Article pubs.acs.org/est
Formation of Semisolid, Oligomerized Aqueous SOA: Lab Simulations of Cloud Processing Lelia N. Hawkins,†,* Molly J. Baril,‡ Nahzaneen Sedehi,‡ Melissa M. Galloway,‡ David O. De Haan,‡ Gregory P. Schill,§ and Margaret A. Tolbert§ †
Department of Chemistry, Harvey Mudd College, 301 Platt Boulevard, Claremont, California 91711 Department of Chemistry and Biochemistry, University of San Diego, 5998 Alcala Park, San Diego, California 92110 § Cooperative Institute for Research in Environmental Sciences and Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309 ‡
ABSTRACT: Glyoxal, methylglyoxal, glycolaldehyde, and hydroxyacetone form N-containing and oligomeric compounds during simulated cloud processing with small amines. Using a novel hygroscopicity tandem differential mobility analysis (HTDMA) system that allows varied humidification times, the hygroscopic growth (HG) of each of the resulting products of simulated cloud processing was measured. Continuous water uptake (gradual deliquescence) was observed beginning at ∼40% RH for all aldehyde-methylamine products. Particles containing ionic reaction products of either glyoxal or glycine were most hygroscopic, with HG between 1.16 and 1.20 at 80% RH. Longer humidification times (up to 20 min) produced an increase in growth factors for glyoxalmethylamine (19% by vol) and methylglyoxal-methylamine (8% by vol) aerosol, indicating that unusually long equilibration times can be required for HTDMA measurements of such particles. Glyoxal- and methylglyoxal-methylamine aerosol particles shattered in Raman microscopy impact-flow experiments, revealing that the particles were semisolid. Similar experiments on glycolaldehyde- and hydroxyacetone-methylamine aerosol found that the aerosol particles were liquid when dried for glyoxal > glycolaldehyde = hydroxyacetone, likely caused by the speed of oligomer formation in each system.
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and data inversion of the growth factor distributions.12,13 HTDMA measurements provide, among other things, particle growth as a function of humidity. Growth curves measured in subsaturated conditions can be used to predict cloud condensation nucleus (CCN) activity (in supersaturated conditions), but recent work has highlighted a disconnect between these two types of measurements. This so-called “hygroscopicity gap” may result from the complex relationship between organic components and aerosol water.14,15 The gap has a directional bias with HTDMA measurements predicting a lower hygroscopicity than direct measurements using CCN counters. One concern that has been presented in several studies is the very short residence time (τres) of particles in the HTDMA humidification chamber.16,17 While τres of less than 1 s are sufficient to equilibrate most inorganic salt particles and many organic particles, minimum required τres of up to 10 s have been reported for particles of varied composition.17−20
INTRODUCTION Aerosol−cloud interactions affect the earth’s radiation budget by modifying precipitation and cloud optical properties1−5 and impacting suspended particulate mass and particulate chemical composition through heterogeneous and aqueous phase chemical processes.6−10 Given this range of influence, there has been a concerted effort to understand the physical processes through which particles accumulate water, droplets evaporate to form particles, and particles maintain equilibrium with atmospheric water vapor. Understanding these processes requires a collection of techniques covering both subsaturated and supersaturated conditions, which provide complementary information, yet are rarely performed simultaneously. As such, measurements of hygroscopic growth in subsaturated conditions remain an important tool for understanding aerosol− water interactions, especially in controlled laboratory conditions where the chemical composition of the particles is constrained. One commonly used technique for measuring hygroscopic growth is the hygroscopicity (or humidified)tandem differential mobility analyzer (HTDMA),11 which entails size-selecting particles, exposing them to a specified humidity, and measuring the resulting size distribution. Subsequent work has focused on the detailed transfer function © 2014 American Chemical Society
Received: Revised: Accepted: Published: 2273
November 7, 2013 January 6, 2014 January 15, 2014 January 15, 2014 dx.doi.org/10.1021/es4049626 | Environ. Sci. Technol. 2014, 48, 2273−2280
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further purification. Methylamine was acidified to pH 4 using oxalic acid before reaction with aldehydes in order to simulate polluted cloudwater conditions. Unless otherwise noted, 18 MΩ water was used to make all solutions. For Raman experiments (described below), aerosol particles were generated from 8 mM fresh mixtures of two reactants. For “reacted” mixtures used in HTDMA experiments, 75 μL of 1 M solutions of each compound (resulting in 0.5 M solutions) were transferred to small glass vials and allowed to evaporate, concentrating the reactants and simulating an evaporating cloud droplet. One key distinction between ambient droplet evaporation and this preparation is the surface to volume ratio of the drying material. Bulk evaporation is necessary to prepare sufficient product material, but if surface reactions vary significantly from the bulk solution, then the product mixtures produced here will be depleted in those surface-generated compounds. Once evaporation was complete, the residual material was dissolved in approximately 30 mL of 18 MΩ water and transferred to a TSI Model 9302 atomizer. The resulting solutions would be 2.5 mM in each reagent if no reaction or organic compound evaporation occurred. We will refer to the products of the reacted mixtures as “aqSOA” to be consistent with terminology presented in ref 10. Modified HTDMA. A tandem differential mobility analyzer12 was constructed using two TSI Model 3080L Differential Mobility Analyzers (DMA1 and DMA2), and one TSI Model 3786 Water-based Condensation Particle Counter (CPC) following the schematic in Figure 1. Particles were
Delayed equilibration has traditionally been attributed to the presence of surface active compounds that reduce the mass accommodation coefficient well below 0.001.16,17,21,22 If the mass accommodation coefficient is in the range of 0.001 to 1, then particles are expected to equilibrate in less than 0.1 s.23 An HTDMA study of atmospheric particles using a novel configuration of the TDMA found that up to 2% of particles required humidfication times longer than 2 s.17 The exact percentage correlated with the sampling time of day, indicating a chemical basis for the slow water uptake observed. The delayed growth was attributed to an organic coating on the surface of the measured particles. However, highly viscous or semisolid particleswith mass accommodation coefficients well above 0.001may have sufficiently low diffusion coefficients for water that molecules are unable to equilibrate into the bulk particle on time scales less than 1 s.16,24,25 We label such particles “(semi)solids” to connote that they are either solid or semisolid phase.26 Given the recent work exploring the low molecular diffusivity of water in (semi-)solid organic particles, it is possible that the slow uptake of water observed in the field study 17 was due to a (semi-)solid organic phase in addition to, or in place of, an organic coating. Amorphous, (semi)solid particles have been reported in one ambient measurement27 and in laboratory-generated particles from biogenic volatile organic carbon (bVOC) and anthropogenic VOC precursor compounds.24,25,27,28 The appearance of (semi)solid organic particles is not surprising given the ubiquity of oligomeric compounds measured in ambient and simulated particles, as well as in cloud and fogwater.29−32 Although a tight correlation between molecular weight and viscosity (as glass transition temperature) has not been established, there is evidence that the two are related.26 Recent work has shown that some specific compounds associated with bVOC oxidation (i.e., glyoxal and methylglyoxal) can form oligomeric products in simulated cloud processed particles when ammonium sulfate, sodium chloride, or primary amine compounds are present.33−40 The products observed in some of those studies and in several others were accompanied by browning,36,41−43 making them similar to oligomeric, lightabsorbing humic-like substances (HULIS) reported in cloud and fogwater.44−47 Previously thought to be too volatile to form measurable SOA, small dicarbonyls such as glyoxal and methylglyoxal are now acknowledged to be capable of significant SOA formation by aqueous phase reactions.48 Therefore, there may be an important connection between the water-soluble oxidation products of common VOCs, atmospheric HULIS, and amorphous (semi)solid SOA particles that should be investigated. To study the interactions of water vapor with the oligomeric product mixtures of glyoxal, methylglyoxal, or glycolaldehyde reactions with small amines, we have measured their hygroscopic growth in a novel HTDMA system that allows for varied humidification time. As a complement, we present Raman microscopy evidence that aldehyde-methylamine reaction products are (semi)solid over a wide relative humidity range.
Figure 1. Modified HTDMA schematic.
generated in the atomizer, dried using two sequential diffusion driers containing Drierite (CaSO4), and size-selected at 100 nm using DMA1 before being sent into the 300-L Teflon chamber for RH equilibration. Air in the Teflon chamber began dry (less than 15% RH) and particle-free (less than 1 cm−3). Sample and sheath air were pulled from the Teflon chamber into DMA2. Sheath air was drawn from the Teflon chamber to ensure that particles did not experience a change in humidity upon entering DMA2. A Vasaila HMT337 sensor was used to monitor temperature and RH of the sample air. Exhaust air leaving DMA2 was filtered and sent back into the Teflon chamber to avoid moisture loss as the humidity was being increased. Humidity was increased by passing the returning, particle-free air through chambers containing wetted vermiculite. Typical residence times of particles in the chamber range from 10 to 25 min and vary by chamber volume. Between experiments, the chamber was flushed repeatedly with dry, particle-free air until particle counts dropped to zero. Size distributions were collected using a Scanning Mobility Particle Sizing (SMPS) system, comprising DMA2 and the CPC. The neutralizer in DMA2 was bypassed to avoid redistributing charge across the
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EXPERIMENTAL METHODS Reagents. Glycolaldehyde (dimer), glycine, and methylamine (40 wt % in water) were obtained from Sigma Aldrich. Methylglyoxal (pyruvic aldehyde, 35 wt % in water) was obtained from Alfa Aesar. Glyoxal (trimer dihydrate) was obtained from Fluka Analytical. All reagents were used without 2274
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Table 1. Summary of Reacted Amine-Aldehyde HTDMA Experiments
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particles. For all runs, the sheath flow rate was kept at 5 LPM and the sample flow rate was kept at 0.6 LPM. For very short residence time runs, particles were mixed with sheath air leaving the Teflon chamber and sent directly to DMA2, bypassing the Teflon chamber completely and resulting in a residence time of approximately 3.4 s for particles in humid air. For intermediate residence time experiments (1 min), particles were passed into a 4-L chamber containing humid air before being sampled. All experiments were carried out under ambient temperature and pressure (22 °C and 1 atm). All HTDMA measurements were analyzed using the TDMA Inversion Toolkit 3.210, written for Wavemetrics Igor Pro software, following the procedure provided with the toolbox.13 This automated inversion algorithm produces growth factor probability distribution functions (GF-PDF) that are independent of the initial guess and are not biased in cases of nonuniform growth. The mean growth factor (GF) for each scan can be determined from the GF-PDF. Although particles used in the experiments presented here are expected to show monomodal, uniform growth because we have only one particle type, we have applied the TDMA inversion algorithm to obtain the most accurate results possible. In addition, the inversion toolkit provides a simple and direct way to determine if doubly charged particles contribute to the GF-PDF. For all experimental conditions, ammonium sulfate particles were used as a standard to ensure consistency with previously reported deliquescence curves.49 Raman Microscopy on Individual Aerosol Particles. Impact-flow experiments50 were performed on solutions containing methylamine and a carbonyl compound. These solutions were atomized, and the resultant aerosol particles were collected onto quartz substrates coated with a commercial hydrophobic silanizing agent and dried for several min at low RH in the Raman microscope environmental chamber (Nicolet Almega XR/Olympus BX51/Linkham THMS 600, see ref 51 for details). After imaging and recording Raman “line maps” across the dried particles (with 1.1 μm resolution), they were briefly pressed with a second coated quartz substrate and imaged again. Particles that cracked and shattered, indicating that they were (semi)solid, were then subjected to increasing RH levels at low temperatures (RH ramp rate 0.1% s−1) and visually recorded throughout the process. The RH level and temperature at which shattered particles began to flow back into spherical shapes was recorded, and the experiment was repeated at different temperatures. Particles that did not crack and shatter during impact were considered liquid. Reaction systems that produced liquid aerosol particles were run again with 20 h ambient drying times before impact-flow analysis, to determine if additional oligomerization time would convert them to (semi)solids.
RESULTS AND DISCUSSION
Hygroscopic Growth Studies. The humidification experiments for amine-aldehyde products (“reacted” mixtures) are listed in Table 1. Hygroscopic growth factors at 80% RH (HGF80) ranged from 1.06 to 1.20, though five of six reaction products had HGF80 clustered above 1.14. These five reactions are all expected to generate products containing ionic functional groups, either carboxylate groups (conserved during aldehyde-glycine reactions), imidazolium rings (formed by glyoxal- and methylglyoxal-amine reactions),38,39 or both (e.g., glyoxal-glycine reactions). The sixth, glycolaldehyde-methylamine products, is not expected to contain ionized functional groups, and at RH > 60% this aerosol shows the smallest increase in size due to water uptake, as expected. Water uptake curves with τres = 3.4 s are shown in Figure 2. For most experiments, particles began to take up water between 30% and 40% RH (growth less than 5%) and continued growth
Figure 2. Humidograms of aerosol particles made from reacted aldehyde-amine solutions, with (a) methylglyoxal, (b) glyoxal, and (c) glycolaldehyde. Panel (d) shows literature data62 for atmospheric HULIS samples or humic acid standards. Aldehyde-amine aerosol uptake measured with 3.4 s τres. Vertical error bars denote the spread of growth factors observed at each RH. Duplicate glycolaldehydemethylamine data are shown in (c) in black. 2275
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Figure 3. Hygroscopic growth factors (D/D0) for reacted solutions of methylamine with glycolaldehyde, glyoxal, and methylglyoxal under 3.4 s (blue markers), 1 min (green markers), or 20 min (red markers) humidification residence times. No measurements were made for the 1 min humidification time for the methylglyoxal-methylamine SOA.
pure, amorphous oxalic acid particles, particle shrinkage upon humidification is more pronounced: HGF falls as low as 0.93 at 41% RH.24 This general behavior has been attributed to hydration-induced restructuring of porous, gel-like structures.24 The differing magnitude of the effect suggests that the dried, (semi)solid particles in our experiments are less porous, or closer to spherical, than oxalic acid aerosol particles. A similar restructuring process is likely behind the occasional flat spots observed in the water uptake curves, most notable in glycolaldehyde-methylamine particles between 55 and 75% RH, and in glyoxal-methylamine particles between 55% and 65% RH (Figure 2). These restructuring effects are only possible in highly viscous liquid or semisolid particles. In summary, these water uptake experiments on aerosol generated from the aqSOA products of amine-aldehyde reactions suggest that (1) particles contain complex mixtures, (2) may be (semi)solid up to at least 60% RH, and (3) have different hygroscopicities depending on the reaction (and therefore the functional groups present). In all reactions, the continuous water uptake behavior is similar to that of atmospheric HULIS. Equilibration Time-Dependent Growth. Measurements of hygroscopic growth of methylamine-aldehyde products were collected under varied equilibration times in the HTDMA system to determine if longer equilibration times produced changes in observed hygroscopic growth. Figure 3 shows the humidograms obtained during 3.4 s, 1 min, and 20 min experiments. Changes in HGF at 80% RH are summarized in Table 1. Because these aqSOA samples all were dried to a brown residue and then redissolved before aerosol generation, any unreacted aldehyde58 or methylamine present in the original samples would have evaporated during the initial drying process. These compounds should be therefore largely absent from the gas phase in the water uptake experiments described here. Thus, particle growth observed at long drying times is almost certainly due to the uptake of water, rather than other molecules, from the gas phase. Glyoxal-methylamine aqSOA showed the largest growth under 3.4 s τres conditions among the aldehydes studied. For 1
beyond 80% RH, with little to no distinct transitions observed (no prompt deliquescence). These humidograms are similar to published measurements of terrestrial humic acid,20,52,53 WSOC extracts including dicarboxylic acids and multifunctional acids,54 and atmospheric HULIS55,56 all of which show gradual growth in response to humidification at low RH and continued growth at higher RH, with no sharp transitions (Figure 2d). Previous studies have shown that continuous water-uptake behavior during humidification is associated with amorphous, noncrystalline materials.24,52,55 The chemical complexity of these mixtures likely hinders individual compounds from forming crystals during drying. At higher RH, even some pure organic substances behave similarly to the aqSOA generated in our study. For example, between 60% and its deliquescence point (95% RH), glycine absorbs a small amount of water, with a HGF at 85% RH that is only 10% higher than the mixtures in this study.57 Thus, the presence of unreacted glycine may contribute to the observed water uptake above (but not below) 60% RH for aldehyde + glycine reactions in Figure 2. Oxalic acid (which may be present as an impurity in all of these experiments due to aldehyde reactions with trace oxidants) forms an amorphous semisolid upon spray drying. Beginning at 40% RH, pure oxalic acid aerosol particles gradually take up water,24 near the observed beginning of water uptake in most experiments in Figure 2. However, since only trace levels of oxalic acid should be produced in the aqSOA particles generated in this study (without oxidants), and because particle phase oxalic acid is likely to evaporate, this substance is expected to be at most a minor contributor to observed water uptake. We instead note that an onset of gradual water uptake at 30−40% RH has been observed for several other amorphous organic solids.24 Another similarity between amorphous pure oxalic acid particles and the aqSOA formed in these experiments is the slight decrease in particle size observed during hydration at RH between 25% and 40% (Figure 2). The largest initial decrease in particle size is observed for methylglyoxal-methylamine aqSOA products, where the HGF reaches a minimum of 0.98 ± 0.02 at 31% RH, just before the onset of water uptake. For 2276
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min τres, no increase in size was observed; however, increasing the τres to 20 min produced particles that were 6% larger (diameter) at 80% RH (equal to 19% increase by volume), indicating that the particles had not fully equilibrated even after 1 min. The change in HGF across the range of 30−80% was assessed using a paired t test. The results show that the change from 3.4 s to 1 min humidification time did not produce a significant change in HGF (95% confidence) while the subsequent increase from 1 to 20 min humidification time did (p value = 0.0002). Methylglyoxal-methylamine aqSOA has lower HGF at all RH than its glyoxal counterpart. Particles did not begin to take up water until RH rose above 45%, even in the 20 min τres experiment, and the HGF at 80% is substantially lower than for the glyoxal-methylamine system. However, when τres is changed from 3.4 s to 20 min, the increase in water uptake is still significant (p value = 0.001). At τres = 20 min, 80% RH particle diameters and volumes are 2.5% and 8% larger, respectively. Glycolaldehyde-methylamine aqSOA is the least hygroscopic of all the systems explored in this work. Under short humidification times (τres = 3.4 s), the HGF is 1.06 at 80% RH, while all other reaction systems are between 1.14 and 1.2. Given such limited water uptake, it is not surprising that increasing τres to 1 or 20 min did not cause a statistically significant increase in HGF across all RH. Growth curves under each of the τres conditions studied show noticeable flat spots and growth onset points, as expected for porous or nonspherical semisolid particles that restructure and eventually liquefy during hydration.24 We note that the onsets of restructuring and phase transitions occur at lower RH under 20 min τres than under 1 min or 3.4 s τres, indicating that the longer equilibration time is needed for water molecules to penetrate into, or eventually dissolve, the (semi)solid material. One would expect HGF data at τres = 1 min to be either intermediate between τres = 3.4 s and 20 min data, or equal to τres = 20 min data, since growth should be continuous until equilibrium is eventually reached. It is clear, however, from Figure 3 that at least between 45 and 65% RH, HGF at τres = 1 min appears to be below HGF measured τres = 3.4 s data for both glyoxal-methylamine and glycolaldehyde-methylamine aqSOA particles. If these differences are significant, then they suggest that particles may sometimes decrease in size during water uptake, whenever continuous growth is interrupted by particle restructuring. For these systems, it appears that (semi)solid particles begin to take up water on the time scale of τres = 3.4 s, restructure and collapse within a τres = 1 min time scale, and then continue growth by further water uptake beyond 1 min. Phase Transitions in aqSOA Particles. The previous sections provided indirect evidence that in aldehyde-methylamine mixed aerosol, drying caused the particles to become (semi)solid. To verify this, “impact-flow” microscopy measurements were performed on aqSOA particles in an environmental chamber. This technique50 allows the direct observation of transitions between (semi)solid and liquid particle phases as a function of RH and temperature. In an impact-flow experiment, (semi)solid particles are shattered as the particles are pressed between quartz substrates, while liquid particles spread out and then bead up again when the top substrate is removed. Shattered (semi)solid aerosol particles are then observed under increasing RH levels until they take up enough water to liquify and flow back into a spherical shape. The RH level (and temperature) where the beginning of flow is observed marks a
point on the phase boundary between liquid and (semi)solid. It is important to note that this phase boundary may depend on the RH ramp rate, which was ∼0.1% RH s−1 in these experiments. When glyoxal or methylglyoxal solutions were mixed with methylamine, sprayed, collected, and pressed after several minutes of drying at low RH, the particles shattered, indicating that they were (semi)solid. The conditions for the onset of flow in these (semi)solid particles are shown in Figure 4 for glyoxal-
Figure 4. Temperature dependence of measurements for aldehydemethylamine products. Glycolaldehyde-methylamine and hydroxyacetone (HA)-methylamine sprays were dried overnight under ambient conditions prior to impact-flow experiment. Glyoxal-methylamine and methylglyoxal-methylamine50 spray were dried 1 h or less prior to impact-flow experiment. The inset shows images of one particle during an impact-flow experiment for glyoxal + methylamine at “250° K”, with a temperature ramp rate of 1° K/min. The black size bar in the lower left hand corner corresponds to 10 um.
and methylglyoxal-methylamine aqSOA as a function of temperature. At RH levels above these data points, the aldehyde-methylamine reaction products took up enough water to change phase from (semi)solid to liquid. As expected, the RH level at which this phase change is observed decreased as temperature was increased. Interestingly, the RH level required to liquify these particles below 250 K is higher for methylglyoxal-methylamine than for glyoxal-methylamine reaction products. This suggests that the hydrophobicity and/or the extent of oligomerization in the methylglyoxal-methylamine product mixture is greater, consistent with HTDMA observations of lower water uptake in this work, faster reaction rates measured by NMR,59 and more light-absorbing products measured by cavity ring-down spectroscopy 60 for this reaction. When glycolaldehyde- or hydroxyacetone-methylamine aqSOA were subjected to impact-flow experiments after several minutes of drying at low RH, particles flowed between the plates and beaded up again after plate removal, indicating that they were liquid phase. However, when the sprayed particles were dried at ambient conditions for 20 h before the impactflow experiment, the aqSOA particles shattered, indicating that they had become (semi)solid. Clearly, the oligomers responsible for solidifying the aqSOA particles did not form immediately upon droplet drying, but sometime during the next 20 h. The conditions for the onset of flow in these aged, (semi)solid particles are shown in Figure 4 as triangles and circles, respectively, as a function of temperature. Even after 20 2277
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processes under kinetic rather than thermodynamic control.61 Ambient particles are likely to include a substantial amount of inorganic material. On the basis of the results of previous work showing that ammonium sulfate and dicarbonyls form oligomeric products, the inclusion of inorganic compounds is expected to result in similar water uptake behavior.39,59 Additional studies of the hygroscopic growth of aminealdehyde-inorganic mixtures are the subject of a manuscript currently in preparation.
h drying times, the glycolaldehyde- and hydroxyacetonemethylamine reaction products undergo humidity-induced liquifaction at lower RH levels than briefly dried glyoxal- or methylglyoxal-methylamine reaction products, indicating that the former products are more hydrophilic and/or less oligomerized than the latter ones. However, since Figure 2 shows that glycolaldehyde-methylamine reaction products are actually less hydrophilic than methylglyoxal- or glyoxalmethylamine reaction products, we conclude that glycolaldehyde-methylamine reaction products must be less oligomerized than methylglyoxal- or glyoxal-methylamine products, even after 20 h extra reaction time. In addition, the data for glycolaldehyde-methylamine aqSOA are consistent with observations by Mikhailov et al.,24 who found that when some organics that form amorphous solids at room temperature are dried, semisolid formation occurred between 30 and 50% RH, consistent with our extrapolated fit. Because the aldehyde + amine reaction time clearly influences (semi)solid formation through oligomer formation, the data in Figure 3 and Figure 4 cannot be directly compared because of differences in aging times between the two experiments. Figure 3 aqSOA was formed from solutions dried in bulk, a process which takes a few days under ambient conditions, while Figure 4 aqSOA was made from fresh aldehyde-amine mixtures that were aged, if at all, for 20 h as collected aerosol on a substrate. For this reason, Figure 3 aqSOA, which was subject to longer aging times, shows signs of slow water uptake (likely due to its (semi)solid nature) all the way past 80% RH at room temperature. If we were to extrapolate the trends in the data in Figure 4 to room temperature, then it would suggest that similarly aged aqSOA samples would liquify at RH no higher than 60%. We attribute the higher RH required to liquify aqSOA inferred from Figure 3 (especially for glyoxal- and methylglyoxal-methylamine) to the longer aging times used in that experiment, causing more extensive oligomer formation.
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AUTHOR INFORMATION
Corresponding Author
*Phone: 909 621-8522; fax: 909 607-7577; e-mail: lhawkins@g. hmc.edu. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Martin Gysel at the Paul Scherrer Institut for assistance with the HTDMA inversion software. This work was supported by NSF Grants ATM-0749145, AGS-1048536, and AGS-1129002.
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REFERENCES
(1) Albrecht, B. Aerosols, cloud microphysics, and fractional cloudiness. Science 1989, 245, 1227−1230. (2) Jones, A.; Roberts, D.; Slingo, A. A climate model study of indirect radiative forcing by anthropogenic sulfate aerosols. Nature 1994, 370, 450−453. (3) Ferek, R.; Garrett, T.; Hobbs, P.; Strader, S.; Johnson, D.; Taylor, J.; Nielsen, K.; Ackerman, A.; Kogan, Y.; Liu, Q. Drizzle suppression in ship tracks. J. Atmos. Sci. 2000, 57, 2707−2728. (4) Ackerman, A.; Toon, O.; Stevens, D.; Coakley, J., Jr. Enhancement of cloud cover and suppression of nocturnal drizzle in stratocumulus polluted by haze. Geophys. Res. Lett. 2003, 30, 1381. (5) Penner, J.; Dong, X.; Chen, Y. Observational evidence of a change in radiative forcing due to the indirect aerosol effect. Nature 2004, 427, 231−234. (6) Hoag, K.; Collett, J.; Pandis, S. The influence of drop sizedependent fog chemistry on aerosol processing by San Joaquin Valley fogs. Atmos. Environ. 1999, 33, 4817−4832. (7) Blando, J.; Turpin, B. Secondary organic aerosol formation in cloud and fog droplets: A literature evaluation of plausibility. Atmos. Environ. 2000, 34, 1623−1632. (8) Collett, J.; Sherman, D.; Moore, K.; Hannigan, M.; Lee, T. Aerosol particle processing and removal by fogs: Observations in chemically heterogeneous central California radiation fogs. Water, Air, Soil Pollut.: Focus 2001, 1, 303−312. (9) Hegg, D.; Covert, D.; Jonsson, H.; Khelif, D.; Friehe, C. Observations of the impact of cloud processing on aerosol lightscattering efficiency. Tellus B 2004, 56, 285−293. (10) Ervens, B.; Turpin, B.; Weber, R. Secondary organic aerosol formation in cloud droplets and aqueous particles (aqSOA): A review of laboratory, field and model studies. Atmos. Chem. Phys 2011, 11, 11069−11102. (11) Liu, B. Y. H.; Pui, D. Y. H.; Whitby, K. T.; Kittelson, D. B.; Kousaka, Y.; McKenzie, R. L. Aerosol mobility chromatograph-new detector for sulfuric-acid aerosols. Atmos. Environ. 1978, 12, 99−104. (12) Rader, D.; McMurry, P. Application of the tandem differential mobility analyzer to studies of droplet growth or evaporation. J. Aerosol Sci. 1986, 17, 771−787. (13) Gysel, M.; McFiggans, G.; Coe, H. Inversion of tandem differential mobility analyser (TDMA) measurements. J. Aerosol Sci. 2009, 40, 134−151.
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ATMOSPHERIC SIGNIFICANCE Evidence from the primary humidified growth experiments and time-dependent hygroscopic growth of the aldehyde-methylamine aqSOA suggests that the products exhibit (semi)solid characteristics, including: (1) very slow water uptake; (2) particle restructuring upon humidification; (3) no prompt deliquescence; (4) time-dependence of RH levels observed to cause the onset of water uptake and phase transitions to the liquid phase; and (5) particle shattering upon impact. From the measurements presented here, we conclude that (semi)solid, oligomeric aqSOA materials are produced by simulated cloud processing of aldehydes with amines at low RH, and these oligomers cause the organic phase to remain (semi)solid, sometimes even at RH as high as 80%. The observed gradual water uptake behavior, and lack of prompt deliquescence, resembles atmospheric HULIS. If we consider this aqSOA material as a proxy for oligomerized SOA material in the atmosphere, then these measurements indicate that traditional HTDMA experiments with short humidification times may underestimate the true HGF of oligomerized SOA. In addition, oligomerized (semi)solid SOA particles in the atmosphere will experience delayed gas-particle equilibration for all chemical species, potentially slowing the uptake of gas-phase organics and oxidants, and hindering evaporation of particle-phase organics. The result would be longer time scales necessary for atmospheric aging of oligomerized aerosol, with particle-phase 2278
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