Atmospheric Environment Vol. 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 NO x 

KATSUYUKI IZUMI and TSUTOMU FUKUYAMA 

Division of Atmospheric Environment, The National Institute for Environmental Studies, P.O. TsukubaGakuen, Ibaraki 305, Japan 

(First received 14 December 1988 and in final form 1 May 1989) 

Abstract By using a 4-m 3 chamber, photochemical aerosol formation from mixtures of aromatic hydrocarbons (AHCs) and nitrogen oxides (NOx) was studied with the main purpose of determining the aerosol yield. 

Altogether, 18 aromatic species were investigated, including toluene, xylenes, ethylbenzene and methoxybenzene. Evolution of the aerosol was monitored through the measurement of the total number ([CN]) 

and volume (I/") concentrations. Aerosol-forming potential of different aromatics was examined in terms of 

the maximum growth rates of I-CN] and V. By putting dry sheath air into an electrical aerosol analyzer, the 

net organic volume concentration (Vd) was obtained; then a relation was established between lid and the C 

mass concentration, and was used to determine the aerosol yield as defined by the ratio of the particulate C 

mass to the C mass of the reacted HC. It was found that the yields were larger for toluene, 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 nonmethane hydrocarbons (NMHC) (Nelson 

et al., 1983; Sexton and Westberg, 1984; Grosjean and 

Fung, 1984). Among them, benzene, toluene, xylenes 

and ethylbenzene are the most common species found 

in the polluted urban air, and comprise more than 

80% of the 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 yielding gaseous products 

has been extensively reviewed by Atkinson and Lloyd 

(1984) and by Atkinson (1985). On the other hand, 

research on the process leading to particulate products was surveyed by Grosjean (1977) up to 1975, and 

following the studies collected in that review paper, 

there were several more investigations on the aerosol 

formation from AHCs. However, the existing data for 

the aerosol yield are rather limited, although there 

were a number of studies in which the yields were 

determined without discriminating whether 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 

particulate pollutants is quite different from that of the 

gaseous ones. The aerosol yields have been determined for 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), 

but they are not reported in a very convenient manner, 

as the yields were defined differently by different 

authors. For example, Leone et al. (1985) gave the 

yield for toluene as a percentage of the initially present 

C, whereas Gery et al. (1985) gave it as percentage of 

the reacted C. The yield reported by Stern et al. (19871 

was based on the total mass, which was estimated by 

assuming that the aerosol particles had a density of 

1 g cm- 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 (Atkinson et at., 1984). It was therefore decided 

to include in this study such minor aromatics as well. 

EXPERIMENTAL" 

All the experiments were carried out by using an 'aerosol 

chamber', a description of which was reported previously 

1433 

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1434 KATSUYUKI IZUMI and TSUTOMU FUKUYAMA 

(Izumi et al., 1984, 1986). Briefly, it is a cylindrical and 

evacuable 4-m 3 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 of water vapor. The reactant gases, NOx and one of 

the AMCs, were measured volumetrically in advance in a 

gas-handling equipment 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 the initial temperature by measuring it with a dew-point meter (Yokogawa 

Electric Works, model 2586). 

The reaction mixture was exposed to light flux from a 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 

photolysis rate constant of NO2 was 0.27 min- t. The irradiation was continued for 60-300 min according to 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 5 min by a 

gas chromatograph with an FID detector by using a I m 

x 3 mm diameter stainless steel column with 20% silicone 

OV-101 on 60--80 mesh AW, DMCS Chromosorb W. Cbemiluminescent analyzers (Monitor Labs., models 8440 and 8410) 

were used to monitor the concentrations of NOt and Ov The 

number concentration of the aerosol particles was continuously 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 

smaller than 0.01 #m contribute to the number concentration 

only in the very early stages of the aerosol evolution. 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 

of the 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 Vd hereafter; as the particles 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 of experiments, which were 

done to determine the aerosol yield from the mixtures of 

about 1 ppm HC and 0.2-0.5 ppm NO~, another series was 

conducted to examine the relation between the concentration 

of aerosol 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 content in the sample was quantified as CO2 by a thermal carbon 

analyzer (TCA; Kimoto Denshi Co.). The details of the AOC 

determination are reported in other paper (Izumi et al., 1988); 

in particular, care was taken to eliminate interference by gasphase organics adsorbed on the filter. In some of the runs in 

this series, the initial concentrations oftbe reactants were set 

somewhat higher than 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 NO x ratio 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 using methylnitrite instead of NO~, because no 

particles were formed from a mixture of benzene 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 matrix air to which only water vapor (r.h. = 50%) and NO,, 

(about 10 ppb) were added. After about 200 min, particulate 

matter was found to be formed up to a volume concentration 

of 3-4 #m 3 cm -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 1 ppm: 

It was found, by another test experiment, that decrease in 

concentration due to wall loss was not significant for the 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 included for comparison with a-methylstyrene, although 

the occurrence oftbe former in the atmosphere is not known. 

All of the HCs were of the research grade supplied by Tokyo 

Kasei Kogyo Co., and were used after distillation in vacuum. 

NO and NO2 were obtained from Matheson Gas Products; 

the former was used after purification by repeated 

freeze-thaw cycles in a bulb containing Ascarite 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). 

RESULTS AND 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 

of the AHCs were set to be about 1 ppm. In most of the 

runs, NO was used as the initial NOx, but in runs 7 

and 9, NO2 was used instead of NO, 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 to 
Photochemical aerosol formation 1435 

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1436 KATSUYUKI IZUMI and TSUTOMU FUKUYAMA 

react with 03 in the dark to exclude reaction with the 

OH radical; the purpose of this experiment was to 

examine the relative contributions of 03 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 aerosol evolution 

in the same run is illustrated in Fig. 2 as observed by 

EAA; here N, S and V refer to 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 of the aerosol formation and became stationary in later stages. The geometric standard deviation, 

try, which is a measure of the 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 except benzene. The results of the 

main experiments are collected in 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 of the initial NO~ was NO, no 

03 was formed (run 8). When NO2 instead of NO was 

used, O3 did appear but its build-up ended shortly 

after the start of the irradiation, reaching a concentration only about a tenth of that for the other aromatics 

(run 9). Moreover, little decrease in benzene concentration and no particle formation were observed in 

both of the runs in spite of the irradiation being longer 

than in the other runs. Slight formation of 03 observed in run 9 was certainly a result of the 

photolysis of NO 2 and not of any reaction of benzene. 

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 time is a rough measure of 

the rapidity of the 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 

the end of the irradiation are listed. From the maximum 

or the terminal values of [Oa], it is seen that [03] had 

much the same level for different aromatics. 

However, the time for [03 ]ma, varied among 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 of the 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]/d t) .... the Ca aromatics were found to have a 

higher aerosol-forming potential than the C a ones; the 

potential was particularly high for m-xylene and ethylbenzene. Comparison of the results from runs 3, 6 and 

7 indicates that an increase in [NO]o and replacement 

of NO by NO 2 both significantly decreased [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-substituted isomers 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 smallest rate. Thus the para-substitution in 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 & 

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Fig. 1. Time variations of toluene, 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 of the aerosol, and geometric / 

mean diameters (dp) on the basis of N. Results from Run 1. 
Photochemical aerosol formation 1437 

(d[CN]/dt)ma~ and (d V/dt) .... were particularly small 

for n-propylbenzene and styrene. 

In the next column of Table 1, the aerosol volume 

concentration at the end of the 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 leveled off when 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 Vf increased compared with the values 

for run 3. In run 7, in which NO was replaced by NO 2, 

the irradiation was interrupted for the reason mentioned above, when d V/dt and dp were still increasing. 

Thus in Table 1, the terminal values are given for 

(d V/dt)max and aYp as the lower limits. 

Net organic volume concentration 

On the analogy of the hygroscopic nature 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 assess the net content of organic 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 significantly less than V. For example, the ratio of 

V d to 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),,a x by multiplying the former by 

the ratio Vd/V; the resultant represents the maximum 

growth rate of the 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 reaction is 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 by using methylnitrite to 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-squares fitting, the following relation 

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 indicates that the particles produced 

from the AHCs and cyclohexene have much the same 

C mass density. Once a relation is established between 

[AOC] and I/d, the former is more readily obtainable 

from the latter than from the filter collection and the 

subsequent analysis by TCA. On the basis of Equation 

(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 for toluene was found to be 35 #g-C m 3 h- 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 of toluene, V is 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 detectable level. 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 with 
1438 KATSUYUKI IZUMI and TSUTOMU FUKUYAMA 

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Photochemical aerosol formation 1439 

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Fig. 3. Relation between mass concentration of aerosol organic carbon (AOC) 

and aerosol volume concentration measured with dry sheath air (Va) observed for 

different HCs: II, benzene; ©, toluene; 

A o-xylene; O, m-xylene; C], p-xylene; 

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Fig. 4. Plot of aerosol volume concentration (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/tin 3 crrr- 3 ppm- ~ for toluene, with two standard 

deviations of the least-squares fitting. By multiplying 

by Vd/V, this is converted into the V d basis yield, i.e. 

the yield on the basis of the net organic volume 

concentration. This was found to be 353 

+ 28 #m 3 cm- 3 ppm- 1. Similar results (335 + 27, 335 

+ 18 and 335 + 19 #m 3 cm- 3 ppm- x) were derived 

from the additional three runs which were conducted 

to check the reproducibility. These values correspond 

to the maximum yield obtained during the 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 Vf to 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 constant 483 is a C mass concentration in a unit of #g-Cm -3 equivalent to an HC 

concentration of 1 ppm-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 toluene is an average over 

the four runs. The maximum AOC 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-xylene was less than half 

of that 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 

observations suggest a lower aerosol formation ability 

of p-methyl-substituted aromatics, 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. According to a recent 

study (Fung and Grosjean, 1984), the aerosol yield 

from the reaction of /~-methylstyrene with 03 is 

2.4-13 %. These values are significantly larger than the 

yield obtained in this study (run 21). This is probably 

because the 0 3 reaction has a larger aerosol yield than 

the OH reaction also in the case of fl-methylstyrene. 

As the relation between V and 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 overall yield (3.0%) determined 

in this study is embedded in the 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 consistent with those of Stern et al. (1987) if the difference 

in the definition of the yield is taken into account. 

As described above, the aerosol yields were obtained for most of the AHCs found in the polluted air. 
1440 KATSUYUKI IZUMI and TSUTOMU FUKUYAMA 

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 situation encountered 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 qualitatively explained 

in the following way: when the initial concentration of 

the HC is lower, the concentrations of the products 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 1 ppm, 

which is two or three orders of magnitude higher than 

the values observed in the field. Therefore if the mode 

of aerosol formation from the aromatics is similar to 

that for cyclohexene, the yield in the actual environment will be 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 observed to level offin later 

stages of the reaction. It is likely that this is due to the 

depletion of NO2, the presence of which is, according 

to Grosjean (1984a), essential for the formation of the 

condensable products: inferring on the basis of the 

report of Leone et al. (1985), a small amount of NO x 

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 NO 2 

exists continuously. However, for the experimental 

reason stated before, detailed examination of the effect 

of increasing NOx on the aerosol yield remains to be 

done. An expedient is to use, as a ceiling value, the 

maximum yield corresponding 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 smaller than those of the 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- 7 ppm 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 of particulate 

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-C m-3 in summertime urban areas. By 

comparing these values with the calculation obtained 

above, one can obtain a rough idea of how large a part 

of organic aerosols is formed from AHCs. Among 

various aromatics, toluene has the largest contribution (1.3/~g-C m- 3). From the standpoint of the pollution control, however, it should be noted that a 

significant portion (2.2 pg-C m- 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 is mainly due to the larger kon of those hydrocarbons. 

SUMMARY AND CONCLUSIONS 

Photochemical aerosol formation from mixtures of 

an AHC and NO x was studied by exposing the gas 

mixtures to simulated solar radiation in a 4-m 3 chamber. 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 of the 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),,a x were particularly large for mxylene and ethylbenzene. Among the isomers of xylene, ethyltoluene or trimethylbenzene, the isomer 

with methyl groups in the para-position was found to 

have the smallest (d V/dt)max, thus suggesting a substitution effect on the aerosol formation. 

By using dry sheath air in the EAA measurement, 

the net organic (exclusive of water) 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 d vs [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 yields were evaluated on three different 

bases, namely on the bases of V, V d and [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 multiplying it by Vd/V, and the per cent AOC 
Photochemical 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 of the 

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. 

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