Atmos. Chem. Phys., 15, 11341–11353, 2015
www.atmos-chem-phys.net/15/11341/2015/
doi:10.5194/acp-15-11341-2015© Author(s) 2015. CC Attribution 3.0 License.

Sea salt aerosols as a reactive surface for inorganic and organic
acidic gases in the Arctic troposphere
J. W. Chi1 , W. J. Li1 , D. Z. Zhang2 , J. C. Zhang3 , Y. T. Lin3 , X. J. Shen4 , J. Y. Sun4 , J. M. Chen1 , X. Y. Zhang4 ,Y. M. Zhang4 , and W. X. Wang1
1 Environment Research Institute, Shandong University, Jinan, Shandong 250100, China
2 Faculty of Environmental and Symbiotic Sciences, Prefectural University of Kumamoto,Kumamoto 862-8502, Japan
3 Key Laboratory of the Earth’ Deep Interior, Institute of Geology and Geophysics,
Chinese Academy of Sciences, Beijing 100029, China
4 Key Laboratory of Atmospheric Chemistry of CMA, Institute of Atmospheric Composition,
Chinese Academy of Meteorological Sciences, Beijing 100081, China
Correspondence to: W. J. Li (liweijun@sdu.edu.cn)
and Y. M. Zhang (ymzhang@cams.cma.gov.cn)
Received: 24 May 2015 – Published in Atmos. Chem. Phys. Discuss.: 18 June 2015
Revised: 17 September 2015 – Accepted: 29 September 2015 – Published: 12 October 2015

Abstract. Sea salt aerosols (SSA) are dominant particles in
the Arctic atmosphere and determine the polar radiative balance. SSA react with acidic pollutants that lead to changes
in physical and chemical properties of their surface, which inturn alter their hygroscopic and optical properties. Transmission electron microscopy with energy-dispersive X-ray spectrometry was used to analyze morphology, composition, size,
and mixing state of individual SSA at Ny-Ålesund, Svalbard,
in summertime. Individual fresh SSA contained cubic NaClcoated by certain amounts of MgCl2 and CaSO4 . Individual
partially aged SSA contained irregular NaCl coated by a mixture of NaNO3 , Na2 SO4 , Mg(NO3 )2 , and MgSO4 . The comparison suggests the hydrophilic MgCl2 coating in fresh SSA
likely intrigued the heterogeneous reactions at the beginningof SSA and acidic gases. Individual fully aged SSA normally
had Na2 SO4 cores and an amorphous coating of NaNO3 . Elemental mappings of individual SSA particles revealed thatas the particles ageing Cl gradually decreased, the C, N, O,
and S content increased. 12 C− mapping from nanoscale secondary ion mass spectrometry indicates that organic matter
increased in the aged SSA compared with the fresh SSA.
12 C− line scan further shows that organic matter was mainly
concentrated on the aged SSA surface. These new findings
indicate that this mixture of organic matter and NaNO3 onparticle surfaces likely determines their hygroscopic and op-tical properties. These abundant SSA as reactive surfaces adsorbing inorganic and organic acidic gases can shorten acidic
gas lifetime and influence the possible gaseous reactions in
the Arctic atmosphere, which need to be incorporated into
atmospheric chemical models in the Arctic troposphere.

1

Introduction

The Arctic atmosphere was long believed to be an extremely
clean background laboratory to research aerosol chemical
processes, transport of atmospheric aerosols, and their impact on global climate (Law and Stohl, 2007). In the last
decades, however, Arctic temperatures have increased attwice the global average rate (Serreze and Francis, 2006),
resulting in a dramatic decrease in Arctic pack ice (Lindsay et al., 2009). This Arctic climate change has been attributed to the increase in greenhouse gases in the troposphere (Vavrus, 2004) and to anthropogenic emissions of air
pollutants from the middle latitudes (Barrie, 1986; Iziomon
et al., 2006; Law and Stohl, 2007). In particular, the “Arctic Haze” phenomenon caused by the long-range transport ofanthropogenic emissions (e.g., organic acids, H2 SO4 /SO2 ,and HNO3 /NOx ) from lower latitudes has received increas-

Published by Copernicus Publications on behalf of the European Geosciences Union.

11342J. W. Chi et al.: Sea salt aerosols as a reactive surface for inorganic and organic acidic gases

ing attention in the last 30 years (Heintzenberg, 1980; Shaw,
1995; Law and Stohl, 2007).
Various aerosol particles from industrial, urban, and marine emissions occur in the Arctic atmosphere (Goto-Azuma
and Koerner, 2001; Ghorai et al., 2014). Black carbon as one
solar radiation absorber in the troposphere and in the snowor ice (Hegg et al., 2009) can amplify the change in the Arctic climate (Sand et al., 2013). In addition, H2 SO4 /SO2 andHNO3 /NOx from anthropogenic sources in middle latitudes
are transported into the central Arctic area (Law and Stohl,
2007), where they react with sea salt aerosols (SSA) (Hara etal., 2003; Geng et al., 2010; Sierau et al., 2014). SSA havereactive surfaces that can easily participate in heterogeneousand multiphase chemical reactions (Rossi, 2003; Ammann etal., 2013) and play a significant role in the global S and N cycle (Pósfai et al., 1994; Sierau et al., 2014). These aged SSA
not only affect incoming solar radiation by scattering or absorbing directly (Murphy et al., 1998), but can also serve as
cloud condensation nuclei (CCN) or ice nuclei (IN) and further alter cloud properties (Hu et al., 2005; Leck and Svensson, 2015). It was also proved that such cloud changes cause
40 % of the Arctic warming (Vavrus, 2004). Understanding
the chemical composition of the Arctic SSA is critical to understanding how they affect the polar climate (Hara et al.,2003).
SSA have been intensively studied in both laboratory andfield experiments in coastal areas (O’Dowd et al., 1997;
O’Dowd and De Leeuw, 2007; Yao and Zhang, 2011; Li etal., 2011b; Laskin et al., 2012; Ault et al., 2012; Ghorai etal., 2014). In laboratory studies Ault et al. (2013a) suggested
spatial redistribution of the cations (Na+ , K+ , Mg2+ , andCa2+ ) between core and surface after heterogeneous reactionof SSA and nitric acids. Deliquescence-mode experimentswith mixed NaCl/MgSO4 aerosol particles show that Mg
salts (e.g., MgSO4 and MgCl2 ) concentrated in the surface
can easily take up water and lower the deliquescence pointof NaCl particles (Woods et al., 2010). Hara et al. (2003)
suggested that most SSA play a pivotal pole in the lower
troposphere (< 3 km) and result in noticeable depletion ofchlorine in the Arctic. Laskin et al. (2012) found that SSA
may effectively react with organic acids, leaving behind particles depleted in chloride and enriched in the corresponding organic salts in one polluted coastal area. Organic acids
can react with SSA and change their hygroscopicity in polluted continental air (Ghorai et al., 2014), which can alter
their heterogeneous reactions and CCN activity (Laskin etal., 2012). In light of the limited microscopic observationsof Arctic aerosols, at least two questions have not been answered. Firstly, detailed information about ageing processesof SSA surfaces has not been determined in the Arctic atmosphere. Secondly, to what degree do organic acids reactwith SSA in the clean atmosphere? This knowledge is criticalto understanding how small amounts of anthropogenic gases(e.g., organic acids, H2 SO4 /SO2 , and HNO3 /NOx ) affect the

Atmos. Chem. Phys., 15, 11341–11353, 2015

hygroscopic and optical properties of SSA in the clean Arctic
atmosphere.
To characterize individual SSA collected on 3–23 August
2012, at the Chinese Arctic Yellow River Station, we applieddifferent microscopic techniques with resolution down to
the nanometer scale: transmission electron microscopy withenergy-dispersive X-ray spectrometry (TEM/EDX), scanning electron microscopy (SEM), scanning TEM (STEM),
and atomic force microscopy (AFM), and nanoscale secondary ion mass spectrometry (nanoSIMS). The different
types of individual SSA were identified based on their morphology and composition. Elemental and ion mappings revealed mixing properties of different species in individual
SSA that further participated in chemical reactions in theclean Arctic atmosphere. The dual aims of our study are to
explain the need to understand the complexity of realistic
atmospheric SSA and to provide fundamental experimental
data to understand heterogeneous reactions of SSA in theclean Arctic atmosphere.
2

Experiments

2.1

Sampling

Svalbard is an archipelago in the Arctic Ocean consisting of
Spitsbergen, Bear Island, and Hopen, which together coverabout 62 000 km2 . Ny-Ålesund, whose geographical coordinates are 78◦ 550 N, 11◦ 560 E, is situated on the western coastof Spitsbergen and is an international center of scientific research and environmental monitoring in the central Arctic.
Aerosol samples were collected from 3 to 23 August 2012,using an individual particle sampler at the Chinese Arctic Yellow River Station, Ny-Ålesund (Fig. S1 in the Supplement). We use copper TEM grids coated with carbon
film (carbon type-B, 300-mesh copper, Tianld Co., China)
to collect aerosols by a single-stage cascade impactor witha 0.5 mm diameter jet nozzle at a flow rate of 1.0 L min−1 .
The collection efficiency of the impactor is 50 % for particles
with an aerodynamic diameter of 0.3 µm and almost 100 % at
0.5 µm if the density of particles is 2 g cm−3 . The sampling
duration of each sample varied from 20 min to 2 h, dependingon the aerosol dispersion on the film that was estimated by
optical microscopy after the sampling. Then we placed thegrids in sealed, dry, plastic capsules to prevent contamination. Finally, the samples were stored in a desiccator at 25 ◦ C
and 20 ± 3 % relative humidity (RH) until analysis (Li et al.,2011a).
According to the sampling time and sample quality,
23 aerosol samples were selected for analysis with TEM.
During the sampling periods, temperatures were 1.6–7.3 ◦ C;
RH, 56–94 %; air pressure, 997.0–1020.7 hPa; and samplingwind speeds, 0–8.9 m s−1 . Detailed sampling information
can be found in Table S1.

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J. W. Chi et al.: Sea salt aerosols as a reactive surface for inorganic and organic acidic gases
2.2

Electron microscopic analyses

Individual particle samples were examined by JEOL JEM2100 transmission electron microscopy operated at 200 kV
with energy-dispersive X-ray spectrometry (TEM/EDX) andscanning electron microscopy (SEM; Philips XL30) operated
at a 20 kV accelerating voltage and a 80 µA filament. EDX
spectra were examined within a maximum time of 30 s tominimize potential beam damage and efficiently collect particle elemental composition. TEM grids were made of copper(Cu) and covered by a carbon-reinforced substrate, so Cu had
to be excluded from the quantitative analyses of the particles
while residual C content was detected and overestimated in
EDX spectra of individual particles. Elemental composition,
morphology, size distribution, and mixing state have beenstudied by the TEM/EDX. Both submicron and supermicronparticles occur on the TEM grids: coarser particles occur near
the center, and finer particles are on the periphery (Li andShao, 2009). To be more representative, 4–5 areas from thecenter to periphery of the sampling spot were chosen to analyze all the particles. Altogether we analyzed 1577 aerosol
particles to understand the details of their mixing states, bothinternal and external. We used iTEM software to analyze theTEM images and obtained area, circularity, perimeter, andequivalent circle diameter of individual aerosol particles.
Some typical aerosol particles were analyzed for elemental mapping with a JEOL JEM-2100F TEM with scanning
TEM (STEM) operation mode. Elemental mappings werecollected in the annular dark-field imaging mode, with theelectron beam focused on a corresponding spot of the sample
but then scanned over this area in a raster. Elemental mapping
has become an important tool in individual particle analysis
in recent years (Ault et al., 2012; Li et al., 2013a, b), because
it clearly displays the distribution of the detectable elements
within each particle.
2.3

11343

NanoSIMS analysis

Individual aerosol particles were analyzed using a nanoscalesecondary ion mass spectrometer (nanoSIMS) 50L(CAMECA Instruments, Geneviers, France), an ultrahigh vacuum technique for surface and thin-film analysis at
the Institute of Geology and Geophysics, Chinese Academyof Sciences. In this study, 12 C− , 16 O− , 12 C14 N− , 14 N16 O−
2,
32 S− , 35 Cl− , and 23 Na16 O− ions in individual particles
were obtained when the Cs+ primary ion beam caused theionization of atoms within the particles. Furthermore, ion
intensity mappings of individual particles with nanometer
resolution can show the distribution of different ions.
12 C− represents the organic matter in individual particles
that excludes the contribution from the carbon substrate.
Laboratory-generated NaNO3 particles on silicon substrate
23 Na16 O− areanalyzed by nanoSIMS show 14 N16 O−
2 andtheir markers (Fig. S3).

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Figure 1. Morphology and relative abundances of typical individual
aerosol particles in summertime Arctic samples: (A) TEM imagesof four types of aerosol particles, (B) relative abundances of different particle size ranges. The number of the analyzed particles indifferent size ranges is shown above each column.

2.4

AFM analysis

Atomic force microscopy (AFM) with a tapping mode analyzed aerosol particles under ambient conditions. AFM,
a digital NanoscopeIIIa Instrument, can detect the threedimensional morphology of particles. The AFM settings contain imaging forces between 1 and 1.5 nN, scanning rates between 0.5 and 0.8 Hz, and scanning range sizes at 10 µm witha resolution of 512 pixels per length. After the AFM analysis, composition of the same particles was confirmed by
TEM, with twelve SSA particles analyzed by this method.
The NanoScope analysis software can automatically obtainbearing area (A) and bearing volume (V ) of each analyzed
particle according to the following formula:
r
 2
π d2
4A
d
A = πr = π ×
=
→d =
,
2
4
π
r
4 3 4 π D3
π D3
3 6V
V = πr = ×
=
→D=
,
3
3
8
6
π
2

(1)
(2)

where d is the equivalent circle diameter and D the equivalent volume diameter.
Additionally, we know the relation between d and D is
D = 0.7487 × d, as shown in Fig. S2. As a result, the equivalent circle diameter (d) of individual aerosol particles meaAtmos. Chem. Phys., 15, 11341–11353, 2015

11344J. W. Chi et al.: Sea salt aerosols as a reactive surface for inorganic and organic acidic gases

Figure 2. Morphology and EDX spectra of the typical fresh SSA. (a) One SEM image, (b–h) TEM images, and (i–l) EDX spectra of NaCl,
MgCl2 , CaSO4 , and MgCa-rich. The main anionic elements are shown in the square brackets.

sured from the iTEM software can be further converted into
equivalent volume diameter (D) based on this relationship.
3
3.1

Results
Types and size distribution of Arctic aerosol
particles

Based on their different morphology and composition,
aerosol particles were divided into four major categories: sea
salt (Fig. 1A-a), S-rich (OC coating) (Fig. 1A-b), Ca/Mg–
S/N/Cl (Fig. 1A-c) and mineral (Fig. 1A-d). TEM observations indicated that SSA were most abundant from 100 nm
to 10 µm in the summer Arctic atmosphere. Figure 1B showsthat the relative abundance of SSA in the samples was about
72 % of the total aerosols. This result is consistent with thedominant SSA in different seasons reported for the Arctic atmosphere (Hara et al., 2003; Geng et al., 2010).
3.2

The microscopic characterization and elements of
SSA

Based on their morphology and composition, we can further
identify three types of SSA: fresh SSA, partially aged SSA,
and fully aged SSA.

3.2.1

Fresh SSA

The fresh SSA refer to particles that have not experienced
any atmospheric chemical modification after emission. TEM
and SEM images clearly show that individual fresh SSA include the cubic NaCl core with MgCl2 and CaSO4 coating(Fig. 2). Figure 2i shows that the NaCl core only contains Na
and Cl, with their ionic concentration ratio (Na / Cl) close
to 1 : 1. The coating contains appropriate quantities of Mg,
Ca, S, O and Cl, which could be defined as CaSO4 , MgCl2 ,and other species (Fig. 2j, k, l). NaCl particles surrounded by
the Mg-rich and Ca-rich coatings have been inferred by SEM
and TEM (Pósfai et al., 1994; Murphy et al., 1998; Hara etal., 2003; Geng et al., 2010), but the details about the coatings have not been revealed in the Arctic atmosphere. In thisstudy, the high-resolution TEM and SEM images displayed
the accurate mixing structures of NaCl and other species.
Here we identified two kinds of fresh SSA: one, a NaCl coreencased by a distinct coating of CaSO4 and Mg-rich (MgCl2 )materials (e.g., Fig. 2a–d), and another, a NaCl core encasedby a mixture of Mg-rich and Ca-rich materials (e.g., Fig. 2e–
f).
3.2.2

Partially aged SSA

We define partially aged SSA as those particles that experience atmospheric chemical modifications on their surfaces
but retain the NaCl core. Figure 3a–f show that individual
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J. W. Chi et al.: Sea salt aerosols as a reactive surface for inorganic and organic acidic gases

11345

Figure 3. TEM images of typical partially aged SSA and EDX spectra of their coatings. (a and b) NaCl partially surrounded by the mixed
species, containing Mg, Ca, Na, S, O, N and minor Cl; (c and d) NaCl completely surrounded by the mixed species with Mg, Ca, Na, S, O, N
and minor Cl; (e and f) the rod-like Na2 SO4 particles (minor Mg, and Ca) associated with NaCl. (g–j) The EDX spectra of one selected areain the internally mixed coating, including Na, Ca, Mg, S, and O with minor Cl and K. N cannot be directly measured but is inferred based onelemental composition in aerosols.

partially aged SSA clearly include NaCl core and coating.
The morphology of the partially aged SSA differs from the
fresh ones, as shown in the TEM images of Figs. 2 and 3.
Figure 3 shows that the core still keeps the crystalline phaseof NaCl with its irregular shape and that the coatings mainly
consist of Na, Mg, Ca, K, O, and S with either negligible orminor Cl. Typical Cl-depletion phenomena suggest that the
SSA likely underwent chemical ageing in the atmosphere, as
below:
MgCl2(s or aq) + HNO3(g or aq) → Mg(NO3 )2(aq) + HCl(g or aq) (R1)

MgCl2(s or aq) + H2 SO4(g or aq) → MgSO4(aq) + HCl(g or aq)(R2)

NaCl(s or aq) + HNO3(g or aq) → NaNO3(aq) + HCl(g or aq)(R3)

NaCl(s or aq) + H2 SO4(g or aq) → Na2 SO4(aq) + HCl(g or aq) (R4)(s, solid; aq, aqueous; and g, gaseous)
Similar chemical reactions on SSA have been detectedin both coastal air and laboratory experiments (Allen et al.,
1996; Gard et al., 1998; Kouyoumdjian and Saliba, 2006;
Ault et al., 2013a). These heterogeneous chemical reactions
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significantly change the morphology and composition of
SSA. Our results suggest that the MgCl2 coatings in fresh
SSA were converted into a more complex coating mixture of
MgSO4 , Mg(NO3 )2 , Na2 SO4 , and NaNO3 in partially aged
SSA.
3.2.3

Fully aged SSA

In fully aged SSA the NaCl core has been completely transformed into NaNO3 and Na2 SO4 through atmospheric heterogeneous chemical reactions with acidic gases. Figure 4
shows that the fully aged SSA have completely lost their
NaCl core, suggesting that the Cl in SSA was completely
depleted through heterogeneous chemical reactions, such
as Reactions (R1)–(R4). The rod-like Na2 SO4 aggregatescomprising the core were frequently internally mixed with
NaNO3 . TEM study indicates that Na2 SO4 and NaNO3 are
crystalline and amorphous materials, respectively. The fully
aged SSA (Fig. 4) on the substrate become more round compared with fresh SSA (Fig. 2) and partially aged SSA (Fig. 3).
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11346J. W. Chi et al.: Sea salt aerosols as a reactive surface for inorganic and organic acidic gases

Figure 4. TEM images and EDX spectra of the typical fully aged SSA. Major species are shown in parentheses, and minor elements are insquare brackets. (a and b) This Na-rich particle contains mostly Na, O, N, and S. The rod-like aggregates are Na2 SO4 and the species of
the non-defined shape are NaNO3 . The NaNO3 -containing coating is more sensitive to the strong electron beam than the Na2 SO4 . (c andd) Na-rich particles mainly contain C, N, O, Na, and S. One typical transparent coating with high C and minor O, Na, S or N was stable
under a strong electron beam, suggesting a possible organic coating. (e and f) The particles mainly contain Na, O, and S. Many small rod-like
Na2 SO4 gather together to form one particle. (g–j) EDX spectra of one selected part within individual SSA marked in TEM images. N cannot
be directly measured but is inferred based on elemental composition in aerosols.

3.3

The SSA ageing and back trajectories of air masses

To summarize three types of SSA, we make one Na–Cl–O
triangular diagram. Figure 5 gives general information about
Cl depletion among the fresh, partially aged, and fully aged
SSA. Three particle types in the triangular diagram display
an interesting distribution: the fresh SSA around the NaCl,the partially aged SSA in the center of triangular (including partial NaCl), the fully aged SSA around NaNO3 , andNa2 SO4 (chloride full depletion).
Three-day (72 h) back trajectories of air masses were generated using the HYSPLIT model at the Chinese Arctic Yellow River Station during 3–23 August 2012, at an altitude of
500 m above sea level (Fig. 6). Most air masses originate in
the Arctic Ocean, and are restricted to this vast marine region during the sampling periods. Figure 6 shows that twogroups of back trajectories exhibit different ageing degrees
Atmos. Chem. Phys., 15, 11341–11353, 2015

of SSA: one group originates from the central Arctic Ocean
and the other one from North America and Greenland. Fractions of the fresh, partially aged, and fully aged SSA are
43.07, 44.53, and 12.41 % in the first group and 18.72, 47.45,
33.83 % in the second group, respectively (Fig. 6). As a result, air masses from North America and Greenland broughtlarge amounts of aged SSA into the Arctic area in summertime.
3.4

Size distribution of individual SSA

Figure 7 shows size distributions of the fresh, partially aged,
and fully aged SSA. The peaks of fresh and partially aged
SSA are 0.68 and 1.30 µm, respectively, and the fully aged
ones display a broad size range from 0.9 to 2.0 µm (Fig. 7).
Our results show that the average particle sizes gradually
increase from fresh SSA, partially aged SSA, to fully aged
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J. W. Chi et al.: Sea salt aerosols as a reactive surface for inorganic and organic acidic gases

Figure 5. Triangular diagram of Na–Cl–O showing EDX data of theelemental composition of 405 SSA particles. The three stars represent the elemental composition of pure NaCl, Na2 SO4 , and NaNO3 ,respectively.

Figure 7. Size distributions of 336 fresh, 607 partially aged, and
255 fully aged SSA collected in Arctic summer. The SSA with diameters lower than 100 nm were not considered in this study.

SSA. Therefore, dry and wet deposition of aerosol particles
could be changed following the degree of the particles ageing.

4
4.1

Figure 6. 72 h back trajectories of air masses at 500 m over Arctic
Yellow River Station in Svalbard during 3–23 August 2012, andarriving time was set according to the sampling time. Air masses
were divided into two groups by the yellow line: one group fromthe central Arctic Ocean and the other one from North America
and Greenland. Pie charts show the number fractions of the fresh,
partially aged, and fully aged SSA.

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11347

Discussion
Sulfate and nitrate formation in individual SSA

TEM observations classified the SSA into fresh, partiallyaged, and fully aged SSA in the Arctic atmosphere. Using
individual particle analysis of the three types of SSA, it isshown that there is a major change in their internal structure
and composition. The STEM further determined elemental
mappings of Na, Mg, Ca, Cl, S, O, C, and N in the threekinds of SSA. Elemental mapping can clearly display theelemental distribution within each particle, which indicatesthe possible heterogeneous chemical reactions on their surfaces (Conny and Norris, 2011; Ault et al., 2012). Figure 8
shows that Cl content decreases and S, O, C and N contents
increase from partially aged (particle B) to fully aged SSA(particle C). The Na–Cl–O triangular diagram further showsthat Cl in SSA has been depleted and O content increases
likely through additional chemical reactions (Fig. 5). Figure 9a shows 35 Cl− in the NaCl core and minor 16 O− and32 S− in the coating in fresh SSA. Figure 9b shows the absence of 35 Cl− and the high intensity of 16 O− , 14 N16 O−
2,
32 S− , and 23 Na16 O− in fully aged SSA. These results arecompletely consistent with the TEM and STEM observationsin Figs. 2, 4, and 8. Based on the mappings of 32 S− (sulfate),
14 N16 O− (nitrate), and 23 Na16 O− in Fig. 9, the nanoSIMS
2
analysis provides direct evidence of Na2 SO4 and NaNO3 formation in fully aged SSA. As a result, sulfate and nitrate accumulated in SSA through atmospheric chemical reactions
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11348J. W. Chi et al.: Sea salt aerosols as a reactive surface for inorganic and organic acidic gases

Figure 8. Bright and dark-field TEM images of the fresh, partially aged, and fully aged SSA and elemental mapping of Na, Mg, Ca,
Cl, S, O, C, and N. (A) One fresh SSA particle, (B) one partially aged SSA particle, (C) one fully aged SSA particle. The dot intensity
represents elemental concentration within an individual particle. The arrows in particles B and C represent increase or decrease in elemental
concentration compared to particle A.

(e.g., Reactions R1–R4) in the Arctic area, which has been
observed in polluted air by Laskin et al. (2012). These mapping results from STEM and nanoSIMS are consistent with
the TEM/EDX results shown in Figs. 2–4 and with investigators who found that individual SSA in the Arctic air contained certain amounts of sulfate or nitrate by SEM/EDX andaerosol time-of-flight mass spectrometry (ATOFMS) (Haraet al., 2003; Geng et al., 2010; Sierau et al., 2014). However, we found that individual aged particles simultaneously
contained sulfate and nitrate, suggesting individual SSA underwent heterogeneous reactions with different acidic gases
during ageing processes in the Arctic atmosphere. A number of studies found that a large fraction of SSA internally
mixed with sulfate and nitrate occurred from coastal to background marine air and from equatorial to high latitudes (Andreae et al., 1986; Pósfai et al., 1994; Middlebrook et al.,
1998; Hara et al., 2003; Geng et al., 2010; Li et al., 2011b;
Yao and Zhang, 2011; Laskin et al., 2012). Interestingly, we
noticed that fully aged SSA (Fig. 8C) had an elevated carbon
content, suggesting that organic matter might occur in theaged SSA (details in Sect. 4.2). Laskin et al. (2012) showedthat SSA can react with secondary organic acids and result information of organic salts in the polluted air.
Na mappings show that Na is absent in the coating of fresh
SSA (Fig. 8A) but is present in minor amounts in the coating of partially aged SSA (Fig. 8B), which is consistent with
the EDX results as shown in Figs. 2–3. In contrast, Mg, S,
and O are absent from the core of fresh SSA (Fig. 8A), but
are present in certain amounts within the core of partiallyaged SSA (Fig. 8B). Therefore, we can deduce that MgCl2
coatings of the fresh SSA firstly reacted with acidic gases
through chemical Reactions (R1) and (R2) as the fresh SSA
were transformed into the partially aged SSA. The presenceof MgCl2 coatings in the fresh SSA is particularly interesting because it is known to readily deliquesce at a considerably lower point (33 % RH at 298 K) than that of NaCl
(75 %) (Wise et al., 2009). We conclude, therefore, that thisAtmos. Chem. Phys., 15, 11341–11353, 2015

MgCl2 coating consistently allowed an aqueous layer to coat
the fresh SSA in the Arctic atmosphere, because the ambient RH (56–94 %) exceeded the deliquescence point. Despite
the relatively low concentration of MgCl2 in SSA, the liquid surface significantly enhances heterogeneous reactions
rates through the uptake of acidic gases (e.g., H2 SO4 /SO2 ,and HNO3 /NOx ) (Liu et al., 2007). Microscopic observations
suggest that MgCl2 on fresh SSA likely initiated and promoted the heterogeneous reactions between SSA and acidic
gases. To our knowledge, this is the first demonstration of theinfluence of minor MgCl2 in individual SSA in field studies, although this phenomenon has been reported in laboratory experiments (Zhao et al., 2006; Liu et al., 2007; Wise etal., 2009). In addition, MgCl2 appears to have greater importance in individual Arctic SSA than elsewhere because theMg2+ / Na+ ratio in SSA increases following a temperature
decrease (Hara et al., 2012). In the partially aged particles,
the new coating containing MgSO4 , Mg(NO3 )2 , and NaNO3(Fig. 3) likely remained as a supersaturated liquid coating on
the surface of the ambient SSA in the Arctic area (Zhao etal., 2006; Li et al., 2008; Woods et al., 2010). Therefore, theMg salts in fresh and partially aged SSA are important surfactants to speed up particle ageing in the Arctic area.
In this study we found an abundance of fully aged SSA in
this Arctic area (Figs. 4–6). Ault et al. (2013a) found that theNa+ , Mg2+ and Ca2+ in individual SSA undergo a spatial
redistribution after heterogeneous reaction with nitric acid in
the laboratory. We noticed a similar phenomenon in which
Na, Mg, and Ca mappings in particle C were different fromparticles A and B (Fig. 8). Na is enhanced at the surface;
Mg and Ca tend to concentrate in the individual particle center. Figure 8C shows that carbon intensity becomes strong infully aged SSA, similar to the C-rich coating in Fig. 4c–d.
These results suggest that the aged SSA probably contain organic matter (details in Sect. 4.2). In addition, most of the
fully aged SSA have a Na2 SO4 core coated with NaNO3(with minor Mg(NO3 )2 and MgSO4 ) (Fig. 4). Hygroscopic

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11349

32 − 35 −23 16 −
Figure 9. NanoSIMS-based ion intensity mappings. Mappings of 12 C− , 16 O− , 12 C14 N− , 14 N16 O−
2 , S , Cl , and Na O fromone fresh SSA particle (a) and one fully aged SSA (b). Lines 1–2 represent the line scanning on the surfaces of individual particles. The redline represents the profile of 12 C− in fully aged SSA.

experiments of individual SSA show that the surfaces of partially and fully aged SSA have earlier deliquescence than thatof fresh SSA based on their hygroscopic growth on the substrate. The results indicate that the nitrate coatings likely influence particle surface deliquescence (Fig. S4). In the laboratory, the pure NaNO3 can take up water starting at 25 %RH and grow continuously with increasing RH (Lee and Hsu,
2000). Hu et al. (2010) also illustrated that NaNO3 did notexhibit an obvious deliquescence phenomenon in the hygroscopic experiment. As a result, some partially and fully aged
Arctic SSA had liquid surfaces in ambient air (56–94 % RH)
and this liquid amounted to certain amounts of water in theseaerosol particles. Park et al. (2014) supplied the evidenceof water in Arctic particles through the hygroscopicity andvolatility tandem differential mobility analyzer. The hygroscopic growth of particles can cause 1.6 to 3.7 times morenegative aerosol direct radiative effects (Sand et al., 2013)
than particles without it.
4.2

Organic matter in aged SSA

Integrated observations of individual SSA through the TEM,
STEM, and nanoSIMS provided direct evidence of the occurrence of sulfate and nitrate in aged SSA in the Arctic atmosphere. Laskin et al. (2012) showed that SSA effectively
reacted with organic acids in polluted coastal air, leaving behind particles depleted in chloride and enriched in the corresponding organic salts. Moreover, substitution of Cl in SSA
by weak organic acids was suggested in the laboratory by Ma
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et al. (2013). However, whether weak organic acids participated in the chloride depletion remains uncertain in the clean
Arctic air.
NanoSIMS technology has been adopted to characterizeorganic matter in SSA through 12 C− and 12 C14 N− mappings(Fig. 9). 12 C− mapping of fresh SSA indicates extremely
small amounts of organic matter in its coating (Fig. 9a).
Tervahattu et al. (2002) found that organic films on SSA
are common in fresh marine aerosols because of bursting
bubbles from the spay of the waves. Quinn et al. (2015)
also found that fresh SSA are internally mixed with organic
matter in the northeastern Atlantic atmosphere using proton
nuclear magnetic resonance (1 HNMR). In comparison withfresh SSA in Fig. 9a, the 12 C− intensity of the aged SSA
in Fig. 9b was much enhanced. The only explanation is that
more organic matter from the atmosphere was incorporated
into the aged SSA. Similar phenomena in aged SSA havebeen observed by laser mass spectrometry at Cape Grim andthe amounts of additional sulfate and organic matter in individual particles may be an indication of particles ageing(Middlebrook et al., 1998). Additionally, with increasing organic matter in SSA, their morphology and crystallization
behavior changes (Ault et al., 2013b).
Interestingly, 12 C− line scan in aged SSA further indicates that organic matter is mostly restricted to their surfaces(Fig. 9b). The near-edge X-ray absorption fine structure spectroscopy (NEXAFS) carbon K-edge spectrum of the organic
phase reveals a dominant contribution from carboxylic acids
on the surface of the aged SSA (Laskin et al., 2012). We fur-Atmos. Chem. Phys., 15, 11341–11353, 2015

11350J. W. Chi et al.: Sea salt aerosols as a reactive surface for inorganic and organic acidic gases

Figure 10. The conceptual model based on our study summarizing the possible SSA ageing processes from fresh, partially aged, and fully
aged SSA. Deliquescence RH (DRH) of NaCl is at 75 %. SSA ageing could have different processes before and after the DRH.

ther found organic matter internally mixed with nitrate on
the surface for aged SSA. This finding supports the resultsfrom laboratory experiments and conforms to predictions ofliquid–liquid phase separation theory (Ault et al., 2013a). As
a result, we obtained one conceptual model based on theseabove findings summarizing the possible SSA ageing processes in the Arctic atmosphere (Fig. 10).
The nanoSIMS analyses of the aged SSA in the Arctic
air show that their surface layers commonly contain organic
matter. The organic layer may influence trace gas uptake, andsubsequently impact trace gas budgets of O3 and NO2 in the
Arctic (McNeill et al., 2006). This is the first demonstrationof organic-coated SSA in clean Arctic air, which extends theapplication of heterogeneous reactions to aerosol particles(Fig. 10). Comparisons of fresh and aged SSA in Fig. 9 suggest that these organic coatings likely took part in the chloride depletion during particle ageing. The chloride depletionin the SSA induced by the presence of organic acids should
be incorporated into the atmospheric chemistry models forclean marine air, in addition to the coastal urban regions reported by Laskin et al. (2012). Randles et al. (2004) reported
that NaCl internally mixed with organic matter can reduce radiative cooling substantially compared to pure NaCl aerosolin the laboratory. In addition, how the organic layer influences hygroscopic and optical properties of the aged SSA in
realistic atmospheres remains unknown. Our measurements
make clear the need to understand SSA’s complexity in Arctic air.

Atmos. Chem. Phys., 15, 11341–11353, 2015

In summary, our results indicate that formation of sulfate, nitrate, and organic salts in the SSA can remove tracegases (e.g., organic acids, H2 SO4 /SO2 , and HNO3 /NOx ) in
the Arctic air, which were mostly emitted by various anthropogenic sources from middle latitudes (Law and Stohl, 2007;
Chang et al., 2011). On the one hand, the results of theseheterogeneous reactions could change the CCN or IN abilities, optical properties, and further dry and wet depositionof SSA. On the other hand, these surface reactions of SSA
could affect the photochemical reactivity in the Arctic air because they release gaseous halogen species (De Haan et al.,
1999; Knipping et al., 2000; von Glasow, 2008; Osthoff etal., 2008; Thornton et al., 2010; Phillips et al., 2012).

5

Conclusions

TEM observations indicated that SSA were most abundantfrom 100 nm to 10 µm in the summer Arctic atmosphere, accounting for about 72 % of the total aerosol particles. Threetypes of SSA, fresh SSA, partially aged SSA, and fully aged
SSA, were identified based on their morphology and composition. The fresh SSA particles exhibited a single core-shellstructure. Containing only Na and Cl, the core consisted of
the cubic NaCl crystal with the coating of Mg, Ca, S, O andCl, identified as CaSO4 and MgCl2 . Individual partially aged
SSA particles consisted of the single irregularly shaped NaClcore coated with Na, Mg, Ca, K, N, O, and S, with or without minor Cl, suggesting a single Cl-depletion phenomenon
in the formation of partially aged SSA. The fully aged SSA
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J. W. Chi et al.: Sea salt aerosols as a reactive surface for inorganic and organic acidic gases
lack the NaCl core, suggesting that the Cl in SSA was completely depleted through heterogeneous chemical reactions.
Nonetheless, most fully aged SSA consist of the Na2 SO4core with the amorphous NaNO3 coating.
STEM further determined the elemental mappings of Na,
Mg, Ca, Cl, S, O, C, and N in the three kinds of SSA. We
found that Cl content decreases and S, O, C and N contents increase along with particles ageing from partially to
fully aged SSA. NanoSIMS technology has been employed
to obtain secondary ion intensity mappings of 12 C− , 16 O− ,12 C14 N− , 14 N16 O− , 32 S− , 35 Cl− , and 23 Na16 O− . 14 N16 O−22
and 23 Na16 O− mappings proved NaNO3 formation in fully
aged SSA. These results show that sulfate and nitrate formedin SSA through atmospheric chemical reactions in the Arctic area. In addition, MgCl2 coating on fresh SSA can induce heterogeneous reactions between SSA and acidic gases
because MgCl2 is known to readily deliquesce at a considerably lower point (33 % RH at 298 K) than that of NaCl
(75 %). The partially and fully aged SSA were expected tohave liquid surfaces (MgSO4 , MgNO3 , and NaNO3 ) in Arctic ambient air (56–94 % RH); such surfaces contain substantial amounts of water, so the coating contents were conduciveto accelerating the heterogeneous reactions. 12 C− mappingof individual fresh SSA obtained by nanoSIMS indicates extremely small amounts of organic matter in the coating of
the particle, although this organic matter was somewhat enhanced in the aged SSA. We also found that organic matter
was mostly limited to the particle surface.
The composition of the SSA and their internal heterogeneity likely have important effects on their hygroscopic andoptical properties, dynamics of phase separations, and heterogeneous reaction with inorganic and organic acidic gases.
These microscopic observations for SSA provide insights
into the system of gas–aerosol–climate in the Arctic atmosphere.
The Supplement related to this article is available online
at doi:10.5194/acp-15-11341-2015-supplement.

Acknowledgements. We appreciate Peter Hyde’s comments andproofreading. This work was funded by the National Natural
Science Foundation of China (41575116), the Chinese Arctic andAntarctic Administration (2012WZ03003), Shandong Natural Science Funds for Distinguished Young Scholar (JQ201413), Young
Scholars Program of Shandong University (2015WLJH37), China
Polar Science Strategy Research Foundation (20140310), and Fundamental Research Funds of Shandong University (2014QY001).
We gratefully acknowledge the NOAA Air Resources Laboratory(ARL) for the provision of the HYSPLIT transport used in thispublication.
Edited by: M. Shiraiwa

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