For the experiments, the chamber was filled with an 80/20-mixture of N₂ (evaporated from liquid N₂; Messer, purity > 99.995 %) and O₂ (Linde 5.0, purity ≥ 99 999 %) at a slight overpressure (ambient conditions +5 hPa) to avoid external contaminations.

The combustion aerosol was produced with a miniCAST propane-diffusion flame soot generator (Model 6204 Type C, Jing Ltd., Switzerland), which allows for repeatable soot generations (Heuser et al., 2025a; Moore et al., 2014). For all experiments, the propane, mixing N₂, quench N₂, oxidation, and dilution air flows were kept at 0.03, 0, 2, 0.75, and 5 L min⁻¹, respectively, as corresponding to the first operation point of the miniCAST, producing a fuel-lean combustion with a global equivalence ratio of 0.91. Fuel-lean miniCAST soot is widely regarded as representative of diesel and aircraft (kerosene-type) emissions (Ess et al., 2021; Moore et al., 2014; Saffaripour et al., 2017). The bulk elemental-to-total carbon ratio (EC / TC) of the generated cast soot (CS), hereafter referred to as CS1, was measured at 0.79 ± 0.11, indicating a BC-dominated composition (Heuser et al., 2025a). The CS1 was injected into CESAM through an activated carbon denuder for a duration between 30 and 390 s, depending on the targeted mass concentration.

The water vapour was injected about 2 h after the soot to reach a relative humidity of ca. 30 %, as steam from a modified pressure cooker filled with ultra-pure water (18.2 MΩ, 3.2 ppb TOC, Milipak) (Wang et al., 2011). Ozone was injected directly out of an ozone generator (BMT 802N, BMT MESSTECHNIK GMBH), using bottled O₂ (Linde 5.0, purity ≥ 99 999 %). About 40 ppb of Butanol-d₉ (1-Butanol D₉, 98 %, Euroisotop) was volatilized and injected, prior to the O₃, using a vacuum line and a glass bulb. The Butanol-d₉ was used as a tracer to monitor the OH-exposure. Following O₃, about 75 to 90 min later, the precursors for the secondary aerosol and coating formation were introduced in the chamber volume. The SO₂ (Merck, 99.95 %) was injected using a calibrated gas-tight syringe and septum inlets. The H₂SO₄ generation itself was then started by initiating the photochemical hydroxyl radical production by turning on the three Xenon-arc lamps. The light of the lamps is introduced via quartz windows equipped with Pyrex^® filters (6.5 mm), generating a spectrum close to solar radiation at ground level (Wang et al., 2011). Temperature fluctuations due to the radiation were limited with a cooling system (LAUDA Integral-T 10 000 W) to about 1 to 2 K, resulting in a temperature range of 294 to 301 K during experiments. The α-pinene (98 %, Sigma-Aldrich) was volatilized using a vacuum line, and a known quantity of the SOA precursor was injected with a glass bubble. Dark conditions and minimal temperature fluctuations between 289 and 291,K required no temperature regulations in the SOA experiments. After 3 h since the start of secondary aerosol formation, the majority of the α-pinene was consumed (< 3 ppb remaining), and the H₂SO₄ coating production was stopped by turning off the lamps. In experiments H₂SO₄-high 1 and 2, SOA-S_ref, and SOA_high-S_high, the coated soot was left in suspension under dark conditions with reduced instrumentation and minimal dilution (about 3 L min⁻¹). After 15 h, the aged soot was again fully characterized.

At the end of each experiment, the CESAM chamber was evacuated to around 3×10-4 hPa to avoid carryover contaminations and stays evacuated between subsequent experiments. The chamber was also additionally cleaned manually at the beginning of each campaign and when changing the chemical system. More details on the experimental protocol for the soot and control experiments are provided in Sect. S1 and Table S1 in the Supplement.

A large complementary set of state-of-the-art instrumentation was used to characterize the physico-chemical and spectral optical properties of the soot aerosol, as well as the thermodynamic state of the volume and its gaseous content. The full instrumentation including details for measurement uncertainties calculations are detailed in Tables S2 and S3, and previously described in Heuser et al. (2025a). The majority of online measurements were performed continuously throughout experiments with the exception of short disconnections at soot and water vapour injections. During the 15 h dark ageing, only the number size distribution was measured continuously. Air sampled out of the chamber during experiments (varying between 3 and 22 L min⁻¹) was compensated with a particle-free N₂ / O₂-mixture, leading to a constant dilution of the compounds in the chamber. The main instruments used to characterise the aerosol properties in this work are presented in the following. The aerosol was measured without drying to preserve realistic RH conditions.

The aerosol number size concentration between 19.5 and 881.7 nm over 64 channels (dN/dlog⁡Dm; Dm, mobility diameter) was continuously measured at 3 min resolution with a scanning mobility particle sizer (SMPS, TSI, Kripton⁸⁵ neutraliser, DMA 3080, CPC 3772, with 2.0/0.2 L min⁻¹ sheath/aerosol flow rates). Measurements were corrected for diffusion losses and multiple charge effects with the instrument software. Total uncertainty on dN/dlog⁡Dm was of the order of 5 %. The number size distributions were fitted with log-normal functions to determine the population count (or number) median diameter (CMD).

The aerosol effective density (ρeff) was derived from measurements with a centrifugal particle mass analyzer (CPMA – Cambustion) and an SMPS (TSI, X-ray neutraliser 3087, DMA 3080, CPC 3775 high). A DMA-CPMA-CPC setup was used to measure the aerosol mass-resolved size distributions as described in Heuser et al. (2025a) and Yon et al. (2015). Multiple measurements between 0.03 to 6 femtograms allowed for the calculation of the effective density over the range of soot diameters ρeff (Dm) (Kasper, 1982; Park et al., 2003). The uncertainty on ρeff (Dm) was estimated to be 10 %. A complete DMA-CPMA-CPC scan to derive a full ρeff (Dm) curve lasted about 35 min. The scans were performed after soot injection in the chamber, after each compound injected in the chamber and for coated soot, before and after long-term ageing. The ρeff (Dm) data were fitted with a power-law relation (Park et al., 2003) 1ρeffDm=aDmDfm-3 to determine the mass-mobility exponent (Dfm) and the pre-factor a. The Dfm is dependent on the morphology of the aerosols (it equals to 3 for spherical-void free particles, and is around 2.2 for soot (Maricq and Xu, 2004; Sorensen, 2011)) and, together with ρeff, was used as a proxy for morphological and chemical evolution of the soot over the simulated ageing processes (Park et al., 2003; Sorensen, 2011). The ρeff (Dm) was used to determine the total aerosol mass concentration (mtotal) as: 2mtotal=∑iπ6Dm,i3dNdlog⁡Dm,idlog⁡Dm,iρeffDm,i where ρeff (Dm,i) indicates the values extrapolated at the Dm values of the SMPS and dN/dlog⁡Dm is the corresponding measured number size distribution (the subscript i in Eq. 2 indicates the SMPS channel). As discussed in Heuser et al. (2025a), the mtotal compared well with gravimetric measurements from a tapered element oscillating microbalance (TEOM 1400a – Thermo Scientific). Noteworthy, the mass concentration calculated with Eq. (2) lacks sensitivity to ongoing aerosol physico-chemical processing due to the coarse time resolution of ρeff measurements.

Morphological information of the soot particles were also retrieved using transmission electron microscopy (TEM). Soot particles samples were collected over different moments of the experiments using three TEM grids (200-mesh copper grid with a Formvar/carbon film, Agar Scientific) glued onto the Teflon-pore filters (47 mm Nuclepore Whatman, 0.8 µm nominal pore size). Sampling was performed with a custom-made stainless-steel filter holder operated at a flow rate of 2–4 L min⁻¹. The total sampling time was adjusted to ensure similar area loadings between different samples. Nevertheless, the total number of TEM images acquired varied considerably among experiments and particularly for the aged conditions, ranging from a few tens to more than 300 images per sampled grid set. The TEM images were acquired with a JEOL^® 100CXII and a JEOL^® JEM2010 microscopes and processed using the same protocol as described in Heuser et al. (2025a). A qualitative visual assessment of the TEM images was performed to characterize the aged sampled soot. However, the variation in the number of images across experiments precluded a statistically robust clustering analysis and, by consequence, a more quantitative comparison across the various experimental stages.

A single particle soot-photometer (SP2 – Droplet Measurement Technologies, 1 min resolution, Stephens et al., 2003) was used to measure the refractory mass concentration (mrBC) and its size distribution between 89 and 567 nm (volume equivalent diameter, Dve), assuming a material density of 1.8 g cm⁻³. The SP2 was calibrated with fullerene-soot material, mass-selected with the CPMA. The calibration curve of fullerene showed comparable coefficients to the most elemental CS1 particles produced at fuel-lean conditions (Heuser et al., 2025a). The measured size distributions from the SP2 were extrapolated based on a lognormal fit to approximate mrBC over the full soot size range (Liu et al., 2014). The uncertainty on mrBC was estimated to be 17 %. The ratio of the mtotal to the SP2 fullerene-calibrated mrBC (R=mtotal/mrBC) was determined for each experiment prior to coating formation and ranged from 1.2 to 1.8. This ratio was used to convert mrBC into the total soot mass (msoot) representing the total carbonaceous mass of soot, including both elemental and organic carbon fractions, according to msoot=R⋅mrBC. This approach enabled continuous tracking of the soot mass concentration throughout the experiments. Due to instrumental limits of the SP2 scattering detector, a detailed quantification of the coating thickness (leading-edge-only, LEO-fit) or frequency of thinly and thickly coated rBC cores (delay-time) was not possible, as reported for the same SP2 instrument in a recent experiment in the AIDA (Atmospheric Interactions and Dynamics in the Atmosphere) chamber (Zanatta et al., 2025).

Spectral optical properties were monitored with a nephelometer (TSI Model 3563 Integrating Nephelometer, 1 s resolution, sampling flow rate 2 L min⁻¹, Anderson et al., 1996), providing the scattering coefficient (bscat) at 450, 550, and 700 nm, and two Cavity Attenuated Phase Shift monitors (CAPS PMex, Aerodyne Research, both operated at 1 s resolution, sampling flow rate 0.85 L min⁻¹, Kebabian et al., 2007; Massoli et al., 2010) measuring the extinction coefficient (bext) at 450 and 630 nm. Two additional CAPS single scattering albedo monitors (CAPS PM_SSA, 1 s resolution, sampling flow rate 0.85 L min⁻¹, Onasch et al., 2015; Yu et al., 2024) were additionally deployed in some experiments to provide bext at 450 and 630 nm to compare against CAPS PMex data (the scattering signal from the CAPS PM_SSA was not considered in this analysis). The bext by the CAPS PMex and PM_SSA agreed within their uncertainty (<6%). The aerosol scattering coefficient measured at angles between 7 and 170° from the nepheleometer was corrected based on Massoli et al. (2009) to obtain the fully integrated 0–180° bscat by using a real refractive index of 1.95 (Bond and Bergstrom, 2006). Total uncertainty on bsca was estimated at about 8 %. The scattering Ångström exponent was calculated via the power-law fit of the spectral bscat and used to interpolate the scattering coefficient at 630 nm.

The aerosol absorption coefficient (babs) at 450 and 630 nm was calculated from online measurements throughout each experiment as bext from the CAPS PMex minus bscat from the nephelometer. The uncertainty on the derived babs was estimated to range within 10 %–30 %. As fully described in Heuser et al. (2025a), the babs derived with extinction-minus-scattering was compared against three filter-based measurements operating between 375 and 850 nm: the Multi Angle Absorption Photometer (MAAP, Petzold et al., 2005), the Multi-Wavelength Absorbance Analyzer (MWAA, Massabò et al., 2013), and the polar photometer of the University of Milano (PP_UniMI, Vecchi et al., 2014). The comparison of the babs at 450 and 630 nm shows an average difference of 7 % and a strong correlation of R2=0.94 between extinction-minus-scattering and filter-based techniques, which is within the combined measurement uncertainty. Greater data dispersion was observed for absorption coefficients below 10 Mm⁻¹, a value that can be considered the lower detection limit for babs. During the experiments, the measured absorption coefficients ranged from 10 to 10³ Mm⁻¹, indicating that the entire range of babs values in the present analysis falls within the reliable measurement range.

The gas phase composition was measured during the experiments using a set of gas monitors measuring the concentrations of SO₂ (Horiba APSA370), O₃ (Horiba APOA370), NO_X / NO₂ / NO (Horiba APNA370), NO₂ (Teledyne T500U), and CO / CO₂ (APEE – ProCeas). To follow the content of α-pinene and butanol-D₉ compounds a Proton Transfer Reaction Time of Flight Mass Spectrometer (PTR-ToF-MS, Kore Technology, Jordan et al., 2009) was used. The atmospheric state of the chamber was tracked throughout the experiments by measurements of the pressure (Baratron type MKS 626AX13TDE (high range)/626AX11TDE (low range)), temperature, and relative humidity (Vaisala HMP234-Hygrometer).

The start point for the experiments is an about two-hour-old soot aerosol for which a complete set of properties was measured to determine an initial reference. In its fresh state the investigated soot has a lacy fractal morphology, as shown in TEM images (Figs. 1e, 2e), quantified by a mass-mobility exponent Dfm of 2.11 ± 0.04. The soot shows a mono-modal size distribution with an initial count median diameter CMD around 100–120 nm, increasing to values between 150 and 250 nm due to coagulation within the first 2 h of experiments (Figs. 1a, 2a). For the CMD of 200 nm the fresh soot particles have an average ρeff of 232 ± 13 kg m⁻³ (Fig. 3). The soot mass concentration at the end of this characterisation and just prior to the water vapour injection ranges between 13 and 169 µg m⁻³ for the different experiments.

As shown in Figs. 1a and 2a, no significant changes in the CMD or number concentration are observed due to water vapour injection, with the exception of minimal loss of particles that can be attributed to a physical washout and the short pressure change in the chamber. Similarly, no significant effect on the soot physico-chemical properties is observed due to ozone injection and the following 2–3 h prior to the H₂SO₄ and SOA generation. Measurements of ρeff (Dm) show only slight differences, within uncertainties, between pre-coating steps and fresh soot (Fig. 3), indicating no significant chemical or morphological changes. The MAC values show a consistent minor declining trend during the pre-coating steps, as indicated by the decrease of Eabs (Figs. 1d, 2d). This decrease remains however well inside one standard deviation of the MAC values of fresh soot (Heuser et al., 2025a, Fig. S1). To account for this minor evolution, as described in Sect. 2.3.1, the reference MAC_prior to calculate Eabs is taken by averaging values over the last undisturbed suspension period prior to the coating generation, corresponding to about 60 to 90 min.

No significant changes in the soot physico-chemical and optical properties are likewise observed for the control experiments for the various conditions tested. The effective density measurements (Fig. 3a) and the mobility exponents determined from them are found to be rather stable compared to the fresh soot and the pre-coating steps. The only exception was a moderate increase in ρeff (Dm) observed during the irradiated humid experiments, both in absence and in presence of ozone (Figs. S22–S23). These changes in effective density indicate that some, albeit minor, physicochemical transformations occurred during the experiments. Specifically, illumination under humid conditions may promote the photooxidation of surface organic carbon, as previously reported by Han et al. (2012). In the humid experiment with ozone, the increase in effective density is likely associated with ozone uptake by the particles, consistent with previous chamber studies (Kamm et al., 1999; Zelenay et al., 2011). The long-term ageing control experiments, named Soot coagulation dry 1 and 2, also showed some deviations for Dfm, with values around 1.96 ± 0.01. These deviations are however attributed to measurement bias due to the very low aerosol concentrations after 24 h of coagulation and dilution. The MAC values remain for all conditions inside one standard deviation of the values of fresh soot as reported in Heuser et al. (2025a) (Fig. S1).

As shown in Fig. 1a and b, the start of the H₂SO₄ generation leads to a progressive increase of soot CMD due to the condensation of coating material on the particles, reflected in a marked increase of the soot ρeff(Dm) (Fig. 3b), as well as to the production of new modes associated with homogeneous particle formation below CMDs of 120 nm. The estimated H₂SO₄ mass deposited on the soot due to condensation, but also coagulation with the H₂SO₄ mode, is quantified via the bulk coating-to-soot mass ratio MR,bulk (Fig. 1c). The H₂SO₄-low condition (lower OH oxidant availability) produces lower coating mass with MR,bulk increasing from a value of about 1.0 roughly 15 min after the beginning of H₂SO₄ generation, to 1.2 about 3 h later. Conversely, the H₂SO₄-high conditions (higher OH availability) lead MR,bulk to a quick rise to around 2.0–2.2 and a continuous increase to 3.5–3.9. The continued accumulation of coating material on soot induces a progressive increase in absorption (Fig. 1c–d). The Eabs values at 630 nm average at a maximum of around 1.3–1.4 (Fig. 1d), with values at 450 nm exceeding 1.5 (Fig. S2).

The retrieved average Dfm ranges from 1.89 to 2.01, and remains comparable with the value for fresh soot, indicating minimal changes to the shape of the lacy fractal particles after the initial coating addition. Nonetheless, TEM images depict a more heterogeneous mixing state, with both partial and fully coated particles, and both compacted and fractal-like soot (Fig. 1f; more illustrative images are shown in Fig. S4).

A decrease in Eabs down to about 1.0–1.3 is measured at both wavelengths after 15 h of simulated dark ageing of the coated soot (grey area Figs. 1b–1d, S2), despite the MR,bulk showing no significant differences. The decrease in Eabs is likely associated to a size reduction of the absorbing soot core, as observed in TEM images analysis (Figs. 1g, S4), showing significant particles' restructuration. This restructuration is assumed to be caused by the continued accumulation of coating on the soot by condensation and coagulation, which homogenises the soot core structure while the MR,bulk remains relatively unchanged. This continuous coating formation is supported by a slight observed increase of both CMD and ρeff(Dm) (Fig. 3b), and a decrease in residual O₃ and soot number concentration.

In contrast with the slower H₂SO₄ experiments, the injection of the α-pinene precursor in the O₃–rich chamber leads to a quasi-instantaneous start of the SOA generation marked by a rapid appearance and growth of a mode below 100 nm mobility diameter associated with new particle formation (Fig. 2a, ρeff(Dm) data Fig. 3c). The start of the SOA generation also marks an increase of MR,bulk due to the condensation of coating material on the soot particles (Fig. 2c), supported by a ρeff(Dm) increase in the soot containing fraction above 100 nm diameter. The MR,bulk rapidly reaches an average value of around 3.3 for SOA-S_low (experiments with the highest precursor-to-soot-ratio), and of 1.2 to 1.3 for the SOA-S_ref (initial soot mass comparable to H₂SO₄ experiments) and SOA_high-S_high experiments (about twice the soot and precursor concentration than SOA-S_ref). The Dfm rises to 2.06 ± 0.04 (SOA_high-S_high), 2.24 ± 0.05 (SOA-S_ref), and 2.61 ± 0.04 (SOA-S_low), indicating that the coating formation induced a transition towards more spherical and compacted particle morphologies. This restructuring is also reflected by the marked and simultaneous decrease in the CMD for SOA-S_ref and SOA_high-S_high. In contrast, the CMD of SOA-S_low increases rapidly after coating formation. The TEM images additionally support the coating-induced soot restructuring in all experiments. They show that the majority of soot collapsed to form small and compact aggregates (Fig. 2f–g; more illustrative images in Fig. S5), and, when visible from microscopy, the coating is identified to encapsulate soot particles. The net increase in CMD observed for SOA-S_low is then attributed to the larger amount of condensed coating material for this experiment, which masks the reduction in particle size associated with soot restructuring (Figs. 2b, S3). After this initial evolution, the soot CMD for SOA-S_ref and SOA_high-S_high stabilizes as a consequence of minimal further SOA deposition (α-pinene precursor consumed) and negligible coagulation (as discussed in Sect. S2, Fig. S10). In contrast, for SOA-S_low the CMD of the soot peak is found to continuously decrease, interpreted as ongoing restructuration.

The Eabs measured at 450 and 630 nm rapidly rises following MR,bulk as the SOA coating on soot establishes. Then, until the ageing, it remains rather stable at values between 1.2 and 1.4 for SOA-S_ref (Fig. 2d) and SOA_high-S_high. Higher Eabs up to 2 are measured for SOA-S_low right after coating starts due to larger MR,bulk, decreasing afterwards following the CMD decrease and its underlying restructuration (Fig. S3).

After 15 h of ageing in the dark, a significant decrease of MR,bulk, and Eabs (0.83–1.04) at rather constant CMD is observed for the SOA-S_ref and SOA_high-S_high experiments. The reduction of MR,bulk, concurrently with the absence of further SOA production, negligible coagulation, invariance of CMD, and a slight reduction in the soot density, indicates a loss of coating mass on the particles. Such a loss can occur during the continuous dilution of the chamber volume, despite minimal, which can lead to the evaporation of some of the more volatile compounds of the SOA (Grieshop et al., 2007; Metcalf et al., 2013), while low-volatile organics remain on the soot and continue to affect the absorption (Zhang et al., 2023). Such a loss of coating may cause further compaction of the soot (Corbin et al., 2023), as TEM pictures show dominantly collapsed particles (Fig. 2g). Therefore, the pronounced decrease in Eabs (down to values lower than 1) after 15 h of ageing is attributed to the combined effects of coating mass loss through evaporation and continued soot restructuring, which further reduced the particle size relative to the pre-coated soot.

The results presented in this work evidence that Eabs depends not only on the quantity of coating material deposited on the soot, but also on size and morphological modifications that affect Eabs magnitude and evolution, modulated by different coating dynamics and ageing.

In the experiments, larger Eabs are observed when aggregate shapes are more widespread and lacy aggregates are present. On the contrary, lower Eabs for given MR,bulk are measured for all conditions with more compacted soot aggregates. This is especially prominent when comparing the clusters of the observations of SOA-S_ref and SOA_high-S_high and can be explained by the collapse of the soot aggregate, reducing particle size and optical absorption cross section (Kahnert and Devasthale, 2011) while the coating mass remains constant. Morphological restructuration is observed both directly at the early stages of the SOA coating and for aged H₂SO₄-coated soot, indicating that Eabs can evolve further after an initial coating formation. This size effect can be strong enough to reduce the Eabs to values lower than the pre-coating state (<1) even with light coatings, as for aged SOA-coated soot.

The shape, i.e. restructuration of the particles, and the coating distribution are found to vary with experiments and ageing time. The slower production of H₂SO₄ impacts the shape and thus the morphology of the soot less in the initial coating formation stages than the much faster SOA formation, which rapidly reduces mobility diameter and compacts soot particles. Only after long term ageing, the majority of particles for H₂SO₄ experiments is restructured, accompanied by a reduced Eabsfor the aged and collapsed H₂SO₄-coated soot. Based on previous flow reactor-type measurements observing soot compaction during rapid H₂SO₄ condensation (Khalizov et al., 2009b; Zhang et al., 2008), it can be assumed that the difference is not purely compound-related but likely a result of the dynamics of the coating formation.

Experimental observations in the present study suggest that the Eabs can be significantly mispredicted if restructuration and size changes are not taken into account, a process frequently observed in ambient observations (Colbeck et al., 1990; Corbin et al., 2023; Fierce et al., 2020; Khalizov et al., 2009a; Wang et al., 2023). However, this is not possible with the commonly used idealized Mie calculations, as they rely on volumetric-based sizing, nor with fixed Eabs approximation. Implications of this drawback are visualized in Fig. 4 (630 nm) and Fig. S6 (450 nm), which each contain two predictions of the Eabs-MR,bulk relation based on Mie core-shell calculations for each of the two coating types. The calculated curves differ in the refractive index assumptions for the soot core, which was selected to represent both pure BC and soot, as well as values with contrasting real and imaginary components. The effect of changes in morphology is best shown in the SOA results (Fig. 4b), where clusters of Eabs at the same MR,bulk can be observed. These can be described as core-shell by varying the refractive index, even though neither the soot's nor the coating's intrinsic optical properties change between experiments. The cases for which the modelled Eabs align more closely with measured ones are conditions with freshly well-established coatings. While for SOA they happen instantly, for H₂SO₄, this only applies for conditions with higher MR,bulk (2<MR,bulk<3.5). This is in line with the observations of Liu et al. (2017) showing that a sufficient enhancement is only possible when a certain mixing ratio or threshold is reached. In this case, a core-shell internal mixing approximation may be potentially suitable for describing it. Observations from this study show that, in the range of MR,bulk investigated, the core-shell approximation fails to predict Eabs for the other conditions, i.e., uncollapsed partially coated soot with significant particle-to-particle heterogeneity and aged coated particles. Previous works using both models and experimental studies have shown that the distribution of the coating and position of the soot in the coated particles (e.g., partial coating, embedded soot) can have a significant impact on the absorption enhancement and that the shape of the particle can influence this distribution (Adachi et al., 2010; Fierce et al., 2016, 2020; Liu et al., 2017; Zhang et al., 2025). Adachi et al. (2010) noted relevant inhomogeneity in particle-coating mixing, particularly for elongated non-collapsed particles. Such an inhomogeneity is likely the cause of the systematic differences between the observed and core-shell predicted Eabs, in particular during a slow coating generation.

Compared to experimental observations, the Eabs predictions at 630 nm overestimate absorption by up to 60 % in the Mie-based retrieval, and by up to 40 % for the, commonly used, fixed Eabs value of 1.5, but increasing up to 75 % for fresh partial coatings or soot that has undergone restructuration. Experimental conditions characterized by established coating and only limited soot restructuring exhibit improved agreement between predicted and measured Eabs values, although underestimations between 2 % and 30 % persist.

Only a limited number of chamber studies have examined both the formation and subsequent ageing of coatings on soot particles. Although their soot generation methods, coating protocols, and the extent of soot and coating characterization differ from those employed in the present study, two studies, the ones by Schnaiter et al. (2005) and by Metcalf et al. (2013), investigated the formation and ageing of organic coatings on polydisperse soot and are discussed below. To the best of our knowledge, no comparable chamber studies examining the formation and ageing of sulfuric acid coatings on soot have been reported.

Schnaiter et al. (2005) investigated the impact of α-pinene SOA coatings on diesel soot using experiments conducted in the 84 m³ AIDA chamber. In their study, the soot underwent five successive coating steps through the in situ ozonolysis of α-pinene before being aged for an additional 10 h in the chamber, resulting in total experiment durations of up to 25 h. They reported absorption enhancement factors of 1.8–2.1 at wavelengths of 450, 550, and 700 nm for collapsed soot aggregates. These values are at the upper end of the range observed in our SOA experiments, particularly for the SOA-S_low experiment, which most closely resembles the conditions in terms of the soot-to-α-pinene precursor ratio as investigated at the AIDA chamber. In contrast to our observations, Schnaiter et al. (2005) found that Eabs remained nearly constant during the ageing period. This discrepancy is likely explained by fundamental differences in the experimental protocols, particularly the ageing conditions. In our experiments, the aerosol was aged under slight continuous dilution and in the absence of additional α-pinene precursor. Under these conditions, the more volatile SOA components gradually evaporated, leading to a reduction in the coating mass. In the AIDA experiments, however, the soot experienced continuous SOA growth throughout the ageing period. This was achieved by first introducing ozone into the chamber and subsequently supplying a continuous flow of air saturated with α-pinene vapor, thereby sustaining ongoing SOA condensation onto the soot particles. These conditions maintained the particle coating and likely explain why Eabs remained nearly constant during the 10 h ageing period. Schnaiter et al. (2005) further showed that the Mie core–shell model cannot accurately reproduce the optical behaviour of thinly coated soot particles, although its agreement with observations improves with increasing coating-to-soot ratios.

Metcalf et al. (2013) performed chamber experiments with fullerene soot, a surrogate for fractal carbonaceous particles, in the 28 m³ Caltech Teflon chamber. The soot was exposed to several types of organic coatings, including SOA formed from the high-NO_x photooxidation of naphthalene and from the high- and low-NO_x photooxidation of α-pinene. The experiments consisted of an initial irradiation period lasting several hours, during which SOA formed and the chamber temperature increased, followed by several hours of dilution accompanied by a gradual decrease in temperature. Consistent with our observations, Metcalf et al. (2013) reported absorption enhancement factors of 1.25–1.35 at 405 nm and 1.15–1.30 at 781 nm for the α-pinene SOA experiments. They also observed that the SOA coating partially evaporated during the course of the experiments, particularly during the dilution phase. The extent of evaporation depended on the chamber temperature, both during the irradiation period, which influenced the formation of more or less semi-volatile SOA components, and during dilution, where the magnitude of the temperature decrease controlled the degree of evaporation. In our experiments, temperature variations during dilution were limited to 1–2 K, suggesting that changes in coating mass were primarily driven by dilution rather than by temperature-induced volatility changes. Unlike our observations, Metcalf et al. (2013) demonstrated that Eabs can be accurately predicted when the coating mass or thickness is known and the particle can be represented by a simple core–shell geometry. However, this approach relies on fullerene spherical particles instead of fractal soot, thereby limiting the representativeness of the results for atmospheric conditions.

Overall, these previous studies have provided evidence that coating-induced absorption enhancement occurs and provide key complementary observations to our study. However, they have important limitations because they provide only limited information on the simultaneous evolution of particle coating and morphology throughout the different stages of the coating process and, in the case of Metcalf et al. (2013), rely on simplified particle morphologies. Consequently, it remains difficult to directly compare these studies or derive predictive relationships applicable to atmospheric soot. The present work addresses this gap by systematically characterizing both coating evolution and soot restructuring throughout the ageing process, so to move towards a mechanistic understanding of Eabs. Although such measurements are experimentally challenging, this approach provides new insights into the underlying processes and demonstrates that this combined characterization is essential for developing a predictive understanding of coating-induced absorption enhancement.