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Atmospheric Environment 131 (2016) 196e208

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Atmospheric Environment

journal homepage: www.elsevier .com/locate/atmosenv

Aerosol optical properties and mixing state of black carbon in thePearl River Delta, China

Haobo Tan a, c, *, Li Liu a, b, Shaojia Fan b, **, Fei Li a, Yan Yin c, Mingfu Cai a, b, P.W. Chan d

a Key Laboratory of Regional Numerical Weather Prediction, Institute of Tropical and Marine Meteorology, China Meteorological Administration,Guangzhou, Guangdong 510080, Chinab Department of Atmospheric Sciences, Sun Yat-sen University, Guangzhou 510275, Chinac Nanjing University of Information Science and Technology, Nanjing 210044, Chinad Hong Kong Observatory, Hong Kong, China

h i g h l i g h t s

� Aerosol optical properties were analysed using in-situ aerosol data.� The external mixture will lead to high ssp and SSA.� Non-carbon-containing particles make a slight difference in calculation of ssp,hbsp.� The mixing state of BC may be between the external mixture and core-shell mixture.

a r t i c l e i n f o

Article history:Received 2 June 2015Received in revised form30 January 2016Accepted 2 February 2016Available online 4 February 2016

Keywords:Aerosol optical propertiesClosure studyMixing state of BCPRD

* Corresponding author. Key Laboratory of Regionation, Institute of Tropical and Marine Meteorology, Cistration, Guangzhou, Guangdong 510080, China.** Corresponding author.

E-mail addresses: hbtan@grmc.gov.cn (H. Tan), ees

http://dx.doi.org/10.1016/j.atmosenv.2016.02.0031352-2310/© 2016 Elsevier Ltd. All rights reserved.

a b s t r a c t

Aerosols contribute the largest uncertainty to the total radiative forcing estimate, and black carbon (BC)that absorbs solar radiation plays an important role in the Earth's energy budget. This study analysed theaerosol optical properties from 22 February to 18 March 2014 at the China Meteorological AdministrationAtmospheric Watch Network (CAWNET) station in the Pearl River Delta (PRD), China. The representativevalues of dry-state particle scattering coefficient (ssp), hemispheric backscattering coefficient (shbsp),absorption coefficient (sabsp), extinction coefficient (sep), hemispheric backscattering fraction (HBF),single scattering albedo (SSA), as well as scattering Ångstr€om exponent (a) were presented. A compar-ison between a polluted day and a clean day shows that the aerosol optical properties depend on particlenumber size distribution, weather conditions and evolution of the mixing layer. To investigate the mixingstate of BC at the surface, an optical closure study of HBF between measurements and calculations basedon a modified Mie model was employed for dry particles. The result shows that the mixing state of BCmight be between the external mixture and the core-shell mixture. The average retrieved ratio of theexternally mixed BC to the total BC mass concentration (rext-BC) was 0.58 ± 0.12, and the diurnal patternof rext-BC can be found. Furthermore, considering that non-light-absorbing particles measured by aVolatility-Tandem Differential Mobility Analyser (V-TDMA) exist independently with core-shell andhomogenously internally mixed BC particles, the calculated optical properties were just slightly differentfrom those based on the assumption that BC exist in each particle. This would help understand theinfluence of the BC mixing state on aerosol optical properties and radiation budget in the PRD.

© 2016 Elsevier Ltd. All rights reserved.

l Numerical Weather Predic-hina Meteorological Admin-

fsj@mail.sysu.edu.cn (S. Fan).

1. Introduction

Aerosol particles interact with solar radiation through absorp-tion and scattering (Charlson et al., 1992), and can serve as cloudcondensation nuclei and ice nuclei upon which cloud droplets andice crystals form (Albrecht, 1989; Rosenfeld, 1999, 2000; Twomey,1974; Zhao et al., 2006). High levels of atmospheric aerosol pol-lutants, including BC, sulfate, nitrate and organic carbon (OC), etc.,have been emitted into and formed in the atmosphere because of

H. Tan et al. / Atmospheric Environment 131 (2016) 196e208 197

human activities. Aerosols of anthropogenic origin are responsiblefor a radiative forcing (RF) of climate change through their inter-action with radiation, and also as a result of their interaction withclouds. The total effective RF due to aerosols was assessed tobe �0.9 Wm�2 (IPCC, 2013). Aerosols dominate in the uncertaintyof the total anthropogenic RF estimate due to their distributionwith spatial and temporal inhomogeneities (Donkelaar et al., 2010;Liu et al., 2009). The aerosol optical properties, including thescattering coefficient (ssp), absorption coefficient (sabsp), singlescattering albedo (SSA) and aerosol optical depth (AOD), are mainlydetermined by the particle number size distribution (PNSD),chemical composition, mixing state and morphology. However, it isstill a challenge to determine the interrelationship between aerosoloptical, physical, and chemical properties.

Even though the majority of aerosol particles scatter solar ra-diation primarily, BC has received increasing attention in recentyears because of its high light-absorbing ability (IPCC, 2013). Today,the concept of “mixing state” has also been widely applied toclassify the mixing of BC and other non(less)-light-absorbingaerosol species such as ammonium, sulfates, nitrates, OC and wa-ter. Condensation of gas-phase compounds on BC and coagulationwith other particles can alter the mixing state of BC. There are threeconceptual models to describe the mixing state of BC: homoge-neously internal mixture, external mixture, and core-shell internalmixture (Jacobson, 2001; Seinfeld and Pandis, 1998). According tomodelling and laboratory studies, a certain amount of anthropo-genic BC particles internally mixed could absorb solar radiationthree times as much as that externally mixed with non-absorbingmaterial (Heintzenberg, 1978). However, the enhancement of ab-sorption for urban mixed soot based on in-situ measurementsmight be much lower than that in models (Cappa et al., 2012;Jacobson, 2013). The BC mixed with other components indifferent ways would lead to various aerosol direct radiative forcing(DRFs). The mixing state must be considered in climate modelswhen investigating aerosol direct effect. It might affect the globalDRF of BC by a factor of 2.9 according to previous researchs(þ0.78 Wm�2 for a homogeneously internal mixture, þ0.54 Wm�2

for a core-shell mixture, and þ0.27 Wm�2 for an external mixture)(Jacobson, 2000, 2001).

With increasing technologies and methods, we can investigatehow BC is mixed with other non(less)-light-absorbing componentsvia various direct experimental measurements and indirect nu-merical closure studies. Transmission electron microscopy (TEM) isoften used for single particle analysis to identify the mixing state ofirregularly aggregated soot particles (Clarke et al., 2004; Katrinaket al., 1992, 1993). A Hygroscopic Tandem Differential MobilityAnalyser (H-TDMA) can identify the hygroscopic and hydrophobiccompositions according to hygroscopic properties of aerosol par-ticles and their mixing state. Similar to H-TDMA, the VolatilityTandem Differential Mobility Analyser (V-TDMA) can reveal themixing state of non-volatile composition. However, the mixingstate obtained from H-TDMA or V-TDMA is not exactly the mixingstate of BC, because there are still some hydrophobic or non-volatile carbonaceous organics in the atmospheric aerosol(Carrico et al., 2005; Cheng et al., 2009; Heintzenberg et al., 2001;Tan et al., 2013; Wehner et al., 2009). Moreover, numerical opticalclosure studies based on the Mie model and in-situ measurementsalso inferred the BC mixing state through the complex relationshipbetween physical, chemical properties and optical properties ofaerosol particles (Ansmann et al., 2002). By means of observedparticle number size distribution (PNSD) and chemical composi-tions and a two-parameter assumption for dry particles, Chenget al. (2006) developed an algorithm to yield the mixing state ofBC. Ma et al. (2012) took the minimum deviation of HBFs betweenmeasurements and calculations as convergence conditions to

retrieve the mass ratio of externally mixed light absorbing carbo-naceous (LAC, also called BC) to the total mass of LAC (rext-LAC) in theNorth China Plain. The result showed that for internally mixedparticles, the assumption of core-shell mixture is more appropriatethan homogenously internal mixture.

The PRD region is one of the most economically invigoratingmegacity clusters in China, but also has a severe air pollutionproblem (Chan and Yao, 2008; Wu et al., 2005). The BC componentin ambient aerosol in the PRD region is more internally mixed withnon-refractory components than that around Houston (Huanget al., 2011). The BC mixing state was strongly dependent on thelocal wind patterns at Xinken, PRD (Cheng et al., 2006). However,there is a lack of research that focuses on the high temporal reso-lution mixing state of BC. In this paper, the aerosol optical prop-erties were analysed, and an optical closure experiment was carriedout using in-situ aerosol data measured at a suburban site in thePRD of China from 22 February to 18March in 2014. The station andaerosol monitoring equipment are briefly described in Sect. 2. Thecharacteristics of aerosol optical properties are analysed in Sect. 3.1.The result of a closure study used to determine the mixing state ofBC is discussed in Sect. 3.2.

2. Measurements

2.1. The field site

The field experiment was carried out at the Panyu station of theChina Meteorological Administration Atmospheric Watch Network(CAWNET) in Guangzhou, China from 22 February to 18 March in2014. The Panyu suburban site, operated by the Guangzhou Insti-tute of Tropical and Marine Meteorology, is located at the top ofDazhengang Mountain (23�00.2360N, 113�21.2920E), with an alti-tude of 145 m, and is about 120 m above the average city elevationof PRD. This site is in the centre of PRD and surrounded by resi-dential neighborhoods without significant pollution sourcesnearby. Thus, it can be well representative of regional anthropo-genic aerosols to a certain extent rather than those dominated bynearby sources. The geographical location of the station is shown inFig. 1. During the period, PNSD, ssp,hbsp, BC mass concentration andaerosol volatility were continuously measured.

2.2. Instrumentations

Ground-based measurements were performed in a measure-ment roomwith a nearly constant temperature of 25 �C. The sampleair was collected with a PM2.5 impactor inlet installed outside theroom, passing through two Nafion dryers (Permapure MD700) tokeep the RH below 30%, and then was split into several flows todifferent instruments.

2.2.1. Particle number size distributionAtmospheric particle number size distribution (PNSD) was

measured using both a scanningmobility particle sizer (SMPS 3936,TSI Inc.) and an aerodynamic particle sizer (APS 3321, TSI Inc.). TheSMPS was comprised a neutraliser (Kr-85), differential mobilityanalyser (DMA3081) anda condensationparticle counter (CPC3772,TSI Inc.). This instrument was responsible for measuring the PNSDranging from 10 to 500 nm in diameter (Stokes diameter). In anSMPS, sheath flow is performed in a closed loop mode; the ratio ofsheath flow to sample flow is 6.5:1. Size-dependent pipe diffusionloss was corrected using the empirical formula defined in Willekeand Baron (1993). APS measured the PNSD with aerodynamicdiameter ranging from500 nm to 2.5 mm,whichwas then convertedto Stokes diameter by assuming an aerosol particle density of 1.7 mg/m3 (McMurry et al., 2010). Thus, the overlap size range is about 400

Fig. 1. The location of Panyu station (CAWNET) in the PRD region.

H. Tan et al. / Atmospheric Environment 131 (2016) 196e208198

nme500nm. Finally, the data of SMPS andAPSweremerged toyieldPNSD with Stokes diameter from 10 nm to 2.5 mm.

2.2.2. Total/hemispheric back scatteringThe ssp and shbsp were measured by a total/back scattering

integrating nephelometer (Model 3563, TSI Inc.) at three wave-lengths (450, 550 and 700 nm) (Anderson et al., 1996; Andersonand Ogren, 2007; Heintzenberg and Charlson, 1996). We used 30-min averaged data in this study, even though the temporal reso-lution of the measurement was 1 min. CO2 was chosen to calibratethe nephelometer every week during the field campaign (Andersonet al., 1996). Particle free air was checked once a day.

2.2.3. BC mass concentrationThe instrument used in this study for BC mass concentration is a

seven-wavelength (370, 470, 520, 590, 660, 880, 950 nm) aethal-ometer (Magee Scientific, AE-31) of high temporal resolution. Thecalculation of BC mass concentration was based on the attenuationof an incident beam, which goes through the quartz filter withparticles (Sandradewi et al., 2008). It is a self-contained instrument,running automatically, without needing calibration, except for pe-riodic checks of the flow rate. The raw datawere recorded as the BCmass concentrations with a 5 min interval. The Eq. (1) was used toconvert the mass concentration of BC to the absorption coefficient.This polynomial fitting was obtained from the inter-comparisonbetween the aethalometer and photoacoustic spectrometer (PAS)(Wu et al., 2009).

sabsp 532 ¼ 8:28*MBC þ 2:23 (1)

In Eq. (1), MBC (mg/m3) is the BC mass concentration measuredby the aethalometer with no corrections for scattering bias, andfilter loading at 880 nm wavelength; sabsp_532 (Mm�1) is the ab-sorption coefficient measured by a photo-acoustic soot spectrom-eter (PASS) at 532 nm. The slope for the regression equationapproximates 8.5 m2 g�1 which was observed in state parks inTexas (Arnott et al., 2003), and is also consistent with 8e10 m2 g�1,measured in metropolitan areas of Mexico City (Barnard et al.,2005). The sabsp at 450 nm, 550 nm, and 700 nm can be calcu-lated by the inverse wavelength ‘Power Law’ (Coen et al., 2004;

Nessler et al., 2005) as,

sabsp l2¼ sabsp l1

*

�l2

l1

��1(2)

where l denotes the wavelength.

2.2.4. Externally mixed non(less)-light-absorbing particlesWhen using rext-BC as an indicator of the mixing state, particles

are usually divided into two kinds: the externally mixed BC and aBC core with non(less)-light-absorbing materials coating. However,in real atmosphere, some particles might be non-light-absorbingsince they do not contain any BC. Aerosols have varying volatilityproperties based on their chemical compositions. Non-volatilematerials (soot) can be internally mixed with volatile materials(VM) that evaporate during thermal treatment at elevated tem-peratures. Particles can be categorised into three groups in terms ofvolatility: namely, low volatility (LV), medium volatility (MV) andhigh volatility (HV) particles in V-TDMA measurements (Wehneret al., 2004). When VM exist as external mixtures with LV, MVand HV, they evaporate completely without leaving any residuals at300 �C. They are referred as CV (completely vaporised) particles.These CV particles are regarded as externally mixed non(less)-light-absorbing particles.

The number fractions of CV particles were obtained from acustom-made V-TDMA. It was based on a H-TDMA (Tan et al., 2013);a heated tube instead of the humidifier between the two differ-ential mobility analysers (DMA, TSI Model 3080L) can effect evap-oration of volatile materials. Firstly, quasi-monodisperse particlesin a polydisperse aerosol are selected by a DMA. Secondly, theseparticles passed through a heating unit where VM were volatilisedat the controlled temperature of 300 �C. Finally, the resultingnumber size distribution of the nonvolatile (LV, MV and HV) par-ticles were measured by another DMA and a condensation particlecounter (CPC, TSI Model 3772). Also, the number fraction of CV(FN,CV) can be obtained from Eq. (3).

NDo*hDo*�1� FN;CV

� ¼ N0 (3)

where ND0 and N0 are number concentrations before heating and

H. Tan et al. / Atmospheric Environment 131 (2016) 196e208 199

after heating at a selected diameter (Do). hDo is the transport effi-ciency of particles, which accounts for particle loss between twoDMAs due to diffusion and thermophoretic forces (Philippin et al.,2004); and h at each particle diameter and V-TDMA temperaturewas determined by laboratory calibrations with NaCl particles,which do not evaporate at the heating temperatures used in ourexperiments. More detail about data treatment of V-TDMA duringthe observation period is given by Cheung et al. (2015).

In Sect. 3.2.1, we compared the model computation with andwithout taking CV particles into consideration.

2.3. Method of optical closure study for dry particles

The closure study of aerosol optical properties is on account ofthe fact that specific particle chemical andmicrophysical propertiescontrol their optical properties. According to the simplified aerosolmodel (Cheng et al., 2006; Ma et al., 2012), an optical closure studywas used to obtain the information of the BC mixing state for dryparticles, and to discuss the uncertainties of different BC mixingstates concerned with estimating optical properties of aerosols inthis work.

A modified BHMIE code and a modified BHCOAT code wereutilised to simulate the optical properties of dry particles forhomogenously internal mixture and core-shell mixture, respec-tively. According to the Lorentz-Mie theory, assuming the particle isspherical, the scattering efficiency can be calculated by integratingthe scattering intensity function from 0� to 180�, and in the sameway, the hemispheric backscattering efficiency can be calculated byintegrating the scattering intensity function from 90� to 180� (Mie,1908). Considering the angular truncations of TSI 3563 nephe-lometer measurements (Anderson et al., 1996; Heintzenberg andCharlson, 1996), the two above codes were applied into calcula-tions to set up a closure experiment between the measured andsimulated optical properties of particles at dry condition(Heintzenberg, 1980; Quinn et al., 1995).

A two-component optical model was assumed in this opticalclosure experiment only for dry particles. In brief, aerosol compo-nents are divided into two classes in terms of their optical prop-erties: BC and non(/less)-light-absorbing species (Wex et al., 2002).For a given spherical particle size, the optical properties aredetermined by the refractive indices ð ~mÞ of the components(Bohren et al., 1983) and the mixing way of different species in theparticle. The refractive indices of BC ð ~mBC ¼ 1:80� 0:54iÞ andnon(less)-light-absorbing components ð ~mnon ¼ 1:55� 10�7iÞ usedin the Mie model calculationwere as same as the values in Ma et al.(2012), which were chosen from related literature (Covert et al.,2007; Hasan and Dzubay, 1983; Ouimette and Flagan, 1982;Seinfeld and Pandis, 1998; Sloane, 1984; Tang and Munkelwitz,1994). Without the information on size-resolved volume concen-tration of BC, the calculation was based on an assumption thatvolume fractions of BC (fBC, BC's volume to total particles' volume)were uniform in each size bin. The refractive indices of homoge-neously internally mixed BC ð ~mint BCÞwere calculated using Eq. (4).

~mint ¼ ~mBC*fBC þ ~mnon*ð1� fBCÞ (4)

For core-shell mixture, ~mcore equals ~mBC and ~mshell equals ~mnon.Three kinds of conceptual models of the BC mixing state were

utilised to calculate the aerosol optical properties by the modifiedMie model in this work. Cheng et al. (2006) considered the externaland homogenously internal mixture as two extreme conditions toget a mixing state, but Ma et al. (2012) viewed external and core-shell mixing states as two limiting cases for the actual mixingstate of BC. As shown in Sect. 3.1, it is not difficult to find that mostHBFs measured with the nephelometer were almost between the

values calculated from the assumption of external and core-shellmixing state. This is also similar to the result in early research(Ma et al., 2012). Similarly, an introduction of rext-BC defined as theratio of externally mixed BC mass (Mext,BC) to total BC mass (MBC) isto quantify the mixing state.

rext;BC ¼ Mext;BC

MBC(5)

Thus, the value of rext-BC for the completely externally mixed BCis 1, and for core-shell internal mixture of BC, it is 0.

Based on the two-component model, PNSD, ~m, l are input pa-rameters for modified Mie models to simulate the aerosol opticalproperties and to retrieve the rext-BC. The PNSD at dry conditions(RH < 30%) was measured with a combination of SMPS and APSranging from 10 nm to 2.5 mm. The PNSD of completely externallymixed BC can be obtained from total PNSD and the volume fractionof BC (fBC) as:

N�logDp

�ext;BC ¼ N

�logDp

�measure*fBC*rext;BC (6)

where N(logDp)measure is the PNSD of all particles, fBC and rext-BC arementioned above. In the same way, the PNSD for core-shell mixedparticles can be described as:

N�logDp

�core�shell ¼ N

�logDp

�measure*

�1� fBC*rext;BC

�(7)

To such a degree, all input parameters are related to rext-BC,leading to the calculated optical results are the functions of rext-BC aswell. Then next step is to find out the best estimation of rext-BC byminimizing the deviation between the simulated and measuredHBF. The HBF was calculated according to HBF ¼ shbsp/ssp usingmeasured shbsp and ssp. The deviation c can be calculated as thelinear least squares as follows:

c2 ¼X3i¼1

HBFcal;i � HBFmeasure;i

HBFmeasure;i

!2

(8)

where i represents three wavelengths of the nephelometer, HBFcal,iis calculated as the external and core-shell mixing state for a givenrext-BC, while HBFmeasure,i is measured HBF from the nephelometer.A comparison between the calculated optical properties and themeasured values are shown in Sect. 3.2. Furthermore, the retrievalmixing state rext-BC is also given.

3. Results and discussion

3.1. Optical properties of aerosol

3.1.1. OverviewFor the observation period, the optical properties of dry particles

including ssp and shbsp are listed in Table 1. For 550 nm, the meanssp was 149.33 ± 89.73 Mm�1 and the mean shbsp was19.05 ± 11.35 Mm�1. Their maximum values can reach up to 675.88and 88.09 Mm�1, respectively. Compared with the results observedin some other fields, the ssp at Panyu was much lower than that inthe North China Plain of 280 ± 253 Mm�1 for spring and379 ± 251 Mm�1 for summer, and the shbsp was lower as well (Maet al., 2011). However, both the ssp and shbsp were approximatelytwice greater than those measured at Melpitz, Germany (Ma et al.,2014). Details are summarised in Table 1.

The average volume concentration of the diameter ranging from10 nm to 2.5 mm was 28 mm3/cm3. Accordingly, fBC (see Sect. 2.3)kept in 0.11 ± 0.03. For 550 nm, the mean sabsp was 45.67 ± 30.39Mm�1dclose to sabsp at Wuqing but half of that at Xinken, while it

Table 1Comparison of aerosol optical properties between Panyu and other monitoring results.

Site Panyu Wuqing Wuqing Xinken Melpitz (Germany)

Reference This study Ma et al. (2011) Ma et al. (2011) Cheng et al. (2008) Ma et al. (2014)

Period 2014.2.22e2014.3.18 2009.3.6e2009.4.5 2009.7.12e2009.8.14 2004.10.4e2004.11.5 2007e2010

ssp (Mm�1) 149.33 ± 89.73 (550 nm) 280 ± 253 (550 nm) 379 ± 251 (550 nm) 333 ± 137 (550 nm) 53.61 ± 58.64 (550 nm)shbsp (Mm�1) 19.05 ± 11.35 (550 nm) 45 ± 37 (550 nm) 49 ± 31 (550 nm) 37 ± 15 (550 nm) 5.97 ± 5.69 (550 nm)sabsp (Mm�1) 45.67 ± 30.39 (550 nm) 47 ± 38 (637 nm) 43 ± 27 (637 nm) 70 ± 42 (550 nm) 5.67 ± 6.95 (637 nm)SSA 0.80 ± 0.06 (550 nm) 0.82 ± 0.05 (637 m) 0.86 ± 0.05 (637 nm) 0.83 ± 0.05 (550 nm) 0.87 ± 0.05 (637 nm)Ångstr€om 1.82 ± 0.18 (450e700 nm) 1.45 ± 0.34 (450e700 nm) 1.33 ± 0.24 (450e700 nm) 1.6 ± 0.15 (450e700 nm) 1.72 ± 0.38 (450e550 nm)

H. Tan et al. / Atmospheric Environment 131 (2016) 196e208200

was much higher than that at Melpitz. Moreover, the maximumvalue was 289.25 Mm�1 and the minimum value was 6.93 Mm�1,respectively. The diurnal pattern of BC volume fraction has abimodal distributiondtwo peaks occur around 9:00 and 18:00corresponding to rush hour.

One of the most important parameters for assessing the directradiative forcing of aerosols is the SSA (SSA¼ ssp/(ssp þ sabsp)). TheSSA depends on sources of aerosol, aging degree of particles and thesurrounding environment (Carrico et al., 2003; Reid et al., 1998),and lower values indicate more absorbing characters. Based on the

Fig. 2. Wind speed and direction dependence map of (a) ssp (b) shbsp (c) sabsp (d) SSA (e) avarying wind speeds (radial direction) and wind directions (transverse direction). The solid

measured ssp and sabsp, an estimation of SSA (550 nm) was0.80 ± 0.06. It should be noted that our measurements arecontrolled at a RH under 30%. Hence, the SSA under ambient con-ditions should be higher due to water uptake of particles.Compared with other sites (Cheng et al., 2008; Ma et al., 2011), theSSA at Panyu was relatively lower, indicating that there were morelight absorption materials in the ambient atmosphere. Deng et al.(2008) reported that biomass burning in Southeast Asia in Marchcould result in high aerosol loading in the PRD region. This may leadto high BC mass fraction and low SSA when the air mass originates

450e700. In each subplot, the shaded contour represents the average of parameters forlines that are the same in each subplot denote frequency of wind direction.

Fig. 3. (a) Average particle number size distributions in two periods. Efficiency ofextinction (xep), scattering (xsp), absorption (xabsp), backscattering (xhbsp) and thecontribution to extinction (Jep), scattering (Jsp), absorption (Jabsp), backscattering(Jhbap) for (b) clean day and (c) polluted day.

H. Tan et al. / Atmospheric Environment 131 (2016) 196e208 201

from southwest of PRD (Cao et al., 2003).The mean value of HBF at 550 nm is 0.129 ± 0.0081. The Ång-

str€om exponent (a) is another important indicative factor in somecases to represent the wavelength dependence of the aerosolextinction, scattering or absorption coefficient, by sz l�a (Seinfeldand Pandis, 1998). a is calculated frommeasured ssp as a function atdifferent wavelengths by:

al1�l2 ¼ �dlogsspdlogl

¼ �log�ssp;l1

�ssp;l2

�logðl1=l2Þ

(9)

For atmospheric aerosols, the normal range of scatteringa450e700 is from 0 to 2.5, with the bigger a the smaller particles(Seinfeld and Pandis, 1998). The average a calculated at wave-lengths of 450 and 700 nm is 1.82 ± 0.18, higher than that at Wuqin(450e700 nm) and also at Xinken (450e700 nm) (Cheng et al.,2008; Ma et al., 2011). In other words, the particles in the PRDtend to be smaller than in the North China Plain.

3.1.2. Impact of wind on aerosol optical propertiesWind rose maps which are used to expose the relationship be-

tween aerosol optical properties and wind characters are presentedin Fig. 2. In each panel, the shaded contour represents the averageof parameters for varying wind speed (radial direction) and winddirection (transverse direction). The black solid lines that are thesame in each panel denote the frequency of wind direction. Theprevailing wind comes from the north (wind speed >2 m/s).

For the whole period, the average ssp and shbsp for westerlyand easterly winds were seemingly higher than that for southerlyand northerly winds. The maximum ssp and shbsp occurred withwinds coming from northwest while wind speed was almostlower than 2 m/s, indicating that local sources contribute toaerosol pollution. In addition, wind from the north and southeastwas almost higher than 2 m/s, so that the dilution effect leads tolow values of ssp and shbsp. The wind map of sabsp is shown inFig. 2c. Similar to ssp and shbsp, the relationship between wind andsabsp showed higher values in the west and lower values in thenorth and southeast.

Fig. 2d and e present the wind dependence of the SSA anda450e700, respectively. The SSA is associated with air mass origin.The value of SSA was lower when north and west wind dominated.That is to say, aerosol transported from the north and west regionscontain a higher fraction of BC, which might be caused by thetransportation of pollutants from the Guangzhou urban area andcity of Foshan. The a accompanied by strong northerly winds washigher than during calm winds, indicating that mean particle sizewas smaller when the strong northerly winds sweep away airpollutants.

3.1.3. Case studyIn order to compare aerosol optical properties under different

PNSDs, two different periods were selected for further research.According to daily mean PM2.5 mass concentration exceeding75 mg/m3, 12 March was chosen for the polluted process, and 9March under 50 mg/m3 was chosen for the clean process.

In Fig. 3a, the average PNSD for two selected periods was takenfor comparison. The crest of the diameter was around 100 nm onthe polluted day (green line), higher than 50 nm on the clean day(blue line). Moreover, the particle number concentration wasobviously much higher on the polluted day than the clean day. Asseen in Fig. 3b and c, the efficiency (x) of light extinction, scattering,absorption and hemispheric backscattering can be calculated withgiven ~m and l based on BHMIE code and BHCOAT code. Assump-tions of core-shell mixture and 550 nm of lwere considered in thecomputation; ~mwas given in Sect. 2.3. The values of contribution to

extinction, scattering, hemispheric backscattering and absorption(J; J ¼ n*x*Dp*DDp, n ¼ dN/dlogDp) are much higher on thepolluted day than the clean day, both with peaks around300e400 nm. Despite the fact that the shapes of PNSD weredifferent, the shapes of contribution to optical properties weresimilar on two days. Thus, the total values of sep, ssp, shbsp and sabspmainly depend on values of total number concentration.

Accordingly, the variations of a450e550 and SSA for cases areportrayed in Fig. 4bdthe blue line and green line represent a450e550and SSA, respectively. As described in Sect. 3.1, the a450e550 wasfrom the measured ssp by a nephlometer and relative wavelengthof 450 nm and 550 nm. Consistent with results of PNSD, the valuesof a450e550 are higher on the clean day than the polluted dayoverall, indicating larger particle size in the polluted condition. Asshown in Fig. 4b, the variations of SSA for two periods were

Fig. 4. Comparison of (a) Wind speed and wind direction (colour dots), (b) Ångstr€om (blue line) exponent and SSA (green line) during two periods.

H. Tan et al. / Atmospheric Environment 131 (2016) 196e208202

denoted as green line. The mean value of SSAwas 0.71 on the cleanday and 0.74 on the polluted day, and slightly lower mean SSA onthe clean day indicated that there were more absorbing compo-nents. Corresponding wind speed and wind direction are alsoshown in Fig. 4a. Themeanwind speedwasmore than 2m/s and airmass mostly came from the north on the clean day, hence, a gooddilution condition was beneficial to particle transportation. At thesame time, the particle size was less and the value of awas greater.On the other hand, wind speed was under 2m/s and wind directionwas mostly from each direction on the polluted day. The land andsea breezes' circulation occurred on 12 March when system windwas weak (Fan et al., 2011; Lu et al., 2009). Bad dilution conditionsbrought about local aerosol particles accumulation. SSA was lowerunder the north wind condition because particles containing moreabsorption components were transported from the city, locatednorth of the site.

The weather charts (http://gb.weather.gov.hk) for two periodsare featured in Fig. 5a and b. The PRD region was influenced by thepassage of a cold front on the clean day with high wind speed,while controlled by the uniform pressure field on the polluted day,with bad diffusion conditions lead to high PM loading. On the otherhand, the evolution of the mixing layer (http://ready.arl.noaa.gov/READYamet.php) is also given in Fig. 5c. A pronounced diurnalpattern that is high during the day and low at night can be found,whereas the maximum of the mixing layer height on a polluted daywas lower than that on a clean day. Overall, bad diffusion condi-tions of uniform pressure, weak wind speed and a lowmixing layerheight gave rise to high particle concentration and many lightextinction effects.

3.2. Closure study for dry particles

3.2.1. Influence of externally mixed non-light-absorbing particles onoptical model computation

As mentioned in Sect. 2.3, the closure study was based on a two-component model. One assumption had been proposeddthat eachparticle could only contain one light-absorbing core. Likewise, BC isconsidered to exist in each particle, either externally mixed orhomogeneously internally mixed and/or coated with other com-ponents. Under this assumption, all particles absorb light (Chenget al., 2009). In fact, in the atmosphere, there could be a situation

where some particles are non(less)-light-absorbing since they arenot carbon-containing. The results from TDMA measurementconfirmed this supposition. Cheung reported that the numberfraction of CV particles obtained from V-TDMA measurementsranged from 0.38 to 0.14 and decreased with the increasing diam-eter of the particle (Cheung et al., 2015).

Fig. 6 shows optical properties efficiency (x) and contribution(J) with mean PNSD for a homogeneously internal (a) and core-shell (b) mixture. The optical properties efficiency x* (Eq. (10))and contribution J* (Eq. (11)) based on the assumption that CVparticles exist independently with other homogeneously internallyor core-shell mixed aerosol are also given.

x* ¼ xCV*FN;CV þ xothers*�1� FN;CV

�(10)

J* ¼ n*x**Dp*DDp (11)

In Eq. (10), x* is determined by the size parameter (p*Dp/l) andthe refractive indexes ~m. Here, either ~mint or ~mcore;shell was used forcalculation of xothers, while ~mnon was used for xcv. FN,CV is numberfraction of CV particles. The FN,CV in 40, 80, 110, 150, 200, 300 nmwere 0.384, 0.181, 0.180, 0.158, 0.143 and 0.137, respectively, ob-tained from V-TDMA measurements. The linear interpolation ofFN,CV with six diameters (40, 80, 110, 150, 200, 300 nm) wasdenoted with a yellow line. The FN,CV of particles with a diameterlarger than 300 nm and less than 40 nmwere set to 0.137 and 0.384,respectively.

For homogenously internal mixture, the particles around600e700 nm had the largest scattering efficiency (xsp), althoughthe crest of PNSD was around 100 nm, and the largest contributionto scattering (Jsp) was around 300e400. Considering independentCV particles, the x* was different from x after 600 nm. However, theJ* showed no significant distinction with J due to low particlenumber concentration. For the core-shell mixture, the J*

ep andJ*

sp were almost equal to Jep and Jsp even though x*ep and x*spwere a little higher than xep and xsp around 500e800 nm, the sameas the homogeneously internal mixture. For the range of size wherex* were mostly sensitive to optical properties, the FN,CV wererelatively low, so the differences between J and J* were alsosmall. In general, considering the externally mixed non(less)-light-absorbing particles would only make a minor difference in the

Fig. 5. Weather charts for (a) clean day and (b) polluted day, (c) diurnal mixing layer in two periods (blue line for clean day and green line for polluted day).

H. Tan et al. / Atmospheric Environment 131 (2016) 196e208 203

Fig. 6. Mean particle number size distribution (n), completely vaporized fraction (FN,CV), efficiency of extinction (xep, x*ep), scattering (xsp, x*sp), absorption (xabsp, x*absp), back-scattering (xhbsp, x*hbsp) and the contribution to extinction (Jep, J*

ep), scattering (Jsp,J*sp), absorption (Jabsp,J*

absp), backscattering (Jhbsp,J*hbsp) for (a) homogenously internal

mixture and (b) core-shell mixture (l ¼ 550). The parameters with symbol “*” represent corresponding efficiencies and contributions based on the assumption that CV particlesexist independently.

H. Tan et al. / Atmospheric Environment 131 (2016) 196e208204

calculation of aerosol optical properties. However, this conclusiononly applies to this study, and different results could be obtainedunder other PNSD, which have lager particles. In order to calculateoptical properties easily, externally mixed non(less)-light-absorbing particles were not considered in the following opticalclosure study.

3.2.2. Comparison of measured and modelled optical propertiesIn this study, 1200 points (30 min average) of aerosol optical

properties were calculated using PNSDs measured by a

combination of SMPS and APS (10 nme2.5 mm). The aerosol wasunder the assumption that the mixing state of BC and non(less)-light-absorbing components are external mixture, core-shell in-ternal mixture and homogeneously internal mixture. A modifiedMie model was employed to calculate the optical properties forthree wavelengths of 450, 550 and 700 nm.

Results show that values calculated from the modified Miemodel display a high correlation with measured values. Linearfitting was used for sep, ssp, shbsp and sabsp to quantify the com-parison of measured and calculated optical properties, based on the

H. Tan et al. / Atmospheric Environment 131 (2016) 196e208 205

hypothesis that the relation between calculated and measuredvalues is scalculate ¼ b*smeasure.

Fig. 7 shows the fitting parameters (b) and the determinationcorrelation coefficient (R2) for sep, ssp, shbsp and sabsp at thewavelength of 550 nm. On average, calculated sep had no obviousdifference among three models of mixing states, inferring sepmight not be sensitive to different mixing states. The calculated sspfor the external mixture was 17.5% and 11.8% higher than thosebased on the assumption of the core-shell and homogeneouslyinternal mixing state, respectively. Ambient aerosol particles werebetween the external mixture and core-shell mixture, or betweenthe external mixture and homogenously internal mixture.Regarding backscattering, the measured shbsp approached calcu-lated shbsp for the external mixing state and core-shell mixing statewhile deviating from calculated shbsp for the homogeneously in-ternal mixing state. They might imply that the sampled aerosolstend to be between the external mixture and core-shell mixture.Large differences between ssp and shbsp for three mixtures showthat shbsp wasmore sensitive to the variations in themixing state oflight-absorbing carbon than ssp (Ma et al., 2012). For the sabsp,values calculated for the core-shell mixture were closed to thosefrom the homogenously internal mixture, and both of them weretwice as much as the values calculated for the external mixture.This result was highly consistent with the study by Cheng et al.(2006) and Ma et al. (2012). That is because BC, which is spreadover all particles with the assumption of homogenously internalmixture, results in enhancement of absorption, while only pure BCparticles contribute to absorption in the external mixture (Malletet al., 2004).

Fig. 7. Comparisons of measured and calculated optical properties (sep, ssp, shbsp and sabs

internal mixture (green dots) of BC at 550 nm, including the 1:1 line (blue line). The straig

3.2.3. Retrieved mixing ratio of BC (rext-BC)As discussed above, the optical properties calculated on the

basis of the modified Mie model with assumptions that the mixingstates of BC include external, homogenously internal and core-shellinternal mixture show high correlation with measured values.During the period of observation, BC mass concentration, PNSDs,ssp,hbsp were continually measured by an aethalometer,SMPS þ APS and nephelometer online.

The time series of volume concentration of total particles and BCvolume fraction are illustrated in Fig. 8a. Their mean values were28 mm3/cm3 and 0.11, respectively. According to the relationshipbetween simulated and measured values of the aerosol opticalproperties, the input and the output parameters for the Mie modelare both the function of rext-BC (the ratio of externally mixed BCmass to total BC mass, which was introduced in Sect. 2.3), thus therext-BC can be retrieved from optical parameters. One of methods toachieve a proper rext-BC is to find a rext-BC which leads to a minimumdeviation between five simulated optical properties (ssp and shbspat 450 and 550, and also sabsp at 630 nm) and their correspondingmeasured values (Cheng et al., 2006). However, the measuredvalues did not always fall into calculated limiting values for externaland homogenously internal mixture. Ma et al. (2012) proposedanother method to retrieve the mixing state of BC using HBF.During the HaChi campaign, it was found that most of HBFsmeasured with a nephelometer fell into the range of values calcu-lated with the assumption of core-shell mixture and externalmixture (Ma et al., 2012).

In the same way, HBFs were simulated based on Mie models forthree mixing states of BC, as shown in Fig. 8b. The mean value

p) for external mixture (black dots), core-shell mixture (red dots) and homogenouslyht lines represent the linear regression fittings to the data.

Fig. 8. Time series of (a) aerosol volume concentration (blue line) and BC volume fraction (green line). (b) Comparison of measured HBF (black line) and calculated HBF with threelimiting mixing states (black line for external, red line for core-shell and green line for homogenously internal mixing state) of BC at 550 nm. (c) Time series of retrieved rext-BC (blackline).

H. Tan et al. / Atmospheric Environment 131 (2016) 196e208206

calculated was 0.141 for the core-shell internal mixing state, about9% higher than that measured from a nephelometer at the wave-length of 550 nm. For the external mixture, the mean value was0.119 and 7.4% lower than the measured value, whereas the lowestHBF from the Mie model was 0.109 for the homogenously internalmixture and 15% lower than measured HBF. Comparing the aboveoptical properties such as ssp and shbsp, it is worth noting thatalmost all measured HBFs fell into the range between the values forthe core-shell and external mixture during the entire period.Hence, the particles might be partially externally mixed andpartially core-shell mixed.

Fig. 8c shows the time series of rext-BC, a parameter to describethe mixing state of BC. Considering a systematic instrumental errorat the wavelength of 700 nm by the TSI 3563 Nephelometer(Heintzenberg et al., 2006), only values of 450 nm and 550 nmwereconducted in the work. For the entire period of observation, rext-BCvaried over time, which is to say that the mixing state of BC variedwith time. The variation range of rext-BC was 0e1, indicating that theBC was partially externally mixed and partially core-shell internallymixed with other non(less)-light-absorbing components.

3.2.4. Diurnal variation of BC mixing stateIn this study, the average value of rext-BC was 0.58 ± 0.12, which

was higher than that in the North China Plain of 0.51 ± 0.21, but theerror bar of one stand was lower, indicating that the mixing state ofBC tended to be externally mixed, and also the variation of rext-BCwas small. Traffic can be considered as the main source of exter-nally mixed BC. In other literature, the rext-BC presented a significantdiurnal pattern that was low in daytime and high at night (Chenget al., 2011; Liu et al., 2011; Ma et al., 2012). A sensitivity study of

a modelled mixing state with the meteorological and chemicalprocesses suggested that the diurnal variation of the aerosol mixingstate was mainly influenced by the evolution of the mixing layer(Liu et al., 2011). The average diurnal variation of rext-BC is displayedin Fig. 9a, as well as the error bar of one standard. The mean rext-BCwas about 0.6 at midnight, and then decreased to the minimumvalue as low as 0.5 at 04:00 am, meaning themixing state of BC wasto be more core-shell. In the early morning, there were relativelyfewer freshly emitted BC particles, which were trapped by theshallow surface layer. Since the measurement site is located in amountain with height of 150 m, measured aerosols mainly con-sisted of aged particles, which had experienced the aging processfor at least one day. With the increasing emissions during rushhour, more fresh BC particles were emitted into the surface atmo-sphere, leading to the enhancement of BC volume fraction (shownin Fig. 9b) and the proportion of externally mixed BC. Then themixing layer height reached a maximum around 02:00 pm, andstrong thermodynamic activity made aerosol particles mix well.Simultaneously, other processes such as condensation, coagulationand the photochemical aging process also contributed to the agingprocess of particles and a low value of rext-BC. The rext-BC presentedthe second peak due to rush hour starting around 05:00 pm, andslightly decreased until the night. At the beginning of the night,some aged particles were trapped due to the sudden collapse of themixing layer, and fresh BC particles emitted into the atmospheremaintained a high value of rext-BC.

4. Summary

The optical properties for dry aerosol particles were

Fig. 9. Average diurnal variation of rext-BC (a) and BC volume fraction (b).

H. Tan et al. / Atmospheric Environment 131 (2016) 196e208 207

investigated at the CAWNET station in the Pearl River Delta ofChina from 22 February to 18 March 2014. A data set of PNSDs bySMPS þ APS, ssp,hbsp by a nephlometer and BC mass concentrationby an aethalometer were collected continuously. During theobservation period, the mean ssp was 149.33 ± 89.73 Mm�1 andthe mean shbsp was 19.05 ± 11.35 Mm�1 at the wavelength of550 nm. An estimation of SSA was 0.80 ± 0.06 and the averagea450e700 was 1.82 ± 0.18. Optical properties were influenced bywind speed and direction, and also sources of air mass. PNSDs,Ångstr€om exponents and SSA were different on clean and pollutedday. The contributions to optical properties depended on particlenumber concentration, and were also influenced by weatherconditions, the evolution of the mixing layer and the compre-hensive aging process.

Considering externally mixed non(less)-light-absorbing parti-cles only made a minor difference on the simulation of aerosoloptical properties. To explore the mixing state of BC in the surfaceatmosphere in the PRD region, a numerical optical closure studywas carried out for dry particles between measurements and cal-culations based on a modified Mie model. Several assumptionswere introduced to simplify the input parameters of the modifiedintegral Mie model and to better simulate the atmospheric aerosoloptical properties: (1) a single particle is spherical; (2) the dryaerosol particles include BC and non(less)-light-absorbing com-ponents; (3) except for BC, all the other aerosol species areconsidered to have the same refractive indices; (4) three conceptualmodels describing the mixing state of BC: external mixture, core-shell internal mixture and homogenously internal mixture. Thus,a modified Mie model was applied for calculating the measuredoptical properties by a nephlometer for three wavelengths of 450,550 and 700 nm. Results showed that values calculated from themodified Mie model displayed a high correlation with themeasured values.

The HBFs calculated based on the assumption of externalmixture were lower than that of core-shell mixture, and higherthan that of homogenously internal mixture. Almost all measuredHBFs fell into the range between values for core-shell and externalmixture throughout the entire period, illustrating that the particlesmight be partially externally mixed and partially core-shell mixed.The average retrieved rext-BC was 0.58 ± 0.12, and showed a diurnal

pattern with a high value at rush hour and with a low value in theearly morning and at noon.

Acknowledgements

This work is supported by the Natural Science Foundation ofChina (41375156, 41275017), the National Key Project of BasicResearch (2011CB403403), the Natural Science Foundation ofGuangdong Province, China (S2013010013265, 2014A030313788),and the Special R&D fund for Research Institutes (2014EG137243).We also acknowledge Dr. Ma and Dr. Shen for their suggestions.

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