Supplement of The influence of photochemical aging on light absorption of atmospheric black carbon and aerosol single-scattering albedo

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1 Supplement of Atmos. Chem. Phys., 8, , 8 Author(s) 8. This work is distributed under the Creative Commons Attribution 4. License. Supplement of The influence of photochemical aging on light absorption of atmospheric black carbon and aerosol single-scattering albedo Xuezhe Xu et al. Correspondence to: Weixiong Zhao and Weijun Zhang The copyright of individual parts of the supplement might differ from the CC BY 4. License.

2 S Flow chart of the instruments PM.5 Sampler Roof (a) (b) (c) Water trap Diffusion dryer TD Thermo CO V DMA 38 Thermo NO x Heating tube Thermo SO 53nm -Albedometer V DMA 38 CPC 3776 Thermo O 3 Figure S: Experimental set-up for the measurements of: (a) aerosol optical properties, (b) volatility of PM.5 particles, and (c) concentrations of pollutant gases.

3 S Estimation of the truncation error of the cavity-enhanced albedometer The geometry of the cavity-enhanced albedometer is shown in Fig. S. The inner diameters of the integrating sphere (L) and the truncation reduction tube (d) were 5 mm and 9 mm, respectively. The length of the sample occupied in the truncation reduction tube (l) was ~ 97 mm. Z is the distance of the particle from the scattering location in the sphere to the exit or entrance aperture of the sphere (Z = is for the centre of the sphere). d = 9 mm Z Truncation reduction tube Integrating sphere Truncation reduction tube l = 97 mm L = 5 mm l = 97 mm Figure S Sample cell geometry of the cavity-enhanced albedometer. The albedometer truncation error was evaluated with Mie theory by considering the geometry of the cell and the scattering phase function of spherical particles (Moosmüller and Arnott, 3; Zhao et al., 4; Onasch et al., 5). The scattering truncation of the particles with a certain diameter (D) inside the cell (T(D, )) is defined as the ratio of the integrated scattering efficiencies of the effective truncation angles to the total scattering efficiency, which can be expressed as following (Onasch et al., 5): I( D, ) T D d (, ) sin( ) Qsca ( m, x, ) (S) where and are the effective truncation angles, I(D, ) is the unpolarized scattering intensity, m is the complex refractive index (CRI, m = n + ik), is the wavelength of incident light, x D is the size parameter, and Q ( m, x, ) is the scattering efficiency. sca The effective truncation angle ( ) depends upon the position of particles within the cell, which can be expressed as (Varma et al., 3; Zhao et al., 4): d, ( Z) tan ( Z) tan L Z d L Z (S)

4 The total scattering truncation can be written as follows (Onasch et al., 5): where the end points of Z integration are defined as: Z ( Z) I( D, ) T ( D) sin( ) d dz Z ( Z) Q ( m, x, ) (S3) sca Z L l, Z L l A plot of the total scattering truncation of PSL particles with different diameters of our cavity-enhanced albedometer and that of PMssa reported by Onasch et al. (5) is shown in Fig S3. By using longer truncation reduction tubes and larger inner diameter integrating sphere, the truncation losses of our albedometer are smaller than that of CAPS PMSSA monitors. (S4). Albedometer PM ssa -45 PM ssa Truncation Diameter (nm) Figure S3 Comparison of the scattering truncations for PSL particles of our albedometer (at λ = 53 nm) and that for CAPS PMssa (at λ = 45, 63 nm). The complex refractive index of PSL was set to be.59 + i.. Three different CRI values were used to represent the scattering truncation of different-type particles: the non-absorbing (.55 + i ); absorbing (.55 + i.); and strongly absorbing (.85 + i.7) particles (treated as black carbon). As shown in Fig. S4, the calculated truncations decreased with increasing of particle size and imaginary part of CRI. 3

5 ..98 m =.55+.i m =.55+.i m =.85+.7i.96 Truncation Diameter (nm) Figure S4 Scattering truncation calculated by the Mie model for three different type particles at λ = 53 nm. The accumulative truncation errors (ATE) based on the SMPS data (integrated with the normalized number size distribution of ambient aerosol) can be expressed as follows: ATE f ( Di )( T ( Di )) ddi (S5) where Di is the particle diameter, f(di) is the normalized particle number distribution, T(Di) is the scattering truncation of the corresponding diameter. As shown in Fig. S5, the accumulative truncation errors for three different type particles are all smaller than.%. Thus, the truncation errors of the measured scattering coefficients were negligible for fine particles in this work. 4

6 Normalized particle number distribution (NPND) NPND m =.55+.i m =.55+.i m =.85+.7i.. 75 Diameter (nm) Accumulative Truncation error (%) Figure S5 Averaged normalized particle number distribution during the observation period and the corresponding accumulative truncation errors at each size. 5

7 S3 Particle losses inside the thermodenuder Laboratory-generated polydisperse NaCl particles (with TSI 376 constant output atomizer) were used to estimate the particle losses inside the thermodenuder (TD). Particles were dried to RH < 5% with a silica gel diffusion dryer (TSI 36), neutralized with an aerosol neutralizer (TSI 377), and then diluted with a buffer to obtain a stable aerosol concentration. The size resolved TD losses are shown in Fig. S6. The measured particle losses agreed with manufactory given data (Dekati Ltd., Technical Note, ). (a) Bypass TD, operating at 3 C 8 (b) TD, operating at 3 C Dekati Ltd., HET 6 8 dn/dlogd p ( 5 cm -3 ) 6 4 Particle loss (%) 4 75 Diameter (nm) 4 6 Diameter (nm) Fig. S6: (a) The changes of particle number distribution of NaCl particles passed through the bypass and the TD operating at 3 C. (b) Size resolved particle losses of the TD. Manufactory given losses (blue line, Dekati Ltd., HET. ) are also shown in the figure. Due to the small absorption cross section, the contribution of small particle to aerosol absorption is negligible. Particle number losses may not exactly the same as the optical losses. In this work, laboratory-generated polydisperse NaCl particles and ambient sample before and after passing through of the TD were used to estimate the actual optical losses inside the TD. The corresponding measured extinction and scattering coefficients for NaCl are shown in the upper panel of Fig. S7. The TD loss was estimated to be ~ 6% for both scattering and extinction. The results for ambient sample were shown in 6

8 the lower panel of Fig. S7. The TD losses for both channels were estimated to be ~ 3%, which are comparable to the estimated losses of absorption of black carbon (3 ± 3% at λ = 37 nm, and 4 ± 6% at λ = 88 nm) (Devi et al., 6). In this work, we use actual TD loss of ambient aerosol 3% for further data analysis. 5 4 R =.998 Intercept = -.6 Slope = R =.999 Intercept =.7 Slope =.63 b ext (Mm - ) 3 b sca (Mm - ) 3 NaCl 3 4 b ext,td (Mm - ) NaCl 3 4 b sca,td (Mm - ) 4 4 b ext (Mm - ) 8 6 R =.96 Intercept = 7.7 Slope =.34 b sca (Mm - ) 8 6 R =.97 Intercept = 7.75 Slope =.38 4 Ambient Ambient b ext,td (Mm - ) b sca,td (Mm - ) Figure S7: Scatter plot of the measured extinction and scattering coefficients of NaCl (upper panel, operating at 3 C) and ambient particles (lower panel, operating at room temperature) before (bext,scat) and after (bext,scat, TD) passing through the TD. 7

9 S4 Meteorological parameters during the measurement period Rainfall (mm) Wind speed (m s - ) Wind direction ( ) Temperature ( C) Temperature RH RH(%) 5 Jun 6 Jun Jun 4 Jun 8 Jul Jul 6 Jul Jul 4 Jul 8 Jul Date Figure S8: Time profiles of rainfall, wind speed, wind direction, temperature, and relative humidity (RH) during the measurement period. 8

10 S5 Air mass backward trajectory Figure S9: The 48 h back trajectories ending at 5 m above ground level at the Shouxian site (calculated every h) were classified into five groups using the clustering method given by HYSPLIT model. 9

11 S6 Case study of the correlation between ω, ωtd, Eabs and Ox Case : CO (ppmv) T ( C) WD ( ) WS (m s - ) RH(%). TD & TD E abs 4 3 6/6/6 : 6/6/6 4: 6/6/6 8: 6/6/6 : 6/6/6 6: 6/6/6 : 6/6/6 4: Date O x (ppbv) Case : CO (ppmv) T ( C) WD ( ) WS (m s - ) RH(%). TD & TD E abs 4 3 6/6/5 : 6/6/5 8: 6/6/5 6: 6/6/6 : 6/6/6 8: 6/6/6 6: 6/6/7 : Date O x (ppbv)

12 Case 3: T ( C) WD ( ) WS (m s - ) RH(%) CO (ppmv) TD & TD Case 4: E abs 4 3 6/7/7 : 6/7/7 8: 6/7/7 6: 6/7/8 : 6/7/8 8: 6/7/8 6: 6/7/9 : WD ( ) T ( C) CO (ppmv) & TD TD Date O x (ppbv) WS (m s - ) RH(%) E abs /7/9 : 6/7/9 : 6/7/ : 6/7/ : 6/7/ : 6/7/ : Date 6/7/ : 6/7/ : 6/7/3 : O x (ppbv) Figure S: Time series of wind direction (WD, colored by wind speed (WS)), air temperature (T) and relative humidity (RH), CO concentrations, ω, ω TD, E abs and O x concentrations for the four selected cases.

13 S7 Mie theory modelling and attribution of light absorption A graphical representation of the calculation procedure is shown in Fig. S. Comparisons of modeling and observation Eabs and SSA are shown as a scatter plot in Fig. S. The calculation of the model is as follows: (a) Initial inputs parameters are the diameter of BC core (Dcore), CRI of core (mbc) and real part of the CRI of shell (nshell); In this work, mbc at λ = 53 nm was fixed at.85 + i.7, and nshell was fixed at.55 (Bond and Bergstrom, 6; Lack et al., ; Saleh et al., 5). kshell varied from (non-absorbing coating; clear shell) to. (strongly absorbing coating) (Lack and Cappa., ). Dcore was constrained between 5 and 4 nm, while Dshell ranged from 5 (for thinly coated BC) to 8 nm (for thickly coated BC). (b) Calculate the extinction, scattering and absorption coefficients (bext, bsca, and babs) of the BC core and coated particles with different kshell and Dshell using the core-shell model; (c) Calculate Eabs and ω from the calculated bext, bsca, and babs of bare BC and coated particles; (d) Determine a set of optimized Dshell and kshell values by minimizing the merit function, : ( ) ( E E ) (S6) calc abs abs, calc E abs Here, ω and Eabs are the observed values, ωcalc and Eabs,calc are the corresponding calculated values, and and Eabs are the measurement uncertainties of ω and Eabs, respectively. (e) The outputs of (d) are the optimized Dshell, kshell, Eabs,calc and ωcalc for a given Dcore in (a). A linear fit (as shown in Fig. S) of the calculated and observed Eabs and with different Dcore was performed. A lookup table (in nm step increments) was built to find the optimized Dcore (bold text in the supplement Table S). In this work, the optimized Dcore was determined to be 6 nm. This result falls within the diameter range of - 65 nm reported by C. Wu et al. (8), as well as the SP measurement results of rbc, which ranged from nm in different megacities in China (Huang et al., ; Wang et al., 5; Gong et al, 6; Y. Wu et al., 8).

14 Table S: Lookup table of the linear fit (intercept, slope, and R ) of calculated and observed Eabs and ω for different BC core diameters (Dcore). Dcore (nm) Intercept- Eabs Slope-Eabs R -Eabs Intercept- Slope- R

15 (a) Given initial diameter of BC core (D core ), m BC, n shell (b) Calculate b ext, b sca, b abs Inputs: D shell, k shell (c) Calculate calc and Eabs,calc (d) Iterate calculating of and E abs until find minimum (e) Iterate D core until get best agreement between calculated and measured values Figure S Optimization process for determining core (Dcore) and shell (Dshell) diameters, and the imaginary part of CRI of the shell (kshell). 4

16 R =.999 y = (-. ±.4) + (. ±.5) x.8 R =.999 y = (-.6±.7) + (.3±.7)x Calculated Calculated E abs Meausred Meausred E abs Figure S The comparisons of the measured and calculated Eabs and. 5

17 Figure S3 The plot of measured and modelled Eabs values with different Dshell/Dcore. Dcore is fixed at 6 nm, and Dshell is ranged from 3 to 48 nm. The imaginary parts (kshell) of complex refractive index ranged from to. respectively present the clear and brown shell. 6

18 Authropogenic SOM (Liu et al., 5) Toluene + OH Toluene + OH + NO x m-xylene + OH m-xylene + OH + NO x Biogenic SOM (Liu et al., 3) a-pinene + O 3 Limonene + O 3 BrC spheres (Alexander et al., 8) Fulvic acid (Liu et al., 5) SRFA (Bluvshtein et al., 6) Black carbon (Kirchstetter et al., 4) BB (Kirchstetter et al., 4) BB (Chakrabarty et al., ) HULIS (Hoffer et al., 6) BB + Authrogenic (Lack et al., ) BB (Lack et al., ) HULIS (Dinar et al., 8) Urban BrC (Cappa et al., ) Fresh BB (Sumlin et al., 7) Aged BB (Sumlin et al., 7) This Work BC - BrC k A-SOM - B-SOM Wavelength (nm) Figure S4: Comparison of the retrieved imaginary part of the coated shell with previously reported values of fresh and aged organic material (adapted from Liu et al. (5) and Sumlin et al. (7)). 7

19 E abs f Lens (%) f Shell (%) f BC (%) Figure S5: Ternary plot of the fractional contribution of lensing effect (flens), the absorption of BC (fbc) and the shell (fshell) to absorption enhancement (the values of Eabs were colour-coded). 8

20 3.5 (a) Observed E abs Mie theory calculation (monodisperse BC core, D=6 nm) Mie theory calculation (polydisperse BC core, GMD=6 nm, =.6) Mie theory calculation (polydisperse BC core, GMD= nm, =.6) Mie theory calculation (polydisperse BC core, GMD= nm, =.6) Mie theory calculation (polydisperse BC core, GMD=8 nm, =.6) 3. E abs.5 E abs (Calc)/E abs (Meas)..3.. (b) Monodisperse BC core, D = 6 nm Polydisperse BC core, GMD=6 nm, =.6 Polydisperse BC core, GMD= nm, =.6 Polydisperse BC core, GMD= nm, =.6 Polydisperse BC core, GMD=8 nm, = Local time (h) Figure S6: Comparison of the Mie theory results of Eabs with monodisperse BC core with 6 nm diameter and polydisperse size distributions with a geometric standard deviation of.6 and mode diameters of 6,,, and 8 nm, respectively. The parameters used for the calculation are the same as in Fig. 8. Polydisperse BC core sizes have larger Eabs values than monodisperse BC core; however, the trends of Eabs values are same. We use an optimization process for determining Dcore, Dshell, and kshell; similar trends show that monodisperse BC core can be used for the interpretation of the measurement data. 9

21 .5 Monodisperse BC core, D = 6 nm Polydisperse BC core, GMD= 6nm, =.6 Polydisperse BC core, GMD= nm, =.6 Polydisperse BC core, GMD= nm, =.6 Polydisperse BC core, GMD= 8nm, =.6.45 f BC f Lens f Shell Local time (h) Figure S7: Fractional contribution of the lensing effect (flens), the absorption of BC (fbc) and the shell (fshell) to absorption enhancement with different BC core sizes as Fig. S6. The trends of the diurnal pattern are similar. The differences between these calculations are less than %.

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