Light absorption property and potential source of particulate brown carbon in 1 the Pearl River Delta region of China 2 3

18 Brown carbon (BrC) is a type of light-absorbing component of organic aerosol (OA), covering 19 from near-ultraviolet (UV) to visible wavelength ranges, and thus may cause additional aerosol 20 radiative effect in the atmosphere. While high concentrations of OA have been observed in the 21 Pearl River Delta (PRD) region of China, optical properties and the corresponding radiative 22 forcing of BrC in PRD are still not well understood. In this work, we conducted a set of 23 comprehensive measurements of atmospheric particulate matters from 29 November 2014 to 5 24 January 2015 to investigate aerosol composition, optical properties, source origins and radiative 25 forcing effects at a suburban station of Guangzhou. Particle absorption Ångström exponent 26 (AAE) was deduced and utilized to differentiate light absorption by BrC from black carbon 27 (BC). The results showed that the average absorption contributions of BrC were 25.9±9.0% at 28 370 nm, 19.7±7.9% at 470 nm, 14.1±6.9% at 520nm, 11.6±5.6% at 590nm and 7.7±4.4% at 29 660nm, respectively. A sensitivity analysis of the evaluation of absorption Ångström exponent 30 of BC (AAEBC) was conducted based on the Mie theory calculation, assuming that the BC31 containing aerosol was internally mixed, with a core-shell configuration. The corresponding 32 uncertainty of BrC absorption contribution was acquired. We found that variations in the 33 imaginary refractive index (RI) of BC core can significantly affect the estimation of BrC 34 absorption contribution. However, BrC absorption contribution was relatively less sensitive to 35 the real part of RI of BC core and was least sensitive to the real part of RI of non-light absorbing 36 shell. BrC absorption was closely related to aerosol potassium cation content (K), a common 37 tracer of biomass burning emission, which was most likely associated with straw burning in the 38 rural area of western PRD. Diurnal variation of BrC absorption revealed that primary organic 39 aerosol had a larger BrC absorption capacity than secondary organic aerosol (SOA) had. 40 Radiative transfer simulations showed that BrC absorption may cause 2.2±2.3 W m radiative 41 forcing at the top of atmosphere (TOA) and contribute 14.2±6.2% of the aerosol warming effect. 42 A chart was constructed to conveniently assess the BrC radiative forcing efficiency in the 43 studied area with reference to a certain aerosol single-scattering albedo (SSA) and BrC 44 absorption contribution at various wavelengths. Evidently, BrC radiative forcing efficiency was 45 higher in shorter wavelength. 46 2 Atmos. Chem. Phys. Discuss., https://doi.org/10.5194/acp-2018-1331 Manuscript under review for journal Atmos. Chem. Phys. Discussion started: 4 January 2019 c © Author(s) 2019. CC BY 4.0 License.


Introduction
BC and organic carbon (OC) are the dominant carbonaceous aerosol components, which are mainly originated from anthropogenic activities and have attracted great environmental concerns in the rapidly developing regions.Carbonaceous aerosols can not only exert adverse impacts on public health like other particulate matters, but also significantly affect terrestrial radiation balance with enormous uncertainties.In previous studies, BC was often considered to be the only light-absorbing species (Andreae and Gelencser, 2006) and OC was believed to be only able to scatter light, i.e., causing the cooling effect (Bond et al., 2011).Nevertheless, it has been reported that some fraction of organic aerosols (OA) may also specifically contribute to light absorption from near-UV to visible wavelength range and is termed as brown carbon (BrC) (Kirchstetter et al., 2004).BrC optical properties are strongly affected by its chemical compositions and physical structures, which are related to different BrC sources.BrC could originate not only from direct emissions, including smoldering biomass burning or any type of incomplete fuel combustion process (Cheng et al., 2011;T. C. Bond et al., 1999), but also from secondary organic aerosol formation process, such as aqueous phase reactions in acidic solutions (Desyaterik et al., 2013) or volatile organic compounds (VOC) oxidation (Laskin et al., 2015;Sareen et al., 2010).In addition, BrC could possess complicated molecular composition and intermix with other substances, such as BC, non-absorbing OA and other inorganic materials, making it complicated to investigate BrC optical properties.
BC absorption is commonly assumed to be wavelength-independent.However, light absorption property of BrC is believed to be wavelength-dependent, which can be represented by distinct absorption Ångström exponent (AAE) values, i.e., the power exponent of light absorption coefficient.A typical threshold AAE of BC (AAE BC ) of 1.6 has been recommend to distinguish BrC from BC (Lack and Cappa, 2010) and the AAE of BrC has been reported with a wider range (2 to 7) (Hoffer et al., 2005).Based on the difference in wavelength dependence of light absorption between BC and BrC, previous studies have applied the AAE method to segregate light absorption by BrC through multi-wavelength optical measuring apparatus, such as 3-wavelength Photoacoustic Soot Spectrometer (PASS-3) (Lack and Langridge, 2013), multiwavelength Aethalometer (Olson et al., 2015), and so on.Based on the AAE method, BrC absorption contribution has been estimated to be about 6 to 41 % of total aerosol light absorption at short wavelengths, for instance, at 370 nm and 405 nm (Washenfelder et al., 2015).
Unity AAE BC is commonly used from the ~300 nm up to ~700 nm (Moosmüller et al., 2011) when evaluating BrC absorption contribution using the AAE method.However, it has been reported that AAE BC can be influenced by the mixing state, BC core size and morphology (Lack and Cappa, 2010).The lensing effect of the coating shell may enhance BC light absorption, the magnitude of which may also depend on wavelength and can alter the value of AAE BC (Liu et al., 2018).Moreover, different values of AAE BC have been found in the NIR and UV ranges (Wang et al., 2018).Therefore, using the default AAE BC = 1 may lead to uncertainty in BrC absorption coefficient estimation.
Quantifying BrC optical absorption accurately is essential to interpret aerosol optical depth (AOD) and the corresponding aerosol direct radiative forcing (DRF) on the atmosphere can also be evaluated, if the SSA and extinction coefficient of aerosols are known.The estimation of the DRF of BrC showed a distinct seasonal variation, indicating the influence from different absorption properties of BrC (Arola et al., 2015).A global simulation study indicated that the averaged warming effect at the TOA caused by the BrC absorption can be up to 0.11 W m −2 , corresponding to ~25% of that predicted from BC absorption only (Feng et al., 2013).
During the last three decades, rapid economic development has led to severe air pollution problems in the PRD region (Chan and Yao, 2008).With rapid increases in automobile population and factories, high loadings of SOA have often been observed (Tan et al., 2016).
Biofuel usage may also play a significant role during the wintertime air pollution events in PRD, indicating that the contribution from BrC light absorption cannot be ignored (Wu et al., 2018).
Recently, BrC light absorption has been quantified by Qin et al. (2018) using the AAE method in the PRD region.OA chemical composition was simultaneously measured by a high resolution time-of-flight aerosol mass spectrometer and it was found that organic aerosol originated from biomass burning possessed the most intense absorption capability and was largely responsible for the BrC absorption.Qin et al. (2018) also suggested that correlations between OA chemical compositions and BrC absorption were wavelength-dependent.
In this paper, we have applied the homologous AAE segregation method to quantify the fraction of aerosol light absorption by BrC using measurements of a seven-wavelengths aethalometer.
The potential error incurred with this methodology has been determined using Mie theory simulations, especially for various complex refractive indexes of BC core and the coating material.The correlation between BrC light absorption and water-soluble ions, used as the source tracer, were employed to identify the potential BrC sources.An atmospheric radiative transfer model also has been applied to evaluate BrC's impact on direct radiative forcing using surface-based aerosol optical properties and satellite-based surface-albedo data.The magnitudes of aerosol radiative forcing at the top of the atmosphere due to BC and BrC were also separately quantified.

Sampling site
Field observation was conducted at the Panyu station (23°00.2360´N,113°21.2920´E),which was one of the monitor sites of the Chinese Meteorological Administration (CMA) Atmospheric Watch Network (CAWNET) located on the summit of the Dazhengang Mountain (about 150 m above sea level) in Guangzhou, China.Fig. 1 shows the location of the Panyu site, which is situated at the center of PRD and is separated from the residential areas by at least 500 m.Some agricultural fields can be found to the west of the site.Although there were no significant pollution sources nearby, this suburban site was strongly affected by the pollutants transported from the urban area of Guangzhou and crop residual fires from the rural area of PRD.Field campaign was conducted from 29 November 2014 to 5 January 2015.During the measurement period, aerosol light scattering and extinction, BC concentration, particle number size distribution (PNSD), OC concentration, along with water-soluble ions concentration of PM 2.5 were continuously monitored.

Measurements and data analysis
All instruments were housed inside the 2 nd floor measurement room of a ~5-m tall, 2-story building.
Ambient sample was pulled in through the roof by a 2-m long, 12.7-mm OD stainless steel inlet and a PM 2.5 cyclone was also used.The metal tubing was thermally insulated and maintained at a constant temperature of ~25℃.A diffusion drier was also used in-line to dry the sample air relative humidity (RH) below 30% before further analyses.

Measurements of relevant species
A TSI-3936 scanning mobility particle sizer (SMPS) and a TSI-3321 aerodynamic particle sizer (APS) were utilized for the measurement of PNSD of 10 to 500 nm in mobility diameter and 0.5 to 2.5 µm in aerodynamic diameter, respectively.The aerodynamic diameters of APS data was converted into mobility diameter using a material density of 1.7 g cm -3 .The detailed data merging method has been described by Cheng et al. (2006).Furthermore, the pipe diffusion loss of SMPS has been corrected using the empirical formula proposed by Kulkarni et al. (1996).
An AE-33 aethalometer (Magee Scientific Inc.) was utilized for BC mass concentration measurement, which was derived from the optical attenuation using a mass absorption cross section (MAC) of 7.77 m 2 g −1 at 880 nm.The sensitivity of the AE-33 was about 0.03 µg m -3 for a 1-min time resolution and a 5.0 liters per minute (LPM) sample flow rate.
OC mass concentration was measured by a Sunset online EC/OC analyzer (Model RT-4) with a laser transmittance-based charring correction (Wu et al., 2018).The sample flow rate of the EC/OC analyzer was maintained at 8 LPM.For each measurement cycle of one hour, samples were collected onto a quartz filter within the first 45 min and then was thermal-optically analyzed during the remaining 15 min.Firstly, OC was completely volatized in oxygen-free helium with a stepwise ramped temperature (600 ℃ and 840 ℃).In the second stage, the temperature was reduced to 550 ℃, and then EC and pyrolyzed carbon (PC) were combusted in an oxidizing atmosphere (10% oxygen in helium), while the temperature was increased up to 870 ℃ step by step.The CO 2 converted from all the carbon components was then quantified by a nondispersive infrared absorption CO 2 sensor (Lin et al., 2009).Internal calibration peak made by 5% methane in helium was applied to quantified OC and EC.In order to correct the PC converted from OC to EC, a tunable pulsed diode laser beam was used to monitor the laser transmittance through the quartz filter during the OC measurement stage (Bauer et al., 2012).

Measurements of optical properties
Light extinction by aerosols at 532 nm was detected using a cavity ring-down aerosol extinction spectrometer (CRDS) (Model XG-1000, Hexin Inc.) by measuring the decay times of laser intensity through the aerosol-containing sample and the filtered background air sample under the same condition.The extinction coefficient (σ ext ) was calculated using the procedure described by Khalizov et al. (2009).
Aerosol total scattering (σ sp ) was measured by a TSI-3563 integrating nephelometer at three wavelengths (i.e., 450 nm, 550 nm, and 700 nm), which was calibrated with CO 2 following the manual instruction.Particle free air was used to check the nephelometer background signal once every two hours.The scattering coefficients at other wavelengths were extrapolated using the following equations: SAE= -ln σ scat,λ 0 -ln σ scat,550nm ln λ 0 -ln(550) (1) where λ 0 =450 nm for wavelengths less than 550 nm and λ 0 =700 nm for wavelengths more than 550 nm.
The aethalometer was also used for multi-wavelengths light absorption measurements in this study.The seven-wavelengths aerosol light attenuation coefficients (σ ATN ) were converted into the aerosol light absorption coefficients (σ abs ) using Eq.(3) (Coen et al., 2010), where k is the parameter to account for the loading effect, ATN is the light attenuation through the filter with sample loading and C ref is a fixed multiple scattering parameter.
Real-time k value was retrieved using the dual-spot loading correction algorithm developed by Drinovec et al. (2015).The detailed formula of ATN can also be found in Drinovec et al. (2015).
C ref is considered a constant that strongly depends on the filter matrix effect.However, some studies have suggested that C ref may vary with wavelength (Arnott et al., 2005;Segura et al., 2014).
C ref at 370 nm was expected to be about 12% and 18% less than C ref at 532 nm for aerosol component mainly from internal combustion engines and biomass burning, respectively (Schmid et al., 2006).Different ambient observations also showed that C ref may have regional specificity, even though they were all retrieved by the same methodology (Coen et al., 2010).
In this study, C ref =3.29 was used in Eq. ( 3) at each wavelength and this value was derived from the slope of σ ATN measured by the aethalometer vs. σ abs , deduced from the CRDS and nephelometer measurements.This C ref was also very similar to the C ref of 3.48 determined from an inter-comparison study between a aethalometer and a photo-acoustic soot spectrometer during a filed campaign conducted in the PRD region in 2004 (Wu et al., 2009).
The BC light absorption at certain wavelength was derived from the absorption coefficients σ abs according to the Beer-Lambert's Law and its variation between different pair of wavelengths (i.e., σ abs,BC,l ) is denoted by the Absorption Ångström exponent (AAE) equation developed by Ångström (1929): It has been suggested that the AAE of BC may vary between short-and long-wavelength ranges (Lack and Cappa, 2010) and hence applying a wavelength-independent AAE BC may lead to uncertainties in BC absorption calculation from one wavelength to another.In this work, the light absorptions of BC at various wavelengths were retrieved by a modified wavelengthdependent AAE segregation method: Here σ abs,BC,λ i (i=1, 2, 3, 4, 5, and 6) stands for the absorption coefficient due to BC alone at λ i =880, 660, 590, 520, 470 and 370 nm, respectively.AAE BC, λ i -λ i+1 represents the AAE of BC between a longer and a shorter wavelength and was calculated as: Accordingly, BrC absorption at a certain wavelength λ (σ abs,BrC,λ ) was the value of the total aerosol absorption (σ abs,λ ) subtracting BC absorption (σ abs,BC,λ ): The data of light absorption at 880 nm (σ abs,BC,880 ) was selected to represent BC absorption, which shall not be affected by BrC (Drinovec et al., 2015).It has been reported that dust-related contribution of PM 2.5 was normally less than 5% in wintertime Guangzhou, therefore the influence from dust could be negligible in this study (Huang et al., 2014).

Estimation of AAE BC
Traditionally, AAE BC was believed to be close to 1.0 (Bodhaine, 1995), which has been commonly used for BC measurements (Olson et al., 2015).However, studies have demonstrated that AAE BC can be affected by the refractive index of coating materials, mixing state, morphology, and BC core size (Liu et al., 2015).Moreover, different values of AAE BC have been found in the NIR and UV ranges (Wang et al., 2018).Therefore, using the default AAE BC = 1 may lead to uncertainty in BrC absorption estimation.In order to obtain the correct AAE BC , a series of Mie theory calculations were conducted using a simplified core-shell model (Bohren and Huffman, 1983;Wang et al., 2018).We used a modified BHCOAT code to calculate aerosol optical properties of core-shell mixture at different wavelengths (Cheng et al., 2006).In the Mie theory, a particle was taken as a perfect homogeneous sphere and its extinction and scattering efficiencies,  '(),*+',, and  -./),*+',, are expressed as (Mie, 1908;Seinfeld and Pandis, 1998): where a =   D  is the size parameter;  ; and  ; are functions of the complex RI and a in the Riccati-Bessel form.Re in Eq. ( 8) denotes that only the real part of RI is taken.The absorption efficiency ( /F-,*+',, ) was thus the difference between extinction and scattering efficiencies: Then the absorption coefficient  /F-,*+',, was obtained following (Bricaud and Morel, 1986): where (log  D ) is the PNSD.A two-component parameterization of dry particles, i.e., the BC core and the non (or less) light-absorbing species, was applied to calculate aerosol optical properties here (Wex et al., 2002).The PNSD of BC core was calculated as: where, ρ BC is the density of BC and is assumed to be 1.5 g cm -3 (Ma et al., 2012); M BC is the BC mass concentration derived from the MAAP, which was obtained by an empirical formula from Aethalometer measured BC concentration (M BC,AE ) proposed by Wu et al. (2009): The diameter of the BC core was calculated as: The σ abs,BC,Mie, λ i of all six wavelengths were calculated through the Mie model, and then the AAE BC of these five wavelengths were obtained using Eq. ( 6).

Atmospheric radiative transfer model
In this work, Santa Barbara DISORT Atmospheric Radiative Transfer (SBDART) model was employed to estimate the DRF of BrC absorption, i.e., its effects on the downward and upward fluxes (F in W m -2 ) of solar radiations at TOA. SBDART is a software tool that can be used to compute plane-parallel radiative transfer under both clear and cloudy conditions within the atmosphere.More details about this model have been described by Ricchiazzi et al. (1998).
Both ground measurements and remote sensing data were used in the simulation.The surface albedo was derived from 500 m resolution MODIS BRDF/Albedo Model Parameters product (MCD43A3, daily).The MCD43A3 products are the total shortwave broadband black-sky albedo (α BSA ) and white-sky albedo (α WSA ), while the actual surface albedo (α) was computed from a linear combination of α WSA and α BSA , weighted by the diffuse ratio (r d ) and direct ratio (1-r d ), respectively: r d was obtained from an exponential fit of Eq. ( 17) based on empirical observations (Roesch, 2004;Stokes and Schwartz, 1994): was calculated as the difference between downward and upward radiation flux: 3 Results and discussion

Aerosol light absorption and uncertainty of BC and BrC optical segregation
When the AAE BC was assumed to be unity, the campaign-averaged σ BrC were respectively 18.0±14.1Mm -1 at 370 nm, 10.0±8.5 Mm -1 at 470 nm, 6.0±5.5 Mm -1 at 520 nm, 4.2±3.7 Mm -1 at 590 nm, and 2.3±2.2Mm -1 at 660 nm.At the corresponding wavelengths, BrC absorption contributed 25.9±9.0%,19.7±7.9%,14.1±6.9%,11.6±5.6%,and 7.7±4.4% to the total aerosol absorption (see Fig. 2).Evidently, aerosol light absorption was predominantly due to BC, however BrC also played a significant part, especially at shorter wavelengths.Table 1 shows the inter-comparison of BrC light absorption in the near UV rang between this work and other studies in the East Asia region.Clearly, the reported values range substantially and our result is toward the lower value end.
In reality AAE BC may vary significantly with the BC containing aerosol of different size, mixing state, and morphology (Lack and Langridge, 2013;Scarnato et al., 2013).In fact, some studies showed that AAE of large size pure BC core may be less than 1.0 (Liu et al., 2018), and that AAE of BC coated with non-absorbing shell may be larger than unity (Lack and Cappa, 2010).
Theoretically, the magnitude of BC absorptions can be apparently affected by both parts of the complex refractive indexes (RI) and thus AAE BC may also vary with RIs of both the BC core and coating shell.In fact, RI was also one of the least known properties of BC and other coating materials of negligibly absorbing capability.So far reported refractive index of BC core (m core ) displays a wide range of variations (Liu et al., 2018).Typically, the real and imaginary parts of RI can vary from 1.5 to 2.0 and 0.5 to 1.1, respectively.In addition, the shell was assumed to be consisting of non-absorbing material, i.e., its imaginary RI was set to be close to zero (10 -7 ).
The real part of the shell RI may vary from 1.35 to 1.6 due to the presence of OA (Redmond and Thompson, 2011;Zhang et al., 2018) and inorganic salts (Erlick et al., 2011).Hence, it is necessary to investigate the uncertainties associated with the variations in AAE BC and the corresponding BrC absorption contribution estimations by varying the RIs of both the BC core and the non-absorbing shell.Based on the core-shell configuration, a suite of Mie theory computation was performed for AAE BC with specific wavelength-independent complex refractive indexes (RI = a -bi).The results are presented in Table 2. Generally, for wavelengths toward the UV range, AAE BC deviated negatively from unity and the shorter the wavelength was the more was the deviation.For wavelengths toward the NIR range, the trend of AAE BC was just reversed.It is also clearly shown that for all wavelength ranges, AAE BC increased with increasing real RI of the BC core but anti-correlated with the imaginary RI of the BC core.For the extreme cases (Model run 19 and 20 in Table 2), the corresponding averaged BrC absorption contribution can be as high as 42.7±7.0%and as low as 14.3±11.4% at 370 nm.Therefore, the estimation of BrC absorption contribution can be significantly affected by the choice of AAE BC .
However, for the cases typically encountered in the atmosphere, especially in the 500 to 600 nm range, AAE BC was very close to one.
Figure 3 shows the impacts of RI on the evaluations of AAE BC and BrC absorption contribution, where the RI of BC core was set constant, i.e., m core =1.80 -0.54i and the real part of shell's RI varied from 1.35 to 1.6 at an interval of 0.05, with the imaginary part of shell's RI set at 10 -7 .
As shown in Fig 3a, the calculated AAE BC was higher than 1.0 at longer wavelength and lower than 1.0 at shorter wavelength (the red line in Fig. 3 denotes AAE BC =1).Even the shell material was assumed non-absorbing, variation in the real RI of the shell still led to changes of shell's refractivity and correspondingly altered its lensing effect, causing the fluctuation in calculated AAE BC values between 520 and 660 nm.In other wavelength intervals, the AAE BC increased with increasing real part of shell's RI.In Fig. 3b, under the same conditions as in Fig. 3a, average BrC absorption contributed 23.7% ~ 27.4% at 370 nm, 13.1% ~ 17.7% at 470 nm, 7.4% ~ 11.0% at 520 nm, 4.8% ~ 8.2% at 590 nm, and -0.1% ~ 4.2% at 660 nm to the total aerosol absorption, respectively.Interestingly, the magnitude of BrC absorption contribution not only decreased with increasing wavelength, but also decreased with increasing real RI of the shell at certain wavelength, which was most likely due to the absorption enhancement of BC core caused by the increased leasing effects of the coating material.
The impacts of BC core on AAE BC and BrC absorption contribution are shown in Fig. 4, where the shell was assumed as non-absorbing (RI= 1.55 -10 -7 i ) and the RIs were wavelengthindependent.The left panels (Figs.4a and 4b) were obtained by fixing the imaginary RI of BC core to 0.54 but varying the real RIs from 1.6 to 2.0 with a step of 0.1; the right panels (Figs.4c and 4d) were done with a constant real RI of BC cores (1.8) but a varying imaginary RI from 0.6 to 1.0 with an incremental of 0.1.As shown in Figs.4a and 4c, the AAE BC at a certain wavelength generally increased with increasing real RIs (Fig. 4a) but decreased with increasing imaginary RI (Fig 4c).The AAE BC appeared to be more sensitive to the imaginary RI than the real RI of BC core, for the fact that the imaginary RI was directly related to light-absorbing properties of particles.In Fig. 4b, within the specified real RI (1.6 to 2.0) of BC core, BrC on average contributed 20.4% ~ 28.7% at 370 nm, 10.1% ~ 18.1% at 470 nm, 4.4% ~ 11.8% at 520 nm, 2.0% ~ 8.2% at 590 nm, and -2.0%~ 3.3% at 660 nm to the total aerosol absorption, respectively.Similarly, in Fig. 4d, within these specific BC core's imaginary RI (0.4 to 1.0), BrC on average contributed 19.9% ~ 37.0 % at 370 nm, 10.7% ~ 24.1% at 470 nm, 5.3% ~ 16.7% at 520 nm, 3.2 % ~ 11.8 % at 590 nm, and 0.01 % ~ 5.6 % at 660 nm to the total aerosol absorption, respectively.wavelength-independent unity AAE BC (Andreae and Gelencser, 2006;Olson et al., 2015).In both Fig. 3 and Fig. 4, negative BrC absorption contributions were obtained at longer wavelengths, which were due to the uncertainties associated with the calculation and the dominance (near 100%) of BC absorption at these wavelengths.
We want to point out that most BC containing particles are often observed as fractal rather than spherical in shape (Katrinak et al., 1993).Since the core-shell Mie model is under the assumption that all particles are spherical, it may lead to potential uncertainty for the estimation of AAE BC and BrC absorption contributions.Moreover, during a closure study in the PRD region Tan et al. (2016) have found that the actual BC mixing state was partially core-shell and partially external mixing.During a test run with the assumption of external mixing, we found that BrC contribution would become significantly higher.

Characteristics of BrC light absorption, water-soluble ion and OC concentration
Globally, BrC have been observed to be highly correlated with biomass and biofuel burning emissions (Laskin et al., 2015).Since large quantity of sylvite is present in biomass burning particles, K + abundancy has often been used as a biomass burning tracer (Levine, 1991).Figure 5 presents the time series of OC mass concentration, K + concentration, and BrC absorption from 29 November 2014 to 5 January 2015 at the Panyu site.The range of OC concentration obtained from the OC/EC online analyzer was from 1.5 to 67.6 µg cm -3 and the campaign average was 12.1 ± 7.8 µg cm -3 .The BrC absorption hourly mean data was between 0.2 and 70.8 Mm -1 and the campaign average was 17.6 ± 12.4 Mm -1 .On the other hand, average K + concentration was 1.0 ± 0.7 µg cm -3 (ranging from 0 to 5.4 µg cm -3 ).Clearly, similar trends among OC, K + , and BrC absorption can be seen during this field campaign (Fig. 5).
In order to investigate the origins of these observed OC, K + , and BrC.Wind rose plots (as shown in Fig. 6) were generated for OC, K + , and BrC absorption, respectively.All three panels of Fig. 6 show consistently that the three substances were associated with the same wind pattern.For the whole campaign period, the highest values of OC, K + , and σ abs,BrC,370nm were mostly associated with southwesterly winds of relatively low wind speed (~2 m s -1 ).The relatively higher OC and K + concentrations were highly related to the seasonal straw burning in the countryside of PRD located to the west of the Panyu station.On the contrary, OC and K + concentrations during periods with easterly winds were substantially lower than those during westerly winds.The wind rose plot of σ abs,BrC,370nm is shown in Fig. 6c.Similar to OC and K + , σ abs,BrC,370nm showed higher values in the weak (<2 m s -1 ) westerly wind and lower values from the north and south, indicating that BrC absorption was likely attributed to local sources and was accumulated under calm wind conditions.However, there was detectable difference among the three rose plots in the maximum concentration direction.The possible explanation was that although biomass burning emission was believed to be the dominant primary source of OC, K + , and BrC, their emission ratios were highly variable and may change with the types of biofuel, burning conditions, and may even vary during different stages of burning (Burling et al., 2012).
Although biomass burning emissions contain substantial light-absorbing BrC, further atmospheric aging process may significantly reduce its light-absorbing capability (Satish et al., 2017).Moreover, secondary formation may also lead to BrC formation inside these primary aerosols, such as humic-like substances formed through aqueous-phase reactions have been suggested to be an important component of BrC (Andreae and Gelencser, 2006).
To further explore possible sources of BrC optical absorption, the diurnal variations of OC, K + , σ abs,BrC,370nm , and σ abs,BrC,370nm /OC values are plotted in Fig. 7. Diurnal variation of OC at the Panyu site appeared to be dominated by the development of planetary boundary layer (PBL) height, i.e., primary emissions were accumulating at night and were swiftly diluted by vertical mixing in the morning.The slight increase of OC in the afternoon indicated that photochemistry may still contribute weakly to the SOA formation.Figure 7b shows the diurnal variation of K + .
Unlike OC, K + shows a distinct peak around 6 AM, which was consistent with the breakfast time and was most likely due to cooking activities using biofuel.It is still a common practice to collect straw as biofuel in the local rural area, which can be visually spotted.The diurnal profile of σ abs,BrC,370nm (see Fig. 7c) shows the combined feature of OC and K + , since both primary and secondary processes will affect its intensity.The nighttime rising trend was most likely attributed to straw burning activities in early winter in nearby rural area that continued to accumulate within the shallow PBL (Jiang et al., 2013).σ abs,BrC,370nm /OC, i.e., the mass absorption coefficient of BrC (MAC BrC ) (Fig. 7d), shows a relatively flat pattern with a pronounced dip in the afternoon and higher values at nighttime, which was likely due to the enhanced primary emissions and stable stratification at nighttime.The declining trends during the late morning and afternoon hours indicated that aging process and photochemical Furthermore, Fig. 8 shows the linear regression analysis results to evaluate the correlations between σ abs,BrC,370nm and OC, K + , Ca 2+ , Mg 2+ , Cl -, SO 4 2-, NO 3 -, NH 4 + concentrations, respectively.The best correlations can be found between σ abs,BrC,370nm and K + and OC (R 2 =0.4889 and 0.4872, respectively), followed by NO 3 -(R 2 =0.3267) and NH 4 + (R 2 =0.3234).
Both nitrogen oxides (NO x ) and ammonia (NH 3 ) can be found in biomass burning plumes (Andreae and Merlet, 2001).Nitrate can be converted from NO x through atmospheric reactions and NO x may originate from many combustion processes, such as biomass burning or any kind of fossil fuel usage (Elliott et al., 2009).Source apportionment analysis of OA and BrC absorption in Beijing and Guangzhou illustrated that biomass burning organic aerosol (BBOA) correlated well with BrC light absorption (Qin et al., 2018;Xie et al., 2018).Thus, the significant correlation between BrC absorption and NO 3 -/NH 4 + reaffirmed that biomass burning was the crucial emission source of BrC observed in this work.Although the geographic location of observation site was situated in a coastal area and K + also could be found in sea salt (Pio et al., 2008), it should be noted that the prevailing wind direction during winter was from the north (see Fig. 3), which will drive marine air parcels away from the site.High concentrations of Ca 2+ and Mg 2+ are often found in dust-related aerosols (Lee et al., 1999).σ abs,BrC,370nm showed poor correlations with both Ca 2+ and Mg 2+ , indicating that dust-related aerosol components contribute insignificantly to the total aerosol mass loading and thus dust may not affect the AAE segregation method used in this work.Although sulfur dioxide (SO 2 ) may also be emitted by biomass burning, SO 4 2-is often believed to be secondary in nature and the presents of other intense SO 2 sources (e.g., automobile and industrial emissions) will further smear the correlation between BrC and SO 4 2-.Sources of Cl -include both combustions and sea salt spray (Waldman et al., 1991).Although the prevailing wintertime wind direction was from the north, sea salt still can be carried to the site by weak sea breeze and thus Cl -may not show considerable correlation with BrC.

BrC radiative forcing efficiency
The radiative effects of aerosol scattering, BrC absorption, and BC absorption were investigated by the SBDART model.For each investigated variable under cloud-free condition, we run the model twice to calculate the DRF at TOA with and without the investigated variable.
Accordingly, the difference of DF between the two simulations was considered as the radiative effect of the investigated variable.The results showed that the average radiative forcing at TOA by scattering, BrC absorption, and BC absorption were respectively -21.0±5.5 W m −2 , 2.2±2.3W m −2 , and 11.3±5.0W m −2 .Furthermore, BrC absorption was attributed to 14.2±6.2% of the warming effect caused by aerosol light-absorption, demonstrating that the nonnegligible role of BrC in radiative forcing evaluation.
We also calculated the BrC radiative forcing efficiency (RFE) under various SSA (ranging from 0.7 to 0.99) at three wavelengths, i.e., 440 nm, 675 nm, and 870 nm.The RFE was denoted as It also should be noted that the simulations were based on SSA measured under dry conditions.
Under the typical ambient conditions of PRD, the SSA might be markedly enhanced by aerosol water uptake (Jung et al., 2009), and then, the BrC radiative forcing efficiency might be less.
) where µ 0 is the cosine of the zenith angle, calculated by the model for any specified date, time, and the latitude and longitude of the site.The surface-based aerosol optical properties, including aerosol light absorption coefficients of both BC and BrC, i.e., segregated from each other under the assumption of unity AAE BC , along with nephelometer measured aerosol scattering Atmos.Chem.Phys.Discuss., https://doi.org/10.5194/acp-2018-1331Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 4 January 2019 c Author(s) 2019.CC BY 4.0 License.coefficients, were used to calculate the SSA at different wavelengths according to Eq. (18), SSA(λ)= σ scat, λ σ abs, BrC,λ +σ abs, BC,λ +σ scat,,λ (18) which was then used in the model calculation.Finally, the AOD and asymmetry factor (ASY) at 440, 675 and 870 nm were derived from the Aerosol Robotic Network (AERONET) measurements at the Hong Kong Polytechnic University site (Holben et al., 1998), which is about 115 km to the southeast of Panyu site.The tropical atmospheric profile was used in the SBDART model based on the prevailing weather conditions in PRD.The aerosol DRF (ΔF) Atmos.Chem.Phys.Discuss., https://doi.org/10.5194/acp-2018-1331Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 4 January 2019 c Author(s) 2019.CC BY 4.0 License.

Figure 4
Figure4demonstrated that the variation of the imaginary RI of BC core will cause the most significant impact on the estimated BrC absorption contributions, indicating that the morphology and structure of BC emitted from different sources will lead to a large uncertainty in BrC estimation.At the same time, the influence arisen from varying real RI of BC core was relatively moderate.Nevertheless, Fig.3demonstrated that alteration of the real RI of the nonabsorbing shell caused the least impact than that caused by the variations of the complex RI of BC core.Please note that σ abs,BrC was extrapolated with an assumed unity AAE BC , which was among a reasonable range at 370 nm but may be significantly overestimated within other longer wavelengths according to Mie theory calculation results.Moreover, many studies demonstrated that BrC showed a stronger light absorbance in the UV-visible wavelength range, where data at 370 nm were often chosen to represent BrC light absorption under the assumption of a Atmos.Chem.Phys.Discuss., https://doi.org/10.5194/acp-2018-1331Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 4 January 2019 c Author(s) 2019.CC BY 4.0 License.production may reduce BrC's light-absorbing capacity (Qin et al., 2018).
the radiative forcing normalized by the AOD.The average AOD and ASY at the three wavelengths were respectively 0.370 and 0.697 at 440 nm, 0.214 and 0.635 at 675 nm, and 0.153 and 0.618 at 870 nm.A solar zenith angle of 55° and an average shortwave broadband surface albedo (0.119) were used in the calculation.The results were plotted as a set of lookup charts of RFE as a function of the surface BrC absorption contribution (see Fig.9).In general, for any wavelength RFE increased with increasing BrC absorption contribution for a certain SSA, indicating BrC was a more efficient radiative forcing agent due to BrC's preferential absorbance in shorter wavelength range.However, for a certain BrC absorption contribution RFE increased with decreasing SSA, i.e., higher portion of light-absorbing aerosol components can lead to more efficient radiative forcing.The trend among panels (a), (b), and (c) in Fig.9demonstrated that the effect of BrC absorption contribution on REF was wavelength-dependent, i.e., BrC was a weaker radiative forcing agent at longer wavelength, which is also consistent with BrC's wavelength-dependent light-absorbing property.The black stars in Fig.9denote the average SSA and BrC absorption contribution conditions during this campaign, i.e., 0.025 W m −2 per unit AOD at 440 nm (Fig.9a), 0.007 W m −2 per unit AOD at 675 nm (Fig.9b), and 0.0002 W m −2 per unit AOD at 870 nm (Fig.9c).These results suggested that the average value of REF decreased distinctly from 440 nm to 870 nm, not only because of the lower BrC absorption contribution, but also due to BrC REF's wavelength-dependence.

Figure 1 .
Figure 1.The location of Panyu station (CAWNET) in the PRD region (indicated by the red 758 dot).The plain areas within the yellow circles are the main rural areas of western PRD.759 760

Figure 2 .
Figure 2. (a) BC and BrC particle average light absorption coefficient at different wavelengths; the whiskers represent the error bar of one standard deviation.(b) Contributions of BC and BrC to the total light absorption coefficient at different wavelengths; the whiskers represent the error bar of one standard deviation.

Figure 3 .
Figure 3. Influence of wavelength-independent refractive index of the non-absorbing shell on the (a) AAEs and (b) BrC absorption contribution with a constant BC core refractive index (m core =1.80-0.54i).The imaginary part of non-(or less-) absorbing shell was set to 10 -7 , while real part varied from 1.6 to 2.0.In each panel, the boundaries of the box represented the 75th and the 25th percentiles; the whiskers above and below each box indicate the error bar of one standard deviation; the black lines among the boxes denote the average values.In panel a, red line indicates where AAE BC =1.In panel b, the red lines indicate the BrC absorption contribution calculated with AAE BC =1 in each wavelength.

Figure 6 .
Figure 6.Wind rose plots of OC (a), K + (b), and σ abs,BrC,370nm (c).In each panel, the black solid lines denote the frequency of wind direction.The shaded contour represents the average values of corresponding species for that wind speed (radial length) and wind direction (transverse direction) in the polar coordinates.

Figure 7 .
Figure 7. Box-whisker plots of diurnal trends of OC concentration (a), diurnal trends of watersoluble K+ concentration (b), diurnal trends of σ abs,BrC,370nm (c), and diurnal trends of σ abs,BrC,370nm /OC (d).Red traces represent the variation of average value.Upper and lower boundaries of the box represent the 75th and the 25th percentiles; the whiskers above and below each box represent the error bar of one standard deviation.

Figure 9 .
Figure 9. BrC radiative forcing efficiencies, defined as the BrC TOA direct radiative forcing divided by AOD, as a function of BrC to BC absorption ratio and SSA measured at surface.The average AOD of the three wavelengths, the average ASY of the three wavelengths, a solar zenith angle of 55°, and the average shortwave broadband surface albedo were used in the calculation.The black star corresponds to average SSA and BrC absorption contribution determined from this campaign.

Table 1 .
table to conveniently assess the BrC radiativeAtmos.Chem.Phys.Discuss., https://doi.org/10.5194/acp-2018-1331Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 4 January 2019 c Author(s) 2019.CC BY 4.0 License.In this work, light absorption due to BrC in the PRD region of China was quantitatively deduced during the winter season of 2014.The average BrC light absorption contribution ranged from 7.7±4.4% at 660 nm up to 25.9±9.0%at 370 nm, when AAE BC was set to unity.The uncertainty in BrC absorption estimation associated with this assumption was further investigated.Using the absorption coefficients of BC calculated according to the Mie theory and the observed total aerosol absorption coefficients, we have estimated the AAE BC and hence the BrC absorption contribution for different core-shell RI configuration.The results showed that at 370 nm Atmos.Chem.Phys.Discuss., https://doi.org/10.5194/acp-2018-1331Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 4 January 2019 c Author(s) 2019.CC BY 4.0 License.Atmos.Chem.Phys.Discuss., https://doi.org/10.5194/acp-2018-1331Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 4 January 2019 c Author(s) 2019.CC BY 4.0 License.Observational studies of the BrC light absorption coefficient and contribution in near 751 ultraviolet wavelength range in East Asia.752 shortwave solar absorption warming effect at TOA, indicating that BrC might be an important climate forcing agent, which was largely neglected in current climate models.To facilitate the estimation of climate effects of BrC, a set of look-up charts were constructed for the investigated area based on the default tropical atmosphere profile, averaged surface albedo, 753 27 Atmos.Chem.Phys.Discuss., https://doi.org/10.5194/acp-2018-1331Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 4 January 2019 c Author(s) 2019.CC BY 4.0 License.

Table 2 .
AAE BC estimation from core-shell Mie theory model.