Interaction of biogenic volatile organic compounds (VOCs) with Anthropogenic VOC (AVOC) affects the physicochemical properties of secondary organic aerosol (SOA). We investigated cloud droplet activation (CCN activity), droplet growth kinetics, and hygroscopicity of mixed anthropogenic and biogenic SOA (ABSOA) compared to pure biogenic SOA (BSOA) and pure anthropogenic SOA (ASOA). Selected monoterpenes and aromatics were used as representative precursors of BSOA and ASOA, respectively.
We found that BSOA, ASOA, and ABSOA had similar CCN activity despite the
higher oxygen to carbon ratio (
In contrast to CCN activity, the hygroscopicity parameter from a hygroscopic tandem differential mobility analyzer (HTDMA) measurement,
Closure analysis of CCN and HTDMA data showed
Secondary organic aerosol (SOA) is an important class of atmospheric aerosol
with impacts on air quality, human health, and climate change (Hallquist et
al., 2009; Kanakidou et al., 2005; Jimenez et al., 2009; Zhang et al., 2011;
Verma et al., 2014). Despite substantial improvements in the understanding
of SOA formation mechanisms and properties, considerable uncertainties
remain about the regional and global budget of SOA (e.g., Goldstein and
Galbally, 2007). Models often do not correctly predict the ambient
concentrations of organic aerosol (OA) (e.g., Spracklen et al., 2011; Heald
et al., 2005), and usually the modeled concentrations underestimate the
observed OA concentrations (Spracklen et al., 2011). Recent studies
suggested that interactions between biogenic volatile organic compounds
(VOCs) and anthropogenic emissions can enhance SOA formation and often,
ambient OA concentrations correlate with anthropogenic tracers such as CO or
isopropyl nitrate (de Gouw et al., 2005, 2008; Weber et al.,
2007; Shilling et al., 2013; Xu et al., 2015). However,
Anthropogenic VOCs (AVOCs), such as aromatic compounds are possibly important
factors that lead to enhanced SOA formation as their oxidation products can
interact with biogenic VOC (BVOC) oxidation products during SOA formation,
as shown by several studies (Hoyle et al., 2011; Emanuelsson et al., 2013;
Flores et al., 2014). In a recent study, Emanuelsson et al. (2013) found
that anthropogenic SOA (ASOA) components reduce the volatility of biogenic
SOA (BSOA) in a non-linear way with respect to the ASOA fraction, possibly
by oligomerization or a phase change such as formation of a glassy state
(Emanuelsson et al., 2013; Virtanen et al., 2010; Koop et al., 2011). The
reduced volatility in the mixed SOA (anthropogenic–biogenic SOA, ABSOA) can
enhance SOA persistence and concentrations in the atmosphere. Flores et al. (2014) investigated the optical properties of BSOA, SOA from simultaneous
addition of BVOC and AVOC and SOA from sequential addition of BVOC and AVOC.
They found that both SOA from mixed AVOC and BVOC show an increase of
scattering component of the refraction index with aging (increase of the
oxygen to carbon ratio (
Besides the thermochemical and optical properties, cloud droplet activation
(cloud condensation nuclei (CCN) activity) and hygroscopicity are important
physicochemical properties that have critical implications for the impact of
aerosol on climate. It is possible that enhanced oligomerization, which
happens in the mixed aerosol particles could modify its CCN activity and
hygroscopicity (Xu et al., 2014). Given that CCN activity and hygroscopicity
correlate with the aerosol
Several field studies found a delay in droplet growth kinetics of the
aerosol from anthropogenic origin when compared with the aerosol from
biogenic origin (Shantz et al., 2010, 2012). ASOA, as an
important anthropogenic aerosol, may contribute to this delay. In addition,
a recent laboratory study suggests limited mixing in SOA formed by
sequentially mixing a biogenic precursor (
In this study, we investigated the effect of the interaction of ASOA and BSOA on CCN activity and hygroscopicity of aerosol. We also studied the kinetics of droplet growth of ASOA, BSOA, and mixed ABSOA.
The experiments were conducted in the atmosphere simulation chamber SAPHIR
(Simulation of Atmospheric PHotochemistry In a large Reaction chamber).
SAPHIR is a double-wall Teflon chamber with a volume of 270 m
Chamber parameters like temperature, relative humidity, flow rate, and photolysis frequencies were also recorded. The actinic flux and the corresponding photolysis frequencies were provided from measurements using a spectral radiometer (Bohn et al., 2005; Bohn and Zilken, 2005).
The number concentration and size distributions of aerosol were measured by a scanning mobility particle sizer (SMPS; DMA model 3081/CPC model 3785, TSI Shoreview, USA) and separate condensation particle counter (CPC; model 3786, TSI) to allow for detection of nucleation particles down to 3 nm.
The chemical composition of aerosol was measured by a high-resolution
time-of-flight aerosol mass spectrometer (HR-ToF-AMS; Aerodyne Research
Inc., USA). To characterize the degree of oxidation of aerosol, the oxygen
to carbon ratio (
Droplet activation and droplet growth were measured using a size scanning
CCN method as described previously (Buchholz, 2010; Zhao et al., 2010). This
method, also known as Scanning Mobility CCN analysis (SCMA; Moore et al.,
2010), has been successfully used in a number of previous studies
(Asa-Awuku et al., 2008, 2009, 2010; Padro et al., 2007; Engelhart et al., 2008, 2011). The
measurement was done by coupling a differential mobility analyzer (DMA;
model 3081, TSI Shoreview, USA) with a cloud condensation nuclei counter
(CCNC; Droplet Measurement Technique, USA) and condensation particle counter
(CPC3786, TSI). Before entering the instruments, the particles were dried
using a silica gel diffusion drier (gradually drying to
For each SS at least three full scans were performed and the resulting
The hygroscopic growth of the aerosol was measured using a home-built
hygroscopic tandem differential mobility analyzer (HTDMA). The details of
the HTDMA were described previously (Buchholz, 2010; Zhao et al., 2010).
Particles were selected using the first DMA and then were exposed to a
prescribed relative humidity to measure the growth factor. Hygroscopic
growth was measured at different RH. The sizes of the humidified particles
were determined by the second DMA, which was operated in a scanning mode in
combination with a CPC (model 3022A, TSI). The size selected aerosol flow
and the sheath air flow of the second DMA were humidified at room
temperature (25–30
SOA samples were collected on PTFE (Polytetrafluoroethylene) filters at the end of different experiments to obtain detailed insight into the chemical composition of the aerosol particles. The details of sample collection and analysis are described in Emanuelsson et al. (2013) and Kristensen and Glasius (2011). Before the filters, the air passed through an annular denuder coated with XAD-4 resin to remove gaseous organic species. The filters were extracted and analyzed using a Dionex Ultimate 3000 HPLC system coupled through an electrospray (ESI) inlet to a q-TOF mass spectrometer (micro-TOFq, Bruker Daltonics GmbH, Bremen, Germany), which was operated in both positive and negative mode. Pinonic acid, cis-pinic acid, terpenylic acid, diaterpenylic acid acetate (DTAA) and 3-methyl butane tri-carboxylic acid (3-MBTCA) were quantified using authentic standards.
Summary of the experiments in this study.
For SOA from part of the experiments (experiment nos. B3, AB4, AB6 as in
Table 1), samples were also collected on quartz fiber filters and analyzed
by ultra-high-resolution mass spectrometry (UHRMS). In this analysis, the
aerosol samples were extracted as described elsewhere (Kourtchev et al.,
2013). The extracts were analyzed using an ultra-high-resolution LTQ Orbitrap
Velos mass spectrometer (Thermo Scientific, Bremen, Germany) equipped with a
TriVersa Nanomate robotic nanoflow chip-based ESI source (Advion
Biosciences, Ithaca NY, USA). The Orbitrap MS instrument calibration,
settings, and mass spectral data interpretation are described in Kourtchev et al. (2014).
The mass accuracy of the instrument was below 1.5 ppm and the
instrument mass resolution was 100 000 at
The VOCs were measured by a high-resolution proton transfer reaction-mass spectrometer (HR-PTR-MS; Ionicon, Innsbruck, Austria) (Jordan et al., 2009) and gas chromatography coupled to a mass spectrometer (GC–MS; PerkinElmer, Waltham, USA) (Apel et al., 2008; Kaminiski, 2014).
The OH concentration was measured directly using laser-induced fluorescence
(LIF) (Fuchs et al., 2012). The OH radicals inside the chamber are mainly
formed by the photolysis of HONO formed via a photolytic process on the
chamber walls, and to a minor fraction by O
The experimental procedures have been described elsewhere in details
(Emanuelsson et al., 2013; Flores et al., 2014) and only a short description
is given here. The chamber was typically humidified to 60–70 % RH in the
beginning of the experiment and relative humidity can vary in the range of
30–70 % due to the ambient temperature change and the dilution by the flow
to compensate the sampling loss. In a typical experiment, VOC was added to
the chamber and then the roof was opened to start the photooxidation. In
some experiments, O
In total, three BSOA experiments (including one using
Droplet activation of BSOA, ASOA and ABSOA at various SS
was parameterized by applying the hygroscopicity parameter
CCN activity of BSOA, ASOA, and ABSOA as a function of OH dose
The droplet activation of BSOA, ABSOA, and ASOA particles represented by
Similarity in CCN activity of ASOA, BSOA, and ABSOA was also observed in the
ABSOA experiments with sequential VOC addition, independent of the order of
addition of AVOC or BVOC. When BVOC was added after AVOC to the chamber,
besides the reaction with OH, BVOC also reacted with O
For BSOA,
Figure 1b shows
CCN activity of ABSOA from sequential VOC addition at various
supersaturations (SS).
For BSOA and ABSOA,
Considering all types of SOA investigated here,
For ASOA systems, particle formation was studied for different aromatic
precursors at low NO
Droplet growth kinetics was investigated using the method of threshold
droplet growth analysis (TDGA), which has been used successfully in many
field and laboratory studies (Engelhart et al., 2008; Asa-Awuku et al.,
2009, 2010; Bougiatioti et al., 2011). In this method, the
droplet growth kinetics was assessed by comparing the droplet sizes from
various SOA with that from ammonium sulfate, which is highly hygroscopic and
rapidly grows under supersaturated conditions. When two particles are
exposed to the same SS, they will grow to droplets of similar size, if their
critical SS and the mass transfer of water vapor are similar. In this study,
the TDGA method was applied to the size-resolved CCNC data and droplet size
was compared for activated particles with SS
Droplet size as a function of SS for BSOA, ASOA, and ABSOA were compared with that of ammonium sulfate (Fig. 3). The droplet sizes of BSOA, ABSOA, or ASOA are similar to those of ammonium sulfate. This indicates the absence of a kinetic barrier for the water uptake of these SOA during droplet activation. Our study is in agreement with several previous studies showing comparable droplet growth kinetics of SOA from monoterpenes with that of ammonium sulfate (Engelhart et al., 2008; Frosch et al., 2011). For SOA from toluene or xylene, no report on droplet growth kinetics was found in the literature. The droplet growth of aerosol from anthropogenic sources in the field containing both organics and ammonium sulfate has been shown to be slower than that of the pure ammonium sulfate, using a static diffusion cloud condensation chamber (Shantz et al., 2010, 2012). Based on our study, ASOA from common aromatics, does not explain such delay and the observations by Shantz and co-workers must have been caused by other aerosol components (Shantz et al., 2010, 2012).
A recent laboratory study by Loza et al. (2013) suggests limited mixing of
different types of SOA components in the particles formed from BSOA
precursor
Droplet sizes of BSOA, ASOA, ABSOA, and ammonium sulfate aerosols at various supersaturations (SS). In the CCNC, all SOA particles reached comparable droplet sizes compared to ammonium sulfate.
Figure 4 shows the hygroscopicity (
The higher
The
The enhanced
Since ASOA has higher
For ABSOA, several cases with non-linear effects were observed. For
ABSOA in experiment no. AB1 and no. AB2 where AVOC was added first,
Morphological effects can also play a role. If the ASOA and BSOA components
were not well mixed in the aerosol particles in the experiments with
sequential VOC additions, there would be more BSOA components on SOA
particle surface in experiments no. AB1 and no. AB2. This could affect
the
The ABSOA filter samples from experiment no. AB4 and no. AB6 were extracted
and analyzed for oligomers. We observed the oligomer formation in these
samples (Fig. S6). Oligomer in SOA has been found by a number of studies (Gao
et al., 2004; Noziere et al., 2015; Tolocka et al., 2004; Kalberer et al.,
2004; Kourtchev et al., 2014, 2015). Small
multi-functional products from aromatics oxidation (Hamilton et al., 2005;
Jenkin et al., 2003; Johnson et al., 2005) may promote oligomerization
between ASOA and BSOA components. But we did not find indications that ABSOA
contained more dimers compared to BSOA. This can be attributed to the low
ASOA fraction
The hygroscopicity parameter
The closure between
Comparison of
Surface tension can also play a role in this discrepancy.
Furthermore, the
We investigated the droplet activation, droplet growth kinetics and hygroscopicity of the BSOA, ASOA, and ABSOA formed from monoterpenes and aromatics used as representative BVOC and AVOC.
We found that BSOA, ASOA, and ABSOA had similar CCN activity although ASOA
had a higher
Analysis of the droplet growth kinetics shows that the droplet sizes from BSOA, ASOA, and ABSOA in supersaturated conditions were similar to those obtained with ammonium sulfate, indicating that none of these SOA has a kinetic barrier for water uptake. The fast water uptake of ASOA indicates that ASOA formed by aromatic precursors is not responsible for the droplet growth delay found in field studies (Shantz et al., 2010, 2012). This finding also suggests that potentially limited mixing between BSOA and ASOA reported in the literature does not hinder the water uptake in supersaturated conditions.
In contrast to CCN activity, the hygroscopicity of ASOA was distinctively
higher than that of BSOA. The higher hygroscopicity was related to the
higher
Comparing hygroscopicity parameter
This study has important implications for assessing the impact of SOA formed
by the interaction of biogenic VOC with anthropogenic VOC emissions on the
radiative forcing and climate. Since the interaction of AVOC with BVOC
reduces the volatility (Emanuelsson et al., 2013), it prolongs particle
persistence, which further enhances the particle concentration. Yet, based
on this study, the CCN activity is not significantly affected. Therefore,
models to assess the climatic effects of SOA formed through the interaction
of biogenic VOC with anthropogenic VOC emissions could use single series of
hygroscopicity parameter
Comparing emission rates of aromatic compounds and isoprenoids (Lamarque et
al., 2010; Guenther et al., 2012) and considering the turnover rates with OH
and O
Based on
From Eq. (A1) and Eq. (A2) one can get
Substituting Eqs. (A4) and (A5) into Eq. (A3) yields
This study was supported by the EUROCHAMP2 (Integration of European Simulation Chambers for Investigating Atmospheric Processes) – EC 7th framework. We thank the SAPHIR team, especially Franz Rohrer, Rolf Häseler, Birger Bohn, Martin Kaminski, Sascha Nehr, Sebastian Schmitt, Anna Lutz, Eva Emanuelsson, and Ismail-Hakki Acir for providing helpful data and supporting our measurements. Marianne Glasius thanks the funding support from NordForsk through the Nordic Centre of Excellence Cryosphere–Atmosphere Interactions in a Changing Arctic Climate (CRAICC) and the VILLUM Foundation. We thank two anonymous reviewers for the constructive comments. The article processing charges for this open-access publication were covered by a Research Centre of the Helmholtz Association. Edited by: H. Su