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Atmospheric Environment 41 (2007) 959–973

www.elsevier.com/locate/atmosenv

The effects of ozone/limonene reactions on indoor secondaryorganic aerosols

Golam Sarwar1, Richard Corsi�

Department of Civil, Architectural and Environmental Engineering, The University of Texas at Austin, Austin, TX 78712, USA

Received 12 April 2006; received in revised form 31 August 2006; accepted 15 September 2006

Abstract

An indoor air quality model was used to predict dynamic particle mass concentrations based on homogeneous chemical

mechanisms and partitioning of semi-volatile products to particles. The ozone–limonene reaction mechanism was

combined with gas-phase chemistry of common atmospheric organic and inorganic compounds and incorporated into the

indoor air quality model. Experiments were conducted in an environmental chamber to investigate secondary particle

formation resulting from ozone/limonene reactions. Experimental results indicate that significant fine particle growth

occurs due to the interaction of ozone and limonene and subsequent intermediate by-products. Secondary particle mass

concentrations were estimated from the measured particle size distribution. Predicted particle mass concentrations were in

good agreement with experimental results—generally within �25% at steady-state conditions. Both experimental and

predicted results suggest that air exchange rate plays a significant role in determining secondary fine particle levels in

indoor environments. Secondary particle mass concentrations are predicted to increase substantially with lower air

exchange rates, an interesting result given a continuing trend toward more energy efficient buildings. Lower air exchange

rates also shifted the particle size distribution toward larger particle diameters. Secondary particle mass concentrations are

also predicted to increase with higher outdoor ozone concentrations, higher outdoor particle concentrations, higher indoor

limonene emission rates, and lower indoor temperatures.

r 2006 Elsevier Ltd. All rights reserved.

Keywords: Indoor chemistry; Ozone; Limonene; Terpenes; Secondary aerosols

1. Introduction

Several recent studies suggest that indoor airchemistry, particularly as related to reactionsbetween terpenes that originate from indoor sources

e front matter r 2006 Elsevier Ltd. All rights reserved

mosenv.2006.09.032

ing author. +1 512 232 3611;

1720.

ess: corsi@mail.utexas.edu (R. Corsi).

ress: USEPA, Mail Drop E243-03,

nder Drive, RTP, NC 27711, USA.

and ozone (O3) that originates outdoors, can be animportant source of indoor fine particles (Weschlerand Shields, 1999, 2003; Wainman et al., 2000; Longet al., 2000; Rohr et al., 2003; Sarwar et al., 2003,2004). These results should raise concerns related tothe indoor contribution of human exposure to fineparticles. The average American spends 18 h in-doors for every hour spent outdoors (Robinsonet al., 1991). Thus, human exposure to fine particlesin indoor environments can be a major fraction oftotal exposure to fine particles.

.

ARTICLE IN PRESSG. Sarwar, R. Corsi / Atmospheric Environment 41 (2007) 959–973960

The use of common terpene-containing consumerproducts in the presence of O3 has been shown tolead to secondary organic aerosol (SOA) formation.Long et al. (2000) conducted six sampling events inBoston homes during which pine oil-based cleanerwas used for mopping floors and toilet cleaning.Particle number concentrations increased by 7–10times relative to the original particle numberconcentrations in five of six sampling events, andmore than 50% of the particles (by volume)generated during these five events were ultra-finein nature. Sarwar et al. (2004) also investigated theeffects of the reactions between terpenes emittedfrom five selected consumer products and O3. Theconsumer products included a lime scented liquidair-freshener, a pine scented solid air-freshener, alemon scented general-purpose cleaner, a woodfloor cleaner, and a perfume. Two chamber experi-ments were performed for each of these five terpene-containing products, one at an elevated O3 con-centration and the other at a lower O3 concentra-tion. Experiments with air fresheners produced thehighest increases in particle number and massconcentrations.

Limonene is known to react with O3 to formSOA. Wainman et al. (2000) used a nested chambersystem to investigate indoor particle formation andgrowth due to O3/limonene reactions. Increases inparticle numbers in the 0.1–0.2 mm size range wereobserved shortly after the first O3 injection, butformation of particles in the 0.2–0.3 mm size rangedid not occur until a second O3 injection. Weschlerand Shields (2003) conducted experiments todetermine the effects of air exchange rates onindoor secondary particle size distribution and massconcentrations. They introduced limonene into twoadjacent unoccupied and identical offices. In one ofthe offices, an O3 generator was operated to raisethe indoor O3 levels to 50–425 ppb. The other officedid not have an O3 generator. The lower airexchange rates shifted the resulting secondaryparticle size distribution toward larger sizes andled to increased particle mass concentrations.

a-Pinene has also been observed to form SOA inindoor environments. Sarwar et al. (2003) investi-gated secondary particles resulting from homoge-neous reactions between O3 and a-pinene.Experimental results indicated that rapid fineparticle growth occurs due to homogeneous reac-tions between O3 and a-pinene, and subsequent gas-to-particle partitioning of the products. A newindoor air quality model (ICEM: indoor chemistry

and exposure model) was used to predict dynamicparticle mass concentrations based on detailedhomogeneous chemical mechanisms and partition-ing of semi-volatile products to particles. Particlemass concentrations were in reasonable agreementwith results predicted from the model. Both experi-mental and model results indicated that secondaryparticle mass concentrations increase substantiallywith lower air exchange rates.

Nøjgaard et al. (2006) recently investigated theeffect of nitrogen dioxide (NO2) on particle forma-tion during oxidation of a-pinene and d-limonene inthe absence of direct sunlight. These experimentswere conducted using dry air in large Tedlar bagsand concentrations representative of the maximumvalues reported for indoor environments. Theyconcluded that the introduction of NO2 reducesthe particle number and volume concentrations fora-pinene as well as d-limonene. However, thereduction was much greater for the case of a-pinenecompared to the case of d-limonene. Nøjgaard et al.(2006) used 33 gas-phase reactions in a box model toanalyze reactant concentrations in the Tedlar bagsand suggested that O3 concentrations were lowerdue to the formation of nitrate radicals (NO3) fromthe reaction between O3 and NO2. The particlenumber concentrations decreased because the nu-cleation potential of NO3 is lower than that of O3.They also reported that reactions between O3 and d-limonene produced about five times more particlenumber concentrations than the reactions betweenO3 and a-pinene.

The studies described above might provide anexplanation for the ‘‘unexplained’’ indoor fineparticles reported by Santannam et al. (1990) andWallace (1996). Santannam et al. (1990) reportedthe results of indoor and outdoor sampling com-pleted at 280 residences in Ohio and Wisconsin. Theaverage ‘‘unexplained’’ percentage of indoor fineparticles, i.e., those that could not be attributed tospecific sources, ranged from 19% to 42%. Similarresults (average of 25% ‘‘unexplained’’ fine parti-cles) were observed during the USEPA’s PTEAMstudy (Wallace, 1996). Wallace concluded that theunexplained fraction of fine particles should be thefocus of future studies.

In this study, the effects of reactions between O3

and limonene on indoor particles were investigatedusing chamber experiments and the ICEM. Theoxidation of limonene produces many gas-phasecompounds, some of which may also be harmful(Clausen et al., 2001; Nøjgaard et al., 2005).

ARTICLE IN PRESSG. Sarwar, R. Corsi / Atmospheric Environment 41 (2007) 959–973 961

However, this study did not consider such productsand focused only on the formation of SOA.

2. Methodology

2.1. Experimental methodology

Detailed experimental procedures have beendescribed elsewhere (Sarwar et al., 2003); only abrief description is provided here. Experiments wereconducted in an 11-m3 stainless steel chamber.A commercial corona discharge ozone generator(Living Air, BORA-IV) was operated in the middleof the chamber for 10–20 h to achieve steady-stateO3 concentrations before to the introduction oflimonene. A 4-ml vial containing pure reagent gradeR-limonene was then placed on the floor of thechamber, allowing for evaporation of limonene.Two fans (SMCs, Model TR12 and Coll-BreezeTM,Model EB24925) were operated on the floor of thechamber to enhance mixing.

Chamber particle number concentrations werecontinuously measured using two particle counters(Particle Measuring Systems Inc., LASAIRs—Model 1002; and TSITM P-TRAKTM—Model8525). The two particle analyzers collectivelyprovided particle number concentrations in ninedifferent diameter ranges: (all in mm) 0.02–0.1,0.1–0.2, 0.2–0.3, 0.3–0.4, 0.4–0.5, 0.5–0.7, 0.7–1.0,1.0–2.0, and42.0 mm. Mass concentration wasestimated from particle number concentrationsand particle density using procedures described bySarwar et al. (2003).

The particle counters were located outside of thechamber during each experiment. The LASAIRs

drew sample air from within the chamber througha 3.5-m long (3mm OD) polyurethane tube. TheP-TRAKTM pulled air through a 5-m long (6mmOD) tygon tube. Particle losses in tubing weredetermined by running the analyzers with andwithout tubing. Particle numbers measured by theseanalyzers were subsequently adjusted to account forlosses in tubing. The details of the procedures usedto adjust the particle losses in tubing can be foundin Sarwar et al. (2003).

Chamber O3 (inside and outside) was measuredusing an O3 analyzer based on ultraviolet absorp-tion (Dasibi Environmental Corporation, Model1003-AH). A solenoid valve was used to switch theintake of the O3 analyzer between the interior andexterior of the chamber. The solenoid valve wasprogrammed to maintain the intake of the O3

analyzer inside the chamber for 50min beforeswitching to outside the chamber for a period of10min for each hour of the experiment. Theanalyzer was placed outside of the chamber anddrew sample air through approximately 3m (6mmOD) of Teflon tubing. Loss of O3 in the Teflontubing was small. A five-point calibration wasperformed on the O3 analyzer using primarystandards 2 weeks before all experiments. Theanalyzer responses were within 1% of the primarystandards. The calibration was also repeated at theend of these experiments; analyzer responses werealso within 1% of the standards.

The chamber air exchange rate was measured byinjecting sulfur hexafluoride (SF6) and monitoring itsdecay using a GC/ECD optimized for analysis of SF6

(Lagus Applied Technology Inc., Model 101, Auto-trac). Chamber temperature and relative humiditywere measured during each experiment using aportable digital hygrometer (VWR, Model 35519-041).

Each experiment began with the measurement ofparticle number concentrations outside of thechamber for 10–30min followed by measurementsinside the chamber for another 10–30min before theintroduction of limonene into the chamber. Experi-ments continued for a period of 10–12 h after sourceintroduction into the chamber.

Limonene concentrations were not measuredduring experiments; however, limonene emissionrates were measured and determined to be constantover the course of an experiment (Table 1). In theabsence of any O3, limonene emitted into thechamber would have resulted in indoor concen-trations of about 70–155 ppb. Thus, limoneneconcentrations used in the study represent upperlimits of the values encountered in indoor environ-ments (Wainman et al., 2000; Singer et al., 2006;Tamas et al., 2006; Wolkoff et al., 2006). Theaverage relative humidity of the chamber was 58%,40%, 39%, 47%, and 67% for experiment ]1, ]2,]3, ]4, and ]5, respectively. The average temp-erature of the chamber during the experimentsranged between 23 and 25 1C. Thus, relativehumidity and temperature during the experimentswere typical of values encountered in indoorenvironments.

2.2. Modeling methodology

Details of the ICEM have been described else-where (Sarwar et al., 2002) and are summarizedhere. The building interior was modeled as a single

ARTICLE IN PRESSTable

1

Summary

ofchamber

experim

entalandmodelingresults

Exptno.

l (h�1)

E (mgmin�1)

InitialO

3

(ppb)

T(1C)

Initial

particle

(]cm�3)

Max

particle

(]cm�3)

Tim

e

(min)

Final

particle

(]cm�3)

Mi

(mgm�3)

Mf,e

(mgm�3)

SOA

(mgm�3)

Mf,p

(mgm�3)

Difference

(%)

10.71

110

163

25

4600

90,200

15

12,000

8.9

199

190

151

24

20.95

85

114

23

7100

56,000

25

13,000

4.3

74.6

70.3

82.6

�11

31.00

88

327

23

3400

160,300

13

14,700

2.5

98.4

95.9

95.9

3

41.17

84

151

23

2700

79,200

19

16,600

1.7

69.4

67.7

71.4

�3

51.21

91

85

24

6300

24,600

33

13,600

5.7

80.5

74.8

68.5

15

No

te:Particle

number

concentrationshavebeenrounded

offto

thenearest

hundred.listheairexchangerate,

Ethelimoneneem

issionrate,Tim

eisthetimeatwhichthemaxim

um

particle

number

concentrationsoccurred

inthechamber,

Mitheinitialparticle

mass

concentrationin

thechamber,

Mf,ethefinalexperim

entalparticle

mass

concentrationin

the

chamber,

Mf,pthefinalpredictedparticle

mass

concentrationin

thechamber,SOA

thesecondary

organic

aerosol¼

Mf,e�

Mi,difference¼

(Mf,e�

Mf,p)/

Mf,e.

G. Sarwar, R. Corsi / Atmospheric Environment 41 (2007) 959–973962

well-mixed environment with homogeneous chem-istry. In its simplest mathematical form, the ICEMcan be represented by

dCi

dt¼ kilCoi � lCi þ F

Ei

V� VdiCiaþ

Xn

j¼1

Rij, (1)

where Ci is the indoor concentration of pollutant i

(ppm), Coi the outdoor concentration of pollutant i

(ppm), ki the outdoor-to-indoor penetration factorfor pollutant i (0–1; unitless), l the fresh airexchange rate (min�1), F is the molar gas volume(m3 kmol�1), Ei the whole-building emission rate (allsources) for pollutant i (ppmm3min�1), Vdi thedeposition velocity for pollutant i to indoormaterials (mmin�1), a the surface area to volumeratio for the indoor environment (1/m), Rij thereaction rate between the pollutants i and j

(ppmmin�1), V the volume of the indoor environ-ment (m3), and t the time (min).

The first term in Eq. (1) corresponds to the timerate of change of pollutant concentration (Ci) withina building environment. The second term representsthe transport of pollutants from outdoors toindoors. The third term represents the transport ofpollutants from indoors to outdoors. The fourthterm corresponds to the whole-building emissionrate for pollutant i. The fifth term corresponds tothe removal of pollutant i by deposition on indoormaterials. The sixth term corresponds to thesummed results of all reactions involving pollutantsi and j.

The ICEM uses the detailed homogeneouschemistry of SAPRC-99 (Carter, 2000). Particlechemistry for limonene (Schell et al., 2001) wasincorporated into ICEM and combined with thehomogeneous reactions of SAPRC-99. The modelcontains more than 300 chemical reactions and 120chemical species.

2.3. Particle chemistry for limonene

Several authors have characterized the gas andparticle-phase products of O3/limonene reactions(Schuetzle and Rasmussen, 1978; Arey et al., 1990;Grosjean et al., 1992, 1993; Hakola et al., 1994;Eusebi, 1996; Calogirou et al., 1999; Ruppert et al.,1999; Glasius et al., 2000; Clausen et al., 2001).Sirakarn et al. (2005) recently proposed a completelimonene oxidation mechanism leading to aerosolformation. However, the reaction mechanism pro-posed by Schell et al. (2001) was used in this study.

ARTICLE IN PRESSG. Sarwar, R. Corsi / Atmospheric Environment 41 (2007) 959–973 963

Studies to investigate the effect of relativehumidity on SOA mass concentrations from mono-terpenes have been inconclusive; some have con-cluded that increased relative humidity increasesSOA mass, while others suggest that it does nothave any effect on SOA mass (Jonsson et al., 2006and references incorporated therein). Jonsson et al.(2006) suggest that enhanced water vapor canincrease SOA mass via: (1) physical water uptakeby the particles and (2) production of additional lowvolatile organic products by gas-phase reactions.The reaction mechanism of Schell et al. (2001) doesnot incorporate water vapor explicitly; thus, theeffect of relative humidity on SOA mass was notevaluated in this study.

2.4. Gas-phase chemistry

The reactions of limonene with O3, hydroxylradical (OH), and NO3 produce two products:CVLIM1 and CVLIM2. The product CVLIM1and CVLIM2 are hypothetical semi-volatile organiccompounds that can partition between gas andparticle phases. The molecular weight of each ofthese compounds is taken to be 200 gmol�1. Thereaction pathways leading to CVLIM1 andCVLIM2 are presented in Eqs. (2)–(4):

LIMþO3 ¼ f O3ð0:163CVLIM1þ 0:247CVLIM2Þ;

(2)

LIMþOH ¼ f OHð0:163CVLIM1þ 0:247CVLIM2Þ;

(3)

LIMþNO3 ¼ f NO3ð0:163CVLIM1þ 0:247CVLIM2Þ;

(4)

A value of 0.778 for f O3, 0.228 for fOH, and 0.0

for f NO3have been suggested by Schell et al. (2001)

and were used in this study. The ICEM containsgas-phase chemical reactions for limonene fromSAPRC-99; however, these reactions do not containany semi-volatile organic compounds or any aero-sol-phase products. The reaction products fromEqs. (2)–(4) were added to the corresponding gas-phase reactions in the ICEM.

2.5. Gas/particle partitioning of semi-volatile

products

Partitioning of semi-volatile organic compoundsbetween air and particle phases was modeled as an

absorption process into the organic mass on theparticle, i.e., nucleation processes were not consid-ered as a particle formation mechanism. The sum ofgas-phase (Cgas,i) and aerosol-phase (Caer,i) com-pounds at any time was assumed equal to the totalconcentrations (Ctot,i), i.e.,

Ctot;i ¼ Cgas;i þ Caer;i. (5)

At thermodynamic equilibrium, the gas-phaseconcentration is equal to the saturation concentra-tion; thus, Eq. (5) can be written as

Caer;i ¼ Ctot;i � Cgas;i ¼ Ctot;i � X i;omCsat;i, (6)

where Xi,om is the mole fraction of compound i andCsat,i the saturation concentration of pure com-pounds i. The mole fraction of compound, Xi,om,can then be expressed as

X i;om ¼Caer;i=miPn

j¼1ðCaer;j=mjÞ þ Cinit=minit, (7)

where mi is the molecular weight of compound i inthe organic solution of the particle, Cinit is anyadditional absorbing material in the particle phase.The saturation concentration of pure compounds i

can be estimated from

Csat;i ¼gip

oi mi 10

6

RT, (8)

where gi is the activity coefficient and is taken asunity, pi

o the saturation vapor pressure (pascal), mi

molecular weight of compound i in the organicsolution of the particle, R the universal gas constant(8.314 Jmol�1K�1), and T the temperature (K) ofair. Combining Eqs. (6) and (7) leads to

Caer;i ¼ Ctot;i � Csat;iCaer;i=miPn

j¼1 Caer;j=mj

� �þ Cinit=minit

.

(9)

The model is first run with gas-phase chemistrythat produces concentrations for Ctot,i. The amountof aerosol that can be formed at any time is thencalculated by iteration using Eq. (9) with arestriction that aerosol concentrations cannot benegative.

In this study, the particle concentrations outsidethe chamber were measured during experiments andwere assigned as input to the model. These particleswere transported to the chamber due to exchange ofair with the chamber. Thirty percent of theseparticles was taken to be organic following thestudy of Russell and Allen (2004), and wereavailable for absorbing additional materials into

ARTICLE IN PRESSG. Sarwar, R. Corsi / Atmospheric Environment 41 (2007) 959–973964

the particle phase. The value, Cinit, was thusassigned as 0.3 times the indoor particle concentra-tions due to the transport of outdoor particles toindoors. The molecular weight of these particles wastaken to be 120 gmol�1 (Kamens et al., 1999).Solution of Eq. (9) required knowledge of thesaturation vapor pressure (pi

o) at the averagechamber temperature, which was calculated usingthe following equation:

poi ¼ poR

i exp �DHvap;i

R

1

T�

1

ToR

� �� �, (10)

where pioR is the reference vapor pressure (pa) of the

pure compound i at the reference temperature, ToR;DHvap,i the enthalpy of vaporization (Jmol�1) andwas taken to be 156,000 Jmol�1 for both com-pounds. The reference vapor pressure, pi

oR, at 298Kfor CVLIM1 and CVLIM2 was taken to be2.5� 10�5 and 1.2� 10�4 Pa, respectively (Schell etal., 2001). It should be noted that the gas/particlepartitioning of semi-volatile products of limonene inthe ICEM is different than that of a-pinene (Sarwaret al., 2003).

Deposition velocities for O3 and particles werepreviously measured in the chamber by comparingdecay rates at specific air exchange rates with aninert tracer gas (SF6) and were used in ICEM. TheO3 deposition rate in the stainless steel chamberused for this study was 0.48 h�1, much lower thanthe reported average O3 deposition rate of 2.8 h�1

for indoor environments (Lee et al., 1999). Thesurface area to volume ratio of the environmentalchamber used in this study was 2.7m�1. With thisvalue, the deposition velocity of O3 in the chamberwas 0.005 cm s�1. The average deposition rate offine particles in the chamber was 0.34 h�1. Themeasured particle deposition rate compares favor-ably with the fine particle deposition rate of 0.33 h�1

reported by Wainman (1999) and 0.39 h�1 reportedby Wallace (1996). The average deposition velocityof particles in the chamber was 0.0035 cm s�1. Thedeposition velocity for compounds CVLIM1 andCVLIM2 was assumed to be equal to that of theparticle deposition velocity.

2.6. Model input parameters

Measured limonene emission rates, O3 emissionrates, chamber outside O3 concentrations, chamberoutside particle mass concentrations, chamberaverage air exchange rate, average chamber tem-perature, average relative humidity, and chamber

surface area and volume were provided as inputdata to the model to estimate chamber particle massconcentrations as a function of time. Ozone emis-sion rates from the generator were observed toincrease with decreased relative humidity. Relativehumidity in the chamber decreased during thecourse of some experiments; thus, the O3 emissionrates increased somewhat. An adjustment factorwas developed to account for the increases in the O3

emission rates with decreased relative humidity andwas used in the model.

Outdoor to indoor transport is the main source ofindoor O3 for most buildings; however, sources ofO3 can also be present in buildings, e.g., photocopymachines, laser printers, electrostatic air filters, andelectrostatic precipitators. Indoor O3 concentrationsusually track outdoor O3 concentrations andincrease with increased air exchange rates, andtypically range between 20% and 70% of theoutdoor O3 levels. Ozone concentrations used inthis study for all but one experiment are about 2–3times greater than levels encountered in indoorenvironments during urban summertime conditions.

3. Results and discussions

A summary of the five experiments involvinglimonene is presented in Table 1. Number concen-trations represent particles in the 0.02–0.7 mmdiameter range, except experiment ]1 for whichparticles in the 0.02–1.0 mm diameter range areshown. Mass concentrations represent particles inthe 0.1–0.7 mm diameter range, except experiment ]1for which particles in the 0.1–1.0 mm diameter rangeare shown.

A typical evolution of particle number concentra-tions is shown in Fig. 1, in this case for experiment]1. A rapid ‘‘burst’’ of small particles in the0.02–0.1 mm size range was detected shortly afterthe introduction of limonene into the chamber.Particle numbers in the 0.02–0.1 mm size range thendecreased before attaining a steady concentration ofapproximately 25% higher than the initial concen-tration. Particle numbers in the next size range thenincreased and subsequently decreased before attain-ing a steady concentration. This process continuedfor particles with diameters up to 0.7–1.0 mm, afterwhich no appreciable increase in particle numberswas observed. The initial ‘‘burst’’ of small particlesfollowed by a decrease in particle numbers in agiven size range and increase in particle numbers insubsequent size ranges created an effective particle

ARTICLE IN PRESS

0

1

10

100

1,000

10,000

0.02-0.1 0.1-0.2 0.2-0.3 0.3-0.4 0.4-0.5 0.5-0.7 0.7-1.0

PARTICLE SIZE [μm]

PAR

TIC

LE

S [#

/cm

3 ]

time = 0 minute

time = 700 minute

Fig. 2. Particle size distributions at time 0 and 700min for experiment ]1.

0.1

1.0

10.0

100.0

1,000.0

10,000.0

100,000.0

-50 50 150 250 350 450 550 650 750

TIME [min]

PAR

TIC

LE

S [#

/cm

3 ]

0.1-0.2

0.2-0.3

0.3-0.4

0.5-0.7

0.4-0.5

0.02-0.1

limonene introduced

0.7-1.0

Fig. 1. Particle number concentrations during experiment ]1.

G. Sarwar, R. Corsi / Atmospheric Environment 41 (2007) 959–973 965

growth ‘‘wave’’ that was observed for all experi-ments. Ultimately, particle number concentrationsreached a steady-state condition controlled primar-ily by the reaction rate of O3 with limonene and theprevailing air exchange rate.

The particle size distribution during experiment]1 at the time of limonene introduction into thechamber (time ¼ 0) and at the steady-state condi-tion (time ¼ 700min) are shown in Fig. 2. Particlenumber concentrations in all size ranges (except0.02–0.1 mm) at steady state were consistently higherthan particle number concentrations at the time oflimonene introduction into the chamber. Similar

results were also observed for other experiments.The increase in particle number concentration isassumed to have occurred via the O3/limonenereactions and partitioning of subsequent reactionby-products.

Particle surface area and volume at time zero forexperiment ]1 are shown in Fig. 3. The maximumparticle surface area and volume occurred in the0.2–0.3 mm size range. The bulk of the particlesurface area and volume were contained in the0.02–0.5 mm size ranges. However, the initial‘‘burst’’ of particles was observed in the0.02–0.1 mm size range. This suggests that the initial

ARTICLE IN PRESS

0

20

40

60

80

0.02-0.1 0.1-0.2 0.2-0.3 0.3-0.4 0.4-0.5 0.5-0.7 0.7-1.0

PARTICLE SIZE [μm]

SUR

FAC

E A

RE

A [

μm2 /c

m3 ]

0

1

2

3

4

VO

LU

ME

[μm

3 /cm

3 ]

Fig. 3. Particle surface area and volume as a function of particle size at time zero during experiment ]1.

0

100

200

300

400

0 150 300 450 600 750

TIME [min]

PM

[μg

/m3 ]

0

40

80

120

160

200

240

OZ

ON

E [

ppb]

predicted particles

predicted ozone

experimental particlesexperimental ozone

Fig. 4. Experimental and predicted particle mass and O3 concentrations during experiment ]1.

G. Sarwar, R. Corsi / Atmospheric Environment 41 (2007) 959–973966

‘‘burst’’ of small particles may not be solely theresult of condensation/absorption of reaction pro-ducts onto smaller particles. It is conceivable thatnucleation of very low vapor pressure reactionproducts also contributed to the particle burst.Exact products that participate in the nucleationprocess are not known. However, the products areexpected to be similar to those associated with O3/a-pinene reactions (Kamens et al., 1999).

An analysis of relative time scales suggests thatcoagulation phenomena played an insignificant rolein defining the particle growth wave. Using proce-dures described in Sarwar et al. (2003), an analysiswas performed to determine estimated and observedgrowth-wave times for each experiment. Estimatedtimes for coagulation were greater than the ob-served experimental growth-wave times scales by1–3 orders of magnitude. Thus, coagulation was not

the dominant process for the reduction of smallerparticle number concentrations (after the initialburst of 0.02–0.1 mm size particles) and subsequentincreases in larger particle number concentrations.

The evolutions of experimental particle mass andO3 concentrations during experiment ]1 are pre-sented in Fig. 4. The initial O3 concentration in thechamber was 163 ppb and decreased with theintroduction of limonene into the chamber. Theinitial particle mass concentration in the chamberfor experiment ]1 was 8.9 mgm�3 and increased withthe introduction of limonene into the chamber. Theparticle mass concentration in the chamber even-tually reached a steady-state value of 199 mgm�3.The SOA mass concentration was, therefore,190 mgm�3.

The evolution of particle number concentrationsfor experiment ]5 is shown in Fig. 5, which was also

ARTICLE IN PRESS

0.1

1.0

10.0

100.0

1,000.0

10,000.0

100,000.0

-50 50 150 250 350 450 550 650 750 850

TIME [min]

PAR

TIC

LE

S [#

/cm

3 ]

0.1-0.20.2-0.30.3-0.4

0.5-0.7

0.4-0.5

0.02-0.1

limonene introduced

Fig. 5. Evolution of particle number concentrations during experiment ]5.

0.1

1.0

10.0

100.0

1,000.0

10,000.0

0.02-0.1 0.1-0.2 0.2-0.3 0.3-0.4 0.4-0.5 0.5-0.7 0.7-1.0

PARTICLE SIZE [μm]

PAR

TIC

LE

S [#

/cm

3 ]

experiment #1

experiment #5

Fig. 6. Particle size distribution during experiments ]1 and ]5 at time zero.

G. Sarwar, R. Corsi / Atmospheric Environment 41 (2007) 959–973 967

conducted at elevated O3 levels. The air exchangerate for this experiment was higher than experiment]1. Similar to experiment ]1, a rapid ‘‘burst’’ ofsmall particles in the 0.02–0.1 mm size range wasdetected shortly after the introduction of limoneneinto the chamber. Particle numbers in the0.02–0.1 mm size range then decreased beforeattaining a steady-state concentration of approxi-mately 10% higher than the initial concentration. Aparticle growth ‘‘wave’’ was also observed duringthis experiment. Particle number concentrationsultimately reached a steady-state condition andwere controlled primarily by the reaction rate of

O3 with limonene and the prevailing air exchangerate.

The particle size distributions during experiments]1 and ]5 at the time of limonene introduction intothe chamber (time ¼ 0) and at steady state (time-600min) are shown in Figs. 6 and 7, respectively.Particle number concentrations in the 0.02–0.1 and0.1–0.2 mm size ranges during experiment ]1 atsteady state were 10–25% lower than the corre-sponding particle number concentrations duringexperiment ]5. Particle number concentration in the0.2–0.3 mm size range during experiment ]1 atsteady state was close to the corresponding particle

ARTICLE IN PRESS

0.1

1.0

10.0

100.0

1,000.0

10,000.0

0.02-0.1 0.1-0.2 0.2-0.3 0.3-0.4 0.4-0.5 0.5-0.7 0.7-1.0

PARTICLE SIZE [μm]

PAR

TIC

LE

S [#

/cm

3 ]

experiment #1

experiment #5

Fig. 7. Particle size distribution during experiments ]1 and ]5 at 600min.

0

40

80

120

160

200

240

280

0 75 150 225 300 375 450 525 600 675 750

TIME [min]

PM

[μg

/m3 ]

Experiment #1, measurements, λ = 0.71 hr-1

Experiment #5, measurements, λ = 1.21 hr-1

Experiment #1, model, λ = 0.71 hr-1

Experiment #5, model, λ = 1.21 hr-1

Fig. 8. Evolutions of particle mass concentrations during experiments ]1 and ]5.

G. Sarwar, R. Corsi / Atmospheric Environment 41 (2007) 959–973968

number concentration for experiment ]5. However,particle number concentrations in the 0.3–0.4,0.4–0.5, 0.5–0.7, and 0.7–1.0 mm size ranges duringexperiment ]1 at steady state were 2–30 times ashigh as the corresponding particle number concen-trations for experiment ]5. Despite the lower initialparticle concentrations during experiment ]1, par-ticles were distributed into the larger size rangesdue to the lower air exchange rate, i.e., greaterreaction time for experiment ]1. These results areconsistent with observations by Weschler andShields (2003).

The resulting particle mass concentrations duringexperiments ]1 and ]5 are shown in Fig. 8. Particlemass concentrations predicted using ICEM are alsoshown in Fig. 8. Particle mass concentrations duringexperiment ]1 were significantly higher than duringexperiment ]5. The higher reaction rate and longerreaction time produced larger particles, whichcontribute significantly to particle mass concentra-tion. In each case, ICEM over-predicted particlemass concentrations during the first 2–3 h, com-pared favorably with steady-state values for experi-ment ] 5 and underestimated steady-state

ARTICLE IN PRESSG. Sarwar, R. Corsi / Atmospheric Environment 41 (2007) 959–973 969

experimental concentrations by 24% for experiment]1. The steady-state results are generally satisfac-tory. The predicted SOA mass may be under-estimated somewhat since the model does notaccount for homogeneous nucleation processes.The SOA mass produced via homogeneous nuclea-tion was about 5% of the total SOA mass for theoxidation of a-pinene (Sarwar et al., 2003). TheSOA mass produced via homogeneous nucleationfor the oxidation of limonene is also likely to besmall. Initial overestimations of experimental resultshave not been resolved and remain an area ofcontinuing research.

Experiment ]3 involved limonene and a chamberO3 concentration that was a factor of two or moregreater than other experiments. Particle concentra-tions during experiment ]3 were higher than particleconcentrations observed during other experimentswith similar air exchange rates. The O3 concentra-tion used for experiment ]3 was unrealistically highfor outdoor-to-indoor penetration, even in theworld’s most polluted urban areas. However, suchan O3 concentration could occur with the use ofcommercial O3 air ‘‘purifiers’’, and underscores thepotential for occupant exposures to significant fineparticle concentrations during the use of suchdevices.

Ozone generators can also produce smallamounts of nitric acid, dinitrogen pentoxide,nitrous acid, and NO2 (Harris et al., 1982).However, concentrations of these species were notmeasured during these experiments. The reaction ofNO2 with O3 also produces some NO3 which canlead to the formation of SOA. Since emissions ofthese additional species from the O3 generator aresmall relative to O3, they are not likely tosignificantly affect the SOA mass concentrations.

Over all experiments, the reasonably close agree-ments between predicted and experimental particlemass concentrations at steady-state conditions areencouraging. The model appears to capture pro-cesses associated with particle formation and/orgrowth. However, additional research will beneeded to improve model performance, particularlyat the start of source events.

3.1. Effects of selected parameters on indoor SOA

Model simulations were performed to investigatethe effects of the following parameters on indoorSOA mass concentrations: air exchange rate, out-door fine particle concentration, outdoor O3 con-

centration, indoor limonene emission rate, andindoor temperature. A base-case scenario wasdefined as follows:

indoor limonene emission rate ¼ 0.46mgmin�1,outdoor O3 concentration ¼ 100 ppbv, andoutdoor fine particle concentration ¼ 15 mgm�3.

An indoor volume of 500m3 was used for thisanalysis. The prescribed indoor emission rate wouldproduce an indoor limonene concentration of about20 ppb at an air exchange rate of 0.5 h�1 in theabsence of O3. While the reported average indoorlimonene concentrations range between 5 and15 ppb (Tamas et al., 2006; Wolkoff et al., 2006),the maximum indoor limonene concentration canbe much higher. For example, indoor limoneneconcentrations of 175 ppb or greater have beenreported by Wainman et al. (2000), Singer et al.(2006), and Wolkoff et al. (2006). The odor thresh-old for limonene is about 40 ppb (Wolkoff et al.,2006).

Limonene odor can often be detected followingapplication of many lemon-scented consumerproducts, indicating that indoor concentrationsindeed exceed odor thresholds, especially in closeproximity to the source. Indoor concentrationsreported in the published literature have typicallynot been measured during the application ofconsumer products and are often based on long-term averaging, thereby yielding lower concentra-tions. Therefore, a base case limonene emissioncorresponding to an indoor concentration of20 ppbv is reasonable for the model simulationdescribed here. For simplicity, it was also assumedthat no pollutants other than O3, limonene,associated gaseous by-products, and fine particlesexisted in the indoor environment.

An average deposition velocity of 0.036 cm s�1 forO3 was used in the model (Nazaroff and Cass, 1986).An average deposition velocity of 0.07 cm s�1 wasused for both hydroxyl and hydroperoxy radicals inthe model (Nazaroff and Cass, 1986). An averagedeposition velocity of 0.004 cm s�1 was used for fineparticles (Wallace, 1996). Removal of pollutants toindoor surfaces is an important term since indoorsurface areas can be greater than those of outdoors.Deposition velocities of semi-volatile organic pro-ducts (CVLIM1 and CVLIM2) in indoor environ-ments have not been experimentally measured. In theabsence of any measured deposition velocities forthese species, we arbitrarily chose a deposition

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velocity equal to the deposition velocity for particles.This may potentially introduce a large uncertainty inthe model; future research should focus on theimprovement of these deposition velocities in indoorenvironments. It was also assumed that no othersources of fine particles were present in the building,and that transport of outdoor fine particles provided‘‘Cinit’’ particles in the model.

All model simulations were performed with anassumed indoor relative humidity of 50%, indoortemperature of 297K (except when the effects ofindoor temperature were investigated), and an airexchange rate of 0.5 h�1 (except when the effects ofair exchange rate were investigated). The predictedindoor O3 concentration for the base-case scenariowas 12 ppb. The total indoor fine particle concen-tration was predicted to be 13.7 mgm�3. Thepredicted indoor SOA concentration resulting fromreactions between O3 and limonene for the base-case scenario was 5.3 mgm�3. As such, for the basecase simulation indoor fine particles derived fromO3 initiated indoor air chemistry are predicted to bea sizeable fraction (�39%) of total indoor fineparticle mass contributed by outdoor particles andindoor chemistry.

Additional model runs were completed to assessthe sensitivity of the predicted SOA mass concen-trations with the prescribed vapor pressures. Whenthe prescribed vapor pressures were increased by afactor of 1.4, the predicted SOA mass concentra-tions decreased by 11%. When the saturation vaporpressures were reduced by dividing these values by

0

5

10

15

20

0.0 0.5 1.0 1

AIR EXCHANG

PM

[μg

/m3 ]

outdoor particles

Fig. 9. Indoor particles mass concentration

the same factor, the predicted SOA mass concentra-tions increased by 13%.

3.2. Effects of air exchange rate on indoor SOA

The air exchange rate was varied from 0.1 to3.0 h�1. Resulting steady-state indoor SOA concen-trations are shown in Fig. 9. Outdoor particleconcentrations as well as indoor particle concentra-tions due to outdoor-to-indoor particle transporta-tion are shown in Fig. 9. Predicted indoor SOAconcentrations increased with lower air exchangerates. Lower air exchange rates were associated withlower indoor O3 levels. However, indoor limoneneconcentrations also increased with lower air ex-change rates. The O3/limonene reaction rate andtime available for reactions increased with thereduction in the air exchange rate. The combinationof higher reaction rates and longer reaction timesresulted in increased indoor SOA concentrations.Interestingly, at air exchange rates below about0.3 h�1, typical of newer energy efficient residentialdwellings, particle mass associated with SOA waspredicted to exceed the indoor particle concentra-tion due to the outdoor-to-indoor particle trans-portations.

3.3. Effects of outdoor fine particle and ozone

concentrations on indoor SOA

Outdoor fine particle concentrations were variedfrom 15 to 100 mgm�3. Predicted indoor SOA mass

.5 2.0 2.5 3.0

E RATES [hr-1]

secondary particles

outdoor-to-indoor transport

s as a function of air exchange rates.

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concentrations increased slightly with higher out-door fine particle mass concentrations. The pre-dicted indoor SOA concentrations increased from5.3 to 7.0 mgm�3. The outdoor O3 concentrationwas varied from 0 to 200 ppb. The higher outdoorO3 concentrations caused an increase in indoorO3 concentrations, which in turn increased theO3/limonene reaction rate. Increased amounts ofreaction products were generated by the higher O3/limonene reaction rate, and resulted in increasedindoor SOA mass concentrations. The predictedindoor SOA mass concentration at an outdoor O3

concentration of 200 ppb was 8.2 mgm�3, approxi-mately 50% greater than the base case SOA massconcentrations.

3.4. Effects of indoor limonene emission rate on

indoor SOA

The indoor limonene emission rate was variedfrom 1 to 3 times the base-case emission rate.Increased indoor limonene emission rates increasedthe O3/limonene reaction rate and resulted inelevated indoor SOA concentrations. The SOAconcentration at a limonene emission rate of1.38mgmin�1 (three times the base case emissions)was 19 mgm�3.

3.5. Effects of indoor temperature on indoor SOA

The indoor temperature was varied from 280 to300K (45–80 F). The indoor SOA concentrationsincreased as indoor temperature was reduced.Lower temperatures reduced the rates of homo-geneous chemical reactions. However, this effectwas more than compensated for by an increase ingas-to-particle partitioning of reaction products atlower temperatures. Predicted indoor SOA concen-trations increased by about 80% as indoor tem-perature was reduced from 300 to 280K.

3.6. Comparisons of SOA concentrations between

O3/a-pinene and O3/limonene reactions

Indoor SOA produced from the oxidation of a-pinene was studied by Sarwar et al. (2003). Theinitial O3 concentrations and the air exchange ratein experiment ]4 in that study were 111 ppb and1.06 h�1, respectively. Experiment ]2 in the currentstudy involved limonene with an initial O3 level of114 ppb and an air exchange rate of 0.95 h�1. The a-pinene emission rate during experiment ]4 was 8%

lower than the limonene emission rate duringexperiment ]2 in this study. The initial particlelevels between these experiments were within 30%.Therefore, conditions between these two experi-ments were not identical; but reasonably similar.The indoor SOA mass concentration during experi-ment ]4 of the a-pinene study was 12 mgm�3

compared to a value of 70 mgm�3 in experiment ]2in the current study. Thus, the indoor SOA massconcentration for experiment ]2 was 5.8 times ashigh as experiment ]4 of the a-pinene study. If theair exchange rate between these experiments hadbeen equal, the difference between the SOAconcentrations would have been slightly lower.

The initial O3 concentration in experiment ]4 ofthis study was 151 ppb and the air exchange rate was1.17 h�1. The a-pinene emission rate during experi-ment ]4 of the a-pinene study was about 7% lowerthan the limonene emission rate during experiment]4 in this study. However, the initial particleconcentrations during experiment ]4 of the a-pinenestudy were at least three times as high as experiment]4 during this study. Despite the higher air exchangerate and lower initial particle concentration, experi-ment ]4 during the current study produced anindoor SOA mass concentration that was 5.6 timesas high as experiment ]4 of the a-pinene study. Ifthe air exchange rate and the initial particle levelsbetween these experiments had been equal, thedifference between the indoor SOA mass concentra-tions would have been greater. Thus, O3/limonenereactions produced 5.6–5.8 times as much indoorSOA mass concentrations as did O3/a-pinenereactions for reasonably similar conditions.

Limonene contains two carbon–carbon doublebonds and is, therefore, more easily attacked by O3

than is a-pinene. The rate constant for the O3/limonenereaction at 298K is 2.0� 10�16 cm3mol�1 s�1 and isapproximately two times as high as the reportedrate constant of 8.66� 10�17 cm3mol�1 s�1 for the O3/a-pinene reaction (Atkinson, 1994). The rate constantfor the OH/limonene reaction at 298K is 1.71�10�10 cm3mol�1 s�1, and is about three times ashigh as the reported rate constant of 5.37�10�11 cm3mol�1 s�1 for the OH/a-pinene reaction(Atkinson, 1994). The primary reaction productsgenerated via oxidation of limonene undergo furtherreaction producing secondary reaction products.Partitioning rates of the primary and secondaryproducts of the O3/limonene reactions will be differentthan the semi-volatile products of O3/a-pinene reac-tions. The combination of higher rate constants and

ARTICLE IN PRESSG. Sarwar, R. Corsi / Atmospheric Environment 41 (2007) 959–973972

different partitioning rates clearly lead to higherSOA levels for O3/limonene reactions compared tothe O3/a-pinene reactions.

4. Conclusions

Five chamber experiments were completed toassess the effects of limonene/O3 reactions onindoor secondary organic particles. Results clearlydemonstrate that fine particle formation/growth canoccur when both limonene and O3 are present inindoor environments at elevated levels. The resultspresented in this paper suggest that O3 reactionswith limonene can lead to elevated exposures toparticles with diameterso1.0 mm. Experimentalresults indicate that significant particle formation/growth can occur indoors due to reactions betweenO3 and limonene. Particle growth occurs through aninitial ‘‘burst’’ of small particles followed by adecrease in particle numbers in a given size rangeand increase in particle numbers in subsequent sizeranges. This process leads to an effective particlegrowth wave. Additional research is recommendedto resolve the issue of particle formation/growthduring the initial phase of the experiment usingparticle analyzers that can measure finer sizedistributions. Indoor SOA may account for someof the previously reported ‘‘unexplained’’ particlemass in indoor environments.

A significant contribution of this research was theprediction of dynamic particle mass concentrationsbased on homogeneous chemical mechanisms andpartitioning of semi-volatile products to particlesusing an indoor air quality model (ICEM). TheICEM allows for the simulation of air exchangeprocesses, indoor emissions, chemical reactions,deposition, and variations in outdoor air quality.Predicted indoor secondary particle mass concen-trations were in reasonable agreement with experi-mental results, which suggest that the model iscapturing the essence of particle formation/growthprocesses. Lower air exchange rates increase indoorsecondary particle mass concentrations, a phenom-enon attributed to longer indoor residence times.The combination of more time to generate reactionproducts and longer particle residence times in-creases particle mass concentrations and distributesthe resulting mass into larger size ranges. Substan-tially higher outdoor fine particle mass concentra-tions are predicted to result in slightly increasedindoor SOA mass concentrations. Higher outdoorO3 concentrations also lead to increased indoor

SOA concentrations. Lower indoor temperaturespromote increased gas-to-particle partitioning oflow vapor pressure by-products of O3/limonenereactions, and increased particle mass concentra-tions. Reactions between limonene and O3 produceat least five times more indoor SOA mass concen-trations than those of a-pinene for reasonablysimilar conditions due to higher rate constants anddifferent partitioning characteristics of the semi-volatile reaction products.

Acknowledgments

The Texas Air Research Center (TARC) providedall necessary funding for this study. The authorswould like to thank Dr. Charles J. Weschler for hisguidance throughout the experimental phase of theresearch.

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