Introduction
Atmospheric fine particulate matter (PM2.5, aerosol with aerodynamic
diameter ≤ 2.5 µm) can scatter and/or absorb solar and terrestrial
radiation as well as influence cloud formation and as a result, can markedly
affect regional and global climate (IPCC, 2013). It is now also established
that exposure to PM2.5 can have an adverse impact on human health
(Dockery et al., 1993; Mauderly and Chow, 2008; Hsu et al., 2011).
Organic matter (OM) comprises the largest mass fraction of PM2.5 and is
derived largely from secondary organic aerosol (SOA) formed through
atmospheric oxidation of volatile organic compounds (VOCs). SOA formation
has been modeled primarily within the framework of absorptive
gas-to-particle partitioning (Pankow, 1994; Odum et al., 1996),
with the products of volatile and semi-volatile organic precursors
decreasing in volatility during multi-generational oxidation, and condensing
onto pre-existing particles or creating new particles through nucleation.
Recent work has demonstrated the importance of heterogeneous (or
particle-phase) chemistry in SOA formation (Jang et al., 2002; Kalberer
et al., 2004; Tolocka et al., 2004; Gao et al., 2004; Surratt et al., 2006);
however, chemical transport models are only just beginning to incorporate
explicit details of this chemistry for specific SOA precursors (Pye et
al., 2013; Karambelas et al., 2014). Although much progress has been made in
recent years in identifying key biogenic and anthropogenic SOA precursors,
significant gaps still remain in our knowledge of the formation mechanisms,
composition and properties of SOA (Hallquist et al., 2009).
Isoprene (2-methyl-1,3-butadiene, C5H8) is the most abundant
non-methane VOC emitted into Earth's atmosphere at ∼ 600 Tg yr-1
(Guenther et al., 2006). The southeastern US during summer
is a particularly strong source of isoprene, primarily through emissions by
broad-leaf trees. Although isoprene is known to influence urban ozone
(O3) formation in the southeastern US, only in the last decade
has hydroxyl radical (OH)-initiated oxidation been recognized as leading to
significant SOA formation, enhanced by the presence of anthropogenic
pollutants such as nitrogen oxides (NOx= NO + NO2) and sulfur
dioxide (SO2) (Claeys et al., 2004; Edney et al., 2005; Surratt
et al., 2006, 2010; Kroll et al., 2006). Previously, the
volatility of the photochemical oxidation products had been assumed to
preclude formation of PM2.5 from isoprene oxidation (Pandis et
al., 1991; Kamens et al., 1982).
Recent studies have made significant strides in identifying critical
intermediates in isoprene SOA formation by varying the levels of NOx
(Kroll et al., 2006; Surratt et al., 2006, 2010) and
acidity of sulfate aerosol (Surratt et al., 2006; Surratt et al., 2010;
Lin et al., 2012, 2013a). The proposed role of isomeric isoprene
epoxydiols (IEPOX) as key intermediates in the formation of isoprene SOA
under low-nitric oxide (NO) conditions (Surratt et al., 2010; Paulot et
al., 2009) has recently been confirmed in studies using authentic compounds
(Lin et al., 2012; Gaston et al., 2014; Nguyen et al., 2014). Under
high-NO conditions, isoprene SOA has been demonstrated to form primarily via
oxidation of methacrolein (MACR) (Surratt et al., 2006) and methacryloylperoxynitrate (MPAN) (Surratt et al., 2010) with methacrylic acid epoxide
(MAE) (Lin et al., 2013b) and hydroxymethyl-methyl-α-lactone (HMML)
(Nguyen et al., 2015) from the further oxidation of MPAN demonstrated as
reactive intermediates. Under both high- and low-NO conditions,
acid-catalyzed reactive uptake and multiphase chemistry of isoprene-derived
epoxides (IEPOX and MAE) as well as aqueous reactions of MACR and methyl
vinyl ketone (MVK) with sulfate radical anion are now known to enhance SOA
formation from isoprene (Surratt et al., 2007b, 2010;
Lin et al., 2013b; Schindelka et al., 2013). Recent flow-tube studies on
reactive uptake kinetics of trans-β-IEPOX (Gaston et al., 2014), the
predominant IEPOX isomer formed in the photochemical oxidation of isoprene
(Bates et al., 2014), have estimated an atmospheric lifetime shorter than
5 h in the presence of highly acidic aqueous aerosol (pH ≤ 1). Since
the predicted atmospheric lifetime of IEPOX for gas-phase oxidation is 8–33 h
at an average OH concentration of 106 molecules cm-3
(Jacobs et al., 2013; Bates et al., 2014) and 11 h for deposition
(Eddingsaas et al., 2010), reactive uptake of IEPOX onto highly acidic
aqueous aerosol would be a competitive or potentially dominant fate of IEPOX
in the atmosphere. Recent field data from sites across the southeastern US
collected by Guo et al. (2015) yielded estimates that aerosol pH ranges
from 0.5–2. Consistent with expectations based on the flow-tube studies
(Gaston et al., 2014; Riedel et al., 2015) and pH estimates from field
data, IEPOX-derived SOA has been observed to account for up to 33 % of the
total fine organic aerosol (OA) mass collected during summer in downtown
Atlanta, GA, by analysis of data acquired on an online Aerodyne Aerosol
Chemical Speciation Monitor (ACSM) (Budisulistiorini et al., 2013). Similar
level of isoprene-derived SOA has been recently observed at other field
sites across the southeastern US using the Aerodyne high-resolution
time-of-flight aerosol mass spectrometer (HR-ToF-AMS) (Xu et al., 2015). In
offline chemical analysis of total fine OA mass at a rural site located in
Yorkville, GA (Lin et al., 2013b), up to 20 % of the OA mass could be
attributed to the known IEPOX-derived SOA tracers, including the
2-methyltetrols (Claeys et al., 2004; Lin et al., 2012), C5-alkene
triols (Wang et al., 2005; Lin et al., 2012), cis- and
trans-3-methyltetrahydrofuran-3,4-diols (Lin et al., 2012; Zhang et al., 2012b)
and IEPOX-derived organosulfates (Surratt et al., 2007a,
2010; Lin et al., 2012).
In addition to examining the effects of NO and aerosol acidity on isoprene
SOA formation, the effect of varying relative humidity (RH) has recently
been examined. In chamber studies on the high-NO pathway under low-RH
conditions, the isoprene SOA constituents 2-methylglyceric acid and
corresponding oligoesters derived from MACR and its associated SOA
precursors (i.e., HMML and MAE) were influenced by RH conditions
(Zhang et al., 2011; Nguyen et al., 2011, 2015).
However, 2-methyltetrols, which are known to be major SOA constituents
formed in the low-NO pathway and minor constituents in the high-NO pathway
(Edney et al., 2005; Surratt et al., 2007b), did not vary significantly
with RH (Zhang et al., 2011). While RH appears to have an effect on the
formation of certain isoprene SOA constituents, recent flow-tube studies
demonstrated that aerosol acidity has a more pronounced effect on IEPOX- and
MAE-derived SOA formation than RH (Gaston et al., 2014; Riedel et al.,
2015). However, field studies have yielded mixed results. At Yorkville, GA,
Lin et al. (2013a) observed no strong correlation of IEPOX-derived SOA
with aerosol acidity, NH3 levels or liquid water content (LWC),
although there was a statistically significant enhancement of IEPOX-derived
SOA under high SO2-sampling scenarios. Similarly, no correlation
between isoprene SOA tracers and aerosol pH or LWC was observed in the
analysis of filter samples collected from field studies in Sacramento, CA,
and Carson City, NV (Worton et al., 2013), and in the isoprene-derived PMF
factor from field study in Centerville, AL (Xu et al., 2015). Another
recent field study by Budisulistiorini et al. (2013) found weak correlation
(r2= 0.22) between aerosol pH and an IEPOX-OA factor resolved by
positive matrix factorization (PMF) from real-time organic aerosol mass
spectra data acquired on an Aerodyne ACSM.
Although isoprene is now recognized as a major source of SOA, the exact
manner in which isoprene-derived SOA is formed in the southeastern US and
how it is affected by anthropogenic pollutants (i.e., NOx level,
aerosol acidity, sulfate and primary aerosol loadings) remains unclear. The
gap in understanding has major public health and policy implications since
isoprene is emitted primarily from terrestrial vegetation and is not
controllable; whereas strategies to control anthropogenic pollutants can be
implemented. Improving our fundamental understanding of the role of
anthropogenic emissions in isoprene SOA formation will be key in improving
existing air quality models, especially in the southeastern US where
models currently under-predict isoprene SOA formation (Foley et al.,
2010; Carlton et al., 2010) and as a result will be critical to developing
efficient control strategies for improving air quality. The study presented
here is part of the 2013 Southeast Oxidant and Aerosol Study (SOAS) spanning
1 June to 17 July 2013 at the Look Rock (LRK), TN ground site (maps are
provided in the Supplement). A major aim of SOAS was to
address the issue of how exactly isoprene SOA formation occurs and the
potential of anthropogenic emissions to enhance SOA formation. At the LRK
ground site we approached this aim by examining the chemical composition of
OA measured in real-time by the Aerodyne ACSM and subsequently applying PMF
for source apportionment. We also collected PM2.5 on filters and
quantified tracers associated with isoprene chemistry to support the
assignment of OA factors resolved from factor analyses of organic mass
spectral data collected by the ACSM. We examined the potential influence of
anthropogenic emissions on isoprene-derived SOA by correlation with temporal
variation of anthropogenic markers monitored by collocated instruments.
Finally, a photochemical box model was employed to further examine the
potential interactions between SOA and anthropogenic emissions. The results
of this study will help to improve model parameterizations required to bring
model predictions closer to ambient observations of isoprene-derived SOA
formation in the southeastern US.
Methods
Site description
Fine aerosol was collected continuously from 1 June to 17 July 2013. LRK is
a ridge-top site located on the northwestern edge of the Great Smoky
Mountains National Park (GSMNP) downwind of Maryville and Knoxville and small
farms with animal grazing areas (Supplement Figs. S1–S2). Up-slope flow carries
pollutants emitted in the valley during early morning to the LRK site by
mid-morning (Tanner et al., 2005). In the evening, down-slope flow
accompanies a shift of wind direction to the south and east during summer
that isolates the site from fresh primary emissions from the valley and
allows aged-secondary species to accumulate (Tanner et al., 2005). As
described in Tanner et al. (2005), particulate sulfate, black carbon (BC),
organic carbon (OC), PM2.5 and PM10 as well as gas-phase sulfur
dioxide (SO2), nitric oxide (NO), nitrogen dioxide (NO2), and sum
of reactive and reservoir nitrogen oxides (NOy) were measured by a
suite of collocated instruments throughout the campaign (Table S1 in the Supplement).
Meteorological measurements (RH, temperature, wind direction, and wind
speed) and O3 concentrations were acquired at a National Park Service
(NPS) shelter across a secondary road opposite the LRK shelter.
ACSM NR-PM1 characterization
Fine ambient aerosol was sampled from the rooftop of the LRK site
air-conditioned building during the SOAS campaign. The sampling inlet was
approximately 6 m above the ground and equipped with a PM2.5 cyclone.
Sample was drawn at 3 L min-1 (residence time < 2 s) and dried
using a Nafion drier (PD-200T-24SS, Perma Pure) to maintain RH below 10 %
and prevent condensation during sampling. ACSM operation parameters followed
those of previous studies (Budisulistiorini et al., 2013, 2014). Briefly, the ACSM scanning rate was set at
200 ms amu-1 and data were averaged over 30 min intervals. Data were
acquired using ACSM DAQ version 1438 and analyzed using ACSM Local version
1532 (Aerodyne Research, Inc.) within Igor Pro 6.3 (WaveMetrics).
Calibrations for sampling flow rate, mass-to-charge ratio (m/z), response
factor of nitrate (RFNO3), and relative ionization efficiencies of both
ammonium (RIENH4) and sulfate (RIESO4) were performed three times
during the campaign. Mass resolution, heater bias and ionizer voltages, and
amplifier zero settings were checked and adjusted daily. A collection
efficiency (CE) of 0.5 calculated based on Middlebrook et al. (2012)
was applied to the ACSM data in order to accommodate composition-dependent
CE. Correlations of combined aerosol mass concentrations of ACSM
non-refractory (NR)-PM1 and collocated black carbon (BC) with aerosol
volume concentrations of PM1 measured by the Scanning Electrical
Mobility Spectrometer/Mixing Condensation Particle Counter (SEMS-MCPC, Brechtel
Manufacturing Inc.) was strong (r2= 0.89) and suggested an aerosol
density of 1.52 g cm-3 (Fig. S3), close to that reported in previous
studies in Pasadena, CA (Hayes et al., 2013) and Atlanta, GA
(Budisulistiorini et al., 2014). If CE of 1 is used, the estimated
aerosol density is 0.78 g cm-3, which is much lower than suggested bulk
organic and inorganic aerosol densities of 1.27 and 1.77 g cm-3, respectively (Cross et al., 2007).
OA source characterization
OA fraction acquired by the ACSM was analyzed using PMF (Paatero and Tapper,
1994) written in PMF Evaluation Tool (PET v2.4) (Ulbrich et al., 2009). In
this study, uncertainty of a selected solution was investigated with Seeds
(varied from 0 to 100, in steps of 5), 100 bootstrapping runs, and Fpeak
parameters. Details of diagnostics for each PMF analysis are given in the
Supplement (Tables S2–S3 and Figs. S4–S8). Evaluation of Q/Qexp time
series and mass spectra and correlation of factor solutions at Fpeak 0 with
collocated measurements (Figs. S4–S5, Table S3) suggests that a 3-factor
solution is optimum. We selected a 3-factor solution at Fpeak -0.09 based
on the quality of PMF fits and interpretability when compared to tracer time
series and reference mass spectra (Table S5). The mass spectrum of a factor
designated IEPOX-OA conforms closely to the IEPOX-SOA factor resolved in
Atlanta, GA (Budisulistiorini et al., 2013). The mass spectrum of the second
factor correlates closely with the factor identified as LV-OOA in previous
studies (Ulbrich et al., 2009; Ng et al., 2011). The third factor is
designated 91Fac, based on the similarity of its mass spectrum to the factor
91Fac, an oxygenated factor resolved in areas dominated by biogenic emissions
(Robinson et al., 2011; Chen et al., 2015).
Gas-phase measurements
High-resolution time-of-flight chemical ionization mass spectrometry
(HR-ToF-CIMS) measurements
Gaseous samples were measured through an approximately 1 m length of polytetrafluoroethylene (PTFE) tubing (1/4′′ outside diameter) from the sidewall of the
building at flow rate of 2 L min-1. The sampling line was placed to
face the valley such that no structures or activity would compromise
sampling. Instrument performance was maintained daily by baseline,
threshold, and single ion area tuning as well as m/z calibration. The
instrument was not operational during some periods of the field campaign
(i.e., 13 to 16 June, 21 June to 4 July, and 14 to 16 July) due to power
outage, broken components, and necessary maintenances. July HR-ToF-CIMS data
were corrected by comparing them to collocated MVK+MACR data measured by
PTR-TOF-MS (proton-transfer-reaction time-of-flight mass spectrometer) (Sect. 2.4.2) and post-campaign calibration in order to derive
a correction factor to account for decay in the micro-channel plate (MCP)
detector.
The HR-ToF-CIMS instrument was operated in the negative ion mode using
acetate ion chemistry for detection of isoprene-derived epoxides. It is
henceforth referred as acetate CIMS. The acetate ion system efficiently
detects small organic acids via deprotonation (Veres et al., 2008;
Bertram et al., 2011), such as MAE, and some vicinal diol species, such as
the IEPOX, as clusters with the reagent ion. MAE is detected as the
[C4H5O3]- ion at m/z 101, whereas IEPOX is detected as the
[CH3COO + C5H10O3]- ion at m/z 177 (Fig. S9).
IEPOX and its gas-phase precursor, hydroxyhydroperoxides (ISOPOOH), were
previously measured by CIMS with triple-quadrupole mass spectrometer that
provides tandem mass spectra, as cluster ion with CF3O- at similar
m/z and were distinguishable through their daughter ions using
collision-induced dissociation (Paulot et al., 2009). Recent field and
laboratory studies using acetate CIMS found that both ISOPOOH and IEPOX were
observed at the same cluster ion at m/z 177. In our measurements, interferences
of ISOPOOH to the cluster ion m/z 177 could not be differentiated because we
could only observe the parent ions unlike Paulot et al. (2009). Our acetate
CIMS sensitivities were measured to be relatively similar towards IEPOX and
ISOPOOH at 10-7 and 9.9 × 10-8 signal ppt-1,
respectively. However, since we operated the acetate CIMS at different
voltage settings than from D. K. Farmer, personal communication (2015),
sensitivity of the deprotonated form of IEPOX is very low, and thus it could
not be used to quantitatively measure IEPOX and/or to define the fractional
contribution of IEPOX and ISOPOOH to the m/z 177 signal. We synthesized ISOPOOH
(see Fig. S10 for nuclear magnetic resonance (NMR) data) and measured CIMS
sensitivities toward ISOPOOH and IEPOX. Results indicated that response
factors of both compounds were similar (see Fig. S11). Investigation of
lowering the IEPOX mixing ratio by a constant factor of total m/z 177 signal,
which will be reported in a future study, showed that SOA tracer model
correlations are not sensitive to this and only tracers mass loadings vary
with the IEPOX:ISOPOOH ratio. The inability to distinguish IEPOX from
ISOPOOH is a limitation in our study. Therefore, we carefully note here that
the m/z 177 ion measured during this study represents the upper limit of the
IEPOX mixing ratio due to ISOPOOH interference at an unknown fraction of the
signal.
Gaseous IEPOX and MAE were quantified with acetate CIMS by applying
laboratory-derived calibration factors. All signals were normalized to
acetate ion [CH3COO]- at m/z 59 to take into account fluctuations in
signal arising from changes in pressure during the course of field sampling
and calibration. Calibrations were performed before and after the SOAS
campaign using synthetic trans-β-IEPOX and MAE standards through dilution
in a dark 10 m3 indoor chamber at the University of North Carolina
(UNC) (Lin et al., 2012). Synthetic procedures for trans-β-IEPOX and MAE
have been described previously (Zhang et al., 2012b; Lin et al., 2013b). A
known concentration of epoxide standard was injected into a 10 mL glass
manifold using glass microliter syringes. The manifold was wrapped with
heating tape and flushed with heated N2 (g) at 5 L min-1 for at
least 2 h to the indoor chamber being sampled by the HR-ToF-CIMS until
ion signals associated with MAE and IEPOX stabilized. We assumed unit
injection efficiency of the epoxides through the glass chamber and into the
chamber in calculating the chamber epoxide mixing ratios. Subsequently, we
performed standard dilution of the acetate CIMS sample flow by using a
T piece in an N2 (g) flow controlled by eight different
micro-orifices to obtain an eight-point calibration curve. During the course
of the calibration experiments, we accounted for the fact that we would lose
14 and 3 % of IEPOX and MAE, respectively. Wall loss rates for IEPOX and
MAE have been measured in the chamber and are 5.91 × 10-5 s-1
and 1.12 × 10-5 s-1, respectively (Riedel et
al., 2015). The chamber was sampled continuously at 2 L min-1 for
measurement of gaseous products by acetate CIMS and at 0.36 L min-1 for
aerosol measurements by SEMS-MCPC to ensure that the chamber was particle
free. Additionally, no particle nucleation events or significant particle
loadings were observed over the course of calibrations. Normalized m/z 177 and
101 ions were plotted against epoxide mixing ratios of eight-point
standards; however, only four-point standards were used for IEPOX
calibration due to non-linearity. Slopes of the fittings were used as
calibration factors for the field measurements (Fig. S12). Field
calibrations were not performed due to the unavailability of IEPOX and MAE
permeation tube systems.
Proton transfer reaction time-of-flight mass spectrometry (PTR-TOF-MS)
A proton-transfer-reaction time-of-flight mass spectrometer (PTR-TOF-MS
8000, Ionicon Analytik GmbH, Austria) equipped with switchable reagent ion
capacity was used to measure the concentrations of gaseous organic species
at the site. Ambient air was sampled from an inlet mounted on a tower ca. 2 m above the rooftop of the LRK site building through a 6.35 mm OD
perfluoroakoxy (PFA) sampling line at 4–5 standard L min-1. A 2.0 µm pore size 47 mm
diameter Zefluor teflon filter (Pall Corporation) at the inlet removed
particles from the sample flow. The PTR-TOF-MS sub-sampled from this flow at
a rate of 0.25 standard L min-1, resulting in a total inlet transit time of ca. 1–2 s.
PTR-TOF-MS has been described previously by Jordan et al. (2009a, b)
and Graus et al. (2010) and was operated in this study as described in Liu
et al. (2013). H3O+ reagent ions were used to selectively
ionize organic molecules in the sample air. A high-resolution TOF detector
(Tofwerk AG, Switzerland) was used to analyze the reagent and product ions
and allowed for exact identification of the ion molecular formula (mass
resolution > 4000). The instrument was operated with a drift tube
temperature of 80 ∘C and a drift tube pressure of 2.35 mbar. In
H3O+ mode, the drift tube voltage was set to 520 V, resulting in
an E/N of 120 Td (E, electric field strength; N, number density of air in
the drift tube; unit, Townsend, Td; 1 Td = 10-17 V cm2).
PTR-TOF-MS spectra were collected at a time resolution of 10 s. Mass
calibration was performed every 2 min with data acquisition using the
Tof-Daq v1.91 software (Tofwerk AG, Switzerland).
A calibration system was used to establish the instrument sensitivities to
VOCs. Gas standards (Scott Specialty Gases) were added into a humidified
zero air flow at controlled flow rates. Every 3 h the inlet flow was
switched to pass through a catalytic converter (platinum on glass wool
heated to 350 ∘C) to remove VOCs and establish background
intensities.
Filter sampling methods and offline chemical analyses
PM2.5 samples were collected on pre-baked
Tissuquartz™ Filters (Pall Life Sciences, 8 × 10 in) with three high-volume
PM2.5 samplers (Tisch Environmental, Inc.). All high-volume
PM2.5 samplers were equipped with cyclones operated at 1 m3 min-1.
One high-volume sampler collected PM2.5 for 23 h
(08:00 to 07:00 the next day, local time), while the two remaining samplers
collected PM2.5 in two cycles. When the sampling schedules were
daytime (08:00–19:00 LT) and nighttime (20:00–07:00 LT), the collection cycle and samples are defined as regular day–night
sampling periods and samples. On selected days (10–12, 14–16 and
29–30 June, and 9–16 July), when high levels of isoprene,
sulfate (SO42-), and NOx were predicted at the LRK site by
FLEXPART and MOZART model simulations (see Supplement), PM2.5 were collected
more frequently (08:00–11:00, 12:00–15:00, 16:00–19:00, and 20:00–07:00 LT)
to capture the effects of anthropogenic pollution on
isoprene SOA formation at higher time resolution by offline techniques. Such
days are defined as intensive sampling periods and the samples as intensive
samples. A total of 47 23 h integrated samples, 2 sets of 64 intensive samples and 59
day–night filter samples were collected over the 6-week period of the
campaign and stored at -20 ∘C until analysis. Field blanks were
collected weekly by placing pre-baked quartz filters into the high-volume
PM2.5 samplers for 15 min and then removing and storing them under the
same conditions as the field samples.
Instrumentation
Gas chromatography/electron ionization-mass spectrometry (GC/EI-MS) was
performed on a Hewlett Packard (HP) 5890 Series II Gas Chromatograph
equipped with an Econo-Cap®
-EC®-5 Capillary Column (30 m × 0.25 mm
ID; 0.25 µm film thickness) coupled to an HP 5971A Mass Selective
Detector. GC/EI-MS operating conditions and temperature program are provided
in Surratt et al. (2010).
Ultra performance liquid chromatography/diode array detector-electrospray
ionization high-resolution quadrupole time-of-flight mass spectrometry
(UPLC/DAD-ESI-HR-QTOFMS) was performed on an Agilent 6500 series system
equipped with a Waters Acquity UPLC HSS T3 column (2.1 × 100 mm,
1.8 µm particle size). UPLC/DAD-ESI-HR-QTOFMS operating conditions are
described in Zhang et al. (2011).
Isoprene-derived SOA tracer quantification
Detailed filter extraction procedures are provided in Lin et al. (2013a).
Briefly, from each filter two 37 mm punches (one for analysis by GC/EI-MS
and one for UPLC/DAD-ESI-HR-QTOFMS analysis) were extracted in separate
pre-cleaned scintillation vials with 20 mL high-purity methanol (LC-MS
Chromasolv-grade®, Sigma Aldrich) by sonication
for 45 min. Filter extracts were then filtered through 0.2 µm syringe
filters (Acrodisc® PTFE membrane, Pall Life
Sciences) to remove suspended filter fibers and insoluble particles, and
then gently blown down to dryness under an N2 (g) stream at room
temperature.
The known IEPOX-derived SOA tracers, 2-methyltetrols (Claeys et al., 2004),
C5-alkene triols (Wang et al., 2005), cis- and
trans-3-methyltetrahydrofuran-3,4-diols (3-MeTHF-3,4-diols) (Lin et al., 2013b),
and IEPOX-derived dimers (Surratt et al., 2006), and the known MAE-derived
SOA tracer, 2-methylglyceric acid (2-MG) (Edney et al., 2005), were
identified by GC/EI-MS immediately following trimethylsilylation.
Derivatization was performed by reaction with 100 µL of BSTFA (N,O-bis(trimethylsilyl)trifluoroacetamide) +
TMCS (trimethylchlorosilane) (99 : 1, v/v, Supelco) and 50 µL of pyridine (anhydrous, 99.8 %, Sigma
Aldrich) at 70 ∘C for 1 h. 1 µL of derivatized sample was
directly analyzed. Base peak ions of the corresponding tracers, m/z 219 for
2-methyltetrols, m/z 231 for C5-alkene triols, m/z 262 for 3-MeTHF-3,4-diols,
m/z 335 for dimers, and m/z 219 for 2-MG, were quantified using authentic
standards of 2-methyltetrols (50 : 50, v/v, 2-C-methylerythritol and
2-C-methylthreitol), cis- and trans-3-MeTHF-3,4-diols, and 2-MG. The C5-alkene
triols and dimers were quantified by the response factor obtained for the
synthetic 2-methyltetrols. Synthetic procedures for cis- and
trans-3-MeTHF-3,4-diols have been described previously by Zhang et al. (2012b).
Synthesis of the tetrol mixture will be described in a forthcoming
publication; the 1H NMR trace (Fig. S13) shows a 1.2 : 1
2-C-methylerythritol and 2-C-methylthreitol of > 99 % purity.
Organosulfates, including the 2-methyltetrol sulfate esters
([C5H11O7S]-, m/z 215), IEPOX dimer sulfate esters
([C10H21O10S]-, m/z 333) (Surratt et al., 2008), and 2-MG
sulfate ester ([C4H7O7S]-, m/z 199) (Lin et al., 2013b),
were analyzed by UPLC/DAD-ESI-HR-QTOFMS. The UPLC/DAD-ESI-HR-QTOFMS was
operated in both negative and positive ion modes; however, only the negative
ion mode data are presented here since the positive ion mode data were
recently described in Lin et al. (2014). Filter extract residues were
reconstituted with 150 µL of a 50 : 50 (v/v) solvent mixture of methanol
(LC-MS Chromasolv-grade, Sigma Aldrich) and laboratory Milli-q water and a 5 µL
aliquot of each sample was eluted with solvent of the same
composition. IEPOX-derived sulfate esters (2-methyltetrol sulfate esters)
were quantified using an authentic standard synthesized at UNC (The University of North Carolina), while sodium
propyl sulfate was used to quantify the remaining isoprene-derived
organosulfates. The 2-methyltetrol sulfate ester standards were obtained and
used as tetrabutylammonium salts. The synthetic procedure will be described
in a forthcoming publication. The 1H NMR trace (Fig. S14) shows the
purity of the sulfate ester mixture is > 99 %. The response
factor of the authentic sulfate ester standards from several analyses is a
factor of 2.25 ± 0.13 lower than that of sodium propyl sulfate used in
previous field studies (Lin et al., 2013a), suggesting that the IEPOX
organosulfates likely make a contribution to mass concentration higher by a
factor of ∼ 2.3 than previously estimated at field sites.
Table 1 summarized data for isoprene-derived SOA tracers quantified from 123
filter samples using the above techniques.
Summary of isoprene-derived SOA tracers measured by GC/EI-MS and
UPLC/DAD-ESI-HR-QTOFMS.
SOA Tracers
Retention Time
# of Samples
Concentration (ng m-3)
Average % among
(min)
Detected*
Maximum
Mean
detected tracers
Tracers by GC/EI-MS
trans-3-MeTHF-3,4-diol
20.5
55
18.8
2.7
0.2 %
cis-3-MeTHF-3,4-diol
21.1
29
5.7
1.7
0.1 %
2-methylglyceric acid
23.4
119
36.7
7.5
1.5 %
2-methylthreitol
32.9
122
329.8
42.4
8.6 %
2-methylerythritol
33.7
122
1269.7
120.7
24.4 %
(Z)-2-methylbut-3-ene-1,2,4-triol
25.6
121
260.0
29.1
5.8 %
2-methylbut-3-ene-1,2,3-triol
26.6
118
162.5
16.5
3.2 %
(E)-2-methylbut-3-ene-1,2,4-triol
26.9
122
1127.0
98.8
19.9 %
Tracers by UPLC/DAD-ESI-HR-QTOFMS
IEPOX-derived organosulfates
1.1–1.7
122
1135.3
169.5
34.2 %
IEPOX-derived dimer organosulfate
2.8
103
14.0
1.4
0.2 %
MAE-derived organosulfate
1.1
114
77.9
10.0
1.9 %
* Total number of samples is 123.
Filter analysis of WSOC and OC constituents
For analysis of water-soluble organic compound (WSOC) concentrations,
additional filter punches (47 mm) were placed in pre-cleaned glass vials and
extracted with 30 or 40 mL ultra pure water by sonication for 40 min at
1 kHz. Extracts were filtered through a syringe filter (0.45 µm,
GE Healthcare UK Limited, UK) to remove insoluble particles. Samples were
extracted batch-wise, with each batch containing 12–21 ambient samples, one
lab blank, and one sample spiked with 1000 µgC L-1. Total
organic carbon (TOC) was analyzed using a 5310 C TOC analyzer and 900
inorganic carbon remover (ICR). The instrument was calibrated by single-point
calibration with 1000 µgC L-1 of potassium hydrogen phthalate
(KHP) and sodium carbonate. The calibration was verified with
1000 µgC L-1 of sucrose, and checked daily with a
1000 µgC L-1 of KHP standard. Standards and samples were run
in triplicate; the first data point was rejected and the following two
averaged.
Total OC and elemental carbon (EC) measurements from filter samples were
conducted at the National Exposure Research Laboratory, US Environmental
Protection Agency, at Research Triangle Park, NC. A 1.5 cm2 punch was
taken from each filter for OC/EC analysis using the thermal-optical method
(Birch and Cary, 1996) on a Sunset Laboratory (Tigard, OR) OC/EC
instrument. Table S4 provides temperature and purge gas settings for the
method. The instrument was calibrated internally using methane gas and the
calibration was verified with sucrose solution at four mass concentrations.
Estimations of aerosol pH and IEPOX-derived SOA tracers
The thermodynamic model, ISORROPIA-II (Fountoukis and Nenes, 2007; Nenes
et al., 1999), is used to estimate aerosol pH. Inputs for the model include
aerosol-phase sulfate, nitrate, and ammonium in µmol m-3, measured
by the ACSM under ambient conditions; RH and temperature obtained from
National Park Service (NPS); and gas-phase ammonia obtained from Ammonia
Monitoring Network (AMoN; TN01/Great Smoky Mountains National Park – Look
Rock). ISORROPIA-II predicted particle hydronium ion concentration per
volume of air (Hair+, µg m-3),
aerosol water (LWC, µg m-3), and aerosol aqueous phase mass
concentration (µg m-3). Aerosol pH is calculated by the following
equation:
pH=-log10aH+=-log10Hair+LMASS×ρaer×1000,
where aH+ is H+ activity in aqueous phase (mol L-1),
LMASS is the total liquid-phase aerosol mass (µg m-3) and ρaer
is aerosol density (g cm-3). The ability of ISORROPIA to
capture pH, LWC and gas-to-particle partitioning of inorganic volatiles
(e.g., NH3, HNO3, HCl) has been the focus of other studies
(Fountoukis et al., 2009; Guo et al., 2015) and is
not further discussed here.
IEPOX-derived SOA tracers are estimated using simpleGAMMA (Woo and
McNeill, 2015). It is a reduced version of GAMMA (Gas Aerosol Model for
Mechanism Analysis), the detailed photochemical box model of aqueous aerosol
SOA (aqSOA) formation developed by McNeill and coworkers (McNeill et al.,
2012). GAMMA and simpleGAMMA represent aqSOA formation in terms of bulk
aqueous uptake followed by aqueous-phase reaction (Schwartz, 1986). For
this study, we utilized only the aqueous aerosol-phase chemistry of IEPOX to
predict IEPOX-derived SOA constituents. We applied the Henry's law constant
of 3 × 107 M atm-1 for IEPOX partitioning based on
measurements by Nguyen et al. (2014) on deliquesced NaCl particles.
Estimation of 2-methyltetrols and IEPOX-derived organosulfate masses in the
aqueous phase was based on the Eddingsaas et al. (2010) mechanism:
IEPOXaq→1-β⋅2methyltetrols+β⋅IEPOXorganosulfate,
where β is a branching ratio between 2-methyltetrols and IEPOX-derived
organosulfate concentration. We applied β= 0.4 based on the
observation of Eddingsaas et al. (2010) for the most concentrated bulk
solution they studied. The rate constant for Eq. (2) (ka) is a function
of aH+ and nucleophile concentrations (Eddingsaas et al., 2010),
modified to include the possible protonation of IEPOX(aq) by ammonium
(Nguyen et al., 2014):
ka=kH+aH++kSO42-SO42-aH++kHSO4-HSO4-+kNH4+NH4+.
Here, kH+=5×10-2 s-1,
kSO42-=2× 10-4 M-1 s-1, and
kHSO4-= 7.3 × 10-4 M-1 s-1.
The ammonium rate constant, kNH4+, was calculated using
GAMMA and the results of the chamber study of Nguyen et al. (2014) are
1.7×10-5 M-1 s-1.
IEPOX uptake and formation of 2-methyltetrols and IEPOX-derived
organosulfate was computed using simpleGAMMA with inputs of SO42-,
HSO4-, NH4+, LWC, aH+ concentrations (mol L-1),
and aerosol pH estimated by ISORROPIA-II simulation of field
conditions, ambient temperature and RH, aerosol surface area (cm2 cm-3)
obtained from SEMS-MCPC measurements, and IEPOX concentration
(mol cm-3) from acetate CIMS (Sect. 2.4). Masses of SOA tracers
formed over 12 h are compared with measurements in Sect. 3.4.2.
Results and discussion
Fine aerosol component mass concentrations
Chemical measurements of fine aerosol made by the ACSM and collocated
instruments are presented in Fig. 1. The ACSM measured a campaign average
7.6 ± 4.7 µg m-3 of NR-PM1, which is predominantly
organic aerosol (64.1 %). Sulfate aerosol (24.3 %) is the most dominant
inorganic aerosol component, followed by ammonium (7.7 %), nitrate
(3.8 %), and chloride (0.1 %). The NR-PM1 mass measured at the site
shows strong association (r2= 0.89) with the SEMS-MCPC PM1 mass
measurements (Figs. 1d and S3).
Time series mass concentration of (a) organic and (b) inorganics
(excluding chloride) measured by ACSM, (c) black carbon (BC) measured by
Aethalometer, and (d) NR-PM1 and PM1 mass concentrations measured
by ACSM and SEMS-MCPC. Collocated sulfate aerosol measured by Thermo
Scientific Sulfate Analyzer was plotted on (b). OC (bars) and WSOC (dots),
both in unit of µgC m-3, measured from filter samples were
plotted on (a) with ACSM organic. EC (bars; in unit of µgC m-3)
measured from filter samples were plotted on (c) along with BC
measurements.
Moderate correlations, depicted in Fig. S15 were observed between ACSM OM
and filter OC and WSOC (r2= 0.54, 0.39, respectively) as well as
between filter OC and WSOC measurements (r2= 0.36), suggesting that
fractions of OM and OC at LRK site are water-soluble as previously observed
(Turpin and Lim, 2001). This water-soluble fraction may be associated with
high isoprene emissions in this area (Zhang et al., 2012a). Lewis et al. (2004)
reported that 56–80 % of total carbon in PM2.5 samples
collected during summer in Nashville, TN, was non-fossil carbon, supporting
the importance of biogenic SOA in the southeastern US during summer. It is
potentially possible that some fraction of this non-fossil carbon is
associated to biomass burning (Ke et al., 2007). A more recent study found
that non-fossil carbon accounts for 50 % of carbon at two urban sites and
70–100 % of carbon at 10 near-urban or remote sites in the US
(Schichtel et al., 2008). In summer 2001, the fraction of non-fossil
carbon was reported to vary from 66–80 % of total carbon at the LRK, TN
site, suggesting the importance of photochemical oxidation of biogenic VOCs
(Tanner et al., 2004). The slope of the linear regression analysis on
Fig. S15a indicates an OM : OC ratio of 2.34 and OM : WSOC ratio of 2.19. Using the
Aiken et al. (2008) parameterization approach, we found an average
(±1-σ) OM : OC ratio of 2.14 (±0.18). The LRK OM:OC
ratios obtained from measurements and parameterization are consistent with a
previous study at Look Rock (2.1) (Turpin and Lim, 2001), but higher than
those measured at Centerville, AL (1.77) (Sun et al., 2011), probably
ascribable to different atmospheric aerosol properties at the two sites.
Elemental analyses of ACSM unit-mass resolution data using the Aiken et al. (2008)
parameterization results in an average O : C ratio of 0.77 ± 0.12. This
is within 0.6–1 of O : C ratio previously observed in the
southeastern US (Centerville, AL) (Sun et al., 2011; Xu et al., 2015).
ACSM sulfate aerosol measurements (average of 1.85 ± 1.23 µg m-3)
agree well (r2= 0.67, slope 1.08) with the collocated
sulfate measurements (Table S1), demonstrating that ACSM performed well when
compared to existing air quality monitoring instruments as previously
reported (Budisulistiorini et al., 2014). Low nitrate concentration is
expected due to the high summer temperatures (15–31 ∘C) and low
prevailing NOx concentrations (0.1–2 ppb) measured at the site. In the
absence of a significant source of chloride, chloride concentrations were
predictably low (0.01 ± 0.01 µg m-3).
On average, mass concentration of BC was 0.23 ± 0.14 µg m-3
or about 3 % of total PM2.5 measured at the site. The low relative
contribution was consistent during the campaign except on 11 to 12 July when
there was a significant increase during a few hours overnight. EC measured
from filters was even lower at 0.06 µg m-3 on average and was
only weakly correlated (r2= 0.32) with BC. Carbon monoxide (CO),
another primary species measured at LRK, was also low (115.62 ± 24.06
ppb on average) throughout the campaign. A previous study found that the
level of primary species increased during mid-morning when the boundary
layer height reached the site, and declined later in the day as a result of
dilution (Tanner et al., 2005). In contrast, secondary species such as
PM2.5 and sulfate do not show significant diurnal variability,
suggesting local meteorological conditions are less influential in
determining concentrations of the long-lived species (Tanner et al.,
2005, 2015). The overall low concentration of primary
emissions at the site (Fig. S16) is consistent with minimum local and/or
regional primary emissions.
Source apportionment of OA from the ACSM
PMF analysis was conducted on the ACSM OA mass spectral data in order to
resolve factors (or source profiles) without a priori assumptions. A 3-factor
solution resolved from PMF analysis, as shown in Figs. 2 and 3, was selected
as the best-fit (see Supplement for details of Q/Qexp, fpeak, etc.),
comprised of the known LV-OOA factor (Jimenez et al., 2009; Ulbrich et al.,
2009), an IEPOX-OA factor (Budisulistiorini et al., 2013; Slowik et al.,
2011; Robinson et al., 2011), and a factor similar to 91Fac, a factor
previously observed in areas dominated by biogenic emissions (Robinson et
al., 2011; Slowik et al., 2011; Chen et al., 2015).
Mass spectra obtained for the 3-factor solution from PMF: IEPOX-OA,
LV-OOA, and 91 Fac.
Left panel shows the PMF 3-factor solution time series mass
contributions measured by ACSM. Top to bottom: left ordinate, IEPOX-OA
(black), LV-OOA (red), and 91 Fac (green); right ordinate, sulfate (orange)
and nitrate (blue). Right panel shows average mass contributions (top) and
diurnal variation (bottom) of factors resolved by PMF.
The IEPOX-OA factor resolved from our data set is more closely correlated to
sulfate measured by the ACSM (r2= 0.58) than by the collocated
instrument (r2= 0.31) (Table S5). Correlation of gaseous IEPOX
measured by acetate CIMS with the IEPOX-OA factor is low (r2= 0.24),
which may be a consequence of the time gap from IEPOX uptake onto sulfate
aerosol process which can take up ∼ 5 h in the presence of
aqueous, highly acidic aerosol (pH ≤ 1) (Gaston et al., 2014). The
time gap between formation of gaseous IEPOX and IEPOX-OA factor could be
wider due to ISOPOOH, which lifetime to OH is 3–5 h (Paulot et al.,
2009), interference on IEPOX signal measured by acetate acetate CIMS.
Importantly, the IEPOX-OA factor correlates strongly with 2-methyltetrols
(r2= 0.80), IEPOX-derived organosulfates (r2= 0.81),
C5-alkene triols (r2= 0.75), and dimers of organosulfates
(r2= 0.73) (Table 2), giving an overall r2 of 0.83 with sum of
IEPOX-derived SOA tracers measured by offline techniques. The high
correlation provides strong evidence that IEPOX chemistry gives rise to the
PMF factor we have designated as the IEPOX-OA factor. The contribution of
this factor to total OM is 32 %, which is strikingly consistent with the
contribution of the factor designated as the IEPOX-OA factor in the PMF
analysis of fine organic aerosol collected in downtown Atlanta, GA
(Budisulistiorini et al., 2013) and across other sites in this region (Xu
et al., 2015). IEPOX-OA was not only formed on site but could also
transported from surrounding forested and isoprene-rich areas. The
reactive-uptake of IEPOX is influenced by aerosol sulfate (Lin et al., 2012)
that is not formed on site, might explain the lack of significant diurnal
variation (Fig. 3) of the IEPOX-OA factor at LRK. WSOC shows fair
correlation with some IEPOX-OA tracers (r2= 0.3–0.4; Table S6) and
IEPOX-OA factor (r2= 0.37; Table S5) the nature of which will be
discussed in more detail below.
Correlation (r2) of PMF factors with isoprene-derived SOA
tracers measured by GC/EI-MS and UPLC/DAD-ESI-HR-QTOFMS.
SOA Tracers
IEPOX-OA
LV-OOA
91Fac
3-methyltetrahydrofuran-3,4-diols
0.12
0.13
0.24
2-methyltetrols
0.80
0.20
0.38
C5-alkene triols
0.75
0.19
0.44
2-methylglyceric acid
0.38
0.44
0.44
IEPOX-derived organosulfates
0.76
0.29
0.42
IEPOX-derived dimer organosulfate
0.71
0.12
0.34
MAE-derived organosulfate
0.37
0.44
0.52
The LV-OOA factor contributes 50 % of OM (Fig. 3). The average f44= 0.22
is comparable to that of the standard LV-OOA profile (Ng et al.,
2011), suggesting it is an oxidized (aged) aerosol. The LV-OOA correlated
well with nitrate (r2= 0.62) but more weakly with sulfate
(r2= 0.39) (Table S5). Correlation with nitrate as well as the high level of
oxidation is consistent with the suggestion above that a fraction of OA
originates from the valley. Located on a ridge top above the morning valley
fog, LRK receives air masses from the valley as the boundary layer rises
during the day (Tanner et al., 2005). Diurnal profile of LV-OOA observed
at LRK is similar to more-oxidized OOA (MO-OOA) observed at Centerville (Xu
et al., 2015), suggesting their regional sources. At LRK average mixing
ratios of monoterpenes and isoprene were < 1 ppb and ∼ 2 ppb
(Fig. 4), respectively. Low anthropogenic emissions at LRK (<1 ppb; Fig. S16) suggests that BVOCs (biogenic VOCs) could be the source of LV-OOA (50 %
of OA) formation. Anthropogenic emissions as well as nitrate chemistry in
the valley could also influence LV-OOA formation that oxidized during
transport to the LRK site.
Diurnal variation of isoprene (left ordinate) as well as isoprene
gaseous 3 photooxidation products (right ordinates), i.e., MVK+MACR, IEPOX
and MAE, measured at LRK site. It should be noted that IEPOX signal includes
interference of ISOPOOH at unknown ratio; thus its mixing ratio represents an
upper limit.
The 91Fac factor is characterized by a distinct ion at m/z 91. At LRK, the
average f44 of 91Fac is 0.12, between the values 0.05 and 0.16 reported
for standard SV-OOA and LV-OOA profiles, respectively (Ng et al., 2011),
indicating that it is likely an oxygenated OA. The LRK 91Fac makes the
smallest contribution to OM (18 %) of the three factors resolved by PMF
analysis. The 91Fac diurnal pattern shows slight increases during noon and
night, suggesting that this factor might be affected by both photochemistry
and nighttime chemistry. Potential sources of 91Fac is discussed in more
detail in the Supplement (Fig. S17) and its association with biogenic SOA chemistry
will be the focus of future studies.
A source apportionment study of organic compounds in PM2.5 at LRK
during August 2002 using the chemical mass balance (CMB) model evaluated
contributions by eight primary sources, chosen as representing the major
contributors to fine primary OC in the southeast US. Primary sources,
consisting largely of wood burning, were estimated to contribute
∼ 14 % of the total OC at LRK (Ke et al., 2007). 14C
Analysis of the LRK PM2.5 in the same study showed that during summer,
∼ 84 % of the OC was non-fossil carbon (Ke et al., 2007).
By contrast, our current study resolved no POA by PMF analysis. However, in
subsequent studies, we will investigate the influence of POA at LRK by
examining the 14C data from filter samples.
Identification and quantification of isoprene-derived SOA tracers
2-Methylglyceric acid, 2-methyltetrols, C5-alkene triols and
IEPOX-derived organosulfates were detected in most filter samples (Table 1).
Among all observed SOA tracers, 2-C-methylerythritol and
2-methylbut-3-ene-1,2,4-triol were the most abundant species identified by
GC/EI-MS, contributing ∼ 24 % (120.7 ng m-3 on average)
and ∼ 20 % (98.8 ng m-3 on average), respectively, of
total quantified mass, while isomeric IEPOX-derived organosulfates accounted
for ∼ 34 % (169.5 ng m-3 on average) of the mass
detected by UPLC/DAD-ESI-HR-QTOFMS. Concentrations of the isomeric
3-MeTHF-3,4-diols were lower (≤ 18.8 ng m-3), often at or below
detection limits. Gaseous IEPOX was on average 1 ppb (maximum 5.8 ppb)
significantly higher than gaseous MAE at 2.8 × 10-3 ppb on
average (maximum 0.02 ppb). This explains the abundance of IEPOX-derived SOA
tracers compared to MAE-derived tracers. It should be noted that IEPOX
quantified here includes the interference of ISOPOOH on its signal; however,
the overall measured IEPOX signal is still substantially higher than the MAE
signal, even if we assume IEPOX only contributes to 1–10 % of the m/z 177
intensity.
In sum, IEPOX- and MAE-derived tracers contributed 96.6 and 3.4 %,
respectively, of total isoprene-derived SOA mass quantified from filter
samples. This observation is consistent with a previous field study in
Yorkville, GA, which reported the summed IEPOX-derived SOA tracers comprised
97.5 % of the quantified isoprene-derived SOA mass (Lin et al., 2013a).
Total IEPOX-derived tracers masses quantified from filter samples were on
average 26.3 % (maximum 48.5 %) of the IEPOX-OA factor mass resolved by
PMF. This is consistent with a recent laboratory study of isoprene
photooxidation under high HO2 conditions that suggested IEPOX isomers
contributed about 50 % of SOA mass formed (Liu et al., 2014).
Masses of IEPOX- and MAE-derived SOA tracers were fairly correlated
(r2= 0.37 and 0.29, respectively) with WSOC (Fig. S15c). Around
26 % of the WSOC mass might be explained by IEPOX-derived SOA tracer
masses, which consist predominantly of 2-methyltetrols, C5-alkene
triols, and IEPOX-derived organosulfates. The tetrols and triols are
hydrophilic compounds owing to the OH groups, and the organosulfates are
ionic polar compounds (Gómez-González et al., 2008).
An interesting and potentially important observation is that oligomeric
IEPOX-derived humic-like substances (HULIS) have been reported in both
reactive uptake experiments onto acidified sulfate seed aerosol and in
ambient fine aerosol from the LRK and Centerville sites during the SOAS
campaign (Lin et al., 2014). The HULIS is a mixture of hydroxylated,
sulfated as well as highly unsaturated, light-absorbing components which may
partition between WSOC and water insoluble organic carbon (WISC) fractions
(Lin et al., 2014). This finding might also in part explain the moderate
correlation between WSOC and the IEPOX-OA factor. However, HULIS has not
been quantified here due to the lack of authentic standards, but will likely
help to close the IEPOX-OA mass budget once appropriate standards are
developed and applied. As quantified by ACSM, summed isoprene-derived SOA
tracers on average accounted for 0.5 µg m-3 or 9.4 % (up to 4.4 µg m-3
or 28.1 %) of the average organic aerosol mass of 5.1 µg m-3 (maximum 15.3 µg m-3) during the campaign. This
contribution is somewhat lower than reported at a different rural site in
the southeast US (13.6–19.4 %) (Lin et al., 2013a) but higher
than reported at a forested site in central Europe (6.8 %) (Kourtchev et
al., 2009) and a rural site in south China (1.6 %) (Ding et al., 2012).
It should be noted that in all previous studies sulfate esters of
2-methyltetrol were quantified by surrogate standard structurally unrelated
to the target analytes (i.e., sodium propyl sulfate). While in current
study, a mixture of authentic 2-methyltetrol sulfate esters was used as a
standard for quantifying IEPOX-derived organosulfates. The use of
structurally unrelated surrogate standards may account in part for
discrepancies between this and previous studies in which total isoprene SOA
mass may have been underestimated as a result of higher instrument response
to surrogates and/or lower recovery in sample preparation. These
possibilities warrant further investigation using the same analytical
protocols and comparison of instrumental responses to authentic and
surrogate standards.
Influence of anthropogenic emissions on isoprene-derived SOA formation at
Look Rock
Effects of aerosol acidity and nitrogen-containing species
The time series of aerosol pH estimated by ISORROPIA-II overlaid on the time
series of the IEPOX-OA factor and IEPOX- and MAE-derived SOA tracers (Fig. 5a, Tables S5–S6) suggests that local aerosol acidity is not correlated with
these measured variables. The correlation coefficients of the IEPOX-OA
factor with ISORROPIA-II estimated pH and LWC bears out this conclusion
(r2 ∼ 0; Table S5). These results are consistent with
recent measurements reported by Xu et al. (2015) across several sites in
the southeastern US. Aerosol acidity can be expected to change during
transport and aging. Further complicating factors may be viscosity or
morphology changes of the aerosol as IEPOX is taken up by heterogeneous
reaction, and thus, slowing of uptake kinetics as the aerosol surface is
coated with a hydrophobic organic layer (Gaston et al., 2014).
Additionally, liquid–liquid phase separation is likely to occur in the
atmosphere due to changes of relative humidity that affect particles water
content (You et al., 2012). Moreover, the effects of aerosol viscosity and
morphology on IEPOX uptake is not well understood and warrant further study
using aerosol of complex mixtures and of atmospheric relevance.
Interpretation of the apparent lack of relationship between SOA and local
aerosol acidity suggests that aerosol acidity is likely not the limiting
factor in isoprene SOA formation at this site, especially since aerosol was
consistently acidic during SOAS. The IEPOX-OA factor is moderately
correlated with aerosol sulfate measured by ACSM (r2= 0.58), while
IEPOX- and MAE-derived SOA tracers are less correlated (r2
∼ 0.4) (Fig. 5b). Correlation between sulfate and IEPOX-OA
factor is consistent with recent measurements by Xu et al. (2015), and
suggests the need for aerosol surface area due to acidic sulfate for these
heterogeneous reactions to occur leading to IEPOX-OA formation.
(a) Time series of IEPOX-OA factor (black bars; darker bars are
intensive filter sampling periods), sum of IEPOX-derived (pink circle) and
MAE-derived (yellow square) SOA tracers, aerosol pH (purple cross) and LWC
(blue triangle) estimated by ISORROPIA-II model. Campaign average pH and LWC
are 1.78 ± 0.53 and 38.71 ± 7.43 mol L-1, respectively.
Correlation plots between IEPOX-OA, summed of IEPOX- and MAE-derived SOA
tracers and (b) sulfate measurements by ACSM and (c) NOy measurements
from NPS.
Correlation of IEPOX-OA and isoprene-derived SOA tracers with NOx,
NOy, and reservoir species (NOz= NOy- NOx) was also
examined. None of the nitrogen species showed significant association with
either the IEPOX-OA factor (r2 < 0.1; Table S5) or the
IEPOX-derived SOA tracers (r2 < 0.3; Table S5). Absence of
correlations suggest the following: (1) the formation of isoprene SOA primarily
through the low-NO pathway of isoprene photooxidation (Paulot et al.,
2009; Surratt et al., 2010), (2) the isoprene oxidation did not happen
locally, and (3) the gas-phase isoprene oxidation is not yet fully
understood. Correlation plot of NOy, a measure of total reactive
nitrogen species including MPAN, with summed MAE tracers is shown in
Fig. 5c and correlation values of NOx with individual compounds are
given in Table S6. Besides being derived solely from the hydrolysis of MAE,
2-MG is also proposed to be the hydrolysis product of
hydroxymethyl-methyl-α-lactone (HMML) (Nguyen et al., 2015) and
fair correlation between NOy and 2-MG (r2= 0.38) is consistent
with this hypothesis. The correlation of the high-NOx isoprene SOA
tracers (2-MG and its corresponding organosulfate) with NOy is
suggesting that other pathways like the uptake and hydrolysis of MAE could
be a source, especially since the further oxidation of MACR has been shown
to yield MAE directly (Lin et al., 2013b). The observation that neither the
summed MAE/HMML-derived tracers nor 2-MG correlated with NOx, is
consistent with the hypothesis that MAE/HMML is formed in urban areas upwind
and transported to the sampling site. Furthermore, it suggests that likely
both HMML and MAE could be sources of these tracers.
In addition to the pattern of daily up-slope transport of air from the
valley, air mass back-trajectory during high IEPOX-derived SOA episodes
(Fig. S18) indicated that air masses also originated west of LRK, in the
direction of the urban areas of Knoxville and Nashville, TN. Yet further
west of the LRK site are the Missouri Ozarks, a large source of isoprene
emissions (referred to as the “isoprene volcano”) (Guenther et al.,
2006). During summer, isoprene emitted in the Ozarks could mix with
anthropogenic emissions from Knoxville and Nashville, undergoing atmospheric
processing during transport. As a consequence, long distance transport and
accompanying oxidative processing may make a contribution to the IEPOX SOA
loading at LRK. During low IEPOX-derived SOA periods (Fig. S19) air masses
originated predominantly from the south and southwest, which are densely
forested, rural areas.
Box modeling supports the impact of aqueous cidic aerosol on IEPOX-derived
SOA
The IEPOX-derived SOA tracers (2-methyltetrols and IEPOX-derived
organosulfate) predicted using simpleGAMMA, taking the locally measured
IEPOX and aerosol parameters as inputs, show good correlation (r2= 0.5–0.7)
with the tracers quantified from filter samples (Table 3, Fig. S20). Slopes of the scatterplots show that the model overestimated the
2-methyltetrols and IEPOX-derived organosulfates by factors of 6.3 and 7.5,
respectively. The simpleGAMMA model calculates Henry's Law gas-aqueous equilibration
at each time step and decouples the subsequent aqueous-phase chemistry of
IEPOX from dissolution (McNeill et al., 2012). In this study, we assumed
an effective Henry's law constant, H*, of 3 × 107 M atm-1
for IEPOX, following the recent laboratory measurements of Nguyen et al. (2014),
whereas previous studies assumed values which ranged 1 order of
magnitude higher (1.3 × 108 M atm-1; Eddingsaas et al.,
2010) or lower (2.7 × 106 M atm-1; Pye et al.,
2013). Replacing the H* with that of Pye et al. (2013), the model
underestimated the 2-methyltetrols and IEPOX-derived organosulfates by
56 and 43 %, respectively. Decreasing the H* by 1 order of magnitude
yielded a factor of ∼ 10 decrease in the predicted IEPOX SOA
tracers mass, which is consistent with Pye et al. (2013) observation in
sensitivity studies that a factor of 7 increase in H* yielded a factor of
∼ 5 increase in predicted IEPOX SOA yield. Similarly, summed
masses of the modeled SOA tracers (Fig. 6) yielded a 141 % (r2= 0.62) overestimate of the IEPOX-OA factor; whereas summed SOA tracers
modeled by assuming H* of 1 order of magnitude lower yielded an 89 %
underestimate of the IEPOX-OA factor (r2= 0.62). The simpleGAMMA model predicts
only a subset of IEPOX-derived SOA tracers; thus underestimation of the
IEPOX-OA factor is expected.
Correlation (r2) of modeled SOA tracers with
measurements.
H* (M atm-1)
2-methyltetrols
IEPOX organosulfates
r2
Slope
r2
Slope
3.0 × 107a
0.45
9.61 ± 0.91
0.66
10.65 ± 0.73
2.7 × 106b
0.44
0.69 ± 0.06
0.66
0.90 ± 0.06
a Nguyen et al. (2014) and b Pye et al. (2013).
In addition to the uncertainty in the H* parameter, several other factors
may also contribute to mass disagreement between the tracer estimated by
simpleGAMMA and the field data. The box model simulations took locally
measured IEPOX and aerosol parameters as inputs, and simulated 12 h of
reactive processing, rather than simulating uptake, reaction, and transport
along a trajectory initiating in the valley. The locally measured IEPOX
signal is noted above to have interference from ISOPOOH; thus the model
outputs likely overestimate the measurements. Examination of IEPOX input
variability to simpleGAMMA tracers estimation will be reported in a future
study. Additionally, C5-alkene triols, the third largest contributor to
the IEPOX-derived SOA tracers, and oligomeric HULIS are not included in the
simpleGAMMA model estimation. Neglect of the C5-alkene triols and
oligomers as well as yet unknown IEPOX-derived SOA formation pathways by
this model could contribute to inaccuracy in estimation of the mass
contribution of 2-methyltetrols and IEPOX-derived organosulfates to the
total amount of IEPOX-derived SOA tracers and reduce the correlation.
Finally, oxidative aging of IEPOX SOA tracers is not included in simpleGAMMA
at this time due to current lack of availability of kinetic and mechanistic
data. Overall, although mass disagreement persists, good correlation between
model and field measurements of tracers suggest that the uptake mechanism of
IEPOX is consistent with acid-catalyzed mechanism proposed from kinetic
(Eddingsaas et al., 2010; Pye et al., 2013) and laboratory studies
(Lin et al., 2012; Nguyen et al., 2014).
Correlation of summed IEPOX-derived SOA tracers estimated by
simpleGAMMA by assuming H* of 3.0 × 107 (Nguyen et al.,
2014) (model a) and 2.7 × 106 (Pye et al., 2013) (model b) and
IEPOX-OA factor from PMF analysis.
Conclusions
Offline chemical analysis of PM2.5 samples collected from LRK, TN,
during the 2013 SOAS campaign show a substantial contribution by
IEPOX-derived SOA tracers to the total OA mass (∼ 9 % on
average, up to 28 %). A larger contribution (32 %) to total OA mass is
estimated by PMF analysis of the real-time ACSM OA mass spectrometric data.
Overall, the importance of IEPOX heterogeneous chemistry in this region is
clearly demonstrable. No association was observed between the gas-phase
constituents NO and NO2 and the IEPOX-derived SOA tracers or the
IEPOX-OA factor, suggesting that IEPOX-derived SOA is formed upwind or distant
from the sampling site. Moderate association between NOy and
MACR-derived SOA tracers was observed, consistent with the proposed
involvement of oxidizing nitrogen compounds in MACR-derived SOA formation
(Lin et al., 2013b; Nguyen et al., 2015). Particle-phase sulfate is fairly
correlated (r2= 0.3–0.4) with both MACR- and IEPOX-derived SOA
tracers, and more strongly correlated (r2 ∼ 0.6) with the
IEPOX-OA factor, overall suggesting that sulfate plays an important role in
isoprene SOA formation. However, this association requires further analysis,
in light of the proposed formation of IEPOX-derived SOA during transport to
LRK from an upwind or down-slope origin. Several explanations may be
proposed for the lack of a strong association between isoprene-derived SOA
mass and particle acidity: (1) isoprene-derived SOA is not strongly limited
by levels of predicted aerosol acidity and LWC even though these are in the
favored ranges (pH < 2) to promote sufficient SOA production based
on recent laboratory kinetic studies (Gaston et al., 2014; Riedel et al.,
2015); thus, other potentially unknown controlling factors in this region
might need to be considered; (2) no strong correlation exists between SOA
mass and local aerosol acidity which estimation is challenging due to
changes in particle composition and characteristics during reactive uptake;
and (3) several key inter-related variables (LWC, aerosol surface area and
aerosol acidity) control SOA yield and thus the correlation of aerosol
acidity and SOA yield will be difficult to deconvolute from complex field
data until modeling can better constrain these effects. Consistent with the
suggestion that IEPOX-derived SOA forms during transport from distant
locations, air mass back-trajectory indicated that westerly flow from
potential sources of oxidation products where biogenic and anthropogenic
emissions can mix, are likely related to episodes of high levels of
IEPOX-derived SOA measured at LRK. In contrast, when air masses originated
mainly from forested and rural areas to the south and southeast of the site,
high levels of IEPOX-derived SOA mass were not observed. Good correlation
between SOA model outputs and field measurements suggests that gaps remain
in our knowledge of isoprene-derived SOA formation. Laboratory studies are
needed to reduce the uncertainty in the effective Henry's Law constant, H*,
for IEPOX. Additional studies are needed to further quantify the
condensed-phase mechanism and kinetics of SOA formation via the IEPOX
pathway so that it may be represented in more detail in models.
Notwithstanding, initial modeling results allow critical insight into how
more explicit treatment of the reactions between anthropogenic pollutants
and isoprene oxidation products may be incorporated into models of SOA
formation. Importantly, by the inclusion of explicit IEPOX- and MAE-derived SOA
formation pathways in a model, Pye et al. (2013) recently demonstrated
that by lowering SOx emissions in the eastern US by 25 % could
lower IEPOX- and MAE-derived SOA formation 35 to 40 %. Future studies
should attempt to improve model predictions of IEPOX-derived SOA formation
and systematically examine effects of implementing stricter SOx
controls in this region.