Nucleation of atmospheric vapours produces more than half of global cloud
condensation nuclei and so has an important influence on climate. Recent
studies show that monoterpene (C10H16) oxidation yields
highly oxygenated products that can nucleate with or without sulfuric acid.
Monoterpenes are emitted mainly by trees, frequently together with isoprene
(C5H8), which has the highest global emission of all organic
vapours. Previous studies have shown that isoprene suppresses new-particle
formation from monoterpenes, but the cause of this suppression is under
debate. Here, in experiments performed under atmospheric conditions in the
CERN CLOUD chamber, we show that isoprene reduces the yield of
highly oxygenated dimers with 19 or 20 carbon atoms – which drive particle
nucleation and early growth – while increasing the production of dimers with
14 or 15 carbon atoms. The dimers (termed C20 and C15,
respectively) are produced by termination reactions between pairs of peroxy
radicals (RO2⚫) arising from monoterpenes or isoprene.
Compared with pure monoterpene conditions, isoprene reduces nucleation rates
at 1.7 nm (depending on the isoprene / monoterpene ratio) and approximately
halves particle growth rates between 1.3 and 3.2 nm. However, above 3.2 nm,
C15 dimers contribute to secondary organic aerosol, and the growth rates
are unaffected by isoprene. We further show that increased hydroxyl radical
(OH⚫) reduces particle formation in our chemical system rather
than enhances it as previously proposed, since it increases isoprene-derived
RO2⚫ radicals that reduce C20 formation.
RO2⚫ termination emerges as the critical step that determines
the highly oxygenated organic molecule (HOM) distribution and the corresponding nucleation capability. Species
that reduce the C20 yield, such as NO, HO2 and as we show
isoprene, can thus effectively reduce biogenic nucleation and early growth.
Therefore the formation rate of organic aerosol in a particular region of
the atmosphere under study will vary according to the precise ambient
conditions.
Introduction
Nucleation of aerosol particles is observed in many environments, ranging
from boreal forests to urban and coastal areas, from polar to tropical
regions, and from the boundary layer to the free troposphere (Kerminen et
al., 2018). Gaseous sulfuric acid, ammonia (Kirkby et al., 2011), amines
(Almeida et al., 2013) and, in coastal regions, iodine
(Sipilä et al., 2016) were shown to contribute to nucleation.
Additionally, a small fraction of the large pool of organic molecules in the
atmosphere, namely highly oxygenated organic molecules (HOMs), some of which
possess extremely low vapour pressures, nucleate together with other
precursors as well as on their own (Riccobono et al., 2014; Kirkby et
al., 2016; Tröstl et al., 2016). This means nature is nucleating
particles on a large scale without pollution, and this may have been
especially pervasive in the pre-industrial atmosphere (Gordon et al.,
2016). HOMs can be formed with molar yields in the single-digit percent
range from the oxidation of monoterpenes (C10H16) with endocyclic
C=C double bonds (Kirkby et al., 2016; Ehn et al., 2014). Monoterpenes
are emitted by a variety of trees in regions ranging from the tropics to
northern latitudes, often reaching mixing ratios of tens to hundreds of
parts per trillion by volume (pptv) (Jardine et al., 2015; Guenther et
al., 2012). Isoprene is a hemiterpene (C5H8) emitted by broadleaf
trees and has the highest emissions of any biogenic organic compound, with
concentrations reaching several parts per billion by volume (ppbv) in the
Amazon rainforest and the southeastern United States despite high reactivity
(Guenther et al., 2012; Martin et al., 2010; Lee et al., 2016). Numerous
studies report suppression of nucleation in isoprene-rich environments, even
if sufficient monoterpenes are present (Lee et al., 2016; Kanawade et
al., 2011; Yu et al., 2014; Kiendler-Scharr et al., 2009, 2012; Varanda Rizzo et al., 2018; Wimmer et al., 2018). This isoprene
suppression effect has been demonstrated in carefully controlled chamber
studies (Kiendler-Scharr et al., 2009, 2012) and
observed in isoprene-rich ambient locations (Kanawade et al., 2011; Lee
et al., 2016; Yu et al., 2014). A recent study also reported a suppression
of secondary organic aerosol (SOA) formation due to isoprene in an
OH⚫-dominated chamber experiment (McFiggans et al., 2019). In
addition to observing suppression of particle formation by isoprene, earlier
studies have proposed mechanisms to explain it. One possibility is
OH⚫ depletion by isoprene, which would reduce the oxidation rate
of monoterpenes and thus supersaturation driving nucleation
(Kiendler-Scharr et al., 2009, 2012; McFiggans et
al., 2019). However, OH⚫ is observed to remain high and
undisturbed in isoprene-rich environments due to atmospheric OH⚫
recycling mechanisms triggered by isoprene (Taraborrelli et al., 2012;
Martinez et al., 2010; Fuchs et al., 2013). Further it was shown that
ozonolysis is crucial for HOM formation (Ehn et al., 2014; Kirkby et al.,
2016). Another proposed possibility for isoprene suppression of nucleation
is the deactivation of sulfuric acid cluster growth due to the addition of
isoprene oxidation products (Lee et al., 2016). However, HOMs
can nucleate without sulfuric acid (Kirkby et al., 2016), and suppression
of nucleation by isoprene is observed in pristine environments such as the
Amazon (Martin et al., 2010).
Isoprene oxidation by OH⚫ triggers complex peroxy-radical
chemistry with a variety of products such as hydroxy-hydroperoxides
(ISOPOOH), hydroperoxy-aldehydes (HPALD) and second-generation
low-volatility compounds (Teng et al., 2017; Berndt et al., 2016).
Isoprene oxidation products with low volatility such as dihydroxyepoxides
(IEPOX) contribute to secondary organic aerosol formation (Carlton et
al., 2009; Krechmer et al., 2015; Paulot et al., 2009; Surratt et al., 2010;
Lin et al., 2011; Budisulistiorini et al., 2013). Recently, the interaction
of isoprene and monoterpene oxidation chemistry was studied, and it was found
that isoprene-derived RO2 molecules can reduce the formation of
monoterpene-derived dimers (Berndt et al., 2018b; McFiggans et al.,
2019). However, the effect of this interaction on nucleation and early
growth of particles under atmospherically relevant conditions remains
unclear so far. One consequence of this is an over-prediction of cloud
condensation nuclei (CCN) in the Amazon by models that simulate pure
biogenic nucleation but neglect the role of isoprene in new-particle
formation (Gordon et al., 2016).
Here, we present experiments performed under atmospherically relevant
conditions at the CERN CLOUD chamber and show on a molecular level how
isoprene affects the chemistry of monoterpene oxidation, thus reducing
nucleation rates as well as early growth rates.
Methods
The Cosmics Leaving Outdoor Droplets (CLOUD) chamber at the European Center
for Nuclear Research (CERN) is a 26.1 m3 stainless-steel aerosol
chamber, in which a large variety of atmospheric conditions can be recreated
under precisely controlled conditions (Kirkby et al., 2011, 2016; Duplissy et al., 2016). The chamber is thermally insulated, and
its temperature can be precisely controlled in the range from -65 to 100 ∘C. In order to reduce contaminations, air
mixed from cryogenic nitrogen and oxygen is used. Trace gases like α-pinene and isoprene can be added and controlled via a two-stage dilution
system at the parts per trillion by volume level. Mixing is ensured by two
magnetically coupled fans. The chamber is equipped with a UV excimer laser
and Hg–Xe UV lamps in order to trigger photochemistry. Ion-free conditions
can be generated by applying a high voltage electric field across the
chamber that sweeps out naturally produced ions (neutral conditions). When
this field is switched off, ions produced by galactic cosmic rays (gcr's)
penetrating the chamber are allowed to stay inside the chamber, and their
effect on nucleation processes can be studied. Using the CERN π+-beam increases the ion concentration artificially (see Supplement
for more detail).
The air inside the chamber is continuously analysed by a variety of
instruments. Organic precursors (α-pinene and isoprene) are measured
by a PTR3 instrument (Breitenlechner et al., 2017). HOMs are
measured by a nitrate chemical-ionization atmospheric-pressure-interface time-of-flight (CI-API-TOF) mass spectrometer (Kürten et al.,
2011) that is connected to the chamber via a 1′′ core sampling probe, where
only the inner part of the flow is sampled into the ion source of the
instrument in order to minimise wall losses. Number concentration and size
distribution of newly formed particles are measured with an array of butanol-based condensation particle counters (CPCs), diethylene-glycol-based
particle size magnifiers (PSMs), and a differential mobility analyser (DMA) train and a scanning
mobility particle sizer (SMPS) (see Supplement for more detail).
A typical experiment starts with the injection of α-pinene into the
particle-free chamber (see Figs. S1 and S2 in the Supplement), while other parameters like
temperature, humidity and ozone levels are already stabilised. Oxidation of
α-pinene by both O3 and OH leads to the formation of HOMs,
which subsequently lead to the formation of particles. The experiment is
continued without intervention until a steady state in HOMs and nucleation
rate has been established. Once the nucleation and growth rates have been
determined, the next experiment is performed under slightly different
conditions. Parameters that were varied are α-pinene and isoprene
levels, ion concentration, UV illumination, sulfuric acid concentration,
temperature and relative humidity.
Results
We performed several experiments at +5 and +25∘C and
relative humidity (RH) ranging from 20 % to 80 % with most of the
experiments being carried out at 38 % RH. Ozone levels ranged from 30 to 50 ppbv. We directly compare experiments performed with α-pinene as
the sole biogenic vapour to experiments with a mixture of α-pinene
and isoprene. α-Pinene levels ranged from 0.33 to 2.5 ppbv, while
isoprene levels ranged from 2.5 to 10 ppbv. We thus could recreate
conditions similar to Kirkby et al. (2016), as well as to regions like
the Amazon (Martin et al., 2010; Yáñez-Serrano et al., 2018) and
southeastern parts of the United States (Lee et al., 2016).
Mass defect plots of neutral HOM molecules
measured with nitrate CI-API-TOF without isoprene (a) and with isoprene
added (b) at +25∘C. α-Pinene levels were 771
and 1326 pptv, respectively. Ozone levels were 49 and 39 ppbv, respectively.
Isoprene was 4.9 ppbv in (b). Relative humidity was 38 % in (a) and (b).
The area of the marker points is linearly scaled with the intensity of the
HOM signals. Colour code shows the relative intensity change for each HOM
peak due to isoprene addition, i.e. the percentage intensity change between
(a) and (b). The colour for each peak is thus the same in (a) and (b). HOM
intensity in (a) was scaled up linearly by 38 % to match [α-pinene] ⋅[O3] levels present in (b) to calculate the
intensity change.
Ozone attack on the endocyclic α-pinene C=C double bond leads to
the well-described formation of highly oxygenated RO2⚫
radicals via intramolecular H shift and autoxidation (mainly
C10H15O4,6,8,10, from now on referred to as RO2(αp)) as well as a wide spectrum of closed-shell monomers (mainly
C10H14,16O5,7,9,11) and covalently bound dimers (mainly
C20H30O8-16 and C19H28O7-11; see Fig. 1a)
(Ehn et al., 2014; Kirkby et al., 2016; Rissanen et al., 2015; Berndt et
al., 2018b; Molteni et al., 2019). These highly oxygenated organic molecules
(HOMs) nucleate at atmospherically relevant concentrations with the help of
ions but without other species (e.g. sulfuric acid or bases) required
(Kirkby et al., 2016). Here, we group the HOMs according to carbon atom
number and define C5, C10, C15 and C20 classes as the sum of
all HOMs with 2–5, 6–10, 11–15 and 16–20 carbon atoms, respectively. This
resembles the basic building block unit of a C5 isoprenoid skeleton.
An isoprene–ozone mixture in the CLOUD chamber produces
C5H9O5-9RO2⚫ radicals (referred to as
RO2(ip)) which terminate to C5H8O5-8 and
C5H10O5-9 monomers and also some C10H18O8-10 dimers under UV-illuminated conditions (see Fig. S5a, b). The
C5H9O5-9 radicals originate presumably from an OH⚫
addition to isoprene and subsequent autoxidation. Under dark conditions,
when the only source of OH⚫ is isoprene ozonolysis at 26 %
yield (Malkin et al., 2010), we observe only C5
monomers. None of these molecules are able to nucleate under atmospherically
relevant conditions despite having an oxygen-to-carbon ratio (O:C) ≥1,
which agrees with earlier observations that products from isoprene
ozonolysis do not drive significant new-particle formation (Kamens et
al., 1982; Riva et al., 2017).
Proposed mechanism for the interference of isoprene in
α-pinene oxidation chemistry. The pathway of HOM formation
of an α-pinene–ozone mixture alone is indicated by red arrows. When
isoprene is present, the green arrows indicate the additional interference
of isoprene in α-pinene oxidation chemistry via RO2⚫
radicals. The oxidation of α-pinene in the
conditions used in our experiments is dominated by ozonolysis. After the
initial ozone attack a C10H15O4, peroxy radical forms via a
vinylhydroperoxide channel (VHP), which can undergo various intramolecular
H shifts and autoxidation steps. Thus the chain of RO2(αp)
mostly consists of C10H15O4,6,8,10. These radicals can
terminate either via reaction with other RO2⚫ radicals, via
reaction with HO2 or via unimolecular processes. The resulting closed-shell products are then either covalently bound C20 class dimers, which
are mostly responsible for nucleation, or C10 class monomers. Possible
fragmentation might also lead to a low number of C5 and C15 class
molecules being formed even without isoprene present. Isoprene oxidation is
dominated by reactions with OH⚫ in the CLOUD chamber, which
produce a series of C5RO2⚫ radicals
(C5H9O3,6,7,8,9). These RO2(ip) radicals can now
interfere in the termination of RO2(αp). The reaction of
RO2(ip) with RO2(αp) can lead to C15 class dimers,
C10 class monomers or C5 class monomers. The reaction of
RO2(ip) with another RO2(ip) can lead to C10 class dimers or
C5 class monomers. The presence of RO2(ip) reduces the steady-state concentration of RO2(αp), as it acts as an additional
sink for RO2(αp). This directly reduces the formation of
C20 class dimers, as two RO2(αp) radicals are needed to
form one C20 class dimer. We link this reduction of C20 class
dimers to the reduction of biogenic nucleation and early growth rates in the
presence of isoprene.
When isoprene is present together with α-pinene and ozone, the HOM
chemistry of α-pinene is altered. We observe the appearance of
C15 and an increase in C5 class molecules compared to α-pinene-only conditions as well as a decrease in C20 and C10
class molecules (see Figs. 1 and S3). Without isoprene, RO2(αp)
can terminate with another RO2(αp), thus forming either one
C20 dimer or two C10 monomers. Monomers can also be formed by
termination with HO2 or unimolecular termination (Rissanen
et al., 2015). The presence of RO2(ip) offers additional termination
channels (Berndt et al., 2018a) (see Fig. 2) and acts as an additional
loss term for RO2(αp). Reactions between RO2(ip) and
RO2(αp) are expected to result in C5 and C10 monomers
as well as C15 dimers. Most importantly, the reduced RO2(αp) steady-state concentrations lead to a reduction of C20 class dimers
by roughly 50 % (depending on detailed conditions) compared to their
level in the absence of isoprene for all studied α-pinene
concentrations (see Fig. S3). To our knowledge the only study that presented
ambient measurements of HOMs for an isoprene-rich region is from the SOAS
campaign (Southern Oxidant and Aerosol Study, Alabama, USA) (Massoli
et al., 2018). When comparing our results with this study, we find good
qualitative agreement for the distribution of HOMs with strong contributions
in the C5 and C10 region and lesser contributions in the C15
and C20 region. We have to caution however that the C15 signal in
the reported HOM distribution could also be caused by sesquiterpene
products. Additionally, the presence of NOx affects HOM chemistry in
Alabama, which also leads to C20 reduction (Lehtipalo et al.,
2018).
Pure biogenic nucleation rates at 1.7 nm diameter (a) and growth rates (b, c) against total HOM concentration with and without
isoprene added at +5 and +25∘C. HOM total
is defined as the sum of C5, C10, C15 and C20 carbon
classes. Relative humidity is 38 % for all data points. (a) Triangles represent Jgcr and circles Jn. Small grey points were
taken from Kirkby et al. (2016). Magenta edges indicate UV-illuminated
conditions at +5∘C; at +25∘C all data points are
with UV light on. Colour shows isoprene-to-monoterpene carbon ratio (R).
Black solid and dashed–dotted lines are parameterisations of Jgcr and
Jn from Kirkby et al. (2016). Red solid and dashed–dotted lines are power-law fits to Jgcr and Jn in the presence of isoprene at +5∘C. Thick solid black and red lines represent power-law fits to
+25∘C data for α-pinene only and α-pinene + isoprene systems. Bars indicate 1σ run-to-run uncertainty. The
overall systematic scale uncertainty of HOMs of +78 % and -68 % and of
J for ±47 % is not shown. In (b) and (c),
triangles represent α-pinene only and circles represent α-pinene + isoprene conditions. Marker colour indicates the size range in which growth
rate was measured: dark blue 1.3–1.9 nm (measured by scanning PSM), light
blue 1.8–3.2 nm, orange 3.2–8.0 nm (both measured by DMA train) and
red 5.0–15 nm (measured by nano-SMPS). Bars indicate 1σ
uncertainties in growth rate estimation. Dashed lines are linear fits to
α-pinene-only data points; solid lines are linear fits to α-pinene + isoprene conditions.
We measured the particle formation rate directly at a 1.7 nm cut-off
diameter with a scanning particle size magnifier (PSM) under neutral (high-voltage field cage switched on; see Supplement for details) and ion
conditions (high-voltage field cage switched off, allowing for galactic
cosmic ray (gcr) ionisation in the chamber), further referred to as
Jn and Jgcr (see Supplement for detail). Figure 3a shows Jn and
Jgcr plotted against the total HOM concentration (the sum of the
C5, C10, C15 and C20 classes) for the α-pinene-only case and α-pinene + isoprene. For +5∘C we find
good agreement with Kirkby et al. (2016). However, the presence of
isoprene and the consequent change in oxidation chemistry reduce
Jgcr by a factor of 2 to 4 and Jn even more by around 1 order
of magnitude at 5 ∘C. The suppression is stronger for lower
α-pinene concentrations and thus higher values of R (the ratio of
isoprene to monoterpene carbon).
The larger gap between Jgcr and Jn with isoprene present compared to
α-pinene-only conditions is direct evidence that isoprene oxidation
products destabilise the nucleating clusters, thus making cluster
stabilisation through the presence of charge more efficient. This also
confirms that C20 class molecules are mainly responsible for pure
biogenic nucleation (Frege et al., 2018). C15 class molecules, which
tend to counteract the losses of the C20 class, do not prevent a
decrease in J. Earlier studies have already suggested that C10 class
molecules do not possess low enough vapour pressure to qualify as extremely
low-volatility organic compounds (Kurtén et al., 2016; Tröstl et
al., 2016) and thus do not drive nucleation, leaving C20 class
molecules as the most likely nucleator molecules. At +25∘C and
UV light illumination, we find that nucleation rates of the pure α-pinene system are reduced by a factor of about 2–3 compared to +5∘C. This is a much smaller reduction in nucleation rate compared
to, e.g. the inorganic sulfuric acid water system, for which the same
temperature increase reduces nucleation rates by around 2 orders of
magnitude (Kirkby et al., 2011) due to an increase in vapour pressure at
warmer temperatures. In our organic system, however, accelerated oxidation
chemistry counters the effect of higher vapour pressures. This includes a
higher rate of initial oxidation of α-pinene by ozone, as well as a
faster autoxidation process, which leads to HOMs with generally higher
oxygen content. When we add isoprene at +25∘C with a constant
ratio of isoprene to monoterpene carbon (R=2), we find a reduction in
Jgcr of around a factor of about 2. Similar to the data at +5∘C where R ranges from 1.6 to 6.5, we expect a stronger decrease
for higher values of R. This can be understood as higher isoprene
concentrations enhance RO2(ip) formation, which in turn reduces
C20 production and subsequent nucleation. R can reach levels around 15
in the Amazon (Greenberg et al., 2004) and around 26 in Michigan
(Kanawade et al., 2011), where we would thus
expect an even stronger isoprene effect on nucleation.
Comparing HOM formation and nucleation for three different α-pinene and isoprene settings, we observe that the addition of 2.7 ppbv of
isoprene to an α-pinene–ozone mixture (770 pptv and 49 ppbv,
respectively) mitigates C20 production and reduces J1.7 from 3.2 to 0.81 cm-3s-1 (see Fig. S6). A rough doubling
of both the α-pinene and isoprene levels to 1326 pptv and 4.87 ppbv,
respectively, increases overall HOM production; however, C20 levels and
consequently J1.7 remain lower than in the original pure α-pinene setting without isoprene. Thus even increasing monoterpene
concentrations can lead to lower J values when isoprene is added as well.
Additional evidence for the important role of C20 is shown in Fig. S9.
Regressing each individual HOM peak with Jgcr gives high coefficients of
determination for C20 class molecules.
It has been argued that OH⚫ depletion by isoprene is responsible
for the absence of nucleation in isoprene-rich environments
(Kiendler-Scharr et al., 2009, 2012); however,
under atmospheric conditions, isoprene-induced OH⚫ recycling can
lead to undisturbed high OH⚫ levels, which might not be true in
chamber experiments (Taraborrelli et al., 2012; Martinez et al., 2010;
Fuchs et al., 2013). In our study we also see an OH⚫ depletion
effect due to isoprene addition (see Fig. S1 and Supplement for detailed
discussion). However, if OH⚫ depletion were the reason for
suppression of nucleation, an increase in OH⚫ would lead to an
increase in the nucleation rate. When we increase OH⚫ levels by
switching on UV lights in the presence of isoprene, this reduces
RO2(αp) further, as well as the C20 and C10 class
molecules, while enhancing the C5 and C15 classes (see Figs. S1, S4
and S5c, d as well as the Supplement for details). Accordingly, J is also
reduced slightly instead of being increased. In the atmosphere with
considerable OH⚫ recycling, this effect, and therefore the
suppression of new-particle formation, would be even stronger. We can
understand this OH⚫ effect by comparing the reactivity of α-pinene and isoprene towards OH⚫ at our given concentrations. For
300 and 1200 pptv the reactivity of α-pinene towards OH⚫
at +5∘C ([αp] ⋅kαpOH) is 25.1 and
6.3 times lower, respectively, than the reactivity of 4 ppbv isoprene
towards OH⚫ ([ip] ⋅kipOH). At +25∘C
these numbers are similar (25.4 and 6.3, respectively). This implies that
any additional OH⚫ provided by UV illumination will favour the
formation of additional RO2(ip) instead of RO2(αp), thus
favouring the formation of C15 over C20 and consequently reducing
nucleation rates. OH⚫ does not enhance nucleation in this chemical
system; it suppresses it.
We performed experiments at +25∘C with three different levels
of relative humidity (20 %, 38 % and 80 %) to probe the effect of water on
new-particle formation. Changes in humidity do not significantly affect HOM
formation and Jgcr (see Fig. S7). Jn increased slightly with
humidity, showing an increased stabilisation of nucleating clusters by
water; however, in gcr conditions, this role is fulfilled more efficiently
by ions.
We further studied the effect of sulfuric acid on nucleation of an α-pinene–isoprene mixture (about 1300 pptv and 4.5 ppbv, respectively) in
experiments with excess ammonia (0.4–2.5 ppbv) in order to reproduce
typical conditions in the eastern parts of the United States
(Lee et al., 2016). We find that sulfuric acid does not
enhance biogenic nucleation up to a concentration of 5×106cm-3 (see Fig. S8). This decoupling of biogenic nucleation from low
sulfuric acid levels is similar to the pure α-pinene system reported
in Kirkby et al. (2016). At sulfuric acid levels higher than 5×106cm-3, nucleation rates depend strongly on sulfuric acid
levels, which agrees with a wide variety of atmospheric measurements
(Kirkby et al., 2016). In the Amazon, sulfuric acid levels are typically
in the range of 1–5×105cm-3
(Kanawade et al., 2011), well below the
threshold value of 5×106cm-3. In Alabama this
threshold was exceeded only three times in a 45 d measurement period due
to transported sulfur plumes, which led to two events of particles growing
to larger sizes (Lee et al., 2016). In Michigan, sulfuric
acid concentrations are typically in the range of 1×106cm-3 (Kanawade et al., 2011). Sulfuric
acid is thus not an important contributor to nucleation in the Amazon as
well as different regions of the eastern United States.
We measured the growth rates of freshly nucleated particles from 1.3 nm
onwards with a scanning particle size magnifier, a DMA train and a nano-SMPS
(see Supplement for details). The change in HOM chemistry caused by
concurrent isoprene oxidation reduces the growth rates of particles in the
range of 1.3–1.9 and 1.8–3.2 nm roughly by a factor of 2 (Fig. 3b, c). This confirms that C15 class molecules have a higher
saturation vapour pressure than C20 class molecules and are thus less
efficient than C20 class molecules at causing growth of the smallest
particles. Likewise, most C10 class molecules are too volatile to
contribute significantly to the early stages of growth (Tröstl et
al., 2016). For the size range from 3.2 to 8.0 nm and larger, we observed no
suppression effect due to isoprene, indicating that molecules smaller than
C20 are capable of condensing onto larger particles. We find a linear
relationship of growth rate vs. C20 for 1.3–1.9 and 1.8–3.2 nm,
regardless of isoprene presence. For larger sizes the linear relationship is
independent of isoprene presence, when plotted against C15+C20; this again indicates that C15 contributes to growth at larger
sizes (Fig. S10). Besides C15 and C20, however, even lighter and
less oxygenated molecules can contribute to particle growth at larger sizes
(Stolzenburg et al., 2018). Growth rates at +25∘C are
typically halved compared to +5∘C due to higher saturation
vapour pressure of the HOMs (Stolzenburg et al., 2018), which leads to a
higher chance of particles being scavenged while growing, even more so in
the presence of isoprene.
Formation rate (gcr) vs. diameter of particles at
+25∘C and 38 % RH. Triangles represent α-pinene only, and circles represent α-pinene + isoprene conditions. α-Pinene levels were 456, 771 and 1442 pptv for triangles and 677, 1326 and
2636 pptv for circles. Ozone levels were 49 ppbv for triangles and 38 to 40 ppbv for circles. Isoprene levels ranged from 2.7 to 9.8 ppbv for circles.
Colour code represents HOM concentration. Bars indicate overall scale
uncertainty for formation rates of ±47 %. The uncertainty in the
diameters is ±0.3 nm. Dashed and solid lines are lines to guide the
eye. The steeper slope at lower-diameter values is caused by the Kelvin
effect, i.e. a smaller growth rate at small sizes that leads to higher
losses of newly formed particles. The formation rate measurements at 2.2 and
2.5 nm for the lowest α-pinene–isoprene setting (cyan circles) are
upper limits.
Figure 4 shows the formation rate of particles measured at diameters of 1.7,
2.2, 2.5 and 6 nm for gcr conditions and six concentration values
(low, middle and high α-pinene mixing ratios with and without isoprene) at
+25∘C. We find that due to the reduced growth rates in the
presence of isoprene, a moderate reduction of formation rates at 1.7 nm
becomes much more pronounced, while the particles grow to larger sizes. When
we compare α-pinene-only data (771 pptv α-pinene, 49 ppbv
O3) with a mixture (1320 pptv α-pinene, 39 ppbv O3 and 4.9 ppbv isoprene, orange data points in Fig. 4), J1.7 is reduced by 45 %, while the corresponding formation rate at 6 nm is reduced by an order
of magnitude. The corresponding precursor concentrations are similar to
conditions found in Alabama for example (Lee et al., 2016). Isoprene
can thus drastically reduce the formation of particles larger than 6 nm even
at relatively warm temperatures like +25∘C. This growth-rate-driven effect becomes stronger when α-pinene concentrations are
reduced. Our measurements agree with observations of small clusters that are
unable to grow efficiently, as has been reported for Alabama
(Lee et al., 2016) and the Amazon (Wimmer et al., 2018).
Increased levels of pre-existing aerosols (i.e. condensation sink) can
scavenge freshly nucleated particles (Dada et al., 2017); however, due
to the reduced initial growth rates, the likelihood for that process at a
given condensation sink is increased when isoprene is present compared to
α-pinene-only conditions.
Discussion
Pure biogenic nucleation was first described for α-pinene oxidation
in the CLOUD chamber (Kirkby et al., 2016). Global evaluation of this
process with the help of atmospheric modelling found an over-prediction of
CCN concentrations in the Amazon, leading to speculation about an as yet
unaccounted-for chemical suppression mechanism for new-particle formation
involving isoprene (Gordon et al., 2016). With our findings, we provide
the molecular understanding for such a mechanism and identify C20 class
molecules as the main drivers of biogenic nucleation and early growth. This
allows us to refine our understanding of biogenic nucleation for
isoprene-rich regions, while at the same time large portions of the
atmosphere where biogenic nucleation is very important remain unaffected by
our findings, especially boreal forests (Gordon et al., 2016).
Suppression of new-particle formation by isoprene was previously attributed
to competition for OH⚫ radicals during the initial oxidation of
volatile organic compounds (VOCs), which was then thought to be followed by independent oxidation
pathways (Kiendler-Scharr et al., 2009). Instead we show that the
suppression takes place via RO2⚫ radical interactions that
strongly couple the oxidation chains of monoterpenes and isoprene.
McFiggans et al. (2019) showed that the same
RO2⚫ mechanism that we describe here is also responsible for
reduced SOA formation, together with an additional OH⚫ scavenging
effect. The oxidation chemistry in McFiggans et al. (2019) was
dominated (>90 %) by OH⚫ for both monoterpenes and
isoprene. In our experimental conditions, monoterpene oxidation was
dominated by ozone, which is more common in the atmosphere, and we
demonstrate the importance of the RO2⚫ mechanism directly in
these conditions. Additionally, while the precursor concentrations in
McFiggans et al. (2019) were much higher than typical
atmospheric levels, the precursor levels in the current study resemble the
atmosphere more closely. This is especially important as HOM formation is
not a linear process and can thus not be scaled down to atmospheric levels
in a straightforward manner.
All extrapolations of chamber experiments to atmospheric conditions must be
treated with care; for example, it has been shown that isoprene
OH⚫ scavenging is stronger in common chamber conditions than in
ambient conditions, where OH⚫ recycling (e.g. by HPALD photolysis)
counters the OH⚫ consumption by isoprene
(Taraborrelli et al., 2012).
McFiggans et al. (2019) also show that OH⚫
scavenging by isoprene is important for reduced SOA formation. We also find
a reduction in OH⚫ levels due to isoprene addition in the CLOUD
chamber. However, this is not the reason for the suppression of nucleation
and early growth rates in our experiments. Quite to the contrary, the
dominant effect of increased OH⚫ in our experiments is to increase
RO2(ip) due to the fast reaction between isoprene and OH⚫;
OH⚫ thus suppresses C20 dimers and nucleation rates in our
chemical system. Thus, while increased OH⚫ levels restore SOA
formation partially in the coupled monoterpene–isoprene system, as shown by
McFiggans et al. (2019), they suppress nucleation in our
experiments. This further highlights the important differences between SOA
formation with pre-existing seed particles on the one hand and the nucleation
of new particles on the other hand. SOA mass production and nucleation are
not the same thing. SOA formation with pre-existing particles can include
molecules possessing comparatively high saturation vapour pressures; however,
due to the Kelvin effect (Tröstl et al., 2016), nucleation depends
critically on molecules with extremely low saturation vapour pressure. Most
of the C20 and C15, many of the C10, and some of the C5
products can form SOA mass, whereas nucleation under atmospheric conditions
is driven largely by the C20 dimers. Even replacing C20 with
C15 dimers suppresses nucleation, as shown in this study.
Our findings are significant beyond the α-pinene–isoprene system, as
they indicate the interaction of a variety of atmospheric VOCs with
monoterpene-derived HOM formation and new-particle formation. Given that
RO2(αp) and RO2 (VOC) reaction rates are competitive (see Supplement for details), VOCs whose RO2⚫ radicals lead to
products that are smaller than C20 when reacting with RO2(αp) (i.e. reduce the ELVOC (extremely low-volatility organic compounds)
fraction in the HOM distribution) are expected to reduce biogenic nucleation
and early growth. On the other hand, VOCs that lead to C20 class or
larger molecules are expected to accelerate both processes.
RO2⚫ termination emerges as the critical step in ELVOC
formation and subsequently biogenic new-particle formation. The suppression
of biogenic new-particle formation by isoprene and potentially other lighter
VOCs, NOx (Lehtipalo et al., 2018) and elevated HO2
concentrations proceeds along the same lines of RO2⚫
termination and subsequent C20 reduction, highlighting the importance
of C20 class molecules for biogenic new-particle formation.
In summary, we find that isoprene interferes with α-pinene HOM
chemistry via RO2⚫ peroxy-radical termination. When isoprene
is present, fewer C20 class molecules are formed, which directly
reduces the nucleation rate. We show that C20 class molecules act as
“nucleator” species. The reduction of nucleation rate becomes stronger
with higher isoprene-to-monoterpene carbon ratio (R), consistent with
earlier observations (Kiendler-Scharr et al., 2009); however, in
the monoterpene–isoprene chemical system, increased OH⚫ does not
enhance nucleation, but, on the contrary, reduces it due to C20 class
reduction. Biogenic nucleation in the α-pinene–isoprene system is
not affected by typical concentrations of sulfuric acid found in the Amazon
or in eastern parts of the United States. The change in monoterpene HOM
chemistry due to isoprene reduces organic growth rates in the 1.3–3.2 nm
range by around 50 %, which strongly reduces the probability that the
smallest, freshly nucleated particles will survive scavenging as they grow
to larger sizes. While other factors can also inhibit nucleation (e.g. NOx, Wildt et al., 2014, or a high condensation sink,
Dada et al., 2017), isoprene can make the difference between
measurable new-particle formation events and their absence under a variety
of atmospheric conditions.
Data availability
Data are available by contacting the
corresponding author.
The supplement related to this article is available online at: https://doi.org/10.5194/acp-20-11809-2020-supplement.
The authors declare that they have no conflict of interest.
Acknowledgements
We thank CERN for supporting CLOUD with technical
and financial resources, and for providing a particle beam from the CERN
Proton Synchrotron. We thank Patrick Carrie, Louis-Philippe De Menezes,
Jonathan Dumollard,
Katja Ivanova,
Francisco Josa,
Ilia Krasin,
Robert Kristic,
Abdelmajid Laassiri,
Osman Maksumov,
Frank Malkemper,
Benjamin Marichy,
Herve Martinati,
Sergey Vitaljevich Mizin,
Robert Sitals,
Albin Wasem and
Mats Wilhelmsson for their contributions to the experiment.
Financial support
This research has been supported by the EC Seventh Framework Programme and European Union's Horizon 2020 programme, Marie Skłodowska Curie (grant nos. 316662
“CLOUD-TRAIN”, no. 764991 “CLOUD-MOTION”, MSCA-IF no. 656994
“nano-CAVa”, MC-COFUND grant no. 600377, ERC projects no. 692891
“DAMOCLES”, no. 638703 “COALA”, no. 616075 “NANODYNAMITE”, no. 335478
“QAPPA”, no. 742206 “ATM-GP”, no. 714621 “GASPARCON”), the German
Federal Ministry of Education and Research (project nos. 01LK0902A,
01LK1222A, 01LK1601A), the Swiss National Science Foundation (projects no. 20020_152907, 200020_172602,
20FI20_159851, 200020_172602,
20FI20_172622), the Academy of Finland (Center of Excellence
no. 307331, project nos. 299574, 296628, 306853, 304013), the Finnish Funding
Agency for Technology and Innovation, the Väisälä Foundation,
the Nessling Foundation, the Austrian Science Fund (FWF; project no. J3951-N36, project no. P27295-N20), the Austrian research funding
association (FFG, project no. 846050), the Portuguese Foundation for Science
and Technology (project no. CERN/FP/116387/2010), the Swedish Research
Council Formas (project number 2015-749), Vetenskapsrådet (grant
2011-5120), the Presidium of the Russian Academy of Sciences and Russian
Foundation for Basic Research (grants 08-02-91006-CERN, 12-02-91522-CERN),
the U.S. National Science Foundation (grants AGS1136479, AGS1447056,
AGS1439551, CHE1012293, AGS1649147, AGS1602086), the Wallace Research
Foundation, the US Department of Energy (grant DE-SC0014469), the NERC GASSP
project NE/J024252/1m, the Royal Society (Wolfson Merit Award), United
Kingdom Natural Environment Research Council grant NE/K015966/1, Dreyfus
Award EP-11-117, the French National Research Agency the Nord-Pas de Calais,
European Funds for Regional Economic Development Labex-Cappa grant
ANR-11-LABX-0005-01).This open-access publication was funded by the Goethe University Frankfurt.
Review statement
This paper was edited by Yafang Cheng and reviewed by two anonymous referees.
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