AtmosphericEnvironmentVol. 24A, No. 6, pp. 1433 1441, 1990. 0004 6981/90 S30(1 + ().IX)

Printed in Great Britain. Pergamon Pres', pie



### PHOTOCHEMICAL AEROSOL FORMATION FROM AROMATIC HYDROCARBONS IN THE PRESENCE OF NOx

KATSUYUKIIZUMIand TSUTOMUFUKUYAMA DivisionofAtmosphericEnvironment,The National Institute for Environmental Studies,P.O.TsukubaGakuen, Ibaraki 305,Japan

(First received 14December 1988and infinalform 1 May 1989)

Abstract Byusinga4-m3chamber,photochemicalaerosolformationfrommixturesofaromatichydrocarbons(AHCs)andnitrogenoxides(NOx)wasstudiedwiththemainpurposeofdeterminingtheaerosolyield. Altogether, 18 aromatic species were investigated, including toluene, xylenes,ethylbenzene and methoxybenzene.Evolutionofthe aerosolwasmonitored through the measurementofthe total number([CN]) and volume(I/")concentrations.Aerosol-forming potential ofdifferentaromatics wasexaminedinterms of the maximumgrowthrates ofI-CN]and V.Byputting dry sheath air intoanelectricalaerosolanalyzer,the net organicvolumeconcentration (Vd) was obtained;then a relationwasestablishedbetweenlidandthe C
massconcentration,andwasusedto determinethe aerosolyieldasdefinedbytheratio oftheparticulate C
massto theCmassofthereactedHC. Itwasfoundthat theyieldswerelargerfortoluene,ethylbenzene,and
o- and m-ethyltoluene (3.0, 3.1, 3.3 and 3.7%, respectively)but smaller for para-substituted toluene derivatives(< 1.5%).
Key word index: Photochemical aerosol, aromatic hydrocarbons, aerosol yield.

INTRODUCTION

Aromatic hydrocarbons (AHCs) are known to form a major category of organic pollutants in the atmosphere, accounting for about one-third of the abundance of nonmethanehydrocarbons (NMHC) (Nelson
et al., 1983; Sexton and Westberg, 1984;Grosjean and Fung, 1984). Among them, benzene, toluene, xylenes and ethylbenzeneare the most common species found in the polluted urban air, and comprise more than 80% ofthe total amount ofAHCs (Harkov et al., 1984 Singh et al., 19851.Accordingly, considerable efforts have been made to throw light upon their fate in atmospheric reaction processes. The present status of knowledge of the reactions yieldinggaseous products has been extensivelyreviewedby Atkinsonand Lloyd (1984) and by Atkinson (1985). On the other hand, research on the process leading to particulate products was surveyedbyGrosjean(1977) up to 1975,and following the studies collected in that review paper, there were several more investigationson the aerosol formationfrom AHCs. However, the existingdata for the aerosol yield are rather limited, although there

were a number of studies in which the yields were determined without discriminatingwhether the products were in gas- or aerosol-phase (Nojima et al., 1975, 1976; Spicer and Jones, 1977; Besemer, 1982; O'Brien et al., 1983). From the standpoint of air pollution control, it is important to determine the yield of the aerosol proper, as the behavior of the particulatepollutantsisquite differentfrom that ofthe gaseous ones. The aerosol yields have been determinedfor toluene(Leone et al., 1985;Gery et al., 1985; Stern et al., 1987) and for some other aromatics (Grosjean, 1984a; Stern et al., 1987;Gery et al., 1987), butthey are notreported ina veryconvenientmanner,
as the yields were defined differently by different authors. For example, Leone et al. (1985) gave the yieldfor tolueneas a percentage ofthe initiallypresent C, whereas Gery et al. (1985) gave it as percentage of the reacted C. The yieldreported by Stern et al. (19871

was based on the total mass, which was estimated by assuming that the aerosol particles had a density of 1gcm- 3. In view of these circumstances, the present study was attempted to redetermine the yields on a consistent basis so as to facilitate quantitative comparison among different aromatic species. Apart from the major aromatics studied thus far, several other species of this category are detected in the ambient air: styrene, trimethyibenzenes,ethyltoluenes and propylbenzenes (Nelson et al., 1983;lsidorov et al., 1983; Grosjean and Fung, 1984). Although their concentrations in the atmosphere are much lower than those of the major species, it is nevertheless important to determine their aerosol yield carefully, as they have a larger rate constant for the reaction with the OH radical, which is the principal reactant that attacks the aromatics in the atmosphere (Atkinsonet at., 1984).It was therefore decided to includein this study such minor aromatics as well.

EXPERIMENTAL"

Allthe experimentswerecarried out by using an 'aerosol chamber', a description of which was reported previously
1433
AE(A} 24:6~G1434 KATSUYUKIIZUMIand TSUTOMUFUKUYAMA

(Izumi et al., 1984, 1986). Briefly, it is a cylindrical and evacuable 4-m3 reaction vessel and has quartz windows on one side for photoirradiation; its inner walls are lined partly with borosilicate glass and partly with Teflon. The surfaceto-volume ratio is 4.9 m-1. The chamber was filled with purified air to a pressure of 820 Torr; the contaminant concentrations in the matrix air were <2 ppb for NOx, 0.04ppm-C for total HCs and 0.1 cm-a for condensation nuclei. Before entering the chamber, the air was made to go through a humidity regulator, which added ~t prescribed amount ofwater vapor. The reactant gases, NOx and one of the AMCs, were measured volumetrically in advance in a gas-handlingequipment and then introduced into the chamber with a stream of ultra high-purity N2. After mixing the gases with electric fans, the temperature of the test atmosphere was adjusted to be 302+0.5 K; the relative humidity (RH) was regulated in a range 50+ 1% at theinitialtemperature by measuring it with a dew-point meter (Yokogawa Electric Works, model 2586). Thereactionmixturewasexposed to lightflux froma solar simulator composed of 12 1.6-kW Xe lamps. Light with wavelengths shorter than 290 nm was eliminated by Pyrex filters. The intensity of the light as determined by the photolysisrateconstantofNO2 was0.27 min- t.Theirradiation was continued for 60-300 min accordingto the reaction progress, which was monitored through the variation of the gas concentrations. Irradiation for 150min, for example, resulted in a temperature rise of about 3 K. Concentration of the HC was measured every 5min by a gas chromatograph with an FID detector by using a I m
x 3mm diameter stainless steel column with 20% silicone OV-101 on 60--80meshAW,DMCS ChromosorbW. Cbemiluminescent analyzers (Monitor Labs., models 8440 and 8410)
were used to monitor the concentrationsofNOt and Ov The numberconcentrationoftheaerosolparticleswascontinuously measured by a condensation nucleus counter (CNC; TSI model 3020), and the size distribution measurement was made intermittently by an electrical aerosol size analyzer (EAA; TSI, model 3030) in the range 0.01-1 #m. Particles smallerthan0.01 #m contributetothenumberconcentration onlyin theveryearly stages oftheaerosolevolution.Then by integration of the size distribution, total aerosol concentrations based on number, surface area and volume were derived. The total number concentration obtained by CNC
was found to be slightly lower than that from EAA, but their difference was less than 20%. As described previously (Izumi et aL, 1986),the dew point ofthe sheath air for EAA was carefully matched with that of the chamber air within +0.5 K during the irradiation, to prevent artificial growth or evaporation of the sampled particles in the EAA. However, after the termination of the irradiation, a few additional EAA measurements were made, in which the dew point of the sheath air was deliberately made lower than 243 K (r.h. < 1% at 302 K). The aerosol volume concentration obtained in these measurements is denoted as Vdhereafter; as theparticles lose water in contact with the dry sheath air, V a represents the volume concentration solely due to organic condensates (Izumi et al., 1988). In addition to the main series ofexperiments, which were done to determine the aerosol yield from the mixtures of about 1ppm HC and 0.2-0.5 ppm NO~, another series was conducted to examinetherelationbetweentheconcentration ofaerosol organic C (AOC) and the total volume concentration. In this series, the EAA measurement was done with dry sheath air and simultaneously the aerosol product was sampled on a filter (Pallflex 2500 QAST, effective size 9 mm diameter; preheated to 973 K). Subsequently, the AOC contentin thesample wasquantifiedasCO2 by a thermalcarbon analyzer (TCA; Kimoto Denshi Co.). The details ofthe AOC determinationare reported inotherpaper (Izumiet al., 1988); in particular, care was taken to eliminateinterferenceby gasphase organics adsorbed on the filter. In some ofthe runs in this series, the initialconcentrationsoftbe reactants were set somewhat higherthan in the main experiments(1-3 ppm for the HCs and 0.2-0.6 ppm for NOx) to increase the AOC concentration,but the HC to NOxratio was kept at five, the
same value as in the main experiments. In the determination of AOC from benzene, the aerosol formation had to be enhanced by usingmethylnitriteinstead ofNO~, because no particles were formed from a mixture ofbenzene and NO or NO2, as described later. For comparison with aromatics, the dark reaction of cyclohexene with 03 was also included in this series of experiments. The initial concentrations of cyclohexene and 03 were 0.35-0.5 and 1.5-2.8 ppm, respectively. A result of the blank test should be noted here. To check the background aerosol formation, the irradiation was done
on matrixair to which onlywatervapor (r.h.= 50%)and NO,, (about 10ppb) were added. After about 200 min, particulate matter wasfound to beformed up to a volume concentration of 3-4 #m3cm-3. This extraneous formation of particles obstructed a trial to reduce the initial HC concentration to a level in the actual environment (0.01 ppm or less). For the present, therefore, all the experiments were carried out with the HC concentrations close to 1ppm: It was found, by another test experiment, that decrease in concentrationdue to wall losswas not significantforthe gasphase components, 03, NO,` and the HCs. In addition, the decay of mass concentration of the aerosol due to wall deposition was about 2% h-~, which is consistent with our previous studies (Izumi et al., 1986, 1988).The correction for the decay was calculated to be less than 3% of the volume concentration measured at the end of the irradiation. Altogether, 17 AHCs and methoxybenzene were investigated in this study. They cover most of aromatics that have
ever been detected in urban polluted air: some of them are found in car exhaust (Greadel et al., 1986). fl-Methylstyrene
was includedforcomparison with a-methylstyrene,although theoccurrence oftbe former in the atmosphereisnot known. All ofthe HCs were ofthe research grade supplied by Tokyo Kasei Kogyo Co., and were used afterdistillation in vacuum. NO and NO2 were obtained from Matheson Gas Products; the former was used after purification by repeated freeze-thawcycles in a bulbcontainingAscarite at 77 K, and the latter was purified by distillation after making it react with 02 evaporated from liquid 02.03 and methylnitrite mentioned above were prepared by procedures reported previously (Izumi et al., 1984, 1988).

RESULTSAND DISCUSSION

Formation of aerosols
Results of the main series of experiments are summarized in Table 1, together with the initial conditions. As already mentioned, the initial concentrations ofthe AHCs were set to be about 1 ppm. In most ofthe
runs, NO was used as the initial NOx, but in runs 7 and 9, NO2 was used instead ofNO, whereas both NO and NO2 were used in run 8. The initial NOx concentration was about 0.2 ppm except for runs 6 and 7, in which it was raised to 0.5 ppm. A few more runs using m-xylene were made with higher initial NO,` concentrations to see the effect of an increase in [NOx]o/[HC]o ratio. In these runs, however, the resultant aerosol size distributions became broader in early stages of the aerosol formation, and the sizes of the larger particles exceeded the upper limit (1/zm) of the EAA range. Therefore it was decided to deal preferentially with the low-NOx case, on which this paper reports. In run 22, ~-methylstyrene was made toPhotochemical aerosol formation 1435

E

0

0 i

m

o

X

i

O',t


## ,o,

~'~

o



## =o 

# o~ • -a .z,~



## ,=,~

= 

## ~$$~q~1

A

I 

## ,~~ I~ ~,~~ A ~ t-,l~ ,



### -°~°°~~~~ I~

~

000000000000000000000 0 .,~

i



# ~ ~ .=.

~ 

# ~ ~

0 ~ '~,



## ~ ~

•~, ~

,.,?: 

### g~-O

~ ~ 00

•,='~= = =..=_= [.- Z -- ~ b-,.

### 1436 KATSUYUKIIZUMIand TSUTOMUFUKUYAMA



### react with 03 in the dark to exclude reaction with the OH radical; the purpose of this experiment was to examine the relative contributions of03 and the OH radical to aerosol formation. As a typical example, time variations of the gas concentrations observed in the experiment with toluene (run 1)are shown in Fig. 1.The aerosolevolution in the same run is illustrated in Fig. 2 as observed by EAA; here N, S and Vreferto total aerosol concentrations based on number, surface area and volume, respectively,and aTp

denotes geometric mean diameter of the particles on the basis of N. To check the reproducibility of the curves in Figs 1 and 2, three

more runs were conducted with similar conditions, and, as a result, it was found that features such as parallelism between [03] and V time profiles were confirmed. In addition, parameters, shown in Table I, which characterize the gas-phase and the aerosolformation processes agreed within 10%. The mean diameter grew up to 0.1 #m in 1 h after the onset ofthe aerosolformation and became stationary in later stages. The geometric standard deviation, try, which is a measure ofthe width of the aerosol size distribution, remained almost constant at 1.4+0.1 throughout the reaction. Apart from minor differences mentioned later, the time variations of the gas and particle concentrations such as shown in Figs I and 2 were generally similar for all the aromatics exceptbenzene. The results ofthe main experimentsare collectedin Table 1,in which are given several parameters descriptive of the gas-phase and the aerosol formation processes. As shown there,, low photochemical reactivity of benzene (Grosjean, 1977, and references cited therein) was confirmed: when more than a half ofthe initialNO~ was NO, no 03 was formed (run 8). When NO2 instead ofNO was used, O3 did appear but its build-up ended shortly after the start ofthe irradiation,reaching a concentration only about a tenth ofthat for the other aromatics (run 9). Moreover, little decrease in benzene concentration and no particle formation were observed in both ofthe runs in spite ofthe irradiationbeing longer than in the other runs. Slight formation of 03 observed in run 9 was certainly a result of the photolysisofNO2and not ofany reaction ofbenzene. The peak concentrations of Oa and CN are listed in Table 1 under the headings [O3],~x and [CN'Jm~. respectively,together with the time when [03] and [CN] attained the peak values.The timeisa rough measure of the rapidity ofthe reaction progress. In runs 10-15, 19, and 21, [03] did not show any definite maximum. For these runs, therefore, the irradiation time and [03] at theend ofthe irradiationarelisted.From themaximum

or the terminal values of [Oa], it is seen that [03] had much the same level for different aromatics. However,the timefor [03 ]ma, variedamong them: 03 formed especially rapidly for m- and p-xylene, and 1,2,3- and 1,3,5-trimethylbenzene, but very slowly for ethylbenzene, ~t-methylstyrene, styrene and propylbenzene isomers. The time for [CN] m,x also varied among the runs but its behavior did not parallel the behavior ofthe time for [O3]m~. The rate of increase in [CN], which is expected to be approximately proportional to the nucleation rate, was maximal in the very early stage of the particle formation; the maximal values are given in Table 1 as 

## (d[CN]/dt)m~. On the basis of [CN]m,~ and (d[CN]/dt)

.... the Ca aromatics were found to have a higher aerosol-formingpotential than the Ca ones; the potential was particularlyhigh for m-xyleneand ethylbenzene. Comparison ofthe results from runs 3, 6 and 7 indicates that an increasein [NO]o and replacement of NO by NO2 both significantlydecreased [CN]m~ and (d[CN]/dt)m~x. In Table 1 is also given the maximum growth rate, (d 

## V/dt)

.... of the aerosol volume concentration, together with the time of its appearance. The rate was largest for m- and o-xylene, and m-ethyltoluene, and

was smallest for the

## para-substitutedisomers among the three isomers of either xylene or ethyltoluene. Furthermore, among the three isomers of trimethylbenzene, the 1,2,4-substituted isomer, which is the only one with methyl groups in the

## para-position,has the smallestrate. Thus the

## para-substitutionin toluene

seems to suppress the volume-basis aerosol formation rate. Both of the aerosol formation rates,

&05
04

03
's Q2

0~ § 20 40 60 80 100 120 140 160

Irradiation Time (rain} E & ~8--

362



### Fig. 1. Time variationsoftoluene, NO, NOx and

0 3 

### concentrations for run 1. 701 ~-"3I 7E I r I , I i r 60

. V 5O

>10 (") Z 1 0.1

0 L 0 L 0 I I 0 I*'~3 i 40 8i0 120 16 [rradiotion Time (rain) Fig. 2. Time variations of total number (N), volume (V), 

### surface (S) concentrations ofthe aerosol, and geometric/
mean diameters (dp)on

### thebasisofN. Resultsfrom Run 1.

### Photochemical aerosol formation 1437

(d[CN]/dt)ma~ and (d

### V/dt)

.... were particularlysmall for n-propylbenzene and styrene. In the next column of Table 1, the aerosol volume concentration at the end ofthe irradiation is given as Vf. It is found to be smaller for p-xylene, 1,2,4- trimethylbenzene and styrene. Thus some extent of parallelism is perceptible between the trends in (d

### V/dt)max and Vf. It should be noted that the volume concentration had not leveledoffwhen the irradiation

was terminated in runs 7 and 10-15; in these runs the particle size distribution was conspicuously broadened during the aerosol evolution, so that the irradiation was ended before the size of the larger particles exceeded the upper limit of the EAA range. As described before, the mean diameter of the particles became stationary in later stages of the reaction. This stationary diameter is given as dp in Table 1. It did not vary much among different HCs, being in a range between 0.08 and 0.11/~m, typical of photochemical aerosols; nor was the mean diameter influenced by the increase in [NOlo. When [NOlo was raised to 0.5 ppm in run 6, both (d 

### V/dt)max and Vfincreasedcompared with the values for run 3. In run 7,in which NO was replacedbyNO2, the irradiation was interrupted for the reason mentioned above, when d 

### V/dt and dpwere stillincreasing. Thus in Table 1, the terminal values are given for (d 

### V/dt)max and aYpas the lower limits.



### Net organic volume concentration On the analogy ofthe hygroscopicnature observed for the aerosol from cyclohexene (Izumi 

### et al., 1988), the volume concentration, V, obtained by EAA was expected to include a contribution of liquid water in

an equilibrium with the water vapor in the gas phase. To assessthe net content oforganic material,the EAA measurement of volume concentration was made by using dry sheath air, as described in the Experimental section. The resultant concentration, Vd,was found to be significantlyless than V. For example, the ratio of V dto Vwas 0.86_+0.04 for toluene; in other words, the water content in the aerosol product was 14% by volume, when RH in the reaction mixture was 50%. The ratios 

### Vd/V are listed in Table 2 for all the aromatics studied here. Uncertainty in this ratio was estimated to be about 5%, which corresponds to twice the standard deviation obtained by repeating the measurement of 

### Vd/K The ratios were much the same for different HCs (in the range 0.83-0.98), and were found to be significantly larger than 0.73 for the aerosol from cyclohexene (Izumi 

### et al., 1988), suggesting that the organic condensates from AHCs are
more hydrophobic. The ratio did not change when [NO]0 was increased. The maximum growth rate 

### (dV/dt)=a~ was converted into (d 

### Vd/dt),,axby multiplying the former by the ratio 

### Vd/V; the resultant represents the maximum growth rate ofthe net organic volume concentration, and values are listed in Table 2. In the table, the rate constant (kon) for the reaction of OH radicals with each HC (Atkinson, 1985) is given in the last column for comparison, as that reactionis the most important loss process for the AHCs in the atmosphere (Atkinson 

### et al., 1984). No parallelism is found between (d 

### Vd/dt)max and koH. As described in the Experimental section, the relation between AOC concentration ([AOC]) and the volume concentration, Vd, was examined for aerosols generated from various HCs, and is shown in Fig. 3. The result for benzene is also included, as it was obtained byusingmethylnitriteto enhance the aerosol formation (see Experimental section). It is seen from the figure that the data from different HCs including cylohexene lie on a single line passing through the origin. By a least-squaresfitting,the followingrelation

was obtained: [AOC] =c V~, (1)
with c=0.49+_0.02 g-Ccm-3, the error limits being twice the standard deviation. This coefficient represents the C mass density in the aerosol particles. The result shown here indicatesthat the particlesproduced from the AHCs and cyclohexene have much the same C mass density. Once a relation isestablishedbetween [AOC] and I/d, the former is more readily obtainable from the latter than from the filter collection and the subsequent analysisby TCA. On the basis ofEquation (1), for example, the maximum growth rate of [AOC]

can be derived from the rate for 

### Vd: (d[AOC]/dt)max= 0.49 x (d 

### Vd/dt)max. (2)
The maximal rate (d[AOC]/dt)max thus calculated is given in Table 2 for each HC. The C mass production rate fortoluene was found to be 35 #g-Cm 3h- 1;this value is comparable to that (31-83pg-Cm-3h 1) observed for o-cresol (Grosjean, 1984a), which is one of the major photo-oxidation products from toluene and is known to be a reactive aerosol precursor.



### Aerosol yields
To find how much aerosol was produced from unit concentration oftoluene, Vis plotted in Fig.4 against the concentration loss, A[HC-], due to the reaction. It should be noted that about 0.05 ppm of toluene was consumed before the aerosol volume concentration

rose to a detectablelevel.The existence of such 'latent consumption' was observed for the other HCs as well, and the amount of the consumption was not affected by the replacement of NO by NO2, indicating that it did not represent the amount of HC required to oxidize NO to NO2. From this result, it was inferred that the aerosol was not produced directly from the HC but secondarily from the gaseous products and such a two-step mechanism caused the latent consumption. This inference is consistent with the report of Grosjean (1984a, 1985), who suggested that the aerosol products from toluene were formed via such intermediates as cresol and nitrocresol, and also with1438 KATSUYUKI IZUMI and TSUTOMU FUKUYAMA

<

0

0

0

e~ 0

e~

ei

b, 

### l= ~

< =

,:~

El

Oo

++.~-



### E? ~ .. .

+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1+11

~~~~ . . .

+1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1

~~=~~ ~ .~

+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1~



### oo~o~ooo~oooo
+1+1+1+1~+1+1+1+1+1+1+1+1+1+1+1+1+1+1 0 '.=.

~d

0

n.

0



### Y=,

x~ 

### ~ ,o ~-
-~ 

### ~ ~.~

~e 

### 8 ~uPhotochemical aerosol formation 1439

160

'E

120 

## :2

80

< 4O i i i i ~ i i

÷

t, "I" •

100 200 300 400
Vd by EAA (lO'6cm3rn3) Fig. 3. Relation betweenmass concentration ofaerosol organiccarbon (AOC) and aerosol volumeconcentration measuredwithdry sheathair(Va)observedfor different HCs: II, benzene; ©, toluene; A o-xylene;O, m-xylene;C], p-xylene; V, 1,3,5-trimethylbenzene; A, ethylbenzene; +, cyclohexene.

80

'E 6O 

### %

4O

> 2O

0A 02 03

[t01uene] (ppm) 04

Fig.4. Plot ofaerosol volumeconcentration (F) against decrement in toluene concentration (A[HC]). Result

from run 1.

the fact that the net organic aerosol production rate, (d 

### Vd/dt )

.... was not correlated with the rate constant of the reaction with OH radicals. As shown in Fig. 4 by a broken line, V depended linearly on A[HC] in a range between 0.08 and 0.20 ppm. The slope of the linear portion gives the volume basis aerosol yield per unit concentration of the reacted HC. It was evaluated to be 411
+ 26/tin3crrr- 3ppm- ~for toluene,with two standard deviations of the least-squares fitting. By multiplying by 

### Vd/V, this is converted into the V dbasis yield, i.e. the yield on the basis of the net organic volume concentration. This was found to be 353
+ 28 #m3cm- 3ppm- 1. Similar results (335+ 27, 335
+ 18 and 335+ 19#m3cm- 3ppm- x) were derived from the additional three runs which were conducted to check the reproducibility.These values correspond to the maximumyieldobtainedduringthe reaction,as they are derived from the maximum slope of the V-A[HC] curve, as shown in Fig. 4. In this sense, the yields on the basis of V and V d are denoted as (AV/A[HC])ma. and (AVJA[HC])

.... respectively,

and are listed in Table 2 together with the values for the other aromatics. The overall yield, which is calculated as a ratio of Vfto the total HC consumption,is much smaller. The maximum aerosol yield on the basis of C mass is also derivable from the volume basis yield:

maximum AOC 

### yield/%=(AVJA[HC])m,,

x (0.49/483n) × 100, (3)

where n denotes the number of C atoms in a HC molecule,and the constant483 is a C mass concentration in a unit of #g-Cm-3 equivalent to an HC concentrationof 1ppm-C at 303 K under 1 arm. The maximum AOC yield obtained in this way is listed in Table 2, together with the overall yield. Here the former yield, 4.9+ 0.4%, for tolueneis an average over the four runs. The maximumAOC yield is the largest for methoxybenzene, 7.5%, but among the HCs, the highest yield was 6.2%, which was obtained for methyltoluene.The yield for p-xylenewas less than half ofthat for the other xylenes. The yields for p-ethyltoluene and 1,2,4-trimethylbenzene are smaller than those for the other corresponding isomers, being similar to the xylene case mentioned above. These observationssuggest a lower aerosol formation ability ofp-methyl-substitutedaromatics, and this is particularly interesting in view of the similar finding concerning the aerosol production rate. The yields for styrene and the two isomers of methylstyrene are significantly smaller than the others. By comparing the results from runs 3 and 6, it was found that the difference in the initial NO concentration did not affect the yield. Comparing the overall yields for ~-methylstyrene obtained in runs 20 and 22, it is seen that the dark reaction of this HC with 03 produces aerosol more effectively than its photo-oxidation. This indicates that the yield from the reaction with Oa is larger than that from the reaction with OH. Accordingto a recent study (Fung and Grosjean, 1984), the aerosol yield from the reaction of /~-methylstyrene with 03 is 2.4-13%. Thesevaluesare significantlylarger than the yield obtained in this study (run 21). This is probably becausethe 0 3reactionhas a larger aerosol yieldthan the OH reaction also in the case of fl-methylstyrene. As the relation between Vand A[HC] is S-shaped,

as shown in Fig. 4, the overall yield depends upon how far the reaction has proceeded. This may be one of the causes of the rather wide range of the yield values for toluene reported so far (Leone 

### et al., 1985; Gery 

### et aL, 1985).The overallyield (3.0%) determined inthis studyisembeddedinthe range (1.4-6.2 %) given by Gery 

### et al. (1985). Our yield data for toluene, mxylene and 1,3,5-trimethylbenzene seem to be consistentwith those of Stern 

### et al. (1987) ifthe difference in the definitionof the yield is taken into account. As described above, the aerosol yields were obtained for most ofthe AHCs found in the polluted air.1440 KATSUYUKIIZUMIand TSUTOMUFUKUYAMA



### Therefore it is now desirable to estimate how much aerosol is formed from those HCs in a typical urban condition. However, application of the present result to the situationencountered in the actual environment is hampered by the limited range of the two experimental parameters, the initial HC concentration [HC]o and the ratio [NOx]o/[HC]o, covered in this study. In our recent study on the aerosol formation from cyclohexene (Izumi et al., 

### 1988), the aerosol yield was shown to decrease when the initial concentration of the HC was decreased. This is qualitativelyexplained in the followingway: when the initialconcentration of the HC islower,theconcentrations oftheproducts are accordingly lower. Hence, the products with lower boiling points cannot attain the concentration level necessary for nucleation, thus resulting in a lower aerosol yield. In this study, all the experiments were done with AHC concentrations equal to about 1ppm, which is two or three orders ofmagnitude higher than the values observed in the field.Therefore ifthe mode of aerosol formation from the aromatics is similar to that for cyclohexene, the yield in the actual environment willbe lower than that determined in this study. Another limitation of this study is that the ratio [NOx]o/[HC]0 used (~0.2) was much lower than in the urban air (> 10). As shown in Fig. 4, the aerosol volume concentration was observedto leveloffin later stages ofthe reaction. It is likelythat this is due to the depletion ofNO2, the presence ofwhich is, according to Grosjean (1984a), essential for the formation ofthe condensable products: inferring on the basis of the report of Leone et al. 

### (1985), a small amount of NOx remaining in later stages of the present experiment (Fig. 1) should be secondarily produced PAN and HNO 3. Hence the volume concentration may rise persistently in the actual environment, where NO2 exists continuously. However, for the experimental

reason stated before,detailedexamination ofthe effect of increasing NOx on the aerosol yield remains to be done. An expedient is to use, as a ceiling value, the maximum yieldcorresponding to the steepest slope of the curve in Fig. 4. Thus a tentative upper-limit estimation was made

on the amount of the aerosol formed from the aromatics in the ambient air. Styrenes need not be considered in this estimation, as their concentrations

are much smallerthan those ofthe others; then all one has to do is to take account of the reaction with the OH radical. As the initial concentrations, the highest values were quoted from the paper of Singh et al. 

### (1985), who reported the concentrations of aromatics averaged over 1979-1984 for twelve cities in the U.S. The OH radical concentration was assumed to be 1

× 10-7 ppm, referring to the values (0.2-2)

× 10-7ppm reported by Hewitt and Harrison (1985)

as the daytime average in urban areas, and the rate constants kon were adopted from Atkinson (1985). Then, by combining these data with the maximum aerosol yield given in Table 2, the mass ofparticulate carbon produced during the day was estimated. The

mass from the major AHCs reaches a total of 5/~gCm -3. It is expected that this value is not very far from the mass concentration due to the aromatics in a pollution episode, as the HC concentrations in such a
case often exceed the highest average values reported by Singh et al. 

### (1985). According to Grosjean (1984b) and Shah et al. 

### (1986), on the other hand, the total concentration of particulate organic carbon is in a range 4-28/ag-Cm-3 in summertime urban areas. By comparing these values with the calculation obtained above, one can obtain a rough ideaofhow largea part of organic aerosols is formed from AHCs. Among various aromatics, toluene has the largest contribution (1.3/~g-Cm- 3). From the standpoint ofthe pollution control, however, it should be noted that a significant portion (2.2 pg-Cm- 3) is made up of the contributions from such HCs as o- and m-xylene, ethylbenzene and 1,3,5-trimethylbenzene, although their concentrations are much lower than toluene. This ismainlydue to the largerkon ofthose hydrocarbons.

SUMMARYAND CONCLUSIONS



### Photochemical aerosol formation from mixtures of

an AHC and NOx was studied by exposing the gas mixtures to simulated solar radiation in a 4-m3chamber. The main purpose was to determine the aerosol yields of different aromatic species on a consistent basis, to facilitate quantitative comparison among them. Altogether 17 AHCs and methoxybenzene were investigated,covering most ofthe aromatics that have been found in urban polluted air. Evolution of the aerosol was monitored by measuring the total number ([CN]) and volume (V)concentrations. Both [CN],~a~ and (d[CN]/dt),,ax were particularly large for mxylene and ethylbenzene. Among the isomers of xylene, ethyltoluene or trimethylbenzene, the isomer with methyl groups in thepara-position 

### was found to have the smallest (dV/dt)max, 

### thus suggesting a substitution effect on the aerosol formation. By using dry sheath air in the EAA measurement, the net organic (exclusiveofwater) volume concentration (lid) was obtained. On the other hand, the AOCmass concentration ([AOC]) was determined by filter sampling and subsequent thermal C analysis of the aerosol products. A plot of V dvs [AOC] for aerosols from different HCs resulted in a single linear relation [Equation (1)], and this relation proved to be useful for deriving [AOC] from Vd, which was more easily measurable than [AOC]. The aerosol yieldswere evaluated on three different bases, namely on the bases of V, V dand [AOC]; these yields represent V, V

d and [AOC], respectively, produced from the unit concentration of the HC lost by the reaction. The second yield was derived from the first by multiplyingit by Vd/V, 

### and the per cent AOCPhotochemical aerosol formation 1441

yield was calculated from the second through Equation (3). For the V basis yield, the maximal value that
turns up on the way of the reaction was obtained from the maximal slope of the V vs A[HC] relation, where A[HC] is the concentration loss of the HC due to the reaction. For AOC yield, the overall value was also calculated from V and A[HC] measured at the end of the irradiation. The overall AOC yield was found to be larger for toluene, ethylbenzene, and o- and m-ethyltoluene. On the basis of the maximum yield, a tentative upper-limit estimation was made on the amount ofthe aerosol formed from the aromatics in the ambient air. As a result it was shown that toluene had the largest contribution and the contributions from o- and mxylene, ethylbenzene and 1,3,5-trimethylbenzene summed up to a significant portion.

Acknowledgement--The authors are very grateful to S. Kosaka and D. Itoh of Nihon Kagaku Kogyo Co. for their technical assistance in performing the chamber experiment.

REFERENCES

AtkinsonR.(1985) Kinetics andmechanismsofthegas-phase reactions ofthe hydroxyl radical with organic compounds under atmospheric conditions. Chem. Rev. 85, 69-201. Atkinson R., Carter W. P. L., Plum C. N., Winer A. M. and Pitts J. N., Jr (1984) Kinetics of the gas-phase reactions of NO3radicals with a seriesofaromatics at 296+2 K. Int. J. Chem. Kinet. 16, 887-898. Atkinson R. and Lloyd A. C. (1984) Evaluation ofkineticand mechanistic data for modeling of photochemical smog../. phys. Chem. Ref Data 13, 315-444. Besemer A. C. (1982) Formation of chemical compounds from irradiated mixtures of aromatic hydrocarbons and nitrogen oxides. Atmospheric Environment 16, 1599-1602. Fung K. and Grosjean D. (1984) Determination of particulate atmospheric benzoic acid by ion chromatography with ultraviolet detection. Anal. Lett. 17, 475-482. Gery M. W., Fox D. L., Jeffries H. E., Stockburger L. and Weathers W. S. (1985) A continuous stirred tank reactor investigationofthegas-phasereactionofhydroxyl radicals with toluene. Int. J. Chem. Kinet. 17, 931-955. Gery M. W., Fox D. L., Kamens R. M., and Stockburger L. (1987) Investigation of hydroxyl radical reactions with oxylene and m-xylene in a continuous stirred tank reactor. Envir. Sci. Technol. 21, 339 348. Greadel T. E., Hawkins D. T. and Claxton L. D. (1986) Atmospheric Chemical Compounds--Sources, Occurrence, and Bioassay, pp. 111 227. Academic Press, New York. Grosjean D. (1977) Aerosols in Ozone and Other Photochemical Oxidants, pp. 45-125. National Academy of Sciences-National Research Council, Washington, DC. Grosjean D. (1984a) Atmospheric reactions of ortho-cresol: gas phase and aerosol products. Atmospheric Environment 

### 18, 1641-1652. Grosjean D. (1984b) Particulate carbon in Los Angeles air. Sci. total Envir. 32, 133-145. Grosjean D. (1985) Reactions ofo-cresol and nitrocresol with NO~ in sunlightand with ozone-nitrogendioxidemixtures in the dark. Envir. Sci. Technol. 19, 968-974. Grosjean D. and Fung K. (1984) Hydrocarbons and carbonyls in Los Angeles air. J. Air Pollut. Control Ass. 34, 537-543. Harkov R., Kebbekus B., Bozzelli J. W., Lioy P. J. and Daisey J. (1984) Comparison of selected volatile organic compounds during the summer and winter at urban sites in New Jersey. Sci, total Envir, 38, 259 274. Hewitt C. N. and Harrison R. M. (1985) Tropospheric concentrations of the hydroxyl radical a review. Atmospheric Environment 19, 545-554. IsidorovV.A.,ZenkevichI.G. and Ioffe B. V.(1983) Methods and results of gas chromatographic-mass spectrometric determination of volatile organic substances in an urban atmosphere. Atmospheric Environment 17, 1347-1353. Izumi K., Mizuochi M., Murano K., Ozaki Y. and Fukuyama T. (1986) Sulfuric acid aerosol formation by the reaction of OH radicals with SO2. Int. J. Envir. Stud. 27, 183-199. Izumi K., Mizuochi M., Yoshioka M., Murano K. and Fukuyama T. (1984) Redetermination ofthe rate constant for the reaction of OH radicals with SO2. Em~ir. Sci. Technol. 18, 116-118. Izumi K., Murano K., Mizuochi M. and Fukuyama T. (1988) Aerosol formation by the photooxidation of cyclohexene in the presence of nitrogen oxides. Envir. Sci. Technol. 22(10), 1207 1215. Leone J. A., Flagan R. C., Grosjean D. and Seinfeld J. H. (1985) An outdoor smog chamber and modeling study of toluene-NOx photooxidation. Int. J. Chem. Kinet. 17, 177-216. Nelson P. F., Quigley S. M. and Smith M. Y. (1983) Sources of atmospheric hydrocarbons in Sydney: a quantitative determination using a source reconciliation technique. Atmospheric Em~ironment 17, 439~149. Nojima K., Fukaya K., Fukui S. and Kanno S. (1975) Studies
on photochemistry of aromatic hydrocarbons lI. Chemosphere 4, 77 82. Nojima K., Fukaya K., Fukui S., Kanno S.,Nishiyama S. and Wada Y. (1976) Studies on photochemistry of aromatic hydrocarbons IIL Chemosphere 5, 25 30. O'Brien R. J., Green P. J., Nguyen N.-L., Doty R. A. and Dumdel B. E. (1983) Carbon balances in simulated atmospheric reactions: aromatic hydrocarbons. Em~ir. Sci. Technol. 17, 183-186. Sexton K. and Westberg H. (1984) Nonmethane hydrocarbon composition of urban and rural atmospheres. Atmospheric Environment 18, 1125-1132. Shah J. J., Johnson R. L., Heyerdahl E. K. and Huntzicker J. J. (1986) Carbonaceous aerosol at urban and rural sites in the United States. J. Air. Pollut. Control Ass. 36, 254 257. Singh H. B., Salas L. J., Cantrell B. K. and Redmond R. M. (1985) Distribution of aromatic hydrocarbons in the ambient air. Atmospheric Environment 19, 191l-1919. Spicer C. W. and Jones P. W. (1977) The fate of aromatic hydrocarbons in photochemical smog systems: toluene. J. Air Pollut. Control Ass. 27, 1122-1125. SternJ. E., Flagan R. C., Grosjean D. and Seinfeld J. H. (1987) Aerosol formation and growth in atmospheric aromatic hydrocarbon photooxidation. Envir. Sci. Tehcnol. 21(12), 1224-1231.