acp-14-13531-2014.pdf

Atmos. Chem. Phys., 14, 13531–13549, 2014

www.atmos-chem-phys.net/14/13531/2014/

doi:10.5194/acp-14-13531-2014

Overview of the Focused Isoprene eXperiment at the California

Institute of Technology (FIXCIT): mechanistic chamber studies on

the oxidation of biogenic compounds

T. B. Nguyen

, J. D. Crounse

, R. H. Schwantes

, A. P. Teng

, K. H. Bates

, X. Zhang

, J. M. St. Clair

, W. H. Brune

G. S. Tyndall

, F. N. Keutsch

, J. H. Seinfeld

2,6

, and P. O. Wennberg

1,6

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

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

Department of Meteorology, Pennsylvania State University, University Park, Pennsylvania, USA

Atmospheric Chemistry Division, National Center for Atmospheric Research, Boulder, Colorado, USA

Department of Chemistry, University of Wisconsin – Madison, Madison, Wisconsin, USA

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

Correspondence to:

T. B. Nguyen (tbn@caltech.edu)

Received: 27 July 2014 – Published in Atmos. Chem. Phys. Discuss.: 25 August 2014

Revised: 10 November 2014 – Accepted: 20 November 2014 – Published: 19 December 2014

Abstract.

The Focused Isoprene eXperiment at the Califor-

nia Institute of Technology (FIXCIT) was a collaborative at-

mospheric chamber campaign that occurred during January

2014. FIXCIT is the laboratory component of a synergistic

field and laboratory effort aimed toward (1) better under-

standing the chemical details behind ambient observations

relevant to the southeastern United States, (2) advancing the

knowledge of atmospheric oxidation mechanisms of impor-

tant biogenic hydrocarbons, and (3) characterizing the behav-

ior of field instrumentation using authentic standards. Ap-

proximately 20 principal scientists from 14 academic and

government institutions performed parallel measurements at

a forested site in Alabama and at the atmospheric cham-

bers at Caltech. During the 4 week campaign period, a se-

ries of chamber experiments was conducted to investigate

the dark- and photo-induced oxidation of isoprene,

-pinene,

methacrolein, pinonaldehyde, acylperoxy nitrates, isoprene

hydroxy nitrates (ISOPN), isoprene hydroxy hydroperoxides

(ISOPOOH), and isoprene epoxydiols (IEPOX) in a highly

controlled and atmospherically relevant manner. Pinonalde-

hyde and isomer-specific standards of ISOPN, ISOPOOH,

and IEPOX were synthesized and contributed by campaign

participants, which enabled explicit exploration into the ox-

idation mechanisms and instrument responses for these im-

portant atmospheric compounds. The present overview de-

scribes the goals, experimental design, instrumental tech-

niques, and preliminary observations from the campaign.

This work provides context for forthcoming publications af-

filiated with the FIXCIT campaign. Insights from FIXCIT

are anticipated to aid significantly in interpretation of field

data and the revision of mechanisms currently implemented

in regional and global atmospheric models.

1 Introduction

1.1 Background

Biogenically produced isoprenoids (hydrocarbons comprised

of C

units) have global emission rates into the atmo-

sphere surpassing those of anthropogenic hydrocarbons and

methane (Guenther et al., 1995, 2012). The biogenic carbon

emission flux is dominated by isoprene (C

) and monoter-

penes (C

), which account for approximately 50 and

30 % of the OH reactivity over land, respectively (Fuentes et

al., 2000). Furthermore, it has been suggested that the atmo-

spheric oxidation of isoprene, in particular, can buffer the ox-

idative capacity of forested regions by maintaining levels of

the hydroxyl radical (OH) under lower nitric oxide (NO) con-

ditions (Lelieveld et al., 2008). Due to their large abundances,

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

13532

T. B. Nguyen et al.: Overview of FIXCIT

isoprene and monoterpenes also dominate the global budget

of secondary organic aerosol (SOA) (Henze et al., 2008).

Thus, the accurate representation of detailed chemistry for

isoprene and monoterpene is necessary for meaningful simu-

lations of atmospheric HO

(OH

), NO

(NO

surface ozone (O

), trace gas lifetimes, and SOA.

Unsaturated hydrocarbons like isoprene and monoterpenes

are primarily oxidized by OH, O

, and the nitrate (NO

) rad-

ical in the atmosphere. OH oxidation is the dominant fate

for isoprene, but O

and NO

oxidation can dominate reac-

tivity for monoterpenes and sesquiterpenes. Our understand-

ing of the OH-initiated isoprene oxidation mechanism has

significantly improved during the last decade, following the

first suggestion of the capacity of isoprene to produce SOA

(Claeys et al., 2004). The mechanistic developments have

been propelled by technological advancements in instrumen-

tation (Hansel et al., 1995; Crounse et al., 2006; Jordan et al.,

2009; Junninen et al., 2010), enabling the detection of more

complex oxidation products derived from isoprene and other

biogenic hydrocarbons. However, the scientific understand-

ing of these biogenic oxidation mechanisms is far from com-

plete. It is outside the scope of this overview to describe com-

prehensively the isoprene and monoterpene oxidation mech-

anisms. Rather, we provide a brief background of the oxida-

tion of biogenic hydrocarbons, which includes “state-of-the-

science” knowledge, to motivate the study. The mechanisms

described here are illustrated in Scheme 1.

1.1.1 OH oxidation

OH predominantly adds to either of the double bonds of iso-

prene, followed by the reversible addition of O

(Peeters et

al., 2009) to produce several isomers of alkylperoxyl radicals

(RO

). In the atmosphere, these RO

react mainly with HO

and NO to form stable products, although self-reaction can

be non-negligible under certain conditions. The stable prod-

ucts are often termed oxidized volatile organic compounds

(OVOCs). In urban-influenced areas, the “high-NO” path-

way is more important and in more pristine environments,

the “low-NO” or HO

-dominated pathway is more impor-

tant. The high-NO pathway generates isoprene hydroxy ni-

trates (ISOPN) that act as reservoirs for NO

, as well as other

products such as methyl vinyl ketone (MVK), methacrolein

(MAC), and hydroxyacetone (HAC) (Paulot et al., 2009a).

For conditions with sufficiently high NO

-to-NO ratios, as is

mainly the case in the atmospheric boundary layer outside of

cities, methacryloyl peroxynitrate (MPAN) is formed from

the photooxidation of MAC. Further oxidation of MPAN can

generate SOA (Chan et al., 2010, Surratt et al., 2010). The

low-NO pathway generates isoprene hydroxy hydroperox-

ides (ISOPOOH) in almost quantitative yields, and further

OH oxidation of ISOPOOH produces the epoxydiols in an

OH-conserving mechanism (Paulot et al., 2009b). In unpol-

luted atmospheres, when the RO

lifetimes are sufficiently

long (

∼

100 s in a forest), isomerization of the RO

followed

by reaction with O

becomes an important fate, producing

the isoprene hydroperoxy aldehydes (HPALDs) and other

products (Peeters et al., 2009; Crounse et al., 2011). These

isomerization reactions are a type of rapid oxygen in-

corporation chemistry (Vereecken et al., 2007; Crounse et

al., 2013; Ehn et al., 2014) that is thought to be responsi-

ble for the prompt generation of low-volatility SOA compo-

nents. Further generations of OH oxidation in isoprene are

currently being explored owing to recent success with chem-

ical syntheses of important OVOCs (Wolfe et al., 2012; Ja-

cobs et al., 2013; Bates et al., 2014; L. Lee et al., 2014). It

has been found that the OH oxidation of IEPOX and ISOPN,

surprisingly under both low-NO and high-NO conditions, re-

sults primarily in fragmentation of the C

skeleton.

Despite extensive work on the isoprene

OH mechanism,

large uncertainties persist, some of which directly translate

into uncertainties in atmospheric model predictions. These

uncertainties stem from, for example, the large range in re-

ported yields for isoprene nitrates (4–15 %) (Paulot et al.,

2009a), disagreements up to 90 % in reported MAC and

MVK yields from the low-NO pathway (Liu et al., 2013, and

references therein), various proposed sources of SOA from

the high-NO pathway (Chan et al., 2010; Kjaergaard et al.,

2012; Lin et al., 2013), missing contributions to SOA mass

from the low-NO pathway (Surratt et al., 2010), uncharac-

terized fates of oxidized species like HPALDs (which may

have isomer dependence), incomplete understanding of oxy-

gen incorporation (Peeters et al., 2009; Crounse et al., 2013),

and under-characterized impact of RO

lifetimes on chamber

results (Wolfe et al., 2012). The OH oxidation of

-pinene

(Eddingsaas et al., 2012) and other monoterpenes is less well

characterized than that of isoprene, but, in general, proceeds

through analogous steps.

1.1.2 Ozone oxidation

Ozonolysis is a significant sink for unsaturated hydrocarbons

and a large nighttime source of OH, particularly in urban-

influenced areas. Reaction with ozone is more important for

monoterpenes than isoprene, due to the faster rate coeffi-

cients (Atkinson and Carter, 1984) and the nighttime emis-

sion profile for the monoterpenes. Furthermore, monoterpene

ozonolysis is highly efficient at converting VOC mass to

SOA (Hoffmann et al., 1997; Griffin et al., 1999). There is

a general consensus that ozonolysis occurs via the Criegee

mechanism (Criegee, 1975), wherein ozone adds to a hydro-

carbon double bond to form a five-member primary ozonide

that quickly decomposes to a stable carbonyl product and

an energy-rich Criegee intermediate (CI). In

-pinene oxi-

dation, ozonolysis, NO

-initiated, and OH-initiated reactions

all produce pinonaldehyde (C

) as a major product

(Wängberg et al., 1997; Atkinson and Arey, 2003), whereas

major first-generation products from isoprene ozonolysis in-

clude MAC, MVK, and formaldehyde. The “hot” Criegee

can promptly lose OH (Kroll et al., 2001) while ejecting an

Atmos. Chem. Phys., 14, 13531–13549, 2014

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T. B. Nguyen et al.: Overview of FIXCIT

13533

Scheme 1.

Representative mechanism from the OH-, O

- and NO

-initated oxidation of isoprene. The most abundant isomers of a partic-

ular pathway are shown. Red and blue arrows in the OH-oxidation scheme denote the NO-dominated and HO

-dominated RO

reactions,

respectively. For the ozonolysis reaction, only the C

sCI and its reaction with water are shown as further-generation chemistry. For the

-oxidation pathway, only one isomer each of R and RO

radicals is shown for brevity. Abbreviations are defined in the text.

alkyl radical, or become stabilized by collision with atmo-

spheric gases to form a stabilized Criegee intermediate (sCI)

with long enough lifetimes to react bimolecularly. The subse-

quent reactions of sCIs produce both carbonyl products and

non-carbonyl products such as hydroperoxides. The

syn

and

anti

conformers of CIs and sCI can have substantially differ-

ent reactivities (Kuwata et al., 2010; Anglada et al., 2011),

with

syn

conformers more likely to decompose unimolecu-

larly, possibly through a vinyl hydroperoxide intermediate

(Donahue et al., 2011).

It has been suggested that reaction with water molecules

is a major (if not dominant) bimolecular fate of sCI in the

atmosphere due to the overwhelming abundance of atmo-

spheric water (Fenske et al., 2000). This suggestion is sup-

ported by observations of high mixing ratios (up to 5 ppbv)

of hydroxymethyl hydroperoxide (HMHP), a characteristic

product of reactions of the smallest sCI (CH

OO) with wa-

ter (Neeb et al., 1997), over forested regions and in biomass

burning plumes (Gäb et al., 1985; Lee et al., 1993, 2000;

Valverde-Canossa et al., 2006). Although HMHP and other

hydroperoxides produced from ozonolysis are important at-

mospheric compounds, their yield estimates are highly un-

certain (Becker et al., 1990; Neeb et al., 1997; Sauer et al.,

1999; Hasson et al., 2001; Huang et al., 2013). This may be

attributable to the fact that hydroperoxide yields have mainly

been determined by offline methods or under conditions with

highly elevated hydrocarbon loadings. Furthermore, few em-

pirical data exist on the humidity dependence of product

branching in this reaction. Lastly, the rate coefficients for the

sCI

O reaction, and other sCI reactions, are still uncer-

tain by several orders of magnitude (Johnson and Marston,

2008; Welz et al., 2012), precluding the assessment of their

atmospheric importance.

1.1.3 Nitrate oxidation

oxidation also produces RO

radicals by addition to

alkenes in the presence of O

. Owing to its high reaction

rate coefficient coupled to atmospheric abundance,

-pinene

is expected to be an important sink for NO

in many areas.

The NO

-derived RO

radicals react with (a) NO

to form

alkoxy radicals (RO) that lead primarily to the production

of nitrooxy carbonyls (b); with other RO

radicals to form

RO radicals, nitrooxy carbonyls, hydroxy nitrates, and ni-

trooxy peroxy dimers; and (c) with HO

to form nitrooxy

hydroperoxides. Further generation NO

-oxidation produces

dinitrates, amongst other products. As the NO

addition ini-

tiates the reaction, the thermodynamically preferred organic

hydroxy nitrates produced through nighttime oxidation may

be structurally different than those produced in the daytime

through OH oxidation. During nighttime oxidation, tropo-

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Atmos. Chem. Phys., 14, 13531–13549, 2014

13534

T. B. Nguyen et al.: Overview of FIXCIT

spheric HO

mixing ratios often surpass those of NO

(Mao

et al., 2012), implying HO

reaction to be a common fate for

-derived RO

. However, previous studies of this reac-

tion have maintained conditions where minimal HO

chemistry occurs and the dominant fate of RO

is reaction

with NO

and RO

(Ng et al., 2008; Perring et al., 2009;

Rollins et al., 2009; Kwan et al., 2012). This may be one of

the reasons why nitrooxy hydroperoxides (the RO

product) are observed with much higher relative abundances

in ambient air (Beaver et al., 2012) than in chamber studies.

1.2 Scientific goals

The 2014 Focused Isoprene eXperiment at the California

Institute of Technology (FIXCIT) is a collaborative atmo-

spheric chamber campaign focused on advancing the un-

derstanding of biogenic hydrocarbon oxidation in the atmo-

sphere. The campaign was motivated by the communal need

for a tight coupling of field and laboratory efforts toward un-

derstanding the mechanistic details responsible for ambient

observations, exploring explicit chemistry as driven by the

fate of RO

radicals through well-controlled experiments,

and fully characterizing instrumental response to important

trace gases using authentic standards to guide data interpre-

tation. To accomplish these goals, a suite of instruments typi-

cally deployed for field missions was used to perform parallel

measurements at a forested site in Alabama and then in the

atmospheric chambers at Caltech. This overview provides an

account of the goals and conditions for the experiments per-

formed during the campaign. A key component of FIXCIT

is the re-design of “typical chamber experiments” to recre-

ate the ambient atmosphere with higher fidelity so that re-

sults from laboratory studies can be implemented in models

and used to interpret ambient observations with higher con-

fidence.

1.2.1 Understanding ambient observations

FIXCIT was designed as a sister investigation to the 2013

Southern Oxidant and Aerosol Study (SOAS). During SOAS

(June–July 2013), a select sub-suite of instruments recorded

ambient observations above the forest canopy on top of a

metal walk-up tower 20 m in height. The sampling site, lo-

cated in Brent, Alabama at the Centreville (CTR) SEARCH

location managed by the Electric Power Research Institute

(CTR, latitude 32.90289 longitude

−

87.24968), was sur-

rounded by a temperate mixed forest (part of the Talladega

National Forest) that was occasionally impacted by anthro-

pogenic emission. CTR was characterized by high atmo-

spheric water content (2.4–3 vol. % typically), elevated tem-

peratures (28–30

◦

C during the day), high SOA loadings

(particulate organics

∼

4–10 μg m

−

; sulfate

∼

2 μg m

−

high isoprene mixing ratios (4–10 ppbv), high ozone (40–

60 ppbv), low-to-moderate nitrogen oxides ([NO]

∼

0.3–

1.5 ppbv, [NO

]

∼

1–5 ppbv), occasional plumes of SO

from

nearby power plants, and occasional biomass burning events

during the SOAS campaign.

The first goal of the chamber campaign was to further in-

vestigate the more interesting observations at SOAS. Due to

the ability of laboratory experiments to study the chemistry

of a single reactive hydrocarbon in a controlled setting, it

was possible to test hypotheses during FIXCIT in a system-

atic manner. Below we list some relevant questions from the

SOAS campaign that were explored during FIXCIT.

1. Which reactions or environmental conditions control

the formation and destruction of OVOCs in the south-

eastern US?

2. Are RO

isomerization and other rapid oxygen incorpo-

ration mechanisms of key hydrocarbons important dur-

ing SOAS?

3. How do anthropogenic influences, e.g., NO

, O

, and

(NH

)

, impact atmospheric chemistry over the for-

est?

4. How much does the NO

-initated reaction control night-

time chemistry during SOAS?

5. How do environmental conditions in the southeastern

US affect ozonolysis end products, which are known to

be water sensitive?

6. Which reactions or environmental conditions most sig-

nificantly impact SOA mass and composition?

1.2.2 Updating the isoprene and monoterpene

mechanisms

Several experiments were designed to “fill in the gaps” of the

isoprene oxidation mechanisms by leveraging the compre-

hensive collection of sophisticated instrumentation at FIX-

CIT. We targeted the following acknowledged open ques-

tions.

7. What are the products of the photochemical reac-

tions stemming from OVOCs like ISOPOOH, IEPOX,

ISOPN, and pinonaldehyde?

8. What is the impact of photolysis vs. photooxidation for

photolabile compounds?

9. What is the true yield of isoprene nitrates from the high-

NO photooxidation pathway?

10 What is the product distribution and true yield of ni-

trooxy hydroperoxides from the NO

oxidation reac-

tion of isoprene and monoterpenes under typical atmo-

spheric conditions?

11. How do products and yields change as RO

lifetimes in

chamber studies approach values estimated to be preva-

lent in the troposphere?

Atmos. Chem. Phys., 14, 13531–13549, 2014

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T. B. Nguyen et al.: Overview of FIXCIT

13535

Figure 2.

Arrangement of instruments at the Caltech Atmospheric

Chamber Facility during the campaign. Instrument IDs are in Ta-

ble 1.

1.2.3 Instrument characterization

A final goal of FIXCIT was to evaluate, compare, and iden-

tify biases in field instrumentation by isolating one variable

at a time. We focused on the following objectives.

12. Identify the causal factor(s) producing the “OH inter-

ference” (Mao et al., 2012) that has been observed in

various biogenically impacted regions by some gas-

expansion laser-induced fluorescence (LIF) techniques.

13. Characterize the performance of newly commercially

available CIMS instrumentation with respect to the de-

tection of OVOCs by using authentic standards.

14. Compare similar measurements (e.g., OH reactivity, hy-

drocarbons, OVOCs) made with different techniques.

2 Scope of the campaign

2.1 Facilities

Experiments were performed in the Caltech Atmospheric

Chamber Facility within a 1 month period in January 2014.

The facility contains several in-house gas- and aerosol-phase

instruments and an 8

5 m insulated enclosure, housing two

side-by-side Teflon atmospheric chambers that are suspended

from the ceiling. The chambers were manufactured from

fluorinated ethylene propylene (FEP) Teflon. The chamber

volume was measured regularly by quantitative transfer of

highly volatile organics such as isoprene by an externally

calibrated GC-FID. Quantitative transfer was checked via

injections of a measured quantity of isoprene (checked by

gravimetric, volumetric, and FT-IR methods) into a pillow

bag with known volume by timing a calibrated mass flow of

air into the pillow bag. For most experiments, the chamber

volume was between 23 and 24 m

. The spatial configura-

tion of instruments in the chamber facility during FIXCIT is

shown in Fig. 1. The instruments, contributors, and identify-

ing abbreviations used in this work are described in Table 1.

A total of 320 UV black lamps (broadband

max

∼

350 nm)

are mounted on the walls of the enclosure. The lamps are

located behind Teflon films so that the heat produced from

the operation of the lamps can be removed by recirculating

cool air. The interior of the enclosure is covered with reflec-

tive aluminum sheets. Light intensities can be tuned to 100,

50, 10, and 1 %.

was measured to be 7

−

100 % light intensity. Light fluxes at several locations within

the chamber (e.g., center, corner, right, left, high, low) did

not vary more than 15 %. Temperature controls in the cham-

ber enclosure are tunable from 10 to 50

◦

C (typically set

at 25

◦

C) and did not fluctuate more than 1

◦

C, except dur-

ing periods when the temperature was explicitly changed or

during a 30 min period immediately following a change in

the light intensities (up to 2

◦

C increase was observed from

switching on 100 % lights.)

The chamber experiments were operated in batch mode

throughout the campaign. Temperature and RH were moni-

tored continuously inside the chamber by a Vaisala HMM211

probe calibrated with saturated salt solutions in the RH range

of 11–95 %. In the range RH < 11 %, water vapor measure-

ments were provided by the TripCIMS. The chambers were

flushed at least 24 h before each use with ultra-purified air

(purified in-house via a series of molecular sieves, activated

carbon, Purafil

™

media, and particulate filters), at elevated

temperature when needed (

∼

◦

C), so that the backgrounds

on gas- and particle-phase instrumentation are at baseline

levels. As a reference, NO levels before each run were typ-

ically less than 100 pptv (from NO–CL measurements) and

particle concentrations were less than 0.01 μg m

−

. Flush-

ing rates, as balanced by exhaust rates, were typically

250 SD L min

−

(SLM) or

∼

0.6 chamber volumes per hour.

Chambers were mixed on the timescale of minutes by in-

jecting high-pressure pulses of air during the beginning of

experiments.

Chamber 1 was reserved for low-NO experiments, so that

the walls did not contact elevated levels of nitric acid and

organic nitrates during the lifetime of the chamber, while

Chamber 2 was reserved for moderate- to high-NO exper-

iments. Experiments were carried out daily in alternating

chambers to allow for the full flushing period of the previ-

ously used chamber. Each chamber was characterized sep-

arately prior to the campaign for vapor and particle wall

loss rates. Typically, wall loss rates for gas-phase species

are slightly higher in the high-NO chamber than the low-NO

chamber due to the greater acidity of the walls. Particle wall

loss rates were not significantly different between chambers.

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Atmos. Chem. Phys., 14, 13531–13549, 2014

13536

T. B. Nguyen et al.: Overview of FIXCIT

Table 1.

List of participating instruments, principle investigators (PIs), and institutions. Key acronyms: laser-induced fluorescence (LIF),

laser-induced phosphorescence (LIP), high-resolution time-of-flight (HRToF), compact time-of-flight (CToF), MS (mass spectrometer), and

CIMS (chemical ionization mass spectrometer).

Instrument

Instr. ID

PI(s)

Institutions

Measurements

Ref.

Ground-based

hydrogen

oxide sensor

GTHOS

W. H. Brune

Pennsylvania

State

University (PSU)

OH, HO

, RO

Brune et al. (1995)

LIF OH reactivity monitor

LIF-OHR

W. H. Brune

PSU

OH reactivity by decay of OH

Mao et al. (2009)

Thermal dissociation LIF

monitor

TDLIF

R. C. Cohen

University of Califor-

nia, Berkeley (UCB)

, sum of organic nitrates

(

ANs), sum of peroxy ni-

trates (

PNs), particulate org.

nitrates (

ANs)

Day et al. (2002)

Switchable iodide and ac-

etate ion HRToF-CIMS

IACIMS

D. K. Farmer

Colorado State Univer-

sity (CSU)

Oxidized VOCs (organic ni-

trates, organic acids, etc.)

Lee et al. (2014a)

−

HRToF- CIMS

CIMS

M. R. Canagaratna,

D. R. Worsnop, J. L. Jimenez

Aerodyne

Research,

Inc. (ARI) and Univ.

of Colorado, Boulder

(CUB)

Low-volatility organic com-

pounds

Junninen et al. (2010)

LIP glyoxal monitor

GlyLIP

F. N. Keutsch

University of Wiscon-

sin, Madison (UWM)

Glyoxal

Huisman et al. (2008)

LIF formaldehyde moni-

tor

FormLIF

F. N. Keutsch

UWM

Formaldehyde

Hottle et al. (2008);

DiGangi et al. (2011)

Comparative rate method

OH reactivity monitor

CRM-OHR

S. Kim, A. B. Guenther

Univ.

California,

Irvine (UCI) and Pa-

cific NW National Lab

(PNNL)

OH reactivity by decay of hy-

drocarbons

Sinha et al. (2008)

Switchable reagent ion

)

HRToF -MS

SRI-ToFMS

A. B. Guenther, J. E. Mak,

A. H. Goldstein

PNNL, SUNY Stony-

brook

(SUNY),

and

UCB

Hydrocarbons, carbonyls, al-

cohols, etc.

Jordan et al., 2009

Chemical

luminescence

NO monitor

NO–CL

G. S. Tyndall, D. D. Montzka,

A. J. Weinheimer

National

Center

for

Atmospheric Research

(NCAR)

NO (> 25 pptv)

Ridley and

Grahek (1990)

−

triple quadrupole

CIMS

TripCIMS

P. O. Wennberg

California Institute of

Technology (Caltech)

ISOPOOH, IEPOX, glyco-

laldehyde, acetic acid, methyl

hydroperoxide

St. Clair et al. (2010)

−

CToF-CIMS

ToFCIMS

P. O. Wennberg

Caltech

Oxygenated VOCs (hydroper-

oxides, organic nitrates, multi-

functional compounds)

Crounse et al. (2006)

Gas chromatograph with

ToFCIMS

GC-ToFCIMS

P. O. Wennberg

Caltech

Isomers for oxygenated VOCs

Bates et al. (2014)

HRToF-aerosol mass

spectrometer

ToF-AMS

J. H. Seinfeld

Caltech

Aerosol composition and size

distribution

DeCarlo et al. (2006);

Canagaratna et al. (2007)

Gas chromatograph with

flame-ionization detector

GCFID

J. H. Seinfeld

Caltech

Isoprene,

methacrolein,

methyl vinyl ketone, cyclo-

hexane

N/A

Thermocouple and

membrane probe

T /

RH probe

J. H. Seinfeld

Caltech

Temperature and relative hu-

midity

N/A

UV-absorption ozone

monitor

J. H. Seinfeld

Caltech

(> 1000 pptv)

N/A

Chemical luminescence

detector

monitor

J. H. Seinfeld

Caltech

NO (> 500 pptv), and NO

(catalytic conversion to NO)

N/A

Measurements of the particle wall loss rates were performed

by injecting ammonium sulfate (AS) seed aerosols into the

chamber and monitoring the decay over the course of 10–

24 h. Particles were injected via atomization of dilute salt

solutions (e.g., AS 0.06 M) through a

210

Po neutralizer and

water trap. Measurements of vapor wall loss rates were per-

formed by injecting OVOC standards (e.g., IEPOX, HMHP,

etc.) into the chamber. Both particle and vapor wall loss

characterizations were performed at several RH conditions

(4–85 % RH). These characterizations have been described

in more detail previously (Loza et al., 2010; Nguyen et al.,

2014).

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T. B. Nguyen et al.: Overview of FIXCIT

13537

Organic compounds were injected into the chamber by two

methods. (1) For volatile compounds, a measured volume

was injected with a micro-syringe through a septum into a

clean glass bulb, and the evaporated standard was quantita-

tively transferred into the chamber by dry purified air. Gas in-

troduction of VOCs (done for isoprene and methacrolein) by

filling an evacuated bulb with the chemical vapor, backfilling

with nitrogen gas, and characterizing with Fourier transform

infrared spectrometry before injecting did not produce signif-

icantly different results than volume injection. (2) For semi-

volatile compounds, the solid or liquid standard was placed

inside a two-neck flask, which was heated by a water bath

(35–65

◦

C), and the headspace was carried into the chamber

by dry purified air. The ToFCIMS or TripCIMS instruments

measured the gas-phase mixing ratio of the semi-volatiles in

real time as the compounds entered the chamber, and injec-

tion was halted when a satisfactory quantity was introduced.

OVOCs were calibrated by the ToFCIMS and TripCIMS by

methods described earlier (Paulot et al., 2009a). The desired

RH inside the chamber was achieved by flowing dry puri-

fied air through a water-permeable (Nafion) membrane hu-

midifier (FC200, Permapure LLC), kept moist by recirculat-

ing 27

◦

C ultra-purified (18 M

, 3 ppb TOC) water (Milli-Q,

Millipore Corp). Particles were atomized into the chamber

as described for particle wall loss experiments. When hy-

drated particles were needed for experiments, particles were

injected via an in-line, heated, wet-wall denuder into a cham-

ber that has RH above the efflorescence point of the particular

salt (Martin, 2000).

2.2 Instrumentation and sampling modifications

Instruments were connected via sampling lines to both cham-

bers through port holes in the enclosure as shown in Fig. 1.

Sampling lines were capped when not in use. Inlet and tub-

ing material were instrument specific, and included stain-

less steel (GTHOS and ToF-AMS), heated stainless steel

and quartz (TDLIF), electro-polished steel and FEP Teflon

(NO

CIMS), polyetheretherketone (PEEK) and Teflon (SRI-

ToFMS), and perfluoroalkoxy polymer (PFA) Teflon (other

instruments).

The duration of each experiment (i.e., the level of oxi-

dation that can be probed) was critically dependent on the

net sampling flow rates at which air was withdrawn from

the chamber. Sampling strategies were developed to mini-

mize the effective sampling flow rate from each instrument,

in such a way that instrument responses were not signifi-

cantly different than during field campaigns. In many cases,

a common high-flow Teflon sampling line was used to mini-

mize the residence time of gases through tubing, and smaller

flows were sampled orthogonally by each instrument. In

some cases, a duty cycle was used as needed.

Several modifications from field designs were utilized

for chamber sampling. The modifications were that (1) the

GTHOS detection system was located between the cham-

bers inside of the enclosure to minimize the residence time of

inside the instrument (Fig. 1). The detection system was

connected to the laser on the outside of the enclosure via a

3 m fiber optic cable fed through the side port hole. The sam-

pling flow rate was similar to field flows (6 SLM); however,

the fast-flow inlet was situated horizontally (

∼

2 m in height)

instead of vertically. The inlet was adapted to each bag di-

rectly, by attaching it to a Teflon plate that was in turn secured

to the chamber walls via a large o-ring. The GTHOS inlet

switched from Chamber 1 to Chamber 2 as needed. Chem-

ical zeroing was performed by releasing hexafluoropropene

) into the inlet as an OH scrubber, and dark zeroing by

measuring the difference between online and offline signals.

Chemical and dark zeroing methods were used to distinguish

between OH present in the chamber or atmosphere (chem-

ical OH) and OH that may have been produced after the

gas stream enters the instrument, which is additional to the

chemical OH signal; (2) LIF-OHR was diluted a factor of 10

with nitrogen gas (effective flow 6 SLM); (3) NO

CIMS was

diluted a factor of 5 with scrubbed zero air (effective flow

2 SLM); (4) GlyLIP and FormLIF both operated at 5 SLM in-

stead of the usual 17 and 10 SLM, respectively; and (5) SRI-

ToFMS (1.5 SLM) and GCFID (0.1 SLM) occasionally sam-

pled through a 0.125–0.25

′′

OD PFA Teflon tube that was

submerged in a cold bath kept at

−

◦

C in order to remove

interferences from certain OVOC (see Sect. 2.3).

GC-ToFCIMS, first described in Bates et al. (2014),

is an extension of the ToFCIMS. Analyte gas samples

were focused with a cold trap onto the head of a RTX

1701 column (Restek) and eluted with a temperature ramp-

ing program (30–130

◦

C) in the oven before reaching the

ToFCIMS for mass spectrometry detection. GC-ToFCIMS

recorded data only when isomer separation was needed,

because its operation took the standard scanning mode of

the ToFCIMS offline. All other instruments operated nor-

mally with the following sampling flows: TDLIF (4 SLM),

ToFCIMS and TripCIMS (2 SLM), CRM-OHR (0.5 SLM),

NO-CL (1 SLM), and IACIMS (2 SLM). Frequencies of ze-

roing (with dry N

or ultrazero air) and calibration (various

methods) were instrument specific, with some instruments

zeroing once per hour and calibrating once every few hours

and others performing zeroing/calibration between experi-

ments.

2.3 Experimental design

The experiments performed at FIXCIT can be divided into

several categories, each probing one or more specific science

questions outlined in Sect. 1.2. Every experiment included

successful elements from past studies, but with a special fo-

cus on extending to atmospheric conditions. One example

is reducing the occurrence of RO

side reactions in

chamber experiments, which can lead to yields of atmospher-

ically relevant products that are biased low. Enabled by the

high sensitivity of field instruments, photooxidation was per-

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T. B. Nguyen et al.: Overview of FIXCIT

Table 2.

Formal experiments and reaction conditions during the campaign. Chemical abbreviations are defined in Table 3. Other ab-

breviations are C1

Chamber 1, C2

Chamber 2, ISOP

isoprene,

-PIN

-pinene, HP

hydrogen peroxide, MN

methyl nitrite,

CHX

cyclohexane, HCHO

formaldehyde, AS

ammonium sulfate seeds. Exp. types are defined in the text. Exp. no. corresponds to the

date in January 2014 when the experiment was performed.

No.

Exp.

type

precursor

[HC]

(ppb)

source

[OH]

(# cm

−

)

]

(ppb)

[NO]

(ppb)

[NO

]

(ppb)

[NO]

[HO

]

Add’l

inj.

Rxn

(

◦

(%)

ISOP

hν

1.5

< 5

< 0.04

< 2

1/7

–

< 5

ISOP

100

hν

2.4

< 5

500

> 100

–

< 5

ISOPOOHs

250

–

< 3

Blank C1

hν

2.0

< 5

< 0.04

< 3

1/6

–

27–33

< 5

ISOPNs

< 13

–

< 3

Blank C2

hν

2.0

< 5

< 0.04

< 2

1/5

–

< 5

ISOP

rxn

[OH]

∼

615

< 0.04

< 3

–

< 5

∗

ISOP

hν

4.1

4.8

< 5

0.08

2, 6

–

40, 40

< 5

ISOP

3.8

0.10

100

2–3

HCHO

< 5

−

hν

2.0

< 5

< 0.04

< 2

1/10

–

< 5

−

hν

2.5

< 5

> 100

–

< 5

−

0.17

150

1.5–8

HCHO

< 5

ISOP

100

rxn

[OH]

∼

605

< 0.04

< 3

–

CHX

< 5

∗

−

hν

< 5

0.08

< 3

2–3,

–

40, 40

< 5

b, i

4,3- ISOPOOH

hν

1.2

< 5

< 0.04

< 3

1/5

–

< 5

∗

ISOP

hν

1.0

4.3

< 5

0.08

< 3

2–3,

> 100

–

25, 26

< 5

b, h

ISOP

hν

1.0

< 5

< 0.04

< 4

1/5

wet AS

ISOP

hν

2.0

< 5

< 0.04

< 2

1/10

–

< 5

ISOP

100

hν

2.3

< 5

430

> 100

–

< 5

ISOP

rxn

[OH]

∼

600

< 0.04

< 3

–

c, h, i

4,3-ISOPN

hν

115

> 100

wet AS

MAC

hν

< 5

< 0.03

< 3

1/10

–

< 5

g, h

MAC

hν

< 5

3.5

10–20

MAE,

wet AS

< 5, 40

d, i

trans

-IEPOX

hν

7.3

< 5

0.25

< 3

2–5

–

< 5

ISOP

rxn

[OH]

∼

610

< 0.04

< 4

–

CHX

g, h, i

Pinonald.

hν

3.5

< 5

0.50

< 3

4–8

–

< 5

∗

1 % lights, 20 % lights, then 100 % lights.

formed with precursor mixing ratios as low as 12 ppbv. Cer-

tain instruments that required extensive dilution in a cham-

ber setting, e.g., LIF-OHR, had poorer-quality data for low-

loading experiments. Experimental durations were typically

4–6 h, with the exception of overnight runs where the major-

ity of instruments sampled briefly to establish starting con-

ditions, then were taken offline during the nighttime and re-

sumed sampling in the morning. The typical reaction time

for an overnight experiment was

∼

15 h. Experimental de-

tails are reported in Table 2. OH concentrations were derived

from hydrocarbon decay data from GCFID, SRI-ToFMS, or

ToFCIMS, when available, using published rate coefficients

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T. B. Nguyen et al.: Overview of FIXCIT

13539

(Atkinson et al., 2006; L. Lee et al., 2014; Bates et al., 2014).

Otherwise, preliminary GTHOS chemical-zeroing data were

used. The following types of experiments were included in

the study:

a. Blank (Exp. 4b and 5b): blank experiments were de-

signed to investigate background signals present in ex-

periments that may have sources other than gas-phase

chemistry of the injected hydrocarbon, e.g., from het-

erogeneous oxidation of residual organics on the cham-

ber walls. OH precursors, such as hydrogen peroxide,

were added to each chamber, the UV lamps were turned

on, and sampling occurred as usual. Furthermore, the

temperatures inside the chambers were ramped from 25

to 35

◦

C to explore the extent to which elevated temper-

atures change the chamber background signals due to

increased volatilization of organics. Blank experiments

were performed under dry conditions. Common back-

ground compounds produced from heterogeneous wall

reactions are formic acid and acetic acid.

b. Low-NO photooxidation (Exp. 2, 10, 17, 19, and 25):

the low-NO experiments that have been extensively in-

vestigated in atmospheric chamber studies were de-

signed to be relevant to the pristine troposphere, and

certain conditions at SOAS, where HO

reactions dom-

inate the RO

fate. Experiments were initiated by H

photolysis as a NO

-free source of OH and HO

→

The execution of these experiments requires precise

engineering to simulate the troposphere closely. One

outstanding challenge of low-NO experiments is the

variation in initial NO levels across different cham-

ber settings and on different days. Because typical

levels in a chamber environment do not typi-

cally exceed

∼

200 pptv from the self-limiting HO

re-

combination, NO should be

∼

40 pptv during the re-

action (a factor of 5 less abundant) in order for the

reactions to be dominated by HO

by a fac-

tor of 10 (

∼

1.6

−

and

∼

8.5

−

molec

−

at 298K (Atkinson et

al., 2006)). Thus, experimental variations in NO that can

lead to discrepancies in low-NO kinetics typically elude

quantification by commercially available NO chemilu-

minesence instruments, owing to their high limits of de-

tection (

∼

500 pptv).

NO levels in the Caltech chambers were suppressed

by continually flushing with filtered air on the inside

and outside the chamber walls. Initial NO levels of

< 40 pptv were typically achieved during experiments.

The NO–CL instrument available during FIXCIT (Ta-

ble 1) has a limit of detection better than 25 pptv, and the

GTHOS instrument provided online HO

quantification

at the pptv level. Another common challenge for low-

NO experiments (even when [NO] is less than [HO

])

is that homogeneous or cross RO

reactions

may dominate the RO

reactivity (

∼

−

–

−

molec

−

at 298 K; Atkinson et al., 2006).

These experiments may be more correctly character-

ized as “low-NO, high-RO

”. For experiments using

] as an OH precursor, RO

reactions were

largely minimized by using reaction conditions that en-

sure [HO

] greater than [RO

] (e.g., [H

]

[ISOP]

∼

and

]

∼

4–5

−

). Thus, the per-

oxy radical self-reaction channels are minor compared

to RO

chemistry. We estimate that the low-NO

experiments were HO

-dominated by at least a fac-

tor of 10 in RO

reactivity by monitoring tracers of

chemistry stemming from high-NO (isoprene nitrates),

high-RO

diols and other products), and low-NO

(ISOPOOH and IEPOX) pathways. The molar yield of

the low-NO products ISOPOOH

IEPOX (measured

within the first 15 min of reaction) was estimated at

95 %, supporting the dominance of RO

chem-

istry over other channels. The structurally isomeric

ISOPOOH and IEPOX that were formed from the HO

dominated isoprene photooxidation were distinguished

by TripCIMS, and the sum was measured by ToFCIMS,

IACIMS, and NO

CIMS. These experiments were per-

formed with isoprene,

-pinene, 4,3-ISOPOOH and

MAC precursors.

c. High-NO photooxidation (Exp. 3, 11, 22, and 24):

high-NO experiments are also commonly performed in

chamber studies. These experiments were designed to

be relevant to the urban-influenced troposphere, such as

some cases at SOAS, where NO can dominate RO

re-

actions. Experiments were typically initiated by H

with added NO during FIXCIT, but have been per-

formed using HONO or other precursors elsewhere. It is

easier to ensure that reaction with NO is the main fate of

, even with higher hydrocarbon loadings, because

NO mixing ratios are typically in excess of both HO

and RO

by hundreds of ppbv. Hydroxy nitrate prod-

ucts were measured by TDLIF, IACIMS, ToFCIMS,

and GC-ToFCIMS. Functionalized carbonyl products

were measured by SRI-ToFMS and ToFCIMS. Glyoxal

and formaldehyde, also important high-NO products,

were measured by the GlyLIP and FormLIF, respec-

tively. This well-studied experiment was important for

multiple reasons, including calibration, diagnostics, and

for determining the hydroxy nitrate yields from alkenes

within the first few minutes of photooxidation. How-

ever, it should be noted that the experimental result rep-

resents a boundary condition that may not fully repre-

sent NO-influenced reactions in the atmosphere due to

the extremely short RO

lifetimes (< 0.01 s at 500 ppbv

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T. B. Nguyen et al.: Overview of FIXCIT

NO). These experiments were performed with isoprene,

-pinene, and the 4,3-ISOPN standard synthesized by

the Caltech group.

d. Slow chemistry photooxidation (Exp. 7, 16, 18, and

27): the slow chemistry experiment is designed to ex-

tend RO

lifetimes closer to atmospheric values when

both NO and HO

impact RO

reactivity (

∼

3–30 s,

assuming 1500–100 pptv NO and 40 pptv HO

). This

was achieved by employing low radical mixing ratios.

With relevant RO

lifetimes, the RO

isomers may be

closer to their equilibrium distribution because of the

reversible addition of oxygen (Peeters et al., 2009).

Figure 2 shows the progress of a representative slow

chemistry experiment. The “slow” portion of exper-

iments was performed under a low light flux (

∼

−

) with methyl nitrite as the OH precursor

(Atkinson et al., 1981):

ONO

→

HCHO

→

These reactions produce a steady-state OH concentra-

tion of [OH]

∼

0.4–1

molec cm

−

and an atmo-

spherically relevant ratio of NO

(2–3) that is sta-

ble throughout the majority of the experiment. Further-

more, we aimed to simulate the summer conditions at

SOAS, where RO

isomerization is competitive with

and RO

NO chemistry. Thus, most ex-

periments of this type were performed at elevated tem-

peratures (

∼

40–45

◦

C) to facilitate the isoprene RO

isomerization to HPALDs (Crounse et al., 2011), as

measured by ToFCIMS. The atmospheric RO

fates

were qualitatively deduced by observations of their re-

spective products during SOAS (forthcoming papers)

and during other campaigns (Paulot et al., 2009b; Wolfe

et al., 2011; Beaver et al., 2012).

The fate of HPALDs is not known, but has been sug-

gested as being strongly influenced by photolysis based

on reactions of chemical analogs (Wolfe et al., 2012).

After the slow chemistry period, 20–100 % lights were

turned on in order to diagnose the effects of direct pho-

tolysis and OH oxidation on the product compounds,

which is especially instructive when coupled with pho-

tochemical modeling. Table 2 reports conditions only

for the

≤

1 % light period and the 20 % light period

due to availability of hydrocarbon decay data. When

ONO experiments were performed with higher

light flux from the start, the NO-to-HO

reactivities

were still competitive, but the OH mixing ratios were

higher. These experiments were performed with iso-

prene,

-pinene, and

trans

-IEPOX precursors.

e. Ozonolysis (Exp. 6, 14, 23, and 29): ozonolysis

reactions were performed in the dark, with and

without the use of excess cyclohexane (50 ppmv)

as a scavenger for OH (Atkinson, 1995). Ozone

reacts with isoprene and

-pinene with rate co-

efficients of

ISO

1.3

−

molec cm

−

and

−

9.0

−

molec cm

−

at 298 K, respec-

tively (Atkinson et al., 2006). After the first few steps

of the reaction, however, little agreement exists in the

literature for product yields, product distribution, or

rate coefficients stemming from reactions of stabilized

Criegee intermediates (sCI). This may be due to the

large differences among studies in the hydrocarbon

loadings ([ISO]

40–10 000 ppbv), ozone-to-isoprene

ratios (< 0.5 to > 100), water vapor content (< 10–

20 000 ppmv), reaction pressures (4–760 torr), analyt-

ical methods used for product analysis (GC, HPLC,

FTIR, direct OH vs. scavenging, etc.), and methods used

to generate sCI (CH

hν

vs. gas-phase ozonolysis)

(Simonaitis et al., 1991; Neeb et al., 1997; Sauer et al.,

1999; Hasson et al., 2001; Kroll et al., 2002; Johnson

and Marston, 2008; Drozd and Donahue, 2011; Welz et

al., 2012; Huang et al., 2013).

We designed the ozonolysis experiments to have sim-

ilar ozone-to-isoprene ratios to those observed during

SOAS (

∼

5–7), and performed the experiments under

dry (RH

∼

4 %) and moderately humid (RH

∼

50 %)

conditions. The ozonolysis experiments at FIXCIT pri-

marily focused on studying unimolecular and bimolec-

ular chemistry of sCI that affects the yields of OH,

hydroperoxides, organic acids, aldehydes and ketones

under humid vs. dry conditions. These experiments

represent the first coupling between direct OH obser-

vations from GTHOS, aldehyde/ketone measurements

from GCFID and SRI-ToFMS, online formaldehyde

measurements from FormLIF, and online hydroperox-

ide measurements from the various CIMS instruments

present to provide the most comprehensive picture thus

far on the humidity-dependent ozonolysis of isoprene.

f. Competitive HO

nitrate (NO

) oxidation (Exp. 9 and

13): the NO

-initiated experiments during the campaign

were performed in the dark, under dry conditions. Ex-

cess formaldehyde ([HCHO]

∼

4–8 ppmv) was used as

a dark HO

precursor in order to elevate the contribu-

tions of RO

reactions in the NO

chemistry:

→

HCHO

→

HNO

HCO

→



This process produces an HO

ratio of approxi-

mately 2 (determined by photochemical modeling from

the mechanism described in Paulot et al., 2009), a ra-

tio more relevant to the troposphere during nighttime

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T. B. Nguyen et al.: Overview of FIXCIT

13541

oxidation. As

-pinene has a higher NO

loss rate com-

pared to isoprene, a factor of 2 greater mixing ratio of

initial formaldehyde was used. The consequence of the

experimental design is that the isoprene nitrooxy hy-

droperoxide (INP) and monoterpene nitrooxy hydroper-

oxide (MTNP) are major products, in contrast to ex-

periments performed under RO

or RO

dominated conditions (Ng et al., 2008; Perring et al.,

2009; Kwan et al., 2012). The focus of these experi-

ments was the quantification of INP and MTNP with

the various CIMS and with TDLIF, and further explo-

ration of their loss channels to OH oxidation (simulat-

ing sunrise) or to dry AS seed particles by measuring

organic aerosol growth on the ToF-AMS. These experi-

ments were performed with isoprene and

-pinene pre-

cursors.

g. High NO

/NO photooxidation (Exp. 26 and 30): the

high NO

-to-NO ratios in the lower troposphere in most

regions of the globe favor the production of acylper-

oxy nitrates (APNs) from the OH-initiated reaction of

aldehydes like methacrolein and pinonaldehyde (Bert-

man and Roberts, 1991; Nozière and Barnes, 1998).

Unlike the APN from methacrolein (MPAN), the APN

from pinonaldehyde has never been measured in the at-

mosphere (Nouaime et al., 1998; Roberts et al., 1998;

Wolfe et al., 2009). The OH oxidations of aldehydes

were performed with an NO

NO ratio greater than

10, and NO

was replenished as it was reacted away.

These reactions were initiated by CH

ONO photol-

ysis under higher light flux, producing [OH] greater

than 3

molec cm

−

. Certain APNs were moni-

tored with ToFCIMS, and total peroxy nitrates (

PNs)

were monitored with TDLIF. A major focus of the high-

experiments was to investigate the SOA-formation

potential and mechanisms from atmospherically rele-

vant APNs, which is expanded in h.

h. SOA-formation chemistry (Exp. 19, 24, 26, and 30):

experiments aimed specifically at studying chemistry

leading to SOA formation have overlapping goals with

those described above. One focus was the evaluation

of the SOA-formation route from APNs by the pro-

posed dioxo ketone, lactone, and epoxide mechanisms

(Chan et al., 2010; Kjaergaard et al., 2012; Lin et al.,

2013), none of which has yet been validated by inde-

pendent studies. However, the proposed epoxide chem-

istry has been integrated into some studies published

soon after the proposal by Lin et al. (2013) (Worton et

al., 2013; Pye et al., 2013). After MPAN was formed

from the high-NO

reaction of MAC

OH, a synthe-

sized standard of methacrylic acid epoxide (MAE, pro-

vided by the UNC group), the proposed epoxide inter-

mediate, was added to discern the SOA-forming po-

tential of MAE vs. other reactive intermediates in the

MPAN reaction. Following the injection and stabiliza-

tion of MAE, water vapor was added until the reaction

mixture reached

∼

40 % RH. Then wet AS seeds were

injected to investigate any SOA mass growth, as quanti-

fied by ToF-AMS.

SOA formation from ISOPN high-NO photooxidation

and isoprene low-NO photooxidation products were in-

vestigated in the presence of wet AS seeds (40–50 %

particle liquid water by volume), meant to simulate the

high particle liquid water and sulfate quantities dur-

ing SOAS. For these experiments, the chambers were

humidified to 40–50 % RH, and hydrated AS particles

were injected through a wet-wall denuder so that the

seed particles retain liquid water above the efflorescence

point of AS (Biskos et al., 2006). In the ISOPN high-NO

photooxidation, the potential for forming organics that

will likely condense onto seed particles, e.g., dinitrates

and IEPOX, was recently suggested (L. Lee et al., 2014;

Jacobs et al., 2014). The dinitrate pathway was investi-

gated as a potential source of particle-phase organic ni-

trogen. In the low-NO isoprene photooxidation, IEPOX

reactive uptake onto acidic Mg

particles (Lin et

al., 2012) and non-acidified AS particles (Nguyen et al.,

2014), both with non-zero liquid water content, were re-

cently demonstrated. We focused on AS particles with

no added acid. The impact of the partitioning of IEPOX

on the gas-phase mixing ratios was examined as a po-

tential reason for the differences in observed IEPOX in

dry and humid regions.

i. Cross-calibrations (Exp. 4a, 5a, 24, 27, and 30): newly

commercially available negative-ion CIMS (Junninen et

al., 2010; B. H. Lee et al., 2014) may become com-

mon tools for monitoring complex OVOCs in the at-

mosphere, similarly to the widespread adoption of pos-

itive ion CIMS (PTR-MS-based instruments). Some of

the new negative ion CIMS instruments were deployed

for the first time in field campaigns occurring in recent

years. During FIXCIT, synthesized standards of eight

isomer-specific compounds were available for cross cal-

ibrations with different CIMS in order to better under-

stand the chemical sources of ambient signals during

SOAS and in other field campaigns. Table 3 shows the

structures, abbreviations, and contributors of the synthe-

sized chemicals. The TripCIMS and the GC-ToFCIMS

separated structural isomers through collision-induced

dissociation (CID) and through chromatography, re-

spectively. Figure 3 shows a GC-ToFCIMS separation

of isomers of the ISOPN synthesized standards, as well

as ISOPNs present in a complex photooxidation mix-

ture. SRI-ToFMS and IACIMS tested the switchable

reagent ion sources for preferential detection of one or

more isomers of compounds with the same molecular

formula.

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