Enhanced light absorption for solid-state brown carbon from wildfires due to organic and water coatings

Domain: earth_science
Published: 2024-11-28
Editorial outcome: minor_revision

Abstract
Wildfires emit solid-state strongly absorptive brown carbon (solid S-BrC, commonly known as tarball), critical to Earth’s radiation budget and climate, but their highly variable light absorption properties are typically not accounted for in climate models. Here, we show that from a Pacific Northwest wildfire, over 90% of particles are solid S-BrC with a mean refractive index of 1.49 + 0.056i at 550 nm. Model sensitivity studies show refractive index variation can cause a ~200% difference in regional absorption aerosol optical depth. We show that ~50% of solid S-BrC particles from this sample uptake water above 97% relative humidity. We hypothesize these results from a hygroscopic organic coating, potentially facilitating solid S-BrC as nuclei for cloud droplets. This water uptake doubles absorption at 550 nm and the organic coating on solid S-BrC can lead to even higher absorption enhancements than water. Incorporating solid S-BrC and water interactions should improve Earth’s radiation budget predictions.

Introduction
Light-absorbing carbon, including black carbon (BC) and brown carbon (BrC), in wildfire smoke has critical climate warming effects1,2. BC is typically considered the dominant light absorber2, but studies have reported that the wildfire smoke particles contain a special type of solid-state strongly absorptive BrC (solid S-BrC) commonly known as tarballs3–5. The number fraction of solid S-BrC in the wildfire smoke particles can vary from <10% to >95% depending on the transport distance and atmospheric aging6–9. Moreover, the imaginary part of the refractive index (RI) at 550 nm wavelengths (k550) reported for solid S-BrC varies over two orders of magnitude (between 10−3 and 10−1), which might be due to the bias in various analytical techniques as well as complex chemical nature and evolution of chemical properties during atmospheric aging and transport6,7,10–18. Besides, solid S-BrC is commonly emitted with other particles, such as soot, other organic aerosol (other OA) which are not solid S-BrC, and inorganic particles1,19,20, which might misrepresent solid S-BrC optical properties based on bulk optical measurements. Furthermore, little is known about the interaction between solid S-BrC and water15,21–23. For example, Semeniuk et al.21 and Adachi and Buseck22 show negligible hygroscopic growth of solid S-BrC at 100% relative humidity (RH). However, Hand et al.15 reported that solid S-BrC can uptake water at ~83% RH. Adachi et al.23 reported abundant solid S-BrC collected in the pyrocumulonimbus cloud to have thin layers or coatings as residual of water mixed with water-soluble species, suggesting solid S-BrC could be hygroscopic and potential nuclei for cloud droplets. These studies show discrepancies in solid S-BrC hygroscopicity, and the relative abundance of hygroscopic and hydrophilic solid S-BrC in the atmosphere is still missing. Moreover, it has been reported that coating on soot can enhance the light absorption properties of soot24,25. However, the effects of these coatings on solid S-BrC light absorption properties have not been investigated, which might contribute to the aerosol optical properties discrepancy between models and observations. Therefore, the regional climate effects of wildfire smoke have significant uncertainties due to unresolved variability of solid S-BrC concentration, optical properties, and hygroscopicity.

Here, we report a comprehensive single-particle and molecular-level analysis of solid S-BrC particles collected during the Pacific Northwest wildfire events on September 5 and 6, 2017, where >90% of particles were solid S-BrC. Given this composition, this event provides a unique opportunity to probe the physical, chemical, and optical properties of solid S-BrC. The experimentally retrieved solid S-BrC optical properties and mass fractions were used as inputs to the Weather Research and Forecasting Model coupled to chemistry (WRF-Chem) to estimate their absorption aerosol optical depth (AAOD) over the Pacific Northwest region. Additionally, we investigated the interactions between solid S-BrC and estimated the lensing enhancement due to water coating. Our results show that solid S-BrC can dominate in wildfire smoke. We found that ~50% of solid S-BrC particles can uptake water above 97% RH, which results in a lensing enhancement at 550 nm by a factor of 2. Furthermore, the light-absorbing organic can coat solid S-BrC, leading to even higher absorption enhancements than water. Additionally, we compare compositional results from a wildfire-impacted plume in the Pacific Northwest (August 2018) to assess the broader applicability of our findings across the region26.

Results
Prevalence of solid S-BrC in wildfire smoke aerosol
An accurate estimate of the solid S-BrC number fraction in wildfire smoke is critical for predicting its climate effects. Figure 1a shows the representative 75° tilted scanning electron microscope (SEM) image of the wildfire particles sample. We manually identified and counted the number fraction of solid S-BrC (spherical shape) and other OA particles that are not solid S-BrC (other OA, dome-like or flat shape), BC particles (fractal or compressed small monomer aggregates), and inorganic particles (crystal or irregular shape)20,27. More than 90% of particles in our samples were solid S-BrC (Fig. 1a). Moreover, 34% of solid S-BrC in these samples are fractal-like aggregates, which might be formed by aggregation during atmospheric aging and transport28.

Figure 1b shows the chemically resolved size distribution of over 3000 particles derived from computer-controlled scanning electron microscopy with an energy-dispersive X-ray spectrometer (CCSEM-EDX). The carbonaceous (CNO), sulfur-containing carbonaceous (CNOS), and potassium-containing carbonaceous (CNOK) particles are dominant in the sample (~92%). Because tilted SEM images show a negligible number of inorganics (<1%) and BC (<1%) and dominated solid S-BrC (>90%), the CNO, CNOS, and CNOK particles are likely solid S-BrC, suggests the complex chemical composition of solid S-BrC. The abundance of the CNOK fraction (~11%) supports that the samples originated from wildfires29. This is consistent with the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model and fire location and intensity data, which show that air masses were influenced by wildfires in western Montana (Fig. S3).

Moreover, the size, shape, mass spectra, and volatility of over 30,000 individual aerosol particles were characterized in situ using the single-particle mass spectrometer (miniSPLAT)30. MiniSPLAT measurements of aerosol morphology and volatility indicated that most particles (>99%) were solid S-BrC because they were spherical and retained ~90% of their volume after 24 h of evaporation at room temperature31. Figure 1c shows an average miniSPLAT mass spectrum of all particles characterized over September 6 and 7. MiniSPLAT mass spectra exhibit mass spectral peaks characteristic of biomass-burning particles (K/KSO3/4) (m/z = 39 and 213) and indicate the presence of organic carbon (OC), including oxidized fragments, e.g., CO, C2H3O, CO2 (m/z = 28, 43, and 44), elemental carbon (EC) (m/z = 12, 24, 36), polycyclic aromatic hydrocarbons (PAHs), sulfates, and organonitrates. Based on single-particle mass spectra, these solid S-BrC can be classified into eight classes (Fig. S1, Section S1), suggesting the complex chemical nature of solid S-BrC. The single-particle mass spectra analysis indicates particle-to-particle variability, mainly due to different mixing ratios of EC, PAHs, organics, and sulfate. This can affect variability in single-particle hygroscopicity and optical properties.

Optical properties and chemical composition of solid S-BrC particles
To understand the direct climate effects of solid S-BrC, we probed the RI of 40 single solid S-BrC particles without any coating or inclusions as a function of wavelength (300–1000 nm) using electron energy-loss spectroscopy coupled to scanning transmission electron microscopy (EELS/STEM). As shown in Fig. 2a, the retrieved real part of solid S-BrC particle RI (n) increased from 1.44 to 1.48 from 200–500 nm while it was relatively constrained and wavelength-independent from 500–1000 nm (reported value of ~1.49). Conversely, the spectrum of retrieved k exhibits a strong wavelength dependence, and the wavelength dependence of k (w, k(λ) = aλ−w) is 0.49 ± 0.5. The average k550 is 0.056 ± 0.003, approximately an order of magnitude lower than BC32. The reported w and k550 fall in the S-BrC class, as suggested by Saleh33. Figure 2a, b shows the literature-reported RI of solid S-BrC7,10,12–18. The high variable RI might contribute to the complex chemical composition and evaluation of their properties during atmospheric aging and transport, as well as bias from the bulk measurements and analytical techniques6,7,10–18,34. As shown in Fig. 2a, b, our n550 and k550 are about 10% and 70% lower than those described in Alexander et al.18, respectively. This discrepancy might be due to our advanced STEM’s lower electron acceleration voltage (80 kV vs. 120 kV). This lower electron acceleration voltage results in lower Cherenkov radiation effects and electron beam-induced knock-on damage, which can overestimate the k18,35,36. Therefore, our optical properties of solid S-BrC can improve the uncertainties of solid S-BrC optical properties in the literature due to measurement limitations. Compared with two other recent studies7,12, our n550 is ~10% higher, and k550 is ~73% higher than Mathai et al.12 but ~29% lower than Chakrabarty et al.7. This variability can be attributed to differences in wildfire sources and chemical properties. The major component of our sample was western Montana wildfire smoke, which transported around 500 miles. In contrast, Mathai et al.12 studied long-range transported smoke, and Chakrabarty et al.7 studied local wildfire smoke. Thus, different transport distances and, consequently, the various extents of transformative atmospheric processes may explain variabilities in the optical properties of solid S-BrC.

In addition, the differences between optical properties reported in this study and in the literature can be attributed to the diversity in the solid S-BrC chemical composition37. MiniSPLAT shows particle-to-particle variability in solid S-BrC due to different mixing states of light-absorbing species, which might result in differences in their optical properties. Moreover, we used scanning transmission X-ray microscopy and near-edge X-ray absorption fine structure (STXM/NEXAFS) to probe the contribution of carbon functional groups of 67 particles. Figure 2c shows the averaged carbon K-edge spectrum of individual solid S-BrC, and the most intense peaks for these selected individual solid S-BrC particles are -COH (0.20 ± 0.02), and -C=C (sp2) (0.20 ± 0.01) peaks, followed by -COOH (0.17 ± 0.02), which is consistent with literature-reported NEXAFS spectrum of solid S-BrC8,12,38,39. The high sp2 fraction in wildfire OA is associated with strong light absorption of solid S-BrC32. The range of the sp2 fraction (0.03–0.34) might explain the variation in k due to the difference in the amount of delocalization in the molecule.

The diversity in molecular composition may shed light on the variation of k. The molecular composition of wildfire aerosols was analyzed by a 21-Tesla Fourier Transform-Ion Cyclotron Resonance mass spectrometer (21-TFT-ICRMS), showing that 50% of the total assigned organic molecular formulas were organonitrate compounds (Fig. 2d), which has been identified as an important component in solid S-BrC40. 98% of detected organonitrates have molecular formulas consistent with nitroaromatics (C≥6, double bond equivalent (DBE)≥4, and O/N≥2). Nitroaromatics are critical species in solid S-BrCs because they enhance light absorption in BrC at short wavelengths41,42. Furthermore, compounds with O/N≥2 and up to four phenol (C6H5O) or two nitrophenol (C6H5NO3) functionalities based on Kendrick mass defects were commonly detected here in the components of BrC wildfire smoke samples41,43–46, providing a molecular basis for the light-absorbing nature of our solid S-BrC samples.

To conclude, our study comprehensively analyzed single solid S-BrC RI and chemical composition to investigate the association between light absorption properties of solid S-BrC, sp2 fraction per particle, and the fraction of organonitrate compounds. Future studies should focus on molecular composition data related to RI from identical individual particles to better understand the facts that affect solid S-BrC’s strong light-absorption properties.

Solid S-BrC absorption aerosol optical depth model sensitivity study
The highly variable RI of solid S-BrC in the literature can lead to significant uncertainties in predicted climate effects. We utilized the WRF-Chem regional model to perform a sensitivity study of RI on the absorption aerosol optical depth (AAOD) in the Pacific Northwest region from August 11 at 1:00 UTC to August 15 at 0:00 UTC, 2018. We show the percentage difference in the AAOD map from August 14 at 19:00 to 23:00 UTC, 2018. We utilized this period because of an existing model domain setup. Moreover, that period had solid S-BrC-rich wildfire smoke events where solid S-BrC’s carbon K-edge spectrum (Fig. S5) and molecular formula similar to those in the 2017 study period26,31, suggesting they have similar properties. We performed a sensitivity analysis of the impact of RI on AAOD by using RI reported by our measurements (Scenario 1), average RI during August 13 and 14, 2018, reported from the local Aerosol Robotic Network (AERONET) site (46.3 N, 119.3 W, Scenario 2, representing the lower bound) and RI from Alexander et al.18 (Scenario 3, representing the upper bound). We chose AERONET because it is commonly used for model input to predict aerosol climate effects47. Figure 3a shows that using AERONET-reported RI resulted in around 50% lower AAOD in wildfire-influenced regions and 56% lower AAOD at the sampling site compared with Scenario 1. Contrary, the percentage difference of AAOD between Scenarios 1 and 3 shows that using RI from Alexander et al.18 can lead to more than 200% higher AAOD at the sampling site (Fig. 3b). The high variation in AAOD highlight the importance of improving the parameterization of the optical properties of solid S-BrC in the models to reduce the uncertainties in wildfire aerosol climate effects.

Absorption enhancement of solid S-BrC due to water and organic coating
Besides the direct climate effects, there is limited knowledge about solid S-BrC indirect effects. Here, we conducted water-uptake experiments in an environmental SEM at 5 °C and analyzed more than 200 solid S-BrC particles. As shown in Fig. 4a, ~50% of the solid S-BrC did not uptake water (indicated by black arrows) even at >99% RH (additional images in Fig. S6). However, the rest of the solid S-BrC uptake water at RH above 97% (indicated by light yellow arrows, hereafter named hygroscopic solid S-BrC), suggesting a hygroscopic behavior and might potentially serve as cloud condensation nuclei (CCN) under supersaturated condition. Based on the aerosol size distribution31 and MOUDI impactor collection efficiency48, and assuming 50% of solid S-BrC particles are hygroscopic and will also activate into droplets at supersaturation conditions, we estimated that these hygroscopic solid S-BrC could result in CCN number concentrations 3692.2 ± 952.1 cm−3 between 131 and 445.1 nm particle size range during the sampling period. These estimates are comparable with modeled CCN in southeast Atlantic biomass-burning aerosol-dominated region49. Future studies are needed to better understand the contribution of hygroscopic solid S-BrC to CCN.

It should be noted that hygroscopic solid S-BrC (indicated by light yellow arrows) is still visible at RH > 97%, suggesting that most of the volume of solid S-BrC is hydrophobic. Moreover, this water forms a shell on hygroscopic solid S-BrC. Although these particles did not form core-shell morphology when collected on the TEM grids, we still can not eliminate the potential for water coatings to fully cover the airborne hygroscopic solid S-BrC due to water surface tension. Moreover, after dehydration, we found that the hygroscopic solid S-BrC did not have obvious deformation. There was a residual surrounding the solid S-BrC (Fig. 4a). Our CCSEM-EDX (Fig. S7), STXM/NEXAFS Carbon speciation maps of solid S-BrC particles (Fig. S8), and the STEM/EDX Elemental map (Fig. S9) only show a negligible amount of inorganic elemental percentage (1.3 ± 1.3%) and inorganic volume fraction (0.04 ± 0.09) in individual particles, respectively. Thus, we hypothesize that those residuals may have resulted from a thin, hygroscopic organic layer on the surface of the hydrophilic solid S-BrC particles that dissolved in water and then remained on the substrate surface after water evaporation. This hydrophilic organic layer may have resulted from condensation and deposition of polar organics and surface oxidation.

Some hygroscopic solid S-BrC particles have an organic coating (indicated by light yellow cycles), implying that hygroscopic organic coating can promote the water uptake of solid S-BrC. These observations support that solid S-BrC-water interaction is preferential, which could be attributed to the chemical heterogeneity of individual solid S-BrC as suggested by single particle analysis. Adachi et al.23 also reported an organic coating on solid S-BrC. These organic coatings might be formed by semi-volatile secondary organic vapor condensing on the solid S-BrC23,50,51 and potentially be secondary BrC due to gas phase oxidation or heterogeneous oxidation after condensing on the solid S-BrC1. Moreover, this hygroscopic organic coating may have resulted from surface oxidation. Ijaz et al.26 reported that using EELS maps, solid S-BrC in the Pacific Northwest in August 2018 had an oxygen outer layer. This conjecture is supported by Hand et al.15 and Tivanski et al.39, which reported that solid S-BrC has oxygenated outer layers, and China et al.20 reported two types of solid S-BrC with different electronic darkness levels due to various degrees of surface oxidation. Therefore, these studies provide evidence of two types of solid S-BrC. However, the link between different types of solid S-BrC and their hygroscopicity is still unrevealed.

Because the hygroscopicity of organics can be predicted based on the O:C ratio52, we retrieved the O:C ratio of individual solid S-BrC to develop a threshold to identify hygroscopic solid S-BrC particles. We also reference the O:C ratio from 21-TFT-ICRMS data. We compare this with the O:C ratio in the literature in Fig. 4c, which overlaps with our results but is highly variable due to diversity in analytical methods, solid S-BrC sources, and oxidation levels4,8,10,13,20,39,53,54. Furthermore, we utilized k-means clustering55 to group solid S-BrC particles based on their O:C ratio (cluster 1: low O:C ratio, cluster 2: high O:C ratio) for both analytical techniques (see Section S2). We suggest that the hydrophobic solid S-BrC belongs to cluster 1 and solid S-BrC with hygroscopic coatings belongs to cluster 2 since organics with higher O:C ratios are more hygroscopic52,56. Therefore, we propose thresholds where individual solid S-BrC might be hydrophilic with an O:C ratio greater than 0.25 and 0.45 based on EDX and STXM, respectively.

The water shell on solid S-BrC can lead to lensing enhancement, increasing the solid S-BrC’s light-absorption properties and warming effect and counterbalancing the cooling effect of solid S-BrC-containing clouds57. We utilized the model developed by Bond et al.24 to estimate the absorption enhancement (Eabs) at 550 nm wavelength of solid S-BrC with diameters of 100–800 nm and water coating thicknesses between 0 and 2.5 µm, using our measured refractive index (RI) of solid S-BrC at 550 nm (Fig. 4b). We assume the solid S-BrC core is located at the center after uptake water since solid S-BrC found in pyrocumulonimbus cloud droplets can located at the center of droplets23. We acknowledge this assumption might overlook the effects of the possibility that the core might not located at the center, which is worth future investigation. As shown in Fig. 4b, water shell can enhance the light-absorption of solid S-BrC by up to a factor of 2.3.

Besides the water shell, we observed organic coating on the solid S-BrC (Fig. S10). We might underestimate the fraction of organic-coated solid S-BrC based on microscopy imaging because these coatings might be evaporated in the high vacuum chamber. Considering this caveat, our finding suggests that solid S-BrC can act as a seed for secondary organic. Moreover, multiple studies have shown that secondary organic aerosols (SOA) can be light-absorbing58,59. Thus, these light-absorbing SOA can be coated on the solid S-BrC and cause even higher absorption enhancement than water coating for the entire particle via both lensing effect and absorption by the coating25. We estimated the absorption enhancement loss (the difference between lensing enhancement with a clear coating and with a light-absorbing coating)25 and the value to be up to a factor of 1.3 (Fig. S11). These results confirm that the organic coating can potentially increase the contribution of solid S-BrC to climate warming60. Further study is needed to better quantify the fraction of organic coated solid S-BrC and the light-absorption enhancement due to light-absorbing SOA coating.

Methods
Biomass-burning particle collection
Wildfire aerosols were collected daily from 9 AM to 2 PM with 50% duty cycle (30 min on and 30 min off) from September 5–6, 2017, and August 9–14, 2018, local time at the Atmospheric Measurement Laboratory in Richland, WA (46.340844 N, 119.278110 W). In this study, all analysis focuses on samples collected from September 5–6, 2017, and ambient temperature and relative humidity during the sampling period are shown in Fig. S12. Details about samples collected from August 9–14, 2018, have been discussed in Ijaz et al.26. Samples for individual particle analysis were collected on Carbon B-film TEM grids (Ted Pella, Inc.) using a MOUDI impactor (model 110-R, MSP, INC) at a flow rate of ~30 LPM. Samples were collected daily on PTFE filters at the same location for offline mass spectrometry analysis. The 48-hour HYSPLIT65 combined with the fire map based on the observation from the Visible Infrared Imaging Radiometer Suite aboard the Suomi National Polar-orbiting Partnership (S-NPP) satellite66 (Fig. S3) indicated that the wildfire in west Montana State was the primary source of Pacific Northwest regional smoke.

Single-particle imaging and analysis
We imaged collected particles at both horizontal and a 75° tilt angle to visualize the deformation that particles undergo upon their impaction on the substrate27. Collected particle samples were analyzed with CCSEM-EDX and STXM/NEXAFS spectroscopy67,68. CCSEM-EDX is an environmental SEM (ESEM) equipped with an FEI Quanta digital field emission gun operated at 20 kV and 480 pA current to probe size (area equivalent diameter), shape, and morphology. The EDX spectrometer (EDAX, Inc.) interfaced with CCSEM was used to probe the chemical composition of individual particles. This study quantified the relative atomic ratios of 15 elements (C, N, O, Na, Mg, Al, Si, P, S, Cl, K, Ca, Mn, Fe, and Zn). The relative atomic fraction of these elements in each particle was used to classify the particle type (see the Supporting Information). This study analyzed more than 100,000 particles. Based on a relative atomic percentage of individual particles, we classified each particle as carbonaceous (CNO), CNO with sulfate (CNOS), CNO with K (CNOK), sodium-rich (Na-rich), dust, bioaerosol, or others (Fig. S4). Samples with a high abundance of solid S-BrC were selected for further analysis via STEM.

We retrieved RI values using the same method as Veghte et al.69, and the STEM/EELS setup was the same as that used by Mathai et al.12. Individual solid S-BrC with a diameter range from 50–200 nm were analyzed with aberration-corrected electron energy-loss spectroscopy (Gatan) coupled to the scanning transmission electron microscope (EELS/STEM) operated at 80 kV. The energy dispersion of the EELS monochromator is 0.025 eV/channel. The collection angle is 45.5 mrad, and the particle thickness was estimated as the diameter of particles. Infrared spectra for individual solid S-BrC at wavelengths from 300 to 1000 nm were retrieved from the single scattering distributions in the low-loss region (0–10 eV) of the EELS spectrum after removing the zero-loss peak. We performed the Kramers−Kronig analysis on extracted single scattering distributions to retrieve the RI70. The electron beam (20 kV for ESEM and 80 kV for STEM) has the potential to modify the particles, and volatile species might be lost in the high vacuum of the ESEM (~2×10−6 torr) and STEM chamber (~2×10−9 torr)69. These caveats should be considered when interpreting TEM results.

Water-uptake experiments were conducted inside the ESEM using a Peltier stage. The temperature was maintained at 278 K, and the RH was gradually increased from 1% to 99% at an interval of 5–10%, then reduced to <10%. Images were captured 2 min after the water vapor pressure reached the set point to ensure the system reached a steady state. We utilized the STEM detector to acquire high-resolution ESEM images. More than 200 particles were analyzed. More water-uptake results can be found in Fig. S6. Because the tilted SEM images show tarballs dominating, these round dark particles are tarballs.

The STXM/NEXAFS spectroscopy at beamline 5.3.2 (Advanced Light Source, the Lawrence Berkley National Laboratory) was utilized to probe the X-ray spectrum and chemical bonding of carbon functional groups of samples. The contribution of functional groups was analyzed by performing spectral deconvolution and nonlinear least-squares fitting of seven peaks (C=C, C=O, -CH, -NH(C=O), -COOH, -C-OH, and -CO3) associated with specific functionality to determine the area under each peak71–73. The masses of C and O were calculated based on the difference between the post- and pre-K-edge optical depth for specific element times, where the projection area of the particle was divided by the difference between the mass absorption coefficients before and after the absorption edge.

To calculate the O:C ratios, we used the C and O elemental ratios of CNO, CNOS, and CNOK particles from CCSEM/EDX and the carbon and oxygen maps of solid S-BrC from STXM/NEXAFS.

21-Tesla Fourier transform ion cyclotron resonance mass spectrometer analysis
A small part of the September 5, 2017, PTFE filter was extracted in 100% acetonitrile by sonicating for 30 min, followed by filtration through a 0.2 μm PTFE syringe filter. The filtrate was diluted with a 70:30 Acetonitrile:H2O solution for mass spectral analysis on a custom-built 21-TFT-ICRMS at the Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington, USA74. Data acquisition details were previously described in Ijaz et al.75. Exact masses were exported to MFAssignR76 for formula assignment using the previously described method75. Briefly, noise was estimated in the mass lists using the KMDNoise(), and monoisotopic and polyisotopic peaks were separated with IsoFiltR(). Preliminary formulae were assigned with C, H, and O only using MFAssignCHO(), and suitable recalibrants were selected from these and used in Recal() for internal recalibration. Molecular formulae were then assigned using MFAssign() as follows: CH O N S 13C 34S ; -13≤DBE-O≤20; 0≤O/C≤2.0; and 0.3≤H/C≤2.5, with a maximum permissible error of 0.5 ppm. Kendrick mass defect analysis was performed to estimate the functional groups present. Structural information of the molecules identified was inferred by calculating the DBE.

WRF-Chem simulation
The regional Weather Research and Forecasting Model coupled with chemistry (WRF-Chem 4.2)77–80 was used to simulate biomass-burning aerosols from August 8 to August 15 00:00 UTC, 2018, in a 2187×2187 km domain with a spatial resolution of 27×27 km in the Pacific Northwest region for the domain shown in Fig. 3 with 72 vertical levels, with the first 3 days used for model spin up. We include primary biomass-burning emissions for gases and aerosols from the 2014 Quick Fire Emissions Database (Darmenov & da Silva, 2015) version 2.5. Biogenic volatile organic compounds emissions are derived from the latest version of the Model of Emissions of Gases and Aerosols from Nature (MEGAN v2.1), recently coupled within the community land model CLM4 (CLM version 4.0) in WRF-Chem81. The CAM-chem global model82 provided initial and boundary conditions for trace gases and aerosols. The Global Forecast System model provides the meteorological initial and boundary conditions. Meteorological and chemical conditions were spun up for 72 hours using WRF-Chem, followed by the 24-h simulation for a given day of interest. Gas-phase chemistry is based on the Statewide Air Pollution Research Center (SAPRC-99) mechanism, which includes 211 reactions of 74 gas-phase species, 18 of which are free radicals. Inorganic aerosol chemistry, and the evolution of aerosol size distribution and microphysics in WRF-Chem are represented by the Model for Simulating Aerosol Interactions and Chemistry (MOSAIC)83. Aerosol species in MOSAIC include sulfate, nitrate, ammonium, other inorganics, EC, organic aerosols, and aerosol water. Aerosols are assumed to be internally mixed and are represented by eight sections with dry particle diameter ranges of 0.039–0.078, 0.078–0.156, 0.156–0.312, 0.312–0.624, 0.624–1.25, 1.25–2.5, 2.5–5.0, and 5–10 µm. Hourly aerosol and trace gas emissions from sources other than biomass-burning and biogenic emissions are derived from the United States Environmental Protection Agency’s 2014 National Emissions Inventory (NEI2014). We represent SOA formed due to the oxidation of biogenic, anthropogenic, and biomass-burning organic gases using our previously documented volatility basis set approach80 that has shown good agreement with field observations. WRF-Chem calculated aerosol optical depth (AOD) and single scattering albedo (SSA) at each vertical layer using the aerosol number density retrieved from the aerosol size distribution31 and aerosol RI using the Mie theory84. The AAOD is calculated as AAOD = AOD × (1-SSA) at each layer and then integrated vertically to get the columnar AAOD. The supporting Information (Tables S2–4) provides further details of the WRF-Chem model configuration. Currently, the WRF-Chem simulation does not include lensing effects.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "agree with previous studies and strongly supports the previous observations using a state-of-the-art instrument. On the other hand, the results are not highly novel. In addition, although the results and discussions of the coating on S-BrC on the radiative forcing are novel, the discussions are not directly supported by their observations. Therefore, I cannot strongly support this paper being published in this multidisciplinary high impact journal.\nSpecific comments.\nLine 69: “collected during the Pacific Northwest wildfire events on September 5 and 6", "reviewer_5": "It is unclear whether the samples were collected only in 2017 or in both 2017 and 2018. In either way, the sampling periods are limited and it is difficult to draw a general conclusion from the limited samples.\nLine 79 “We manually identified particle types based on their morphology” Please define the morphological criteria. It will also affect the discussion of the hygroscopicity study in Fig. 4.\nLine 130-133: “As shown in Figs. 2a and 2b, our n550 and k550 are about 10% and 70% lower than those described in Alexander et al., respectively. This discrepancy might be due to our advanced STEM's lower electron acceleration (80 kV vs. 120 kV), which results in lower Cherenkov radiation effects and electron beam-induced knock-on damage, which can overestimate the k” This discussion suggests that the detailed comparison of refractive indices from this study and those of Alexander et al in Fig. 3 is meaningless, since the difference is due to the uncertainties of the measurements.\nLine 127-160: In the second paragraph of this section (line 127-139), the authors explain the difference in refractive indices due to different atmospheric processes. In the third and fourth paragraphs, they discuss the differences based on SP2 fractions and molecular compositions, respectively. Although all these processes may contribute to the refractive indices, a comprehensive discussion is helpful.\nLine 174-177: “We show the percentage difference in the AAOD map from August 14 at 19:00 to 23:00 UTC, 2018. We utilized this period because of an existing model domain setup and because that period had solid S-BrC-rich wildfire smoke events with similar molecular compositions to the 2017 study period.” The authors explain why they chose a different time period between their observations and modeling; however, it is better to use the same time period to draw a solid conclusion.\nLine 233-237: “We might underestimate the fraction of organic-coated solid S-BrC based on microscopy imaging because these coatings might be evaporated in the high vacuum chamber. Considering this caveat, our finding suggests that solid S-BrC can act as a seed for secondary organic aerosol formation and that the organic coating can potentially increase the contribution of solid S-BrC to climate warming.” The discussion in the second sentence is based on a hypothesis and lacks evidence. In the abstract, they mention that “Furthermore, the organic coating on solid S-BrC can lead to even higher lensing enhancements than water,” which is not supported by the evidence.\nFig. 4a and S5. “solid S-BrC that uptake water” Some of them look like they are deliquescing rather than absorbing water. This result may suggest that inorganic salt that attached to or internally mixed with the S-BrC has deliquesced. If the authors have considered this possibility, I suggest adding some discussion. At least, I found many sulfate in Fig. S1.\nFig. S4. Is the first question (Yes/No for Na>1%) correct?", "reviewer_3": "Review of \"Enhance light absorption from solid-state brown carbon from wildfires due to organic and water coatings\" by Cheng et al.\nThe study focused on a wildfire event on September 5 and 6, 2017 at a single location. The researchers conducted an extensive and comprehensive analysis of the wildfire smoke, offering a very detailed picture of the physical properties of particles from this wildfire event, including optical properties and some aspects of hygroscopicity. Most of the data and analysis are high quality and should be published. However, I wonder if the paper fits within Nature Communications' mandate. This aspect is up to the Editor.\nThe paper consists of four main sections: 1) prevalence of solid S-BrC in wildfire smoke aerosol, 2) optical properties and chemical composition of solid S-BrC particles, 3) sensitivity study of solid S-BrC absorption aerosol optical depth using model simulations, and 4) absorption enhancement due to water-solid S-BrC interactions and organic coatings. Sections 1-3 are not novel but still significant. Section 4 presents the most novel results, but the conclusions is less convincing than the other sections.\nDetailed comments:\n1) Clarity and context: I had to read the paper a couple of times to appreciate what is novel in the paper. Also, the presentation feels a little disjointed in places. For example, Sections 1-3 seem separate from Section 4. I think Sections 1-3 could be a separate paper.\n2) Title: The title only highlights Section 4. I.e., the title does not represent all the content of the manuscript.\n3) Environmental implications: The environmental implications only cover the results in Section 4. This is one reason for arguing that Sections 1-3 could be a separate paper.\n4) Significance: the most novel aspect of the paper is the observation that approximately half of the solid S-BrC are hydrophobic, and half take up water at 97 % RH. Even this observation is not completely shocking since a) one previous paper by the authors observed two types of solid S-BrC in a previous field campaign, b) hydrophobic and hydrophilic solid S-BrC have been observed previously in separate studies, as pointed out by the authors, and c) a recent study observed solid S-BrC in cloud residuals (Adachi et al. Egusph. 2024) indicating that at least some solid S-BrC are hydrophilic. These previous studies may compromise the manuscript’s significance.\n5) Validity of the data: I have questions about the validity of the data interpretations and conclusions, especially the results in Section 4. See below.\n6) Page 7, lines 198 to 201: \"As shown in Fig. 4a, ~50% of the solid S-BrC did not uptake water (indicated by black arrows) even at >99% RH (additional images in Fig. S5). However, the rest of the solid S-BrC uptake water at RH above 97% to form a core-shell morphology with an aqueous shell (indicated by light yellow arrows, hereafter named hygroscopic solid S-BrC), suggesting a potential function as cloud condensation nuclei.\" Based on Fig. 4a, it doesn't look like the particles form core-shell morphology, contrary to the suggestions of the authors. It looks more like the solid S-BrC often remains at the interface of the water droplets. Also, one of the particles with a light yellow circle doesn't appear to take up any water (the particle with a light yellow circle at the top right-hand corner of the figure).\n7) Page 7, lines 202-203: \"Some hygroscopic solid S-BrC particles have an organic coating (indicated by light yellow circles), implying that hygroscopic organic coating can promote the water uptake of solid S-BrC.\" What is the proof of an organic coating? Is the proof the observation of a larger \"residual\" after evaporation? One of these particles with a light yellow circle did not leave a larger \"residual\" after evaporation. Also, in some cases, the residual may have been caused by a water-soluble particle nearby. Also, how can the authors differentiate between partial dissolution of the solid S-BrC and the presence of an organic coating? The solid S-BrC particles could partially or completely dissolve if enough water is added.\n8) Page 8, lines 235-237: \"Considering this caveat, our finding suggests that solid S-BrC can act as a seed for secondary organic aerosol formation and that the organic coating can potentially increase the contribution of solid S-BrC to climate warming.\" I don't see conclusive evidence for an organic coating since alternative explanations are also possible for the observations. So, I don't think the results provide convincing evidence that solid S-BrC can act as a seed for secondary organic aerosol formation.\n9) Page 9, lines 254-256. \"Our study established a previously unrecognized concept by showing that water coating on solid S-BrC can cause lensing enhancement up to 2.3 at k550.\" This assumes the solid S-BrC is in the center of the water droplets (i.e., core-shell), and the solid S-BrC doesn't dissolve in water. However, the images (Figure 4) suggest they will not form a core-shell structure, and some of the images indicate that the solid S-BrC may have partially dissolved. In the current experiments, the water uptake is less than what will happen in a cloud droplet. Perhaps in a cloud droplet, the solid S-BrC will completely dissolve.\n10) The authors argue that some of the solid S-BrC have an organic coating. How does this affect their optical constants measurements? I guess the authors assumed the solid S-BrC did not have an organic coating when extracting the optical constants."}, "author_response": ""}, {"round": 2, "reviewer_comments": {"reviewer_1": "I carefully evaluate the response. I am happy on the revision. I basically go to accept this article in NC. Here I found one minor point.\nL302-303, How did the author determine 240%. The authors should cite the sources.\nHere I do want make challenge this number. The light absorbing organic coating might have shield effect instead of the light enhancement. How did the author find even bigger Eabs? How do you set up the optical refractive index of light absorbing organic? Please make more suitable implication here.", "reviewer_2": "The authors have considered the reviewers' comments and provided appropriate responses and discussion in the revised paper. I agree with the authors' responses and have no further comments. I hope that this paper makes a significant contribution to the understanding of brown carbon from wildfires.", "reviewer_3": "Second review of “enhanced light absorption for solid-state brown carbon from wildfires due to organic and water coatings”.\nI thank the authors for taking my comments seriously. However, I still don’t think the paper warrants publication in Nature – Communications, which publishes papers that represent “important advances of significance to specialists within the field”.\nThe paper is a good one, but doesn’t quite reach the standard of other papers from the same field that have been published in Nature – Communications.\nSpecific comments:\n1) The clarity of the paper still needs to be improved, especially if the authors are considering a higher impact and more general journal. In several places it is still not clear what is new and what has already been done.\n2) the number of samples still seems two small to make strong general comments.\n3) Significance: In most cases, the results are not new. The new aspects do not seem significant enough for a higher impact more general journal.\n4) Some of the implications are uncertain due to speculation. For example, the authors assume a core-shell model when discussing the effect of solid S-BrC on cloud droplets, but whether or not this is valid is not clear.\nI do think the paper is a nice contribution to the field of atmospheric science and should be published. I also commend the authors for the large body of work."}, "author_response": "We appreciate that the reviewer considers the high quality of the manuscript and deserving publication. As the reviewer suggested, we modified the highlights of the manuscript (please see below). Below are our responses to each comment:\n\nAbstract\n1 (uO]ZbZVcgZ[gVXi^kZ^cYZmd[/,27).,.37^Vi33.cb,v?djaYi]ZVji]dghX]ZX`ZYi]ZcjbWZgd[.,.37* l]nDXdjaYcdi[^cY^i^cbV^cXdciZmi,>Vh^XVaan*i]ZcjbWZgh]djaYWZZm^hi^c\\^cXdciZmiWZ[dgZDXVc\\d id VWhigVXi, KaZVhZ bV`Z hjgZ i]Z cjbWZg ^h g^\\]i, Dc [VXi* i]Z gZ[gVXi^kZ ^cYZm ^h cdi hjgeg^h^c\\ [dg bZ* WZXVjhZi]ZgZlZgZhdbZhijY^ZhgZedgiZYi]ZhZcjbWZg,\nWe want to thank the reviewer for pointing this out. We have corrected the value in the abstract. It should be 0.056. Please see our revised text below:\nRL35-37: “CZgZ* lZ h]dl i]Vi [gdb V KVX^[^X Idgi]lZhi l^aY[^gZ ^c NZeiZbWZg 0./5* \\gZViZgi]Vc7.$ 'WncjbWZg(d[eVgi^XaZhlZgZhda^YN+>g?l^i]VbZVc gZ[gVXi^kZ^cYZm d[/,27).,.34iVi33.cb,v\n\n0( G3.+30*dcZgZXZci hijYn lZaa gZk^ZlZY i]ZVWhdgei^kZ eVgi^XaZh, ce_ ?a^bViZ VcY=ibdhe]Zg^XNX^ZcXZ 0.02*5'/(843,\nWe thank the reviewer for sharing this reference. We have cited this paper in the revised manuscript.\nRL51-53: u>? ^h ine^XVaan Xdch^YZgZY i]Z Ydb^cVci a^\\]i VWhdgWZg*0 Wji hijY~Zh ]Vk~Z gZedgiZY i]Vi i]Z l^aY[^gZ hbd`Z eVgi^XaZh XdciV^c V heZX^Va ineZ d[ hda^Y+hiViZ higdc\\an VWhdgei^kZ>g?'hda^YN+>g?(Xdbbdcan`cdlcVhiVgWVaah,1t3v\n\n1(G/03*eaZVhZF33.^h.,.34^chiZVYd[.,.37^cVWhigVXi,\nThis has been corrected in the abstract.\n\n2( G/3/+/36* >Vh^XVaan* dcZ hijYn lZaa gZedgiZY i]Z >g? XdciV^c dg\\Vc^X c^igViZ WVhZY dc dcZ heZX^Va iZX]c^fjZTjVc Zi Va,* Ack^gdc, NX^,OZX]cda, GZii, 0.0/* 6'/(8 /4+01,O]Z Vji]dgh XVc X^iZ i]^h id hjeedgi XdcXajh^dc[gdbWja`hVbeaZh,\nWe appreciate the reviewer for sharing this relevant reference. We have added this reference and a relevant discussion to the discussion section in the revised manuscript.\nRL160-1648uO]ZbdaZXjaVgXdbedh^i^dcd[l^aY[^gZVZgdhdahlVhVcVanoZYWnV0/+OZhaV Bdjg^Zg OgVch[dgb+Ddc ?nXadigdc MZhdcVcXZ MZhdcVcXZ bVhh heZXigdbZiZg '0/+O BO+D?M HN(* h]dl^c\\ i]Vi 3.$ d[ i]Z 7*62/ idiVa Vhh^\\cZY dg\\Vc^X bdaZXjaVg [dgbjaVh lZgZ dg\\Vcdc^igViZXdbedjcYh'B^\\,0Y(*l]^X]]VhWZZc^YZci^[^ZYVhVc^bedgiVci XdbedcZci ^chda^YN+>g?,39”\n\n3( G/77+0.1,O]Z B^\\jgZ 2V* XdjaY WZ `Zn ^c i]^h hijYn, D ldjaY a^`Z id Vh` id X]ZX` l]Zi]Zg i]Z iVgWVaah ^cY^XViZY Wn i]Z a^\\]i nZaadl XnXaZh ^ciZgcVaan b^mZY l^i] b^cdg hZXdcYVgn heZX^Zh hjX] Vh hja[ViZ dg F+ hja[ViZ,CZgZi]ZB^\\jgZ2V^hcdiXaZVgVcY]^\\]Zcdj\\]bV\\c^[^XVi^dchidY^heaVni]ZXaZVghigjXijgZ,DVb V[gV^Yi]Vii]ZVji]dgZbe]Vh^oZi]ZeVgiYjZidndjgiZX]c^fjZ^chiZVY d[i]Z[VXi,O]ViXdjaYbV`Zlgdc\\ h^YZ,\nJust of course, I finally agree with your conclusions. In fact, most of particles are more or less coated by secondary species or the condensed organic species. As the Proc. Natl. Acad. Sci. U.S.A. 2013, they found these particles dynamically changed of the surface depending the environmental condition. Following the transport distance, these tarballs mostly internally mixed with others. Finally, they can be CCN. However, I am still worried about the hygroscopicity is not from the surface of the solid tarball.\nWe appreciate the reviewer’s constructive comments. We agree with the reviewer that atmospheric-aged solid S-BrC can be internally mixed with hygroscopic species such as K and sulfate (see ref.1–4). We also agree that our SEM images might not have high enough resolution to see the detailed internal structure of individual particles. However, we observed that most tar balls did not have inorganics based on CCSEM-EDX, STXM/NEXAFS, and TEM results. We acknowledge the potential effects of inorganic inclusions, but this might not be the case in our study since our CCSEM-EDX and STXM/NEXAFS only show a negligible amount of inorganic elemental percentage (1.3±1.3%) and inorganic volume fraction (0.04±0.09) in individual particles, respectively. In addition, due to the hydrophobic nature of solid S-BrC, water might not be able to easily diffuse into the particle. Since we observe different hygroscopicity between different solid S-BrCs, we believe the K and S might not be the reason for that different hygroscopicity. Figure 4 a shows water (lighter gray, indicated by white arrow) coated on the Solid S-BrC (black, indicated by dark gray arrow) in 97% RH and 100%RH. This suggests that the Solid S-BrC does not dissolve in water after water uptake, and the water uptake happens on the surface of Solid S+BrC, However, we have not eliminated the trace amount of inorganics as a potential cause for the water uptake. We will add some discussion below:\nRL225-2378uDih]djaYWZcdiZYi]Vi]n\\gdhXde^Xhda^YN+>g?'^cY^XViZYWnl]^iZXnXaZh( ^h hi^aa k^h^WaZ Vi MC;75$* hj\\Zhi^c\\ i]Vi bdhi d[ i]Z kdajbZ d[ hda^Y N+>g? ^h cdc+ ]nYgde]dW^X, HdgZdkZg* i]^h lVi~Zg [dgbh V h]Zaa dc ]n\\gdhXde^X hda^Y N+>g?,=ai]dj\\] i]ZhZ eVgi^XaZh Y^Y cdi [dgbXdgZ+h]Zaa bdge]dad\n l]Zc XdaaZXiZY dc i]ZOAH\\g^Yh* lZ hi^aa XVccdi Za^b^cViZ i]Z ediZci^Va [dg lViZg XdVi^c\\h id [jaan XdkZg i]Z V^gWdgcZ ]n\\gdhXde^X hda^Y N+>g? YjZ id lViZg hjg[VXZ iZch^dc, HdgZdkZg* V[iZg YZ]nYgVi^dc* lZ [djcY i]Vi i]Z ]n\\gdhXde^X hda^Y N+>g? Y^Y cdi ]VkZ dWk^djh YZ[dgbVi^dc, O]ZgZ lVh V gZh^YjVahjggdjcY^c\\i]Zhda^YN+>g?'B^\\,2V(,Jjg??NAH+A@S'B^\\,N5(*i]ZNXVcc^c\nRL176-178: “Fig. 2 | Refractive index, carbon chemical bonding, and molecular composition of solid S-BrC. HZVc 'V( gZVa eVgi 'c( VcY 'W( ^bV\\^cVgn eVgi '`( d[ MDh V\\V^chilVkZaZc\\i][dghda^YN+>g?eVgi^XaZh[gdbi]^hhijYnVcYa^i~ZgVijgZ,5*/.*/0t/6”\nRL279-290: “Fig. 4. |Wateruptake by solid S-BrC, lensing enhancement of solid S-BrClightabsorptionproperties,andoxygen-to-carbonratioforsolidS-BrC.'V(Nda^Y N+>g? lViZg jeiV`Z ZmeZg^bZci Vi 3s? h]dlh i]Vi hdbZ hda^Y N+>g? XVc jeiV`Z lViZg 'ZmVbeaZh ^cY^XViZY Wn WaVX` Vggdlh(* VcY hdbZ VgZ ]nYgde]^a^X 'ZmVbeaZh ^cY^XViZY Wn a^\\]inZaadlVggdlh(,HdgZdkZg*i]ZhZhda^YN+>g?*l]^X]XVcjeiV`ZlViZg*YdcdiY^hhdakZ ^clViZgVcY[dgbVXdgZ+h]Zaabdge]dad\nVi]^\\]MCXdcY^i^dch,O]Zhda^YN+>g?hZaZXiZY Wni]Za^\\]inZaadlXnXaZhVgZhda^YN+>g?l^i]i]^cdg\\Vc^XXdVi^c\\hVcYXVcjeiV`ZlViZg, O]ZhXVaZWVg^h/rb, 'W(GZch^c\\Zc]VcXZbZcid[hda^YN+>g?XdgZh'Y^VbZiZgh[gdb/.. id6..cb*MD :/,27).,.34^(XdViZYl^i]lViZg'.t03..cbi]^X`cZhh*MD * hda^YN+>g?*33. lViZg33. :/,11).^(Vi33.cb'E (*l]^X]XVckVgnWZilZZc/,..2VcY0,63/'hZZNZXi^dcN1(, VWh*?G 'X(J8?ZaZbZciVagVi^d[gdbi]^hhijYnVcYa^i~ZgVijgZ,2*6*/.*/4*0.*16*31t33/*3t/0RZVahdgZ[ZgZcXZ i]^h hijYn&h J8? gVi^d jh^c\\ 0/+O BOD?M HN YViV, O]Z gZY a^cZh ^cY^XViZ i]Z bZVch* i]Z WaVX`YdihVgZi]ZbZY^Vch*i]Z\\gVngZXiVc\\aZhVgZi]Z^ci~ZgfjVgi^aZgVc\\Zh*i]Z\\gVnkZgi^XVa a^cZh VgZ i]Z 73 $ Xdc[^YZcXZ ^ci~ZgkVah* VcY i]Z k^da^c+h]VYZY VgZVh h]dl i]Z YViV Y^hig^Wji^dc,v\n\n5(B^cVaan*i]Zi^iaZ*l]ni]ZgZ^hcd^cdg\\Vc^X,Dcdg\\Vc^X^cV\\ZYiVgWVaaVahdXdcig^WjiZi]Zdei^XVaVWhdgei^dc, KaZVhZ Xdch^YZg ]dl id gZk^hZ ^i VcY l^YZan VXXZeiZY Wn i]Z gZVYZgh, D i]^c` i]Vi XjggZci dcZ b^\\]i bV`Z b^hiV`Zndjgldg`,\nWe agree with the reviewer that inorganics in aged solid S-BrC can potentially contribute to light absorption. However, in response to your earlier comment 5, we elaborated that for this specific study, we observed a minor contribution of inorganics and that they might not play a major role in the water update. This is also surprising to us and emphasizes that our finding is novel since organics facilitate water uptake. Therefore, we decided to keep the same title.\n\nReviewer#2(RemarkstotheAuthor):\nDci]^heVeZg*?]Zc\\ZiVa]VkZbVYZhZkZgVa]^\\]fjVa^inbZVhjgZbZcihdchda^Y+hiViZhigdc\\anVWhdgei^kZ Wgdlc XVgWdc 'hda^Y N+>g?( ^c iZgbh d[ Xdbedh^i^dc* dei^XVa egdeZgi^Zh* VcY ]n\\gdhXde^X^in, O]Z^g bZVhjgZbZcihegdk^YZkVg^VWaZ`cdlaZY\\Z d[N+>g?*l]^X]^hVc^bedgiVci[VXidgd[Xa^bViZ,O]jh*D]VkZ cdYdjWii]Vii]^hldg`l^aaXdcig^WjiZidVibdhe]Zg^XVcYXa^bViZhX^ZcXZ,Jci]Zdi]Zg]VcY*Vhi]Z_djgcVa V^bhid#gZegZhZci^bedgiVci VYkVcXZhd[ h^\\c^[^XVcXZidheZX^Va^hih l^i]^c ZVX][^ZaY#*DfjZhi^dc l]Zi]Zg i]Z gZhjaih d[ i]^h eVeZg gZVX] i]Vi aZkZa, O]Zn [djcY V h^\\c^[^XVci cjbWZg d[ N+>g? [gdb V hbVaa hZi d[ hVbeaZh'ild+YVnhVbea^c\\(VcYh]dlZYi]Z^gXdbedh^i^dchVcYdei^XVaegdeZgi^Zh,O]ZhZgZhjaihV\\gZZl^i] egZk^djhhijY^ZhVcYhigdc\\anhjeedgihi]ZegZk^djhdWhZgkVi^dchjh^c\\VhiViZ+d[+i]Z+Vgi^chigjbZci,Jci]Z di]Zg]VcY*i]ZgZhjaihVgZcdi]^\\]ancdkZa,DcVYY^i^dc*Vai]dj\\]i]ZgZhjaihVcYY^hXjhh^dchd[i]ZXdVi^c\\ dcN+>g?dci]ZgVY^Vi^kZ[dgX^c\\VgZcdkZa*i]ZY^hXjhh^dchVgZcdiY^gZXianhjeedgiZYWni]Z^gdWhZgkVi^dch, O]ZgZ[dgZ*DXVccdihigdc\\anhjeedgii]^heVeZgWZ^c\\ejWa^h]ZY^ci]^hbjai^Y^hX^ea^cVgn]^\\]^beVXi_djgcVa,\nWe appreciate the reviewer for providing insightful comments and noting our high quality and valuable measurements. We understand that maybe the novelty part was not evident in the manuscript. We revised some parts of the manuscript to clarify the study's novelty. We also outline below the novel aspect of the manuscript:\n1. SolidS-BrCwithdifferentsurfacelayersmaydictatethehygroscopicity.Weshowed that Solid S-BrC with an oxygenated layer can uptake water (hydrophilic Solid S- BrC), and those without are hydrophobic. This finding along with quantifying number fraction of these particles is important for parameterizing climate models' hydrophobicandhydrophilicsolidS-BrC.\n2. Water coating on hydrophilic solid S-BrC can enhance the light absorption of solid S-BrCby afactorof two.This novelfinding alsosuggeststhecurrentmodelsmight underestimatetheclimateeffectsofwildfiresmoke.\n3. SolidS-BrCcanbecoatedwithorganics,whichenhanceslightabsorptionmorethan watercoating.\nWe understand the reviewer’s concerns about the number of samples since we focused our results on limited samples collected from September 2017. However, we conducted high- resolutionmassspectrometryandsingleparticleanalysis(STXM/NAXFSandCCSEM/EDX) on solid S-BrC samples collected in 2018 (see Ijaz et al., 3 and our response to specific comments below), and their chemical composition were similar. Therefore, we expect these samples to have solid S-BrC with similar properties, supporting our findings and having broadimplications.Weagreewiththereviewerthatourpaperstillhasroomtoimprove.We believeaddressingthesecommentscanhelpimproveourmanuscriptsignificantly.Therefore, we modified the highlights of the manuscript (please see below). Below are ourresponses to eachcomment"}]

Decision letter