the alkoxy radical ( mechanism shown 
 the si figure s6 ) . in the subsequent reaction 
 forming a ketone , the bond that is broken can be predicted via structure 
 activity relationships ( e.g. , table 3 in vereecken 
 and peeters , shown in black in si figure s6 ) . because a -alkyl substitution 
 destabilizes the adjacent bond more than a -nitrate , the ketones 
 predicted to be formed from breaking the weakest c  c bond in 
 -pinene and -3-carene are structurally very different . 
 in -pinene , the predicted product has the radical at the opposite 
 end of the molecule from a terminal nitrate . in -3-carene , 

limonene , and -caryophyllene , the radical ends up adjacent 
 to the nitrate group , a structure which will stabilize the radical , 
 possibly making it more likely to undergo subsequent radical - initiated 
 oligomer formation rather than decomposition , and explaining high 
 organonitrate and aerosol yields .
however , we note that the predicted 
 pathway of -pinene bond scission is inconsistent with the observed 
 dominant product channel of pinonaldehyde ( table 1 ) , which is consistent with our observed low nitrate yield 
 from -pinene and potentially explanatory of low aerosol yield
. 
 additional observations of noy balance 
 relevant to the understanding this subsequent radical chemistry are 
 discussed below .
the importance of the nitrate radical in initiating 
 rapid soa growth 
 was tested by conducting another limonene experiment ( expt .
8) , where 
 no3 was generated by o3 + no2 ( more 
 typical in the atmosphere ) rather than n2o5 dissociation . 
 in this case ,
limonene was oxidized by a mixture of o3 and 
 no3 , rather than only the no3 radical .
oh 
 scrubber was used , so oh recycling may have also contributed to oxidative 
 chemistry in this case , given the 86% oh yield from ozonolysis of 
 limonene .
we observed in this mixed - oxidant 
 case both a 3-fold lower peak number concentration and slower initial 
 volume growth than in the case of no3 alone at nominally 
 the same initial oxidant concentration ( [ n2o5]i = 10 ppb vs [ o3]i = 12 ppb and [ no2]i  = 6.3 ppb , in both cases with [ limonene]i = 10 ppb ) .
the rate constant of o3 + limonene is 2.1  10 cm molec s , in contrast to no3 + limonene s 1.22  10 cm molec s. in the observed volume growth curves , 
 later in the experiment the total soa volume in the mixed - oxidant 
 case  catches up  to the no3-only experiment . 

hence , limonene ozonolysis and nitrate oxidation produce similar eventual 
 soa mass yields at comparable initial concentrations , but no3-initiated chemistry appears more effective at nucleating new particles . 

this is consistent with the initial nitrate oxidation products being 
 higher mass and lower - volatility than their ozone and oh counterparts 
 under dry conditions , but both reactions producing substantial yields 
 of products of intermediate volatility that begin to condense as aerosol 
 loading increases .
the wide range of 
 observed soa yields 
 from no3 oxidation of various bvoc is provocative , but 
 perhaps most surprising is the observation that -pinene reacting 
 with no3 produced no soa . because -pinene 
 is frequently used as a model monoterpene in chamber studies , this 
 exceptional behavior relative to all other monoterpenes tested is 
 particularly important .
we seek an explanation in the initial oxidation 
 steps of this reaction , and note that -pinene could be expected 
 to produce an internal nitrate rather than a terminal nitrate , such 
 as would be expected in -pinene , -3-carene , or sabinene 
 ( see si figure s6 ) .
it has been shown that 
 terminal functional groups result in lower vapor pressure than internal , 
 but this observation would seem to be a very extreme manifestation 
 of that trend .
we note that the no3 + -pinene reaction 
 forms little organonitrate ( table 4 ) , and this 
 organonitrate was not observed to partition to the aerosol phase . 

most of the no3 returns to the gas phase as no2 , consistent with previous observations that -pinene chemistry 
 instead produces the diketone pinonaldehyde ( si figure s6 , red pathway ) , which is expected to be more volatile than 
 the organonitrate alternatives .
an example of 
 observed gas- and aerosol - phase organonitrate production is shown 
 in the si ( figure s7 ) .
the nitrogen balance 
 of all high concentration experiments , for which organonitrate measurements 
 were available , are reported in table 4 and 
 described below .
bvoc - based molar organic nitrate 
 yield , aerosol fraction of organonitrates , and organonitrate fraction 
 of total aerosol mass are all evaluated at 2 h after chamber experiment 
 initiation ( data are all from the high - concentration experiments , 
 when the td - lif instrument was available ) ; n2o5-based no2 and ans + pns production are evaluated at 30 
 min after chamber experiment initiation . for -pinene , limonene , 
 and -pinene , no3 decays slowly enough that this 
 n balance analysis can be reliably repeated at 2 h. these are the 
 numbers reported in italics .
several parameters are not available 
 for -caryophyllene , because the n2o5 decay 
 was very rapid and not well - characterized .
we determined molar organonitrate yields from bvoc 
 ( [ans 
 + pns]/[voc ] in ppb units , using 
method 1  voc ) , 
 fraction of total organonitrates in the aerosol phase ( [ans 
 + pns]aero/[ans + pns ] in ppb 
 units ) , and the fraction of total aerosol mass that is due to organonitrates 
 ( mans , aero / mo in g m ) , all evaluated at 2 h into the experiment to capture longer - term 
 trends .
the calculation of total organonitrates requires subtraction 
 from the total td - lif ans + pns signal the fraction of chamber n2o5 that is detected in this channel ( empirically 
 determined by comparison to crds to be 11% ) , and if dinitrates are 
 present , then they would appear as 2 nitrates .
the calculation of 
 total and organonitrate aerosol mass concentrations require ( a ) the 
 conversion of gas phase organonitrates to mass concentration , using 
 assumed values for the average organonitrate molecular weight ; and 
 ( b ) assumptions about average density of smps - measured aerosol and 
 the average organonitrate molecular weight to convert mixing ratios 
 to mass concentrations for comparison to smps - derived total aerosol 
 mass concentration .
we assume an aerosol density of 1.4 g cm , and that ans+pns are on average hydroxynitrates of molecular weight 
 214 g mol ( 286 g mol for -caryophyllene ) , 
 hence at the experiments ambient pressure ( patm = 0.8 atm ) , 1 ppb = 7.0 g m ( 9.4 g m for -caryophyllene ) .
in addition , we investigate the observed nitrogen balance in these 
 high - concentration experiments by comparing molar no2 release 
 from n2o5 ( [no2]/[n2o5 ] in ppb units ) and organonitrate formation ( [ans 
 + pns]/[n2o5 ] in ppb units ) to 
 assess closure . for these measurements , we use data 30 min after initiation 
 of the chamber experiment , to ensure that [n2o5 ] is still well - constrained .
first , we note that in all experiments except -pinene , 
 organonitrates constitute a substantial fraction of total aerosol 
 mass ( greater than 60% ) , and a large fraction ( 50100% ) of 
 observed organonitrates are in the aerosol phase .
this indicates that 
 the organonitrates produced in no3 + monoterpene or sesquiterpene 
 reactions tend to be of low volatility , with the largest aerosol - phase 
 fraction ( lowest volatility ) nitrate arising from the oxidation of 
 the single sesquiterpene tested , -caryophyllene . 

there is a correlation between organic 
 nitrate molar yields and 
 aerosol mass yields ( see table 3 ) , with -pinene s 
 low organonitrate yield corresponding to no soa production , while 
 -3-carene and limonene have both larger organonitrate yields 
 and soa yields .
however , the correlation is not strong ( consistent 
 with previous observations by hallquist et al . ) .
for example , -caryophyllene does not fit the trend : 
 despite the largest soa yield , it has the second - lowest organonitrate 
 yield .
we note that the -caryophyllene experiment differed 
 from others in terms of initial conditions , with [ bvoc ]  [ n2o5 ] , which may explain this discrepancy .
in addition , 
 as a sesquiterpene ( c15 ) , -caryophyllene starts 
 at a lower volatility than other precursors , which may also affect 
 the degree to which non - nitrate products might also partition to the 
 aerosol phase .
its larger size also may affect vapor - phase wall losses , 
 which are observed to be faster for larger carbon number molecules , potentially depressing apparent soa yield relative 
 to the monoterpenes ( c10 ) .
the nitrogen balance provides additional indications of differences 
 among precursors ( a schematic of n balance is shown in si figure s8 ) . no2
release greater 
 than 1 indicates that some of the no3 + bvoc products fragment 
 to rerelease no2 ( the first no2 molar equivalent 
 comes from the initial dissociation of n2o5 ) . 

this no2 release is greatest for -pinene , which 
 does not produce soa , suggesting that the non - nitrate / fragmentation 
 products are less condensable . in all cases , the total n balance 
 after 30 min reaction time sums 
 to less than two total , the total that would be expected for closure 
 ( as there are two molar equivalents of n in each n2o5 precursor upon which the n balance is based ) .
this shortfall 
 could be due to faster wall losses of organonitrates , or to the production 
 of noy species not measured , most notably , 
 hno3 .
previous chamber studies of no3 + -pinene 
 and limonene have demonstrated substantial hno3 formation ; in these experiments , -pinene and limonene exhibit the least 
 total n closure .
furthermore , as can be seen in the three available 
 time - dependent n balances , the n shortfall is largest at the beginning 
 of the experiments , when [ no3 ] most dramatically exceeds 
 [ bvoc ] , resulting in the greater likelihood of other later - generation 
 no3 reactions such as h abstraction to produce hno3 . in summary ,
soa yield is greater for the sesquiterpene 
 than monoterpenes , 
 and within the monoterpenes is roughly correlated with the production 
 of low - volatility nitrates .
soa formation is hindered by excessive 
 fragmentation and no2 release ( as observed in -pinene ) .
the observed soa 
 yields range widely , from 0 to 86% at 10 g 
 m background aerosol , but display consistent trends 
 across experiments at different initial concentrations , suggesting 
 that differences are a function of bvoc precursor molecular structure . 
 in past global modeling efforts ,
a single yield has been applied for 
 aggregate no3 + terpenes , for example , pye et al .
, which gives soa yield at 10 g m of 26% and find that adding in no3 oxidation of terpenes 
 increased global soa production by 3 tg / yr . if we apply our speciated 
 yields to the case of a rocky mountain front range forest , where the 
 mix of bvoc is 1:1:1 -pinene/-pinene/-carene , then we obtain a bvoc - weighted yield of 24%36% 
 ( range based on yields from two voc methods ) , which falls in 
 line with the pye et al .
however , in parts of 
 the country where -pinene is dominant among terpenes ( coastal 
 pacific northwest , deep south ) , this 
 aggregate number may overestimate no3-initiated soa production , 
 while areas where other terpenes dominate ( central plains , northeastern 
 u.s . ) , the aggregate may underestimate no3-initiated soa 
 production . an additional factor to note when comparing to the 
 real atmosphere 

is that the relative importance of no3 oxidation vs o3 varies among these molecules : at 10 ppt no3 and 
 30 ppb o3 , nitrate reaction rates proceed 2090 
 times faster than ozonolysis for all the monoterpenes , but for -caryophyllene , 
 only half as fast . thus , -caryophyllene 
 will be relatively less likely to be oxidized by no3 .
a 
 true assessment of the role of these reactions requires more detailed 
 regional modeling that can capture the spatial overlap of speciated 
 natural bvoc emissions with anthropogenic nox plumes and resulting oxidation chemistry .
 the 
 secondary organic aerosol ( soa ) mass yields from no3 oxidation 
 of a series of biogenic volatile organic compounds ( bvocs ) , 
 consisting of five monoterpenes and one sesquiterpene ( -pinene , 
 -pinene , -3-carene , limonene , sabinene , and -caryophyllene ) , 
 were investigated in a series of continuous flow experiments in a 
 10 m3 indoor teflon chamber . by making in situ measurements 
 of the nitrate radical and employing a kinetics box model , 
 we generate 
 time - dependent yield curves as a function of reacted bvoc . 
 soa yields 
 varied dramatically among the different bvocs , from zero for -pinene 
 to 3865% for -3-carene and 86% for -caryophyllene 
 at mass loading of 10 g m3 , suggesting that 
 model mechanisms that treat all no3 + monoterpene reactions 
 equally will lead to errors in predicted soa depending on each location s 
 mix of bvoc emissions . in most cases , organonitrate is a dominant 
 component of the aerosol produced , but in the case of -pinene , 
 little organonitrate and no aerosol is formed . 
Introduction
Experimental
and Theoretical Methods
Results
Implications for Modeling SOA Formation