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Atmos. Chem. Phys., 18, 65–79, 2018

https://doi.org/10.5194/acp-18-65-2018

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Influence of temperature on the molecular composition of ions

and charged clusters during pure biogenic nucleation

Carla Frege

, Ismael K. Ortega

, Matti P. Rissanen

, Arnaud P. Praplan

, Gerhard Steiner

3,4,5

, Martin Heinritzi

Lauri Ahonen

, António Amorim

, Anne-Kathrin Bernhammer

4,18

, Federico Bianchi

1,3

, Sophia Brilke

4,5,6

Martin Breitenlechner

4,a

, Lubna Dada

, António Dias

, Jonathan Duplissy

3,8

, Sebastian Ehrhart

8,b

Imad El-Haddad

, Lukas Fischer

, Claudia Fuchs

, Olga Garmash

, Marc Gonin

, Armin Hansel

4,18

Christopher R. Hoyle

, Tuija Jokinen

, Heikki Junninen

3,17

, Jasper Kirkby

6,8

, Andreas Kürten

Katrianne Lehtipalo

1,3

, Markus Leiminger

4,6

, Roy Lee Mauldin

3,16

, Ugo Molteni

, Leonid Nichman

Tuukka Petäjä

, Nina Sarnela

, Siegfried Schobesberger

3,14

, Mario Simon

, Mikko Sipilä

, Dominik Stolzenburg

António Tomé

, Alexander L. Vogel

1,8

, Andrea C. Wagner

, Robert Wagner

, Mao Xiao

, Chao Yan

, Penglin Ye

12,15

Joachim Curtius

, Neil M. Donahue

, Richard C. Flagan

, Markku Kulmala

, Douglas R. Worsnop

3,14,15

Paul M. Winkler

, Josef Dommen

, and Urs Baltensperger

Paul Scherrer Institute, Laboratory of Atmospheric Chemistry, 5232 Villigen, Switzerland

ONERA – The French Aerospace Lab, 91123 Palaiseau, France

University of Helsinki, Department of Physics, P.O. Box 64, University of Helsinki, 00014 Helsinki, Finland

University of Innsbruck, Institute of Ion Physics and Applied Physics, Technikerstraße 25, 6020 Innsbruck, Austria

University of Vienna, Faculty of Physics, Boltzmanngasse 5, 1090 Vienna, Austria

Institute for Atmospheric and Environmental Sciences, Goethe University Frankfurt, 60438 Frankfurt am Main, Germany

Universidade de Lisboa, Ed. C8, Campo Grande, 1749-016 Lisbon, Portugal

CERN, Geneva, Switzerland

Tofwerk AG, 3600 Thun, Switzerland

School of Earth and Environmental Sciences, University of Manchester, Manchester, M13 9PL, UK

IDL – Universidade da Beira Interior, Av. Marquês D’Avila e Bolama, 6201-001 Covilhã, Portugal

Center for Atmospheric Particle Studies, Carnegie Mellon University, Pittsburgh, Pennsylvania, 15213, USA

Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California, 91125, USA

University of Eastern Finland, Department of Applied Physics, 70211 Kuopio, Finland

Aerodyne Research Inc., Billerica, Massachusetts, 01821, USA

Department of Atmospheric and Oceanic Sciences, University of Colorado, Boulder, Colorado, 80309-0311, USA

University of Tartu, Institute of Physics, 50090 Tartu, Estonia

Ionicon Analytik GmbH, Eduard-Bodem Gasse 3, 6020 Innsbruck, Austria

now at: Harvard University, School of Engineering and Applied Sciences, Cambridge, MA 02138, USA

now at: Max-Planck Institute of Chemistry, Atmospheric Chemistry Department, 55128 Mainz, Germany

Correspondence:

Josef Dommen (josef.dommen@psi.ch)

Received: 5 May 2017 – Discussion started: 29 May 2017

Revised: 31 October 2017 – Accepted: 10 November 2017 – Published: 4 January 2018

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

C. Frege et al.: Influence of temperature on the molecular composition of ions and charged clusters

Abstract.

was

recently

shown

the

CERN

CLOUD experiment that biogenic highly oxygenated

molecules (HOMs) form particles under atmospheric condi-

tions in the absence of sulfuric acid, where ions enhance the

nucleation rate by 1–2 orders of magnitude. The biogenic

HOMs were produced from ozonolysis of

-pinene at 5

◦

Here we extend this study to compare the molecular com-

position of positive and negative HOM clusters measured

with atmospheric pressure interface time-of-flight mass

spectrometers (APi-TOFs), at three different temperatures

(25, 5 and

−

◦

C). Most negative HOM clusters include

a nitrate (NO

−

) ion, and the spectra are similar to those

seen in the nighttime boreal forest. On the other hand, most

positive HOM clusters include an ammonium (NH

) ion,

and the spectra are characterized by mass bands that differ in

their molecular weight by

∼

20 C atoms, corresponding to

HOM dimers. At lower temperatures the average oxygen to

carbon (O : C) ratio of the HOM clusters decreases for both

polarities, reflecting an overall reduction of HOM formation

with decreasing temperature. This indicates a decrease in

the rate of autoxidation with temperature due to a rather

high activation energy as has previously been determined by

quantum chemical calculations. Furthermore, at the lowest

temperature (

−

◦

C), the presence of C

clusters shows

that HOM monomers start to contribute to the nucleation of

positive clusters. These experimental findings are supported

by quantum chemical calculations of the binding energies of

representative neutral and charged clusters.

1 Introduction

Atmospheric aerosol particles directly affect climate by in-

fluencing the transfer of radiant energy through the atmo-

sphere (Boucher et al., 2013). In addition, aerosol particles

can indirectly affect climate, by serving as cloud conden-

sation nuclei (CCN) and ice nuclei (IN). They are of natu-

ral or anthropogenic origin, and result from direct emissions

(primary particles) or from oxidation of gaseous precursors

(secondary particles). Understanding particle formation pro-

cesses in the atmosphere is important since more than half of

the atmospheric aerosol particles may originate from nucle-

ation (Dunne et al., 2016; Merikanto et al., 2009).

Due to its widespread presence and low saturation va-

por pressure, sulfuric acid is believed to be the main vapor

responsible for new particle formation (NPF) in the atmo-

sphere. Indeed, particle nucleation is dependent on its con-

centration, albeit with large variability (Kulmala et al., 2004).

The combination of sulfuric acid with ammonia and amines

increases nucleation rates due to a higher stability of the

initial clusters (Almeida et al., 2013; Kirkby et al., 2011;

Kürten et al., 2016). However, these clusters alone cannot ex-

plain the particle formation rates observed in the atmosphere.

Nucleation rates are greatly enhanced when oxidized organ-

ics are present together with sulfuric acid, resulting in NPF

rates that closely match those observed in the atmosphere

(Metzger et al., 2010; Riccobono et al., 2014). An important

characteristic of the organic molecules participating in nu-

cleation is their high oxygen content and consequently low

vapor pressure. The formation of these highly oxygenated

molecules (HOMs) has been described by Ehn et al. (2014),

who found that, following the well-known initial steps of

pinene ozonolysis through a Criegee intermediate leading to

the formation of an RO

radical, several repeated cycles of

intramolecular hydrogen abstractions and O

additions pro-

duce progressively more oxygenated RO

radicals, a mech-

anism called autoxidation (Crounse et al., 2013). The (ex-

tremely) low volatility of the HOMs results in efficient NPF

and growth, even in the absence of sulfuric acid (Kirkby et

al., 2016; Tröstl et al., 2016). The chemical composition of

HOMs during NPF has been identified from

-pinene and

pinanediol oxidation by Praplan et al. (2015) and Schobes-

berger et al. (2013), respectively.

Charge has also been shown to enhance nucleation

(Kirkby et al., 2011). Ions are produced in the atmosphere

mainly by galactic cosmic rays and radon. The primary

ions are N

, N

, O

, H

, O

−

and O

−

(Shu-

man et al., 2015). These generally form clusters with water

(e.g., (H

O)H

); after further collisions the positive and

negative charges are transferred to trace species with highest

and lowest proton affinities, respectively (Ehn et al., 2010).

Ions are expected to promote NPF by increasing the cluster

binding energy and reducing evaporation rates (Hirsikko et

al., 2011). Recent laboratory experiments showed that ions

increase the nucleation rates of HOMs from the oxidation of

-pinene by 1–2 orders of magnitude compared to neutral

conditions (Kirkby et al., 2016). This is due to two effects,

of which the first is more important: (1) an increase in clus-

ter binding energy, which decreases evaporation, and (2) an

enhanced collision probability, which increases the conden-

sation of polar vapors on the charged clusters (Lehtipalo et

al., 2016; Nadykto, 2003).

Temperature plays an important role in nucleation, result-

ing in strong variations of NPF at different altitudes. Kürten

et al. (2016) studied the effect of temperature on nucleation

for the sulfuric-acid–ammonia system, finding that low tem-

peratures decrease the needed concentration of H

maintain a certain nucleation rate. Similar results have been

found for sulfuric-acid–water binary nucleation (Duplissy et

al., 2016; Merikanto et al., 2016), where temperatures below

◦

C were needed for NPF to occur at atmospheric concen-

trations. Up to now, no studies have addressed the tempera-

ture effect on NPF driven by HOMs from biogenic precursors

such as

-pinene.

In this study we focus on the chemical characterization

of the ions and the influence of temperature on their chemi-

cal composition during organic nucleation in the absence of

sulfuric acid. The importance of such sulfuric-acid-free clus-

ters for NPF has been shown in the laboratory (Kirkby et al.,

Atmos. Chem. Phys., 18, 65–79, 2018

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C. Frege et al.: Influence of temperature on the molecular composition of ions and charged clusters

2016; Tröstl et al., 2016) as well as in the field (Bianchi et al.,

2016). We present measurements of the NPF process from

the detection of primary ions (e.g., N

, O

, NO

) to the

formation of clusters in the size range of small particles, all

under atmospherically relevant conditions. The experiments

were conducted at three different temperatures (

−

25, 5 and

◦

C) enabling the simulation of pure biogenic NPF repre-

sentative of different tropospheric conditions. This spans the

temperature range where NPF might occur in tropical or sub-

tropical latitudes (25

◦

C), high-latitude boreal regions (5

◦

and the free troposphere (

−

◦

C). For example, NPF events

were reported to occur in an Australian Eucalypt forest (Suni

et al., 2008) and at the boreal station in Hyytiälä (Kulmala et

al., 2013). Nucleation by organic vapors was also observed at

a high mountain station (Bianchi et al, 2016). High aerosol

particle concentrations were measured in the upper tropo-

sphere over the Amazon Basin and tentatively attributed to

the oxidation of biogenic volatile organic compounds (An-

dreae et al., 2017).

2 Methods

2.1 The CLOUD chamber

We conducted experiments at the CERN CLOUD cham-

ber (Cosmics Leaving Outdoor Droplets). With a volume

of 26.1 m

, the chamber is built of electropolished stain-

less steel and equipped with a precisely controlled gas sys-

tem. The temperature inside the chamber is measured with

a string of six thermocouples (TC, type K), which were

mounted horizontally between the chamber wall and the cen-

ter of the chamber at distances of 100, 170, 270, 400, 650

and 950 mm from the chamber wall (Hoyle et al., 2016).

The temperature is controlled accurately (with a precision of

0.1

◦

C) at any tropospheric temperature between

−

65 and

◦

C (in addition, the temperature can be raised to 100

◦

for cleaning). The chamber enables atmospheric simulations

under highly stable experimental conditions with low parti-

cle wall loss and low contamination levels (more details of

the CLOUD chamber can be found in Kirkby et al., 2011

and Duplissy et al., 2016). At the beginning of the campaign

the CLOUD chamber was cleaned by rinsing the walls with

ultra-pure water, followed by heating to 100

◦

C and flush-

ing at a high rate with humidified synthetic air and elevated

ozone (several ppmv) (Kirkby et al., 2016). This resulted in

and H

concentrations that were below the detec-

tion limit (

15 pptv and

−

, respectively), and

total organics (largely comprising high volatility C

–C

com-

pounds) that were below 150 pptv.

The air in the chamber is ionized by galactic cosmic

rays (GCRs); higher ion generation rates can be induced by

a pion beam (

) from the CERN Proton Synchrotron en-

abling controlled simulation of galactic cosmic rays through-

out the troposphere. Therefore, the total ion-pair production

rate in the chamber is between 2 (no beam) and 100 cm

−

(maximum available beam intensity, Franchin et al., 2015).

2.2 Instrumentation

The main instruments employed for this study were atmo-

spheric pressure interface time-of-flight (APi-TOF, Aero-

dyne Research Inc. & Tofwerk AG) mass spectrometers. The

instrument has two main parts. The first is the atmospheric

pressure interface (APi), where ions are transferred from at-

mospheric pressure to low pressures via three differentially

pumped vacuum stages. Ions are focused and guided by

two quadrupoles and ion lenses. The second is the time-of-

flight mass analyzer (TOF), where the pressure is approxi-

mately 10

−

mbar. The sample flow from the chamber was

10 L min

−

, and the core-sampled flow into the APi was

0.8 L min

−

, with the remaining flow being discarded.

There is no direct chemical ionization in front of the in-

strument. The APi-TOF measures the positive or negative

ions and cluster ions as they are present in the ambient atmo-

sphere. As described above, in the CLOUD chamber ions are

formed by GCRs or deliberately by the

beam, leading to

ion concentrations of a few hundred to thousands per cm

, re-

spectively. In our chamber the dominant ionizing species are

and NO

−

(see below). These ions mainly form clusters

with the organic molecules, which is driven by the cluster en-

ergies. Therefore, the signals obtained do not provide a quan-

titative measure of the concentrations of the compounds. The

higher the cluster energy with certain compounds, the higher

the ion cluster concentration will be.

calibrated

the

APi-TOF

using

trioctylammo-

nium

bis(trifluoromethylsulfonyl)imide

(MTOA-B3FI,

) to facilitate exact ion mass determination

in both positive and negative ion modes. We employed two

calibration methods, the first one by nebulizing MTOA-B3FI

and separating cluster ions with a high-resolution ultra-fine

differential mobility analyzer (UDMA) (see Steiner et

al., 2014 for more information); the second one by using

electrospray ionization of a MTOA-B3FI solution. The cali-

bration with the electrospray ionization was performed three

times, one for each temperature. These calibrations enabled

mass/charge (

m/z

) measurements with high accuracy up to

1500 Th in the positive ion mode and 900 Th in the negative

ion mode.

Additionally, two peaks in the positive ion mode were

identified as contaminants and also used for calibration pur-

poses at the three different temperatures: C

and

. These peaks were present before the addi-

tion of ozone in the chamber (therefore being most likely

not products of

-pinene ozonolysis) and were also detected

by a proton transfer reaction time-of-flight mass spectrome-

ter (PTR-TOF-MS). Both peaks appeared at the same

m/z

all three temperatures. Therefore, based on the calibrations

with the UDMA, the electrospray and the two organic cali-

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Atmos. Chem. Phys., 18, 65–79, 2018

C. Frege et al.: Influence of temperature on the molecular composition of ions and charged clusters

bration peaks, we expect an accurate mass calibration at the

three temperatures.

2.3 Experimental conditions

All ambient ion composition data reported here were ob-

tained during nucleation experiments from pure

-pinene

ozonolysis. The experiments were conducted under dark con-

ditions, at a relative humidity (RH) of 38 % with an O

mixing ratio between 33 and 43 ppbv (Table 1). The APi-

TOF measurements were made under both galactic cosmic

ray (GCR) and

beam conditions, with ion-pair concen-

trations around 700 and 4000 cm

−

, respectively.

2.4 Quantum chemical calculations

Quantum chemical calculations were performed on the clus-

ter ion formation from the oxidation products of

-pinene.

The Gibbs free energies of formation of representative HOM

clusters were calculated using the MO62X functional (Zhao

and Truhlar, 2008), and the 6-31

G(d) basis set (Ditchfield,

1971) using the Gaussian09 program (Frisch et al., 2009).

This method has been previously applied for clusters con-

taining large organic molecules (Kirkby et al., 2016).

3 Results and discussion

3.1 Ion composition

Under dry conditions (RH

0 %) and GCR ionization, the

main detected positive ions were N

and O

. With in-

creasing RH up to

∼

30 % we observed the water clusters

, (H

and (H

as well as NH

(protonated pyridine), Na

and K

(Fig. 1a). The

concentrations of the precursors of some of the latter ions

are expected to be very low; for example, NH

mixing ra-

tios were previously found to be in the range of 0.3 pptv (at

−

◦

C), 2 pptv (at 5

◦

C) and 4.3 pptv (at 25

◦

C) (Kürten et

al., 2016). However, in a freshly cleaned chamber we ex-

pect ammonia levels below 1 pptv even at the higher tem-

peratures. For the negative ions, NO

−

was the main detected

background signal. Before adding any trace gas to the cham-

ber the signal of HSO

−

was at a level of 1 % of the NO

−

sig-

nal (corresponding to

−

molecules cm

−

, Kirkby et

al., 2016), excluding any contribution of sulfuric acid to nu-

cleation in our experiments.

After initiating

-pinene ozonolysis, more than 460 dif-

ferent peaks from organic ions were identified in the positive

spectrum. The majority of peaks were clustered with NH

while only 10.2 % of the identified peaks were composed of

protonated organic molecules. In both cases the organic core

was of the type C

−

for the monomer region

and C

−

for the dimer region.

In the negative spectrum we identified more than

530 HOMs, of which

∼

62 % corresponded to organic

clusters with NO

−

or, to a lesser degree, HNO

−

The rest of the peaks were negatively charged organic

molecules. In general, the organic core of the molecules

was of the type C

−

in the monomer region

and C

−

in the dimer region. For brevity

we refer to the monomer, dimer (and

-mer) as C

, C

(and C

), respectively. Here, the subscript indicates the

maximum number of carbon atoms in these molecules, even

though the bands include species with slightly fewer carbon

atoms.

3.1.1 Positive spectrum

The positive spectrum is characterized by bands of high in-

tensity at C

intervals, as shown in Fig. 1b. Although we

detected the monomer band (C

), its integrated intensity

was much lower than the C

band; furthermore, the trimer

and pentamer bands were almost completely absent. Based

on chemical ionization mass spectrometry measurements,

Kirkby et al. (2016) calculated that the HOM molar yield

at 5

◦

C was 3.2 % for the ozonolysis of

-pinene, with a frac-

tional yield of 10 to 20 % for dimers. A combination reac-

tion of two oxidized peroxy radicals has been previously re-

ported to explain the rapid formation of dimers resulting in

covalently bound molecules (see Sect. 3.3). The pronounced

dimer signal with NH

indicates that (low-volatility) dimers

are necessary for positive ion nucleation and initial growth.

We observed growth by dimer steps up to C

and possibly

even C

100

. A cluster of two dimers, C

, with a mass/charge

in the range of

∼

700–1100 Th, has a mobility diameter

around 1.5 nm (based on Ehn et al., 2011).

Our observation of HOM–NH

clusters implies strong

hydrogen bonding between the two species. This is con-

firmed by quantum chemical calculations which shall be

discussed in Sect. 3.3. Although hydrogen bonding could

also be expected between HOMs and H

, we do not ob-

serve such clusters. This probably arises from the higher pro-

ton affinity of NH

, 203.6 kcal mol

−

, compared with H

164.8 kcal mol

−

(Hunter and Lias, 1998). Thus, most H

ions in CLOUD will transfer their proton to NH

to form

3.1.2 Negative spectrum

In the negative spectra, the monomer, dimer, and trimer

bands are observed during nucleation (Fig. 2). Monomers

and dimers have similar signal intensities, whereas the trimer

intensity is at least 10 times lower (Fig. 2a and b). The trimer

signal is reduced since it is a cluster of two gas phase species

). Additionally, a lower transmission in the APi-

TOF may also be a reason for the reduced signal.

In Fig. 2, we compare the CLOUD negative-ion spectrum

with the one from nocturnal atmospheric measurements

from the boreal forest at Hyytiälä as reported by Ehn et

al. (2010). Figure 2a and b show the negative spectrum of

Atmos. Chem. Phys., 18, 65–79, 2018

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C. Frege et al.: Influence of temperature on the molecular composition of ions and charged clusters

Table 1.

Experiments performed at the CLOUD chamber.

Campaign

Experiment

Ionization

-pinene

Mass spectrometer

Temperature

no.

(pptv)

(ppbv)

polarity

(

◦

CLOUD 8

1211.02

GCR

258

33.8

Negative

CLOUD 10

1710.04

beam

618

41.5

Positive

CLOUD 10

1712.04

beam

511

40.3

Negative and positive

CLOUD 10

1727.04

beam

312

43.3

Negative and positive

−

Figure 1.

Positive spectra at 5

◦

(a)

Low mass region, where primary ions from galactic cosmic rays are observed, as well as secondary

ions such as NH

, which are formed by charge transfer to contaminants.

(b)

Higher mass region during pure biogenic nucleation, which

shows broad bands in steps of C

. Most of the peaks represent clusters with NH

-pinene ozonolysis in the CLOUD chamber on logarithmic

and linear scales, respectively. Figure 2c shows the Hyytiälä

spectrum for comparison. Although the figure shows unit

mass resolution, the high-resolution analysis confirms the

identical composition for the main peaks: C

−

and C

−

(marked in the

monomer region), and C

, C

−

, C

−

, C

−

, C

−

, C

−

and C

−

(marked in the dimer region). The

close correspondence in terms of composition of the main

HOMs from the lab and the field both in the monomer and

dimer region indicates a close reproduction of the atmo-

spheric nighttime conditions at Hyytiälä by the CLOUD

experiment. In both cases the ion composition was dom-

inated by HOMs clustered with NO

−

. However, Ehn et

al. (2010) did not report nocturnal nucleation, possibly

because of a higher ambient condensation sink than in the

CLOUD chamber.

3.2 Temperature dependence

Experiments at three different temperatures (25, 5 and

−

◦

C) were conducted at similar relative humidity and

ozone mixing ratios (Table 1 and Fig. 3). Mass defect plots

are shown for the same data in Fig. 4. The mass defect is

the difference between the exact and the integer mass and

is shown on the

axis versus the mass/charge on the

axis. Each point represents a distinct atomic composition of

a molecule or cluster. Although the observations described

in the following are valid for both polarities, the trends at the

three temperatures are better seen in the positive mass spectra

due to a higher sensitivity at high

m/z

The first point to note is the change in the distribution of

the signal intensity seen in Fig. 3 (height of the peaks) and

in Fig. 4 (size of the dots) with temperature. In the positive

ion mode, the dimer band has the highest intensity at 25 and

◦

C (see also Fig. 1b), while at

−

◦

C the intensity of the

monomer becomes comparable to that of the dimer. This in-

dicates a reduced rate of dimer formation at

−

◦

C, or that

the intensity of the ion signal depends on both the concen-

tration of the neutral compound and on the stability of the

ion cluster. Although the monomer concentration is higher

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C. Frege et al.: Influence of temperature on the molecular composition of ions and charged clusters

Figure 2.

Comparison of the negative ion composition during

-pinene ozonolysis in CLOUD and during nighttime in the boreal forest at

Hyytiälä (Finland).

(a)

CLOUD spectrum on a logarithmic scale.

(b)

CLOUD spectrum on a linear scale.

(c)

Typical nighttime spectrum

from the boreal forest at Hyytiälä (Finland), adapted from Ehn et al. (2010).

than that of the dimers (Tröstl et al., 2016), the C

ions are

the more stable ion clusters as they can form more easily

two hydrogen bonds with NH

(see Sect. 3.3). Thus, positive

clusters formed from monomers may not be stable enough at

higher temperatures. Moreover, charge transfer to dimers is

also favored.

The data also show a “shift” in all band distributions to-

wards higher masses with increasing temperature, denot-

ing a higher concentration of the more highly oxygenated

molecules and the appearance of progressively more oxy-

genated compounds at higher temperatures. The shift is

even more pronounced in the higher mass bands, as clearly

seen in the C

band of the positive ion mode in Fig. 3a–

c. In this case the combination of two HOM dimers to a

cluster essentially doubles the shift of the band towards

higher mass/charge at higher temperatures compared to the

band. Moreover, the width of each band increases with

temperature, as clearly seen in the positive ion mode in

Fig. 4, especially for the C

band. At high temperatures,

the production of more highly oxygenated HOMs seems to

increase the possible combinations of clusters, resulting in a

wider band distribution.

This trend in the spectra indicates that the unimolec-

ular autoxidation reaction accelerates at higher tempera-

tures in competition to the bimolecular termination reac-

tions with HO

and RO

. This is expected. If unimolecular

and bimolecular reactions are competitive, the unimolecu-

lar process will have a much higher barrier because the pre-

exponential term for a unimolecular process is a vibrational

frequency while the pre-exponential term for the bimolec-

ular process is at most the bimolecular collision frequency,

which is 4 orders of magnitude lower. Quantum chemical

calculations determine activation energies between 22.56 and

29.46 kcal mol

−

for the autoxidation of different RO

radi-

cals from

-pinene (Rissanen et al., 2015). Thus, such a high

barrier will strongly reduce the autoxidation rate at the low

temperatures.

The change in the rate of autoxidation is also reflected

in the O : C ratio, both in the positive ion mode (Fig. 4a–

c), and the negative ion mode (Fig. 4d–f), showing a clear

increase with increasing temperature. The average O : C ra-

tios (weighted by the peak intensities) are presented in Ta-

ble 2 for both polarities and the three temperatures, for all the

identified peaks (total) and separately for the monomer and

dimer bands. For a temperature change from 25 to

−

◦

the O : C ratio decreases for monomers, dimers and total

number of peaks. At high masses (e.g., for the C

and

bands), the O : C ratio may be slightly biased since accu-

rate identification of the molecules is less straightforward:

as an example, C

has an exact mass-to-

charge ratio of 942.34 Th (O

0.64), which is very sim-

ilar to C

at 942.38 Th (O

0.60). How-

ever, such possible misidentification would not influence the

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C. Frege et al.: Influence of temperature on the molecular composition of ions and charged clusters

0.20

0.15

0.10

0.05

0.00

1200

1100

1000

900

800

700

600

500

400

300

200

100

Mass/charge (Th)

0.20

0.15

0.10

0.05

0.00

(

)

0.20

0.15

0.10

0.05

0.00

T = 25 °C

T = 5 °C

T = -25 °C

(a)

(b)

(c)

0.20

0.15

0.10

0.05

0.00

1200

1100

1000

900

800

700

600

500

400

300

200

100

Mass/charge (Th)

0.20

0.15

0.10

0.05

0.00

(

)

0.20

0.15

0.10

0.05

0.00

T = 25 °C

T = 5 °C

T = -25 °C

(d)

(e)

(f)

Figure 3.

Positive

(a–c)

and negative

(d–f)

mass spectra during pure biogenic nucleation induced by ozonolysis of

-pinene at three tempera-

tures: 25

◦

(a, d)

, 5

◦

(b, e)

and

−

◦

(c, f)

. A progressive shift towards a lower oxygen content and lower masses is observed in all bands

as the temperature decreases. Moreover, the appearance of C

species can be seen in the positive spectrum at the lowest temperature

(c)

Table 2.

Signal weighted average O : C ratios for positive and negative spectra at 25, 5 and

−

◦

Temperature

Positive mode

Negative mode

(

◦

Monomer

Dimer

Total

Monomer

Dimer

Total

0.37

0.57

0.54

0.94

0.81

0.90

0.34

0.51

0.49

0.88

0.66

0.75

−

0.31

0.38

0.36

0.79

0.65

0.68

calculated total O

C by more than 0.05, and the main con-

clusions presented here remain robust.

The O : C ratios are higher for the negative ions than for

the positive ions at any of the three temperatures. Although

some of the organic cores are the same in the positive and

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Atmos. Chem. Phys., 18, 65–79, 2018

C. Frege et al.: Influence of temperature on the molecular composition of ions and charged clusters

Figure 4.

Mass defect plots with the color code denoting the O : C ratio (of the organic core) at 25, 5 and

−

◦

C for positive

(a–c)

and

negative ion mode

(d–f)

. A lower O : C ratio is observed in the positive ion mode than in the negative ion mode. The intensity of the main

peaks (linearly proportional to the size of the dots) changes with temperature for both polarities due to a lower degree of oxygenation at

lower temperature.

Figure 5.

Comparison of the positive ion mode spectrum measured (blue), the C

band obtained by the combination of all C

molecules

(light gray), and the C

band obtained by combination of only the C

molecules with O

≥

0.4 (dark gray). The low or absent signals at

the lower masses obtained by permutation suggest that only the highly oxygenated dimers are able to cluster and form C

negative ion mode, the intensity of the peaks of the most

oxygenated species is higher in the negative spectra. While

the measured O : C ratio ranges between 0.4 and 1.2 in the

negative ion mode, it is between 0.1 and 1.2 in the pos-

itive ion mode. An O : C ratio of 0.1, which was detected

only in the positive ion mode, corresponds to monomers and

dimers with two oxygen atoms. The presence of molecules

with such low oxygen content was also confirmed with a pro-

Atmos. Chem. Phys., 18, 65–79, 2018

www.atmos-chem-phys.net/18/65/2018/

C. Frege et al.: Influence of temperature on the molecular composition of ions and charged clusters

ton transfer reaction time-of-flight mass spectrometer (PTR-

TOF-MS), at least in the monomer region. Ions with O : C

ratio less than 0.3 are probably from the main known ox-

idation products like pinonaldehyde, pinonic acid, etc., but

also from minor products like pinene oxide and other not yet

identified compounds. It is likely that these molecules, which

were detected only in the positive mode, contribute only to

the growth of the newly formed particles (if at all) rather

than to nucleation, owing to their high volatility (Tröstl et

al., 2016). In this sense, the positive spectrum could reveal

both the molecules that participate in the new particle for-

mation and those that contribute to growth. The differences

in the O : C ratios between the two polarities are a result of

the affinities of the organic molecules to form clusters either

with NO

−

or NH

, which, in turn, depends on the molecu-

lar structure and the functional groups. Hyttinen et al. (2015)

reported the binding energies of selected highly oxygenated

products of cyclohexene detected by a nitrate CIMS, finding

that the addition of OOH groups to the HOM strengthens the

binding of the organic core with NO

−

. Even when the num-

ber of H-bonds between NO

−

and HOM remains the same,

the addition of more oxygen atoms to the organic compound

could strengthen the binding with the NO

−

ion. Thus, the less

oxygenated HOMs were not detected in those experiments,

neither in ours, in the negative mode. The binding energies

were calculated for the positive mode HOMs–NH

and are

discussed in Sect. 3.3.

We also tested to which extent the formation of the

band could be reproduced by permutation of the po-

tential C

molecules weighted by the dimer signal inten-

sity. Figure 5 shows the measured spectrum (blue) and two

types of modeled tetramers: one combining all peaks from

the C

band (light gray) and one combining only those peaks

with an organic core with O

≥

0.4 – i.e., likely represent-

ing non-volatile C

molecules (dark gray). The better agree-

ment of the latter modeled mass spectrum of the tetramer

band with the measured one suggests that only the molecules

with O

≥

0.4 are able to form the tetramer cluster. This

would mean that C

molecules with 2–7 oxygen atoms are

likely not to contribute to the nucleation, but only to the

growth of the newly formed particles. One has to note that

the comparison of modeled and measured spectra relies on

the assumption that the charge distribution of dimers is also

reflected in the tetramers.

These two observations (change in signal distribution and

band “shift”) are not only valid for positive and negative ions,

but also for the neutral molecules as observed by two nitrate

chemical-ionization atmospheric-pressure-interface time-of-

flight mass spectrometers (CI-APi-TOF; Aerodyne Research

Inc. and Tofwerk AG). This confirms that there is indeed a

change in the HOM composition with different temperature

rather than a charge redistribution effect, which would only

be observed for the ions (APi-TOF). The detailed analysis of

the neutral molecules detected by these CI-APi-TOFs will be

the subject of another paper and is not discussed here.

G =

17.98 kcal mol

G =

17.32 kcal mol

G =

17.46 kcal mol

(a)

(b)

(c)

Figure 6.

Quantum chemical calculations of the free energy related

to the cluster formation between NH

and three structurally sim-

ilar molecules with different functional groups:

(a)

acetaldehyde,

(b)

acetic acid and

(c)

peracetic acid.

A third distinctive trend in the positive mode spectra at the

three temperatures is the increase in signal intensity of the

band at

−

◦

C. The increase in the signal of the trimer

also seems to occur in the negative ion mode when compar-

ing Fig. 3d and f. For this polarity, data from two campaigns

were combined (Table 1). To avoid a bias by possible dif-

ferences in the APi-TOF settings, we only compare the tem-

peratures from the same campaign, CLOUD 10, therefore ex-

periments at 25 and

−

◦

C. The increase in the trimer signal

may be due to greater stability of the monomer–dimer clus-

ters or even of three C

molecules at low temperatures, as

further discussed below.

3.3 Quantum chemical calculations

Three points were addressed in the quantum chemical calcu-

lations to elucidate the most likely formation pathway for the

first clusters, and its temperature dependence. These included

(i) the stability of the organic cores with NO

−

and NH

de-

pending on the binding functional group, (ii) the difference

between charged and neutral clusters in terms of clustering

energies, and finally (iii) the possible nature of clusters in the

dimer and trimer region.

The calculations showed that among the different func-

tional groups the best interacting groups with NO

−

are

in order of importance carboxylic acids (R–C(

O)–OH),

hydroxyls (R–OH), peroxy acids (R–C(

O)–O–OH), hy-

droperoxides (R–O–OH) and carbonyls (R–(R

′

–) C

O).

On the other hand, NH

preferably forms a hydrogen bond

with the carbonyl group independent of which functional

group the carbonyl group is linked to; Fig. 6 shows ex-

amples of NH

clusters with corresponding free energies

of formation for carbonyls (

=−

17.98 kcal mol

−

), car-

boxylic acid (

=−

17.32 kcal mol

−

) and peroxy acid

(

=−

17.46 kcal mol

−

). For the three examples shown,

the interaction of one hydrogen from NH

with a C

group is already very stable with a free energy of cluster ion

formation close to

−

18 kcal mol

−

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Atmos. Chem. Phys., 18, 65–79, 2018