ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-16-5191-2016Biogenic, anthropogenic and sea salt sulfate size-segregated aerosols
in the Arctic summerGhahremanRoyahttps://orcid.org/0000-0001-7855-8571NormanAnn-Lisealnorman@ucalgary.caAbbattJonathan P. D.https://orcid.org/0000-0002-3372-334XLevasseurMauriceThomasJennie L.Department of Physics and Astronomy, University of Calgary,
Calgary, CanadaDepartment of Chemistry, University of Toronto,
Toronto, CanadaDepartment of Biology, Laval University,
Québec, CanadaSorbonne Universités, UPMC Univ. Paris 06,
Université Versailles St-Quentin, CNRS/INSU, UMR8190, LATMOS-IPSL, Paris,
FranceAnn-Lise Norman (alnorman@ucalgary.ca)26April20161685191520212December20154February20168April201615April2016This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://acp.copernicus.org/articles/16/5191/2016/acp-16-5191-2016.htmlThe full text article is available as a PDF file from https://acp.copernicus.org/articles/16/5191/2016/acp-16-5191-2016.pdf
Size-segregated aerosol
sulfate concentrations were measured on board the Canadian Coast Guard Ship
(CCGS) Amundsen in the Arctic during July 2014. The objective of
this study was to utilize the isotopic composition of sulfate to address the
contribution of anthropogenic and biogenic sources of aerosols to the growth
of the different aerosol size fractions in the Arctic atmosphere.
Non-sea-salt sulfate is divided into biogenic and anthropogenic sulfate using
stable isotope apportionment techniques. A considerable amount of the average
sulfate concentration in the fine aerosols with a diameter
< 0.49 µm was from biogenic sources (> 63 %), which is
higher than in previous Arctic studies measuring above the ocean during fall
(< 15 %) (Rempillo et al., 2011) and total aerosol sulfate at higher
latitudes at Alert in summer (> 30 %) (Norman et al., 1999). The
anthropogenic sulfate concentration was less than that of biogenic sulfate,
with potential sources being long-range transport and, more locally, the
Amundsen's emissions. Despite attempts to minimize the influence of
ship stack emissions, evidence from larger-sized particles demonstrates a
contribution from local pollution.
A comparison of δ34S values for SO2 and fine aerosols was used
to show that gas-to-particle conversion likely occurred during most sampling
periods. δ34S values for SO2 and fine aerosols were similar,
suggesting the same source for SO2and aerosol sulfate, except for two
samples with a relatively high anthropogenic fraction in particles
< 0.49 µm in diameter (15–17 and 17–19 July). The high
biogenic fraction of sulfate fine aerosol and similar isotope ratio values of
these particles and SO2 emphasize the role of marine organisms (e.g.,
phytoplankton, algae, bacteria) in the formation of fine particles above the
Arctic Ocean during the productive summer months.
Introduction
Climate is changing in the Arctic faster than at lower latitudes (IPCC,
2013), and it has the potential to influence the Arctic Ocean and aerosols
that form above it. The Arctic Ocean is considered a source of primary
aerosol, such as sea salt and organics, as well as secondary particles from the oxidation of
SO2 to sulfate (SO42-) (Bates et al., 1987; Charlson et al.,
1987; Andreae, 1990; Yin et al., 1990; Leck and Bigg, 2005a, b; Barnes et
al., 2006; Ayers and Cainey, 2007). Aerosols drive significant radiative
forcing and influence climate directly (by the scattering of short- or
long-wave radiation) and indirectly (by changing the number and size of cloud
droplets and altering precipitation efficiency) (Shindell, 2007). Recently,
it has been shown that their net effect is cooling the Arctic, which offsets
around 60 % of the warming effect of greenhouse gases (Najafi et al.,
2015). However, there are key uncertainties in the estimation of aerosol
effects and their sources which arise from limited information on their
spatial and temporal distribution.
Sulfate in the Arctic atmosphere originates from anthropogenic, sea salt and
biogenic sources. Anthropogenic aerosols, with a winter-to-springtime maximum
known as Arctic haze, contain particulate organic matter, nitrate, sulfate
and black carbon which originate from North America and Eurasia (Sirois and
Barrie, 1999; Quinn et al., 2002; Stone et al., 2014). Sea salt enters the
atmosphere via mechanical processes such as sea spray and bubble bursting
(Leck and Bigg, 2005a). The formation of breaking waves on the ocean surface
(at wind speeds higher than 5 m s-1) leads to the entrainment of air
as bubbles into surface ocean water. These bubbles rise to the surface due to
their buoyancy and start to scavenge organic matter. They burst at the
air–sea interface and release sea spray aerosol (SSA), which includes
organic matter and inorganic sea salt (Quinn et al., 2015). Although, sea
salt is generally found in coarse-mode particles, it is sometimes found in
smaller sizes as well (Bates et al., 2006). Several mechanisms are
responsible for the formation of SSA with different sizes. Small film drops
are generated by the shattering of the film caps. Larger jet drops (with a
size range of 1 to 25 µm) are formed by the collapse of the bubble
cavity. Spume drops are torn from the crests of waves and enter the
atmosphere directly at high wind speeds above 10 m s-1 (Lewis and
Schwartz, 2004; Quinn et al., 2015).
The most important source of biogenic sulfate aerosols in the Arctic summer
is the oxidation of dimethyl sulfide (DMS) (Norman et al., 1999). DMS is
mostly produced by the breakdown of its algal precursor
dimethylsulfonopropionate (DMSP) by phytoplankton and bacteria DMSP lyases
and transported from the ocean to the atmosphere via turbulence and diffusion
which depends on sea surface temperature, salinity and wind speed
(Nightingale et al., 2000). Gaseous sulfur compounds from DMS oxidation are
able to form new particles or condense onto preexisting aerosols in the
atmosphere and thereby become large enough to act as cloud condensation
nuclei (CCN) (Charlson et al., 1987). However, there are crucial
uncertainties in the details of the potential impact of DMS on climate on a
global scale (Quinn and Bates, 2011).
The formation of new particles and CCN is particularly important during the
summer when anthropogenic aerosols are scarce, scavenging is efficient and
sea–atmosphere gas exchange produces considerable DMS in the Arctic (Gabric
et al., 2005; Elliott et al., 2012; Li and Barrie, 1993; Leaitch et al.,
2013). Some studies have suggested an increase in biological activity, DMS
production and emission with an increase in temperature and a decrease in
sea-ice cover during summer (Sharma et al., 2012; Levasseur, 2013). However,
modeling results from Browse et al. (2014) suggest that increased DMS
emissions during summertime will not cause a strong climate feedback due to
the efficient removal processes for aerosol particles. Such results are
highly dependent on aerosol size distributions, which are relatively
unconstrained particularly with respect to DMS oxidation (Bigg and Leck,
2001; Matrai et al., 2008;
Quinn et al., 2009; Leaitch et al., 2013).
Tracers, such as DMS and methanesulfonate (MSA) for biogenic activities
(Savoie et al., 2002), have been used in some studies to indicate different
sources for sulfate. Other studies have assumed that non-sea-salt sulfur
originates from biogenic sources in clean areas with low anthropogenic sulfur
emissions (Bates et al., 1992; Hewitt and Davison, 1997). These methods may
overestimate the role of biogenic sources if anthropogenic sulfate is
present. The isotopic differences of various sources present a way to
determine the oceanic DMS contribution to aerosol growth (Norman et al.,
1999, 2004; Seguin et al., 2010, 2011; Rempillo et al., 2011).
Size-segregated aerosols were collected in July 2014 during an extended
transect going from the strait of Belle Isle to Lancaster Sound in the
Canadian Arctic, permitting comparison with measurements from other seasons.
Sulfate aerosols have been apportioned into biogenic, anthropogenic and sea
salt sulfate using sulfur isotopes, to find the contribution of each source
in aerosol formation and growth.
Field description and methods
Particles were collected on board the Canadian Coast Guard Ship (CCGS)
Amundsen in the Arctic during July 2014 as part of the NETCARE
(Network on Climate and Aerosols: Addressing Key Uncertainties in Remote
Canadian Environments) project. The route of this expedition, which took
place from 8 to 24 July 2014, and sampling intervals are shown in Fig. 1.
The route of CCGS Amundsen from 8 to 24 July 2014. Circles
indicate sampling intervals for the high-volume sampler from 9 to 22 July
(9–11, 11–13, 13–15, 15–17, 17–19, 20–22 July). The high-volume sampler was off because of stormy
weather from 10:00 on 19 July to 10:00 on 20 July.
Wind speed and sea surface and air temperatures were documented every minute
and averaged over 10 min using the Automatic Voluntary Observing Ships
System (AVOS) system available onboard the Amundsen at ∼ 23 m
above the sea surface. In addition, a version of the Lagrangian particle
model, FLEXPART-WRF (FLEXible PARTicle dispersion model, Weather Research and
Forecasting; Brioude et al., 2013), was used to estimate potential emission
sensitivities. More details and figures of FLEXPART-WRF are published in
other studies from the same campaign (NETCARE 2014; e.g., Mungall et al.,
2015; Wentworth et al., 2016).
A high-volume sampler was used to collect aerosol samples at a calibrated
flow rate of 1.08 ± 0.05 m3 min-1. This high-volume sampler
was placed facing the bow above the bridge of the ship, around 30 m above
the sea surface. It was fitted with a cascade impactor to collect
size-fractionated particles on quartz filters as well as SO2. The
SO2 was trapped on a cellulose filter pretreated with potassium
carbonate (K2CO3) and a glycerol solution (Saltzman et al., 1983;
Norman et al., 2004; Seguin et al., 2010). The sampling interval was 2 days,
starting from 10:00 UTC. The high-volume sampler was turned off manually to
avoid contamination when the ship emissions toward the sampler were observed
or at times when the ship was stationary. Periods greater than 30 min are
reported in Table 1. Figure 1 shows sampling intervals: the high-volume
sampler was off because of stormy weather from 10:00 on 19 July to 10:00 on
20 July. The particle size was cut off at a flow rate of 1.13 m3 min-1, and standard temperature and
pressure (25 ∘C and 1 atm) for spherical particles is at 50 %
collection efficiency, and the six ranges of particle aerodynamic diameter of
the cascade impactor are as follows: A (> 7.2 µm), B
(3.0–7.2 µm), C (1.5–3.0 µm), D
(0.95–1.5 µm), E (0.49–0.95 µm) and F
(< 0.49 µm). Temperature and pressure effects are negligible;
however, the lower flow rate increases the cut off diameter slightly for each
size range (Tisch Environmental, Inc., 2004). TOTAL sulfate refers to the sum
of sulfate in each of the size fractions. Field blanks were collected on two
separate occasions and loaded and unloaded with the same method as used to
process the samples except that the high-volume sampler was turned off to
assess whether and how much contamination occurred from procedural handling
and analyses. Filters were stored in sealed ziplock bags at < 4∘C
before analysis in the lab.
Periods greater than 30 min when the high-volume sampler was off to
avoid contamination from ship emissions. The sampling interval was 2 days,
starting from 10:00.
Sampling intervalTurn off–on time (UTC)Reason for turning off(July 2014)of the high-volume samplerthe high-volume sampler9–1110 July: 12:40–13:10Ship emissions toward the sampler11–1311 July: 11:20–13:30Exchange of the sampler exhaust13–1515 July: 06:30–08:00The ship was stationary15–1717 July: 08:00–10:00The ship was stationary17–1918 July: 22:00–07:00∗The ship was stationary2–2221 July: 15:30–16:10Ship emissions toward the sampler
∗ 07:00 on the following day 19 July.
A LI-COR 7000 CO2/H2O Analyzer, with an inlet near the location of
the high-volume sampler (∼ 3 m) and at the same height was used to
measure the atmospheric CO2 mixing ratios. The objective of the
CO2 measurement was to determine the influence of smoke stack emissions
from the ship for quality assurance–quality control (QA/QC) of aerosol samples. The CO2 concentrations are
shown in Fig. 2a. There were two periods when CO2 measurements were not
saved due to a computer malfunction: 10:30 on 10 July to 09:00 on 11 July and 14:00 on 15 July to 10:35 on
17 July. The observation shows a relatively constant CO2 mixing ratio
with some peaks, indicating relatively little smoke stack contamination.
Panel (a): CO2 mixing ratio (ppm); panel (b):
wind speed (m s-1); panel (c): sea surface and air
temperatures (∘C). CO2 measurements were not reported from 10:30
on 10 July to 09:00 on 11 July and 14:00 on 15 July to 10:35 on 17 July. Wind
speed and temperatures were not recorded before 11 July.
Once back in the laboratory, sulfate extracted from filter extracts was
analyzed for sulfate isotopes and concentration. Filter papers were shredded
in distilled deionized water and sonicated for 30 min. Then, filter paper
fibers were removed by 0.45 mm Millipore filtration, and a portion of the
filtrate samples (2 × 10 mL) was used for ion concentration
measurements. Remaining filtrate was treated with 5 mL of 10 %
BaCl2 and 1 mL HCl to precipitate BaSO4. In addition to BaCl2
and HCl, 2 mL of 30 % hydrogen peroxide was added to SO2 filter
solutions to oxidize the SO2 to sulfate. After extraction, BaSO4
was dried and samples were packed into tin cups and analyzed with a PRISM II
continuous flow isotope ratio mass spectrometer (CF-IRMS) to obtain
δ34S values in parts per thousand (‰) (relative to VCDT,
Vienna Cañon Diablo Triolite) (Seguin et al., 2010). δ34S for sulfur isotopes is shown by the
abundance ratio of the two principal sulfur isotopes (34S/32S)
(Krouse et al., 1991).
δ34S(‰)={34S/32Ssample/34S/32Sstandard-1}×1000
The uncertainty for δ34S values (±0.3 ‰) was
determined by the standard deviation of the δ34S values of a suite
of internal standards bracketing the δ34S values of the samples.
Concentrations of cations (Ca2+, K+, Na+, Mg2+) and
anions (Cl-, SO42-, PO43-, NO3-) were obtained
by ion chromatography with a detection limit of 0.1 mg L-1. No peaks
were detected for sulfate in the blank filters, and the average concentration
of Na+ in the blank filters was 1.2 mg L-1 after extraction
(which is around 5 and 20 % of the maximum and minimum of the Na+
concentration in filter A with the most sea salt).
Three different sources – anthropogenic, biogenic and sea salt – was
considered for sulfur aerosols and the fraction of each source was obtained
using
[SO42-]total=[SO42-]bio+[SO42-]anthro+[SO42-]SS[SO42-]totalδ34Stotal=[SO42-]bioδ34Sbio+[SO42-]anthroδ34Santhro+[SO42-]SSδ34SSS.
Also, δ34SNSS was determined using the expression for
two-source mixing:
NSSδ34SNSS=measuredδ34Smeasured-SSδ34SSS,
where SS and NSS refer to sea salt and non-sea-salt sulfate, respectively, and
quantities in brackets, [X], indicate concentrations.
The amount of sea salt sulfate in sea water was calculated by
SO42-and Na+ mass ratios:
[SO42-]SS=0.252[Na+].
Sulfur isotope apportionment in the Arctic assumes a δ34S value of
+21 ‰ ± 0.1 (Rees et al., 1978),
+18.6 ‰ ± 0.9 (Sanusi et al., 2006; Patris et al., 2002) and +3 ‰ ± 3 (Li and Barrie, 1993; Nriagu and Coker, 1978;
Norman et al., 1999) for sea salt, biogenic and anthropogenic δ34S values, respectively. These values were used to find sea salt,
biogenic and anthropogenic fractions in this study. The partial derivative
rule for error propagation and standard deviation were considered for
uncertainties.
Average TOTAL, sea salt and non-sea-salt sulfate concentrations
(ng m-3), sulfur isotopic values (‰), and non-sea-salt fraction
(%) for size-segregated aerosol filters. Standard deviations are reported
in parentheses.
Interaction of wind at the ocean's surface may lead to the formation of
primary coarse-mode sea salt particles. DMS oxidation pathways, the formation
of biogenic SO2 and the production of new particles are influenced by
wind speed and temperature. Wind speed and sea and air temperatures from the
Amundsen's AVOS system are shown in Fig. 2b and c.
Sulfate aerosols
Total, sea salt and non-sea-salt sulfate concentrations and their standard
deviations for the entire sampling program for different size fractions are
summarized in Table 2.
Similar average sulfate concentrations were found for aerosols in
A> 7.2µm (113 ng m-3),
B3.0–7.2µm (100 ng m-3) and
D0.95–1.5µm (110 ng m-3) size
fractions. An average sulfate concentration of 34 ng m-3 was found for
the C1.5–3.0µm size aerosols. On the other
hand, the F< 0.49µm filter (fine aerosol) has the
highest average sulfate concentration (∼ 214 ng m-3) and
contains less than 3 % sea salt sulfate (6 ng m-3).
Sea salt sulfate
Table 2 includes average sea salt sulfate concentrations for aerosols for
different size fractions for this study. As expected, coarse-size filters
A> 7.2µm and B3.0–7.2 in this study
contain more sea salt sulfate than smaller-diameter aerosols and the average
sea salt sulfate is approximately 6 times higher than non-sea-salt sulfate.
In contrast, smaller aerosols on the
D0.95–1.5µm filter contain lower but
significant amounts of sea salt sulfate (∼ 55 ng m-3). Although,
on average, more than 75 percent of sulfate for the
C1.5–3.0µm filter is from sea salt, a
considerable decrease in concentration is observed compared to
A> 7.2µm, B3.0–7.2µm
and D0.95–1.5µm filters. Sea salt sulfate
concentrations are low for aerosols collected on the
E0.49–0.95µm and
F< 0.49µm filters (∼ 5 to 6 ng m-3). The
spatial variability of TOTAL sulfate and sea salt concentrations is shown in
Fig. 3a.
TOTAL sulfate, sea salt (a) and non-sea-salt (b)
sulfate concentrations (ng m-3) of aerosols on
A> 7.2µm–F< 0.49µm filters.
Numbers in the figure show TOTAL, sea salt and non-sea-salt sulfate
concentrations (ng m-3) in gray, blue and red colors,
respectively.
Non-sea-salt sulfate
The average non-sea-salt sulfate concentrations for the entire study are
reported in Table 2 (spatial variation in non-sea-salt sulfate is shown in
Fig. 3b). Results show approximately uniform TOTAL non-sea-salt sulfate
concentrations (average 130 ± 21 ng m-3; range from 102 to
152 ng m-3), except the first sample collected nearby the Gulf of St
Lawrence (8 to 10 July) which contains the highest non-sea-salt sulfate
concentration. The majority of sulfate for small aerosols in the
D0.95–1.5µm (∼ 55 ng m-3,
50 %), E0.49–0.95µm
(∼ 66 ng m-3, 93 %) and F< 0.49µm
(∼ 208 ng m-3, 97 %) fractions is from non-sea-salt sources.
DiscussionSea salt sulfate
Sea salt concentrations are variable with season and depend on atmospheric
stability (Lewis and Schwartz, 2004). Although wind is considered an
important factor in the sea–air exchange of sea salt, correlations in this
study between wind speed and sea salt sulfate concentrations for coarse- and
fine-mode aerosols were not significant (R2≅0.1), which is
consistent with previous studies (Lewis and Schwartz, 2004; Rempillo et al.,
2011; Seguin et al., 2011; Jaeglé et al., 2011).
Non-sea-salt sulfate
The spatial variation of non-sea-salt sulfate (anthropogenic plus biogenic
aerosols) is shown in Fig. 3b. Results show approximately uniform
non-sea-salt sulfate concentrations for samples in the Labrador Sea and
further north (130 ± 21 ng m-3). Sulfate concentrations,
especially non-sea-salt sulfate, in this research were found to be higher
than previous Arctic studies above the ocean during fall (2007–2008)
(Rempillo et al., 2011) and at higher latitudes at Alert in summer
(1993–1994) (Norman et al., 1999) and about the same as at Barrow, Alaska
during July (1997–2008) (Quinn et al., 2009). One reason could be higher
biological activity and biogenic aerosols from phytoplankton during summer,
as addressed in the next section.
Sulfur isotope apportionment
Total δ34S versus the percentage of sea salt sulfate of
size-fractionated aerosols is shown in Fig. 4. The mixing lines for sea
salt–biogenic sulfate (solid line) and sea salt–anthropogenic sulfate
(dashed line) are shown to demonstrate mixing for each pair of sources. Data
from this study fall mainly within the mixing lines, which suggests that the
assignment of the end-member δ34S values is appropriate. However,
it can also be seen that the data lie in two groups. One cluster has a high
percentage of sea salt sulfate (> 40 to > 95 %) and the second has
a very low percentage of (< 10 %) sea salt sulfate. There is a high
contribution of sea salt sulfate for aerosols on filters
A> 7.2µm and B3.0–7.2, and this
decreases for smaller-size aerosols. Sulfate aerosols on the
A> 7.2µm filter lie along the sea salt–anthropogenic
mixing line and are consistent with sea spray and a small contribution from
the ship's stack emission. Aerosols on the
B3.0–7.2µm,
C1.5–3.0µm and
D0.95–1.5µm filters and most of the
E0.49–0.95µm filters lie between the upper and
lower mixing line near the right-hand side of the Fig. 4. This indicates that
sulfate is dominated by sea salt for these samples, and the remainder is a
mixture of biogenic and anthropogenic sulfate. The δ34S value for
aerosols < 0.49 µm (F< 0.49µm filter) is
more variable, it indicates that very little sea salt sulfate is present, and
the majority of the sulfate is derived from a mixture of biogenic and
anthropogenic sulfate. Norman et al. (1999) showed that most data from Alert
during spring, fall and winter lie between 0 and +7 ‰, which
demonstrates a combination of anthropogenic and sea salt sulfate aerosols.
Also, their data show an increase in δ34S values during summer
(between +7 and +15 ‰) and confirm the importance of biogenic
sulfate. The δ34S data for non-sea-salt sulfate from Rempillo et
al. (2011) illustrate the dominance of anthropogenic sources (more than
70 %) during fall 2007 and 2008. In addition, Rempillo et al. (2011)
introduced a new sulfate source, the Smoking Hills
(δ34S =-30 ‰). This new source altered background
δ34S to -30 ‰ near the Smoking Hills on Cape Bathurst,
Northwest Territories (Fig. 1) and δ34S =-5 ‰
further away. There is no evidence from the isotope data for a significant
contribution of sulfate from the Smoking Hills in this study; however,
results from FLEXPART-WRF modeling show that several potential emissions
originated in or passed near the Smoking Hills (Fig. 5).
Total δ34S versus the percentage of sea salt sulfate of
size-fractionated aerosols. The mixing lines show sea salt–biogenic sulfate
(solid line) and sea salt–anthropogenic sulfate (dashed line) contributions.
The standard deviations of each run were taken as the uncertainty for
δ34S values.
FLEXPART-WRF backward configuration of potential emission
sensitivity plots for (a) 13 July (12:01:00), (b) 20 July
(12:17:00) and (c) 21 July (12:01:00). The black line shows the ship
track (note that these panels include the ship track after 23 July 2014 when
high-volume sampling was not performed). The air mass residence time
(seconds) before arriving at the ship location is shown with different
colors. Numbers on the panels show the approximate lifetime and the center of
the plume locations.
Anthropogenic and biogenic sulfate
The concentration of sulfate for aerosol samples derived from apportionment
calculations for non-sea-salt sulfate, anthropogenic and biogenic sources is
shown in Fig. 6. Results show an approximately uniform concentration
(130 ± 21 ng m-3) for sulfate aerosols in the Arctic region,
aside from the Gulf of the St Lawrence, which has around 4 times higher
concentrations (Fig. 6a). In addition, the highest concentration for both
anthropogenic and biogenic sulfate was found in the
F< 0.49µm filter in the Arctic region.
Non-sea salt (a), anthropogenic (b) and biogenic (c) sulfate concentrations for size-segregated aerosols in the Arctic and subarctic. Strictly speaking, Arctic samples
include those collected after 13 July. Inserts contain the first sampling
period (9–11 July) in the Gulf of St Lawrence.
Two possible sources for anthropogenic sulfate are ship emissions and
long-range transport (LRT). In the Arctic CO2 above background is likely
from ship emissions. The question is what is the appropriate background
CO2 mixing ratio? Analyses were performed assuming three different
levels for background CO2 (380, 385, 400 ppm). The result of these
analyses indicates that CO2 mixing ratios (Fig. 2a) reached 380, 385 and
400 ppm for less than 1.5, 0.5 and 0.1 % of sampling time, respectively,
and were relatively uniform in comparison with similar measurements by
Rempillo et al. (2011), which reached more than 2000 ppm when stack
emissions impacted the samples (on average, 5 % of the sampling time;
O. Rempillo, personal communication, June 2015). Therefore, the direct impact
of ship stack emissions on most aerosol samples in this study collected is
expected to be small. This was confirmed by nearly white filter samples after
collection for all size fractions during this study compared to filters which
appeared gray or black when contaminated by ship stack sulfate in the Surface
Ocean – Lower Atmosphere Study (SOLAS) study from 2007 to 2008 (O. Rempillo,
personal communication, June 2015; Rempillo et al., 2011). Furthermore, weak
correlations were observed between anthropogenic sulfate and CO2 for the
A> 7.2µm, B3.0–7.2µm,
D0.95–1.5µm,
E0.49–0.95µm and
F< 0.49µm samples, suggesting that some portion of
the anthropogenic sulfate was locally derived from the ship's emissions.
However, the correlations were poor, so CO2 is not considered an
adequate tracer to distinguish local sulfate from LRT.
Long-range transport of SO2 and particles is a second potential
mechanism affecting the concentration of anthropogenic sulfate during this
study. The lifetime of SO2 in the Arctic is more than 1 week (Thornton
et al., 1989), and this means that SO2 potentially acts as a reservoir from which new anthropogenic aerosols could
form. Long-range transport of anthropogenic sulfur dominates in the Arctic
winter and early spring because of the stable atmosphere and weak removal of
particles, and concentrations significantly decrease during summer because of
a lower number of sources within the polar front and stronger scavenging
(Quinn et al., 2002; Stone et al., 2014). The backward configuration modeling
of FLEXPART-WRF shows that potential emissions originated from the east for
the first 2 days (12 and 13 July), and expanded to cover a broader region
after that (Fig. 5 shows some examples of backward configuration results of
FLEXPART-WRF). The Hudson Bay area is an important source of DMS (Richards et
al., 1994), and air parcels originating from Hudson Bay may contain more
biogenic SO2 and sulfate. On the other hand, air parcels originating
from the south (North America) may contain more pollution from LRT.
Figure 6b shows the time series of anthropogenic sulfate concentrations for
size-segregated aerosols. The size fraction of aerosols is different for two
distinct anthropogenic sources: long-range transport and ship emissions. The
contribution of anthropogenic sulfate from long-range transport is highest
for the first sample collected in the Gulf of St Lawrence and is pronounced
in the E0.49–0.95µm and
F< 0.49µm filters. On the other hand, the
anthropogenic aerosol sulfate concentrations on filters
A> 7.2µm, B3.0–7.2µm
and C1.5–3.0µm were highest for samples
collected from 17 to 19 July, which suggests more sulfate from the ship's
emissions. Although the high-volume sampler was turned off when the ship was
stationary on each of these days, some anthropogenic aerosols from ship
emissions may have influenced the results for aerosol sulfate in that time
period (17 to 19 July).
Biogenic fraction of non-sea-salt sulfate (%) for each size range
of filter. There was not enough sample for isotope analysis for some
periods.
A considerable amount of the sulfate concentration, ranging from 18 to
625 ng m-3 for F< 0.49µm filters, is from
biogenic sources. These values are higher than previously measured in the
Arctic. For example, the average biogenic TOTAL sulfate concentration at
Alert was around 30 ngS m-3 during July (Norman et al., 1999). Also,
Rempillo et al. (2011) reported low biogenic sulfate concentrations with a
maximum and median equal to 115.2 and 0 ng m-3, respectively, above
the Arctic Ocean in the Canadian Arctic Archipelago during fall 2007 and
2008.
Figure 6b and c show that filter F< 0.49µm contains
the highest biogenic and anthropogenic sulfate concentrations for all samples
(except anthropogenic sulfate for 11–13 July). The biogenic fraction of
non-sea-salt sulfate for each size range is reported in Table 3: high
fractions of sulfate on filter F< 0.49µm were from
biogenic sources (73, 95, 92, 65 %), except for two samples collected on
July 15–17 (25 %) and 17–19 (41 %) (see Sect. 4.5).
Aerosol growth
The oxidation of SO2 occurs in the gas phase, the aqueous phase and also
on the surface of particles. The rate of this oxidation depends on factors
such as the presence of the aqueous phase in the form of clouds and fogs, the
concentration of oxidants such as H2O2 and O3, cloud pH, and
sunlight intensity. The δ34S value of aerosols reflects the
proportion of δ34S values for preexisting aerosols and SO2 by
the oxidation of local SO2 on the surface of, or within, preexisting
aerosols (Seguin et al., 2011). Although the δ34S value for
preexisting aerosols is not clear, it is reasonable to assume that particles
with different sizes and the same δ34S value originate from the
same source (Seguin et al., 2011). However, sulfur isotope fractionation can
confound apportionment. Harris et al. (2013) reported sulfur isotope
fractionation due to SO2 oxidation, which depends on temperature and
oxidation pathways. By solving isotope fractionation equations (Harris et
al., 2013) for the average temperature during sampling for this study
(∼ 5 ∘C), δ34S values of sulfate are
10.6 ± 0.7 ‰, 16.1 ± 0.1 ‰ and
-6.22 ± 0.02 ‰ for homogeneous, heterogeneous and transition
metal ion (TMI) oxidation, respectively. However, a comparison of the δ34S values for SO2 and the F< 0.49µm filter
(or any other size fractions) does not support consistent isotope
fractionation during SO2 oxidation for samples collected during this
campaign.
The isotope ratios (δ34S value) for
F< 0.49µm and SO2 filters are shown in Fig. 7
along with the 1 : 1 line. Four of six samples lay close to the 1 : 1
line, which suggests that they have the same source or mixture of sources
(and the same isotope ratio value). However, there are two samples, collected
on 15–17 and 17–19 July, with different δ34S values for SO2
and F< 0.49µm filter sulfate, which are shown with an
asterisk on Fig. 7. The anthropogenic fraction of sulfate for the
F< 0.49µm filter for these two sampling periods is
relatively high. Although the anthropogenic fraction of sulfate in
F< 0.49µm filters for these two sampling periods was
higher than the remainder of samples (refer to Sect. 4.4), SO2 was
predominantly biogenic (more than 80 %).
The isotope ratio (δ34S value) for
F< 0.49µm and SO2 filters along with the 1 : 1
line. Two samples with different δ34S values for SO2 and
F< 0.49µm filter sulfate are shown with asterisks.
Conditions for aerosol nucleation based on biogenic SO2 concentrations
were evaluated by Rempillo et al. (2011). They showed that the threshold
value for biogenic SO2 to form new particles was 11 nmol m-3 for
the clean Arctic atmosphere in fall. Sulfur dioxide concentrations in this
study were higher than this threshold throughout the July 2014 campaign
(average around 32 nmol m-3) except for 11–13 July. This is
consistent with the measurements of Mungall et al. (2015), who reported high
DMS concentrations in both the ocean and atmosphere during the same cruise.
When δ34S values for aerosol size fractions and SO2 are
similar, then it is likely that local SO2 oxidation lead to substantial
sulfate content. There are two periods where this is clearly the case and
biogenic sulfate was dominant:
11–13 July, with δ34S
values for E0.49–0.95µm and
D0.95–1.5µm filters of +14.2 and
+13.1 ‰, respectively, and
13–15 July, with δ34S values for SO2, F< 0.49µm
and E0.49–0.95µm filters of +16.7, +16.8
and +15.8 ‰, respectively.
In contrast, anthropogenic sulfate contributed to aerosol growth on
9–11 July with δ34S values for
E0.49–0.95µm and
D0.95–1.5µm filters equal to +5.4 and
+5.0 ‰, respectively.
It is interesting to note that δ34S values for 17–19 July on the
E0.49–0.95µm filters
(0.49–0.95 µm) and SO2 indicate almost pure biogenic
sulfur (δ34SE=+17.8 ‰,
δ34SSO2=+17.6 ‰). However, the
δ34S value for sulfate on the F< 0.49µm
filters (< 0.49 µm) was lower (+10.2 ‰). This
suggests that aerosols < 0.49 µm (F) for this sampling period
originated, in part, from anthropogenic sources, but aerosol growth from 0.49
to 0.95 µm (E) was dominated by the oxidation of biogenic SO2
at this time.
Conclusion
Size-segregated aerosol sulfate concentrations were measured in the Arctic
and subarctic during July 2014. Sulfate was apportioned between sea salt,
biogenic and anthropogenic sources using sulfur isotopes. Around 85 % of
coarse-mode (> 0.95 µm) aerosol sulfate was from sea salt.
However, there was little to no sea salt sulfate in fine aerosols
(< 0.49 µm), and more than 97 % of the sulfate in these
aerosols was non-sea-salt. Approximately uniform non-sea-salt sulfate
concentrations were found for TOTAL sulfate (130 ± 21 ng m-3) in
the Arctic atmosphere. The dominant source for fine aerosols and SO2 was
biogenic sulfur, arising from the oxidation of DMS, which is likely due to a
high ocean–atmosphere gas exchange and the large ice-free surface in the
Arctic during July (Levasseur, 2013).
A comparison of δ34S values for fine (< 0.49 µm)
aerosols and SO2 samples was used to show that the growth of preexisting
fine particles occurred primarily due to the oxidation of SO2 from DMS
during all sampling events except for two where a relatively high
anthropogenic fraction in the smallest submicron size
(< 0.49 µm, F filter) was found (15–17 and 17–19 July). The
dominance of ocean biogenic sources in fine-aerosol sulfate and the
similarity of the sulfur isotope composition for SO2 and these fine
particles highlight the contribution of marine life to the formation and
growth of fine particles above the Arctic Ocean during the productive month
of July.
Acknowledgements
This study was part of the NETCARE (Network on Climate and Aerosols:
Addressing Key Uncertainties in Remote Canadian Environments) project and was
supported by funding from NSERC. The authors would also like to thank the
crew of the Amundsen and fellow scientists. Edited by: B. Ervens
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