acp-12-6489-2012.pdf

Atmos. Chem. Phys., 12, 6489–

6504

, 2012

www.atmos-chem-phys.net/12/6489/2012/

doi:10.5194/acp-12-6489-2012

Atmospheric

Chemistry

and Physics

α

-pinene photooxidation under controlled chemical conditions –

Part 1: Gas-phase composition in low- and high-NO

environments

N. C. Eddingsaas

, C. L. Loza

, L. D. Yee

, J. H. Seinfeld

1,2

, and P. O. Wennberg

2,3

Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA, USA

Division of Engineering and Applied Science, California Institute of Technology, Pasadena, CA, USA

Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, USA

Correspondence to:

N. C. Eddingsaas (eddingsa@caltech.edu)

Received: 29 December 2011 – Published in Atmos. Chem. Phys. Discuss.: 1 March 2012

Revised: 21 June 2012 – Accepted: 1 July 2012 – Published: 25 July 2012

Abstract.

The OH oxidation of

-pinene under both low-

and high-NO

environments was studied in the Caltech atmo-

spheric chambers. Ozone was kept low to ensure OH was the

oxidant. The initial

-pinene concentration was 20–50 ppb

to ensure that the dominant peroxy radical pathway under

low-NO

conditions is reaction with HO

, produced from re-

action of OH with H

, and under high-NO

conditions,

reactions with NO. Here we present the gas-phase results ob-

served. Under low-NO

conditions the main first generation

oxidation products are a number of

-pinene hydroxy hy-

droperoxides and pinonaldehyde, accounting for over 40 %

of the yield. In all, 65–75 % of the carbon can be accounted

for in the gas phase; this excludes first-generation products

that enter the particle phase. We suggest that pinonaldehyde

forms from RO

+ HO

through an alkoxy radical channel

that regenerates OH, a mechanism typically associated with

acyl peroxy radicals, not alkyl peroxy radicals. The OH oxi-

dation and photolysis of

-pinene hydroxy hydroperoxides

leads to further production of pinonaldehyde, resulting in

total pinonaldehyde yield from low-NO

OH oxidation of

∼

33 %. The low-NO

OH oxidation of pinonaldehyde pro-

duces a number of carboxylic acids and peroxyacids known

to be important secondary organic aerosol components. Un-

der high-NO

conditions, pinonaldehyde was also found to

be the major first-generation OH oxidation product. The

high-NO

OH oxidation of pinonaldehyde did not produce

carboxylic acids and peroxyacids. A number of organoni-

trates and peroxyacyl nitrates are observed and identified

from

-pinene and pinonaldehyde.

1 Introduction

The emissions of biogenic volatile organic compounds

(BVOCs) far outnumber those of anthropogenic VOCs

(

Guenther et al.

1995

;

Steinbrecher et al.

2009

;

Monks

et al.

2009

). Excluding methane, BVOCs are estimated to

account for a flux of

∼

1150 Tg C yr

−

, while anthropogenic

VOCs account for only

∼

140 Tg C yr

−

(

Guenther et al.

1995

;

Goldstein and Galbally

2007

). Important BVOCs in-

clude isoprene, (flux of

∼

500 Tg C yr

−

) and the monoter-

penes (

∼

127 Tg C yr

−

) of which

-pinene accounts for

∼

50 Tg C yr

−

(

Guenther et al.

1995

;

Chung and Seinfeld

2002

). Because they are unsaturated, these compounds are

highly reactive towards OH, O

, and NO

and thus play an

important role in tropospheric chemistry. The atmospheric

oxidation of BVOCs also results in the formation of sec-

ondary organic aerosol (SOA). Monoterpenes are significant

sources of SOA due to their large emission rate and high SOA

yield (

Hoffmann et al.

1997

;

Pye et al.

2010

The gas-phase oxidation of simple alkanes and alkenes

is well understood and most tropospheric chemical mecha-

nisms use laboratory studies of reactions of these species to

inform the parameterization of the atmospheric oxidation of

VOCs. Many BVOCs, including isoprene, which has a conju-

gated double bond system, and

-pinene which is a bicyclic

hydrocarbon with an endocyclic double bond, however, have

much more complicated chemistry than the simple alkanes

and alkenes. For example, recent experimental and theoret-

ical studies with isoprene have shown that its atmospheric

chemistry is not well modeled by the reactions of simple

Published by Copernicus Publications on behalf of the European Geosciences Union.

6490

N. C. Eddingsaas et al.:

α

-pinene photooxidation gas-phase composition

Table 1.

Experimental conditions.

Date

Hydrocarbon

Concentration

OH Source

Aerosol

Length of

(ppb)

seed

experiment (hours)

5 May 2010

-pinene

No seed

6 May 2010

-pinene

HONO

No seed

7 May 2010

-pinene

9 May 2010

-pinene

HONO

10 May 2010

-pinene

AS+SA

11 May 2010

-pinene

HONO

AS+SA

12 May 2010

-pinene

No seed

13 May 2010

-pinene

ONO

No seed

14 May 2010

-pinene

17 May 2010

-pinene

ONO

18 May 2010

-pinene

AS+SA

19 May 2010

-pinene

ONO

AS+SA

2 June 2010

-pinene

3 June 2010

-pinene

ONO

4 June 2010

Pinonaldehyde

–

No Seed

8 Feb 2011

-pinene

No seed

14 Feb 2011

Pinonaldehyde

–

No seed

16 Feb 2011

-pinene

ONO

No seed

17 Feb 2011

Pinonaldehyde

–

ONO

No seed

AS: ammonium sulfate, AS+SA: ammonium sulfate and sulfuric acid.

The initial concentration of pinonaldehyde was not determined.

alkenes, especially under low-NO

(NO and NO

) condi-

tions (

Paulot et al.

2009b

;

Peeters and Muller

2010

;

Crounse

et al.

2011

-pinene is a ten-carbon bicyclic hydrocarbon with an en-

docyclic double bond and, therefore, has the potential to re-

act in ways not represented by simple alkenes. In addition,

-pinene is highly reactive with both O

and OH, reacting in

the atmosphere with

-pinene nearly equally, adding to the

richness of its atmospheric photochemistry (

Capouet et al.

2008

). Despite a number of studies of OH oxidation of

pinene, large uncertainties in the identity and yields of its

reaction products remain. For instance, the reported yield of

pinonaldehyde from the photooxidation of

-pinene ranges

from 28 to 87 % in the presence of NO

and from 3 to

37 % in the absence of NO

(

Arey et al.

1990

;

Hatakeyama

et al.

1991

;

Noziere et al.

1999

;

Jaoui and Kamens

2001

;

Wisthaler et al.

2001

;

Aschmann et al.

2002

;

Lee et al.

2006

). In addition, the molecular structures of major products

identified by mass (e.g. 184 and 200 daltons) are subject to

debate (

Aschmann et al.

2002

;

Vereecken et al.

2007

). The

photooxidation product with molecular mass 184 has been

assigned to an unsaturated hydroperoxy carbonyl (

Vereecken

et al.

2007

) or a dihydroxy carbonyl (

Aschmann et al.

2002

A better understanding of

-pinene gas-phase chemistry will

increase the accuracy of

-pinene atmospheric chemistry

models and provide insight into atmospheric photooxidation

mechanisms in general.

In this study, we isolate the peroxy radical reaction path-

ways to investigate the photochemistry of

-pinene. We have

studied these reactions under low-NO

conditions similar

to those found in the atmosphere, where RO

+ HO

is the

dominant peroxy radical reaction, and other reactions are

suppressed (RO

+ RO

and reactions with O

). We contrast

these conditions with results from the gas-phase photoox-

idation of

-pinene under high-NO

(with varied amounts

of NO

to study both RO

+ NO and RO

+ NO

). We fo-

cus our analysis on low-NO

rather than high-NO

chem-

istry because the low-NO

chemistry of

-pinene is less well-

characterized yet it is more atmospherically-relevant (

Pye

et al.

2010

). In a forthcoming paper, SOA yields and com-

position formed from the controlled chemical conditions de-

scribed here will be presented.

2 Experimental

Photooxidation experiments of

-pinene were performed in

the Caltech dual 28 m

Teflon chambers. Details of the cham-

ber facilities have been described elsewhere (

Cocker et al.

2001

;

Keywood et al.

2004

). A few photooxidation experi-

ments were performed in a

∼

1 m

bag enclosed in a small,

black walled chamber with UV-lights lining one wall, as de-

scribed by

Crounse et al.

(

2011

). 40 W black lights (Sylvania

F40/350BL) with emission peak emission at 352 nm were

used in both chambers. The light intensity as a function of

wavelength (300–800 nm) was measured using a Licor (LI-

1800) spectroradiometer. Prior to each run, the chamber was

flushed for a minimum of 24 h with dry purified air. While

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, 2012

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N. C. Eddingsaas et al.:

α

-pinene photooxidation gas-phase composition

6491

being flushed, the chamber was irradiated with the chamber

lights for a minimum of six hours. The temperature, relative

humidity (RH), and concentrations of O

, and NO

(NO and

) were continuously monitored. In all experiments the

RH was kept below 10 %. Aerosol size distribution and num-

ber concentration were measured continuously by a differ-

ential mobility analyzer (DMA, TSI model 3081) coupled to

a condensation nucleus counter (TSI model 3760). Informa-

tion on aerosol measurements can be found in part 2, where

aerosol formation, growth, and composition are discussed.

Table

shows a list of conditions for all experiments used in

this study. Experiments ran for 3 to 19 h. No gas phase losses

to the chamber walls of either

-pinene prior to light irradia-

tion or oxidation products after lights were extinguished was

observed.

Experiments were performed under low- and high-NO

conditions. Under low-NO

conditions, photolysis of hydro-

gen peroxide (H

) was the OH source, while for the high-

experiments the photolysis of nitrous acid (HONO) or

methyl nitrite (CH

ONO) produced OH. For low-NO

ex-

periments, 280 μL of 50 wt % H

was injected into the

chamber resulting in an H

concentration

∼

4 ppm. The

two different OH sources used during the high-NO

experi-

ments provided the mechanism to vary the NO to NO

ratio,

with a higher quantity of NO

in the CH

ONO experiments.

HONO was prepared daily by dropwise addition of 15 mL

of 1 wt % NaNO

into 30 mL of 10 wt % H

in a glass

bulb, and then introduced into the chamber with dry air. This

process produces NO and NO

as side products, which are

also introduced to the chamber. CH

ONO was synthesized,

purified, and stored according to the procedure outlined by

Taylor et al.

(

1980

). CH

ONO was warmed from liquid ni-

trogen temperatures and vaporized into an evacuated 500 mL

glass bulb and introduced into the chamber with an air stream

of 5 L min

−

. After addition of CH

ONO, 300–400 ppb of

NO was added to the chamber to suppress the formation of

. Determination of exact NO and NO

concentrations us-

ing the commercial NO

monitor was precluded due to in-

terferences by both HONO and CH

ONO. At the start of all

high-NO

experiments the total NO

reading (NO, NO

, and

interference from HONO or CH

ONO) was 800 ppb and NO

concentration throughout the experiments was such that the

concentration of O

never exceeded 5 ppb.

-pinene was added to the chamber to achieve a concen-

tration of 20–50 ppb by passing dry air through a bulb con-

taining a known volume of

-pinene. The mixing ratio of

-pinene was monitored by gas chromatography (Agilent

6890N) coupled with a flame ionization detector (GC-FID).

The GC-FID was calibrated using a 55 L Teflon bag contain-

ing a known concentration of pure

-pinene. Gas phase pho-

tooxidation products were monitored using triple-quadrupole

chemical ionization mass spectrometry (CIMS) (

St. Clair

et al.

2010

Details of the operation of the CIMS can be found in

a number of previous reports (

Crounse et al.

2006

;

Paulot

et al.

2009a

;

St. Clair et al.

2010

) and therefore only a brief

description is presented here. The CIMS was operated in neg-

ative ion mode using CF

−

as the reagent ion, and in the

positive ion mode using H

for proton transfer mass spec-

trometry (PTR-MS). In negative mode, CF

−

is sensitive to

the detection of polar and acidic compounds by either clus-

tering with the analyte (R) resulting in an ion with a mass-

to-charge ratio (

m/z

) MW+85

(

−

)

or via fluorine ion

transfer resulting in

m/z

MW+19

(

−

)

. The dominant

ionization mechanism depends mostly on the acidity of the

neutral species; highly acidic species such as nitric acid only

form the fluorine transfer ion, while non-acidic species such

as methyl hydrogen peroxide form only the cluster ion. This

separation aids both in the determination of the structure of

a molecule and in the identification of isomers. In negative

mode, tandem mass spectrometry (MS/MS) was used to help

identify functional groups of an analyte. In brief, a parent ion

selected in the first quadrupole is exposed to an elevated pres-

sure of N

resulting in collision-induced dissociation (CID)

in the second quadrupole, and the resulting fragmentation

ions are detected in the third quadrupole. Molecules with dif-

ferent functional groups have been shown to fragment differ-

ently by CID and thus the detection of certain fragment ions

in quadrupole three can aid in the identification of an analyte.

For instance, hydroperoxides form a characteristic fragment

m/z

63 (

Paulot et al.

2009b

Standards are not available for most of the VOCs described

here and thus the sensitivity of the CIMS is related to the

thermal capture rate and the binding energy of the cluster

(

VOC

−

)

. Details on calculating the sensitivity of the

CIMS to a given analyte can be found in previous publica-

tions (

Paulot et al.

2009a

3 Results and discussion

The goal of this study is to determine the gas-phase re-

action products and mechanism of the photooxidation of

-pinene by OH under both low-NO

and high-NO

con-

ditions. In both cases, the experiment is designed such

that

-pinene only reacts with OH and that a single per-

oxy radical reaction is dominant. Under ambient con-

ditions,

-pinene reacts at an approximately equal rate

with O

(

−

molecules

−

)

and OH

(

−

molecules

−

)

(

Sander et al.

2006

;

Atkinson et al.

2006

). To isolate the OH chemistry,

the formation of O

was suppressed.

Under low-NO

conditions, photolysis of H

resulted in

steady state OH concentration of

∼

−

and an HO

concentration

∼

−

. The OH and HO

concentra-

tions were determined from a kinetic molecular model of

pinene OH reaction which is described in detail in Sect. 3.2

and in the Supplement. In brief, OH is produced by the pho-

tolysis of H

and is primarily consumed by reactions with

VOCs in the chamber and with H

. The reaction of OH

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Atmos. Chem. Phys., 12, 6489–

6504

, 2012

6492

N. C. Eddingsaas et al.:

α

-pinene photooxidation gas-phase composition

(

)

(

)

m/z

= (-) 253, ~6% yield

m/z

= (-) 303

m/z

= (-) 271

m/z

= (-) 271

m/z

= (-) 271

m/z

= (-) 303

m/z

= (-) 269

/-HO

, HO

-O

Fig. 1.

Pathway of photooxidation of

-pinene under low-NO

conditions. Species that were observed are labeled in red. The

m/z

of all

species observed in the negative mode are of the complex with CF

−

(molecular mass + 85). Pinonaldehyde is observed at molecular mass

+ 1 in the positive mode and

-pinene oxide is observed at molecular mass + 1 and + 19. The observed percent yield for first-generation

products is also indicated.

and H

produces HO

. The photolysis rate of H

used

in the model and the OH concentration determined by the

model were confirmed by comparison of the gas-phase con-

centrations of H

and

-pinene from the experiment and

the model (Fig. S1). These OH concentrations are similar to

those observed in the troposphere; however, the HO

concen-

trations are about an order of magnitude greater than levels

typically observed in rural or remote areas (

Lelieveld et al.

2008

;

Ren et al.

2008

;

Wolfe et al.

2011

). The O

concen-

tration at the start of each experiment was

4 ppb and typi-

cally did not exceed 12 ppb over the course of an experiment

(in one experiment, conducted for 20 h, the O

concentra-

tion increased nearly linearly from 4 to 25 ppb). Modeling of

these oxidant concentrations indicated that reaction with O

accounted for

3 % of the

-pinene loss.

In the oxidation of

-pinene by OH, OH adds predomi-

nantly to the endocyclic double bond, followed by addition

of O

to the resulting

-hydroxy alkyl radicals producing a

number of hydroxy peroxy radicals. OH abstraction of a hy-

drogen from

-pinene occurs with a yield of

∼

12 %, result-

ing in a peroxy radical when O

reacts with the alkyl rad-

ical (

Capouet et al.

2004

). Under low-NO

conditions the

peroxy radicals can react with either HO

or with another

(self or cross-reactions). Here we seek to emulate atmo-

spheric conditions where RO

+ HO

dominates. To confirm

that the RO

+ HO

pathway dominated in our experiments, a

kinetic model was constructed, as described in Sect. 3.2 and

the Supplement. For the experimental conditions (with initial

concentration of

-pinene of 20–50 ppb), the kinetic model

indicates that less than 1 % of the peroxy radical reactions

proceed via RO

+ RO

For high-NO

photooxidation experiments, OH is gener-

ated from the photolysis of either HONO or methyl nitrite.

When HONO is synthesized, NO and NO

are produced as

side products, which are also introduced to the chamber when

HONO is injected. Methyl nitrite is synthesized pure and

upon photolysis eventually produces OH and NO

ONO

hν

→

q

(R1)

q

→

q

(R2)

q

→

q

(R3)

Atmos. Chem. Phys., 12, 6489–

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, 2012

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N. C. Eddingsaas et al.:

α

-pinene photooxidation gas-phase composition

6493

Pinonic peroxyacid

m/z

= (-) 285, 265, 219

H-abstraction could produce

pinonic hydroperoxide (

m/z

(-) 285)

Pinonic acid

m/z

= (-) 203, 269

Pinonaldehyde

m/z

= (+) 169

Norpinonaldehyde

m/z

= (+) 155

m/z

= (-) 257

Fig. 2.

Pathway of photooxidation of pinonaldehyde under low-NO

conditions. Species that were observed are labeled in red. Pinonic acid

and pinonic peracid were observed in the negative mode as both the transfer ion (molecular mass + 19) and the complex (molecular mass

+ 85), pinonic peracid is also observed as the complex minus HF (molecular mass + 65), norpinonaldehyde hydroperoxide was observed as

the complex ion, pinonaldehyde and norpinonaldehyde are observed in the positive mode at their molecular mass +1.

When methyl nitrite was used as the OH source, 300 to

400 ppb of NO was added to suppress O

production. Un-

der the conditions used, the peroxy radicals formed from the

OH reaction of

-pinene react with NO and not RO

or HO

This is confirmed by the lack of RO

+ HO

or RO

+ RO

reaction products during high-NO

photooxidation. The ex-

act ratio of NO to NO

is not known as both HONO and

methyl nitrite interfere with the signals from the chemilumi-

nescence NO

instrument; however, it was determined that a

higher NO

concentration exists during methyl nitrite pho-

tolysis due to the increased production of peroxyacyl nitrates

(PANs), as detected by CIMS in the negative mode, which

result from the RO

+ NO

reaction. More details on the de-

tection of, as well as the specific PANs detected will be dis-

cussed in Sect. 3.3.

3.1 Gas phase composition from low-NO

photooxidation of

α

-pinene

The principal first-generation oxidation products observed

from the low-NO

photooxidation of

-pinene were a num-

ber of

-pinene hydroxy hydroperoxides and pinonaldehyde.

Production of

-pinene hydroperoxide from H-abstraction

and

-pinene oxide are also observed (see Fig.

for proposed

reaction mechanism for

-pinene and Fig.

for pinonalde-

hyde). The time traces from the CIMS are shown in Fig.

along with those from a number of the minor oxidation prod-

ucts. The identification of each of these signals is discussed

below.

The

-pinene hydroxy hydroperoxides are observed at

m/z

(

−

)271 (molecular weight + CF

−

) by the CIMS. They

are expected to be a major products of the reaction channel of

+ HO

. Three different

-pinene hydroxy hydroperox-

ide isomers are formed; two

-hydroxy hydroperoxides and

one ring opened hydroxy hydroperoxide containing a double

bond (see Fig.

). The subsequent reaction pathways of these

three

-pinene hydroxy hydroperoxides are expected to be

different and will lead to distinct reaction products, as dis-

cussed in Sect. 3.2. The overall estimated initial yield of the

-pinene hydroxy hydroperoxides is

∼

23 %. This estimate

accounts for the small yield of pinic acid, which has the same

molecular weight as these peroxides (MW

186). Similar to

other carboxylic acids, ionization of pinic acid by CF

−

yields approximately equal amounts of signal at

m/z

(

−

)271

and 205 (MW

−

+ HF). This allows us to use the signal at

m/z

(

−

)205 to insure that the pinic acid concentration does

not significantly impact the hydroperoxide yield estimate.

Pinonaldehyde is neither acidic nor is the complex with

−

significantly strong to be detected in the negative

mode and thus is observed only in the positive mode of the

CIMS. Pinonaldehyde was synthesized to directly observe

its OH oxidation; however, the synthesized sample was not

of sufficient purity to calibrate the CIMS response. While

we cannot report an absolute yield for pinonaldehyde, we

observe that the yield under low-NO

conditions is 2/3 of

that under high-NO

conditions. As pinonaldehyde is one

of the main products of

-pinene oxidation by any atmo-

spheric oxidant, it has been widely studied. Although the

reported yield of pinonaldehyde under high-NO

conditions

varies widely from 27–87 % (

Arey et al.

1990

;

Hatakeyama

et al.

1991

;

Noziere et al.

1999

;

Wisthaler et al.

2001

;

Jaoui

and Kamens

2001

;

Aschmann et al.

2002

;

Lee et al.

2006

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Atmos. Chem. Phys., 12, 6489–

6504

, 2012

6494

N. C. Eddingsaas et al.:

α

-pinene photooxidation gas-phase composition

(

)

(

)

(

)

Fig. 3.

CIMS time traces of a number of important low-NO

photooxidation products of

-pinene.

) Main first generation

products,

m/z

(

−

)271

-pinene hydroxy hydroperoxide (black),

m/z

(+)169 pinonaldehyde (red),

m/z

(

−

)253

-pinene hydroperox-

ide (blue)

(B)

Signals of species that are formed in multiple gener-

ations

m/z

(

−

)285 (black),

m/z

(

−

)269 (red),

m/z

(

−

)303 (green),

m/z

(

−

)301 (orange), and purely second-generation

m/z

(

−

)203

pinonic acid (blue). The signal from

m/z

(+)169 is the only signal

shown from the positive mode and the intensity was divided by 2

for clarity.

most of the measured yields are between 27–35 %. The very

high yields were measured by FTIR which may be biased

by interference from other carbonyls (

Noziere et al.

1999

;

Hatakeyama et al.

1991

). Assuming that under high-NO

conditions the pinonaldehyde yield is between 27–35 %, we

estimate that the low-NO

yield is about 20 %.

Pinonaldehyde has been observed from the low-NO

pho-

tooxidation of

-pinene previously; however, it is typically

assigned as a product of RO

+ RO

chemistry (

Noziere

et al.

1999

;

Larsen et al.

2001

). In the present set of ex-

periments, the chemistry is overwhelmingly dominated by

+ HO

(i.e. low

-pinene concentration and relatively

high HO

concentration). This is confirmed by the photoox-

idation products observed. The two main reaction channels

for alkyl peroxy radicals with other RO

are:

q

′

q

→

R=O

′

(R4)

q

′

q

→

q

′

q

(R5)

resulting in the formation of alcohols and carbonyls from

Reaction (R4) and alkoxy radicals from Reaction (R5). In

the case of

-pinene, these would be

-diols,

-hydroxy

carbonyl, and hydroxy alkoxy radicals. The six membered

ring of the hydroxy alkoxy radical will then open forming

pinonaldehyde. There is no indication of either

-diols or

-hydroxyl carbonyl being formed during the present experi-

ments. In the case of alkyl peroxy radicals reacting with HO

the main reaction channel is:

q

q

→

ROOH

(R6)

which forms hydroperoxides. For

-pinene, Reaction (R6)

will produce the observed

-pinene hydroxy hydroperoxides.

For many small alkyl peroxy radicals it has been shown to be

the only channel (

Hasson et al.

2004

;

Raventos-Duran et al.

2007

;

Noell et al.

2010

If Reaction (R6) was the only channel, there would be

no route to pinonaldehyde production. One possible route to

pinonaldehyde is a radical channel (see Fig.

q

q

→

q

q

(R7)

similar to those known to be important for acetyl peroxy rad-

icals (

Hasson et al.

2004

;

Dillon and Crowley

2008

) and

possibly for the reaction of OH with toluene (

Birdsall et al.

2010

). If this is the route to pinonaldehyde formation, an OH

recycling channel would also be of importance in the reaction

of OH with

-pinene under low-NO

conditions.

-pinene hydroperoxide and

-pinene oxide are also ob-

served products of the OH oxidation of

-pinene under

low-NO

conditions.

-pinene hydroperoxide can be formed

from H-abstraction by OH, which is estimated to account for

about 12 % of the OH reaction with

-pinene (

Capouet et al.

2004

). Assuming similar CIMS sensitivity to

-pinene hy-

droperoxide and

-pinene hydroxy hydroperoxide, the yield

-pinene hydroperoxide is estimated to be

∼

6 %. The re-

mainder of the H-abstraction branching ratio cannot be ac-

counted for at this time.

-pinene oxide is neither acidic nor

is the complex with CF

−

significantly strong to be de-

tected in the negative mode and thus is observed only in the

positive mode at

m/z

(

)153 and 171 as confirmed by di-

rect injection of

-pinene oxide into the chamber. Further

evidence of

-pinene oxide formation is presented in part 2

of this series of paper where SOA composition is discussed.

-pinene oxide appears to be a minor product, and the mech-

anism for its formation by OH oxidation is not known. The

mechanism for the formation from O

oxidation is known

and yields of a few percent have been reported (

Alvarado

et al.

1998

;

Berndt et al.

2003

); however, in the present ex-

periments O

accounts for less than 3 % of the oxidation of

-pinene.

The oxidation products of pinonaldehyde have been shown

to be important in the formation of SOA from the pho-

tooxidation of

-pinene. Specifically, pinonic acid, 10-

hydroxypinonic acid, and pinic acid have been observed

in ambient SOA samples (

Kavouras et al.

1998

1999

;

Atmos. Chem. Phys., 12, 6489–

6504

, 2012

www.atmos-chem-phys.net/12/6489/2012/

N. C. Eddingsaas et al.:

α

-pinene photooxidation gas-phase composition

6495

et al.

1999

;

Laaksonen et al.

2008

;

Zhang et al.

2010

)

and in laboratory studies of

-pinene photooxidation (

Larsen

et al.

2001

;

Jaoui and Kamens

2001

;

Librando and Tringali

2005

). The low-NO

photooxidation of pinonaldehyde was

carried out to determine the contribution of pinonaldehyde

oxidation products to the gas and particle phases of

-pinene

photooxidation.

Photooxidation of pinonaldehyde accounts for a number

of the products observed from

-pinene photooxidation. OH

oxidation of pinonaldehyde occurs by H-abstraction, typi-

cally from the aldehydic group (59–86 %) (

Kwok and Atkin-

son

1995

;

Vereecken and Peeters

2002

) forming an acyl per-

oxy radical after the addition of O

. The reaction of HO

with an acyl peroxy radical produces carboxylic acids, per-

oxyacids, and acetoxy radicals that will decompose to pro-

duce CO

and an alkyl radical.

Hasson et al.

(

2004

) have

shown that these three pathways occur in roughly equal

yields. The reaction pathways and products from pinonalde-

hyde are shown in Fig.

. Pinonic acid is observed by the

CIMS mostly by its transfer product ion (ratio of

m/z

(

−

)203

m/z

(

−

)269 is

∼

9:1), typical of acidic species, while the

peroxyacid is observed more by its complex ion (ratio of

m/z

(

−

)219 to

m/z

(

−

)285 is

∼

1:4) (Fig.

). In addition, we

have observed, using synthesized peracetic acid, that peroxy-

acids are also observed in the negative mode as its molecular

mass +65 (complex with CF

−

HF) at 10–20 % that of

the complex ion (+85). A signal at

m/z

(

−

)265,

∼

10 % the

intensity of

m/z

(

−

)285, has been assigned to pinonic perox-

yacid providing further evidence of its formation. These two

products are formed in roughly equal yields. H-abstraction

at carbons other than the aldehydic carbon could also form

pinonaldehyde hydroperoxide which would also be detected

m/z

(

−

)285, as would 10-hydroxy pinonic acid (product

found in SOA). A signal at

m/z

(

−

)257 that has nine carbons

was observed and has been assigned to the hydroperoxide

formed from the alkyl radical formed from the decomposi-

tion of the acetoxy radical. The formation of norpinaldehyde

was also detected. Pinic acid was observed in equal amounts

m/z

(

−

)205 and

m/z

(

−

)271, but is a minor product.

Low-NO

photooxidation of

-pinene also leads to

the pinonaldehyde oxidation products (Fig.

). Ions at

m/z

(

−

)203 and (

−

)219 assigned to the transfer ions of

pinonic acid and pinonic peroxyacid are observed to be sec-

ond generation products from

-pinene photooxidation, as

would be expected arising from the oxidation of pinonalde-

hyde. The ion at

m/z

(

−

)257 assigned to the C

hydroperox-

ide produced from pinonaldehyde photooxidation is also ob-

served to be second generation. Pinonic acid and pinonic per-

oxyacid are also observed as the complex ion at

m/z

(

−

)269

and (

−

)285 respectively. The time traces for

m/z

(

−

)269 and

m/z

(

−

)285 from the photooxidation of

-pinene are not

purely from first or second generation products (see Figs.

3 and 4) but a combination of the two. The second genera-

tion products observed as these two ions are pinonic acid and

pinonic peroxyacid, evident from their transfer ions, while

lights on (hours)

(

)

-

(

)

Fig. 4.

Comparison of CIMS signals from low

−

photoox-

idation of

(A)

-pinene and

(B)

pinonaldehyde;

m/z

(

−

)285

(black),

m/z

(

−

)269 (red),

m/z

(

−

)257 (orange),

m/z

(

−

)219 (blue),

and

m/z

(

−

)203 (green). Pinonaldehyde photooxidation produces

pinonic peroxyacid observed at both

m/z

(

−

)285 and (

−

)219,

pinonic acid observed at both

m/z

(

−

)269 and (

−

)203, and a C

hy-

droperoxide observed at

m/z

(

−

)257. The pinonaldehyde photoox-

idation products are observed from

-pinene photooxidation along

with additional first-generation signals at

m/z

(

−

)269 and (

−

)285

from first-generation photooxidation products.

the first generation products arise from the photooxidation

-pinene. Oxidation products of these molecular mass

(184 and 200) have been observed and theoretically proposed

from the photooxidation of

-pinene in the presence of NO

(

Aschmann et al.

2002

;

Vereecken et al.

2007

). Under high-

conditions,

Aschmann et al.

(

2002

) observed molecules

at these masses and assigned them to a dihydroxy carbonyl

and a trihydroxy carbonyl that would be formed by a num-

ber of isomerization steps as well as multiple reactions of

peroxy radicals with NO to form alkoxy radicals.

Vereecken

et al.

(

2007

) indicate that molecules with these molecular

weights could be formed by isomerization reactions after

ring opening, while the molecule with MW

200 is pro-

duced from a reaction of NO with the peroxy radical. In both

mechanisms, alkoxy radicals are essential. Alkoxy radicals

appear to form from the reaction of the initially formed

pinene hydroxy peroxy radical intermediates with HO

, and

this is a possible route; however, at this point, the structure

of these molecules is unknown. These are, however, clearly

polyoxygenated species formed from the primary oxidation

www.atmos-chem-phys.net/12/6489/2012/

Atmos. Chem. Phys., 12, 6489–

6504

, 2012

6496

N. C. Eddingsaas et al.:

α

-pinene photooxidation gas-phase composition

-pinene and their most likely molecular formulas are

and C

Highly oxidized species at

m/z

(

−

)301 and (

−

)303 are also

observed as first generation products (see Fig.

). Both sig-

nals decay similarly to the signals at

m/z

(

−

)269 and (

−

)285,

indicating that they are also formed as later generation prod-

ucts. The most likely structure for the first-generation species

m/z

(

−

)303 is an

-pinene hydroxy hydroperoxide with

a bridging peroxy group that results from the ring closing

channel of the ring opened hydroxy hydroperoxide as shown

in Fig.

. This mechanism has been postulated by

Vereecken

et al.

(

2007

). The addition of one hydroxyl group and two hy-

droperoxyl groups decreases the vapor pressure about eight

orders of magnitude making this product a likely aerosol

phase component (

Capouet and M

uller

2006

). The photoox-

idation of

-pinene hydroperoxide will produce a hydroxy

dihydroperoxide that will also be observed at

m/z

(

−

)303

thus accounting for the slow decay of the signal. We do

not know the structure of the photooxidation product(s) at

m/z

(

−

)301. Given the molecular weight, the structure po-

tentially includes one carbonyl and two hydroperoxyl groups.

Whatever the structure, the vapor pressure will be greatly re-

duced from that of

-pinene, once again making it a potential

SOA component.

Another route to the formation of the low vapor pressure

oxidation products at

m/z

(

−

)301 and (

−

)303 is peroxy rad-

ical isomerization. Peroxy radical isomerization has recently

been extensively studied for the isoprene system both theo-

retically (

Peeters et al.

2009

;

da Silva et al.

2010

;

Nguyen

et al.

2010

) as well as experimentally (

Crounse et al.

2011

It has been shown that peroxy radical isomerization most of-

ten occurs via a six or seven membered intermediate (

Per-

rin et al.

1998

;

Blin-Simiand et al.

2001

;

Jorand et al.

2003

). When OH adds to the tertiary carbon of the double

bond the peroxy radical on the secondary carbon can form

a seven membered ring with the hydrogen of either the sec-

ondary carbon in the four membered ring or one of primary

carbons attached to the four membered ring. After isomer-

ization, reaction with O

and HO

will result in the forma-

tion of

-pinene hydroxy dihydroperoxide which would be

detected at

m/z

(

−

)303. See Fig. S2 in the Supplement for

proposed mechanism. When OH adds to the secondary car-

bon of the double bond, a seven membered intermediate can

form with the other tertiary carbon of the four membered ring

ultimetly forming

-pinene hydroxy dihydroperoxide. Sim-

ilarly, the

-pinene hydroxy hydroperoxide peroxy radical

formed from isomerization from the addition of OH to the

secondary carbon can form a six membered ring with the hy-

drogen on the carbon

to the hydroxyl group. This isomer-

ization would result in the formation of

-pinene carbonyl di-

hydroperoxide which would be observed at

m/z

(

−

)301. For

the species observed at

m/z

(

−

)301 and (

−

)303 to be formed

via isomerization, the rate of peroxy radical isomerization

would have to be competitive with that of RO

+ HO

. In the

present experiments [HO

] is

∼

molecules cm

−

the rate of loss of

-pinene hydroxy peroxy radical by reac-

tion with HO

∼

0.2 s

−

. In the atmosphere the HO

con-

centration is about an order of magnitude less, so for isomer-

ization to be competitive (10 % of peroxy radical reaction)

the rate would have to be

∼

0.002 s

−

. In the isoprene system

the isomerization rate has been found to be 0.002 s

−

with

conformer specific rates being much faster (

Crounse et al.

2011

). At this time there have been no experimental stud-

ies on the isomerization rate in the

-pinene system so it is

unknown what the rate is and therefor its atmospheric rel-

evance. If the rate of isomerization is competitive with re-

action with HO

in the atmosphere, more low vapor pres-

sure species capable of partitioning into the aerosol will be

formed than were observed in this study.

3.2 Kinetic model of low-NO

photooxidation of

α

-pinene

To gain a better understanding of the low-NO

photooxida-

tion of

-pinene, a kinetic model was assembled and com-

pared to the time traces of

-pinene and the reaction products

measured during the photooxidation of 19.8 ppb

-pinene

and 4 ppm H

. The kinetic model was constructed using

Kintecus modeling software (

Ianni

2002

). When available,

rate constants from literature were used; however, a major-

ity of the rate constants are not known. In these cases, reac-

tion rates with respect to OH oxidation were estimated us-

ing the structural activity relationship derived by Kwok and

Atkinson (

Kwok and Atkinson

1995

). This method has been

shown to predict rate constants to within a factor of two for

most species (see Tables S1 and S2 for lists of photooxidation

products, reactions, and reaction rates used in this model).

The rate constants were then modified slightly to best fit

the data. The rate constants for RO

+ HO

and RO

+ RO

were taken from the estimates used for the Master Chemical

Mechanism (

Saunders et al.

2003

). Photolysis was included

for hydrogen peroxide, the organic hydroperoxides, and for

pinonaldehyde. The photodissociation frequencies (

) were

determined from the spectral radiance measured from the

chamber lights and the absorption cross sections reported in

the literature for hydrogen peroxide and pinonaldehyde and

estimated for the organic hydroperoxides from reported cross

sections of similar hydroperoxides (

Atkinson et al.

2006

;

Sander et al.

2006

For the model to be accurate, the concentration of OH and

need to be correct. OH is produced from photolysis of

which initially reacts with hydrocarbons in the cham-

ber as well as H

. It is the reaction of OH with H

that

produces HO

. The value of

was confirmed by com-

paring the simulated loss of H

(photolysis and reaction

with OH) to the time trace observed from the CIMS (see SI

Fig. S1). The comparison of the simulated loss of

-pinene,

using the recommended rate constant for the reaction of

-pinene with OH from the IUPAC database (

Atkinson et al.

2006

), with the the observed loss of

-pinene also confirms

Atmos. Chem. Phys., 12, 6489–

6504

, 2012