Brownness of organics in aerosols from biomass burning linked to their black carbon content

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1 SUPPLEMENTARY INFORMATION DOI: /NGEO2220 Brownness of organics in aerosols from biomass burning linked to their black carbon content Rawad Saleh 1, Ellis S. Robinson 1, Daniel T. Tkacik 1, Adam Ahern 1, Shang Liu 2, Allison Aiken 2, Ryan C. Sullivan 1, Albert A. Presto 1, Manvendra K. Dubey 2, Robert J. Yokelson 3, Neil M. Donahue 1, and Allen L. Robinson 1 1 Center for Atmospheric Particle Studies, Carnegie Mellon University, Pittsburgh, PA, USA 2 Earth and Environmental Sciences, Los Alamos National Laboratory, Los Alamos, NM, USA 3 Department of Chemistry, University of Montana, Missoula, MT, USA NATURE GEOSCIENCE

2 1. FLAME 4 experimental setup and procedures Experiments were performed at the Fire Science Laboratory in Missoula, Montana. The facility and burn procedure have been described previously. 1,2 Briefly, the Fire Science Laboratory combustion chamber (3000 m 3 ) was filled with emissions from a small scale burn (0.3 1 kg), including both the flaming and smoldering phases. The burn conditions varied across experiments, leading to a wide range of black carbon-to-organic aerosol (BC-to-OA) mass ratios. For example, the BC-to-OA ratios for black spruce in the smog chamber ranged between and 0.04 in different experiments. Twenty to thirty minutes after the burn completion, the emissions were well-mixed in the combustion chamber, which was inferred from the stable readings of a suite of gas and particle phase measurement instruments. The aerosol mass loadings in the combustion chamber were 1 10 mg/m 3, depending on the experiment. Emissions were then sampled from the burn room into two smog chambers via 1/2" stainless steel sampling lines (heated to 60 C to minimize vapor losses), and (two per chamber) ejector diluters (Dekati, Helsinki, Finland). 3 The smog chambers were 7 m 3 Teflon bags, one of which was the chemistry chamber where the emissions were aged via either photo-oxidation or dark ozonolysis; the second chamber served as a control. The dilution ratio of the emissions was approximately 100 relative to the burn room, leading to mass loading in the smog chambers of μg/m 3. Sampling alternated between the smog chambers every 30 minutes via an automatic 3-way valve. 1

3 A suite of gas and particle phase instruments sampled from the smog chambers. The key instruments which pertain to the optical analysis in this paper include: 1. Scanning mobility particle sizer (AMS, TSI) to determine the particle size distribution. 2. Single article soot photometer (SP2, DMT) to determine the BC size distribution. 3. High resolution aerosol mass spectrometer (HR-AMS, Aerodyne) to determine the chemical composition of the non-refractory components (operationally defined as material which evaporates at the vaporizer temperature of 600 C) of the particles. The HR-AMS was mainly used to assess the extent of chemical aging of the emissions (see section 5). 4. Aethalometer (Magee Scientific) to determine the absorption coefficients at seven wavelengths (370, 470, 520, 590, 660, 880, and 950 nm). 5. Photo-acoustic soot spectrometer (PASS-3, DMT) to determine absorption and scattering coefficients at three wavelengths (405, 532, and 780 nm). 6. Thermodenuder, which is a heated tube used to strip (thermally denude) a portion of the OA from the particle phase in order to investigate its contribution to light absorption (see section 4 for details). We investigated three different fuel types of global importance: 4 boreal forests, grasslands, and croplands. Boreal forests are major sources of carbon in the atmosphere, as they are usually consumed in wild-land and prescribed fires. 5 7 In this study, we investigated two species commonly found in boreal forests in North America, namely black spruce (leaves and branches) and ponderosa pine (needles and branches). 2 During periods of draught, fires can spread to grasslands. 2 We investigated two grasses common in the southeastern United States, saw grass 2

4 and wire grass. 2 Finally, we investigated a cropland fuel consumed in prescribed fires (organic hay), and an agricultural waste product usually burned after harvest in east Asia (rice straw) Optical closure procedure In this section, we present a detailed description of the optical closure procedure performed to determine the effective imaginary part of the refractive index of the organics (koa) in biomass burning emissions. As described in the subsequent sections, optical closure consists of the following steps: 1. We measure the absorption/scattering coefficients (the absorption/scattering cross-section of the ensemble of particles per unit volume of air; Mm -1 ) of the total particulate emissions. 2. We measure the size distribution of the total particulate emissions and the size distribution of BC. Using these measurements and condensation dynamics simulations, and assuming that some OA is internally-mixed with BC, we estimate the mixing state of BC and OA (what fraction of the OA is internally-mixed with BC and what fraction of the OA exists in externallymixed particles that do not contain BC), as well as the coating thickness of OA on BC cores, assuming that the internally-mixed OA forms a coating around the BC particles. We also perform optical closure with the assumption that BC and OA are completely externally-mixed. 3. Using Mie theory, we calculate theoretical scattering/absorption coefficients (bsca,mie / babs,mie). Mie theory calculations require the following inputs: BC and total OA size distributions and mixing state (obtained from step 2), BC refractive index ( i; obtained from literature), 9 and OA refractive index (unknown). The real part of OA refractive index (noa) was retrieved 3

5 from comparing measured and calculated scattering coefficients. The value obtained for noa was 1.7±0.2, and was wavelength-independent in the visible spectrum, which is in good agreement with literature values for biomass burning OA koa was then retrieved from comparing measured and calculated absorption coefficients. Below are the details for each step. The example illustrated in Supplementary Fig 6 is for photo-chemically aged ponderosa pine emissions (experiment #7; see Supplementary Table 1 and Table 2). This experiment also involved heating in a thermodenuder (experiment #8). We present the analysis for both non-heated and heated data. Step 1. Derivation of absorption coefficients Absorption (babs) and scattering (bsca) coefficients were measured using a 3-wavelength (405, 532, and 781 nm) Photoacoustic Spectrometer (PASS-3, DMT). babs values were also estimated from absorption coefficient measurements (babs,aet,raw(λ)) from a 7-wavelength (370, 470, 520, 590, 660, 880, and 950 nm) aethalometer (Magee Scientific) measurements: babs,aet,raw(λ) = MACAET CBC,AET (1) Where λ is the wavelength, MACAET = 14625/λ (m 2 /g) is the manufacturer s specified mass absorption cross-section, and CBC,AET is the BC concentration reported by the instrument. The aethalometer data (babs,aet,raw) was corrected for two artifacts. The first was absorption enhancement due to multiple scattering in the collection filter. Following Weingartner et al., 13 we used a correction factor of The second artifact was the decrease in the aethalometer response as the particle loading increases. This can be due to diminishing the enhancement of filter scattering as particles deposit on it 14,15 or due to some particles being shadowed by 4

6 others. 13 We used the method of Kirchstetter and Novakov 14 to correct for this artifact. Measurements of constant light absorbing material (e.g. BC) concentrations over a period long enough to witness a substantial decrease in instrument transmission (Tr) can be used to derive a linear correction factor as a function of Tr. The constant light-absorbing material concentration was obtained by atomizing and drying a suspension of aquadag (BC surrogate) in deionized water. We verified that the output concentration was constant by measuring the total aquadag particle number concentration using a scanning mobility particle sizer (SMPS). The combined correction due to multiple scattering and particle loading used in this study is: b abs, AET babs, AET, raw 2.14(0.55Tr 0.42) (2) where babs,aet,raw and babs,aet are the measured and corrected absorption coefficients respectively. The wavelength-dependence of babs (or the Absorption Ångström Exponent; AAE) was derived from the aethalometer and PASS-3 data by applying power law fits to the measured babs. As shown in Supplementary Fig 5, AAE values from the two instruments were within 15%. The AAE from the aethalometer was modestly smaller than PASS-3, in agreement with the findings of Ajtai et al. 16 However, likely due to artifacts associated with OA loading on the filter, 17,18 babs estimated from Aethalometer measurements (equation 3) was a factor of larger than reported by PASS-3. The PASS-3 is thought to provide more robust absorption coefficient measurements because it does not involve collection of the sample on a filter. Since the AAE measured by the two instruments were quite similar (within 15%), and because the aethalometer covers a wider 5

7 range of wavelengths (providing better constraint in the optical closure analysis), we scaled the aethalometer measurements using the PASS-3, and used the scaled aethalometer-derived babs for optical closure analysis. For some of the experiments PASS-3 measurements were not available; therefore we used a scaling factor of 2 (the upper bound of the observed range). Using the upper bound is conservative from the perspective of OA absorption, because it minimizes the estimated contribution of OA to absorption, and provides a lower estimate of the retrieved koa values. The two instruments report measurements at different wavelengths. To perform the scaling, we interpolated the aethalometer measurements (using cubic spline interpolation) to the wavelengths which correspond to PASS-3, and used these points for scaling by minimizing the difference between the two instruments. We did not alter the wavelength-dependence (or the AAE) of the aethalometer measurements, which is conservative from the perspective of estimating OA absorption (it provides a lower estimate of koa). Step 2. Size distributions and mixing state The size distribution of the total aerosol was obtained from scanning mobility particle sizer (SMPS, TSI) measurements. The SMPS measures particle electrical mobility, and the size distribution was determined by assuming spherical particles. BC size distributions were measured using a single particle soot photometer (SP2, DMT). The mass of individual BC particles was calculated from their incandescence signals detected by the SP2, and the size distributions were obtained assuming spherical particles and a density of 1.8 g/cm 3. 9 To extend the BC size distribution to sizes smaller than 90 nm (the SP2 detection limit), 6

8 we applied lognormal fits. 19,20 The SP2 was calibrated using aquadag. Aquadag suspension in deionized water was atomized and dried, then size-selected using a differential mobility analyzer. The size selected steam was then split into two; one stream was sampled by an SMPS and the other was sent to the SP2. The SMPS was used to double-check particle size, as well as the number concentration of the selected particles. The number counting of the SP2 was within 10% of the SMPS for particles larger than 90 nm. The size calibration was obtained by using size-dependent aquadag effective density values reported by Gysel et al. 21 Different calibration standards can affect SP2 measurements 22 leading to potentially significant uncertainty. However, the upper limit on BC particle size (mass concentration) was well-constrained as discussed in Supplementary section 3. For the internally-mixed case, the OA coating on BC cores was distributed according to the condensation sink of the BC particles by simulating condensation kinetics of OA on BC particles and using the SMPS measurements as a constraint (particles cannot grow beyond the total size distribution). 20 This is illustrated in Supplementary Fig 6a for non-heated and heated (downstream of the thermodenuder) particles. The condensation simulation starts with size distribution of the BC cores measured using the SP2 (solid red curve). The BC cores grow via condensation of organics until they hit the total measured size distribution (blue curve for nonheated particles and green curve for heated particles). The dotted curves in Supplementary Figure 6a correspond to the internally-mixed BC with OA. The externally-mixed OA particles (do not contain BC) are then defined as the difference between the total size distributions and the internally-mixed size distributions. By comparing the BC core sizes with the internallymixed particls sizes, we obtain the coating thickness as a function of BC core size. We note that 7

9 this approach yields an upper limit on the coating thickness which from an optical closure perspective, is a conservative approach. The thicker the coating, the larger is the contribution to absorption enhancement by lensing, thus the smaller are the retrieved koa values. For the externally-mixed case, the OA size distribution was estimated as the difference between total (SMPS) and BC (SP2) size distributions. Step 3. Mie theory calculations and optical closure An illustration of the optical closure procedure is shown in Supplementary Fig 6b for the non-heated (experiment #7) and heated particles (experiment #8). The measured absorption coefficients of the non-heated and heated particles are shown by the black circles and triangles, respectively. Calculations of absorption coefficients were performed using a size-resolved core-shell Mie theory model based on the formulation of Bohren and Huffmann 23 for coated spheres. We extended the computer code for light absorption cross-sections of a single spherical particle by Mätzler 24 to calculate absorption coefficients of a polydisperse particle distribution, and to account for coating using equation 8.2 in Bohren and Huffmann. Inputs to the Mie theory model were: 1) BC and OA size distributions, mixing state, and OA coating thickness around BC cores. These were obtained from SP2 and SMPS measurements as described in Step 2. 2) BC refractive index, mbc = i. 9 3) OA real part of the refractive index, noa = 1.7, obtained from PASS-3 scattering coefficient measurements. 8

10 The OA imaginary part of the refractive index (koa) was the free parameter in the model. We assumed that koa had a power-law wavelength dependence, 11,25 and expressed it as koa,550 (550/λ) w, where koa,550 is the imaginary part of the refractive index at λ = 550 nm. We applied brute force optimization to determine the values of koa,550 and w which result in the best theoretical fit (blue lines in Supplementary Fig 6b) to measured absorption coefficients (black symbols). The retrieved koa,550 and w values for all experiments are given in Supplementary Table 1 and plotted in Fig 1 in the main text and Supplementary Fig 1. Supplementary Fig 6b also illustrates the relative contribution to absorption by BC and OA. The red line shows calculated absorption coefficient of BC alone, i.e. assuming OA is completely externally-mixed and non-absorbing (koa = 0). The green lines (solid for non-heated and dotted for heated) are absorption coefficients calculated using the same mixing state and coating thickness values as the blue lines, but assuming that OA is non-absorbing. Therefore, the green lines are absorption coefficients of BC + lensing. The difference between the red and green lines is the enhancement in BC absorption due to lensing by the OA coating. For the internally-mixed case, BrC contribution to absorption is the difference between the green and blue lines. For the externally-mixed case, there is no lensing enhancement, and thus the contribution of BrC to absorption is the difference between the red and blue lines. Clearly, the externally-mixed case attributes more absorption to OA, and thus yields larger (by 35% on average) koa values (see Supplementary Table 1). 9

11 3. Uncertainty analysis The major source of uncertainty in the optical closure analysis is the BC mass concentration / size distribution derived from the SP2 measurement due to the calibration and detection limit issues described in section 2, both of which might lead to underestimation of BC particle mass. If BC mass concentration is underestimated, absorption would be misattributed to OA leading to an overestimation of the retrieved koa values. This bias would particularly influence experiments with large BC-to-OA ratios. To avoid such bias, we adjusted BC particle mass to yield a conservative upper bound on BC mass concentration, and used the adjusted BC size distributions in the optical closure analysis. The mass (size) of each BC particle was increased while conserving the total number concentration. The BC adjustment was performed using absorption coefficient measurements at 950 nm as a constraint. The absorption coefficient at 950 nm due to BC alone (excluding lensing and BrC absorption) cannot exceed the measured absorption coefficient. Thus, a scaling factor that would yield the maximum BC mass concentration was calculated as the scaling factor which satisfied babs,bc(950 nm) = babs,measured(950 nm) for at least one experiment. This condition, illustrated in Supplementary Fig 7, was satisfied for Experiment #13 (see Supplementary Table 1) and yielded a scaling factor of 3.1 (assumed to be size-independent), which was used to adjust BC mass concentrations for all experiments. We note that a factor of 3.1 is the largest possible adjustment in BC mass concentration that is consistent with our experimental data. A larger adjustment would yield unphysical negative OA absorption for at least one experiment (Experiment #13). Furthermore, we reiterate that this adjustment is conservative (upper bound) because it assumes no absorption enhancement by lensing, nor BrC absorption. 10

12 Another important source of uncertainty is the mixing state of OA and BC, and the morphology of the internally-mixed particles. To address this issue, we retrieved koa using two limiting cases: 1) BC and OA are completely externally-mixed (they exist in different particles); and 2) a fraction of the OA is internally-mixed with and exists as a coating over the BC, with maximum possible coating thickness (see Supplementary section 2). In case 2, BC absorption is enhanced relative to case 1 due to lensing, thus the absorption attributed to OA in case 2 is smaller than in case 1, leading to retrieved koa values smaller by 35% on average (see Supplementary Table 1). We also considered the propagation of uncertainties associated with the following: a. SMPS measurements: We considered uncertainty of 20% in particle mass. 26 b. Complex refractive index of BC: The mean and uncertainty bounds were taken as the mean ( i) and range ( i, i) reported by Bond and Bergstrom. 9 c. The real part of the refractive index of OA was retrieved from PASS-3 scattering measurements. The mean and bounds were 1.7 and (1.5, 1.9). d. Aethalometer: The mean and uncertainty bounds were taken as the average and standard deviation (typically 10-15%) of 10 measurement points, after scaling to match PASS-3 measurements. The best estimate of koa was determined using the mean values of the model inputs described above in the optical closure analysis. The lower bound of koa was calculated using the minimum of (d) and the maximum of (a), (b), and (c). The upper bound of koa corresponds to the maximum of (d) and the minimum of (a), (b), and (c). The upper and lower bounds are 11

13 represented by the whiskers in Figs 1a and 1b in the main text and Supplementary Figs 1a and 1b. 4. Thermodenuder measurements Six experiments included sampling the emissions through a thermodenuder to investigate the dependence of koa on volatility. The thermodenuder used in this study is a stainless steel tube (length = 100 cm, diameter = 2.54 cm). The aerosol flow rate was 3 SLPM, corresponding to an average residence time of 5.8 seconds at the operating thermodenuder temperature of 250 C. The cooling section was a 0.6 cm diameter copper tube, and did not include an activated carbon denuder. The particle transmission in the thermodenuder at the operating temperature (250 C) and flow rate (3 SLPM) was determined using atomized and dried NaCl particles. The NaCl particle volume concentrations upstream (non-heated) and downstream (heated) of the thermodenuder were estimated from SMPS measurements, and the transmission was calculated as the ratio of the heated to non-heated volume concentrations. The transmission, 94%, was used to correct heated particle size distributions and absorption coefficients measured in this study. The SMPS (1 SLPM) and aethalometer (2 SLPM) alternated measurements between the thermodenuder and a bypass line, typically every 10 minutes. A flow rate of 3 SLPM was maintained through the idle line (while sampling through the other line) to prevent accumulation of aerosol in the dead volume. The SP2, PASS-3, and AMS did not sample through the thermodenuder. The heated BC size distribution was assumed to be the same as the non-heated size distribution, which is justified 12

14 because the refractory BC does not evaporate at the thermodenuder temperature of 250 C. To scale the heated aethalometer measurements, we used the same scaling factor as the non-heated measurements. This is justified because the aethalometer measurement artifacts are due to accumulated organics on the aethalometer filters, 17,18 as described in section 2, which changed negligibly within the switching time of 10 minutes. Therefore, the scaling factor did not change between heated and non-heated measurements. A typical simulation of OA evaporation in the thermodenuder is shown in Supplementary Fig 8. As evident in Fig 8, the residual OA (which does not evaporate in the TD) should have effective saturation concentration of 10-4 µg/m 3 or less to survive heating in the TD, and was thus characterized as ELVOCs. The volume concentrations of the non-heated OA (which bypassed the thermodenuder) and heated OA (ELVOCs; which did not evaporate in the thermodenuder) were estimated as the difference between total volume concentrations (from SMPS measurements) and BC volume concentration (from SP2 measurements). OA mass concentrations were calculated assuming a density of 1 g/cm 3. The cooling section walls had a much larger condensation sink than the aerosol, with a coupling number 27 less than 0.02 for all experiments. At these conditions, the recondensation fraction in the cooling section (the fraction of vapors that evaporate in the thermodenuder and condense back on the particles in the cooling section) is less than 1%. 27 Therefore, we were confident that the organic residue in the particles downstream of the thermodenuder was predominantly comprised of ELVOCs that did not evaporate in the thermodenuder. 13

15 5. Determination of SOA fraction using High Resolution Aerosol Mass Spectrometer (HR-AMS) As described in the main text, in some of the experiments the emissions were chemically aged via either photo-oxidation or ozonolysis. To estimate the mass fraction of SOA in these experiments, we used data from a High Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS), which measured sub-micron aerosol composition. We note that this estimate was not used in any of the quantitative analyses, but simply as an indicator of the extent of chemical processing. All HR-ToF-AMS data were collected in single-reflection mode (V-mode), providing high sensitivity while allowing for the separation of ions at the same unit mass. The HR-ToF-AMS was operated according to the standard protocol with a vaporizer temperature at 600 C for all experiments. Separation of chemically aged OA into a primary (POA) and secondary (SOA) factors was done according to the residual analysis method of Sage et al. 28 as further applied to biomass burning emissions by Grieshop et al. 29 The residual analysis method relies on a single MS peak as a tracer for the primary biomass-burning organic aerosol. Assuming that the mass spectra of POA is constant throughout the experiment, the OA mass can be decomposed into two components: MS residual = MS t - f ion MS POA (3) 14

16 where fion is the maximum fraction of POA mass (MSPOA) that contributes to the total OA mass (MSt), and is calculated according to: f ion = m m/z=ion (t) m m/z=ion (t 0 ) (4) In this two-factor solution, we attribute all residual mass (MSresidual) to SOA. We performed the analysis using three prominent fragments in biomass burning POA as the tracer: C4H9 + (nominal mass 57), C2H4O + (nominal mass 60), and C3H5O2 + (nominal mass 73). All of these tracers provided similar estimate for the SOA production (see Supplementary Fig 9). Note that the specific molecular ions (e.g. C2H4O + ) at each m/z (e.g. 60) were selected, as there were other ions contributing to the nominal mass. None of the ions selected showed any signs of enhancement due to secondary chemistry, though we cannot rule out heterogeneous oxidation of compounds contributing to these ions, which would artificially increase the fraction of SOA to total mass. We also verified that the residual method did not estimate any attribution of SOA in control experiments that did not chemically age the biomass burning emissions (see Supplementary Figure 10). 6. Diesel experiments The diesel experiments were performed at Carnegie Mellon University (CMU). The diesel generator (Yanmar L-A series, 4-cycle 6.6 HP) was operated at low load (approximately 20% of rated capacity), under which conditions the emissions have high OA loadings. 30 Following the procedures of Grieshop et al., 31 the emissions were diluted and injected into a smog chamber. Characterization of the fresh emissions was performed on measurements 30 minutes prior to 15

17 the onset of photo-oxidation. The emissions were exposed to UV-lights to initiate photooxidation. For the photo-chemically aged emissions, analysis was performed on two measurement points at 20 minutes and 90 minutes after turning on the UV lights. 7. Direct radiative forcing (DRF) calculations To estimate the contribution of biomass burning OA to DRF, we calculated the ratio of DRF of biomass burning emissions (BC + OA) to the DRF of BC alone. First, we calculated the simple forcing efficiency (SFE, W/g) using the formulation of Chen and Bond: 32 dsfe 1 ds SFE F a a 2 2 atm (1 ) c 2 1 s MSC 4 smac d (5) 4 d Where, ds/dλ is the solar irradiance (taken from ASTM G reference spectra), τatm is the atmospheric transmission (0.79), Fc is the cloud fraction (0.6), as is the surface albedo (0.19), β is the backscatter fraction (0.17), MSC and MAC are the mass scattering cross-section and the mass absorption cross-section of the particles, respectively, calculated using Mie theory, where koa values were obtained based on the parameterization shown in Fig 1 in the main text and Supplementary Fig 1. The ratio of DRF of biomass burning emissions to DRF of BC alone was calculated as: DRF SFE 1 1 DRF SFE BC-to-OA ratio BC BC (6) The calculations were performed for a BC core size of 100 nm (which is representative of the values observed in this study). The OA was distributed between internally-mixed with BC (coating) and externally-mixed (pure OA). We considered two cases to determine the coating 16

18 thickness and the fraction of externally-mixed OA, which cover a wide range of possible mixing states. In the first case, the coating thickness was held constant (the total particle to BC core diameter ratio was 2.6, which is representative of the values observed in this study), and the concentration of externally-mixed OA varied in the BC-to-OA ratio space (as the BC-to-OA ratio decreased, more externally-mixed OA was added to the system). In the second case, the fractions of internally- and externally-mixed OA were held constant at 50%, while the coating thickness varied (the coating thickness decreased with increasing BC-to-OA ratio). Furthermore, calculations were performed assuming either non-absorbing OA (koa = 0) or koa values based on the parameterization in Fig 1a and 1b. References 1. Yokelson, R. J., Christian, T. J., Karl, T. G. & Guenther, A. The tropical forest and fire emissions experiment : laboratory fire measurements and synthesis of campaign data. Atmos. Chem. Phys. 8, (2008). 2. McMeeking, G. R. et al. Emissions of trace gases and aerosols during the open combustion of biomass in the laboratory. J. Geophys. Res. 114, D19210 (2009). 3. Hennigan, C. J. et al. Chemical and physical transformations of organic aerosol from the photo-oxidation of open biomass burning emissions in an environmental chamber. Atmos. Chem. Phys. 11, (2011). 4. Wiedinmyer, C. et al. The Fire INventory from NCAR (FINN): a high resolution global model to estimate the emissions from open burning. Geosci. Model Dev. 4, (2011). 5. O Neill, N. T. O. et al. Optical properties of boreal forest fire smoke derived from Sun photometry. J. Geophys. Res. 107, 4125 (2002). 6. French, N. H. F., Kasischke, E. S. & Williams, D. G. Variability in the emission of carbon-based trace gases from wildfire in the Alaskan boreal forest. J. Geophys. Res. 108, 8151 (2002). 17

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22 Figures a - different colors correspond to different fuels - closed symbols: fresh - open symbols: aged koa, wavelength-dependence b diesel OA 0 c DRF / DRFBC atmosphericallyrelevant absorbing OA non-absorbing OA BC-to-OA ratio Figure 1 Same as Fig 1 in the main text, but for externally-mixed case. The fits for the externally-mixed case are: y = x , R-square = 0.714, for koa,550nm vs log10(bc-to-oa ratio); and y = / (x ), R-square = 0.65, for w vs BC-to-OA ratio. 21

23 Composition (%) mass fraction of SVOCs in the condensed phase imaginary part of the refractive index a BC ELVOC 0.4 ELVOC + LVOC + SVOC LVOC + SVOC 0.3 this study 0.2 TD 0.1 this study koa fulvic acid (Dinar et al.) 570 Da 740 Da Kirchstetter et al. Chen and Bond Lack et al. Saleh et al. Alexander et al. b wavelength (nm) Figure 2 Same as Fig 2 in the main text, but for externally-mixed case condensed phase organic loading ( g/m 3 ) Figure 3 The effect of OA loading on the imaginary part of the refractive index (koa). Partitioning of SVOCs towards the condensed phase as OA loading increases (blue line and left y-axis), which leads to a decrease in the effective koa (green line and right y-axis). Volatility distribution used in the calculations is from May et al

24 AAE (PASS-3) mass fraction in the particle C * = 10-4 g/m 3 C * = 1 g/m 3 C * = 10 g/m 3 C * = 100 g/m residence time in TEM (min) Figure 4 Simulation of evaporation kinetics of a single organic particle in a TEM. We assume that 10% of the particle mass is comprised of ELVOCs with effective saturation concentration (C * ) of 10-4 μg/m 3, and the rest of the mass is distributed in volatility bins according to May et al. 37 This simulation does not account for particle heating due to bombardment by the electron beam, which would accelerate stripping of the semi-volatile components. Initial and final diameters (after 60 minutes) are 400 nm and 190 nm, respectively. 3 1: :1 1: AAE (Aethalometer) Figure 5 Comparison of the Absorption Ångström Exponent (AAE) values calculated from the aethalometer and the PASS-3 measurements from 16 experiments. 23

25 absorption coefficient (Mm -1 ) dn/dlogd p (cm -3 ) total (heated) total (non-heated) a BC internally-mixed (heated) internally-mixed (non-heated) d p (nm) non-heated b heated heated measurements heated non-heated BC + non-absorbing OA BC only BC + absorbing OA non-heated measurements wavelength (nm) Figure 6 An example showing the steps of the optical closure procedure for non-heated and heated emissions (experiments #7 and #8. See Supplementary Table 2). a) Size distributions measured with the SMPS for non-heated (blue) and heated (green) particles, and BC size distribution obtained from lognormal fit to SP2 measurements (red). The dotted curves correspond to simulations of particles initially consisting of BC that grow by OA condensation. This represents the maximum amount of coating, as the coated BC distribution would otherwise 24

26 absorption coefficient (Mm -1 ) exceed the distribution measured by the SMPS at a certain size. b) Absorption coefficients as a function of wavelength. Black circles and triangles are measurements for non-heated and heated emissions, respectively. Lines are Mie-theory calculations. The red line corresponds to BC and (non-absorbing) OA being externally-mixed. The green lines correspond to BC coated with nonabsorbing OA for non-heated (solid) and heated (dotted) emissions. The blue lines are the fits to the data, which yield the value of the imaginary part of the refractive index of OA (koa) for nonheated (solid) and heated (dotted) emissions measurements 10 5 BC (adjusted) BC (original) wavelength (nm) Figure 7 Adjusting the BC mass concentration for Experiment #13. The BC mass concentration for Experiment #13 was adjusted (scaled up) to yield zero OA absorption at wavelength of 950 nm, assuming that BC and OA are completely externally-mixed. The scaling factor derived here was the smallest among all experiments, and was used to adjust BC mass concentration in all other experiments. 25

27 MFR, mass fraction in condensed phase MFR C * = 10-4 g/m 3 C * = 1 g/m 3 C * = 10 g/m 3 C * = 100 g/m residence time in the thermodenuder (sec) Figure 8 Simulation of evaporation kinetics of OA in the thermodenuder for typical OA mass concentration and size distribution. MFR is the mass fraction remaining at the exit of the thermodenuder. In order to reproduce observations, namely that approximately 10% of the OA do not evaporate in the thermodenuder, we need to assign a maximum effective saturation concentration (C * ) of approximately 10-4 μg/m 3 to the 10% residual. The rest of the mass is distributed in volatility bins according to May et al C 4 H 9 + (mz 57) C 2 H 4 O 2 + (mz 60) C 3 H 5 O 2 + (mz 73) 0.6 f ion Hours from chemistry 3 Figure 9 Example of time series of fraction of POA mass that contributes to total OA mass for three different tracer fragments (for Experiment 11/01/12; see Supplementary Table 1 for 26

28 details). fion is computed for three high-resolution ions commonly associated with the primary organic aerosol spectrum for biomass-burning, as in Grieship et al. 29 C4H4O2 + was used for calculating the fraction of POA to total OA mass for this study. Figure 10 Example of fraction of POA to total OA time series (for Experiment 11/01/12; see Supplementary Table 1 for details). The fraction of POA to the total OA mass was calculated using C2H4O + as a POA tracer ion. Tables Table 1 Summary of experiments. BS: black spruce; PP: ponderosa pine; OH: organic hay; SG: saw grass; WG: wire grass; RS: rice straw. Results are shown for the two extreme cases used in the analysis: externally mixed, and internally-mixed with maximum coating. # Date Fuel TD Chemistry SOA mass fraction OA mass loading (μg/m 3 ) 27 BC-to-OA ratio Internally-mixed Externally-mixed k OA,550 w k OA,550 w 1 10/29/12 BS N N/A N/A /29/12 BS Y N/A N/A /29/12 BS N Ozone /29/12 BS N Ozone /30/12 BS N N/A N/A /31/12 PP N N/A N/A /31/12 PP N UV /31/12 PP Y UV

29 9 11/01/12 PP N N/A N/A /01/12 PP Y N/A N/A /01/12 PP N Ozone /01/12 PP N Ozone /01/12 PP Y Ozone /03/12 PP N N/A N/A /03/12 PP N Ozone /03/12 PP N UV /03/12 PP Y UV /04/12 OH N N/A N/A /04/12 OH Y N/A N/A /04/12 OH N UV /05/12 SG N N/A N/A /05/12 SG N UV /06/12 WG N N/A N/A /07/12 RS N N/A N/A /07/12 RS N UV /10/12 BS N N/A N/A /11/12 BS N N/A N/A /12/12 BS N N/A N/A /25/11 D N N/A N/A /25/11 D N UV /25/11 D N UV

30 Table 2. Composition of PM for the six experiments that involved heating in the thermodenuder obtained via the analysis described in section 2. The values shown in brackets are for the heated particles. More details on the experiments are shown in Table 1 in the main text. # Date Fuel BC-to-OA ratio ELVOCs-to-OA ratio 1 (2) 10/29/12 BS (0.29) (8) 10/31/12 PP (0.1) (10) 11/01/12 PP 0.11 (0.37) (13) 11/01/12 PP (0.38) (17) 11/03/12 PP (0.13) (19) 11/04/12 OH (0.13)