ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-16-1761-2016Hydroxyl radicals from secondary organic aerosol decomposition in waterTongHaijiehttps://orcid.org/0000-0001-9887-7836ArangioAndrea M.LakeyPascale S. J.BerkemeierThomashttps://orcid.org/0000-0001-6390-6465LiuFobangKampfChristopher J.BruneWilliam H.https://orcid.org/0000-0002-1609-4051PöschlUlrichhttps://orcid.org/0000-0003-1412-3557ShiraiwaManabum.shiraiwa@mpic.dehttps://orcid.org/0000-0003-2532-5373Multiphase Chemistry Department, Max Planck Institute for
Chemistry, Mainz, GermanyInstitute for Inorganic and Analytical Chemistry, Johannes Gutenberg
University Mainz, Mainz, GermanyDepartment of Meteorology, Pennsylvania State University, University
Park, PA 16802, USAManabu Shiraiwa (m.shiraiwa@mpic.de)15February20161631761177122October20153November201529January20162February2016This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://acp.copernicus.org/articles/16/1761/2016/acp-16-1761-2016.htmlThe full text article is available as a PDF file from https://acp.copernicus.org/articles/16/1761/2016/acp-16-1761-2016.pdf
We found that ambient and laboratory-generated secondary organic aerosols
(SOA) form substantial amounts of OH radicals upon interaction with liquid
water, which can be explained by the decomposition of organic
hydroperoxides. The molar OH yield from SOA formed by ozonolysis of terpenes
(α-pinene, β-pinene, limonene) is ∼ 0.1 %
upon extraction with pure water and increases to ∼ 1.5 % in
the presence of Fe2+ ions due to Fenton-like reactions. Upon extraction
of SOA samples from OH photooxidation of isoprene, we also detected OH
yields of around ∼ 0.1 %, which increases upon addition of
Fe2+. Our findings imply that the chemical reactivity and aging of SOA
particles is strongly enhanced upon interaction with water and iron. In
cloud droplets under dark conditions, SOA decomposition can compete with the
classical H2O2 Fenton reaction as the source of OH radicals. Also
in the human respiratory tract, the inhalation and deposition of SOA
particles may lead to a substantial release of OH radicals, which may
contribute to oxidative stress and play an important role in the adverse
health effects of atmospheric aerosols.
Introduction
Secondary organic aerosols (SOA) account for a major fraction of fine air
particulate matter and have a strong influence on climate and public health
(Jimenez et al., 2009; Pöschl et al., 2010; Huang et al., 2014).
Formation of SOA is triggered by oxidation of volatile organic compounds
followed by condensation of semi-volatile oxidation products (Hallquist
et al., 2009; Donahue et al., 2012). Recently, it has been shown that
extremely low volatility organic compounds contribute significantly
to SOA growth (Ehn et al., 2014; Jokinen et al., 2015; Mentel et al.,
2015).
Particle phase chemistry and cloud processing are also efficient pathways
for SOA formation and aging (Kalberer et al., 2004; Herrmann et al.,
2005; Ervens et al., 2011; Shiraiwa et al., 2013). Evolution of SOA is one
of the largest uncertainties in the current understanding of air quality,
climate and public health (Kanakidou et al., 2005; Solomon, 2007). With
regard to SOA health effects, substantial amounts of reactive oxygen species
including organic radicals are detected in ambient and laboratory-generated
SOA (Venkatachari and Hopke, 2008; Chen and Hopke, 2010; Chen et al.,
2010; Fuller et al., 2014). Despite intensive research, multiphase chemical
reactions of SOA in the atmosphere and upon interaction with the human
respiratory tract are not well understood (Pöschl and Shiraiwa,
2015).
Schematics of the experimental setup for generation and collection
of SOA particles.
OH radicals in atmospheric droplets originate from the uptake of gaseous OH
radicals (Jacob, 1986; Arakaki et al., 2013) as well as photolysis of
ozone (Anglada et al., 2014). A recent study has shown that
SOA can form OH radicals in the aqueous phase under light conditions
(Badali et al., 2015). Under dark conditions, Fenton reactions
between H2O2 and iron ions have been regarded as the main source
of OH radicals so far (Herrmann et al., 2005). In this study, we
found that OH radicals are formed by decomposition of SOA upon interactions
of water and iron ions under dark conditions.
MethodsSOA formation and particle collection
Figure 1 shows the experimental setup for generation of secondary organic
aerosols (SOA). O3 was used as oxidant for oxidation of α-pinene, β-pinene and limonene, and OH radicals were used for
naphthalene. O3 was generated via synthetic air (Westfalen AG,
1.8–2.1 L min-1)
passing through a 185 nm UV light (O3 generator, L.O.T.-Oriel GmbH &
Co. KG). The typical ozone concentrations were 600 ppb for α-pinene,
β-pinene and limonene, and 1200 ppb for naphthalene. A total of 1 mL of α-pinene (98 %, Sigma Aldrich), β-pinene (99 %, Sigma Aldrich)
or limonene (99 %, Sigma Aldrich) was kept in a 1.5 mL amber glass vial
(VWR International GmbH), and 5–10 g of naphthalene crystals (99.6 %,
Alfa Aesar GmbH & Co. KG) was put in a 100 mL glass bottle (DURAN Group
GmbH) as SOA precursor sources. A total of 1 bar and
50–150 ccm min-1
N2 (99.999 %, Westfalen AG) flow was passed through these sources,
and the evaporated volatile organic compound (VOC) vapours were introduced into a 7 L quartz flow tube
reactor for gas-phase oxidation reaction with O3 or OH radicals with a
reaction time of ∼ 3 min. SOA by α-pinene, β-pinene
and limonene were generated under dark and dry conditions. The flow tube
reactor is surrounded by four UV lights (wavelength of 254 nm, LightTech Lamp
Technology Ltd.), which were turned on to generate OH radicals by photolysis
of ozone and water vapour. The relative humidity in the flow tube was
30 % for generating naphthalene SOA, and other experiments were conducted
under dry conditions. Isoprene SOA was produced in a potential aerosol mass
(PAM) chamber through the reaction of gas phase OH radicals and isoprene. The
detailed information about this chamber has been described elsewhere (Kang et
al., 2007; Lambe et al., 2011), and the SOA generated by the PAM chamber have
been shown to be similar to SOA generated in large environmental chambers
(Bruns et al., 2015; Lambe et al., 2015) and the atmosphere (Ortega et al.,
2015) in terms of oxidation state and chemical composition. Briefly the
isoprene vapour was taken into the chamber by N2 gas with an estimated
concentration of tens of parts per million (ppm). Ozone concentration in the PAM was 6–15 ppm, and
relative humidity was 30–40 %.
Number concentration and size distribution of the generated SOA particles
were characterized using a scanning mobility particle sizer (SMPS, GRIMM
Aerosol Technik GmbH & Co. KG). The typical size of the SOA ranged from 50
to 400 nm. The median diameters of the mass size distribution were
100–200 nm. MnO2 (copper mesh covered with MnO2 from ANSYCO
Analytische Systeme und Componenten GmbH fixed in Gelman filter) and charcoal
(4–8 mesh, Sigma Aldrich) denuders were used to remove unreacted O3
before the collection of SOA particles on a filter. SOA was collected on
47 mm Omnipore Teflon filters (100 nm pore size, Merck Chemicals GmbH). The
concentration of O3 was monitored after an ozone denuder with an ozone
analyser (typically 0–20 ppb, model 49i, Thermo Fisher Scientific Inc.).
Two silica gel (2–4 mm, Carl Roth GmbH & Co. KG) denuders were used to dry the naphthalene SOA before
collection.
Blank tests confirmed that no radicals were produced without SOA particles on
a filter. Condensation of water vapour on a filter during SOA collection was
negligible. A Teflon filter with particle loading was weighed using a
XSE105DU balance with accuracy of ±20 µg. It was then immersed
into a 0.5–1 mL 10 mM BMPO water solution and stirred with a vortex shaker
(Heidolph Reax 1) for 2–7 min for particle extraction. A typical extraction
efficiency of > 70 % in weight can be obtained with 7 min extraction
time. After extraction, the filter was dried under 2–3 bar N2 for
∼ 10 min and the filter was weighed. The weight difference was
regarded as the weight of extracted particles. The final SOA concentration
depends on the extraction time, and the average molar mass of SOA was assumed
to be 200 g mol-1 in calculating SOA concentrations. The pH of SOA
solutions was in the range of 4.8–6.4.
A micro-orifice uniform deposit impactor (MOUDI, 110-R mode, MSP
Corporation) was used for collection of ambient particles on the roof of the
Max Planck Institute for Chemistry (Mainz, Germany) in 24 h time resolution
with a flow rate of 30 L min-1 from 17:30 UTC + 1 4 June 2015 to
17:30 5 June 2015 and from 17:30 7 June 2015 to 17:30
8 June 2015. Particles within the diameter range of 180–320 nm, which is
the size range dominated by organic aerosols in Mainz (Faber et al., 2013),
were used for further analysis. The mass loading of these two samples on
filters were ∼ 70 and 80 µg, respectively. Teflon filters of 47 mm diameter (100 nm pore size, Merck Chemicals GmbH) were used to collect
the roof particles. Filters were cleaned with pure ethanol and ultra-pure
water and dried by nitrogen gas before sampling and weighing. The extraction
procedure is the same as that for laboratory SOA, and the field particle
extracts were concentrated with a N2 flux to obtain high signal-to-noise
ratio spectra. Concentrations of field particles in water extracts for EPR
measurements were ∼ 0.3 g L-1, which is of the same order of
magnitude as extracts of laboratory-generated SOA.
CW-EPR
Continuous-wave electron paramagnetic resonance (CW-EPR) spectroscopy
(EMXplus-10/12, Bruker, Germany) was applied for detection of radicals. A
total of 15–30 µL sample solutions were kept in a 50 µL capacity
micropipette and inserted into a highly sensitive cavity (E4119001 HS-W1) for
analysis. The set of EPR parameters used for this study was as follows: a
modulation frequency of 100 kHz; a modulation amplitude of 0.6 or 1;
microwave power of 2.149 mW (20 dB) or 21.17 mW (10 dB); a receiver gain
of 40 dB; a time constant of 0.01 ms; and a magnetic field scan of 100 G.
After the SOA extraction, the samples were immediately analysed by an EPR.
The spin trap 5-tert-butoxycarbonyl-5-methyl-1-pyrroline-N-oxide (BMPO, high
purity, Enzo Life Sciences GmbH) was used as a trapping agent of OH radicals.
Compared to other spin-trapping agents such as 5, 5-dimethyl-1-pyrroline
N-oxide (DMPO), BMPO has the following advantages: high purity and
stability in the crystalline phase; highly distinguishable EPR spectra for
different structure of the trapped radicals; and spectra with high signal-to-noise ratio. Buffer solutions are often used in the spin-trapping technique,
but they were not used in this study to avoid changing the real acidity
environment of SOA solutions. A BMPO concentration of 10 mM was used. No
significant difference was observed among 10, 20, 30, 40 and 50 mM BMPO
solutions, confirming that a BMPO concentration of 10 mM is sufficient to
achieve the maximum trapping efficiency. The influence of the BMPO
concentration on the aqueous phase OH radical trapping efficiency for β-pinene SOA was investigated as shown in Fig. S3. Further blank tests
confirmed that H2O2 (30 %, Sigma Aldrich), Fe2+ and
Fe3+ (Fe2O12S3⚫ xH2O, 97 %, Sigma
Aldrich) do not induce OH radical formation when each of them is mixed with
BMPO in water (Fig. S4).
The spin-counting method was applied for quantification of OH radicals using
the embedded subroutine of the Bruker Xenon software (Weber, 2012). For
better quantification of detected radicals, the spin-fitting method (Bruker
Xenon software, chapter 13; Weber, 2012) was used to increase the signal-to-noise ratio especially for low radical concentrations. The required
parameters are hyperfine splitting parameters for OH radicals, which were
taken from Zhao et al. (2001). Spectral simulations for radical adducts were
carried out using the Matlab-based computational package Easyspin (Stoll and
Schweiger, 2006). A global optimization (genetic algorithm) was conducted to
obtain parameters for simulating the EPR spectrum. The parameter set was
further optimized using the particle swarm method within the Easyspin
program. The function “garlic” for cw EPR spectra in isotropic and fast
motion regimes was chosen for simulation. The hyperfine splitting constants
for simulation were taken from the Zhu et al. (2009).
EPR spectra of sample solutions mixed with the spin-trapping agent
BMPO: (a) α-pinene SOA, (b) β-pinene SOA,
(c) limonene SOA, (d) isoprene SOA,
(e) naphthalene SOA, (f) 180–320 nm size field particles,
(g) 180–320 nm size field particles mixed with Fe2+,
(h) tert-butyl hydroperoxide solution and (i) H2O2 solution
with Fe2+. The four peaks (dotted lines) are characteristic of BMPO-OH
adducts.
LC-MS/MS analysis. (a) LC-MS chromatogram of aqueous BMPO
solution (black line) and BMPO mixed with β-pinene SOA water extracts
(red line). The downward triangle indicates the retention time of m/z 216
(BMPO-OH). (b) MS spectrum of [BMPO + H+]+ with
nominal m/z 200. (c) MS2 spectrum of m/z 200, with the
characteristic fragment ion m/z 144.0639
([BMPO + H+]+-m/z 56.0626). (d) Proposed
fragmentation pathway for m/z 200. The most abundant fragment ion present
in (c) corresponds to the loss of C4H8 from
[BMPO + H+]+. (e) MS spectrum of [BMPO-OH]+ with
m/z 216. (f) The MS2 spectrum of m/z 216, with the
characteristic fragment ion m/z 160.0590
([BMPO-OH]+-m/z 56.0624). (g) Proposed fragmentation
pathway for m/z 216. The observed loss of C4H8 is characteristic
of the fragmentation of the t-butoxycarbonyl function of BMPO.
LC-MS/MS
The SOA extracts mixed with spin-trapping agent BMPO were also analysed with
a nanoHPLC-chip-MS/MS system (Agilent), which consists of a nano pump
(G2226A) with four-channel micro-vacuum degasser (G1379B), a microfluidic chip
cube with electrospray ionization (ESI) source (G4240-62010) interfaced to a
Q-TOF mass spectrometer (6540; nominal mass resolution 30 000 at a scan rate
of 5 s-1), a capillary pump (G1376A) with degasser (G1379B), and an
auto-sampler with thermostat (G1377A). All modules were controlled by Mass
Hunter software (Rev. B.05.01, Agilent). Eluents used were 3 % (v/v)
acetone nitrile (Chromasolv, Sigma, Seelze, Germany) in water / formic acid
(0.1 % v/v, Chromasolv, Sigma, Seelze, Germany) (Eluent A) and 3 %
water / formic acid (0.1 % v/v) in acetone nitrile (Eluent B). The flow
rate was 400 nL min-1 with a gradient program that starting with
3 % B for 3 min followed by a 36 min step that raised eluent B to
60 %. Further, eluent B was increased to 80 % at 40 min and
returned to initial conditions within 0.1 min, followed by column
re-equilibration for 9.9 min before the next run. The ESI-Q-TOF instrument
was operated in the positive ionization mode (ESI+) with an ionization
voltage of 1900 V. Fragmentation of protonated ions was conducted using the
automatic MS/MS mode. Spectra were recorded over the mass range of
m/z 100–3000. Data analysis was performed using the qualitative data
analysis software (Rev. B. 06.00, Agilent).
Kinetic modelling
The chemical reactions used to describe the BMPO/SOA/Fe2+/H2O
system, including Fenton-like reactions, are listed along with their rate
coefficients in Table S1. From this set of 25 reactions, 16 were optimized
using the MCGA method and parameter ranges are given in Table S1 to
illustrate the uncertainty arising from global optimization. For all other
parameters reference values were taken from the literature, which remained fixed
during optimization. Kinetic rate coefficients of a large set of chemical
reactions were determined using a uniformly sampled Monte Carlo search
seeding a genetic algorithm (MCGA method; Berkemeier et al., 2013; Arangio et
al., 2015) as the global optimization method. This algorithm optimizes the correlation between
a kinetic model and experimental data in order to constrain the input parameters of the model.
Genetic algorithms mimic processes known from natural evolution and offer mechanisms such as crossover and
mutation to avoid convergence towards local minima. In the kinetic model, ROOH represents
all organic hydroperoxides without resolving individual structures. This is a
simplification, which is necessary for the kinetic modelling but seems to
return consistent results.
OH formation efficiency by SOA. (a) Concentrations of OH
radicals formed in water extracts of SOA of β-pinene (black), α-pinene (blue), limonene (red), isoprene (purple) and naphthalene (pink) as
a function of SOA concentrations in the aqueous phase. The formation
efficiency of OH (molar concentration ratio of OH to SOA:
[BMPO-OH] / [SOA], in %) in iron containing SOA water extracts against
molar concentration ratios of FeSO4 and SOA ([Fe2+] / [SOA]) by
(b)β-pinene, (c)α-pinene and
(d) limonene. The markers are experimental data, and the solid curves
with shaded area are modelled with uncertainty.
Results and discussion
Figure 2 indicates that EPR spectra of laboratory generated SOA by α-pinene (spectrum a), β-pinene (spectrum b), limonene (spectrum c) and
isoprene (spectrum d) SOA were composed of four major peaks, whereas naphthalene
SOA (spectrum e) exhibited no significant signals. These four peaks were also
found for field samples (spectrum f) and became more prominent in the presence
of Fe2+ (spectrum g). In addition, the same splitting was also observed in
a solution of tert-butyl hydroperoxide (spectrum h). Four-line signals
generated by hyperfine splittings are characteristic of BMPO-trapped OH
radicals in water solution, as shown in the spectrum (spectrum i) for solutions
of H2O2 and Fe2+, generating OH via the Fenton reaction
(Fe2++ H2O2→ Fe3++ OH-+⚫OH (Zhao et al., 2001).
Figure 3 shows LC-MS chromatograms of the BMPO-OH adduct (m/z 216.121) for
aqueous BMPO solutions (black line) and for BMPO in aqueous β-pinene
SOA extract (red line). A strong peak is observed at a retention time of
11.6 min for BMPO in aqueous β-pinene SOA extract, but not for the
aqueous BMPO solution, which served as a blank. Confirmation of the BMPO
structure for m/z 216.121 was achieved by comparing MS2 spectra of
[BMPO + H+]+ (m/z 200.126) from the aqueous standard and
m/z 216.121. In both cases the loss of a characteristic fragment with a
mass of 56.062 Da is observed (panel c and f), which corresponds to the loss
of C4H8 from the t-butoxycarbonyl function of BMPO. The above LC-MS/MS
analysis confirms the presence of OH radicals in β-pinene SOA extracts
observed by EPR shown in Fig. 2.
The EPR and LC-MS/MS observations provide strong evidence that OH radicals
are generated in water extracts of SOA by α-pinene, β-pinene,
limonene and isoprene as well as field fine particles, which can be enhanced
by Fe2+. Note that additional hyperfine splitting is observed for
monoterpene and isoprene SOA and especially for field samples, indicating the
presence of organic radicals. Figure 4a shows that the amount of OH radicals
trapped by BMPO increases as the SOA concentration increases in the aqueous
phase. The OH yield from β-pinene SOA is the highest generating
∼ 1.5 µM of OH radicals at 1.5 mM SOA concentration,
followed by α-pinene, isoprene and limonene SOA. Naphthalene SOA has
a negligible yield of OH radicals.
For assessment of potential interferences from trace amounts of impurities
such as transition metals in water, the OH yield was also measured in water
with three different purity grades – Milli-Q water (18.2 M, Thermo
Scientific™
Barnstead™
GenPure™ xCAD Plus ultrapure water system),
TraceSELECT® Ultra ACS reagent water (Sigma
Aldrich) and Savillex water (DST-1000 Acid Purification System) – which
results in excellent agreement (Fig. 5) confirming that OH radicals can be
formed in the absence of transition metals.
OH yield of β-pinene SOA in three different kinds of pure
water: Milli-Q (squares), Savillex (triangles) and TraceSELECT (Sigma,
crosses).
Ambient particulate matter is often associated with iron ions, which play an
important role in aerosol chemistry via Fenton-like reactions (Deguillaume et
al., 2005). To investigate the effects of transition metals on OH formation
by SOA, different concentrations of Fe2+ were added in SOA water
extracts. Figure 4b–d show the OH formation efficiency (molar concentration
ratio of OH and SOA: [BMPO-OH] / [SOA], in %) of β-pinene,
α-pinene and limonene SOA as a function of molar concentration ratio
of FeSO4 to SOA ([Fe2+] / [SOA]). The OH formation efficiency
reaches maximum values of 1.5 % for β-pinene SOA, 1.1 % for
α-pinene SOA and 0.5 % for limonene. Different behaviours in OH
formation efficiency of limonene compared to α-pinene and β-pinene may be induced by different organic hydroperoxide concentrations and
different R subgroup structure of ROOH. This order is the same as the order
of the relative contribution of organic peroxides in these types of SOA
(Docherty et al., 2005). For isoprene SOA, the first results of ongoing
experiments indicate a significant increase of OH yield with increasing
Fe2+ concentrations. The EPR spectra of the isoprene SOA show a
dependence on the oxidant concentration level in the PAM chamber. The more
complex behaviour of the isoprene SOA from OH photooxidation is under
investigation and will be presented in a follow-up study.
The observed formation of OH radicals is most likely due to hydrolysis and
thermal decomposition of organic hydroperoxides (ROOH), which account for the
predominant fraction of terpene SOA (Docherty et al., 2005; Epstein et al.,
2014) as well as in rain water (Hellpointner and Gäb, 1989), but they have
little contribution for naphthalene SOA (Kautzman et al., 2010). ROOH are
formed via multigenerational gas-phase oxidation and autoxidation,
introducing multiple hydroperoxy functional groups forming extremely low
volatility organic compounds (Crounse et al., 2013; Ehn et al., 2014). Due to
the low binding energy of the O–O bond induced by the electron-donating R
group, ROOH are well-known to undergo thermal homolytic cleavage (ROOH → RO⚫+⚫OH; Nam et al., 2000). In the presence
of Fe2+, it has been reported that decomposition of ROOH can be enhanced
mainly via Fenton-like reactions leading to heterolytic cleavage of the O–O
bond in the following two ways depending on the pH and reaction environments:
ROOH + Fe2+→ RO⚫+ OH-+ Fe3+
or
ROOH + Fe2+→ RO-+⚫OH + Fe3+
(Goldstein and Meyerstein, 1999; Deguillaume et al., 2005). Note that
homolytic cleavage can be catalysed by iron ions (Foster and Caradonna,
2003). The formed alkoxy radicals (RO⚫) were trapped by BMPO and
found to increase as the Fe2+ concentration increases (Fig. 6). The
formation of organic radicals in α-pinene and limonene SOA has been
also detected in the previous studies (Pavlovic and Hopke, 2010; Chen et al.,
2011). As shown in Fig. 4, the chemical box model including the above three
ROOH decomposition pathways reproduces experimental data very well, strongly
suggesting that the source of OH radicals is decomposition of ROOH. The
decrease of OH radical production with increasing Fe2+ concentration is
supposedly induced by reaction of the BMPO-OH adduct with Fe2+ (Yamazaki
and Piette, 1990) (see also Supplement).
Formation efficiency of organic radicals. Molar concentration ratio
of organic radicals to SOA ([BMPO-OR] / [SOA], in %) in mixtures of
Fe2+ and SOA solutions.
It has been suggested that hydrogen peroxide (H2O2) can be
generated from α- and β-pinene SOA in water, but the mass
yield of H2O2 is ∼ 0.2 % (Wang et al., 2011). In the
presence of Fe2+, H2O2 can yield OH radicals via the Fenton
reaction, and the formation efficiency of BMPO-OH adduct by mixtures of
H2O2 with Fe2+ was measured to be ∼ 0.6 % (Fig. S2).
Thus, the potential contribution of generated H2O2 to OH yields in
β- and α-pinene SOA extracts is much lower than the observed
OH radicals. Moreover, the OH yield was not affected, even if β-pinene
SOA was dried under a N2 flow before the water extraction to evaporate
particle-phase H2O2. Hence it is clear that the H2O2 in
SOA should not be the dominant source of OH radicals observed in this study.
OH production rate in cloud droplets and lung lining fluid.
(a) The OH production rate in cloud droplets by SOA decomposition
compared to the classical Fenton reaction. The data points were measured in
the absence of Fe2+ for different precursors of β-pinene (black
squares), α-pinene (blue circles), limonene (red upward triangles)
and isoprene (purple downward triangles). The shaded green area represents
the possible range in the presence of iron as a function of SOA concentration
in the aqueous phase, which is based on the minimum and maximum OH radical
production efficiency of SOA in Fig. 4. The dashed lines represent OH
production rates due to the Fenton reaction from H2O2 with typical
dissolved iron concentrations (Fe2+ : Fe3+= 1 : 1) of 0.1
and 2.5 µM. (b) The OH production rate in lung lining
fluid by SOA decomposition as a function of ambient SOA concentrations, and by
the classical Fenton reaction as a function of H2O2 concentrations
with typical dissolved iron concentrations
(Fe2+ : Fe3+= 1 : 1) of 100 and 1 nM. The purple shaded
area represents patients with respiratory disease exhibiting high
H2O2 concentrations in the bronchoalveolar lavage (Corradi et al.,
2008).
Implications
The implications of this finding are illustrated in Figs. 7 and 8. The orange
area in Fig. 7a shows OH production rate by Fenton reactions between
Fe2+ and H2O2 forming OH radicals as a function of
H2O2 concentration with typical dissolved iron concentrations in
cloud droplets of 0.1–2.5 µM (Deguillaume et al., 2005). The green
area shows the OH production rate by SOA decomposition in cloud or fog
droplets, which ranges of ∼ 0.01–100 nM s-1 depending on SOA
precursors and the Fe2+ and SOA concentrations (see Supplement). It
clearly shows that SOA decomposition is comparably important to the Fenton
reaction in most conditions and that SOA can be the main source of OH radicals at
low concentrations of H2O2 and Fe2+. Water-soluble gases such
as aldehydes taken up by deliquesced particles may undergo reactions in the
presence of OH radicals to form low-volatility products, including organic
acids, peroxides, peroxyhemiacetals and oligomers (Lim et al., 2010; Ervens
et al., 2011; Liu et al., 2012; Ervens, 2015; Lim and Turpin, 2015; McNeill,
2015). Thus, the formed OH radicals would promote chemical aging of SOA
especially in the presence of iron ions (e.g. SOA-coated mineral dust
particles) (Chu et al., 2014) and may also induce aqueous-phase oxidation of
sulfur dioxide forming sulfuric acid (Harris et al., 2013).
Implications of OH formation by SOA. Formation of OH radicals upon
decomposition of organic hydroperoxides (ROOH) in secondary organic aerosol
leads to rapid chemical aging of SOA particles upon deliquescence and cloud
or fog processing in the atmosphere as well as oxidative stress upon
inhalation and deposition in the human respiratory tract. Mixing and
Fenton-like reactions of iron with ROOH from SOA can occur both in
atmospheric particles and in the lung lining fluid.
Recent studies have shown that OH radicals can trigger autoxidation reactions
in the gas phase, generating highly oxidized and extremely low volatility
compounds (Crounse et al., 2013; Ehn et al., 2014). In addition, it has been
shown that some radicals can be long-lived in the condensed phase (Shiraiwa
et al., 2011b; Gehling and Dellinger, 2013) by interacting with transition
metals (Truong et al., 2010). We hypothesize that OH radicals formed from SOA
decomposition could also trigger autoxidation in the condensed phase. Such a
self-amplification cycle of SOA formation and aging may be relevant for
example in the Amazon, where cloud and fog processing are important pathways
forming a high fraction of SOA with high O : C ratio, resulting in an
enhancement of cloud condensation nuclei activity of particles (Pöschl et
al., 2010; Pöhlker et al., 2012). Organic peroxides are often used as the
agent of the vulcanization processes to initiate the radical polymerization
by forming free radicals, which abstract hydrogen atoms from the elastomer
molecules converting them into radicals that undergo oligomerization to form
elastic polymer or rubber. Similar processes might also occur in SOA
particles (“SOA vulcanization”), which may contribute to formation of
dimers and oligomers observed in SOA particles (Kalberer et al., 2004)
possibly leading to the occurrence of an amorphous solid state (Virtanen et
al., 2010; Koop et al., 2011; Shiraiwa et al., 2011a; Renbaum-Wolff et al.,
2013; Kidd et al., 2014).
In indoor air, terpenes are commonly found at higher concentrations than in
the ambient air due to their widespread use as solvents and odorants in
cleaning products and air fresheners (Weschler, 2011). Depending on precursor
concentrations, the SOA concentration in indoor air can reach up to
30 µg m-3 with the highest contribution from limonene SOA
(Waring, 2014). To evaluate potential adverse health effects by SOA
deposition into the lungs, we estimated the OH production rate by SOA within
the lung lining fluid (LLF) as a function of ambient SOA concentration
considering breathing and deposition rates (see Supplement) (Fig. 7b). The pH
of lung lining fluid for healthy people is about 7.4. Our recent experiments
have shown that the formation of OH radicals was increased by
∼ 20 % at a pH of 7.4 in a phosphate-buffered saline solution.
Thus, the OH production rate by SOA decomposition shown in Fig. 7b may
represent the lower limit. We intend to investigate pH effects on OH
formation in detail in follow-up studies.
Figure 7b also shows the OH production rate by the Fenton reaction with
typical iron (Gutteridge et al., 1996) and H2O2 concentrations in
the LLF (Corradi et al., 2008). Patients with respiratory diseases are
reported to have high H2O2 concentrations in the bronchoalveolar
lavage (Corradi et al., 2008) (as shown in shaded purple area), and the Fenton
reaction may be the main source of OH radicals for such patients. However,
for healthy people with low H2O2 and Fe2+ concentrations, SOA
decomposition can be more important than the Fenton process under high
ambient or indoor SOA concentrations. Excess concentrations of reactive
oxygen species including hydrogen peroxide, OH radicals (and potentially also
organic radicals) are shown to cause oxidative stress to human lung
fibroblasts, alveolar cells and tissues (Pöschl and Shiraiwa, 2015).
Thus, in polluted indoor or urban megacities with high SOA concentration such
as in Beijing, SOA particles may play a critical role in adverse aerosol
health effects.
The Supplement related to this article is available online at doi:10.5194/acp-16-1761-2016-supplement.
Acknowledgements
This work was funded by the Max Planck Society. C. J. Kampf acknowledges financial support by the German Research Foundation (DFG project KA 4008/1-1).The article processing charges for this open-access
publication were covered by the Max Planck
Society. Edited by: F. Keutsch
References
Anglada, J. M., Martins-Costa, M., Ruiz-López, M. F., and Francisco, J.
S.: Spectroscopic signatures of ozone at the air–water interface and
photochemistry implications, P. Natl. Acad. Sci. USA, 111, 11618–11623,
2014.
Arakaki, T., Anastasio, C., Kuroki, Y., Nakajima, H., Okada, K., Kotani, Y.,
Handa, D., Azechi, S., Kimura, T., and Tsuhako, A.: A general scavenging rate
constant for reaction of hydroxyl radical with organic carbon in atmospheric
waters, Environ. Sci. Technol., 47, 8196–8203, 2013.
Arangio, A. M., Slade, J. H., Berkemeier, T., Pöschl, U., Knopf, D. A.,
and Shiraiwa, M.: Multiphase Chemical Kinetics of OH Radical Uptake by
Molecular Organic Markers of Biomass Burning Aerosols: Humidity and
Temperature Dependence, Surface Reaction, and Bulk Diffusion, J. Phys. Chem.
A, 119, 4533–4544, 2015.Badali, K. M., Zhou, S., Aljawhary, D., Antiñolo, M., Chen, W. J., Lok,
A., Mungall, E., Wong, J. P. S., Zhao, R., and Abbatt, J. P. D.: Formation of
hydroxyl radicals from photolysis of secondary organic aerosol material,
Atmos. Chem. Phys., 15, 7831–7840, 10.5194/acp-15-7831-2015, 2015.Berkemeier, T., Huisman, A. J., Ammann, M., Shiraiwa, M., Koop, T., and
Pöschl, U.: Kinetic regimes and limiting cases of gas uptake and
heterogeneous reactions in atmospheric aerosols and clouds: a general
classification scheme, Atmos. Chem. Phys., 13, 6663–6686,
10.5194/acp-13-6663-2013, 2013.Bruns, E. A., El Haddad, I., Keller, A., Klein, F., Kumar, N. K., Pieber, S.
M., Corbin, J. C., Slowik, J. G., Brune, W. H., Baltensperger, U., and
Prévôt, A. S. H.: Inter-comparison of laboratory smog chamber and
flow reactor systems on organic aerosol yield and composition, Atmos. Meas.
Tech., 8, 2315–2332, 10.5194/amt-8-2315-2015, 2015.
Chen, X. and Hopke, P.: A chamber study of secondary organic aerosol
formation by limonene ozonolysis, Indoor air, 20, 320–328, 2010.
Chen, X., Hopke, P. K., and Carter, W. P.: Secondary organic aerosol from
ozonolysis of biogenic volatile organic compounds: chamber studies of
particle and reactive oxygen species formation, Environ. Sci. Technol., 45,
276–282, 2010.
Chen, X., Hopke, P. K., and Carter, W. P. L.: Secondary Organic Aerosol from
Ozonolysis of Biogenic Volatile Organic Compounds: Chamber Studies of
Particle and Reactive Oxygen Species Formation, Environ. Sci. Technol., 45,
276–282, 2011.Chu, B., Liu, Y., Li, J., Takekawa, H., Liggio, J., Li, S.-M., Jiang, J.,
Hao, J., and He, H.: Decreasing effect and mechanism of FeSO4 seed
particles on secondary organic aerosol in α-pinene photooxidation,
Environ. Pollut., 193, 88–93, 2014.
Corradi, M., Pignatti, P., Brunetti, G., Goldoni, M., Caglieri, A., Nava, S.,
Moscato, G., and Balbi, B.: Comparison between exhaled and bronchoalveolar
lavage levels of hydrogen peroxide in patients with diffuse interstitial lung
diseases, Acta Biomed., 79, 73–78, 2008.
Crounse, J. D., Nielsen, L. B., Jørgensen, S., Kjaergaard, H. G., and
Wennberg, P. O.: Autoxidation of organic compounds in the atmosphere, J.
Phys. Chem. Lett., 4, 3513–3520, 2013.
Deguillaume, L., Leriche, M., Desboeufs, K., Mailhot, G., George, C., and
Chaumerliac, N.: Transition Metals in Atmospheric Liquid Phases: Sources,
Reactivity, and Sensitive Parameters, Chem. Rev., 105, 3388–3431, 2005.Docherty, K. S., Wu, W., Lim, Y. B., and Ziemann, P. J.: Contributions of
organic peroxides to secondary aerosol formed from reactions of monoterpenes
with O3, Environ. Sci. Technol., 39, 4049–4059, 2005.
Donahue, N. M., Henry, K. M., Mentel, T. F., Kiendler-Scharr, A., Spindler,
C., Bohn, B., Brauers, T., Dorn, H. P., Fuchs, H., Tillmann, R., Wahner, A.,
Saathoff, H., Naumann, K.-H., Möhler, O., Leisner, T., Müller, L.,
Reinnig, M.-C., Hoffmann, T., Salo, K., Hallquist, M., Frosch, M., Bilde, M.,
Tritscher, T., Barmet, P., Praplan, A. P., DeCarlo, P. F., Dommen, J.,
Prévôt, A. S. H., and Baltensperger, U.: Aging of biogenic secondary
organic aerosol via gas-phase OH radical reactions, P. Natl. Acad. Sci. USA,
109, 13503–13508, 2012.
Ehn, M., Thornton, J. A., Kleist, E., Sipila, M., Junninen, H., Pullinen, I.,
Springer, M., Rubach, F., Tillmann, R., Lee, B., Lopez-Hilfiker, F., Andres,
S., Acir, I.-H., Rissanen, M., Jokinen, T., Schobesberger, S., Kangasluoma,
J., Kontkanen, J., Nieminen, T., Kurten, T., Nielsen, L. B., Jorgensen, S.,
Kjaergaard, H. G., Canagaratna, M., Dal Maso, M., Berndt, T., Petaja, T.,
Wahner, A., Kerminen, V.-M., Kulmala, M., Worsnop, D. R., Wildt, J., and
Mentel, T. F.: A large source of low-volatility secondary organic aerosol,
Nature, 506, 476–479, 2014.Epstein, S. A., Blair, S. L., and Nizkorodov, S. A.: Direct photolysis of
α-pinene ozonolysis secondary organic aerosol: effect on particle
mass and peroxide content, Environ. Sci. Technol., 48, 11251–11258, 2014.
Ervens, B.: Modeling the Processing of Aerosol and Trace Gases in Clouds and
Fogs, Chem. Rev., 115, 4157–4198, 2015.Ervens, B., Turpin, B. J., and Weber, R. J.: Secondary organic aerosol
formation in cloud droplets and aqueous particles (aqSOA): a review of
laboratory, field and model studies, Atmos. Chem. Phys., 11, 11069–11102,
10.5194/acp-11-11069-2011, 2011.
Faber, P., Drewnick, F., Veres, P. R., Williams, J., and Borrmann, S.:
Anthropogenic sources of aerosol particles in a football stadium: Real-time
characterization of emissions from cigarette smoking, cooking, hand flares,
and color smoke bombs by high-resolution aerosol mass spectrometry, Atmos.
Environ., 77, 1043–1051, 2013.Foster, T. L. and Caradonna, J. P.: Fe2+-catalyzed heterolytic RO-OH
bond cleavage and substrate oxidation: A functional synthetic non-heme iron
monooxygenase system, J. Am. Chem. Soc., 125, 3678–3679, 2003.
Fuller, S., Wragg, F., Nutter, J., and Kalberer, M.: Comparison of on-line
and off-line methods to quantify reactive oxygen species (ROS) in atmospheric
aerosols, Atmos. Environ., 92, 97–103, 2014.Gehling, W. and Dellinger, B.: Environmentally Persistent Free Radicals and
Their Lifetimes in PM2.5, Environ. Sci. Technol., 47, 8172–8178, 2013.
Goldstein, S. and Meyerstein, D.: Comments on the mechanism of the
“Fenton-like” reaction, Acc. Chem. Res., 32, 547–550, 1999.
Gutteridge, J. M. C., Mumby, S., Quinlan, G. J., Chung, K. F., and Evans, T.
W.: Pro-oxidant iron is present in human pulmonary epithelial lining fluid:
implications for oxidative stress in the lung, Biochem. Biophys. Res.
Commun., 220, 1024–1027, 1996.Hallquist, M., Wenger, J. C., Baltensperger, U., Rudich, Y., Simpson, D.,
Claeys, M., Dommen, J., Donahue, N. M., George, C., Goldstein, A. H.,
Hamilton, J. F., Herrmann, H., Hoffmann, T., Iinuma, Y., Jang, M., Jenkin, M.
E., Jimenez, J. L., Kiendler-Scharr, A., Maenhaut, W., McFiggans, G., Mentel,
Th. F., Monod, A., Prévôt, A. S. H., Seinfeld, J. H., Surratt, J. D.,
Szmigielski, R., and Wildt, J.: The formation, properties and impact of
secondary organic aerosol: current and emerging issues, Atmos. Chem. Phys.,
9, 5155–5236, 10.5194/acp-9-5155-2009, 2009.Harris, E., Sinha, B., van Pinxteren, D., Tilgner, A., Fomba, K. W.,
Schneider, J., Roth, A., Gnauk, T., Fahlbusch, B., Mertes, S., Lee, T.,
Collett, J., Foley, S., Borrmann, S., Hoppe, P., and Herrmann, H.: Enhanced
Role of Transition Metal Ion Catalysis During In-Cloud Oxidation of SO2,
Science, 340, 727–730, 2013.
Hellpointner, E. and Gäb, S.: Detection of methyl, hydroxymethyl and
hydroxyethyl hydroperoxides in air and precipitation, Nature, 337, 631–634,
1989.
Herrmann, H., Tilgner, A., Barzaghi, P., Majdik, Z., Gligorovski, S.,
Poulain, L., and Monod, A.: Towards a more detailed description of
tropospheric aqueous phase organic chemistry: CAPRAM 3.0, Atmos. Environ.,
39, 4351–4363, 2005.
Huang, R.-J., Zhang, Y., Bozzetti, C., Ho, K.-F., Cao, J.-J., Han, Y.,
Daellenbach, K. R., Slowik, J. G., Platt, S. M., Canonaco, F., Zotter, P.,
Wolf, R., Pieber, S. M., Bruns, E. A., Crippa, M., Ciarelli, G., Piazzalunga,
A., Schwikowski, M., Abbaszade, G., Schnelle-Kreis, J., Zimmermann, R., An,
Z., Szidat, S., Baltensperger, U., Haddad, I. E., and Prevot, A. S. H.: High
secondary aerosol contribution to particulate pollution during haze events in
China, Nature, 514, 218–222, 2014.
Jacob, D. J.: Chemistry of OH in remote clouds and its role in the production
of formic acid and peroxymonosulfate, J. Geophys. Res.-Atmos., 91,
9807–9826, 1986.
Jimenez, J. L., Canagaratna, M. R., Donahue, N. M., Prevot, A. S. H., Zhang,
Q., Kroll, J. H., DeCarlo, P. F., Allan, J. D., Coe, H., Ng, N. L., Aiken, A.
C., Docherty, K. S., Ulbrich, I. M., Grieshop, A. P., Robinson, A. L.,
Duplissy, J., Smith, J. D., Wilson, K. R., Lanz, V. A., Hueglin, C., Sun, Y.
L., Tian, J., Laaksonen, A., Raatikainen, T., Rautiainen, J., Vaattovaara,
P., Ehn, M., Kulmala, M., Tomlinson, J. M., Collins, D. R., Cubison, M. J.,
Dunlea, E. J., Huffman, J. A., Onasch, T. B., Alfarra, M. R., Williams, P.
I., Bower, K., Kondo, Y., Schneider, J., Drewnick, F., Borrmann, S., Weimer,
S., Demerjian, K., Salcedo, D., Cottrell, L., Griffin, R., Takami, A.,
Miyoshi, T., Hatakeyama, S., Shimono, A., Sun, J. Y., Zhang, Y. M., Dzepina,
K., Kimmel, J. R., Sueper, D., Jayne, J. T., Herndon, S. C., Trimborn, A. M.,
Williams, L. R., Wood, E. C., Middlebrook, A. M., Kolb, C. E., Baltensperger,
U., and Worsnop, D. R.: Evolution of organic aerosols in the atmosphere,
Science, 326, 1525–1529, 2009.
Jokinen, T., Berndt, T., Makkonen, R., Kerminen, V.-M., Junninen, H.,
Paasonen, P., Stratmann, F., Herrmann, H., Guenther, A. B., Worsnop, D. R.,
Kulmala, M., Ehn, M., and Sipilä, M.: Production of extremely low
volatile organic compounds from biogenic emissions: Measured yields and
atmospheric implications, P. Natl. Acad. Sci. USA, 112, 7123–7128, 2015.
Kalberer, M., Paulsen, D., Sax, M., Steinbacher, M., Dommen, J., Prevot, A.,
Fisseha, R., Weingartner, E., Frankevich, V., and Zenobi, R.: Identification
of polymers as major components of atmospheric organic aerosols, Science,
303, 1659–1662, 2004.Kanakidou, M., Seinfeld, J. H., Pandis, S. N., Barnes, I., Dentener, F. J.,
Facchini, M. C., Van Dingenen, R., Ervens, B., Nenes, A., Nielsen, C. J.,
Swietlicki, E., Putaud, J. P., Balkanski, Y., Fuzzi, S., Horth, J., Moortgat,
G. K., Winterhalter, R., Myhre, C. E. L., Tsigaridis, K., Vignati, E.,
Stephanou, E. G., and Wilson, J.: Organic aerosol and global climate
modelling: a review, Atmos. Chem. Phys., 5, 1053–1123,
10.5194/acp-5-1053-2005, 2005.Kang, E., Root, M. J., Toohey, D. W., and Brune, W. H.: Introducing the
concept of Potential Aerosol Mass (PAM), Atmos. Chem. Phys., 7, 5727–5744,
10.5194/acp-7-5727-2007, 2007.
Kautzman, K. E., Surratt, J. D., Chan, M. N., Chan, A. W. H., Hersey, S. P.,
Chhabra, P. S., Dalleska, N. F., Wennberg, P. O., Flagan, R. C., and
Seinfeld, J. H.: Chemical Composition of Gas- and Aerosol-Phase Products from
the Photooxidation of Naphthalene, J. Phys. Chem. A, 114, 913–934, 2010.
Kidd, C., Perraud, V., Wingen, L. M., and Finlayson-Pitts, B. J.: Integrating
phase and composition of secondary organic aerosol from the ozonolysis of
alpha-pinene, P. Natl. Acad. Sci. USA, 111, 7552–7557, 2014.
Koop, T., Bookhold, J., Shiraiwa, M., and Pöschl, U.: Glass transition
and phase state of organic compounds: dependency on molecular properties and
implications for secondary organic aerosols in the atmosphere, Phys. Chem.
Chem. Phys., 13, 19238–19255, 2011.Lambe, A. T., Ahern, A. T., Williams, L. R., Slowik, J. G., Wong, J. P. S.,
Abbatt, J. P. D., Brune, W. H., Ng, N. L., Wright, J. P., Croasdale, D. R.,
Worsnop, D. R., Davidovits, P., and Onasch, T. B.: Characterization of
aerosol photooxidation flow reactors: heterogeneous oxidation, secondary
organic aerosol formation and cloud condensation nuclei activity
measurements, Atmos. Meas. Tech., 4, 445–461, 10.5194/amt-4-445-2011,
2011.Lambe, A. T., Chhabra, P. S., Onasch, T. B., Brune, W. H., Hunter, J. F.,
Kroll, J. H., Cummings, M. J., Brogan, J. F., Parmar, Y., Worsnop, D. R.,
Kolb, C. E., and Davidovits, P.: Effect of oxidant concentration, exposure
time, and seed particles on secondary organic aerosol chemical composition
and yield, Atmos. Chem. Phys., 15, 3063–3075, 10.5194/acp-15-3063-2015,
2015.Lim, Y. B. and Turpin, B. J.: Laboratory evidence of organic peroxide and
peroxyhemiacetal formation in the aqueous phase and implications for aqueous
OH, Atmos. Chem. Phys., 15, 12867–12877, 10.5194/acp-15-12867-2015,
2015.Lim, Y. B., Tan, Y., Perri, M. J., Seitzinger, S. P., and Turpin, B. J.:
Aqueous chemistry and its role in secondary organic aerosol (SOA) formation,
Atmos. Chem. Phys., 10, 10521–10539, 10.5194/acp-10-10521-2010, 2010.Liu, Y., Monod, A., Tritscher, T., Praplan, A. P., DeCarlo, P. F.,
Temime-Roussel, B., Quivet, E., Marchand, N., Dommen, J., and Baltensperger,
U.: Aqueous phase processing of secondary organic aerosol from isoprene
photooxidation, Atmos. Chem. Phys., 12, 5879–5895,
10.5194/acp-12-5879-2012, 2012.
McNeill, V. F.: Aqueous organic chemistry in the atmosphere: Sources and
chemical processing of organic aerosols, Environ. Sci. Technol., 49,
1237–1244, 2015.Mentel, T. F., Springer, M., Ehn, M., Kleist, E., Pullinen, I., Kurtén, T.,
Rissanen, M., Wahner, A., and Wildt, J.: Formation of highly oxidized
multifunctional compounds: autoxidation of peroxy radicals formed in the
ozonolysis of alkenes – deduced from structure–product relationships, Atmos.
Chem. Phys., 15, 6745–6765, 10.5194/acp-15-6745-2015, 2015.
Nam, W., Han, H. J., Oh, S.-Y., Lee, Y. J., Choi, M.-H., Han, S.-Y., Kim, C.,
Woo, S. K., and Shin, W.: New insights into the mechanisms of OO bond
cleavage of hydrogen peroxide and tert-alkyl hydroperoxides by iron (III)
porphyrin complexes, J. Am. Chem. Soc., 122, 8677–8684, 2000.Ortega, A. M., Hayes, P. L., Peng, Z., Palm, B. B., Hu, W., Day, D. A., Li,
R., Cubison, M. J., Brune, W. H., Graus, M., Warneke, C., Gilman, J. B.,
Kuster, W. C., de Gouw, J. A., and Jimenez, J. L.: Real-time measurements of
secondary organic aerosol formation and aging from ambient air in an
oxidation flow reactor in the Los Angeles area, Atmos. Chem. Phys. Discuss.,
15, 21907–21958, 10.5194/acpd-15-21907-2015, 2015.Pavlovic, J. and Hopke, P. K.: Detection of radical species formed by the
ozonolysis of α-pinene, J. Atmos. Chem., 66, 137–155, 2010.
Pöhlker, C., Wiedemann, K. T., Sinha, B., Shiraiwa, M., Gunthe, S. S.,
Smith, M., Su, H., Artaxo, P., Chen, Q., Cheng, Y., Elbert, W., Gilles, M.
K., Kilcoyne, A. L. D., Moffet, R. C., Weigand, M., Martin, S. T.,
Pöschl, U., and Andreae, M. O.: Biogenic potassium salt particles as
seeds for secondary organic aerosol in the Amazon, Science, 337, 1075–1078,
2012.
Pöschl, U. and Shiraiwa, M.: Multiphase Chemistry at the
Atmosphere–Biosphere Interface Influencing Climate and Public Health in the
Anthropocene, Chem. Rev., 115, 4440–4475, 2015.
Pöschl, U., Martin, S. T., Sinha, B., Chen, Q., Gunthe, S. S., Huffman,
J. A., Borrmann, S., Farmer, D. K., Garland, R. M., Helas, G., Jimenez, J.
L., King, S. M., Manzi, A., Mikhailov, E., Pauliquevis, T., Petters, M. D.,
Prenni, A. J., Roldin, P., Rose, D., Schneider, J., Su, H., Zorn, S. R.,
Artaxo, P., and Andreae, M. O.: Rainforest Aerosols as Biogenic Nuclei of
Clouds and Precipitation in the Amazon, Science, 329, 1513–1516, 2010.Renbaum-Wolff, L., Grayson, J. W., Bateman, A. P., Kuwata, K., Sellier, M.,
Murray, B. J., Schilling, J. E., Martin, S. T., and Bertram, A. K.: Viscosity
of α-pinene secondary organic material and implications for particle
growth and reactivity, P. Natl. Acad. Sci. USA, 110, 8014–8019, 2013.
Shiraiwa, M., Ammann, M., Koop, T., and Pöschl, U.: Gas uptake and
chemical aging of semisolid organic aerosol particles, P. Natl. Acad. Sci.
USA, 108, 11003–11008, 2011a.
Shiraiwa, M., Sosedova, Y., Rouvière, A., Yang, H., Zhang, Y., Abbatt, J.
P., Ammann, M., and Pöschl, U.: The role of long-lived reactive oxygen
intermediates in the reaction of ozone with aerosol particles, Nat. Chem., 3,
291–295, 2011b.
Shiraiwa, M., Yee, L. D., Schilling, K. A., Loza, C. L., Craven, J. S.,
Zuend, A., Ziemann, P. J., and Seinfeld, J. H.: Size distribution dynamics
reveal particle-phase chemistry in organic aerosol formation, P. Natl. Acad.
Sci. USA, 110, 11746–11750, 2013.
Solomon, S.: Climate change 2007-the physical science basis: Working group I
contribution to the fourth assessment report of the IPCC, Cambridge
University Press, Cambridge, 2007.
Stoll, S. and Schweiger, A.: EasySpin, a comprehensive software package for
spectral simulation and analysis in EPR, J. Magn. Reson., 178, 42–55, 2006.
Truong, H., Lomnicki, S., and Dellinger, B.: Potential for misidentification
of environmentally persistent free radicals as molecular pollutants in
particulate matter, Environ. Sci. Technol., 44, 1933–1939, 2010.
Venkatachari, P. and Hopke, P. K.: Development and evaluation of a
particle-bound reactive oxygen species generator, J. Aerosol. Sci., 39,
168–174, 2008.
Virtanen, A., Joutsensaari, J., Koop, T., Kannosto, J., YliPirilä, P.,
Leskinen, J., Mäkelä, J. M., Holopainen, J. K., Pöschl, U.,
Kulmala, M., Worsnop, D. R., and Laaksonen, A.: An amorphous solid state of
biogenic secondary organic aerosol particles, Nature, 467, 824–827, 2010.
Wang, Y., Kim, H., and Paulson, S. E.: Hydrogen peroxide generation from
alpha- and beta-pinene and toluene secondary organic aerosols, Atmos.
Environ., 45, 3149–3156, 2011.
Waring, M. S.: Secondary organic aerosol in residences: predicting its
fraction of fine particle mass and determinants of formation strength, Indoor
Air, 24, 376–389, 2014.
Weber, R. T.: Xenon Data Processing Reference, Bruker BioSpin Corporation: Billerica, MA USA, chapter 14,
1–24, 2012.
Weschler, C. J.: Chemistry in indoor environments: 20 years of research,
Indoor Air, 21, 205–218, 2011.Yamazaki, I. and Piette, L. H.: ESR spin-trapping studies on the reaction of
Fe2+ ions with H2O2-reactive species in oxygen toxicity in
biology, J. Biol. Chem., 265, 13589–13594, 1990.
Zhao, H., Joseph, J., Zhang, H., Karoui, H., and Kalyanaraman, B.: Synthesis
and biochemical applications of a solid cyclic nitrone spin trap: a
relatively superior trap for detecting superoxide anions and glutathiyl
radicals, Free Radic. Biol. Med., 31, 599–606, 2001.
Zhu, B.-Z., Shan, G.-Q., Huang, C.-H., Kalyanaraman, B., Mao, L., and Du,
Y.-G.: Metal-independent decomposition of hydroperoxides by halogenated
quinones: Detection and identification of a quinone ketoxy radical, P. Natl.
Acad. Sci. USA, 106, 11466–11471, 2009.