Surrogate polydisperse BBA particles were generated by atomizing 1 wt % aqueous solutions of single-component particles LEV, MNC, KS, and particle mixtures of LEV : MNC : KS in 1:1:0, 0:1:1, 1:0:1, 1:1:1, and 1:0.03:0.3 mass ratios in a flow of ultra-high purity (UHP) N2 using a commercial atomizer (TSI Inc. model 3076). To simulate the partitioning of MNC from the gas phase to the particulate phase, first reagent MNC was heated (up to ∼ 70 ∘C) and volatilized, and then condensed onto KS seed particles. Growth of the KS seed particles by MNC condensation was achieved by gradually cooling the mixed MNC–KS flow downstream of the heating section before entering the flow reactor. The atomized particles were dried by passing the atomized flow through two diffusion dryers prior to entering the AFR. After exiting the AFR, the particles were subsequently dried in two additional diffusion dryers, where the overall sample flow RH ≤5 %, before the size analysis and CCN activity measurements. This second drying stage was included in the experimental setup because the derivation of κ requires knowledge of dry particle size. The dry particle size distribution was determined with a differential mobility analyzer (DMA, TSI Inc. model 3081) and a condensation particle counter (CPC, TSI Inc. model 3772), and sampled at a total flow rate of 1.3 standard liters per minute (standard L min-1). Number-weighted mean particle diameters, D‾p, for all of the particles investigated in this study ranged from ∼ 40 to 150 nm.

OH radicals were generated via O3 photolysis in the presence of water vapor in a 60 cm long and 5 cm inner diameter (i.d.) temperature-controlled Pyrex flow reactor as shown in Fig.  . O3 was produced by flowing 2–25 sccm (standard cubic centimeters per minute) of UHP O2 through an O3-producing lamp (Jelight model 600; emission wavelength λ=185 nm). O3 concentrations ranged from 250 ppb to 20 ppm and were monitored throughout the experiment using an O3 photometric analyzer (2B Technologies model 202), which sampled at ∼ 850 sccm. An O3 denuder containing a Carulite 200 catalyst was connected to the outlet of the AFR to convert O3 to O2 before entering the aerosol charge neutralizer and other sensitive instrumentation. A 50–600 sccm flow of UHP N2 was bubbled in a 500 mL Erlenmeyer flask filled with distilled/deionized Millipore water (resistivity >18.2 MΩ cm) to generate humidified conditions in the AFR. The RH for all of the experiments was measured with an RH probe (Vaisala model HM70) and varied from 30 to 45 %. The humidified and O3 flows were mixed in a 4.5 L glass vessel before entering with the particles into the AFR. The mixed N2/O2/O3/H2O and particle flow was then passed over a 60 cmO3-free quartz tube containing a 60 cm long mercury pen-ray lamp (λ>220 nm) to photolyze O3. The lamp was cooled with a flow of compressed air. Total flow rates in the flow reactor ranged from ∼ 2.2 to 3 standard L min-1, corresponding to a range in residence times of 26–39 s. Flows were laminar, with Reynolds numbers between 60 and 80. OH concentrations were determined by applying a photochemical box model validated based on isoprene loss measurements in the presence of OH as described previously . OH concentrations ranged from ∼ 0.2×1010 to 2×1010 moleculecm-3 and were varied by changing either RH or [O3]. As previous studies have indicated, neither UV light nor O3 introduction in this manner leads to particle degradation or a significant change in particle mass or chemistry . The temperature inside the flow reactor was maintained near 298 K by a cooling jacket. A slight temperature gradient of ∼ 3 ∘C from the leading edge of the sheath flow tube containing the lamp to the inner walls of the AFR was observed, but has no significant effect on [OH]. OH equivalent atmospheric exposures were determined from the product of the residence time in the AFR and applied [OH], which was then normalized to a daily averaged ambient [OH]=2×106 moleculecm-3. Using this method allowed varying atmospheric OH exposures equivalent to <1 day up to ∼ 1 week. At the given [OH], residence time, total pressure of 1 atm, and particle sizes, we assume that OH mass transfer to the particles is sufficiently fast to maximize the exposure. At 40 % RH, the reactive uptake coefficient, γ, of LEV+OH would be 0.65 for atmospheric OH concentrations . However, the presence of higher [OH] in the AFR decreases γ to ∼ 0.2 . OH diffusion impacts γ by only ∼ 7 % , implying that OH exposure is not diffusion limited. At RH >15 %, MNC is less reactive with OH, exhibiting γ<0.07 due to competitive co-adsorption of water and OH . Similar suppressions in gas uptake, due to competitive adsorption processes, have been observed in the case of OH and NO3 uptake by BBA surrogate films , O3 uptake by benzo[a]pyrene in the presence of water vapor , and O3 and NO2 uptake by benzo[a]pyrene-coated soot . The presence of higher [O3] may further decrease the OH reactivity of OA . Under the applied experimental conditions, the multiphase reaction kinetics involving highly viscous organic material are likely limited by surface–bulk exchange .

The CCNc and operating conditions are described in more detail in . CCN activity data were acquired following procedures similar to previous studies , whereby the dry particle diameter is scanned while keeping the CCN chamber supersaturation fixed. A more detailed description of this approach is given in . Briefly, particles first passed through a Kr-85 aerosol neutralizer (TSI 3077A), were size-selected using a DMA (TSI 3081), and processed in a CCNc (Droplet Measurement Technologies, Inc., single-column CCNc) , while in tandem the total particle concentration was measured with a CPC. The CCNc was operated at a 0.3 standard L min-1 total flow rate and 10:1 sheath-to-sample flow rate ratio. The total sample flow rate, which includes a 1 standard L min-1 CPC flow rate, was 1.3 standard L min-1, and a 10:1.3 sheath-to-sample flow rate ratio was applied for the DMA. The temperature gradient in the CCNc column was set by custom-programmed Labview software and operated at ΔT=6.5, 8, 10, and 12 K, corresponding to chamber supersaturations S=0.2, 0.27, 0.35, and 0.425 %, based on routine calibrations applying atomized ammonium sulfate particles. The temperature gradient was stepped successively, from 6.5 to 12 K and in reverse. Each temperature gradient was maintained for a total of 14 min to allow an up and down scan of the particle size distribution by the DMA. The aerosol size distributions and size-resolved CCN concentrations were acquired by applying an inversion method described in , which implicitly accounts for multiply charged particles. The ratio of the aerosol size distribution and CCN size distribution provided size-resolved CCN activated fractions (i.e., the fraction of particles that become CCN at a given supersaturation and particle size).

The hygroscopicity and CCN activity can be described by κ-Köhler theory (KT) , which relates dry and wet particle diameter to the particle's critical supersaturation (RH above 100 %, at which the particle grows to a cloud droplet size) based on a single hygroscopicity parameter, κ. In κ-Köhler theory, the water vapor saturation ratio over an aqueous solution droplet as a function of droplet diameter, S(D), is given by S(D)=D3-Dd3D3-Dd3(1-κ)exp⁡4σMwRTρwD, where D is wet particle diameter, Dd is dry particle diameter, σ is droplet surface tension, Mw is the molecular weight of water, R is the universal gas constant, T is temperature, and ρw is density of water. κ ranges typically from ∼ 0.5 to 1.4 for hygroscopic inorganic species and from ∼ 0.01 to 0.5 for less hygroscopic organic species; κ=0 represents an insoluble but wettable particle, and thus Eq. (1) reduces to the Kelvin equation .

An alternative, approximate expression for determining κ is given as follows : κ=4A327Dd3ln⁡2Sc, where

A=4σs/aMwRTρw. Sc represents the critical supersaturation, i.e., point of supersaturation where more than 50 % of the initial dry particles are activated to CCN. Here, we assume σs/a is equivalent to that of water. While aqueous solutions of LEV and KS exhibit surface tensions approximately equal to the surface tension of water , to our knowledge no previous surface tension measurements of MNC aqueous solutions have been made. Our assumption applying the surface tension of water at all OH exposures could result in an overestimation of κ since the presence of surface-active organics can decrease σs/a . We do not have surface tension data of the different mixtures applied in this study. However, we anticipate that increasing OH exposure may decrease σs/a, thus enhancing the particle's CCN activity as demonstrated in and .

Hygroscopic growth of compounds exhibiting moderate to weak solubility in water can be limited by their low water solubility , and thus cannot be treated as either fully dissolvable or insoluble substances. A theoretical treatment of κ, which includes solubility limitations, has been detailed in . Here, κ=εiκiH(xi)H(xi)=1if xi≥1xiif xi<1, where ε is the volume fraction of the solute i in the dry particle. κi is the theoretical κ of solute i in the absence of solubility limitations and is given by κi=νρimwρwmi, where ν is the Van't Hoff factor, ρi is the density of the solute, ρw is the density of water, mi is the molar mass of the solute, and mw is the molar mass of water. xi is defined as the dissolved volume fraction of the solute and given as xi=CiVwVi, where Ci is the water solubility of the solute, expressed as the solute volume per unit water volume at equilibrium with saturation, and Vi is the volume of the solute. For complete dissociation, xi is equal to unity. The parameters listed in Table 1 were used in predicting κ.

Exemplary activated fractions, i.e., fractions of initial dry particle sizes activated to CCN, for LEV, MNC, KS, and the ternary particle mixtures at a chamber supersaturation of 0.425 %, are shown in Fig. 2. The activated fraction curves were fit to a cumulative Gaussian distribution function as described in detail previously : f(x)=12erfcx2, where x=(Dd–Dd,50)/σD. In the fitting procedure, Dd is the dependent variable and Dd,50 and σD are adjustable parameters to minimize the root mean square error between f(x) and the data. Dd,50 is the dry diameter interpreted as being where 50 % of the dry particles have activated into cloud droplets, also referred to as the critical particle diameter, Dp,c.

KS particles exhibit the smallest particle activation diameter of ∼ 50 nm, followed by LEV particles at ∼ 75 nm, and MNC particles at ∼ 210 nm at S=0.425 %. In this study, κ is derived from Eq. (2), where S is evaluated at 0.2, 0.27, 0.35, and 0.425 % is used in place of Sc, and Dd is the determined Dp,c. At lower S, the activated fraction curves are shifted to larger sizes since the smaller particles do not activate at lower S.

Table 2 lists the derived κ values for all of the particle types employed in this study in comparison to literature values. The reported uncertainties in κ are ±1σ from the mean κ derived at each S. The derived κ values for LEV and KS are consistent with κ for LEV and KS given in the literature. The critical diameter of LEV (∼ 70 nm at S=0.425 %) is in good agreement with the critical diameter of LEV measured by at the same S. κ ranges from 0.149 (±0.008) to 0.176 (±0.009) for LEV over all S, in agreement with the humidified tandem DMA (HT-DMA) derived κ=0.165 . Within experimental uncertainty, κ for KS is in agreement with the value derived in , but exhibits a marginal increase as a function of S, possibly due to an increasing van't Hoff factor at higher S, similar to ammonium sulfate. To our knowledge, no previous hygroscopicity measurements of MNC have been made. κ ranges from 0.008 (±0.002) to 0.013 (±0.003) for MNC. However, κ could not be derived for MNC at S = 0.2%, given the applied aerosol size distribution. In other words, the Dp,c is likely too large, and the scanned particle size range is not sufficient to give the complete activation curve. As a result, the Dp,c could not be reliably derived. For comparison, humic-like substances (HULIS), which is known to contain nitrocatechols , exhibits a κ value of 0.05 . In addition, κ for NO3 oxidized oleic acid particles, comprising similar chemical functionalities as MNC (i.e., nitrogen oxides and conjugated double bonds), is ∼ 0.01 .

On average, κ for all of the binary and ternary mixed particles range from 0.111 (±0.010) to 0.373 (±0.034). Due to constraints in water uptake and water solubility, mixed particles comprising a significant fraction of MNC (i.e., 1:1:0, 0:1:1, 1:1:1) exhibit relatively lower κ than the 1:0:1 mixture. The 1:0.03:0.3 ternary-component particles exhibit a slightly lower κ compared to the other particle mixtures, due to the relatively low KS content. It is not entirely clear why the particles exhibit lower κ at S=0.2 %. This implies that either the supersaturation used to derive κ was artificially high, the van't Hoff factor is lower, or limitations in solubility are more pronounced at this lower supersaturation than at higher supersaturations. The calibration data applying ammonium sulfate particles yield a variability in the derived supersaturation at S=0.2 % of ∼ 5 %, which still results in less variability in the derived κ values at that supersaturation compared to what was observed.

As listed in Table 2, the derived κ values are reasonably predicted by applying the volume mixing rule : κ=κOrg×εOrg+κInorg(1-εOrg), where κOrg and κInorg are the κ values of the organic and inorganic particles, respectively, and εOrg is the organic volume fraction of the particles. However, the quality of the estimate depends on whether the effects of solubility are to be included. For example, applying the experimentally derived κ of MNC particles in the volume mixing rule results in a significant underprediction of κ for the 1:1:0, 0:1:1, and 1:1:1 particle mixtures. This deviation in κ suggests that water uptake by the pure-component MNC particles is mechanistically different than water uptake by the mixed particles, which contain a significant MNC volume fraction. The volume mixing rule is applicable over a range of mixtures and hygroscopicity. However, when the particles contain both soluble and sparingly soluble compounds, predicted κ can deviate significantly from derived κ . MNC is significantly less water-soluble than pure LEV and KS. During CCN activation, the most water-soluble component preferentially dissolves, enhancing the solute effect in the Köhler equation. Since water-soluble components in aerosol particles can retain greater liquid water content during water uptake, the less water-soluble component can more easily dissolve . This solubility constraint in the volume mixing rule is described in more detail in . As a result and depending on the volume fraction of the sparingly soluble compounds in the particle, the peak of the Köhler curve may occur at a sufficiently large droplet size when all compounds, including the sparingly soluble compounds, are completely dissolved. The same can be applied here for the mixtures containing an appreciable MNC volume fraction. The water solubility of MNC is approximated as C=0.003, which is categorized as sparingly water-soluble . To verify whether MNC behaves as if it is infinitely soluble in a solution with KS, Fig.  shows derived Köhler curves of pure MNC and mixtures containing variable KS volume fractions and the MNC dissolved fraction, xMNC. In Fig. , the critical supersaturation, i.e., the maximum in the Köhler curve, decreases with increasing KS volume fraction. Accordingly, the MNC dissolved fraction increases with an increasing KS volume fraction. At a KS volume fraction of ∼ 36 % (MNC volume fraction of ∼ 64 %) indicated by the orange curves in Fig. , the maximum in the Köhler curve corresponds to xMNC≈1, implying that CCN activation is not limited by MNC solubility. This MNC volume fraction corresponds to the 1:1 by mass MNC : KS particles, which suggests that for this particular mixture, MNC behaves as if there are no solubility limitations during CCN activation (i.e., infinitely soluble, equivalent to C=∞) and κ of MNC can be predicted using Eq. (6). This result is consistent with the 1:1:1 by mass LEV : MNC : KS particles. In the presence of LEV, alone, MNC remains slightly insoluble during CCN activation.

Figure  shows the predicted κ plotted against experimentally derived κ for all of the particle mixtures and S applied in this study. κ was predicted by applying the volume mixing rule under three different scenarios: (1) calculated from experimentally derived single-component κ (κ = limit; color-scale symbols), (2) calculated from Köhler theory (κ= KT; gray-scale symbols), and (3) predicted from Eq. (6), which assumes no solubility limitations (i.e., C=∞; horizontal lines). For all particle mixtures containing equal by mass MNC (i.e., 1:1:0, 0:1:1, and 1:1:1), applying the experimentally derived single-component κ underpredicts the experimentally derived κ of the mixtures. Note that when assuming solubility limitations, κ could not be predicted at S=0.2 % for the mixtures containing MNC since κ for MNC, alone, could not be derived experimentally at that supersaturation. As discussed previously, this underprediction is due to the enhancement in MNC water solubility when in the presence of water-soluble LEV and KS, which is not accounted for applying the experimentally derived single-component κ. Applying Köhler theory results in better agreement with the experimentally derived κ, particularly for the 1:1:0 particle mixture. Predicted κ assuming no solubility limitations results in an overprediction for the 1:1:0 particle mixture, but is in best agreement with the experimentally derived κ for all other mixtures. The ability to predict experimentally derived κ of the mixtures applying Köhler theory depends on the solubility and volume fractions of the different particle components, which have not all been measured. The solubility of MNC was estimated from the US Environmental Protection Agency's Estimation Program Interface (EPI) suite . Applying this estimated solubility results in an underprediction in the 0:1:1 and 1:1:1 κ, but is in best agreement for the 1:1:0 particle mixture. These results support the finding that the volume mixing rule is most accurate when accounting for the changes to water solubility when the components are mixed. In the presence of LEV, MNC remains slightly insoluble. However, in the presence of KS, MNC behaves as if there are no solubility limitations during CCN activation.

Surrogate single-component BBA particles were oxidized in the presence of O3 (mixing ratio, χO3 = 0.76–20 ppm) and in the presence of OH radicals (0.2×1010–2×1010 moleculecm-3), corresponding to <1 day up to ∼ 1 week of a 12 h daytime OH exposure at [OH]=2×106 moleculecm-3. κ as a function of O3 exposure is presented in the Supplement. Upon exposure to OH, both LEV and MNC particles exhibited significant chemical erosion due to molecular fragmentation and volatilization . Figure  shows the evolution of LEV and MNC particle volume in the presence of OH, V (Hg lamp on, with O3), normalized to the initial particle volume just before switching on the Hg lamp, V0 (Hg lamp off, with O3), as a function of OH exposure. Following OH exposure, the average decrease in particle volume for all OH exposures for LEV and MNC particles was 36 (±7) and 19 (±7) %, respectively. In general, OH exposure led to an increase in LEV modal particle diameter and a decrease in MNC modal particle diameter. The increase in LEV modal particle diameter in combination with a decrease in total particle volume suggests the smallest LEV particles experienced the most chemical erosion. Occasionally, a second smaller size mode developed following OH oxidation of pure MNC particles. While the exact mechanism for the formation of the smaller mode is not clear, we speculate that OH oxidation of gas-phase MNC could lead to in situ particle formation in the flow reactor. Particle size is not expected to alter κ directly unless a change in particle size coincides with a change in particle composition. Given that there are two different particle populations and presumably two different particle compositions following OH oxidation of MNC, the newly formed particles may affect the derived κ. Clearly, more careful control and study of the particle size distribution are needed to resolve the impacts of volatilization, but are beyond the scope of this study.

κ was determined as a function of OH exposure and S for the single-component organic particles LEV and MNC as shown in the top panels of Fig. . The bottom panels of Fig.  correspond to the critical particle diameter as a function of OH exposure. It should be noted that the critical particle diameter decreases with increasing S. For the same exposure, smaller particles become more oxidized due to their larger effective surface area to volume ratio. As demonstrated in Fig.  for both LEV and MNC, at a fixed OH exposure, the largest κ corresponds to the smallest critical particle diameter. While it is clear that κ depends on the particle size at a fixed OH exposure, we are interested in the resulting changes to κ due to increasing OH exposure at the applied S. For both LEV and MNC, the trend in κ as a function of OH exposure does not significantly deviate for the applied particle sizes. For LEV particles, κ at the lowest OH exposure is not significantly different to κ derived at the highest OH exposure. Conversely, MNC κ increases significantly from ∼ 0.01 to ∼ 0.1 with increasing OH exposure at all applied S.

The reactive uptake, condensed-phase reaction products, and volatilized reaction products resulting from heterogeneous OH oxidation of LEV are well documented . However, there are no direct measurements of its CCN activity following OH oxidation. showed that following OH exposure, particle volatilization accounts for a ∼ 20 % by-mass loss of LEV. This suggests that the majority of the reaction products, which include carboxylic and aldehydic species , remain in the condensed phase. Although volatilization due to high OH exposures has been linked to an increase in the critical supersaturation and thus suppression in the CCN activity of oxidized squalane particles , the results here suggest that, regardless of volatilization, the condensed-phase reaction products are just as or somewhat more active CCN than pure LEV. On average, there is only a slight increase in κ for LEV particles, with increasing OH exposure as indicated by the positive slope in the linear fit to the data at all applied S. Such an incremental enhancement in κ may be a result of similar κ between LEV and its oxidation products. The hygroscopicity of several carboxylic acids that may represent levoglucosan OH oxidation products, including malonic, glutaric, glutamic, succinic, and adipic acid, exhibits κ values between 0.088 and 0.248 , similar to κ of oxidized and pure LEV. Furthermore, the hygroscopicity of organic compounds containing hydroxyl functionalities similar to LEV or carboxylic groups are nearly equivalent . We also cannot rule out that volatilization, while reducing particle mass, also removes newly formed reaction products from the aerosol phase, leaving the parent organic (i.e., LEV) and thus κ unchanged.

The CCN activity of MNC aerosol particles increases with OH exposure as shown in the top right panel of Fig. . MNC becomes more CCN active with increasing OH exposure and κ transitions from ∼ 0.01 in the absence of OH to ∼ 0.1 for OH exposures equivalent to a few days in the atmosphere. Further exposure (≥4×1011 moleculescm-3s-1) does not significantly enhance MNC κ, which suggests that MNC or the particle surface is fully oxidized and that the reaction products reach a maximum in κ. Similar enhancements in κ and subsequent constant κ values with increasing OH exposure have been observed for organic aerosol with initially low hygroscopicity . For example, observed that κ of BES increased from ∼ 0.008 to ∼ 0.08 for an OH exposure of ∼ 1.5×1012 moleculecm-3 s, and κ of stearic acid increased from ∼ 0.004 to ∼ 0.04 due to an OH exposure of ∼ 7.5×1011 moleculecm-3 s.

The enhancement in MNC κ following OH exposure may be linked to the formation of more hydrophilic chemical functionalities. Strongly linked to enhancements in OA hygroscopicity are larger O : C ratios . Neglecting the oxygen atoms in the -nitro functionality of MNC , the O : C ratio of pure MNC is ∼ 0.29, close to the lower end in O : C where transitions from low κ to high κ typically occur . The presence of -methyl, unsaturated, and -nitro functionalities are also linked to low hygroscopicity . As proposed in and observed for other nitro-phenolic species, OH oxidation of MNC can favor removal of the -nitro functionality by electrophilic substitution of OH . OH substitution at the -methyl position and addition to the double bonds is also possible . OH addition to the -nitro or -methyl functionality would increase O : C to ∼ 0.43 or ∼ 0.5, respectively. OH substitution at both positions would enhance O : C to ∼ 0.67. showed that hydroxyl-dominated OA with an O : C of less than ∼ 0.3 has an apparent κ of ≤10-3. However, an increase in O : C to 0.4 or 0.6 due to the addition of hydroxyl, aldehydic, or carboxylic functionalities results in an enhanced κ of ∼ 0.1. Thus, small changes in O : C can significantly affect κ. Pure MNC is also sparingly soluble in water and thus κ is strongly dependent on its actual solubility, which can change depending on the oxidation level and the presence of other compounds having different solubility . Consequently, the conversion from low to high κ following OH oxidation is consistent with the addition of more hydrophilic functionalities and a molecular transition from sparingly soluble to sufficiently water-soluble. Interestingly, MNC, while having a 10 times smaller OH uptake coefficient compared to LEV at the same RH , exhibits a greater change in κ than LEV following OH oxidation. Under dry conditions, we understand uptake is limited by surface–bulk processes . In that case, due to the low hygroscopicity and low water solubility of MNC, its viscosity may be sufficiently high that oxidation is limited to the particle surface. Consequently, MNC surface molecules may undergo several generations of oxidation as opposed to LEV, which is known to undergo a semi-solid to liquid-phase transformation at the same RH = 40 % . However, assessing the effects of RH or bulk diffusivity on hygroscopicity following OH exposure is beyond the scope of the current work.

Binary-component particles consisting of LEV : MNC, LEV : KS, and MNC : KS in 1:1 mass ratios were exposed to OH and analyzed for their hygroscopicity as a function of OH exposure. The approach here is to determine whether the presence of more than one component can influence the hygroscopicity of another following OH and oxidation; i.e., are the observed changes in hygroscopicity of the pure-component particles following OH oxidation retained when mixed? Figure  shows κ and the critical particle diameter for the different binary aerosol mixtures as a function of OH exposure for each applied S. κ as a function of O3 exposure is presented in the Supplement. The dotted and dashed lines in Fig.  display the predicted κ as a function of OH exposure using the volume mixing rule including and excluding MNC solubility limitations, respectively, based on the linear fits of κ as a function of OH exposure for pure LEV and MNC particles (Fig. ) at each S. Modeled κ as a function of OH exposure excluding MNC solubility limitations (i.e., black dashed lines in Fig. ) assumes that κ for MNC of the mixed particles is 0.16.

There are two important points to be made regarding the results from Fig. . (1) Hygroscopicity of the mixed particles is virtually unchanged as a function of OH exposure; i.e., while OH exposure significantly impacts MNC hygroscopicity alone, it does not significantly influence κ for the binary-component particles containing MNC as predicted by the volume mixing rule applying the single-component experimentally derived κ for LEV, MNC, or KS. (2) κ and the trend in κ with OH exposure are significantly underpredicted assuming MNC solubility limitations are applicable in the volume mixing rule (dotted lines in Fig. ). As discussed previously and demonstrated in Fig. , the presence of either KS or LEV influences the extent that MNC solubility impacts particle activation. We have shown that MNC exhibits no solubility limitations for the volume fractions applied here. Larger MNC volume fractions are expected to have a greater influence on κ following OH exposure. The organic content of BBA was shown to dominate hygroscopic growth, in particular the water-soluble organic content (WSOC), which is largely levoglucosan . Other studies have indicated that sparingly soluble organic compounds have limited importance in atmospheric aerosol CCN activity , although they are, besides completely insoluble organic material, the most likely class of compounds susceptible to hygroscopic changes following oxidation, due to their low water solubility. In other words, there is more room for an enhancement in the solute effect of sparingly soluble organic particles than there is for more water-soluble particles. Our results show that oxidative aging impacts on the hygroscopicity of pure-component particles can be vastly different if the particles are internally mixed with substances having different water solubility.

Here we investigate the CCN activity of internally mixed LEV, MNC, and KS particles with 1:1:1 and with an atmospherically relevant mass ratio of 1:0.03:0.3 (LEV : MNC : KS) following exposure to OH. The results for the OH exposure are shown in Fig.  and κ as a function of O3 exposure is presented in the Supplement.

Within the uncertainty of the derived κ values for the 1:1:1 and 1:0.03:0.3 ternary-component particles, their hygroscopicities are virtually unaffected by OH exposure, similar to the binary mixtures. However, on average the 1:1:1 particle mixture exhibits a slight enhancement in hygroscopicity. The predicted κ values for the 1:1:1 mixture, which include MNC solubility limitations (dotted lines), significantly underestimate κ, and only after removing these limitations (black dashed line) does the predicted κ agree with the experimentally derived κ. This is not surprising, given that the more water-soluble components LEV and KS are present at equal mass to MNC. Thus MNC behaves as if it is infinitely soluble during CCN activation. One possible explanation for the slight enhancement in κ with OH exposure, which differs from the binary mixed particles, is the presence of both MNC and LEV, which both exhibit enhancements in κ following OH oxidation. However, the range in derived κ at a given OH exposure is sufficiently large that, within experimental uncertainty, there is no significant trend in κ with OH exposure.

The WSOC, mostly LEV, is known to dominate the BBA volume fraction . MNC constitutes ≤5 % by mass of the BBA organic fraction as determined from both field and lab chamber studies . The remaining fraction can be largely composed of inorganic salts, including KS . To simulate atmospheric BBA, we atomized a mixed aqueous solution of LEV, MNC, and KS in a mass ratio of 1:0.03:0.3 and determined its CCN activity unexposed and after exposure to OH and O3. The resulting κ of this mixture as a function of OH exposure is displayed in the top right panel of Fig. . As anticipated, since pure LEV shows little enhancement in CCN activity with OH exposure (Fig. ) and dominates the volume fraction of this mixture, and KS is unreactive to OH, no enhancements in κ following OH exposure were observed. A similar observation was made from laboratory-controlled burns, whereby following several hours of photo-oxidation, there were very slight enhancements in κ of the particles . Larger enhancements in κ were observed only for the SOA particles generated from oxidative aging of gas-phase volatiles emitted during the controlled burns, in the absence of seed particles . This implies that photo-oxidative aging of BBA may contribute little to changes in its hygroscopicity, unless the entire aerosol population is comprised of SOA material (e.g., MNC). Furthermore, both predicted κ including solubility limitations and without solubility limitations are in agreement with the derived values. This is due to the low mass fraction of MNC present, which has sufficiently low impact on both the solubility and oxidation level of the mixed aerosol particles.

Internally mixed organic–inorganic atmospheric aerosol particles can exhibit phase separations, i.e., a core-shell structure, which often contains an insoluble or solid inorganic core with a less viscous organic outer layer . The presence of an organic coating has been shown to impact CCN activity and water uptake , ice nucleation efficiency , and heterogeneous chemistry . Because MNC originates from gas-phase chemical reactions, and its concentration determined in BBA particles, MNC must partition from the gas to the particulate phase. In this section, we investigate whether the mixing state of mixed MNC and KS particles has an effect on its CCN activity following OH exposure by the application of MNC-coated particles in comparison to the atomized MNC : KS binary-component particles. For example, observed a complete deactivation in the CCN activity of ammonium sulfate particles when thickly coated with stearic acid.

The CCN activity of KS particles coated with MNC was derived as a function of the organic volume fraction (Vf,org) of MNC, and before and after OH exposure as shown in Figs. a–c. Figure a displays a color map of the dry KS particle size distribution evolution following exposure to MNC in the absence of OH, where time = 0 min is the point at which KS particle growth by MNC condensation begins. The 25th, 50th, and 75th percentiles of the number-weighted particle size distribution grew in size by ∼ 20 nm as indicated by the red, black, and blue circles in Fig. a, respectively. For the 50th percentile, this corresponds to an enhancement in the MNC Vf,org from 0 % at time = 0 min to ∼ 70 % shortly after, close to the Vf,org of the atomized MNC : KS binary-component particles of 64 %. The similar Vf,org between the atomized and coated MNC-KS particles enables a direct intercomparison of their hygroscopicity, since relatively larger MNC Vf,org would bias towards lower κ and vice versa, as indicated in the colored dashed and dotted lines in Fig. b.

The particles' hygroscopicity was analyzed throughout the period of condensational growth as demonstrated in Fig. b and shown as the black circles. It is important to note that the particle size distribution is scanned (up and down voltage scans) at four different S (0.2, 0.27, 0.35, and 0.425 %) in ascending and descending order, a process that takes roughly 90 min. In contrast to the atomized binary-component particles, here KS particles grow due to MNC condensation over this experimental time period. Hence, the DMA and CCNc capture the size distribution and CCN activity of a time-dependent and compositionally different particle population at each scan. The black line in Fig. b displays the steps in S over the course of the experiment. The first two κ values are of pure KS particles evaluated at S=0.2 %. Subsequent κ values are of MNC-coated KS particles, which increase in Vf,org with time. The change in Vf,org with time is indicated by the red, black, and blue circles according to the 25th, 50th, and 75th percentiles of the particle population, respectively. Vf,org allows one to compare derived κ with that predicted using the volume mixing rule. As previously discussed, the solubility limitations of pure MNC can be neglected when predicting κ of the atomized 1:1 mass ratio MNC : KS binary-component particles. To determine whether the solubility of MNC impacts the MNC-coated KS particles similarly to the atomized mixture, κ is predicted using the volume mixing rule and applying the experimentally derived pure MNC κ corresponding to a specific S (i.e., including solubility limitations), as indicated by the dotted lines in Fig. b, and compared to predicted κ applying a pure MNC κ of 0.16 (i.e., pure MNC κ in the absence of solubility limitations calculated from Eq. 6), as indicated by the dashed lines in Fig. b. The predicted κ with increasing Vf,org generally captures the trend in experimentally derived κ with increasing Vf,org; however, similarly to the atomized MNC : KS binary-component particles, assuming MNC CCN activity is limited by its solubility, the volume mixing rule underpredicts derived κ. When applying a pure MNC κ=0.16 in the absence of solubility limitations, the volume mixing rule is in slightly better agreement with the derived κ values. However, there are notable deviations between derived κ and predicted κ in both cases, which depend on S. For example, in Fig. b, the predicted κ including MNC solubility limitations (dotted line) is in better agreement with the derived κ at S=0.425 % than at lower S. At higher S, the particles that activate first are smaller in diameter than the particles that activate first at lower S. Assuming differently sized KS particles were exposed to an equal quantity of gas-phase MNC, the larger particles, having relatively larger surface areas than the smaller KS particles, would acquire a thinner organic coating, and thus relatively smaller Vf,org. As a result, the particles that activate at S = 0.425 % possess a larger Vf,org compared to the particles that activate at, e.g., S = 0.2 %. This corresponds to a decrease in derived κ at S = 0.425 % (i.e., better agreement with predicted κ including MNC solubility limitations) relative to other S as indicated in Fig. b. The generally better agreement in the predicted κ excluding MNC solubility limitations with the experimentally derived κ indicates that MNC is sufficiently water-soluble to not deactivate KS, in contrast to the particle systems studied by . One possible explanation for the higher than expected κ when KS is coated with MNC is that the particle-phase diffusivity is sufficiently high to allow water molecules to penetrate the KS core .

The effects of OH exposure on the CCN activity of MNC-coated KS particles as a function of Vf,org are given in Fig. c. κ is plotted as a function of MNC Vf,org. The κ values resulting from an OH exposure of 3.3 × 1011 moleculecm-3 s are given by the filled circles, whereby the different colors represent the applied S during the experiment. Open circles correspond to κ in the absence of OH. At Vf,org = 0 %, κ is ∼ 0.55 and independent of OH exposure. κ decreases when Vf,org≈70 %, but undergoes a slight enhancement following OH exposure. The dotted lines indicate the modeled change in κ as a function of Vf,org applying the volume mixing rule and applying the experimentally derived κ for MNC and KS at a given S (i.e., including MNC solubility limitations). Similar to the atomized 1:1 MNC : KS binary-component particles, modeled κ as a function of Vf,org underpredicts the experimentally derived OH-unexposed κ, even after accounting for the enhancements in pure-component MNC κ due to the high OH exposure. This suggests that in the presence of KS at this Vf,org≈70 %, MNC may not be limited by its solubility, similar to the atomized 1:1 mass ratio MNC : KS binary-component particles, and that OH exposure can have very little impact on the CCN activity of sparingly soluble organics coated on water-soluble compounds. However, the dashed black line shows the modeled κ as a function of Vf,org applying the volume mixing rule and assuming MNC is not limited by its solubility, i.e., MNC κ = 0.16, which slightly overpredicts the derived κ, but is in better agreement with the trend in experimentally derived κ for the OH-exposed particles (filled circles, Fig. c). A reasonable explanation for this is that Vf,org≈70 % is sufficiently large such that MNC solubility limitations on the CCN activity of MNC-coated KS particles are partially exhibited. While OH exposure has a significant impact on the CCN activity of pure MNC, its impact on the CCN activity of MNC-coated KS particles is significantly less. The higher water solubility of KS appears to govern hygroscopic growth, similar to the atomized MNC : KS binary-component particles. This suggests that the water solubility of the more soluble component of mixed-component aerosol particles can be more important for CCN activation than the actual mixing state of the particle.