Aerosols are ubiquitous in the atmosphere and are key components of the climate system (Pöschl, 2005; Stevens and Feingold, 2009). They present a large uncertainty when it comes to predicting their effect on the global climate (Boucher et al., 2013). Aerosols can act as pollutants and affect air quality and human health, especially when considering the urban environment (Chan and Yao, 2008; Guarnieri and Balmes, 2014). A large proportion of atmospheric aerosols are organic, (Jimenez et al., 2009) some of which are surface-active (Cheng et al., 2004). Unsaturated fatty acids are a major class of surface-active organic compounds found in the atmosphere, with oleic acid (18-carbon backbone) as a widely studied example (Gallimore et al., 2017; King et al., 2010; Zahardis and Petrucci, 2007). Sources of atmospheric oleic acid include marine (Fu et al., 2013; Osterroht, 1993) and cooking emissions (Allan et al., 2010; Alves et al., 2020; Ots et al., 2016; Vicente et al., 2018; Zhao et al., 2015). The reaction of organic compounds with the key initiators of atmospheric oxidation – hydroxyl radicals (OH), nitrate radicals (NO3) and ozone (O3) – is an important factor in the evolution of these aerosols (Estillore et al., 2016). Oleic acid, along with the other unsaturated fatty acids, can be oxidised by these species, and its reaction with O3 in particular is well studied and has made it the model system for both theoretical and experimental studies (Gallimore et al., 2017; King et al., 2004, 2009; Last et al., 2009; Milsom et al., 2021a; Morris et al., 2002; Pfrang et al., 2017; Schwier et al., 2011; Shiraiwa et al., 2010; Zahardis and Petrucci, 2007).

The phase state of organic aerosols can vary significantly and has been identified as an important factor in determining atmospheric lifetimes (Shiraiwa et al., 2017; Slade et al., 2019; Virtanen et al., 2010), with particle viscosity being a key property (Reid et al., 2018). Indeed, the chemical lifetime of an organic species in the atmosphere could increase from seconds to days due to temperature- and humidity-induced changes in particle viscosity and the diffusion coefficient of molecules through the particle (Shiraiwa et al., 2010, 2011a). Glassy and semi-solid states of organic aerosols have been postulated, and studies have shown that a phase transition causes a drastic change in physical properties such as the uptake of water and reactive gases (Berkemeier et al., 2016; Knopf et al., 2005; Koop et al., 2011; Mikhailov et al., 2009; Zobrist et al., 2011). Diffusion gradients may arise within a viscous organic aerosol particle being exposed to humidity changes and are due to the kinetic limitation of water uptake and loss to and from the viscous particle (Bastelberger et al., 2018; Zobrist et al., 2011). The long equilibration times for these viscous aerosols imply similarly long evolutions in key aerosol properties. Oleic acid, harmful polycyclic aromatic hydrocarbons (PAHs) and phthalates have been identified in marine aerosols that have been heavily influenced by urban emissions (Kang et al., 2017), suggesting that long-range transport of these molecules does happen. PAH reactivity has been shown to be strongly affected by interactions with particle surfaces (Chu et al., 2010). It has also been indicated that coatings of organic aerosol shield PAHs, increasing their ability to be transported and cause harm, and this has been linked to the phase state of the aerosol (Mu et al., 2018; Shrivastava et al., 2017).

As a surface-active molecule, oleic acid is able to form, in contact with water, complex self-assembled structures such as organogels (in organic solvents) (Nikiforidis et al., 2015), vesicles (Blöchliger et al., 1998) and even helices (Ishimaru et al., 2005). Mixed with its sodium salt (sodium oleate) and water, oleic acid can also form lyotropic liquid crystalline (LLC) phases (Tiddy, 1980). These phases bring with them a range of different physical properties such as directionally dependent water diffusion, viscosity differences and different optical characteristics. The LLC-phase behaviour of oleic acid–sodium oleate has been extensively studied in a biological and cosmetic context and has been shown to have a diverse set of accessible phases, ranging from a simple micellar solution through to hexagonal arrays of water channels formed by cylindrical assemblies of the fatty acid (Engblom et al., 1995; Mele et al., 2018; Seddon et al., 1990).

Fatty acids, mixed with their fatty acid soap (salt), can form another set of unique structures called acid–soap complexes (Lynch, 1997). These complexes result from the strong hydrogen bonding between the carboxylate head group of the soap and the carboxylic acid group of the fatty acid and the interactions between the fatty acid alkyl chains. They are stoichiometrically discrete compounds. For the palmitic acid–sodium palmitate acid–soap complex, the sodium ion is shared between adjacent carboxylate anions. Carboxylic acid groups are associated mainly via hydrogen bonding to the carboxylate anions (Lynch et al., 2002). Key properties of acid–soap complexes include (i) crystallinity (in an atmospheric context this is important; as discussed previously there is a strong link between phase state and the atmospheric properties of an aerosol particle), (ii) unique hydrogen bonding exhibited by their distinct infrared (IR) spectra (compared to their constituent parts), and (iii) ordered alkyl chain packing deduced spectroscopically and using X-ray techniques such as small-angle and wide-angle X-ray scattering (SAXS and WAXS). They are also known to form a range of LLC phases upon addition of water, further demonstrating the versatile nature of the oleic acid–sodium oleate system (Cistola et al., 1986).

Previous work has demonstrated that these LLC phases were present in a levitated unsaturated fatty acid aerosol proxy (Seddon et al., 2016) and that self-assembly drastically reduces the oleic acid ozonolysis reaction rate (Pfrang et al., 2017). The formation of one of these phases (lamellar phase) was found to decrease the ozonolysis reaction rate by ca. an order of magnitude (Milsom et al., 2021a).

In this work, the importance of the oleic acid–sodium oleate acid–soap complex in atmospheric conditions is investigated. This complex has previously been studied in a biological context (Ananthapadmanabhan and Somasundaran, 1988; Tandon et al., 2001). The hydrocarbon chain and head group of the fatty acid can be characterised by complementary Raman and IR spectroscopy: the acid–soap complex has characteristic peaks in both the IR and Raman spectra, allowing confirmation of the structure of the complex (Lynch et al., 1996; Tandon et al., 2001). SAXS and WAXS have also been used to confirm the lamellar packing of the acid–soap complex and to reveal the sub-cell packing arrangement of the alkyl chains unique to the acid–soap complex (Tandon et al., 2001). We also employed polarising optical microscopy (POM) in order to visualise structural changes with temperature and humidity.

The oleic acid–sodium oleate acid–soap complex is studied in acoustically levitated droplets and analysed by simultaneous SAXS–WAXS and Raman microscopy. Ozonolysis is followed by Raman, while the effect of oxidative ageing on self-assembly is investigated using SAXS–WAXS. We first carried out a detailed structural characterisation of the acid–soap complex to confirm its presence in the levitated particles. We then probed the effects of exposure to humidity and ozone. We employed a micrometre-sized X-ray beam (16 µm × 12 µm) available on the I22 beamline at the Diamond Light Source (UK) to follow structural changes throughout the particle during controlled humidity changes and atmospheric ageing. This enabled us to build a spatially resolved SAXS–WAXS picture of the particle as a function of time exposed to humidity and ozone. Using this technique, we observed the emergence of a diffusion gradient across the humidifying and dehumidifying acid–soap particle and described this effect. We also investigated if exposure to ozone destroys the acid–soap structure and if the crystalline structure affects the reaction kinetics, drawing atmospheric implications.

A 1:1 wt ratio of oleic acid and sodium oleate was chosen to afford a system with a molar excess (7.8 %) of oleic acid, simulating the acidic nature of urban (Zhang et al., 2007) and marine (Keene et al., 2004) aerosols.

The acid–soap complex reported here is a crystalline organic material that is made up of surface-active molecules. The observations in this study suggest that water uptake is in line with the crystalline water uptake model described in the literature (Koop et al., 2011; Mikhailov et al., 2009). The addition of water gradually broke down the acid–soap complex and created a majority inverse micellar phase initially on the outside of the particle, then throughout the particle by the end of the experiment. This inverse micellar phase will have a different bulk diffusion coefficient compared to the crystalline solid (Shiraiwa et al., 2011a). Our results suggest that water uptake into these separate crystalline and liquid crystalline phases is markedly different due to viscosity-related differences in diffusivity. Equilibration times are longer than what would be expected for a well-mixed liquid particle, explaining the observation of a water diffusion and particle-phase gradient throughout the humidifying and dehumidifying particle (Fig. 1c–l). A similar trend is expected for other atmospheric trace gases such as ozone, implying that the reactive lifetime of this aerosol would be affected by water uptake. These observations were backed up by the use of a simple model of water uptake and loss, which revealed that water diffusivity is reduced ∼ 33-fold in the crystalline lamellar phase compared with the inverse micellar phase.

Previous work has not considered the formation of inverse micelles, though there has been a theoretical study on aggregate formation in organic aerosols where the formation of micelles was postulated (Tabazadeh, 2005). However, recent atmospheric (Pfrang et al., 2017) and biological (Mele et al., 2018) studies into the oleic acid–sodium oleate–water self-assembled system have shown that the inverse micellar phase can form for this system due to a significant non-polar “oily” fraction in the mixture (i.e. oleic acid). This is because the majority of head groups in the mixture are protonated (i.e. uncharged). The larger hydrophobic tail region of the oleic acid molecule compared with its hydrophilic head drives interface curvature towards water and therefore the formation of inverse phases, even in excess water. Inverted micellar and other inverted topology phases have been observed for oleic acid–sodium oleate mixtures in excess water (Seddon et al., 1990). Normal topology micelles (polar head groups at the micelle surface) form in systems with larger and charged headgroups and are only observed within the sodium oleate–oleic acid system at high sodium oleate content (> 80 wt %) (Seddon et al., 1990). It is therefore likely that the micellar phase observed in this system has an inverse rather than normal topology, as suggested in the preceding atmospheric literature (see Fig. 3 for a cartoon representation). Further addition of water to this phase would decrease the oil fraction in the mixture to a point where the water cavities, formed inside the inverse micelles, transform into hexagonal arrays of cylindrical water channels (Lisiecki et al., 1999). Our week-long humidity experiments, which revealed this inverse hexagonal phase, suggest that the inverse micellar phase observed in these levitated particles is a transient phase on its way to becoming the inverse hexagonal phase observed under the polarising microscope – consistent with the excess water phase observed in bulk mixtures with water of the same organic composition (Figs. S9 and S12, which includes a cartoon of the inverse hexagonal phase). These phases are known to have differing physical characteristics such as viscosities (Mezzenga et al., 2005; Tiddy, 1980), which have a significant effect on diffusion through the particle phase, affecting reactive gas and water uptake and the rate of these processes (Koop et al., 2011; Marshall et al., 2016; Mikhailov et al., 2009; Reid et al., 2018; Shiraiwa et al., 2011).

There are implications for the atmospheric lifetime of such particles in locations where oleic acid is likely to be found, such as in cooking emissions in urban areas of the UK (Allan et al., 2010) and China (Zhao et al., 2015) and even emissions from a university cafeteria (Alves et al., 2020). We have shown that the dry acid–soap complex is much more stable towards ozonolysis than liquid oleic acid (Fig. 2i), having an ∼ 80 % lower reactivity. If humidity-dependent phase changes take ca. a few hours to occur (∼ 4 h for the large particle studied here), viscosity and diffusion through the particle would also change on a similar timescale. Reactive uptake of oxidants such as ozone would therefore evolve slowly, resulting in a varying particle lifetime in the atmosphere that is dependent on its surrounding humidity – one of the uncertainties that need to be considered in atmospheric models (Abbatt et al., 2012; Gallimore et al., 2011; Liao et al., 2004).

Ozonolysis eventually breaks down the acid–soap complex. The reaction was stopped after 6+ h (400 min) exposure to a high ozone concentration (Fig. 2i); 34 ± 8 % of the double bonds remain in the particle, as demonstrated by simultaneous Raman microscopy, while the acid–soap structure was no longer observable by SAXS from ∼ 300 min onwards, with 59 ± 8 % of the double bonds still remaining at that point. Atmospheric ageing thus changes the relative amounts of oleic acid and sodium oleate in the particle, limiting the ability of the acid–soap to remain stable and slowly breaking down the structure. The fact that the reaction remains slow after the acid–soap complex has disappeared indicates that the diffusivity of oleic acid and ozone continues to be severely impeded, thus maintaining the low reaction rate.

The aged particle is a more complicated mixture of reactants and products, some of which could be surface-active and affect the phases observed within the particle. The presence of surface-active organic material in atmospheric aerosols is known to affect aerosol hygroscopicity by affecting properties such as the surface tension of aqueous droplets (Bzdek et al., 2020; Facchini et al., 1999, 2000; Ovadnevaite et al., 2017). Two of the primary products of oleic acid ozonolysis have been reported to be surface-active: azelaic acid (Tuckermann, 2007) and nonanoic acid (King et al., 2009). If a significant portion of oleic acid remains in the particle as we have reported, it is likely that some of these surface-active products also remain in the viscous particle, although formation of high-molecular-weight products derived from these molecules would act as a sink (Zahardis et al., 2006). This implies that in addition to the preservation of oleic acid in the particle, other surface-active products could also be preserved. Increasing the residence time of these surfactant molecules could have significant effects on cloud formation (Facchini et al., 1999; Prisle et al., 2012). These findings are also consistent with organic residues observed after oxidation of unsaturated organic films at the air–water interface, with similar implications (King et al., 2009; Pfrang et al., 2014; Sebastiani et al., 2018; Woden et al., 2018, 2021).

Products from oleic acid ozonolysis are known to take part in oligomerisation reactions with Criegee intermediates (Lee et al., 2012; Reynolds et al., 2006; Wang et al., 2016; Zahardis et al., 2006). These high-molecular-weight species are likely to have different physical characteristics, e.g. lower bulk diffusion coefficients and higher viscosities, compared to their precursors and other components in the particle. The higher-molecular-weight products may therefore impede reactant diffusion and act as a “crust”, as proposed in previous modelling studies (Pfrang et al., 2011; Zhou et al., 2019). This is consistent with the continued slow ozonolysis after the acid–soap complex degradation we report here.

We suggest that there is a phase separation between viscous products and unreacted oleic acid at the end of the ozonolysis experiment, as suggested in the schematic in Fig. 3. This separation is in the form of a core–shell arrangement where the shell consists of a majority high-molecular-weight, viscous product phase (evidenced by the low-q scattering observed in the oxidised particle; Figs. 2g and S8), and the core is a majority unreacted disordered oleic acid phase. This is opposite to the viscous core–less viscous shell behaviour observed during humidification. Unlike the core–shell behaviour observed during humidification, where both phases are observable by SAXS, the unreacted (and disordered) oleic acid is not detectable by SAXS. However, simultaneous WAXS has enabled us to determine that unreacted free oleic acid (with a characteristic WAXS peak; Fig. S11) does exist both in the core and the shell of the particle at the end of ozonolysis (Fig. 2h). The fact that (i) the Raman laser was focussed on the bulk of the sample, showing only disordered oleic acid by the end of the reaction, and (ii) ozonolysis does not speed up after the acid–soap SAXS signal has gone means it is most likely that a core–shell morphology prevails and that the reaction becomes limited by this inert shell (Pfrang et al., 2011). This also fits with the previous suggestion that viscous and large particles are not well mixed and that the reaction occurs primarily in the surface layers of the particle (Moise and Rudich, 2002). The boundary between these phases is not thought to be as distinct as in the core–shell system observed during humidification due to the presence of an oleic acid WAXS signal both in the centre and at the edge of the particle.

The phase state of organic aerosols has been shown to affect the chemistry and transport of harmful pollutants, such as polycyclic aromatic hydrocarbons (PAHs) (Mu et al., 2018). A coating of organic material on an aerosol particle has been used to explain the long-range atmospheric transport of toxic PAHs, which are products of combustion, increasing lung cancer risks (Shrivastava et al., 2017). The authors stated that the shielding was viscosity-dependent, therefore depending on the phase of the organic layer. In this study, atmospheric processing of the proxy (whether by humidity change or oxidation) significantly affects the phase of the particle by either changing the 3-D molecular arrangement or destroying the self-assembly altogether. Additionally, an inert crust layer formed as a result of oxidation may contribute to the shielding effect. As oleic acid has been observed in the urban environment, viscous oleic-acid-derived phases (including the acid–soap complex) could contribute to the effect organic films have on the shielding and transport of pollutants and thus on public health. Indeed, oleic acid has been observed in PAH-containing marine organic aerosols with significant urban influences, suggesting that this shielding effect could contribute to the transport of such aerosols (Kang et al., 2017).

Temperature has been identified as a key factor in determining phase state, reactivity and atmospheric transport of a reactive aerosol species (Mu et al., 2018). The acid–soap complex is stable at room temperature (∼ 22 ∘C). Our POM temperature experiments also show that the acid–soap complex is thermally stable until degradation at ∼ 32 ∘C, i.e. in the atmospherically relevant range (Fig. S5), consistent with the literature (Tandon et al., 2001). This suggests that this crystalline phase could exist in warm and dry environments. Although temperature would affect key parameters such as reaction rate constants and reactive gas surface accommodation times, the observation of a crystalline phase of oleic acid up to ∼ 32 ∘C implies that reactant diffusion arguments made here are valid in many atmospheric temperature conditions.

Heterogeneous oleic acid ozonolysis has been reported to be a surface reaction with a small reacto-diffusive length of ∼ 10–20 nm (Mendez et al., 2014; Moise and Rudich, 2002; Morris et al., 2002; Smith et al., 2002). A study on a different solid (in this case frozen) form of oleic acid showed that the reactive uptake of ozone is significantly decreased due to its solidity; the reaction happens only at the surface, and diffusion to and from the near-surface and bulk layers is severely impeded (Moise and Rudich, 2002). The same argument can be applied to the acid–soap complex, which is stable at significantly higher temperatures than the frozen oleic acid investigated previously.

These observations emphasise the importance of the effect that solid and semi-solid species have on the viscosity and diffusion of reactants and therefore oxidation kinetics. Atmospheric lifetimes of organic aerosols can be significantly increased as a result of viscous-phase formation (Shiraiwa et al., 2010, 2011a; Virtanen et al., 2010). Here, in addition to previously reported liquid crystalline phases (Pfrang et al., 2017), a solid crystalline state of an unsaturated fatty acid aerosol proxy was oxidised, and results clearly show that the crystalline nature of the particle is the reason for the retardation of the reaction rate, which we estimate as being ∼ 80 % slower for the acid–soap complex compared to an oleic acid droplet of similar size.

Atmospheric particulate matter is not always well mixed, and its composition can be rather heterogeneous (Laskin et al., 2019). Fatty acids, including oleic acid, have been characterised on the surface of marine aerosols (Kirpes et al., 2019; Tervahattu et al., 2002, 2005). This therefore means that they can concentrate in a specific region of a particle, and we suggest that 1:1 acid soap complexes can exist in some regions in coexistence with other sections of different compositions. The fact that this acid–soap complex still forms at a molar excess of oleic acid shows that the sample does not need to be at an exact 1:1 composition and that there is a composition window where it can form, where the 1:1 molar complex coexists with the excess component. Our results suggest that it is possible to form such a phase at room humidity (∼ 50 % RH). Formation of this phase could contribute to an explanation for the extended lifetime of oleic acid in the atmosphere compared with laboratory investigations (Robinson et al., 2006; Rudich et al., 2007).

As pH is variable in atmospheric aerosols (Paglione et al., 2021), so too would the ratio of oleic acid and sodium oleate be. SAXS data from a 2:1 wt (oleic acid : sodium oleate) levitated particle, representing more acidic conditions, demonstrate that an inverse micellar phase forms at ∼ 50 % H (Fig. S14). Where exactly in the phase diagram this transition would occur is difficult to determine, requiring small changes in composition not practicable for a beamline experiment. This observation shows that a change in aerosol pH could affect particle viscosity via a change in nanostructure. The addition of other molecules does not prevent self-assembled phase formation, and composition-dependent phase changes have been qualitatively observed (Pfrang et al., 2017) and are explored in follow-on work.

We are now able to spatially resolve SAXS–WAXS patterns through an acoustically levitated particle during humidity changes, revealing structural and physical changes as a result. This has provided a droplet-level picture of a diffusion front forming in a humidifying particle whereby an inverse micellar phase starts to form on the outside of the particle before eventually forming the dominant particle phase. This is in line with literature observations of diffusion fronts in highly viscous aerosol particles (Bastelberger et al., 2018; Zobrist et al., 2011). If diffusion varies across a particle, it follows that the diffusion coefficients of atmospheric trace gases would also vary throughout the particle. Some gases, such as ozone, are more soluble in hydrophobic than hydrophilic solvents (Panich and Ershov, 2019). The inverse micellar phase is a “water-in-oil” phase where pockets of water are enclosed by surfactant molecules, with their hydrophobic chains forming the majority of the hydrophobic “oil” domain (see Fig. 3). This means that ozone uptake is expected to increase upon inverse micelle formation for two reasons: (i) the phase is less viscous than the solid acid–soap complex, increasing the rate of ozone dissolution, and (ii) the majority of the particle is hydrophobic (water-in-oil), suggesting that ozone is more likely to dissolve and diffuse through the hydrophobic region of the particle rather than being constrained inside pockets of water.

The oleic acid–sodium oleate acid–soap complex has been identified in an unsaturated fatty acid aerosol proxy. Raman and IR spectroscopy, along with SAXS–WAXS, were used to confirm the formation of the acid–soap complex in acoustically levitated particles. The acid–soap complex was also identified by Raman microscopy on microscope slide deposits.

We observed a clear phase gradient in a humidifying levitated acid–soap complex consistent with impeded diffusion of water through highly viscous (in this case, solid and semi-solid) aerosols (Koop et al., 2011; Mikhailov et al., 2009; Pfrang et al., 2011; Reid et al., 2018). This is the first time that a spatially resolved phase gradient throughout a levitated humidifying aerosol particle has been reported using an X-ray-based technique. While we are unable to quantify the viscosity of the particle directly, it is possible to probe changes at the molecular organisational level, identifying clear differences in the way oleic acid moieties organise themselves during atmospheric processing. Preceding literature has reported mapped viscosity changes across a humidifying and oxidising aerosol particle using a fluorescence-based technique, fluorescence lifetime imaging microscopy (FLIM) (Hosny et al., 2013, 2016), with some experiments on optically levitated particles (Athanasiadis et al., 2016; Fitzgerald et al., 2016). An advantage of an X-ray-based technique in comparison to FLIM is that there is no need to add a molecular marker to the sample in order to measure physical changes. This is especially important when considering self-assembled systems as adding other molecules to the system is likely to change the self-assembled structure (Salentinig et al., 2010). The SAXS–WAXS experiment can also be used on samples in any physical state found in the atmosphere, allowing for phase changes to be monitored under a variety of conditions. Signals in the WAXS pattern correlate with crystallinity for this system and also contain information about the packing of alkyl chains via characteristic scattering peaks (Figs. 1b and S13). We have also shown that it is possible to observe product formation via low-q scattering intensity increases, opening a new avenue of inquiry.

An aerosol particle will experience changes in humidity during its lifetime in the atmosphere. A gradual phase change throughout the humidifying acid–soap complex suggests that the particle may rarely be in a homogeneous state; rather, there would likely be humidity-dependent physical differences within the particle as its atmospheric environment is changing.

Week-long exposure to high humidity revealed the inverse hexagonal liquid crystal phase observed under the polarising microscope, correlating with the inverse hexagonal phase observed in bulk mixtures of oleic acid–sodium oleate with excess water. This is therefore believed to be the equilibrium phase at saturated humidity. Molecular diffusion through liquid crystal phases, such as the inverse hexagonal phase, can vary significantly and has been used for this reason with regard to drug delivery (Zabara and Mezzenga, 2014). An analogy can be drawn with the dissolution of atmospheric species, whereby uptake of atmospheric trace gases could vary significantly depending on the 3-D organisation of the surfactant molecules.

The present study demonstrates that oleic acid will have a longer atmospheric lifetime if incorporated in an acid–soap complex. Aerosols have significant impacts on urban air pollution with organic aerosol emissions as key components (Chan and Yao, 2008). In the UK, cooking organic aerosols have been estimated as an additional 10 % of anthropogenic PM2.5 emissions (Ots et al., 2016). As previously discussed, phase-dependent viscous organic coatings have been reported to shield harmful carcinogenic combustion products (Shrivastava et al., 2017). Inert organic layer formation, such as what was observed in this study, may therefore contribute to this shielding effect since a gas-phase oxidant cannot reach even highly reactive compounds in our particles.

The modelling of organic atmospheric aerosols and their effect on the climate is of great importance to the climate community (Jimenez et al., 2009; Kanakidou et al., 2005). If an aerosol has an extended atmospheric lifetime, it is more likely to affect the climate and urban environment. The physical-state-dependent reactivity of the unsaturated fatty acid aerosol proxy presented here suggests that its atmospheric lifetime is variable and that this variance is significant. The addition of the humidity-dependent phase changes observed in the levitated proxy further enhances the dynamic nature of this system, highlighting how physical state is of utmost importance when considering reactivity and atmospheric lifetimes.

In summary, we have shown that the acid–soap complex is formed in an unsaturated fatty acid atmospheric aerosol proxy. This acid–soap complex is stable under atmospheric conditions and can be formed from an inverse micellar phase as demonstrated by reversible phase changes during a humidification–dehumidification cycle. A core–shell effect was observed during this cycle, and phase-dependent diffusivity was estimated, with a ∼ 33-fold difference between crystalline core and liquid crystalline shell. The proxy's ozone reactivity reduces significantly in the acid–soap complex state, and this remained so even after the acid–soap complex broke down. Ozonolysis does not go to completion after the reaction was allowed to continue for ∼ 6 h at a high ozone concentration: 34 ± 8 % of initial oleic acid remains in the particle, and low-q scattering is observed in the SAXS pattern, suggesting that high-molecular-weight and oligomeric products are present, and these products exhibit some order. This is evidence for an inert crust formation, inhibiting particle reactivity and protecting surface-active molecules from ageing, with implications for aerosol processes such as cloud formation. This layer may also help protect toxic aerosol components such as PAHs, enabling them to travel farther. This study presents a novel way of obtaining a spatially resolved phase picture of single aerosol particles, with the addition of WAXS to the list of simultaneous experiments possible on an acoustically levitated particle. We continue to demonstrate the versatile nature of oleic acid as an unsaturated fatty acid aerosol proxy.