Microsoft Word - 4_AAQR-16-09-OA-0383

Aerosol and Air Quality Research, 17: 878–887, 2017

ISSN: 1680-8584 print / 2071-1409 online

doi: 10.4209/aaqr.2016.09.0383

Mixed Chloride Aerosols and their Atmospheric Implications: A Review

Hao Wang

, Xinfeng Wang

, Xue Yang

, Weijun Li

, Likun Xue

, Tao Wang

1,2

, Jianmin Chen

Wenxing Wang

Environment Research Institute, Shandong University, Ji’nan, China

Department of Civil and Environmental Engineering, the Hong Kong Polytechnic University, Hong Kong, China

ABSTRACT

Natural and anthropogenic chloride aerosols make up a significant fraction of atmospheric particulate matter and play

important roles in the boundary layer chemistry. Here we provide a review of the mixing characteristics of chloride

aerosols and the subsequent atmospheric implications, which are rarely considered in current field and modeling studies.

Single-particle analytical techniques have shown that a large fraction of chlorides mix internally with other components, in

particular inorganic salts and organic matters, instead of existing separately. In marine and coastal regions, high

proportions of chloride aerosols usually mix with inorganic substances (e.g., Mg, Ca, K, N, S), while small quantities of

them are coated by organic matter. In forest, grassland, and agricultural areas, most chlorides in biomass burning particles

mix with or are coated by organics. In industrialized urban areas, the chloride aerosols often co-exist with heavy/transition

metals (e.g., Zn, Pb) and are coated by organic materials in aged plumes. Moreover, secondary chlorides also mix with

mineral dusts, nitrates, and sulfates. The mixing of chloride aerosols with insoluble substances can inhibit their

hygroscopic properties, which in turn affects the cloud condensation nuclei activation and heterogeneous reactivity. The

encasing of chloride aerosols within light-absorbing substances changes their optical properties and subsequently causes

atmospheric warming. This paper emphasizes the complexity of the mixing of chloride aerosols, as well as the potential

atmospheric implications thereof, and proposes some research topics deserving future study.

Keywords: Chloride aerosol; Mixing state; Hygroscopicity; Reactivity; Optical properties.

INTRODUCTION

Chloride aerosols have been identified as a major

constituent of atmospheric particulate matter, and play

important roles in tropospheric chemistry (Erickson and

Duce 1988; Carslaw et al., 2010; Laskin et al., 2012).

They exist in various chemical forms and different sizes in

diverse locations. In marine and coastal regions, sea salts,

dominated by sodium chloride, contribute a large fraction

to the mass and number concentrations of aerosols, most of

which exist in super-micro particles (Erickson and Duce

1988; Gong et al., 2002; O’Dowd and de Leeuw, 2007). In

forest, grassland, and agricultural areas, aerosols

containing potassium chloride contribute almost half of the

number concentration of fine particles in biomass burning

plumes (Pratt et al., 2011). In industrialized urban areas,

aerosols containing metal chlorides contribute as much as

73% to the fine particles emitted in industrial smoke

Corresponding author.

Tel.: 86-531-88364675; Fax: 86-531-88361990

E-mail address: xinfengwa[email protected]u.cn

(Moffet et al., 2008a). Chloride aerosols can be activated

via reactions with strong acids or acid anhydrides, and in

turn further anticipate the tropospheric chemistry. Previous

field studies have confirmed that particulate-phase chlorides

(e.g., sodium chloride and potassium chloride) react with

sulfuric acid and/or nitric acid to release gas-phase HCl,

which further reacts with NH

to produce ammonium

chloride or with dust to produce chloride-dust (Sullivan et al.,

2007a). Recent studies have revealed that the heterogeneous

reactions of N

on chloride aerosols release ClNO

which serves as an important source of Cl atoms (Osthoff

et al., 2008; Thornton et al., 2010; Wang et al., 2016).

With the development and application of single-particle

analytical techniques, detailed information has been

obtained on the chemical constituents and mixing states of

individual aerosols, showing that a large fraction of chloride

aerosols mix with other substances, which alters the

behavior of chloride aerosols. Scanning electron microscopy

(SEM) and transmission electron microscopy (TEM) can

be used to identify the size and elemental distribution of

individual chloride aerosols in the atmosphere, together with

the mixing characteristics with other constituents (Barkay et

al., 2005; Laskin et al., 2006; Adachi and Buseck, 2008;

Chi et al., 2015). The recently developed aerosol time-of-

Wang et al., Aerosol and Air Quality Research, 17: 878–887, 2017

879

fight mass spectrometry (ATOFMS) can provide the size

and chemical composition of a large number of individual

aerosols in real time (Moffet et al., 2008b; Bi et al., 2011;

Prather et al., 2013). These single-particle techniques have

provided new insights and a deeper understanding of the

mixing characteristics of chlorine-rich aerosols including

sea salts, particles in biomass burning plumes and industrial

smoke, and secondary chloride aerosols. Andreae et al.

(1986) unexpectedly found that a large fraction of mineral

substances and sulfates in the marine boundary layer were

internally mixed with sodium chlorides. Subsequently, several

field studies have confirmed that a significant proportion of

sea salts are internally mixed with sulfates and nitrates (Mouri

et al., 1993; Laskin et al., 2002; Li et al., 2003a), while

other research has revealed that the chloride components in

sea salts acquire a coating of organic surfactants in polluted

coastal environments or even at remote islands (Tervahattu

et al., 2002a, b; Laskin et al., 2012; Chi et al., 2015).

Electron microscopy and single-particle mass spectrometry

have indicated that potassium chlorides in biomass burning

smoke mix with soot and organics, and some even acquire

an organic surface coating (Silva et al., 1999; Li et al.,

2003b). In urban areas, field studies have detected plentiful

submicron metal chloride containing particles, which are

typically mixed with carbonaceous materials in combustion

emissions (Moffet et al., 2008b; Li et al., 2009; Geng et al.,

2010; Hu et al., 2015). Additionally, in long-range transport

dust plumes over the Pacific, Sullivan et al. (2007b) observed

that a large amount of secondary particulate chlorides

(produced from gas-phase HCl) was internally mixed with

dust particles. The mixing of various chloride aerosols with

other substances, particularly insoluble and light-absorbing

materials, may significantly alter the hygroscopic, reactivity,

and optical properties, as well as their effect on the climate,

thus causing the apparent deviation between model

predictions and ambient observations. In spite of the above

studies, the current knowledge on the mixing state of chloride

aerosols is generally fragmentary, and a comprehensive

understanding is required.

This study reviews the currently available, scattered

studies on mixed chloride aerosols (the major field campaigns

are shown in Fig. 1) to obtain a clearer understanding of

their mixing properties and the subsequent atmospheric

implications. First, we provide a brief introduction to the

commonly used single-particle analytical methods. Then, we

summarize the previous results on the composition of mixed

chloride aerosols, their mixing characteristics, the co-existing

constituents and the necessary conditions for aerosol

mixing, according to a classification into sea salt, biomass

burning particle, industrial smoke dust, and secondary

chloride particle. Finally, we summarize and discuss the effect

of the mixing of chloride aerosols on the hygroscopicity,

reactivity, and optical properties of the particles.

ANALYTICAL TECHNIQUES FOR PARTICLE

MIXING STATE

The development of single-particle techniques has allowed

substantial progress in research into particle mixing states,

and offers considerable advantages compared with the

Fig. 1. Map showing the previous field studies on mixing characteristics of chloride aerosols with various techniques.

Wang et al., Aerosol and Air Quality Research, 17: 878–887, 2017

880

traditional analytical methods for bulk aerosols. Bulk

aerosol analytical methods (e.g., online and offline ion

chromatography and aerosol mass spectrometry) provide only

information on the concentrations of different components

for large amounts of particles. To obtain the mixing

characteristics of different constituents in individual particles,

several advanced single-particle techniques have been applied

or developed in the past decades, including SEM, TEM,

ATOFMS, scanning transmission X-ray microscopy with

near-edge X-ray absorption fine-structure spectroscopy

(STXM-NEXAFS), nanoscale secondary-ion mass

spectrometry (NanoSIMS), and time-of-flight secondary-

ion mass spectrometry (TOF-SIMS).

SEM is a widely used technique in which the surface of

a single particle is imaged by scanning it with a high-energy

beam of electrons in a raster scan pattern. SEM equipped

with energy-dispersive X-ray spectrometry (EDX) is an

effective approach for identifying the surface composition

(elemental mapping) of individual particles with diameters

greater than 100 nm. The mixing characteristics of different

constituents in the surface layer of the particle can be

obtained from information on the shape and chemical

composition by SEM-EDX (Barkay et al., 2005; Laskin et

al., 2006; Pachauri et al., 2013).

TEM is an electron microscopic technique in which a

beam of electrons is transmitted through the particle.

TEM-EDX can be used to readily observe the mixing state,

structure, and elemental composition of the entire particle

(Li et al., 2003a; Adachi and Buseck, 2008; Li et al., 2016a).

Note that the ordinary EDX can only detect elements heavier

than carbon and the TEM requires labor-intensive operation.

The application of ultra-thin window detectors in SEM and

TEM permits the detection of light elements and thus

achieves nearly total element analysis. In addition, the TEM

and SEM suffer from a defect of possible changes of particle

samples in the microstructure, composition, and mass (e.g.,

semi-volatile compounds) during examination.

STXM-NEXAFS can also image the particle shape,

albeit with poor resolution. This combined method provides

information on the spatially resolved bonding and oxidation

state of the investigated particles with enhanced chemical

sensitivity. STXM-NEXAFS can be used to obtain the

mixing properties of certain specific constituents, particularly

organics and light-absorbing components (Moffet et al.,

2010; Laskin et al., 2012; Ault et al., 2013a).

ATOFMS is an online, field-deployable form of aerosol

mass spectrometry that couples aerodynamic particle sizing

with time-of-flight mass spectrometry in a single instrument.

ATOFMS provides real-time, high-time-resolution data on

the sizes, compositions, and mixing properties of individual

particles (Dall’Osto et al., 2004; Dall’Osto and Harrison,

2006; DeCarlo et al., 2006; Moffet et al., 2008a, b; Bi et

al., 2011). It provides good statistic information on single

particles; however, the specific components of the particles

are difficult to identify from the mass spectra.

Recently, TOF-SIMS and NanoSIMS have been used to

study single particles. In these methods, a primary-ion

beam is fired at the particle surface, and the secondary ions

produced by the impact are detected by advanced mass

spectrometers. TOF-SIMS and NanoSIMS provide the

distributions of all elements and isotopes, as well as certain

molecules, at the particle surface with very high sensitivity,

accuracy, and lateral resolution. Compared with TOF-SIMS,

NanoSIMS generally has higher sensitivity and a higher

lateral and mass resolution and obtains more detailed

information on the mixing of particles (Hagenhoff, 2000;

Tervahattu et al., 2002b, 2005; Marino et al., 2006; Ghosal

et al., 2014; Chi et al., 2015; Li et al., 2016b).

MIXING CHARACTERISTICS OF CHLORIDE

AEROSOLS IN ATMOSPHERE

Mixing of Chlorides in Sea Salts

Generally, a large fraction of the chlorides in sea salts are

internally mixed with inorganic materials in the atmosphere

and co-exist with many different substances (see the

schematic diagrams in Fig. 2). In the relatively clean

atmosphere over the Pacific Ocean, Atlantic Ocean, and

coastal Antarctica, field observations have indicated that

much of the sodium chloride in fresh sea salts is capped by

a small quantity of other inorganic elements (as shown in

Fig. 2(a)) (Andreae et al., 1986; Murphy et al., 1998; Li et

al., 2003a; Hara et al., 2005; Li et al., 2016a; Young et al.,

2016). For example, Andreae et al. (1986) observed by

SEM that regular sodium chloride crystals contacted with

various silicates and sulfates in the equatorial Pacific

Ocean between Ecuador and Hawaii, and Li et al. (2016a)

observed by SEM and TEM that square sodium chloride

particles were associated with certain amounts of magnesium

chlorides and calcium sulfates in the Yellow Sea of China.

In coastal regions with anthropogenic influences, the

compositions and mixing characteristics of sea salts

significantly change along with the atmospheric transport

and aging processes. A fraction of the particulate chlorides

in sea salts is converted into gas-phase compounds (e.g.,

HCl and ClNO

), with most of the remaining chlorides

completely encased within a large quantity of inorganic

elements (as shown in Fig. 2(b)) (e.g., Mg and S at Cape

Grim (Murphy et al., 1998), K, Ca, and S in Incheon (Geng

et al., 2010), Mg, K, Ca, and S in Antarctica (Hara et al.,

2005), and Mg, N, and S in Houston, Macao, and Hong Kong

(Laskin et al., 2002; Li et al., 2010, 2016a)). Amorphous

sodium chloride particles were found to be completely bound

with sodium nitrates and sodium sulfates in coastal Hong

Kong (Li et al., 2016a). An investigation of sea spray

aerosols generated in the laboratory by Ault et al. (2013b)

showed that elemental redistribution occurred within the

sea salt particles, and their internal structures were altered

by reaction with nitric acid. In addition, field studies have

found that the chlorides in sea salts were closely bound

with aluminum compounds in the coastal town of M’Bour,

Senegal (Deboudt et al., 2010), and iron compounds in the

industrial region of Dunkerque (Choël et al., 2010).

Besides co-existing with other inorganic matter, a

significant fraction of the chlorides in sea salts also mix

with organic compounds in polluted coastal regions (see the

schematic diagrams in Fig. 3). The identified organic species

co-existing with sea salts varies by location (e.g., fatty acids

Wang et al., Aerosol and Air Quality Research, 17: 878–887, 2017

881

Fig. 2. Schematic diagrams showing the mixing of chlorides in sea salts with inorganic substances, referred from Li et al.

(2016a). (a) Typical square NaCl attaching with the mixture of Mg, K, Ca and S in the fresh sea salt; (b) Shapeless NaCl

enclosed by Na

and NaNO

in the aged sea salt.

Fig. 3. Schematic diagrams showing the chlorides in sea salts coated by organic matters, referred from Ault et al. (2013a)

and Chi et al. (2015). (a) A typical fresh sea salt (square NaCl) and (b) an aged sea salt (NaCl core) coated by organic

materials.

in Helsinki (Tervahattu et al., 2002a, b), methanesulfonates

in King George Island, Antarctica (Maskey et al., 2011),

and multiple organic compounds at Cape Grim (Middlebrook

et al., 1998; Murphy et al., 1998) and in coastal central

California (De Bock et al., 2000)). In the sea salts mixed

with organics, most of the sodium chlorides were surface-

coated by organic materials. Laskin et al. (2012) found by

SEM-EDX and STXM-NEXAFS that the chloride core in

aged sea salts was coated by a substantial C-containing

constituent layer. Subsequent ocean-in-lab experiments with

sea spray aerosols generated from sea water after the addition

of biological and organic materials also discovered an

apparent organic coating layer at the surface of the sodium

chloride core (Ault et al., 2013a; Prather et al., 2013).

Mixing of Chlorides in Biomass Burning Particles

Chloride aerosols from biomass burning normally mix with

carbonaceous materials in young smoke and are completely

coated by organics after partial replacement in aged smoke

(see the schematic diagrams in Fig. 4). Li et al. (2003b)

investigated chloride aerosol particles from biomass

burning in the southern African boundary layer and free

troposphere using TEM. In fresh biomass burning plumes,

most of the chlorides existed as potassium chloride, and

the potassium chloride crystals contacted with carbonaceous

materials. Further biomass burning experiments in the open

field by Li et al. (2016a) confirmed that the co-existing

carbonaceous materials could be either organic matter or

soot, and their mixing characteristics varied depending on

the type of biomass fuel and the burning conditions. Geng

et al. (2010) also observed chlorides in tar ball particles

generated from household wood combustion in Incheon in

winter. In aged biomass burning smoke, potassium chlorides

were partly replaced by potassium sulfates and potassium

nitrates through heterogeneous reactions, and the remaining

chlorides and potassium were completely coated by organic

compounds (Li et al., 2003b).

Mixing of Chlorides in Industrial Smoke

Chloride aerosol particles directly emitted in industrial

smoke mostly co-exist with transition/heavy metals. Geng

et al. (2010), using TEM, observed abundant Zn/Pb/Cl-

Wang et al., Aerosol and Air Quality Research, 17: 878–887, 2017

882

Fig. 4. Schematic diagrams showing the mixing characteristics of chlorides in biomass burning particles, referred from Li

et al. (2003b) and Li et al. (2016a). (a) KCl crystals touching with organics and soot in young biomass burning smokes. (b)

Shapeless KCl, and secondary K

and KNO

coated by organic compounds after aging processes in aged smokes.

containing particles

in polluted urban Incheon, which were

attributed to industrial processes and coal-fired power

generation. The gas-phase zinc and lead chlorides emitted

from high-temperature combustion sources condensed into

solid-phase submicron Cl-containing particles upon cooling

(Hu et al., 2003). Online measurements of single particles

by ATOFMS in an industrial/residential section of Mexico

City by Moffet et al. (2008a, b) demonstrated that the

metal/Cl-containing particles accounted for a large fraction

of the fine-mode particles, and the fraction reached 73% in

the early morning hours. Other studies have also observed

numerous micron-sized Zn/Pb/Cl-containing particles in

lead smelter plumes (Ettler et al., 2005), in incinerator fly

ash (Tan et al., 2002; Zhang et al., 2009), in mixed urban

plumes (Lu et al., 2012), and in haze episodes (Li et al., 2009;

Hu et al., 2015). The chloride particles from industrial smoke

were often coated by organic matter after aging (see the

schematic diagram in Fig. 5). TEM images of aerosol

particles collected in urban Shanghai by Fu et al. (2012)

clearly showed that zinc chloride particles from industrial

plumes were completely enclosed within organic compounds.

Fig. 5. Schematic diagram showing the typical industrial

ZnCl

particle coated by organic matters, referred from Fu

et al. (2012).

Mixing of Secondary Chloride Particles

The secondary chloride aerosols produced by the reaction

of HCl with alkaline substances (e.g., NH

and mineral

dusts) often mix with dust, nitrate, and sometimes sulfate.

Sullivan et al. (2007b) observed a large fraction of chlorides

mixing with mineral dusts (i.e., Al, Ca, and Fe compounds)

in long-range transport dust plumes by using ATOFMS

during shipboard measurements over the Pacific Ocean

between Hawaii and Japan. Further study by Sullivan et al.

(2007a) showed that the abundant chlorides in dust particles

originated from the heterogeneous reactions of HCl released

from chemically aged sea salt particles. The newly added

secondary chlorides in the mixed dust particles contributed

4–9% of the individual dust particle mass, and secondary

chloride-containing dust particles constituted as much as

65% of the total dust particles during a major dust storm.

Tobo et al. (2010) also noticed the chloride depletion in

sea salt particles and a coatings containing CaCl

on the

surface of dust core at Kanazawa site during an Asian dust

storm period. Moreover, Sullivan et al. (2007b) observed a

significant fraction of secondary chloride particles mixing

with nitrates (14.8% of the chloride-dust and nitrate-dust

particles) and a small proportion mixing with sulfates

(0.8% of the chloride-dust and sulfate-dust particles).

ATMOSPHERIC IMPLICATIONS OF MIXED

CHLORIDE AEROSOLS

Alteration to Hygroscopicity and Cloud Condensation

Nuclei Activation

The mixing of chloride aerosols with other inorganic

and organic constituents (as summarized in Table 1) can

significantly alter their hygroscopicity and further influence

the activation of cloud condensation nuclei (CCN) when

compared with the pure chloride salts. Salt particles primarily

consisting of pure hydrophilic chlorides are hygroscopic,

with deliquescence relative humidity (DRH) values of around

80% (e.g., 75.3% for NaCl, 84.2% for KCl, and 80.0% for

Cl at 298 K) (Tang et al., 1978; Tang and Munkelwitz,

1993). The mixing of chloride salts with other inorganic salts,

Wang et al., Aerosol and Air Quality Research, 17: 878–887, 2017

883

Table 1. Summary of mixed chloride aerosols in atmosphere.

Type Location Chloride form Mixed element or

substance

Mixing pattern

Sea salts Marine and coastal

region

NaCl, minor MgCl

Mg, Ca, K, N, S,

organics

Enclosed with inorganics, coated

by organics

Biomass burning

particles

Vegetation covered

area

KCl N, S, K, soot,

organics

Touching with or coated by

carbons, replaced by N and S

Industrial smokes Industrialized city Zn-Pb-Cl Metals, organics Co-existing with metals, coated

by organics

Secondary chloride

particles

Marine and other

locations

Cl, CaCl

Al, Ca, Fe, N, S Mixed with dust, nitrates, and

minor sulfates

particularly low-DRH salts like nitrates and hydrosulfates,

heavily decreases the DRH of the mixed aerosol particles

and thus enhances their CCN activation. Wexler and

Seinfeld (1991) compared the DRHs of mixtures of nitrates,

chlorides, and sulfates and those of the individual salts,

and found apparent decreases in DRH after mixing. Wise

et al. (2009) observed a significant reduction in deliquescence

humidity when sodium chloride was mixed with a small

amount of various inorganic substances, with the DRH

dropping to 66% for laboratory-synthesized sea salts and

to 57% for natural sea salts. For sulfate-coated and Mg-

rich chloride-coated sea salt particles collected from the

ambient atmosphere, Semeniuk et al. (2007a) also observed

that the initial water uptake occurred at lower relative

humidity (RH) than for common marine aerosol particles,

and the coated sea salts underwent a complex multi-step

deliquescence. Nevertheless, the mixing of chlorides with

less hygroscopic (i.e., organic compounds) and insoluble

substances (e.g., soot) can significantly suppress their

hygroscopicity and CCN efficiency. Semeniuk et al. (2007b)

investigated the hygroscopic behavior of single particles of

potassium chlorides mixing with organic matter at RH

values of 0–100%, and found that the mixed chloride particles

did not appear to deliquesce. For ambient particles comprising

organic substances and chloride and/or other inorganic

components, online field measurements indicated that the

CCN efficiency decreased from 12.8 ± 6.1% to 4.5 ± 2.6%

when the organic fraction increased from 10–20% to 30–

40% (Zhang et al., 2012).

Change in Heterogeneous Reactivity

Changes in the liquid water and soluble chloride contents

in mixed chloride aerosols subsequently affect the reactivity

of the chlorides in related heterogeneous reactions. Generally,

the mixing of chlorides with inorganic salts tends to promote

reactivity, while mixing with or coating by less hygroscopic

and insoluble substances is prone to suppress reactivity.

Saul et al. (2006) determined the reactive uptake coefficient

of nitric acid on both pure sodium chlorides and sodium

chlorides mixed with magnesium chlorides at RH values of

10–85%, and found that the uptake coefficient on the mixed

chloride aerosols remained high (> 0.1) even at 10% RH, and

was much higher than that on pure sodium chloride aerosols.

This enhancement suggests that the displacement of chloride

by nitrate is facilitated by the mixing of chlorides. Stewart

et al. (2004) also observed a higher N

uptake coefficient

on natural sea salts (e.g., 0.048 at 50% RH) than that on

pure sodium chlorides (e.g., 0.033 at 50% RH), but they

attributed this phenomenon to the variation in the salt size,

which led to a limitation in the uptake rate on small particles.

An investigation of the heterogeneous loss of N

submicron sea salt particles with a hexanoic acid coating

by Thornton and Abbatt (2005) showed that the presence of

millimolar levels of hexanoic acid greatly decreased the N

uptake coefficient by a factor of 3–4 (i.e., approximately from

0.025 to 0.008, at 70% RH).

Effects on Optical Properties and Radiative Forcing

The mixing of chloride aerosols with organic matter alters

not only the hygroscopicity but also the optical properties

of the aerosols, which has a further effect on the radiative

forcing. Generally, mixing with non-absorbing organic matter

suppresses the uptake of water, particularly at high humidity,

and thus reduces the aerosol scattering efficiency and the

subsequent cooling effect on the atmosphere. Abo Riziq et

al. (2007) measured the optical properties of mixed chloride

particles and found that the extinction efficiency of mixed

particles of sodium chloride with glutaric acid decreased

significantly in comparison with pure sodium chloride

particles. A modeling study by Randles et al. (2004) predicted

that an internal mixture of 90% sodium chloride with 10%

non-absorbing organics would cause 3% less cooling than

100% sodium chloride particles in the visible spectrum

over clear-sky oceans. The mixing of chloride with light-

absorbing substances (e.g., black carbon (soot) and brown

carbon (nitroaromatic compounds, humic-like substances,

and imidazoles)) generally enhances the absorption of light

and/or decreases the scattering of light by aerosols when

compared with the individual components, and thus causes

a warming effect. Mallet et al. (2004) compared the single-

scattering albedos of internally mixed sea salt with black

carbon particles, the corresponding externally mixed particles,

and unmixed sea salts at a coastal industrialized region in

France, and found that the single-scattering albedo of the

internally mixed particles (0.75) was significantly lower than

those of the externally mixed particles (0.85) and unmixed

sea salts (0.99). A modeling study by Randles et al. (2004)

predicted that the internal mixing of 90% NaCl with 10%

mildly absorbing organic compounds would substantially

reduce the radiative cooling compared with 100% NaCl

aerosols. In addition, Hoffer et al. (2006) found that humic-

like substances in biomass burning particles at a tropical

Wang et al., Aerosol and Air Quality Research, 17: 878–887, 2017

884

pasture site in Brazil contributed 50% to the absorption of

light by aerosols at 300 nm and 7% for the entire spectrum.

Chakrabarty et al. (2010) also reported that the presence of

brown carbon in biomass burning particles collected from

a laboratory chamber led to an increase in light absorption and

the radiative forcing efficiency. The global mean radiative

forcing of biomass burning aerosols is approximately 0.04

W m

–2

, with a median of 0.02 W m

–2

(Forster et al., 2007).

FUTURE PERSPECTIVES

A number of field studies on single particles have

confirmed that a large fraction of the chloride aerosols from

various sources are internally mixed with other inorganic

and organic substances in diverse locations. Laboratory

experiments and a few field measurements and modeling

simulations have provided some evidence that the mixing

of chloride aerosols with other constituents has significant

effects on their hygroscopicity, CCN activity, heterogeneous

reactivity, optical properties, and radiative forcing when

compared with the pure chloride salts. However, additional

studies are required to obtain a comprehensive understanding

of the mixing characteristics of chloride particles and their

subsequent atmospheric effects, including (a) more field

studies on the mixing properties of chloride aerosols in

megacity centers, suburban areas, and rural locations to

fully understand the influences of anthropogenic activities

on the mixing state as well as the mixing processes; (b)

further laboratory studies to clarify the complex effects of

different mixing patterns on the size, density, morphology,

porosity, hygroscopicity, reactivity, and optical properties

of chloride aerosols; (c) the development and improvement

of modeling techniques by taking into account the mixing

state of the chloride aerosols and the subsequent changes

in their physical and chemical properties; and (d) modeling

simulations to evaluate the potential environmental effects

(e.g., radiative forcing, CCN activation, and precipitation)

of mixed chloride aerosols.

CONCLUSION

In this review, we have summarized the existing studies

and findings on the mixing characteristics of chloride

aerosols, and the atmospheric consequences of this mixing.

Besides the traditionally used SEM and TEM, several new

single-particle techniques have been developed and applied

in research on the mixing state and chemical composition

of aerosol particles, including STXM-NEXAFS, ATOFMS,

TOF-SIMS, and NanoSIMS. Field studies in various locations

have shown that a large proportion of chlorides internally

mix with other inorganic and organic constituents rather

than existing alone. Specifically, sodium chlorides in fresh

sea salts are usually attached to a small quantity of other

inorganic substances, while in aged sea salts they are

completely encased within a large quantity of inorganic

materials. Sodium chlorides in polluted coastal regions also

co-exist with multiple organic compounds, with an organic

layer coated on the surface. Potassium chlorides in biomass

burning particles are generally attached to carbonaceous

materials in young smoke, and they are completely coated by

organic matter in aged smoke. Most chloride aerosols emitted

in industrial smoke are mixed with transition/heavy metals,

and a coating of organics and/or sulfates appears on aged

particles. In addition, some secondary chlorides are produced

by the reactions of HCl with alkaline substances, and can

mix with mineral dusts, nitrates, and sulfates. The mixing

of chloride with other components can significantly alter

the physical and chemical properties of the aerosol

particles. Generally, mixing with inorganic salts enhances

the hygroscopicity, heterogeneous reactivity, and CCN

activation of the aerosols, while mixing with organic matter

suppresses these traits. The mixing of chloride aerosols

with organic compounds can decrease the scattering of light

or increase its absorption by aerosols, thus leading to a

warming effect to the atmosphere. With consideration of

the apparent effects of mixing on the properties of chloride

aerosols and subsequently on the climate and the limited

current studies in this community, we also provide

suggestions for necessary future research.

ACKNOWLDEGEMENTS

This work was supported by the National Natural Science

Foundation of China (Nos. 91544213, 41275123, 21407094)

and the Natural Science Foundation of Shandong Province

(No. ZR2014BQ031).

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Received for review, September 2, 2016

Revised, December 19, 2016

Accepted, December 19, 2016