Collapse Mechanisms of Nascent and Aged Sea Spray Aerosol Proxy Films

Sea spray aerosol (SSA) is highly enriched in marine-derived organic compounds during seasons of high biological productivity, and saturated fatty acids comprise one of the most abundant classes of molecules. Fatty acids and other organic compounds form a film on SSA surfaces, and SSA particle surface-area-to-volume ratios are altered during aging in the marine boundary layer (MBL). To understand SSA surface organization and its role during dynamic atmospheric conditions, an SSA proxy fatty acid film and its individual components stearic acid (SA), palmitic acid (PA), and myristic acid (MA) are studied separately using surface pressure–area (Π − A) isotherms and Brewster angle microscopy (BAM). The films were spread on an aqueous NaCl subphase at pH 8.2, 5.6, and 2.0 to mimic nascent to aged SSA aqueous core composition in the MBL, respectively. We show that the individual fatty acid behavior differs from that of the SSA proxy film, and at nascent SSA pH the mixture yields a monolayer with intermediate rigidity that folds upon film compression to the collapse state. Acidification causes the SSA proxy film to become more rigid and form 3D nuclei. Our results reveal film morphology alterations, which are related to SSA reflectivity, throughout various stages of SSA aging and provide a better understanding of SSA impacts on climate.

The chemical diversity of nascent SSA is caused in part by the production mechanism and the biological productivity of the source seawater [1,[41][42][43][44].Organic matter that is transferred to SSA is derived from marine biota, primarily from phytoplankton blooms [45][46][47][48][49][50][51][52][53].Surface active material such as lipids, proteins, and polysaccharides [37,[54][55][56][57][58] partition to the air-sea interface where they can be encapsulated into bubbles and burst to form SSA particles between 10 nm and 5 µm in diameter [1,49,59].Supermicron SSA composition more closely resembles that of upper surface seawater, so larger SSA particles contain more oxygenated compounds and have a higher fraction of soluble organic material, including short-to medium-chain fatty acids.Submicron SSA composition tends to be enriched in aliphatic compounds, such as long-chain fatty acids [49,54,57,58,60,61].In both supermicron and submicron SSA, fatty acids of moderate aliphaticity are throught to form films on SSA surfaces [37,54,62,63].The 2D organization of these films modulate gas and water uptake into the aqueous bulk of the particle [22,23,36,52,64,65].As a result, experiments involving ocean-relevant SSA film proxies are important for better predictions of aerosol reactivity and nucleation capability in climate models.
During phytoplankton blooms, organic enrichment within SSA is increased such that the competition for space at the SSA surface is high due to the surface activity of the marine-derived organic compounds.Fatty acids, which represent some of the most surface active components in SSA, condense to accommodate for the high surface density of organic molecules at the interface.The ability of the film (i.e., monolayer) to condense and resist transformation from the 2D film into 3D structures is dependent upon the film rigidity which is modulated by lateral interactions between the fatty acid alkyl chains and by interactions between the underlying aqueous core and carboxylic acid head groups [66][67][68][69][70]. Shorter chain fatty acids in nascent SSA are expected to be more soluble [71,72] given that the particles have an initial seawater pH of ∼8.2.As a result, the longer fatty acids will dominate the lateral interactions in the SSA film, making the film more rigid due to the greater sum of dispersion interactions [66,73,74].Carboxylic acid head groups of long chain fatty acids (>C 14 ) are only partially deprotonated at high pH due to their relatively high surface pK a values [75,76].However, cation-head group interactions slightly moderate the pK a increase associated with increasing alkyl chain length [76].As SSA ages in the marine boundary layer (MBL), the particles interact with trace gases that lower the pH of the particle aqueous core.Consequently, shorter fatty acids are less soluble and more surface active, so the SSA film is fluidized [77][78][79][80].SSA film behavior can be represented by 2D Langmuir isotherms in which a fatty acid monolayer is compressed beginning at low surface coverage, and the monolayer surface pressure is measured with decreasing surface area [64].Film rigidity dictates the maximum monolayer surface pressure (Π c ) attainable before collapse into 3D structures [81][82][83][84][85], thereby changing the phase state of the film.
SSA proxy film collapse mechanisms are examined on aqueous NaCl subphases to elucidate changes in film morphology with particle aging in the MBL.A fatty acid mixture composed of myristic acid (C 14 , MA), palmitic acid (C 16 , PA), and stearic acid (C 18 , SA) at the molar ratio of 3 SA:4 PA:2 MA is studied to mimic the dominant surfactant composition in fine SSA (diameter ≤ 2.5 µm).PA and SA constitute about two-thirds of the saturated fatty acids in fine SSA, and MA is the third most abundant species [54].The aqueous core of nascent SSA is modeled by a 0.4 M NaCl subphase at pH 8.2 [86], and aged SSA is modeled with the aqueous NaCl subphase adjusted to pH 5.6 and 2.0 [87].Surface pressure-area (Π − A) isotherms are conducted to measure the SSA proxy film phase behavior, and Brewster angle microscopy (BAM) images of the SSA proxy film collapse are taken at each pH.Differences in monolayer phase behavior and collapse between the SSA proxy film and its individual components are observed, highlighting the importance of using more complex model systems to elucidate atmospherically-relevant SSA film behavior.Additionally, changes in SSA film collapse mechanisms as a function of pH are measured.From our results, we expect that subphase composition and pH will play a major role in modulating SSA film morphology which will then impact particle trace gas uptake and nucleation ability in the MBL.

Materials
Myristic acid (C 14 H 28 O 2 , MA, ≥99%, Sigma-Aldrich, Saint Louis, MO, USA), palmitic acid (C 16 H 32 O 2 , PA, ≥99%, Sigma-Aldrich), and stearic acid (C 18 H 36 O 2 , SA, ≥99%, Sigma-Aldrich) were used without further purification.Each fatty acid was dissolved in chloroform (HPLC Grade, Fisher Scientific, Fair Lawn, NJ, USA) at a concentration of 3 mM.Aliquots of the individual fatty acid solutions were mixed according to the molar ratio of 2 MA:4 PA:3 SA to produce the mixed lipid stock solution in duplicate.NaCl (Sodium chloride, 99+%, ACS reagent, Acr ōs Organics, Fair Lawn, NJ, USA) was baked at 650 • C in a furnace (Fisher Scientific Isotemp R Muffle Furnace, Dubuque, IA, USA) for at least 10 h to remove residual organics [88].The salt was dissolved in ultrapure water with a resistivity of 18.2 MΩ•cm (Milli-Q Advantage A10, EMD Millipore, Billerica, MA, USA) and with a pH of 5.6 because of acidification by atmospheric CO 2 .Acidic subphases were prepared at pH 2.0 via the addition of HCl (Hydrochloric Acid, TraceMetal TM Grade, Fisher Scientific).Only the 0.4 M NaCl solution could be adjusted to pH 8.2 via NaOH (Sodium Hydroxide Pellets, Mallinckrodt Analytical Reagent, Paris, KY, USA) addition.Water at pH 8.2 acidified rapidly over the course of an experiment, but the 0.4 M NaCl solution acidified more slowly.Therefore, the solution was initially prepared at pH 8.5 so that the pH would drop to 8.2 ± 0.1 while data was recorded.

Surface Pressure-Area Isotherms
Surface pressure-area (Π − A) isotherms were performed in triplicate on a Teflon Langmuir Mini-Micro trough (KSV NIMA, Biolin Scientific, Espoo, Finland, area 144.5 cm 2 ) and Delrin barriers (KSV NIMA).Both the trough and barriers were thoroughly cleaned with reagent alcohol (Histological Grade, Fisher Scientific, Fair Lawn, NJ, USA) and ultrapure water.Surface pressure was measured by the Wilhelmy plate method using a filter paper plate (Ashless Grade 41, Whatman, GE Healthcare, Chicago, IL, USA), and the paper plate was fully wetted prior to running an isotherm.The barriers were swept across the surface at the maximum compression speed (270 mm/min/barrier) to check for surface cleanliness, indicated by a surface pressure value ≤ 0.20 mN/m.A microsyringe (50 µL, Hamilton, Reno, NV, USA) was used to spread the lipid solution dropwise onto the aqueous subphase.Ten minutes were allowed for solvent evaporation, and then the monolayer was symmetrically compressed at a rate of 10 mm/minute (5 mm/min/barrier).All Π − A isotherms were conducted at 21.8 ± 0.5 • C and a relative humidity of 37 ± 6 %. (Relative humidity is not expected to change the monolayer collapse mechanism.)

Brewster Angle Microscopy (BAM)
Brewster angle microscope images were collected simultaneously with Π − A isotherms using a custom-built BAM setup similar to that described previously in the literature [89,90].Plane-polarized light at 543 nm was produced by a 1.5 mW He-Ne laser source (Meredith Instruments, Peoria, AZ, USA) with linear 500:1 polarization, and the p-polarized light was further purified by a Glan-laser calcite polarizer (Thorlabs, Newton, NJ, USA).The BAM was built on a goniometer and positioned along the Brewster angle of the air-water interface at 53.1 • .Reflected light was then sent through a 10X infinity-corrected super long working distance objective lens (CFI60 TU Plan EPI, Nikon Instruments, Melville, NY, USA).The magnified image was further collimated and focused using a tube lens (MXA22018, Nikon Instruments; focal length 200 mm) before being sent to the back-illuminated EM-CCD camera (iXon DV887-BV, Andor Technology USA, Concord, MA, USA; 512 × 512 active pixels, 16 µm × 16 µm pixel size).The BAM images were processed using ImageJ software (version 1.52a, National Institutes of Health, Bethesda, MD, USA) [91] and cropped from their original size of 8.2 × 8.2 mm to show only the region of highest resolution.

Nascent SSA Proxy Film Phase Behavior
Although cations tend to be enriched in SSA relative to their concentrations in seawater, Na + is not enriched [57,58].Consequently, 0.4 M NaCl was added to the Π − A isotherm subphase to model the Na + ion concentration in seawater in order to understand how the aqueous core of SSA impacts lipid organization in its organic coating [86,92].Additionally, the pH of the subphase was adjusted to mimic that of seawater at pH 8.2.Π − A isotherms of the SSA proxy film (2 MA: 4 PA:3 SA) and its individual fatty acid components on 0.4 M NaCl at pH 8.2 are shown in Figure 1 [86].A BAM image of the SSA proxy monolayer collapse is also displayed as an inset in Figure 1a.Fatty acid mixtures containing different chain lengths have been previously shown to be fully miscible [93], so the the SSA proxy film should be entirely homogeneous.However, isotherms of the individual fatty acids were measured to evaluate how their phase behavior differs from the SSA proxy film mixture.If differences are observed in phase behavior, this suggests that individual fatty acid monolayers do not fully capture MBL SSA surface organization and dynamics.The SSA proxy monolayer (Figure 1a) undergoes a 2D phase transition from a gas-tilted condensed (G-TC) coexistence phase to a tilted condensed (TC) phase at 24 Å 2 /molecule, known as the lift-off point [66,94,95].Upon further compression in the TC phase, the monolayer transitions into the untilted condensed (UC) phase at a surface pressure of 18 mN/m.The film reaches a maximum surface pressure around 70 mN/m.The existence of only one collapse pressure (Π c ) observed at ∼60 mN/m also suggests that the SSA proxy film is completely miscible.A monolayer containing immiscible components have different Π c points for each component, whereas miscible monolayers have a single Π c that is dependent upon the molar fraction of each component in the mixture [66,77].Such a high Π c indicates that the SSA film on model seawater is resistant to collapse.This phase behavior could be caused in part by Coulombic repulsion between the negatively-charged carboxylate moieties and by contact ion pair formation between the carboxylate and Na + cation [96].Alternatively, a surface pressure plateau could be the result of slow relaxation kinetics caused by strong cohesive forces between the lipids [70].Further insight into the intermolecular interactions contributing to the surface pressure plateau can be obtained from the individual fatty acid isotherms.
MA (Figure 1b) is the shortest of the long chain fatty acids, yielding a smaller sum of dispersion interactions between the lipids.As a result, MA films at the air-water interface are more disordered and pack less tightly [66,97,98].MA is partially soluble in water [71,72], and its rate of desorption increases with carboxylic acid headgroup deprotonation because of the strong electrostatic interactions between the carboxylate moiety and water molecules.MA is largely deprotonated at pH 8.2 based on the surface pK a value of 7.88 at 20 • C [75], so a stable monolayer could not be obtained for the model seawater subphase due to dissolution.Increasing the amount of fatty acid spread onto the air-water interface helped to overcome the diffusion-mediated desorption, but the mechanical forcing from lateral barrier compression during the Π − A isotherm experiment promoted desorption.Hence, the monolayer was not stable, so collapse could not be reached.
Both PA (Figure 1c) and SA (Figure 1d) achieve a surface pressure plateau at collapse around ∼67 mN/m on 0.4 M NaCl at pH 8.2.PA and SA should be partially deprotonated on this model seawater subphase given their surface pK a values of 8.34 at 20 • C and 9.89 at 20 • C, respectively [75].However, the lipids are largely insoluble because of the greater dispersion interactions between the alkyl chains which generates a more rigid film.The PA lift-off point was measured at 25.5 Å 2 /molecule, and SA lift-off occurred at 23.5 Å 2 /molecule.PA and SA underwent the tilted condensed-untilted condensed (TC-UC) phase transition at ∼20 mN/m.PA and SA Π − A isotherms are similar to the SSA proxy isotherm, suggesting that the mixture film dynamic behavior resembles that of the individual fatty acids.The SSA proxy lift-off point is intermediate between the PA and SA lift-off points, further supporting that the monolayer mixture is miscible.Miscible monolayers have phase transitions that are dependent upon the molar fraction of each component in the mixture; thus, the monolayer mixture lift-off occurs at a pressure between that of its individual components [66,77].While PA and SA monolayers are good models of SSA film behavior on 0.4 M NaCl at pH 8.2, the SSA proxy monolayer has a different isotherm slope at collapse which could indicate small differences in the collapse mechanisms.
SSA film rigidity partially governs the monolayer collapse mechanism into 3D structures.Fluid monolayers lose material by desorption into the aqueous subphase and have low Π c .Rigid or highly ordered monolayers tend to collapse by forming multilayer aggregates in the air phase or by desorption into the aqueous phase, and the Π c is often high.When the monolayer possesses intermediate rigidity, it can buckle and form protrusions into the subphase, known as a folding collapse.Defects in the monolayer act as folding nucleation sites, leading to random protrusions that coexist with the monolayer.Continuous compression after protrusion formation increases the fraction of monolayer in the folds relative to the flat regions such that a constant Π c is maintained.Monolayer folding is a reversible process unlike film desorption and fracturing, so the monolayer molecular organization is maintained throughout dynamic compression and expansion [82, 84,85,99,100].Changes in film morphology can affect SSA uptake of trace gases and water vapor in the MBL, so understanding SSA film collapse mechanisms is important for better predictions of SSA radiative effects and surface reactivity in climate models.
BAM was used to visualize the SSA proxy collapse structure that gives rise to the surface pressure plateau on 0.4 M NaCl at pH 8.2 (Figure 1a).Small white spots approximately 5-10 µm in diameter appeared in the condensed phase at surface pressures below the Π c (image not shown).These spots are indicative of monolayer-bound vesicle budding [101].At Π c , bright white lines began to encompass many of the budding vesicles, and further compression caused the lines to become more interconnected.The presence of lines suggest a folding collapse mechanism in which the budding vesicles are likely incorporated into the folds [85,[101][102][103].A surface pressure plateau is also indicative of monolayer folding.Film material is incorporated into the folds with compression, and the surface pressure rises only minimally because molecular organization within the film itself does not change.Instead, the surface pressure slowly rises as new folding nuclei form upon compression [85].This suggests that nascent SSA films are resistant to collapse into 3D structures and may be able to retain their molecular organization during dynamic compression.SSA particle shrinkage could then create an even greater aerosol surface-to-volume ratio due to film folding at the interface.

Effects of SSA Proxy Aqueous Composition on the Film Phase Behavior
In order to understand how SSA interfacial organization changes throughout SSA residence in the MBL, the 0.4 M NaCl aqueous subphase pH was lowered to 5.6 in the Π − A isotherm and BAM experiments (Figure 2).The SSA proxy monolayer lift-off point (Figure 2a) occurs at 25 Å 2 /molecule, and the film undergoes the TC-UC phase transition at ∼18 mN/m.A surface pressure plateau at collapse occurs at ∼64 mN/m.The Π c on 0.4 M NaCl at pH 5.6 is slightly lower than the Π c at pH 8.2 (Figure 1a), indicating that the monolayer begins to fold at a lower surface pressure at pH 5.6.BAM also suggests that part of the SSA proxy monolayer on 0.4 M NaCl at pH 5.6 folds at collapse (inset in Figure 1a).Small 3D nuclei begin to form in the UC phase just before the Π c , and some of these nuclei become connected by thin white lines upon further compression at collapse.Line structures are indicative of monolayer folding [85].However, a significant number of these small 3D nuclei do not coalesce into line structures, and the number of 3D nuclei increases with compression.The lower collapse pressure at pH 5.6 is likely caused by a fraction of more rigid film material being incorporated into vesicles that detach from the monolayer, meaning that less of the film is incorporated into folds [104].Changes in the SSA proxy film organization and fluidity on subphases with differing pH suggest that aerosol particle pH is important in modulating SSA film dynamics in the MBL.
While the SSA proxy monolayer folds at collapse, its individual fatty acid components do not fold on the same subphase conditions.MA (Figure 2b) lifts off around 41 Å 2 /molecule and enters a TC-UC coexistence phase [80,105,106] at ∼15 mN/m.The monolayer exists in the UC phase at surface pressures higher than 15 mN/m until the Π c is reached at ∼48 mN/m, and then the surface pressure drops with further compression.PA (Figure 2c) and SA (Figure 2d) isotherms also exhibit a similar collapse mechanism to MA with Π c values at ∼62 mN/m and ∼67 mN/m, respectively, and their surface pressures decrease after collapse.The other PA and SA phase transitions remain relatively unchanged in comparison to the individual fatty acid isotherms on 0.4 M NaCl at pH 8.2.The lift-off point is increased by 1 Å 2 /molecule for both PA and SA at pH 5.6, and the TC-UC phase transition is increased by ∼2-3 mN/m for both fatty acids, suggesting that the monolayer is less fluid on salt water at pH 5.6 than on model seawater.Both the Π − A isotherms and BAM images demonstrate significant differences between the SSA proxy film collapse on aqueous NaCl as compared to ultrapure water (Figure 3).A surface pressure plateau is observed at ∼64 mN/m on 0.4 M NaCl, whereas the Π c on H 2 O is ∼53 mN/m.Both line structures and 3D nuclei are observed in the BAM images on salt water, suggesting that NaCl introduces sufficient fluidity to promote a folding collapse mechanism.On water at pH 5.6 (Figure 2 inset), the SSA proxy film initially forms small 3D nuclei at a surface pressure less than the Π c .The 3D nuclei aggregate into larger, irregular structures upon reaching monolayer collapse.Both the formation of larger 3D aggregates and the decrease in surface pressure following further compression indicates that the SSA proxy monolayer on water fractionates upon reaching Π c .Although the monolayer is partially deprotonated at pH 5.6, the dispersion interactions between the alkyl chains are likely greater than the subphase-head group interactions.Consequently, the film is more rigid and readily breaks when the size and concentration of 3D nuclei becomes sufficiently large [70].Thus, a folding collapse is likely water mediated and promoted by contact ion pair interactions between the carboxylate head group and Na + ion which fluidize the monolayer sufficiently [96,107].At pH 5.6, the head groups are still deprotonated such that these ion pairs form easily, but the partial protonation likely decreases the magnitude of these subphase-head group interactions which increases the rigidity of the film.

Aged SSA Proxy Film Phase Behavior
When the SSA proxy film is completely protonated on 0.4 M NaCl at pH 2.0, the Π − A isotherm and BAM image indicate that the film no longer folds at collapse (Figure 4).The lift-off point is significantly higher at 28 Å 2 /molecule, caused by decreased MA solubility at low pH.Like the other SSA proxy isotherms, the TC-UC phase transition occurs around ∼20 mN/m.Upon monolayer collapse, the Π − A isotherm reaches a Π c of ∼58 mN/m before the surface pressure decreases.BAM images reveal that the film forms 3D nuclei 5-10 µm in diameter and that the nuclei do not aggregate even beyond the Π c .This collapse structure suggests that the monolayer is too rigid to fold due to weaker subphase-head group interactions, so the film collapses into 3D vesicles.Consequently, SSA acidification in the MBL is expected to induce significant changes in film organization, particularly during seasons of high biological productivity.

SSA Film Collapse Mechanisms and Their Atmospheric Implications
A comparison of the collapse pressures for the SSA proxy mixture and its individual fatty acid components on various subphase conditions is plotted in Figure 5. Resistance to a 2D to 3D transformation increases with increasing pH.The SSA proxy mixture on 0.4 M NaCl exhibits the highest Π c at pH 8.2, and the Π c steadily decreases with subphase acidification.However, the Π c of the SSA proxy film on ultrapure water at pH 5.6 is significantly lower than that of the film on salt water at the same pH, indicating that both salt and pH are important in stabilizing the 2D monolayer at the air-water interface.Additionally, the strength of the dispersion interactions between the alkyl chains of the fatty acids are important in controlling film rigidity.MA exhibited the most fluid monolayer because of the weaker dispersion interactions, albeit the monolayer was unstable.SA was highly rigid as shown by the nearly identical collapse surface pressures at both pH 5.6 and 8.2.PA and SA had collapse surface pressures close to that of the SSA proxy mixture, but the individual fatty acids alone were not able to completely reproduce the collapse behavior at all pH values.Therefore, while the lateral dispersion interactions are crucial in regulating film rigidity, the subphase-head group interactions modulated by pH and salt are the most important factors in determining the collapse mechanism for the SSA proxy film.Further studies are needed to evaluate how other marine-relevant salts and organic compounds impact the mechanical properties of the SSA proxy film.Probable film collapse mechanisms on SSA surfaces in the MBL are summarized in Figure 6.The organic coating of nascent SSA is likely to fold upon particle contraction during periods of high marine biological activity.As a result, nascent SSA is dynamic and retains its interfacial molecular organization.However, as the particle interacts with trace gases in the MBL and acidifies, the film becomes more rigid.Monolayer folding is still observed, but 3D nuclei also begin to form at the surface as the interfacial area is reduced.Folding ceases when the monolayer is fully protonated, and only the 3D nuclei are observed.This change in collapse indicates that the SSA film fractionates and undergoes significant reorganization at low pH.Monolayers on acidic subphases, such as the films on aged SSA, are less dynamic and less resistant to 3D transformations, so some film material is removed from the surface when the available area is too small.It is therefore reasonable to assert that interfacial reactivity and the nucleation ability of SSA particles change as the interfacial molecular environment is altered via pH and salt concentration, thereby yielding different impacts on climate upon aerosol aging in the

Figure 1 .
Figure 1.Π − A isotherms of the sea spray aerosol (SSA) proxy film and its individual components at pH 8.2: (a) SSA proxy film and a Brewster angle microscope (BAM) image of the monolayer collapse; (b) myristic acid (MA) spread at different amounts at the air-water interface; (c) palmitic acid (PA); and (d) stearic acid (SA).The BAM image scale bar is 50 µm (see Supplementary Materials).

Figure 2 .
Figure 2. Π − A isotherms of the SSA proxy film and its individual components at pH 5.6: (a) SSA proxy film and a BAM image of the monolayer collapse, (b) myristic acid (MA), (c) palmitic acid (PA), and (d) stearic acid (SA).The BAM image scale bar is 50 µm (see Supplementary Materials).

Figure 3 .
Figure 3. Π − A isotherm of the SSA proxy film on H 2 O at pH 5.6 and a BAM image of the monolayer collapse.The BAM image scale bar is 50 µm (see Supplementary Materials).

Figure 4 .
Figure 4. Π − A isotherm of the SSA proxy film on 0.4 M NaCl at pH 2.0 and a BAM image of the monolayer collapse.The BAM image scale bar is 50 µm (see Supplementary Materials).

Figure 5 .
Figure 5. Collapse pressures of the SSA proxy film and its individual fatty acid components plotted as a function of subphase pH (see Supplementary Materials).