Atmos. Chem. Phys., 7, 3909–3922, 2007

www.atmos-chem-phys.net/7/3909/2007/

© Author(s) 2007. This work is licensed

under a Creative Commons License.

Atmospheric

Chemistry

and Physics

Secondary organic aerosol formation from m-xylene, toluene, and

benzene

N. L. Ng1, J. H. Kroll1,*, A. W. H. Chan1, P. S. Chhabra1, R. C. Flagan1, and J. H. Seinfeld1

1Departments of Chemical Engineering and Environmental Science and Engineering, California Institute of Technology,

Pasadena, CA 91125, USA

*now at: Aerodyne Research, Inc. 45 Manning Road, Billerica, MA 01821, USA

Received: 1 March 2007 – Published in Atmos. Chem. Phys. Discuss.: 28 March 2007

Revised: 28 June 2007 – Accepted: 13 July 2007 – Published: 24 July 2007

Abstract. Secondary organic aerosol (SOA) formation from

the photooxidation of m-xylene, toluene, and benzene is investigated in the Caltech environmental chambers. Experiments are performed under two limiting NOx conditions;

under high-NOx conditions the peroxy radicals (RO2) react

only with NO, while under low-NOx conditions they react

only with HO2. For all three aromatics studied (m-xylene,

toluene, and benzene), the SOA yields (defined as the ratio of the mass of organic aerosol formed to the mass of

parent hydrocarbon reacted) under low-NOx conditions substantially exceed those under high-NOx conditions, suggesting the importance of peroxy radical chemistry in SOA formation. Under low-NOx conditions, the SOA yields for mxylene, toluene, and benzene are constant (36%, 30%, and

37%, respectively), indicating that the SOA formed is effectively nonvolatile under the range of Mo(>10μg m−3

) studied. Under high-NOx conditions, aerosol growth occurs essentially immediately, even when NO concentration is high.

The SOA yield curves exhibit behavior similar to that observed by Odum et al. (1996, 1997a, b), although the values

are somewhat higher than in the earlier study. The yields

measured under high-NOx conditions are higher than previous measurements, suggesting a “rate effect” in SOA formation, in which SOA yields are higher when the oxidation rate

is faster. Experiments carried out in the presence of acidic

seed aerosol reveal no change of SOA yields from the aromatics as compared with those using neutral seed aerosol.

1 Introduction

Aromatic hydrocarbons contribute an important fraction

(∼20–30%) of total volatile organic compounds in the urban

atmosphere (Calvert et al., 2002). Atmospheric oxidation of

Correspondence to: J. H. Seinfeld

(seinfeld@caltech.edu)

aromatic hydrocarbons leads to the production of ozone as

well as low-volatility species which then partition into the

condensed phase, forming secondary organic aerosol (SOA).

The anthropogenic contribution to global SOA formation

is currently estimated to be small, roughly about 10% (Tsigaridis and Kanakidou, 2003). Ambient measurements suggest that SOA formation in the atmosphere is higher than that

predicted by current models (Heald et al., 2005, 2006; de

Gouw et al., 2005; Volkamer et al., 2006). In addition, it

has been suggested that SOA formation from anthropogenic

sources is substantially higher than currently thought (de

Gouw et al., 2005; Volkamer et al., 2006).

Gas-phase chemistry of aromatic hydrocarbons is dominated by reaction with the OH radical (Calvert et al., 2002).

Despite considerable study of the oxidation chemistry of aromatic hydrocarbons, the basic underlying mechanisms of

SOA formation and growth from aromatic precursors remain poorly understood. There have been few studies on the

molecular composition of SOA from aromatic hydrocarbons

(Forstner et al., 1997; Jang and Kamens, 2001; Kleindienst et

al., 2004). The carbon balance is poorly constrained; generally, only about 50% of the reacted carbon has been identified

as products (Calvert et al., 2002).

SOA formation from individual precursors is typically

studied in laboratory chamber experiments. Aerosol yields

from the photooxidation of aromatic hydrocarbons have been

shown to be highly sensitive to the NOx level (Hurley et al.,

2001; Johnson et al., 2004, 2005; Song et al., 2005); generally, a higher SOA yield is observed under low-NOx conditions. This general dependence of SOA formation on the

NOx level has been proposed to be the result of differences

in concentrations of different oxidants (OH, O3, and NO3)

(Hurley et al., 2001), or in changes in peroxy radical chemistry (Hatakeyama et al., 1991; Johnson et al., 2004, 2005;

Presto et al., 2005; Kroll et al., 2006). In addition, particlephase reactions have been found to be important processes in

SOA formation (Kalberer et al., 2004; Gao et al., 2004a, b;

Published by Copernicus Publications on behalf of the European Geosciences Union.
3910 N. L. Ng et al.: Secondary organic aerosol formation from aromatics

Tolocka et al., 2004), and the presence of sulfuric acid seed

has been shown to lead to increased SOA yields in a number

of systems (Jang et al., 2002; Iinuma et al., 2004; Gao et al.,

2004a, b; Edney et al., 2005). Odum et al. (1996, 1997a, b)

performed an extensive study on aromatic SOA formation.

In light of the recent findings on the NOx dependence and

effect of seed aerosol acidity on SOA yields, it is important

that SOA formation from aromatics be restudied to establish

fully the NOx dependence and effect of particle acidity on

SOA formation.

Most chamber experiments of SOA formation by aromatics involve the irradiation of aromatic/NOx mixtures (Izumi

and Fukuyama, 1990; Odum et al., 1996, 1997a, b; Hurley et

al., 2001; Johnson et al., 2004; Song et al., 2005). In these

classical photooxidation experiments, the NO and NO2 concentrations (hence NO/NO2 ratio) constantly change, making

it difficult to isolate the effect of NOx on SOA formation. For

example, the decreasing NO concentration over the course of

the experiment may lead to a switch from “high-NOx” conditions to “low-NOx” conditions for the peroxy radical chemistry (Johnson et al., 2004). Another potential complication

in interpreting SOA data is that a delay in aerosol formation from the onset of photooxidation has been frequently

observed in aromatic systems; in particular, aerosol does not

form until the concentration of NO approaches zero. When

extrapolating to urban areas where the NOx level is usually

high, this would suggest that aromatics (and other hydrocarbons) do not produce SOA in the atmosphere. The observation that SOA does not form until [NO] approaches zero is

not universal, however; in a study of toluene photooxidation

by Stroud et al. (2004), aerosol growth is observed even at

NO concentrations of 1–3 ppm. Thus the NOx dependence

of SOA yields, which is a crucial parameter for atmospheric

modeling, is very poorly understood.

In this work, SOA formation from the photooxidation of

m-xylene, toluene, and benzene is investigated. A main goal

of this study is to establish the NOx dependence of SOA formation for these aromatic hydrocarbons. In the experiments,

SOA formation under two NOx conditions is studied: (1)

high-NOx experiments in which HONO is used as the OH

precursor and the NOx level at the inception of photooxidation is ∼1 ppm; and (2) low-NOx experiments in which

H2O2 is used as the OH precursor and no NOx is introduced

into the chambers. The background NOx concentration is

<1 ppb, which is within the uncertainty of the NOx monitor. By performing experiments at these extreme NOx limits, the oxidation conditions (initiating oxidant and fate of

peroxy radicals) can be maintained relatively constant over

the course of the experiment, allowing for the evaluation of

the effect of NOx level on SOA formation. Additionally, the

effect of seed aerosol acidity on SOA formation is studied

under both high- and low-NOx conditions. The SOA yield

parameters obtained at the two NOx limits allow one to parameterize the NOx dependence of SOA formation for use in

atmospheric models (Presto et al., 2006).

2 Experimental section

Experiments are performed in Caltech’s indoor, dual 28 m3

Teflon environmental chambers. Details of the facilities

have been given elsewhere (Cocker et al., 2001; Keywood

et al., 2004). Before each experiment, the chambers are

flushed continuously with dry purified air for ∼24 h. Each

chamber has a dedicated Differential Mobility Analyzer

(DMA, TSI model 3081) coupled with a condensation nucleus counter (TSI model 3760) for measuring aerosol size

distribution, number concentration, and volume concentration. All aerosol growth data are corrected for wall loss, in

which size-dependent coefficients determined from inert particle wall loss experiments are applied to the aerosol volume

data (Keywood et al., 2004). Generally, the wall loss coefficients range from 10−4

to 10−2 particles min−1. Temperature, relative humidity (RH), O3, NO, and NOx are continuously monitored. Half of the available black lights are used

in the experiments. The initial temperature of the chamber

is ∼20◦ C. Heating from the lights leads to a temperature

increase of approximately 5◦ C inside the chamber over the

course of the experiment. The analytical instruments are located outside the chamber enclosure and are at the temperature of the surrounding room (∼20–22◦ C). The air may cool

slightly as it is sampled from the chamber into the instruments, and the measured aerosol likely corresponds to the

gas-particle partitioning at the temperature of the instruments

rather than the chamber enclosure. Such small temperature

differences are unlikely to affect results significantly.

Seed particles are introduced into the chamber to act as a

substrate onto which the gas-phase products may condense.

In an earlier work, we have shown that without seed particles, there is an “induction period” in which hydrocarbon is

reacted but no aerosol is formed, which has the effect of biasing SOA yield measurements low (Kroll et al., 2007). Therefore, for all experiments in this study seed particles are used

to eliminate this effect. Seed particles are generated by atomizing an aqueous solution with a constant-rate atomizer.

The nonacid seed consists of 0.015 M (NH4)2SO4, while

the acidic seed contains a mixture of 0.015 M (NH4)2SO4

and 0.015 M H2SO4. Since all experiments are performed at

RH∼5%, which is lower than the crystallization RH (35%)

of ammonium sulfate, the nonacid seed is likely a solid (Seinfeld and Pandis, 2006). The initial particle number concentration is about 30 000 particles cm−3

, with a geometric mean diameter of about 50 nm. Initial aerosol seed volume is about 15μ m3

cm−3. After introduction of the seed

aerosol, a known volume of the parent hydrocarbon is injected into a glass bulb, and introduced into the chambers

by an air stream. The concentration (mixing ratio) of the

parent hydrocarbon is monitored with a Hewlett Packard gas

chromatograph (model 5890) with flame ionization detection

(GC-FID).

In most of the high-NOx experiments nitrous acid

(HONO) serves as the OH precursor. It is introduced into the

Atmos. Chem. Phys., 7, 3909–3922, 2007 www.atmos-chem-phys.net/7/3909/2007/
N. L. Ng et al.: Secondary organic aerosol formation from aromatics 3911

Table 1. Aromatic hydrocarbons studied. Parent 

Hydrocarbon Structure Formula (MW) kOHa

 (cm3 molec-1 s-1) 

m-xylene C8H10 (106) 2.31 x 10-11

toluene C7H8 (92) 5.63 x 10-12

benzene C6H6 (78) 1.22 x 10-12

a

: Rate constants were obtained from Calvert et al. (2002). 

a

: Rate constants were obtained from Calvert et al. (2002).

Table 2. Initial conditions and data for high-NOx (HONO) experiments.

Expt.

No.

Parent

Hydrocarbon

T

(K)

RH

(%)

NO

(ppb)

NO2

(ppb)

Seed 1HC

(ppb)

1Mo

(μg/m3)

SOA Yield

(%) a

1 m-xylene 297 5.5 470 473 (NH4)2SO4 70.9 18.2±1.8 5.9±0.4

2 m-xylene 298 5.7 451 494 (NH4)2SO4 28.1 4.3±1.2 3.5±0.7

3 m-xylene 298 5.9 432 511 (NH4)2SO4 132.5 46.4±3.7 8.0±0.3

4 m-xylene 297 5.1 431 514 (NH4)2SO4 106.1 36.7±2.8 8.0±0.2

5 toluene 298 3.8 421 524 (NH4)2SO4 30.1 9.1±1.3 8.0±0.7

6 toluene 298 4.3 414 532 (NH4)2SO4 56.7 23.8±2.2 11.1±0.4

7 toluene 298 4.9 388 559 (NH4)2SO4 80.2 38.7±3.3 12.8±0.4

8 toluene 298 4.4 373 568 (NH4)2SO4 50.7 20.9±1.9 10.9±0.5

9 benzene 297 5.2 83 86 (NH4)2SO4 39.4 35.4±2.7b 28.1±0.9

a

: Stated uncertainties (1σ ) are from scatter in particle volume measurements.

b

: Assuming an SOA density of 1.4 g cm−3.

chamber after injection of the seed aerosol and parent hydrocarbon. HONO is prepared by dropwise addition of 15 mL of

1% NaNO2 into 30 mL of 10% H2SO4 in a glass bulb. The

bulb is then attached to the chamber and a stream of dry air

is passed through the bulb into the chamber. NO and NO2,

formed as side products in the preparation of HONO, are also

introduced into the chamber, and are measured by a commercial NOx monitor (Horiba APNA-360, Irvine, CA). Additional NO from a 500 ppm gas cylinder (Scott Marrin, Inc.)

is introduced into the chamber after the addition of HONO

to achieve a total NOx level in the chamber of about 1 ppm

(upper limit of the NOx monitor). In some high-NOx experiments, only NO and NO2 (from gas cylinders) are added

to the chamber. To differentiate these experiments from the

high-NOx experiments in which HONO is used as the OH

precursor, we refer to these experiments as classical photooxidation experiments. The majority of the high-NOx experiments in this study are performed with HONO; only a few

classical photooxidation experiments are performed for comparison purposes.

For low-NOx experiments, H2O2 is used as the OH precursor. The background NOx level in the chamber during the

experiment is < 1 ppb. H2O2 is introduced into the chamber

(prior to introduction of seed particles and parent hydrocarbon) by bubbling air through a 50% H2O2 solution for 2.5 h

at 5 L/min. The air stream then passes through a particle filter to remove any droplets. The concentration of H2O2 in

the chamber is not measured; based on the rate of hydrocarbon decay and literature values of σH2O2 and kOH+H2O2, we

estimate [H2O2] to be ∼3–5 ppm (Kroll et al., 2006).

Once the seed, parent hydrocarbon, and NOx concentrations stabilize, reaction is initiated by irradiating the chamber

with blacklights. Output from the lights is between 300 and

400 nm, with a maximum at 354 nm. At these wavelengths

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3912 N. L. Ng et al.: Secondary organic aerosol formation from aromatics

Table 3. Initial conditions and data for low-NOx (H2O2) experiments.

Expt. No. Parent

Hydrocarbon

T

(K)

RH

(%)

Seed 1HC

(ppb)

1Mo

(μg/m3)

SOA Yield

(%)a

1 m-xylene 298 5.1 (NH4)2SO4 32.5 53.0±4.2 37.7±0.8

2 m-xylene 298 5.2 (NH4)2SO4 16.1 24.6±2.2 35.2±1.8

3 m-xylene 298 5.1 (NH4)2SO4 8.0 12.8±1.7 36.7±2.6

4 m-xylene 297 6.2 (NH4)2SO4 26.1 40.5±3.4 35.7±1.0

5 toluene 297 6.8 (NH4)2SO4 32.1 37.4±2.8 30.8±1.7

6 toluene 297 6.2 (NH4)2SO4 63.9 73.1±5.6 30.2±0.7

7 toluene 298 5.2 (NH4)2SO4 10.0 11.5±1.6 30.4±4.1

8 toluene 298 5.9 (NH4)2SO4 23.8 26.7±2.5 29.8±1.6

9 benzene 298 6.6 (NH4)2SO4 64.7 76.4±5.8b 36.9±0.9

a

: Stated uncertainties (1σ ) are from scatter in particle volume measurements.

b

: Assuming an SOA density of 1.4 g cm−3.

Table 4. Initial conditions and data for acid/nonacid experiments.

Expt.

No.

Parent

Hydrocarbon

T

(K)

RH

(%)

NOx

Conditiona

Seed 1HC

(ppb)

1Mo

(μg/m3)

SOA Yield

(%)b

1 m-xylene 297 4.3 Low NOx (NH4)2SO4 60.2 101.3±7.8 38.6±0.5

2 m-xylene 297 4.5 Low NOx (NH4)2SO4+H2SO4 58.8 103.5±8.0 40.4±0.6

3 m-xylene 297 5.0 High NOx (NH4)2SO4 68.9 78.9±5.6c 26.3±0.5

4 m-xylene 298 4.2 High NOx (NH4)2SO4+H2SO4 68.5 78.3±5.4c 26.3±0.4

5 toluene 298 5.9 Low NOx (NH4)2SO4 37.9 41.0±3.0 28.7±0.6

6 toluene 299 4.6 Low NOx (NH4)2SO4+H2SO4 38.9 43.2±3.2 29.5±0.7

7 toluene 296 4.9 High NOx (NH4)2SO4 60.0 43.8±3.6c 19.3±0.4

8 toluene 298 4.9 High NOx (NH4)2SO4+H2SO4 58.2 38.3±3.2c 17.4±0.5

a

: Low NOx (H2O2 only); high NOx (H2O2+about 100 ppb NO added).

b

: Stated uncertainties (1σ ) are from scatter in particle volume measurements.

c

: Assuming SOA densities are the same as those determined for HONO experiments (see Table 5).

Table 5. Estimated effective SOA densities.

Parent

Hydrocarbon

NOx Condition Effective

Density (g cm−3)

a

m-xylene Low NOx 1.33±0.10

m-xylene High NOx 1.48±0.10

toluene Low NOx 1.45±0.10

toluene High NOx 1.24±0.10

a

: Stated uncertainties (1σ ) are from repeated measurements of ammonium sulfate seed densities.

HONO efficiently photolyzes to OH and NO. By contrast

H2O2 absorbs only weakly in this wavelength range, requiring the use of ppm concentrations of H2O2 to achieve reasonable levels of OH.

The parent aromatics studied (shown in Table 1) and their

stated purities are as follows: m-xylene (Aldrich, anhydrous,

99+%), toluene (Aldrich, anhydrous, 99.8%), and benzene

(Aldrich, anhydrous, 99.8%). Experimental conditions and

results for high-NOx and low-NOx experiments are given in

Tables 2 and 3, while those for studying the effect of seed

acidity on SOA growth are given in Table 4. In calculating SOA yield (defined as the ratio of the mass of organic

aerosol formed to the mass of parent hydrocarbon reacted),

knowledge of the SOA density is required. By comparing

volume distributions from the DMA and mass distributions

from an Aerodyne quadrupole Aerosol Mass Spectrometer

(AMS), the effective densities for the SOA formed can be

estimated (Bahreini et al., 2005). The estimated densities of

the SOA formed from different systems are given in Table 5.

Atmos. Chem. Phys., 7, 3909–3922, 2007 www.atmos-chem-phys.net/7/3909/2007/
N. L. Ng et al.: Secondary organic aerosol formation from aromatics 3913

Figure 1: 

500

400

300

200

100

0

Gas-phase concentration (ppb)

0 50 100 150 200 250

Time (min)

25

20

15

10

5

0

∆

M

o (

μg/m

3

)

 NO

m-xylene

 SOA

Fig. 1. Typical reaction profile of a high-NOx experiment in which

HONO is used as the OH precursor (initial conditions: 89.3 ppb of

m-xylene, 470 ppb NO, and 473 ppb NO2).

3 Results

3.1 High-NOx conditions

Figure 1 shows a typical reaction profile under high-NOx

conditions in which HONO is used as the OH precursor. In

this experiment, 89 ppb of m-xylene is reacted, and initial

NO and NO2 concentrations are 470 ppb and 473 ppb, respectively. The efficient photolysis of HONO generates high

concentrations of OH (∼3×107 molecules cm−3initially),

leading to a rapid hydrocarbon decay. This decay slows

down after ∼1 h, suggesting that the HONO is consumed

and OH radicals are instead generated through recycling via

NOx/HOx chemistry. Aerosol growth occurs essentially immediately, even when [NO] is high (100’s of ppb). With the

high NO concentration, formation of ozone (and hence NO3

radicals) is suppressed.

Concentration (mixing ratio) profiles from two classical

photooxidation experiments with different initial NOx concentrations are shown in Fig. 2. Figure 2a shows the reaction

profile from the photooxidation of 101.6 ppb m-xylene, with

initial NO and NO2 concentrations of 97 ppb and 26 ppb,

respectively. The hydroxyl radical source in classical photooxidation experiments is likely from the photolysis of

HONO, which is formed from the heterogeneous reaction of

NO2 on the chamber wall. The m-xylene-OH reaction leads

to formation of RO2 radicals, which react with NO rapidly,

converting NO to NO2. Ozone is formed from the photolysis of NO2, with its concentration increasing rapidly when

[NO] falls below ∼50 ppb. Only when the NO concentration approaches zero does aerosol growth begin, consistent

with other classical photooxidation experiments (Izumi and

Fukuyama, 1990; Hurley et al., 2001; Johnson et al., 2004;

Martin-Rev ́ıejo and Wirtz, 2005; Song et al., 2005). The

difference between high-NOx experiments and classical photooxidation experiments will be discussed in Sect. 4.2. Figure 2b shows the reaction profile for the photooxidation of

Figure 2 (a): 

250

200

150

100

50

Gas-phase concentration (ppb)

0

0 100 200 300 400 500

Time (min)

30

25

20

15

10

5

0

∆

M

o (

μg/m

3

)

 NO

 O3

m-xylene

 SOA

Fig. 2a. Reaction profile of a typical classical photooxidation experiment (initial conditions: 101.6 ppb m-xylene, 97 ppb NO, and

26 ppb NO2).

Figure 2 (b): 

1000

800

600

400

200

0

NO, O3 concentration (ppb)

0 400 800 1200

Time (min)

100

80

60

40

20

0

Hydrocarbon concentration (ppb)

 NO

 O3

m-xylene

Fig. 2b. Reaction profile of a classical photooxidation experiment

in the presence of ∼1 ppm NOx (initial conditions: 94.8 ppb mxylene, 878 ppb NO, and 65 ppb NO2). A negligible amount of

ozone is formed during the experiment, and no SOA is formed.

94.8 ppb m-xylene, with initial NO and NO2 concentrations

of 878 ppb and 65 ppb, respectively. The NO concentration

decreases over the course of the experiment, but does not fall

below 100 ppb, even after 20 h. A negligible amount of ozone

is formed during the experiment, and no SOA is formed.

For m-xylene and toluene, a series of high-NOx experiments (HONO experiments) with varying initial hydrocarbon

concentrations are carried out. The time-dependent “growth

curves” (organic aerosol 1Mo as a function of hydrocarbon

reacted 1HC) over the course of the experiment, for four

m-xylene experiments, with initial hydrocarbon concentrations ranging from 42 to 172 ppb, are shown in Fig. 3. In

these experiments, 67–79% of the initial m-xylene is consumed. Most of the parent hydrocarbon is consumed in the

first hour and the maximum aerosol yield is reached. After

that hydrocarbon continues to decay slowly and there is little

or no SOA growth, as a result the aerosol yield decreases.

Only SOA growth data up to the maximum aerosol yield are

shown.

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3914 N. L. Ng et al.: Secondary organic aerosol formation from aromatics

Figure 3: 

50

40

30

20

10

0

∆

M

o (μg/m

3

)

0 100 200 300 400 500 600

∆HC (μg/m3)

m-xylene

 132.5 ppb

 106.1 ppb

 70.9 ppb

 28.1 ppb

Fig. 3. Time-dependent growth curves for m-xylene photooxidation

under high-NOx conditions. The concentrations in the legend refer

to the amount of m-xylene reacted in each experiment.

Figure 4: 

40

30

20

10

0

∆

M

o (μg/m

3

)

0 50 100 150 200 250 300 350

∆HC (μg/m3)

toluene

 80.2 ppb

 56.7 ppb

 50.7 ppb

 30.1 ppb

Fig. 4. Time-dependent growth curves for toluene photooxidation

under high-NOx conditions. The concentrations in the legend refer

to the amount of toluene reacted in each experiment.

The time-dependent growth curves for four toluene experiments are shown in Fig. 4. The initial toluene concentration ranges from 88 to 270 ppb. Since the toluene-OH reaction rate constant is ∼4 times lower than that of m-xyleneOH, more initial toluene is needed relative to m-xylene; only

about 30–37% of the initial toluene injected is consumed

at the point of maximum aerosol yield. Photooxidation of

toluene under high-NOx conditions results in slightly more

SOA growth than for m-xylene.

Because benzene reacts slowly with OH radicals

(k=1.22×10−12 cm3 molecule−1s

−1

, Calvert et al., 2002), it

is not feasible to carry out photooxidation experiments over

a range of initial benzene concentrations unless high levels

(ppm) of benzene are used. Thus only a single benzene photooxidation experiment at high NOx was carried out; at an

initial benzene concentration of 337 ppb, only 12% is reacted

at the point of maximum aerosol yield. The time-dependent

Figure 5: 

80

60

40

20

0

∆

M

o (μg/m

3

)

0 50 100 150 200 250

∆HC (μg/m3)

benzene

 low-NOx

 high-NOx

Fig. 5. Time-dependent growth curves for benzene photooxidation

under high- and low-NOx conditions. Under high-NOx conditions,

the initial benzene concentration is 337 ppb (12% reacted). Under

low-NOx conditions, the initial benzene concentration is 395 ppb

(16% reacted) and the system has a constant yield of 37%.

growth curve from benzene under high-NOx conditions (as

well as under low-NOx conditions) is shown in Fig. 5. Hydrocarbon decay slows down at 1HC ≈90μg m−3

, suggesting that HONO is almost completely consumed. After that,

OH is generated mainly via NOx/HOx chemistry, and hydrocarbon decays at a slower rate resulting in the more closely

spaced data points. The slower hydrocarbon oxidation rate

after the consumption of HONO may affect the kinetics of

SOA growth and contribute to the slight change in the shape

of the growth curves.

3.2 Low-NOx conditions

Under low-NOx conditions, aerosol growth is observed immediately after initiation of irradiation. The parent hydrocarbon decays at a much slower rate than under highNOx conditions, due to the slow production of OH radicals by H2O2 photolysis and lack of OH regeneration by

NOx/HOx cycling. As OH radicals are continually produced, the OH concentration is constant throughout the experiment (∼3×106 molecules cm−3

). Ozone formation of

∼10–15 ppb is observed, possibly owing to residual material

released from the chamber walls.

Time-dependent growth curves for four m-xylene lowNOx experiments are shown in Fig. 6, with initial m-xylene

concentrations ranging from 9 to 37 ppb. About 83–89% of

the initial hydrocarbon injected is consumed at the point at

which the SOA yield reaches its maximum. From Fig. 6 it

is clear that the SOA yield from m-xylene photooxidation

is constant under low-NOx conditions, at 36%. Since the mxylene SOA yield is much higher under low-NOx conditions,

a smaller amount of initial parent hydrocarbon is needed to

produce the same amount of SOA than under high-NOx conditions. Comparable time-dependent growth curves for four

Atmos. Chem. Phys., 7, 3909–3922, 2007 www.atmos-chem-phys.net/7/3909/2007/
N. L. Ng et al.: Secondary organic aerosol formation from aromatics 3915

Figure 6: 

60

50

40

30

20

10

0

∆

M

o (μg/m

3

)

0 20 40 60 80 100 120 140

∆HC (μg/m3)

m-xylene

 32.5 ppb

 26.1 ppb

 16.1 ppb

 8.0 ppb

Fig. 6. Time-dependent growth curves for m-xylene photooxidation

under low-NOx conditions. The concentrations in the legend refer

to the amount of m-xylene reacted in each experiment. The system

exhibits a constant yield of 36%.

Figure 7: 

80

60

40

20

0

∆

M

o (μg/m

3

)

0 50 100 150 200 250

∆HC (μg/m3)

toluene

 63.9 ppb

 32.1 ppb

 23.8 ppb

 10.0 ppb

Fig. 7. Time-dependent growth curves for toluene photooxidation

under low-NOx conditions. The concentrations in the legend refer

to the amount of toluene reacted in each experiment. The system

exhibits a constant yield of 30%.

toluene low-NOx experiments are shown in Fig. 7. The initial toluene concentration ranges from 21 to 140 ppb. With

the slower reactivity of toluene relative to m-xylene, only

about 45–48% of the initial toluene is consumed. As with mxylene, the aerosol yield (30%) is substantially higher than

under high-NOx conditions.

The time-dependent growth curve for benzene photooxidation is shown in Fig. 5. Similar to m-xylene and toluene,

benzene photooxidation under low-NOx conditions results in

a constant SOA yield (37%).

3.3 SOA yield parameters

SOA yield has traditionally been described by a semiempirical model based on absorptive gas-particle partitioning of two semivolatile products (Odum et al., 1996, 1997a,

b):

Figure 8: 

0.16

0.14

0.12

0.10

0.08

0.06

0.04

0.02

0.00

Yield 

0 10 20 30 40 50

Mo (μg/m3)

 toluene

-xylene

Fig. 8. Yield curves for toluene and m-xylene under high-NOx conditions. The parameters for fitting the yield curves are, toluene:

α1=0.058, Kom,1=0.430, α2= 0.113, and Kom,2= 0.047; m-xylene:

α1=0.031, Kom,1=0.761, α2=0.090, and Kom,2=0.029.

Y=Mo



α1Kom,1

1+Kom,1Mo

+

α2Kom,2

1+Kom,2Mo



(1)

in which Y is the aerosol yield, Mo is the organic aerosol

mass present (equal to 1Mo in chamber experiments with

no absorbing organic mass present initially), αiis the massbased gas-phase stoichiometric fraction for semivolatile

species i, and Kom,i is the gas-particle partitioning coefficient for species i. With this two-product model, Eq. (1) can

be fit to experimental yield data to determine values for αi

and Kom,i, and the resulting plot (Y versus Mo) is generally

referred to as a “yield curve”.

For m-xylene and toluene, the final SOA yield for each

high-NOx (HONO) experiment is calculated, and the data

are fitted to Eq. (1) to obtain the SOA yield parameters. The

high-NOx yield curves for m-xylene and toluene are shown

in Fig. 8. For all three aromatics (m-xylene, toluene, and benzene), the low-NOx experiments result in a constant aerosol

yield (the slope of the “growth curve”), implying the SOA

formed can be represented by a single product with very low

volatility. Under these conditions the yield curve is simply

a horizontal line, and the constant yield corresponds to α1

in Eq. (1). SOA growth parameters for the three compounds

under high-NOx and low-NOx conditions are summarized in

Table 6.

3.4 Acid/nonacid seed experiments

Several m-xylene and toluene photooxidation experiments

are performed in the presence of acid seed to study the effect of seed acidity on SOA growth. Growth curves for

toluene photooxidation under acidic and nonacid conditions

are shown in Fig. 9. Regardless of the NOx level, the timedependent growth curves from the acid and nonacid experiments are essentially indistinguishable. Therefore, there is

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3916 N. L. Ng et al.: Secondary organic aerosol formation from aromatics

Table 6. Aerosol yield parameters.

Parent

Hydrocarbon

NOx Condition α1 Kom,1

(m3 μg

−1

)

α2 Kom,2

(m3 μg

−1

)

m-xylene Low NOx 0.30 N/A N/A N/A

m-xylene High NOx 0.031 0.761 0.090 0.029

toluene Low NOx 0.36 N/A N/A N/A

toluene High NOx 0.058 0.430 0.113 0.047

benzene Low NOx 0.37 N/A N/A N/A

benzene High NOx 0.072 3.315 0.888 0.009

N/A: not applicable

Figure 9: 

30

25

20

15

10

5

0

∆

M

o (μg/m

3

)

0 20 40 60 80 100 120 140

∆HC (μg/m3)

 High NOx (acid)

 High NOx (nonacid)

 Low NOx (acid)

 Low NOx (nonacid)

Fig. 9. Time-dependent growth curves for toluene photooxidation

in the presence of neutral seed versus acidic seed.

no evidence that the presence of acidic seed enhances SOA

growth in the photooxidation of toluene; similar results are

observed for m-xylene.

4 Discussion

4.1 Effect of NOx on SOA yields

Experiments have been performed under two limiting NOx

conditions: (1) high-NOx experiments in which HONO is

used as the OH precursor; and (2) low-NOx experiments in

which H2O2 is used as the OH precursor. For all hydrocarbons, considerably more SOA is formed under low-NOx than

high-NOx conditions (Figs. 3–7). Under high-NOx conditions, the SOA yields from m-xylene and toluene photooxidation are about 10%; they are 36% and 30%, respectively,

under low-NOx conditions. Similar NOx dependences have

been observed in other SOA-forming systems (Hatakeyama

et al., 1991; Zhang et al., 1992; Hurley et al., 2001; Johnson

et al., 2004; Song et al., 2005; Presto et al., 2005; Kroll et al.,

2006). Since O3 and NO3 are not formed appreciably under

either set of conditions, oxidation is dominated by OH radicals for all experiments, so this NOx effect cannot be a result

of differences in relative oxidant levels (Hurley et al., 2001).

Instead the NOx level likely governs the fate of the organic

peroxy radicals formed subsequent to the hydrocarbon-OH

reaction, which in turn controls the volatility of molecular

products and hence the amount of SOA formed.

Shown in Fig. 10 is the simplified mechanism of the initial steps of toluene-OH reaction, leading to the formation of

condensable products. One mechanism by which NOx levels

may affect the products formed in the oxidation of aromatic

hydrocarbons is by reaction with the aromatic-OH adduct.

The aromatic-OH adduct reacts predominately with O2 under atmospheric conditions. Under very high levels of NOx

(ppm levels), the adduct+NO can still be neglected (Koch et

al., 2006) while the adduct+NO2 reaction increases in importance, leading to nitrogen-containing ring-retaining products such as nitrobenzene and nitrotoluene (Atkinson et al.,

1989; Atkinson and Aschmann, 1994; Koch et al., 2006).

The presence of a high concentration of NOx in the chamber

also has an impact on the formation yields of phenol-type

compounds, possibly due to the reactions of the aromaticOH adduct with NO and NO2. For example, the phenol yield

from benzene photooxidation starts to decrease dramatically

for [NO2] >100 ppb (Volkamer et al., 2002). In the current

work, photooxidation of m-xylene and toluene is carried out

in the presence of ∼500 ppb each of NO and NO2; while for

photooxidation of benzene the initial NO2 concentration is

<100 ppb. At these NOx levels, the reaction of the aromaticOH adduct with NO2 is not expected to be significant (Volkamer et al., 2002; Koch et al., 2006), so such reactions cannot

be responsible for the observed effect of NOx on SOA yield.

Reaction of the aromatic-OH adduct with O2 results in

the formation of peroxy radicals. Theoretical studies have

shown that the peroxy radicals preferentially cyclize to form

bicyclic radicals, which then react with O2 to form bicyclic

peroxy radicals (Andino et al., 1996; Lay et al., 1996; Suh et

al., 2003; Fan et al., 2006), even in the presence of ppm levels of NO (Zhao et al., 2005; Fan et al., 2006). Because of the

high energy barrier, the isomerization of bicyclic radicals to

form epoxide radicals is unlikely for toluene; only a small of

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N. L. Ng et al.: Secondary organic aerosol formation from aromatics 3917

fraction of expoxy carbonyls are observed in m-xylene photooxidation, suggesting that the epoxide radical channel is

not important either (Suh et al., 2003; Zhao et al., 2005).

As is typical for RO2 radicals, the fate of the bicyclic peroxy radicals depends mainly on the relative concentrations

of NO, HO2, and RO2. At the two limiting NOx conditions

of this study, the peroxy radical chemistry is straightforward; under high-NOx conditions, RO2 reacts virtually entirely with NO, as NO concentration is high throughout the

entire experiment, while under low-NOx conditions, RO2 reacts predominantly with HO2. Based on the Master Chemical Mechanism version 3.1 (MCM v 3.1, http://www.chem.

leeds.ac.uk/Atmospheric/MCM/mcmproj.html), a simple

kinetic simulation shows that under low-NOx conditions, the RO2+RO2 reaction accounts for less than 1%

of the RO2 reacted because of the relative reaction

rate constants (kRO2+RO2=8.8×10−13 cm3 molecule−1s

−1

,

kRO2+HO2=2.3×10−11 cm3 molecule−1s

−1

) as well as the

high HO2 concentration. Thus the larger SOA yields obtained under low-NOx conditions imply that the products

formed via the RO2+HO2 partition much more readily into

the aerosol phase than those formed via the RO2+NO reaction. This conclusion is the same as that reached by previous studies (Hatakeyama et al., 1991; Johnson et al., 2004,

2005; Presto et al., 2005; Kroll et al., 2006). That SOA yields

are constant under low-NOx conditions implies that the SOA

formed by this channel is effectively nonvolatile under the

range of Mo(>10μg m−3) studied. It is possible that under

lower SOA loadings the yields may decrease as some of the

products partition back into the gas phase.

In the classical photooxidation experiments carried out in

this study and by other researchers (Izumi and Fukuyama,

1990; Hurley et al., 2001; Johnson et al., 2004; MartinRev ́ıejo and Wirtz, 2005; Song et al., 2005), an “induction period”, a delay between the onset of oxidation and

SOA formation, was observed. This too is a likely result of the role of NOx in peroxy radical chemistry, and

hence in product volatility and SOA formation. As illustrated in Fig. 2a, only when [NO] approaches zero

does aerosol growth commence. As [NO] approaches

zero (from the RO2+NO and HO2+NO reactions), the

RO2+HO2 reaction starts to compete with the RO2+NO

reaction. The fraction of RO2 which reacts with HO2,

kRO2+HO2[HO2]/(kRO2+NO[NO]+kRO2+HO2[HO2]), can be

calculated based on the rate reaction rate constants

(kRO2+HO2=2.3×10−11 cm3 molecule−1s

−1

; kRO2+NO

=8.5×10−12 cm3 molecule−1s

−1

) (MCM v 3.1). For instance, at ∼1 ppb of NO, it only requires 42 ppt of HO2 for

10% of the RO2 to react via RO2+HO2. Thus it is likely that

initial SOA formation results from the RO2+HO2 reaction,

consistent with the simulations of SOA formation from classical photooxidation experiments of toluene (Johnson et al.,

2004). To further study the role of peroxy radical chemistry

in the “induction period”, in one of the experiments additional NO was injected after its initial consumption. SOA

growth slows down immediately. This provides strong evidence that the presence of NO suppresses the formation of

relatively nonvolatile hydroperoxides (and hence further reactions of hydroperoxides) from the RO2+HO2 reaction and

results in a lower SOA yield (Johnson et al., 2004, 2005).

The time-dependent growth curves for benzene photooxidation under high- and low-NOx conditions exhibit the same

trend as that of m-xylene and toluene, in which more SOA

is formed under low-NOx conditions. Martin-Rev ́ıejo and

Wirtz (2005) studied the formation of SOA from benzene

photooxidation under different NOx conditions. However,

as the NOx dependence of SOA formation was not systematically studied, it is difficult to draw a definite conclusion

on the effect of NOx on SOA yields from their data. Additionally, the experimental conditions are somewhat different; in particular, the experiments in that study were performed in the absence of seed aerosol while in the current

work ammonium sulfate seed aerosol is employed. Kroll et

al. (2007) find that SOA yields from the photooxidation of

aromatic hydrocarbons are lower when inorganic seed particles are not present initially. The absence of seed particles

results in a period in which the hydrocarbon is reacted but

no aerosol is formed. The length of the “seed induction period” (and hence the amount of hydrocarbon reacted in this

period, 1HC) is likely to be affected by the NOx levels. Thus

the aerosol yields (1Mo/1HC) obtained by Martin-Rev ́ıejo

and Wirtz (2005) under different NOx conditions may be affected by this “seed induction period” and cannot be directly

compared.

4.2 Effect of oxidation rate

Under high-NOx conditions, using HONO as an OH source,

we observe SOA formation even when [NO] is several hundreds of ppb (Fig. 1). This is in contrast to results from

classical photooxidation experiments, in which essentially no

SOA is generated at high NOx levels, as observed both in

the present study (Fig. 2b) and in work by other researchers

(Johnson et al., 2004; Song et al., 2005).

The observation of SOA at high NOx levels suggests that

products from the RO2+NO reactions, such as organic nitrates, are indeed condensable; however they apparently do

not partition into the aerosol phase appreciably in the classical photooxidation experiments. A major difference between

the HONO experiments (Fig. 1) and classical oxidation experiments (e.g. Fig. 2) is OH level, and hence oxidation rate.

HONO photolysis is an efficient source of OH in the wavelength range of our blacklights; thus there is a burst of OH

once the lights are turned on, resulting in a rapid rate of mxylene oxidation. In the classical photooxidation experiment,

OH is generated mainly from recycling through NOx and

HOx cycles, and OH concentrations are generally far lower

than in the HONO experiments. The large difference in SOA

formation in these two cases suggests that SOA yields are

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3918 N. L. Ng et al.: Secondary organic aerosol formation from aromatics

Figure 10: 

OH

OH

H

H

OH

H

H

OO

O2

H

OH O

O

OO

Isomerization, O2

HO2 NO

NO2

1st generation products

(possibly semivolatile)

OH OH

Condensable products

SOA

X

 ROOH

(possibly semivolatile)

Fig. 10. A simplified SOA formation mechanism for toluene photooxidation. X represents the generic non particle-phase product

from all gas-phase loss processes.

Figure 11: 

0.10

0.08

0.06

0.04

0.02

0.00

Yield 

0 10 20 30 40 50 60

Mo (μg/m3)

 This work

 Odum et al. (1996)

Fig. 11. Comparison of m-xylene high-NOx yield curve obtained in

the current work to that from Odum et al. (1996). The yield curve

from Odum et al. (1996) has been corrected for the temperature

(25◦ C) of this study and density (1.48 g cm−3) of the SOA.

dependent on the oxidation rate, with faster oxidation rates

resulting in higher SOA yields.

This “rate effect” may arise as a result of loss of

semivolatiles through processes other than the simple formation and partitioning of semivolatile organics. The loss of

semivolatiles has been proposed as one of the mechanisms

for higher SOA yields observed from aromatic photooxidation in the presence of seed aerosols as compared to nucleation experiments (Kroll et al., 2007). In the mechanism

shown in Fig. 10, X represents the generic non particlephase product of all gas-phase loss processes. Possibilities include loss of organic species to the chamber walls,

photolysis, and further reactions to form volatile products.

With a rapid oxidation rate, the gas-phase concentration of

semivolatiles (formed, for example, from the RO2+NO reaction) increases quickly, and the high concentrations of

semivolatiles ensure that aerosol growth ensues even in the

presence of semivolatile loss processes (i.e. paths not forming SOA). On the other hand, in classical photooxidation experiments, the slower formation of semivolatiles in the presence of semivolatile sinks will lead to lower SOA growth. If

the dominant loss of semivolatiles is deposition to the chamber walls, then conditions in which the effect of wall loss

is minimized (i.e. when the reaction is fastest) are expected

to be most representative of atmospheric SOA formation.

Recently, the effect of particle- and gas-phase reactions of

semivolatile products on SOA formation has been examined

in a modeling study by Chan et al. (2007), in which the experimental data presented in this work (among other literature

data) are fitted to simple kinetic models. Given the importance of the loss of semivolatiles, these loss processes merit

further investigation and experimental evidence.

This rate effect also can explain higher SOA yields in the

high-NOx experiments (HONO) compared to the classical

photooxidation experiments (Odum et al., 1996), illustrated

in Fig. 11. Several classical photooxidation experiments with

similar NOx levels as in Odum et al. (1996) were performed

in the present study, with SOA yields comparable to those

of Odum et al. (1996). The aerosol yields from Odum et

al. (1996) were obtained at a temperature of 35–40◦ C with

an assumed SOA density of 1 g cm−3. In Fig. 11, the yield

curve from Odum et al. (1996) has been adjusted based on the

temperature (25◦ C) and SOA density (1.48 g cm−3) obtained

in this study. The enthalpy of vaporization employed to convert the SOA yields from the higher temperature in Odum et

al. (1996) to the lower temperature in this study is assumed to

be 42 kJ mol−1(Chung and Seinfeld, 2002), which is in the

range of limited experimental data (Offenberg et al., 2006).

The NOx concentrations (several hundreds of ppb) used by

Odum et al. (1996) are lower than those in the current experiments (∼1 ppm). The higher SOA yields obtained here

suggest that the enhancement in SOA yield from a faster oxidation rate is greater than the suppression in SOA yield at a

higher NOx concentration.

Significant SOA formation from the rapid oxidation of

aromatics under high (ppm) levels of NOx has also been observed by Stroud et al. (2004), using isopropyl nitrite as an

OH source to oxidize toluene. Such results could not be reproduced in the simulations of Johnson et al. (2004), which

predicted no SOA formation in the presence of such high

NOx levels, as the RO2+NO reaction products were found

Atmos. Chem. Phys., 7, 3909–3922, 2007 www.atmos-chem-phys.net/7/3909/2007/
N. L. Ng et al.: Secondary organic aerosol formation from aromatics 3919

to be too volatile to partition substantially into the aerosol

phase. However, the present work suggests that products

of the RO2+NO reaction are in fact condensable, but contribute to aerosol mass only when oxidation reactions are

fast, minimizing the effect of other loss processes. This underscores the need for a better understanding of the behavior

of semivolatile organics in the atmosphere, in terms of both

physical properties (partitioning coefficients) and chemical

reactions (reactive loss processes).

Finally, we note that the pair of high-NOx experiments

(Fig. 1 and Fig. 2b) also provides insight into the extent to

which the hydrocarbon/NOx (HC/NOx) ratio characterizes

the NOx level in chamber experiments. Song et al. (2005),

for example, report a series of classical photooxidation experiments to study the effect of HC/NOx ratio on SOA formation from m-xylene. It was found that aerosol yields

increase with increasing HC/NOx ratio (i.e. more aerosol

growth at lower NOx levels), which is consistent with the

present study. However, the experiments in Fig. 1 and 2b

have essentially similar HC/NOx ratios but exhibit very different SOA growth. Therefore, while the HC/NOx ratio may

be a useful metric for photochemistry for experiments with

similar oxidation conditions, it is less useful when comparing systems with very different oxidative conditions, as both

oxidation rate and fate of peroxy radicals may differ. Given

the importance of the peroxy radical in SOA formation, the

NO/HO2 ratio would be the more appropriate measure of different NOx levels (Kroll et al., 2006), although continuous

measurement of HO2 is not currently feasible.

4.3 General mechanism of SOA formation

Despite uncertainties in the detailed chemical mechanism of

aromatic photooxidation beyond the initial peroxy radical

chemistry, we can gain insights into the general SOA formation mechanism and growth kinetics from the shapes of

the growth curves under both high-NOx and low-NOx conditions (Figs. 3–7). Such curves allow for the identification

of the rate-determining steps in SOA formation (Ng et al.,

2006). In cases where the initial oxidation step determines

the rate of SOA formation (condensable products are firstgeneration, or are formed extremely rapidly after the initial

oxidation), SOA is formed at the same rate of hydrocarbon

oxidation, and aerosol growth ceases once the parent hydrocarbon is consumed. In this case, time-dependent growthcurves from experiments with different initial hydrocarbon

concentrations would overlap, as is the case for α-pinene

ozonolysis (Ng et al., 2006). On the other hand, when there

are further rate-limiting steps to the formation of condensable

products, there is a time lag between hydrocarbon oxidation

and SOA formation and the growth curve exhibits a vertical

section at the end, as is the case for the ozonolysis and photooxidation of compounds with multiple double bonds (Ng

et al., 2006).

Figures 3–7 show the time-dependent growth curves for

m-xylene, toluene, and benzene photooxidation under highNOx and low-NOx conditions. In all cases, SOA growth is

observed immediately after photooxidation commences, resulting in a smooth growth curve from the onset of oxidation.

Some hydrocarbon remains unreacted by the end of these experiments. In experiments in which all the hydrocarbon is reacted (not shown), no vertical section in the growth curve is

observed, indicating there is no further SOA formation after

the complete consumption of the parent hydrocarbon. This

indicates that the first oxidation step (oxidation of the parent

hydrocarbon) governs the rate of SOA formation.

However, unlike with α-pinene ozonolysis (Ng et al.,

2006), the time-dependent growth curves from experiments

with different initial aromatic hydrocarbon concentrations

do not overlap, and aerosol growth is not the same for a

given amount of hydrocarbon reacted (1HC). Instead, SOA

growth at a given value of 1HC depends on the initial hydrocarbon concentration (HC0): experiments with higher

HC0 reach a given 1HC in a shorter time than those with

smaller HC0. At smaller HC0, during the longer time required to reach the same 1HC, the first generation products have more time to be oxidized to form condensable

products. Therefore, the divergence in growth curves at different initial hydrocarbon concentrations indicates that even

though the first step is the rate-limiting in SOA formation,

subsequent oxidation steps also occur prior to SOA formation. As discussed in Ng et al. (2006), if subsequent oxidation steps are essentially instantaneous relative to the initial oxidation step, then SOA formation can be treated as a

single step and all the growth curves will overlap. In the

present study, the growth curves diverge only slightly, indicating that although the second oxidation step in not instantaneous, it is still substantially faster than the initial oxidation step. The formation of SOA by multiple oxidation steps,

in which the later steps are substantially faster than the initial oxidation, is consistent with available kinetic data and

current understanding of the photooxidation of aromatic hydrocarbons. In general, first-generation products of aromatic

photooxidation react about an order of magnitude faster

with OH than do their parent hydrocarbons (Calvert et al.,

2002). For example, whereas the benzene-OH rate constant

is 1.22×10−12 cm−3 molecule−1s

−1

, the rate constant of the

reaction of OH with phenol, a major first-generation reaction

product, is 2.7×10−11 cm−3 molecule−1s

−1

(Calvert et al.,

2002). Our study does not exclude the possibility of formation of SOA from the condensation of first-generation products; however, it suggests that second (or further) generation

products contribute significantly to SOA growth.

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3920 N. L. Ng et al.: Secondary organic aerosol formation from aromatics

4.4 Effect of seed acidity

To our knowledge, there are no published data on the effect

of seed aerosol acidity on SOA formed from the photooxidation of aromatic VOCs. As shown in Fig. 9, seed particle

acidity does not enhance the SOA yield under different NOx

conditions. The composition of the acid and nonacid seed

particles, as well as the RH (∼5%), are the same as those

previously employed in the study of isoprene SOA formation (Surratt et al., 2007); yet, an enhancement in SOA yield

is observed for isoprene experiments but not for the aromatics. The chambers are maintained at RH ∼5% in this study

and so the nonacid seed particles are dry, whereas for acidic

seed particles some water might be present. If aerosol water

is essential for the acidity effect, its absence might explain

the lack of an observed effect in the current study. The dependence of SOA growth on RH is beyond the scope of this

work but warrants future investigation.

5 Implication for SOA growth from aromatic hydrocarbons

We report a series of chamber experiments investigating the

NOx dependence and effect of seed aerosol acidity on SOA

formation from the photooxidation of aromatic compounds.

High-NOx experiments are performed with HONO as the OH

precursor at initial NOx of ∼1 ppm. By performing experiments with HONO, SOA is formed under truly high-NOx

conditions, as the NO concentration remains high throughout the entire experiment. In low-NOx experiments, H2O2

is used as the OH precursor and the initial NOx is <1 ppb.

For each of the aromatic hydrocarbons studied (m-xylene,

toluene, and benzene), the SOA yields under low-NOx conditions are significantly larger than those under high-NOx

conditions; this is likely a result of the competition between

RO2+NO and RO2+HO2 reactions, similar to what has been

observed in other studies (Hatakeyama et al., 1991; Johnson

et al., 2004, 2005; Presto et al., 2005; Kroll et al., 2006).

In assessing the contribution of aromatic compounds to

total ambient SOA, it is important that laboratory conditions are representative of those in the atmosphere. The aromatic SOA yield parameters (αi and Kom,i) currently employed in atmospheric models are those obtained by Odum

et al. (1996, 1997a, b) almost a decade ago by irradiation

of hydrocarbon/NOx mixtures. With the profound dependence of NOx on SOA formation, it is necessary that the

effect of NOx on SOA yields be included in atmospheric

models. For instance, compounds like benzene are mainly

emitted in source-rich regions; with its slow reactivity, however, benzene can be transported to areas with a lower NOx

level before it react substantially, resulting in a higher SOA

yield than if it reacted in the immediate vicinity of its sources.

By performing experiments at two extreme NOx conditions,

we are able to obtain SOA yield parameters under high- and

low-NOx conditions, allowing for the parameterization of the

NOx dependence for atmospheric models, based upon the

reactivity of organic peroxy radicals. Other than the NOx

effect, the loss of semivolatiles also affects the SOA yields

measured. The depletion of semivolatiles by chemical reaction or wall loss may lead to lower SOA formation in the

chambers than occurs in the atmosphere. Due to the slower

oxidation rate in the classical photooxidation experiments

conducted by Odum et al. (1996, 1997a, b), it is likely that

their SOA yield parameters underestimate SOA formation

from aromatic hydrocarbons in the atmosphere.

Finally, detailed analysis of the chemical composition of

aromatic SOA will assist in unraveling the detailed aromatic

SOA formation mechanism under both high and low-NOx

conditions. In a forthcoming publication, the chemical composition of aromatic SOA will be explored. Since a wide

array of sulfate esters are observed only when acidified inorganic seed aerosols are employed (Surratt et al., 2007), the

chemical composition of the SOA formed in nonacid/acid

seed experiments may also provide insights into the lack of a

seed acidity effect on SOA yields in the present study.

Acknowledgements. This research was funded by the U.S. Environmental Protection Agency Science to Achieve Results (STAR)

Program grant number RD-83107501-0, managed by EPA’s

Office of Research and Development (ORD), National Center

for Environmental Research (NCER), and by U.S. Department

of Energy Biological and Environmental Research Program

DE-FG02-05ER63983.

Edited by: S. Martin

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