Geophysical Research Letters Supporting Information for The effect of complex black carbon microphysics on the determination of the optical properties of brown carbon Dantong Liu1, Jonathan W. Taylor1, Dominque E. Young1, Michael J. Flynn1, Hugh Coe1 and James D. Allan1,2 1 School of Earth, Atmospheric and Environmental Science, University of Manchester, Manchester, UK 2 National Centre for Atmospheric Science, University of Manchester, Manchester, UK Contents of this file Supplementary sections S1-S5. Supplementary figures S1-S5. Introduction The supporting information for our article “The effect of complex black carbon microphysics on the determination of the optical properties of brown carbon” contains 5 sections with one figure in each section. Section S1 contains details on the sensitivity of scattering response to the particle geometry for the SP2 instrument. Section S2 shows the overview of the BC-related properties during the experiment. Section S3 gives the detailed measurements of absorbing properties under thermos-denuded and ambient conditions. Section S4 demonstrates the procedures applying Mie calculation to the SP2 measurements. Section S5 shows the model sensitivities on the derived optical parameters of brown carbon. S1. The sensitivity of SP2 scattering response to particle geometry The particle geometry on the SP2 scattering response is tested by applying the Rayleigh-Debye-Gans (RDG) approximation as an extreme case for fractal particle shape, where the total particle scattering is equal with the number square of monomer scattering, expressed in Equation (S1): CSca(ΔΩ)=Ns2*Cs,Sca(ΔΩ) (S1) Where Cs,sca denotes the scattering cross section of monomer, which has been tested to be insensitive to the monomer size at λ =1064nm. ΔΩ denotes the Csca is integrated over the SP2 collection solid angle. The number of monomer Ns is determined by equation (2), where the measured BC core is composed of the sum of monomer volumes. Fig. S1 shows the sensitivity of the SP2 detected C Sca to particle geometry becomes larger for larger particle with thicker coatings. For the extreme case of this study at D c=197nm and Dp/Dc=2.1, the uncertainty is <20%, leading to a Dp/Dc determination <6%. Figure S1. The Mie and RDG modelled scattering cross section (Csca) as a function of BC core size at the SP2 operation λ=1064nm. The coated BC with Dp/Dc=1.1 and Dp/Dc=2.1 are shown. Note that the scattering of monomer is almost independent of monomer size at λ=1064nm. S2. An overview of BC and BrC related parameters Figure S2. Time series for A) BC mass from solid fuel burning (BC sf) and traffic sources (BCtr); B) SFOA mass; C) BCsf mass fraction; D) BC core mass median diameter; E) BC-containing particles (BCc) coating thickness Dp/Dc. S3. The measured MAC under different regimes Figure S3.A) the PASS measured absorption coefficient at 532nm as a function of SP2 measured rBC mass under ambient and TD250°C for different separated regimes. The MAC is obtained via least square linear regression for each environment; B) the derived MAC under regime I to VI, with solid and open sphere markers denoting the ambient and TD250°C respectively, the colour scheme is identical with in the main text, denoting the three PASS operating λ, the upper panel shows the coating thickness Dp/Dc under ambient and TD250°C; C) MAC as a function of Dp/Dc with ambient and TD250°C in one plot. S4. Mie modelled MACBCc based on SP2 measured single particle data The MACBCc is averaged over the entire Dc distribution to get the bulk MACBCc for each time period, as equation (S2) expresses: = ∑( × ) ∑ (S2) Where i denotes the ith Dc size bin, MACi and Mi denotes the Mie modelled MAC and rBC mass at the ith Dc size bin. Figure S4. BC core size distributions under regime I to VI, and Mie modelled MAC BCc corresponding with each BC core size bin, using ncore=1.85+0.71i and ncoating=1.5+0i. For clarity, only regime I, IV and VI and uncoated BC are shown for Mie results. S5. The calculation of BrC optical parameters for regime VI Figure S5. The kBrC as a function of MACBrC, the dotted curve shows the Mie calculated MACBrC-kBrC function at λ=405nm, assuming BrC sphere with density of 1.4 g cm-3. The dots denote the MACBrC obtained from different models at regime VI.