ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-18-3937-2018Secondary sulfate is internally mixed with sea spray aerosol and organic aerosol in the winter ArcticSecondary sulfate in the winter ArcticKirpesRachel M.https://orcid.org/0000-0002-2998-0108BondyAmy L.BonannoDanielMoffetRyan C.WangBingbingLaskinAlexanderhttps://orcid.org/0000-0002-7836-8417AultAndrew P.aulta@umich.eduhttps://orcid.org/0000-0002-7313-8559PrattKerri A.prattka@umich.eduhttps://orcid.org/0000-0003-4707-2290Department of Chemistry, University of Michigan, Ann Arbor, Michigan, USADepartment of Chemistry, University of the Pacific, Stockton, California, USAEnvironmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington, USAState Key Lab of Marine and Environmental Science & College of Ocean and Earth Sciences, Xiamen University, Xiamen, ChinaDepartment of Environmental Health Sciences, University of Michigan, Ann Arbor, Michigan, USADepartment of Earth & Environmental Sciences, University of Michigan, Ann Arbor, Michigan, USAcurrently at: Sonoma Technology, Petaluma, California, USAcurrently at: Department of Chemistry, Purdue University, West Lafayette, Indiana, USAKerri A. Pratt (prattka@umich.edu) and Andrew P. Ault (aulta@umich.edu)20March20181863937394927October20171November201717January201811February2018This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/This article is available from https://acp.copernicus.org/articles/18/3937/2018/acp-18-3937-2018.htmlThe full text article is available as a PDF file from https://acp.copernicus.org/articles/18/3937/2018/acp-18-3937-2018.pdf
Few measurements of aerosol chemical composition have been made during the
winter–spring transition (following polar sunrise) to constrain Arctic
aerosol–cloud–climate feedbacks. Herein, we report the first measurements
of individual particle chemical composition near Utqiaġvik (Barrow),
Alaska, in winter (seven sample days in January and February 2014).
Individual particles were analyzed by computer-controlled scanning electron
microscopy with energy dispersive X-ray spectroscopy (CCSEM-EDX, 24 847
particles), Raman microspectroscopy (300 particles), and scanning
transmission X-ray microscopy with near-edge X-ray absorption fine structure
spectroscopy (STXM-NEXAFS, 290 particles). Sea spray aerosol (SSA) was
observed in all samples, with fresh and aged SSA comprising 99 %, by
number, of 2.5–7.5 µm
diameter particles, 65–95 % from 0.5–2.5 µm, and
50–60 % from 0.1–0.5 µm, indicating SSA is the dominant
contributor to accumulation and coarse-mode aerosol during the winter. The
aged SSA particles were characterized by reduced chlorine content with
94 %, by number, internally mixed with secondary sulfate (39 %, by
number, internally mixed with both nitrate and sulfate), indicative of
multiphase aging reactions during transport. There was a large number
fraction (40 % of 1.0–4.0 µm diameter particles) of aged SSA
during periods when particles were transported from near Prudhoe Bay,
consistent with pollutant emissions from the oil fields participating in
atmospheric processing of aerosol particles. Organic carbon and sulfate
particles were observed in all samples and comprised 40–50 %, by number,
of 0.1–0.4 µm diameter particles, indicative of Arctic haze
influence. Soot was internally mixed with organic and sulfate components. All
sulfate was mixed with organic carbon or SSA particles. Therefore, aerosol
sources in the Alaskan Arctic and resulting aerosol chemical mixing states
need to be considered when predicting aerosol climate effects, particularly
cloud formation, in the winter Arctic.
Introduction
The Arctic region is experiencing warming at a greater rate than elsewhere
on Earth (Pachauri et al., 2014) and undergoing substantial
transformations, including rapid loss of sea ice (Overland and
Wang, 2013). This is leading to increased aerosol emissions, resulting in
changes to atmospheric aerosol budgets and associated climate feedbacks
(Struthers et al., 2011). Characterizing the chemical
composition and morphology of individual Arctic aerosol particles is
important for understanding the influence of local and transported aerosols
on climate (Leck et al., 2002; Leck and Svensson, 2015), which remains
one of the largest uncertainties in radiative forcing (Boucher et al.,
2013). Aerosol mixing state, the distribution of chemical species across an
aerosol population and within each individual particle, determines particle
reactivity, hygroscopicity, cloud activation efficiency, and optical
properties (Prather et al., 2008; Ault and Axson, 2017). However, the few
studies that have used single-particle analysis techniques to characterize
the chemical mixing state of the full aerosol population have been limited
to Svalbard (Weinbruch et al., 2012; Hara et al., 2003; Geng et al., 2010; Chi et al., 2015;
Moroni et al., 2015, 2017; Young et al., 2016), the summertime Canadian archipelago
(Köllner et al., 2017), the summertime central Arctic (Hamacher-Barth et al., 2016; Sierau et al., 2014), and the Alaskan Arctic during spring (Brock et al., 2011; Parungo et al., 1990; Parungo et al., 1993) and summer (Gunsch et al., 2017). Evaluating aerosol
impacts on climate across the Arctic region is of particular importance
given rapid changes in aerosol sources. Therefore, there is an urgent need
to study the chemical composition of individual Arctic aerosol particles.
Aerosol influences on cloud formation and cloud–climate feedbacks in the
Arctic are highly uncertain during winter, when there is little direct solar
radiation and longwave radiative forcing dominates (Holland and Bitz, 2003;
Letterly et al., 2016; Pithan and Mauritsen, 2014; Garrett and Zhao, 2006).
Few studies have characterized Arctic aerosols, particularly those that may
act as cloud condensation nuclei (CCN) and ice-nucleating particles (INP),
during this period. Most studies in the winter–spring have focused on the
components of Arctic haze, long-range transported pollution from the
midlatitudes present in the Arctic after polar sunrise, including
non-sea-salt sulfate, soot, organics, and metals (e.g., Sturges and Barrie,
1988; Norman et al., 1999; Sirois and Barrie, 1999; Quinn et al., 2002;
Polissar et al., 1999; Hara et al., 2002b; Fisher et al., 2011). Notably,
particulate sulfate concentrations in the Alaskan Arctic during haze season
are 0.1–0.4 µgm-3 on average, and much higher than average
nitrate concentrations of 0.01–0.03 µgm-3 (Quinn et al.,
2007). Sea spray aerosol (SSA) has also been identified as a significant
contributor to the winter–spring aerosol budget by mass (10–30 %) in
the Canadian Arctic (Sirois and Barrie, 1999; Norman et al., 1999; Quinn et
al., 2002) and by number (55–85 %) in the Norwegian Arctic (Weinbruch et
al., 2012). SSA are efficient CCN (Collins et al., 2013; Quinn et al., 2014)
and can act as INP (DeMott et al., 2016), resulting in complex sea
ice–aerosol–cloud interactions in the Arctic (Browse et al., 2014). Gaseous
sulfuric acid or sulfur dioxide associated with Arctic haze has been shown to
react with SSA, resulting in sulfate formation and internally mixed
SSA–sulfate particles (Hara et al., 2002a, 2003). While less commonly
observed in the Arctic, reactions between gaseous HNO3 or N2O5
and SSA can also form mixed SSA–nitrate particles (Hara et al., 1999). These
multiphase reactions result in chlorine (HCl, ClNO2, Cl2)
liberation from SSA, contributing to atmospheric halogen chemistry (Sturges
and Barrie, 1988; Barrie and Barrie, 1990; Hara et al., 2002c, a). Given
changing marine emissions coupled with transported pollution, it is important
to understand aerosol chemical composition and heterogeneous processing to
determine impacts on climate in the winter Arctic.
To improve our understanding of Arctic aerosol chemical mixing state under
the changing radiation and sea ice conditions during the winter–spring
transition (following polar sunrise), atmospheric particles were collected
near Utqiaġvik (Barrow), Alaska, during January and February 2014.
Scanning electron microscopy with energy dispersive X-ray spectroscopy
(SEM-EDX), Raman microspectroscopy, and scanning transmission X-ray
microscopy with near-edge X-ray absorption fine structure spectroscopy
(STXM-NEXAFS) were utilized to characterize individual particle chemical
composition and mixing state. To our knowledge, these are the first
measurements of individual particle chemical composition in the Alaskan
Arctic during winter. The relative contributions of regional Arctic haze and
SSA on the aerosol budget during this winter–spring transition were
examined, and the mixing states of individual aerosol particles were
evaluated to examine atmospheric aging by multiphase reactions forming
sulfate and nitrate.
Methods
Atmospheric particle sampling was conducted from 23 to 28 January and 24 to 28 February
2014 near Utqiaġvik (Barrow), Alaska at a tundra field site
(71.28∘ N, 156.64∘ W) located ∼ 5 km
inland from the Arctic Ocean. Ozone concentrations and meteorological data,
including wind speed, wind direction, and solar radiation, were obtained
from the NOAA Barrow Observatory (71.32∘ N, 156.61∘ W),
located 5 km to the northeast of the sampling site and separated only by
flat tundra. Atmospheric particles were collected using a rotating
micro-orifice uniform deposition impactor (MOUDI, MSP Corp., model 110)
sampling at 30 LPM through a 10 µm cut-point cyclone (URG-2000-30EA)
located ∼ 2 m above the snow surface. The 50 % particle
collection efficiency size cuts for the six MOUDI stages used were 3.2, 1.8,
1.0, 0.56, 0.32, and 0.18 µm aerodynamic diameter (Da).
Particles were impacted on transmission electron microscopy (TEM) grids (Carbon
Type-B film copper grids, Ted Pella, Inc.) and silicon substrates (Ted
Pella, Inc.) for SEM analysis, and quartz substrates (Ted Pella, Inc.) for
Raman microspectroscopy analysis. Particle samples were stored frozen prior
to analysis to keep near the ambient temperature at collection. Samples
selected for analysis were collected for ∼ 24 h on 24–25 January
(10:15–10:00 AKST) and 27–28 January (11:00–10:30 AKST),
∼ 18 h on 26 January (11:00–17:15 AKST), ∼ 12 h
during 26 February daytime (09:00–19:30 AKST), 26 February nighttime (19:45–08:30 AKST),
27 February daytime (09:00–19:30 AKST), and 27 February nighttime (20:00–07:30 AKST).
These time periods were characterized by wind directions of
75–225∘ such that the town of Utqiaġvik was not upwind during
sampling. Polar sunrise occurred at Utqiaġvik on 22 January 2014.
Computer-controlled SEM (CCSEM) analysis of individual atmospheric particles
was completed using a FEI Quanta environmental SEM with a field emission gun
operating at 20 keV with a high-angle annular dark field (HAADF) detector
(Laskin et al., 2006, 2012). An EDX spectrometer (EDAX,
Inc.) collected X-ray spectra from elements with atomic numbers higher than
Be (Z=4). A total of 24 847 individual particles, typically ∼ 1000 per
substrate, were analyzed by CCSEM-EDX. A size distribution showing the number
of particles analyzed by CCSEM-EDX is shown in Fig. S1 in the Supplement. Morphological data,
including projected area diameter (Dpa) and perimeter, were collected
for each particle, in addition to the relative abundance of the following
elements quantified from the EDX spectra: C, N, O, Na, Mg, Al, Si, P, S, Cl,
K, Ca, and Fe. Individual particle data were analyzed using K-means clustering
of the EDX spectra (Ault et al., 2012; Shen et al., 2016; Axson et al.,
2016). K-means cluster analysis resulted in 50 clusters, which were then
grouped into five particle classes (fresh SSA, partially aged SSA,
organic + sulfate aerosol, fly ash aerosol, and mineral dust aerosol), based
on comparisons of cluster EDX spectra with particle classes identified in
previous studies. Prior ambient aerosol CCSEM-EDX studies have established
EDX spectral signatures for fresh and aged SSA (Ault et al., 2013a; Hara
et al., 2002c, 2003), organic + sulfate aerosol (Moffet et
al., 2010b; Laskin et al., 2006; Allen et al., 2015), fly ash
(Ault et al., 2012), and mineral dust
(Coz et al., 2009; Sobanska et al., 2003; Axson et al., 2016; Creamean et
al., 2016).
Individual particles from two MOUDI stages (1.0–1.8 and 0.56–1.0 µm
aerodynamic diameter size ranges) for each of the seven samples
were also analyzed by Raman microspectroscopy using a Horiba Scientific
Labram HR Evolution spectrometer coupled with a confocal optical microscope
(100× Olympus objective, 0.9 numerical aperture) equipped with a Nd : YAG
laser source (50 mW, 532 nm) and CCD detector. A 600 groove mm-1 diffraction
grating was used, yielding spectral resolution of 1.8 cm-1. The laser
power was adjusted between 25 and 100 % by varying a neutral density
filter to prevent damage to the sample. Raman spectra were obtained over the
500–4000 cm-1 range for ∼ 300 particles. Spectra were
compared with prior Raman studies of nascent and reacted sea spray aerosol
(Ault et al., 2013c, 2014).
Beamline 5.3.2 on the Advanced Light Source at Lawrence Berkeley National
Laboratory (Berkeley, CA) was used for STXM-NEXAFS analysis over the carbon
K edge (280–320 eV), as previously described by Moffet
et al. (2010a). Briefly, X-rays from the synchrotron were energy-selected
using a monochromator, focused on the sample, and raster scanned across a
selected area. The sample was rescanned at closely spaced X-ray energies to
complete a spectral image stack. After the X-ray spectra were converted to
optical density using the Beer–Lambert law, STXM-NEXAFS maps were generated
to show the distribution of organic carbon, soot, and inorganic components
in individual aerosol particles, based on the X-ray absorptions at 288.5,
285.4, and 283 eV, respectively. From the 26 February nighttime
sample (0.10–0.18 µmDa), 290 particles were analyzed for detection of
organic carbon. Dpa was measured by CCSEM-EDX, Raman, and STXM-NEXAFS;
therefore, it is the parameter reported for all data herein. Dpa is often
larger than geometric diameter due to particle deformation upon impaction
(Sobanska et al., 2014; Hinds, 2012; O'Brien et al., 2014), indicating
that particle size reported here is an upper bound and could represent
smaller diameter in the atmosphere.
Results and discussionChemical composition and size distribution of observed particle
types
Five individual particle classes, including fresh SSA,
partially aged SSA, organic + sulfate particles, fly ash, and mineral dust
particles, were identified from the CCSEM-EDX data (Fig. 1). SSA (both fresh
and partially aged) and organic (with and without sulfate) particles were
the most commonly observed types, indicating that mixing of sulfate with SSA
and organic aerosol may be significant in the winter Arctic. Fresh and
partially aged SSA comprised 99 %, by number, of the observed supermicron
particles (1.0–7.5 µmDpa) (Fig. 2). Across the submicron size
range (0.1–1.0 µmDpa), the majority of particles were also SSA
(50–75 %, by number) (Fig. 2). The prevalence of SSA particles, even in
the winter, may be a result of changing conditions in the Arctic, with
previous work showing local SSA influence in Utqiaġvik, Alaska, from nearby
sea ice leads, even during winter (May et al., 2016).
Organic particles (with and without sulfate) were also a significant
fraction (25–50 %, by number) of submicron particles. Only a limited
fraction of particles (∼ 1 % by number across the entire
size range) were classified as fly ash or mineral dust, characterized by
silicon and oxygen, with trace amounts of aluminum, sodium, and iron (Coz
et al., 2009; Sobanska et al., 2003).
Representative SEM images and EDX spectra of individual particles
corresponding to the main particle types observed by CCSEM-EDX, and the
average EDX spectrum for each particle type. Average spectra show the
relative peak areas of all elements analyzed by CCSEM-EDX. (a) Fresh SSA
particle comprised of sodium chloride core (red) and magnesium chloride
shell (black). The spectrum for the core is offset for clarity.
(b) Partially aged SSA particle containing sodium and more sulfur than chlorine.
(c) Organic + sulfate particle. (d) Organic + sulfate particle on silicon
substrate. (e) Aluminum- and silicon-containing dust particle. *Carbon and
oxygen peaks include some signal from TEM grid substrate background for
particles (a), (b), (c), and (e). Aluminum and silicon peaks are due to sample
holder and silicon substrate background, respectively, for particle (d).
Size-resolved CCSEM-EDX number fraction distributions of
observed particle types for all samples. Particles were sorted into 16 bins
(logarithmic) from 0.1 to 10.0 µm projected area diameter (8 bins per
decade). Organic + sulfate class includes a small fraction of internally
mixed soot.
Size-resolved number fractions of individual fresh SSA, partially
aged SSA, and organic + sulfate particles containing Cl, S, and N, in
addition to average atomic (mole) ratios of Cl / Na, S / Na, and
N / Na for individual fresh and partially aged SSA.
Particles classified as fresh SSA, based on grouping by chemical composition
by K-means analysis, contained sodium, magnesium, sulfur, and chlorine in
similar mole ratios (Table 1) to those found in seawater (Cl / Na = 1.2,
Mg / Na = 0.11, S / Na = 0.06) (Quinn et al., 2015; Pilson, 2013),
indicating these particles had not undergone chemical aging processes during
atmospheric transport. Some SSA particles were observed with a sodium
chloride core and magnesium chloride outer coating (Fig. 1), which is likely
due to the particle undergoing efflorescence after collection
(Ault et al., 2013b); this morphology has been
previously observed for Arctic SSA particles
(Chi et al., 2015). The partially aged
SSA particles contained sulfur and/or nitrogen and were characterized by
Cl / (Na + 0.5 Mg) ratios of less than 1 (Laskin et
al., 2012). This indicates that multiphase reactions had occurred, releasing
chlorine-containing trace gases, primarily hydrochloric acid (Laskin et
al., 2002, 2003; Gard et al., 1998), and resulting in the
formation of sulfate and nitrate in the particles. SSA chemical mixing state
information is further discussed in Sect. 3.2. SSA aging was observed for few
1.0–7.5 µm particles (7 %, by number, aged SSA and 90 % fresh
SSA), with a greater fraction of submicron 0.1–1.0 µm SSA particles
having undergone aging (18 %, by number, aged SSA and 42 % fresh SSA)
(Fig. 2). Compared to supermicron particles, submicron particles have longer
atmospheric lifetimes, a smaller Cl reservoir, and greater surface area to
volume ratios, which are conducive to increased atmospheric processing
(Hara et al., 2002a; Leck et al., 2002; Williams et al., 2002; Ault et
al., 2014). While concentrations of sulfur- and nitrogen-containing gases
are lower in the Arctic winter compared to the peak of spring haze season,
allowing for SSA particles to remain chemically fresh further from the
emission point, aged SSA particles have also been observed during winter at
Svalbard (Hara et al., 1999, 2002a). Overall, fresh and aged
SSA were significant contributors to the winter Arctic aerosol budget (Figs. 2 and S2).
This observation is consistent with studies of annual Arctic
aerosol trends that have shown a large influence of SSA in the winter by
mass: constituting up to 40 % of supermicron mass at Barrow
(Quinn et al., 2002) and 60–90 % of 0.5–10 µm
particles, by number, for winter samples at Svalbard (Weinbruch et al.,
2012).
Organic particles, classified by K-means analysis, were characterized by
spherical morphology and carbon and oxygen in the single-particle EDX
spectra. Since there is background C and O EDX signal from the TEM grid
substrate film, the contribution of C and O to this particle class was
confirmed by CCSEM-EDX analysis of 110 particles that had been collected
simultaneously on silicon substrates that do not have these interferences.
Figure 1 shows the representative EDX spectra of organic particles analyzed
on TEM grids and silicon substrates for comparison. Sulfur was present in 47 %,
by number, of organic particles, at levels of at least 2 % atomic
content in the EDX spectrum; therefore, these organic particles will be
discussed together as an organic + sulfate particle class (Laskin et al.,
2006; Moffet et al., 2010b). Example organic + sulfate particles are shown
in Fig. 1c and d. Organic + sulfate particles were primarily observed in the
submicron size range (Fig. 2). Overall, 40–50 % of the particles 0.1–0.5 µm
in diameter and 15–25 % of the 0.5–1.0 µm particles,
by number, were classified as organic + sulfate (Fig. 2). The
detailed chemical mixing states of these organic + sulfate particles will be
discussed in Sect. 3.3. The presence of a large number fraction of submicron
organic + sulfate particles is consistent with previous winter–spring Arctic
studies, which have observed organic particles contributing up to 30 % of
submicron aerosol by mass and greater than 80 %, by number, at Barrow
(Shaw et al., 2010; Hiranuma et al., 2013) and greater than 80 %, by
number, of 0.1–0.5 µm (aerodynamic diameter) particles at Svalbard
(Weinbruch et al., 2012). Internal mixing of organic and sulfate aerosol
has previously been observed in the Arctic winter–spring at Svalbard, with
most 0.2–2.0 µm (aerodynamic diameter) organic particles containing
sulfate (Hara et al., 2002b). Internally mixed
organic + sulfate aerosol is now being observed across the Arctic during the
winter, highlighting the importance of considering sulfate mixing states
during this period.
Internal mixing of SSA with sulfate and nitrate
Raman microspectroscopic analysis of individual aged SSA particles confirmed
that the sulfur and nitrogen detected by EDX in SSA were in the forms of
sulfate and nitrate, respectively, based on the presence of sharp peaks
corresponding to characteristic symmetric stretches at ∼ 1000 cm-1
for νs(SO42-) and ∼ 1050 cm-1 for
νs(NO3-) (Fig. 3) (Ault et al., 2014;
Deng et al., 2014; Eom et al., 2016). In addition, these particles were
characterized by broad peaks in the 3000–3500 cm-1 range (Fig. 3),
corresponding to O–H stretching, likely due to particle-phase water
(Ault et al., 2014), confirmed by the frequency of
the νs(NO3-)(aq) mode at ∼ 1050 cm-1.
Raman C–H stretching peaks in the 2800–3000 cm-1 range
indicated that organic compounds were present in both fresh and aged SSA
(Ault et al., 2013c; Baustian et al., 2012; Eom et al., 2016); the
organic functional groups present will be discussed further in a future
publication.
Optical images and Raman spectra of three representative
SSA particles containing nitrate and/or sulfate and hydroxyl groups. A total
of ∼ 300 individual particles were analyzed by Raman
microspectroscopy. * The 790–796 cm-1 peak is due to quartz substrate
background. Scale bar for all images is 5 µm.
Based on the CCSEM-EDX analysis, SSA aging by sulfur species (e.g., sulfuric
acid) was more prevalent than aging by nitrogen species (e.g., nitric acid)
in the submicron size range, consistent with previous measurements of SSA
during Arctic haze periods in the Norwegian Arctic (Hara et al., 2002c). A
total of 73 % of partially aged SSA, by number, in the
0.1–1.0 µm size range contained secondary sulfate. This was
determined by a S / Na ratio at least 25 % greater than the seawater
mole ratio 0.06 (Pilson, 2013), with these particles having an average
S / Na ratio of 1.07 (Table 1). In comparison, only 22 % of
0.1–1.0 µm particles contained nitrate (Table 1). The
diffusion-limited uptake of SO2 in submicron particles is favored over
the thermodynamically controlled uptake of HNO3, resulting in a
preference for sulfate in submicron aged SSA (Liu et al., 2007; Zhuang et
al., 1999; Kerminen et al., 1998). However, sulfate was also more prevalent
than nitrate in supermicron SSA (Table 1), where kinetically favorable uptake
of HNO3 would be expected to dominate, suggesting that higher
concentrations of H2SO4, compared to HNO3, and aqueous-phase
sulfate formation influenced
particle aging. The prevalence of SSA aging by sulfur species near
Utqiaġvik is consistent with the appearance of springtime Arctic haze, as
30 % of submicron particle mass corresponds to sulfate during haze season
(January to May) (Quinn et al., 2007, 2002). Sulfate mass concentrations peak
in winter–spring near Utqiaġvik, while methanesulfonic acid mass is
greatest in the summer and has not been observed during winter months (Quinn
et al., 2007). Therefore, the prevalence of mixed SSA–sulfate suggests that
reactions with sulfuric acid from Arctic haze are an important source of SSA
sulfate (Hara et al., 2002a; Barrie and Barrie, 1990). SSA aging through
sulfate addition was likely also due to influence from Prudhoe Bay SO2
emissions (Peters et al., 2011; Gunsch et al., 2017), discussed further in
Sect. 3.4.2.
Organic particle mixing states
Organic particles and internally mixed organic + sulfate particles composed
a significant number fraction of submicron particles, which is consistent
with the presence of organic aerosol, sulfuric acid, and ammonium sulfate in
Arctic haze (Hara et al., 2002b; Hirdman et al., 2010). STXM-NEXAFS
indicated the presence of organic carbon in these particles, based on X-ray
absorption at 288.5 eV, characteristic of carboxylic acids
(Moffet et al., 2010a). Additionally, STXM-NEXAFS
analysis confirmed that organic and inorganic (likely sulfate, based on
sulfur detected during CCSEM-EDX analyses) components were internally mixed
within individual particles (Fig. 4), with particles showing an internal mix
of both inorganic-dominant (> 50 %) and organic-dominant
regions. The pre- and post-edge ratio of inorganic to organic components also
indicated that most analyzed particles contained both inorganic and organic
species (Fig. 4b). Raman analysis confirmed sulfur was present in the form
of sulfate. Nitrogen (nitrate, according to Raman analysis) was also present
in 15 % of 0.1–1.0 µm organic + sulfate particles, by number.
Representative STXM-NEXAFS map from 26 February nighttime
showing (a) the distributions of inorganic dominant
(blue, > 50 % by mass), organic carbon dominant
(green, > 50 % by
mass), and soot (red, sp2 > 35 %) and (b) the ratio of
inorganic (pre-) and organic (post-edge) components between populations of
individual particles sampled during a period with a high fraction of
organic + sulfate particles.
Chemical mixing state analysis determined that a small fraction of particles
classified as organic + sulfate (7 % of this particle class, by number)
by CCSEM-EDX were primarily carbon-containing particles with less than 5 %
oxygen and sulfur. For the 26 February nighttime sample analyzed by
STXM-NEXAFS, elevated levels of sp2 carbon, indicative of soot, were
observed in some particles (Fig. 4) (Moffet et al.,
2010a). These small soot particles observed by STXM-NEXAFS were likely
members of the “primarily carbon” group identified by CCSEM-EDX and were
internally mixed with organic carbon and inorganic species (likely sulfate,
based on sulfur detected during CCSEM-EDX analyses). Therefore, these
particles were included in the organic + sulfate class. Externally mixed
soot particles, comprised solely of elemental carbon with no organic or
sulfate component, were not observed in any sample, indicating that all soot
was internally mixed with organic + sulfate particles. Soot present in
Arctic haze (Quinn et al., 2007; Law and Stohl, 2007) has previously been
observed to be internally mixed with sulfate for winter–spring Arctic
aerosol, with soot-sulfate particles contributing ∼ 10–20 %
of observed particles sampled (< 2.0 µm), by number, at
Svalbard (Hara et al., 2003).
Influence of marine- and Prudhoe Bay-influenced air masses on particle
composition
There was no clear dependence or trend with wind speed or month (January vs.
February) for SSA S / Na or Cl / Na ratios, with average wind speeds ranging from
5–12 m s-1 for the selected sampling periods, but some variability in
particle composition between samples could be attributed to the influence of
different air masses. Though all samples experienced some degree of Arctic
Ocean air mass influence due to the sampling location and prevailing wind
direction from the north over the Beaufort Sea to the sampling site, using
NOAA HYSPLIT 48 h backward air mass trajectory analysis (Rolph, 2016), two main air mass
source regions (Arctic Ocean and Prudhoe Bay influence) were determined for
the seven analyzed sample periods. Most notably, the 26 February daytime sample
was influenced by air from the north and east over the Arctic Ocean within
the boundary layer for the 6–7 h prior to arrival at the sampling site,
whereas the 27 January sample had prolonged surface influence (18 h) along
the air mass trajectory from the east to the southeast, during which the air mass
passed over Prudhoe Bay, the third largest oilfield in North America
(U.S. Energy Information Administration, 2015) (Fig. 5).
Prudhoe Bay influence was determined by HYSPLIT trajectories that passed
within 1∘ (∼ 50 km) of the Prudhoe Bay emissions box,
described in Kolesar et al. (2017) as the area significantly
influenced by combustion emissions from the oilfields. The air mass
trajectories for the remaining samples (24 January, 26 January day, 26 February
night, 27 February day, 27 February night) fell in between the two
regions (Arctic Ocean and Prudhoe Bay influence).
Size-resolved number fractions of observed particle types
(CCSEM-EDX), for example sample periods influenced by (a) the Arctic Ocean (26 February day, 4490 particles)
and (b) Prudhoe Bay (27 January, 1475 particles).
Air mass influence is shown for (c) 26 February daytime and (d) 27 January as
determined by NOAA HYSPLIT 48 h backward air mass trajectories. Both
ensemble (dotted line) and single representative trajectories are shown.
Color scale indicates air mass altitude, and markers are placed at 6 h
intervals. Red line shows extent of Prudhoe Bay emissions influence box
(Kolesar et al., 2017). Yellow diamond indicates sampling
site near Utqiaġvik.
Comparison of particle type contributions as a function of size for the
representative Arctic Ocean-influenced (26 February day) and Prudhoe Bay-influenced (27 January) samples are shown in Fig. 5 (with results of
additional samples shown in Fig. S2). The Arctic Ocean-influenced sample was
characterized by a large fraction (95 %, by number) of fresh SSA in the 1.0–7.5 µm
size range. In comparison, the Prudhoe Bay-influenced sample was
characterized by 55 % fresh SSA and 40–45 % partially aged SSA, by
number, in the supermicron range. This is indicative of multiphase reactions
between SSA and gaseous emissions from combustion at the oilfields (e.g.,
SO2, NOx) (Jaffe et al., 1991; Peters et al., 2011; Gunsch et
al., 2017), contributing to a greater number fraction of aged SSA during
Prudhoe Bay-influenced periods. The Prudhoe Bay-influenced sample also had a
greater number fraction of organic + sulfate particles in the 0.1–0.5 µm
range (60–70 %) compared to the Arctic Ocean-influenced
sample (40–50 %). Given that organic + sulfate particles were a
significant fraction of submicron particles in all samples, including
ocean-influenced periods, these samples were likely influenced by long-range
transported pollution from the midlatitudes, consistent with regional
background haze (Quinn et al., 2007). However, it is
likely that gas-particle partitioning of oxidation products from and multiphase reactions of Prudhoe Bay
oilfield combustion emissions, including volatile organic compounds and
SO2 (Peters et al., 2011; Jaffe et al., 1991; Gunsch et al., 2017),
also results in the formation of organic + sulfate particles, including
particles internally mixed with soot (Sect. 3.3), contributing to the
increased number fraction of organic + sulfate particles observed during
Prudhoe Bay-influenced periods.
Conclusions
For atmospheric particles collected in January and February 2014 near
Utqiaġvik, Alaska, SSA was observed to be the most prevalent particle
type, composing 50–75 and 99 %, by number, of particles in the
0.1–1.0 and 1.0–7.5 µm projected area diameter ranges,
respectively. Internal mixing of sulfate and nitrate with SSA particles was
observed in all samples, regardless of air mass influence, suggesting
prevalent regional pollution, such as Arctic haze influence, for secondary
inorganic aerosol formation. Prudhoe Bay-influenced air masses were
characterized by higher number fractions of partially aged SSA, however,
suggesting that oilfield emissions also contribute significantly to
multiphase reactions with SSA. Most global and regional climate models
assume that Arctic haze components (sulfate, organic aerosol, black carbon)
and natural aerosols are externally mixed and do not predict climate impacts
of internally mixed species (Eckhardt et al., 2015; Alterskjaer et al.,
2010; Korhonen et al., 2008). However, no externally mixed sulfate or
sulfuric acid particles were observed during January or February sampling in
Utqiaġvik, Alaska; all sulfate was internally mixed with organic aerosol
particles or with SSA. Internal mixing of SSA and sulfate reduces CCN
efficiencies compared to externally mixed sulfate aerosol or SSA, as sodium
sulfate is less hygroscopic than sodium chloride or sulfuric acid (Gong
and Barrie, 2003; Petters and Kreidenweis, 2007). The prevalence of SSA
internally mixed with sulfate should be considered in the interpretation of
elevated sulfate concentrations in the winter–spring Arctic atmosphere
(Sturges and Barrie, 1988; Sirois and Barrie, 1999; Hara et al., 2002a).
While SSA comprised 50–60 % of 0.1–0.5 µm particles, by number,
organic + sulfate particles made up 40–50 %, by number, in this particle
diameter range and were present in similar number fractions in all samples,
suggesting the importance of Arctic haze as a source of submicron particles
in January and February in Utqiaġvik, Alaska. Internal mixing of sulfate
and nitrate with organic aerosol is consistent with previous single-particle
measurements at Svalbard, where organic aerosol mixed with sulfate and
nitrate was observed to be the dominant particle type in the submicron size
range in the winter and spring (Weinbruch et al., 2012). Weinbruch et
al. (2012) also observed soot particles internally mixed with organics,
sulfate, and nitrate, consistent with the small fraction of internally mixed
organic + sulfate and soot particles (∼ 2–3 % of total
observed particles, by number) observed in this study. The internal mixing
of sulfate with organic aerosol is important to consider in climate
predictions, as the CCN activity of internally mixed organic + sulfate
aerosol is reduced relative to externally mixed sulfate, due to the lower
hygroscopicity of the organic fraction (Wang et al., 2015; Petters and
Petters, 2016). Continuing oil and gas development in the Arctic region will
influence both SSA and organic aerosol composition
(Peters et al., 2011), as well as mixing state, due to
secondary inorganic aerosol formation.
Data are available by contacting the corresponding authors.
The supplement related to this article is available online at: https://doi.org/10.5194/acp-18-3937-2018-supplement.
RMK, KAP, and APA prepared the manuscript and
led data interpretation. KAP collected the samples. RMK
analyzed the samples, with assistance from ALB. DB and RM conducted
the STXM-NEXAFS analysis. AL and BW assisted with
CCSEM-EDX analysis. APA provided guidance with CCSEM-EDX and Raman
microspectroscopy analysis.
The authors declare that they have no conflict of interest.
Acknowledgements
CCSEM-EDX analyses were performed at the Environmental Molecular
Sciences Laboratory (EMSL), a national scientific user facility located at
the Pacific Northwest National Laboratory (PNNL) and sponsored by the Office
of Biological and Environmental Research of the US Department of Energy
(DOE). PNNL is operated for DOE by Battelle Memorial Institute under
contract no. DE-AC06-76RL0 1830. Travel funds to PNNL and Alaska were
provided by the University of Michigan College of Literature, Science, and
the Arts and Department of Chemistry. Additional travel funds and logistics
support for sampling in Alaska were provided by the National Science
Foundation (PLR-1107695). Ryan C. Moffet acknowledges funding by US DOE's
Atmospheric System Research Program, BER under grant DE-SC0008643. The
STXM-NEXAFS particle analysis was performed at beamlines 5.3.2 at the
Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory. The
work at the ALS was supported by the Director, Office of Science, Office of
Basic Energy Sciences, of the US DOE under contract DE-AC02-05CH11231.
Bingbing Wang acknowledges the support by Chinese Fundamental Research Funds for the
Central Universities (no. 20720160111) and the Recruitment Program of Global
Youth Experts of China. Rachel M. Kirpes received funding in part from a University
of Michigan Davis Graduate Fellowship. Meteorological data were obtained
from the NOAA Earth System Research Laboratory Barrow Observatory. The
authors gratefully acknowledge the NOAA Air Resources Laboratory for the
provision of the HYSPLIT transport and dispersion model and READY website
(http://www.ready.noaa.gov) (Rolph, 2016) used in this publication.
Edited by: Annmarie Carlton
Reviewed by: two anonymous referees
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