The experimental setup used here has been detailed in . Beech-wood logs (Fagus sylvatica) were burnt in a modern logwood stove (9kW, Rüegg Mars, Switzerland) according to official Swiss type-approval protocols at the officially certified testing facility of the University of Northwestern Switzerland (Swiss register STS 396, European register NB 2113). The stove was operated by facility personnel resulting in burns that were, relative to household usage, extremely reproducible.

Wood logs were added to the stove in batches of ∼ 3 kg (see inset of Fig. ). The first batch of each measurement day was ignited using a small amount of tinder (a commercial product containing paraffin wax with the appearance of wood wool) and kindling (small pieces of wood). The tinder and kindling were placed on top of the first batch of wood logs, which therefore burnt top-down. Subsequent batches were allowed to self-ignite upon the embers of the previous batch, burning bottom-up. In both cases, the stove operator increased the airflow into the stove at the beginning of each burn to facilitate the onset of flaming combustion. The consequential increase in dilution was corrected for in but not herein, for reasons discussed below.

Wood-combustion emissions were directed through an indoor chimney where a heated sampling line (433K) extracted samples for analysis. After eightfold dilution, samples were either measured as is, oxidized (see below), or filtered and oxidized for a given burn. The discussion below focusses on the first case (fresh emissions) and oxidized case (aged emissions).

Aging was performed using the Micro-Smog Chamber MSC,. The MSC is described in detail in and , so only a brief description is given here. The reactor consists of three UV-grade quartz tubes totalling only ∼ 225 cm3 in volume. Losses of particles and low-volatility gases to the UV-heated MSC walls are expected to be negligible . Aerosols entering the MSC are exposed to 4W of UVC irradiation (254nm and 185nm emission lines) generated by five low-pressure mercury lamps (Heraeus, type GPH212T5VH/2), forming O3 from O2 and subsequently OH⚫ from background water vapour. In the second tube, 30 W of UVA radiates from a high-pressure halogen lamp (Panacol-Elosol, type UV-H 255) to drive further OH⚫ chemistry. Some NOx (=NO + NO2) or SO2 chemistry is also possible depending on the combustion emissions. The last MSC tube allows the aerosol to cool slightly while reaction continues. Some experiments were performed with the intermediate UVA tube removed, resulting in no observed differences in this study . For this study, OH exposures were estimated at roughly 109 moleccm-3h-1 based on follow-up experiments . This represents an extreme upper limit for atmospheric OH exposure.

The SP-AMS used in this study is an HR-ToF-AMS (hereafter shortened to AMS) equipped with a switchable 1064nm continuous-wave soot-particle vaporization module . The SP module was switched on and off periodically, whereas the AMS vaporizer (described below) remained on continuously. Thus the data reported here represent either standard AMS measurements or dual-vaporizer SP-AMS measurements. Here, we use these terms to indicate the laser state where relevant, also referring to the instrument itself as the SP-AMS.

The design of the SP-AMS has been described in detail elsewhere . In brief, the instrument samples aerosol through an aerodynamic lens, which efficiently focusses particles with aerodynamic diameters between approximately 60 and 600 nm into a narrow beam. The lens opens into a vacuum-pumped chamber, where gas expansion accelerates particles to their size-dependent terminal velocity. Particles then transit a sizing chamber to be vaporized within an ionization chamber. Vaporization is achieved by a switchable 1064nm continuous-wave laser (SP-AMS mode) or by impaction upon a porous-tungsten thermal vaporizer held at 873K (600 ∘C; AMS mode).

In SP-AMS mode, 1064nm light-absorbing refractory PM (LR-PM) is heated to vaporization by the continuous-wave laser. Due to this heating, any internally mixed material which is refractory below the LR-PM vaporization temperature may also be vaporized. In this study, the LR-PM was rBC, which vaporizes at ∼ 4000 K , so that NR-PM (here mostly OM; ), ash, or BC surface functionalities are vaporized when internally mixed with rBC. NR-PM that is not internally mixed with rBC will pass through the SP-AMS laser and be vaporized on the AMS thermal vaporizer. In AMS mode, only NR-PM is vaporized. In both cases, the vapour is ionized by electron impact. The resulting ions are mass analyzed by pulsed extraction into an ion-time-of-flight chamber.

The SP-AMS can provide free-molecular-regime aerodynamic-size measurements by modulating the sampled particle beam with a mechanical “chopper” and monitoring the time taken for signals to be observed (particle time-of-flight, or PToF, mode). In this study, PToF data were accumulated over 15–20 s min-1 and were further averaged before analysis. For the remainder of each minute, mass spectrum (MS) mode data were acquired by removing the chopper entirely (“open” position) for 5 s of particle-beam measurements, then replacing it entirely (“closed” position) for 5 s of background measurements. The closed signals were subtracted from the corresponding open signals after data analysis (described below). Using a camera, the laser was observed to stabilize after roughly 5 s when switching on (going from AMS to SP-AMS mode) and almost instantly when switching off (vice versa). So, since each chopper cycle began with a closed measurement (no rBC), no additional wait time was specified when switching the laser on or off. The ion-time-of-flight chamber was operated in single-reflectron “V” mode with a resolution of 4200 at m/z 91. This mode was sufficient for the unambiguous distinction and determination of elemental composition for virtually all ions, with exceptions discussed below. In SP-AMS mode, the instrument was operated in two separate mass-spectral configurations, one of which extended to 1000 m/z to monitor for the fullerene-ion signals that have been previously observed . No fullerenic signals were observed, so these data are not presented. Therefore, in the data presented below, one SP-AMS measurement is available for every two AMS measurements.

Ion signals in the SP-AMS/AMS were identified and integrated using a modified version of the open-source PIKA software version 1.10H; as well as custom code written in Igor Pro (version 6.2 and 6.3; WaveMetrics, OR, USA). Two major modifications were made to PIKA. First, the robustness of the peak-shape and peak-width-calibration determinations were improved as described in the Supplement. Second, peak-integration uncertainties were estimated as discussed in and described briefly in Sect. .

PMF assumes that a matrix of data can be explained by a linear combination of factors with characteristic profiles and varying temporal contributions . The PMF model has been widely and successfully applied to AMS data . Applying PMF to AMS or SP-AMS data entails the assumption that the overall mass spectrum can be described by a small number of characteristic mass spectra , which is evaluated during PMF analysis by inspection of the model residuals .

PMF analysis was conducted using the PMF Evaluation Tool . Before analysis, signals were integrated and exported using the peak-integration approach and new uncertainty model described in the Supplement and in Sect. . Ash signals, defined as ions containing Cl, Si, K, or other metals, were excluded from the model. The number of PMF factors used, P, was chosen based on the degree to which the model improved with increased P, as evaluated by the structure of model residuals in temporal and m/z space. In general, increasing P beyond the values presented herein only served to better explain spikes in OM concentration at the start of each burn. Therefore, the conclusions discussed below are insensitive to the chosen P.

The large number of zeroes in this data set meant that ambiguity due to possible linear transformations of the PMF solution (“rotational ambiguity”) was negligible in this data set . Similarly, there was no evidence for local minima (see Supplement).

The large range of signals observed in the present study led PMF to report “unique factors” containing a single ion such as CO+ when using the old uncertainty model. This was corrected by the uncertainty model (Sect. ), which provides a more even weighting for high and low signals. We note that the linear uncertainty term in the model is similar to the ad hoc “C3” parameter of PMF labelled “model error” in PET. The difference is that the value of the linear uncertainty term was estimated from the data, and that it was added in quadrature rather than linearly to the pre-existing Poisson uncertainties, since its physical origin is independent of counting uncertainties .

The new uncertainty model also generally reduced signal-to-noise ratios (SNRs) such that previous outliers (in weighted-residual space) from major ions such as CO+ and C3+ became more comparable to other species. show that such outliers may arise purely from the omission of a linear uncertainty term when such an uncertainty exists. The outliers therefore do not indicate measurement errors, peak-integration errors, or transient signals. The new uncertainty model also increased the number of “weak” variables, defined as having SNR below 2 , which were downweighted by a factor of 2. No variables were “bad” in the sense of having SNR < 0.2 . Additional details on the PMF analysis, including residual plots, are available in the Supplement.

Figures  and  illustrate the temporal evolution of the fresh and aged emissions using selected SP-AMS and AMS marker ions. Also shown in the figure is the pre-dilution CO2(g) concentration, which is proportional to the combustion rate. The CO2(g) also indicates the airflow into the chimney, which implicitly dilutes the emissions to a varying degree. Following the addition of a batch of wood (black arrows along the abscissa), CO2(g) increases rapidly (“starting phase”) before reaching a relatively stable level (“flaming phase”) and finally dying off (“smouldering phase” with negligible emissions).

During the starting phase, the wood was only partially aflame, typically in only one region. Both fresh (Fig. ) and aged (Fig. ) OM emissions were highest during this phase. The degree of oxidation and emission factors of this OM has been reported in detail elsewhere .

The starting-phase OM emissions are illustrated by the C2H4O2+ signal in Fig. . This ion is correlated with cellulose/hemicellulose pyrolysis products such as levoglucosan and other anhydrosugars and its signal spiked when each batch of wood was added (black arrows on the abscissa) and died away thereafter. Similar trends were seen for the oxidized emissions (Fig. ), although the C2H4O2+ signal was much lower in that case. The SP-AMS and C2H4O2+ signals were not always well correlated, as discussed in Sect. .

The starting phase ended once flames had completely engulfed the wood. In this “flaming phase” most volatilized organics were destroyed in the flames such that emissions were comprised of little OM but significant amounts of refractory black carbon. This is illustrated in Figs.  and  by the C3+ ion, which in the SP-AMS represents rBC .

Black carbon was not the only SP-AMS species observed during the flaming phase. Signals from two other species, K+ and COx+ (=CO++CO2+), remained extremely high during this phase (Figs.  and ). These signals dropped to negligible levels in the AMS (dashed lines in Fig. ), confirming that they were generated from LR-PM particles. Although high K+ signals were only observed in the SP-AMS, relatively high COx+ signals were observed in both AMS and SP-AMS. These signals therefore appear to have originated from both OM and rBC.

The CO+:CO2+ ratio for these signals provided a useful metric of comparison between SP-AMS and AMS across burn periods. In the AMS, this ratio was 3.92±0.01 for fresh emissions but 1.22±0.01 for aged emissions . This latter ratio is in good agreement with observations of atmospherically oxidized OM 0.9–1.25,, indicating a relative increase of CO2+ due to the thermal degradation of oxidation-formed carboxylic acids or peroxides on the AMS vaporizer . In contrast, the phenols, alcohols, and aldehydes that are abundant in fresh wood are more likely to yield CO+ than CO2+ when vaporized, giving the high CO+:CO2+ ratio of 3.92.

The SP-AMS CO+:CO2+ ratio was not different from the AMS ratio during the starting phases (Figs.  and ) nor during the filtered and oxidized experiment (not shown). This suggests that OM fragmentation in the SP-AMS and AMS was comparable, at least in terms of COx+ fragments.

In contrast to the OM-dominated starting phase, the SP-AMS CO+:CO2+ ratio increased significantly during the flaming phase, increasing from ∼4 to ∼6 for the fresh case and from ∼1 to ∼2 for the oxidized case.

The discussion in Sect.  highlighted selected ions and mass spectra from selected time periods in the burn cycle. To generalize this discussion to the entire mass spectrum and the entire burn cycle, PMF was performed on the fresh and aged data. The utility of PMF was to reduce 399 measured ions in over 100 mass spectra to 3–4 “factors” with mass spectra and time series describing > 92 % of the variance in the data. (Over 97 % of the variance would have been explained had the failed-start burn been excluded.) These PMF factors are discussed below.

The combination of non-refractory AMS and refractory SP-AMS measurements of CO+ and CO2+ (Figs. c–d and c–d) clearly showed that the majority of these signals originated from refractory species (rCOx+). These refractory CO2+ signals became more intense after oxidative aging, indicating that they originated from incompletely oxidized species. The change in the ratio of the rCOx+ signals after aging indicated a change in the chemical species producing these signals.

These two observations reduce the likelihood of confounding sources of rCOx+ generating the majority of the signal. Carbonates such as CaCO3 cannot be oxidized as observed. Refractory organics such as large, oxidized polyaromatic hydrocarbons may generate COx+ but would also produce other, hydrogen-containing, ions in the mass spectrum. Additionally, if the refractory organics had not contained hydrogen, the observed increase in COx+ after aging would be impossible.

This increase in COx+ is consistent with soot formation mechanisms: soot only forms in oxygen-deprived environments and initially condenses as PAH-like nanoparticles before graphitizing via loss of hydrogen . Oxygen is normally gained during later oxidation, before exiting the flame, for example by flame-produced radicals like OH⚫ .

Since well over 90 % of soot produced in a flame is destroyed the abovementioned in-flame oxidation, all soot is expected to contain oxygenated surface groups . The BC surface functional groups are observed as COx+ and not, say, C3CO+ because BC surface groups thermally decompose at much lower temperatures than the vaporization temperature of BC .

We cannot completely rule out the role of adsorbed CO(g) or CO2(g) in forming rCOx+. This is an inherent weakness of the destructive SP-AMS technique relative to FTIR and NEXAFS. However, the significant change in COx+ upon oxidation indicates that such a role was not dominant. Additionally, if the thermal stability of covalently bonded or adsorbed species at the BC is similar, reaction mechanisms may be also be similar in both cases.

Two atmospherically relevant observations can be made regarding BC surface groups. First, the PMF results show that the relative amount of these surface groups can be regarded as constant throughout the burn. Regardless of burn stage, stove temperature (cf. first burn in Fig. ), or the concentration of emitted BC, a single PMF factor adequately represented these signals. It may therefore be possible to model the surface of beech-wood BC as a single chemical species. This single chemical species is likely to undergo complex chemistry given that laboratory-generated alkane soot samples display different regimes of reactivity and are sensitive to photochemistry . Soot surrogates such as the n-hexane soot recommended by the International Steering Committee on Black Carbon Reference Materials may therefore not provide an accurate representation of wood-combustion soot.

Second, the raw data, supported by PMF, show that the BC surface functionality changed considerably after aging. This is demonstrated by the change of CO+:CO2+ ratio after aging. Based on the PMF factors, this ratio decreased from 5.4 to 3.5 (a 54 % decrease). As mentioned above, these rCOx+ signals originate from the thermally driven desorption of different functional groups on the BC surface as CO(g) or CO2(g) . Whether CO(g) or CO2(g) desorbs is governed by the nature of the functional groups themselves. The mass of desorbed gases can be directly related to the mass of the initial functional groups . Although the exact path of decomposition is sensitive to the heating rate , the decrease of the CO+:CO2+ ratio after aging indicates an increased average oxidation of the BC surface groups. For example, the decreased ratio may have corresponded to the oxidation of phenolic or carbonyl groups at the BC surface . In addition to the increased average oxidation of the BC surface upon aging, the absolute quantity of BC surface groups changed. This can be considered in terms of the C3+:CO2+ ratio, which decreased from 3.5 to 2.4 (a 46 % decrease). This corresponds to an increase in the amount of functionalized carbon and indicates that the BC surface became more oxidized upon exposure to OH⚫ and O3. However, the data cannot be used to estimate the absolute oxygen content of the BC until the SP-AMS sensitivity to rCOx+ is established by future studies (Sect. ).

Our data represent the first time-resolved measurements of in situ BC surface aging. The SP-AMS may therefore be useful in the online measurement of BC surface groups in atmospheric studies. This would allow the competition between BC oxidation and organic oxidation to be investigated, although oxidant concentrations in this study were higher than would be expected in the atmosphere (Sect. ).

The majority of BC emissions always occurred in the flaming phase, during which organic concentrations were at their lowest in both the particle phase (Figs.  and ) and the gas phase . This is because on the one hand large amounts of organic vapours are yielded by pyrolysis (Sect. ) at temperatures lower than the ignition point of the fire, while on the other hand ignition triggers the flaming combustion which simultaneously generates soot and converts the emitted organics to CO(g) and CO2(g) . The organic vapours which are emitted by flaming-phase combustion appear may have followed trajectories that avoid the flames, given that their mass spectra resemble the aliphatic products of lignin pyrolysis ().

Given that most BC is emitted when little organics are emitted, and that BC was overall the major species emitted by this fire , the BC surface may represent a significant oxidant sink during the initial aging of a similar combustion plume. The actual significance of the BC in this case would depend on the degree of mixing between starting and flaming phases after emission and on the fate of co-emitted nitrogen and sulfur oxides.

It might be hypothesized that a functionalized BC particle would become more hygroscopic and therefore more likely to act as a CCN or ice nucleus. However, while our measurements are the first to directly observe the BC functionalizations in a wood-combustion aerosol, they do not directly affect the observations of previous studies on beech-wood soot which suggest a minor role in this regard . In the case of CCN, these and other studies have indicated that mixing of beech-combustion soot with other aerosols via condensation or coagulation is the most likely pathway for their becoming CCN active.

In general, our conclusions apply only to a well-operated stove and, moreover, only to wood stoves. Under different operating conditions, stove emissions may change considerably . Less-efficient combustion systems such as open burning may produce more primary as well as secondary OM . These higher organic emissions lead to a greater role of secondary OM in the evolution of particle hygroscopicity, which is also strongly dependent on the fuel .

In general, the SP-AMS and AMS mass spectra were highly similar. Excluding Cx+, the uncentred correlation coefficients (rUC) between the two POM–Start and POM–Flame mass spectra were 0.99 and 0.93, respectively. Excluding all ions below m/z 44 (to account for refractory COx+ signals and to reduce the influence of the highest signals) increased this rUC to 0.993 and 0.997 for POM–Start and POM–Flame, respectively.

The mass spectrum of POM–Start was similar to that of pure levoglucosan , as shown in Table . This is consistent with the fact that C2H4O2+ signals (m/z 60), commonly used as a tracer for biomass-burning pyrolysis products like levoglucosan , were almost entirely explained by POM–Start in the PMF model. POM–Start is thus interpreted as reflecting wood pyrolysis, in particular the pyrolysis of cellulose, hemicellulose, and other carbohydrates (“holocellulose”). Holocellulose pyrolysis produces levoglucosan and other anhydrosugars and occurs at appreciably lower temperatures than pyrolysis of the other major polymer in wood, lignin . This thermal instability, together with the fact that holocellulose comprises about 70 % of beech wood , explains why POM–Start was the more-abundant POM factor. The high CO+ signals associated with POM–Start (Fig. c) may therefore be explained as originating from the polyalcoholic sugars such as glucose and xylose which comprise holocellulose. The relatively low signal of CO2+ from POM–Start may be related to decarboxylation reactions within the stove during pyrolysis , similarly to the decarboxylation which produces CO2+ within the AMS and SP-AMS during vaporization .

An interesting feature of the POM–Start mass spectrum was the presence of aromatic ions such as C6H5+ (phenyl) and C7H7+ (benzyl). The fact that these ions originated from aromatic molecules was confirmed by their persistence in the OOM–Start mass spectrum due to the unusual stability of oxygenated aromatic molecules against fragmentation upon electron impact . These aromatics may have formed during pyrolysis or from the flame itself . Higher starting-phase signals of aromatics, including PAHs, have been observed by previous studies .

Since POM–Start was ascribed to holocellulose pyrolysis, it was hypothesized that POM–Flame may have been more closely associated with lignin pyrolysis, which generally requires higher temperatures . The alkyl fragments CnH2n-1+ and CnH2n+1+ seen in the POM–Flame mass spectrum (Fig. b) may be related to the cyclic aliphatic molecules, phytosterols, emitted together with substituted phenols during lignin pyrolysis .

The uncentred correlation coefficient between the mass spectra of pure burnt lignin and POM–Flame for the SP-AMS and AMS was relatively high (Table ), suggesting that this association was reasonable. However, the POM–Flame mass spectra were also well correlated with levoglucosan (Table ), suggesting that either POM–Flame was not clearly separated from POM–Start during factor analysis or that POM–Start contained contributions from both lignin and holocellulose. Both are likely to be true to some degree (the factor separation issue is further discussed in the Supplement). In particular, although holocellulose pyrolyzes at lower temperatures, both holocellulose and lignin pyrolyze across a range of overlapping temperatures , which would have led to a range of mass spectra being observed. Moreover, ash species such as potassium catalyze the pyrolysis process , so the pure-lignin mass spectrum used here is not an ideal reference.

A second, distinct hypothesis for the origin of POM–Flame is the in-flame synthesis of aliphatic functionalities, as has recently been observed by . This hypothesis is considered unlikely given the degree of oxygenation of POM–Flame.

The interpretation of the POM mass spectra as dominated by pyrolysis products provides insight into the relationship between the SP-AMS and AMS data, in particular the excellent correlation between the two sets of mass spectra in spite of the SP-AMS signals being frequently higher (Figs. b–c and b–d).

The excellent mass-spectral correlation is much better than expected given the possibility of different vaporization temperatures in SP-AMS and AMS (Sect. ). Differences in vaporization temperature have previously been invoked to explain fragmentation differences in the SP-AMS relative to the AMS for OM coatings of diesel-exhaust soot and of a branched-chain laboratory diester . Since fresh diesel-exhaust-soot coatings consist mostly of lubricating oil , both of these samples are chemically distinct from pyrolysis-generated OM. The similarity in AMS and SP-AMS pyrolysis-OM mass spectra may be due to fact that pyrolysis products have already undergone thermal bond rearrangement and dehydration reactions and are thus less likely to do so when heated in the AMS or SP-AMS.

The higher SP-AMS signals in the OM time series in Fig. b–c might be hypothesized to reflect a difference in the SP-AMS sensitivity to OM when it is internally mixed with BC due to a change in either vaporization temperature or physical position of the vaporized particle (Sect. ; ). However, this difference was observed even in the absence of rBC, as shown by the second fresh-emissions burn. In addition, the difference between SP-AMS and AMS was smaller for the aged-emissions experiment (Fig. b–c) than the fresh-emissions experiment (Fig. b–c). The apparent influence of aging is unlikely to be related to a change in mixing state: both aerodynamic- and mobility-size distributions were unimodal. A difference in particle focussing efficiency (Sect. ) also does not explain the difference, as aging would have increased the size of the particles and therefore focussed a larger fraction of them into the SP-AMS laser. The difference between fresh and aged samples may therefore reflect an influence of the chemical composition of the OM.

It is therefore hypothesized that the observed discrepancy was caused by 1064nm light-absorbing carbonaceous species other than rBC brown carbon;. Brown-carbon absorption would explain why the SP-AMS/AMS discrepancy was reduced after aging (Figs. b–c and b–c), since oxidation may reduce the conjugated or aromatic bonds required for light absorption. Lignin and its pyrolysis products contain the majority of the aromatic species from wood and is itself brown . The water-soluble component of aerosol from inefficient beech-wood combustion is also brown . Brown carbon would explain why the SP-AMS/AMS discrepancy in the aged experiment was highest for the anomalously high emissions of the second, failed-start burn (Fig. b and Fig. S2).

Potassium ions, K+, were observed as a dominant species in the SP-AMS mass spectrum but were negligible in the AMS. The SP-AMS therefore provides the possibility to specifically measure rBC-bound K+ with high sensitivity, allowing its use as an atmospheric tracer for biomass-combustion aerosols. The requirement of internal mixing with rBC would avoid interference from other atmospheric sources of potassium like dust, vegetative debris, and sea salt , although a minor contribution to K+ from vehicular emissions may be expected .

As discussed in Sect. , H2O+ quantification in the SP-AMS is routinely confounded by background signals from water vapour and particulate water. The size-resolving PToF mode of the SP-AMS was used to separate these background signals from particulate signals, which was only possible when mass loadings were sufficiently high (Fig. S8). The particulate H2O+ signals were virtually negligible in the AMS but extremely high in the SP-AMS: a factor of 40 higher than C3+ (even after including the C3+ RIE; Sect. ) during the flaming phase. Unlike COx+, the H2O+ intensity did not change after aging and was constant (when normalized to C3+) to within 10 % between fresh and aged experiments.

The fact that the relative amount of H2O+ did not change after aging suggests that the dominant source was not surface functional groups . Rather, the large amount of H2O+ observed and its thermal stability in the AMS point towards an origin of liquid water adsorbed in pores on the BC itself . Such water may have been produced during combustion or evaporated from the logs, which had a moisture content of 12–20 % according to Swiss testing standards. Such adsorbed water is known to be thermally stable and does not evaporate easily due to the inverse Kelvin effect .

has proposed that the nucleation of ice on soot particles which are not immersed in water droplets (deposition-mode nucleation) is governed by such in-pore water. In a study of deposition-mode ice nucleation on beech-wood soot produced by a stove similar to ours, as well as diesel soot, found that oxidative aging did not influence the ice-nucleating activity of the soot particles, although the activity of the diesel and wood soot particles was different. Our observation that the H2O+ signal was unchanged upon aging is consistent with those results and suggests that the SP-AMS H2O+ signals may relate to the ice-nucleating potential of a given soot sample. More work is needed to explore this hypothesis.