Partitioning of semivolatile surface‐active compounds ... - OpenSky

GEOPHYSICAL RESEARCH LETTERS, VOL. 38, L03807, doi:10.1029/2010GL046147, 2011

Partitioning of semivolatile surface‐active compounds between bulk, surface and gas phase S. Romakkaniemi,1 H. Kokkola,2 J. N. Smith,1,2,3 N. L. Prisle,2,4 A. N. Schwier,5 V. F. McNeill,5 and A. Laaksonen1,6 Received 10 November 2010; revised 17 December 2010; accepted 5 January 2011; published 5 February 2011.

[1] We present a model study demonstrating that surface partitioning of volatile surfactants enhances their uptake by submicron liquid droplets. In submicron‐sized droplets, surface partitioning of a surface‐active volatile species may significantly decrease its equilibrium partial pressure, thus increasing the total flux of the surfactant from gas phase to aqueous phase. Such uptake of volatile organic species into aqueous aerosols can be followed by aqueous‐phase chemistry to form low‐volatility secondary organic aerosol material, leading to increased aerosol mass. In the study, we used an air parcel model that includes simplified aqueous‐ and gas‐phase chemistry, condensation/evaporation, and a model of aqueous‐phase thermodynamics that takes into account the partitioning of surfactants between the bulk and surface phases. We modeled the uptake and aqueous‐ phase chemical reactions of methylglyoxal, as it is a moderate surfactant that forms less volatile secondary organic material via aqueous‐phase chemical reactions with the hydroxyl radical as well as hydronium and ammonium ions. Our model simulations show an order of magnitude higher uptake of methylglyoxal in aqueous aerosols of cloud condensation nuclei sizes (less than 200 nm in radius) when surface partitioning is taken into account, compared to when surface partitioning is neglected. As a consequence, the production of SOA through the aqueous‐phase chemical processing of methylglyoxal is also enhanced, but to a lesser degree, because condensation of the hydroxyl radical from gas phase limits the production. Citation: Romakkaniemi, S., H. Kokkola, J. N. Smith, N. L. Prisle, A. N. Schwier, V. F. McNeill, and A. Laaksonen (2011), Partitioning of semivolatile surface‐active compounds between bulk, surface and gas phase, Geophys. Res. Lett., 38, L03807, doi:10.1029/2010GL046147.

1. Introduction [2] Atmospheric secondary organic aerosol (SOA) is formed when gas phase organics oxidize to form less volatile compounds, which condense on particles and increase aerosol mass [Seinfeld and Pankow, 2003; Kroll and Seinfeld, 2008]. Alternatively, SOA can form through reactions of 1 Department of Applied Physics, University of Eastern Finland, Kuopio, Finland. 2 Kuopio Unit, Finnish Meteorological Institute, Kuopio, Finland. 3 National Center for Atmospheric Research, Boulder, Colorado, USA. 4 Department of Physics, University of Helsinki, Helsinki, Finland. 5 Department of Chemical Engineering, Columbia University, New York, New York, USA. 6 Finnish Meteorological Institute, Helsinki, Finland.

Copyright 2011 by the American Geophysical Union. 0094‐8276/11/2010GL046147

gas phase organic compounds that are absorbed into the aqueous phase of deliquesced aerosols or cloud droplets [Jang and Kamens, 2001; Kroll et al., 2005; Carlton et al., 2006]. It has been estimated that 20–90% of aerosol in the lower troposphere is actually organic and up to 80% of that can be SOA; thus, its predicted effect on climate is notable [Kanakidou et al., 2005]. SOA formation has been an active area of research for over two decades, but a large gap exists between modeled and measured atmospheric SOA concentrations, indicating that SOA formation is still poorly understood [Hallquist et al., 2009]. [3] Some organic compounds, known as surface‐active organics, contain both hydrophobic and hydrophilic moieties. These species tend to partition to the gas‐liquid interface in aqueous solution, with potentially important consequences for atmospheric aerosol chemistry [Djikaev and Tabazadeh, 2003], and for the surface tension of aqueous aerosol particles and droplets [e.g., Facchini et al., 1999]. The partitioning of volatile species between aqueous and gas phases is described by Henry’s law, which does not include surface‐bulk partitioning. Although surfactant concentrations are enhanced on the droplet surface, the equilibrium vapor pressures of absorbed species are still determined by aqueous‐phase concentrations in the bulk phase. Thus, due to the high surface area‐to‐volume ratio of submicron aerosols, the total uptake of volatile species is enhanced when surface partitioning is taken into account and the effective Henry’s law coefficient for these aerosols is higher than that measured with bulk samples. [4] Köhler theory states that a decrease in surface tension increases the likelihood that an aerosol will activate into cloud droplets [Köhler, 1936]. When surface partitioning is taken into account, the supersaturation needed for individual droplets to activate is decreased compared to a system in which surface partitioning is neglected [Sorjamaa et al., 2004]. This has been observed for particles containing up to 95 wt% sodium salts of fatty acids [Prisle et al., 2008] and up to 80 wt% humic‐like substances (HULIS) [Kokkola et al., 2006]. However, these substances have very low volatility, meaning that their vapor pressure is negligible and no transfer between gas and aqueous phase takes place. Volatile or semi‐volatile surfactants represent a special case which has received relatively little attention. Large uncertainties related to their partitioning between gas and particle phase can lead to several orders of magnitude difference in the mass of formed SOA [e.g., McFiggans et al., 2010]. [5] Sareen et al. [2010] recently showed that aqueous‐phase (nonoxidative) reactions of the volatile dicarbonyl compound methylglyoxal (MG) results in the formation of oligomeric secondary organic material and a significant decrease in surface tension. As gas‐phase MG exists in equilibrium with the

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[8] Products of the reaction between MG and OH include pyruvic acid, oxalic acid, and oligomeric material that are likely to be surface‐active [Altieri et al., 2008]. The surface excess of the surface active compound is calculated by solving the Gibbs adsorption equation, as presented by Sorjamaa et al. [2004]. This approach differs from the one presented by Djikaev and Tabazadeh [2003] in the sense that we do not consider gas‐adsorption isotherms as a parameterization for surface coverage [Donaldson, 1999], but instead solve the Gibbs adsorption equation in the aqueous phase simultaneously with the bulk aqueous‐phase thermodynamical equilibrium. Also, in our approach the Kelvin effect is taken into account, allowing us to study submicron particles as well as cloud droplets. As the surface partitioning of several components is numerically difficult and we do not know the exact surface activities of all products, we assume that the surface activity of the MG‐product mixture is represented well by the data of Sareen et al. [2010]. Figure 1. (a) Total concentration of methylglyoxal in ammonium sulfate – water droplets as a function of droplet size. Volume mean value is denoted as a diamond. (b) The ratio of increase in the total concentration as a function of surface tension parameters A and B in the Szyszkowski‐Langmuir equation (equation (1)) for the same aerosol size distribution. aqueous phase, and the net effect of addition of MG to the aqueous phase is surface tension depression, we refer to MG as a volatile surfactant. The reaction between MG and OH in the aqueous phase has also been shown to lead to SOA formation [Altieri et al., 2008; Tan et al., 2010]. In aerosol particles with sizes relevant to cloud droplet formation, the enhanced uptake of dicarbonyl compounds due to surface partitioning could lead to increased SOA yields, and the particles may reach the CCN size faster. [6] In this paper, we use modeling tools to determine how surface partitioning affects the uptake of MG and aerosol‐ phase chemistry. We also study the effect of uptake and subsequent chemistry on the surface tension of particles in sub‐ and supersaturated conditions.

3. Results [9] To demonstrate how surface partitioning would increase the uptake of MG to the aqueous phase, we calculated the equilibrium concentration of MG in droplets consisting of ammonium sulfate and water at relative humidity of 95% and temperature of 293.15K. The size distribution of the dry ammonium sulfate particles was assumed to be log‐normal with mean radius of 100 nm and standard deviation of 2.2. [10] Figure 1a shows the concentration of MG as a function of wet droplet size when the mixing ratio of MG in the gas phase is 1 ppb. In previous studies, the Henry’s law constant for MG has been estimated to be 3.2 × 104 M atm−1 [Zhou and Mopper, 1990], so the equilibrium concentration (neglecting Kelvin effect) is approximately 3.2 × 10−5M. This Henry’s law coefficient is actually measured for seawater, but as the wet aerosol particles considered in this study have a higher ionic content than sea‐water this is preferred over the pure water coefficient. In our calculations we use surface tension parameterizations of Schwier et al. [2010] fit to data presented by Sareen et al. [2010] using the Szyszkowski‐Langmuir equation:

2. Methods


¼
0  A lnð1 þ BC Þ;

[7] We used an adiabatic cloud parcel model to examine the effects of surface partitioning on the reactive uptake of MG to liquid droplets consisting of ammonium sulfate and water. The model was previously described by Kokkola et al. [2006] and is modified here to include partitioning between the gas and aqueous phases of volatile organic surface active compounds. In short, the model solves time‐ dependent growth of aerosol particles due to condensation of water and trace gases, such as MG, and aqueous phase chemical kinetics. For condensation of different gases we have assumed accommodation coefficient of unity and the vapor pressure of MG at the surface of particle is assumed to follow Henry’s law, i.e., we have assumed that organics form ideal solute with water. Aqueous phase chemical reactions solved in the model include reactions of MG with ammonium and hydronium ions according to Sareen et al. [2010] and MG with OH radicals according to Ervens et al. [2003]. All these processes are described by differential equations and solved by a numerical solver DVODE designed for stiff differential equations.

A ¼ T

ð1Þ

where a = 1.85 × 10−5 Nm−1K−1 and B = 140 kg H2O (mol C)−1. In the equation, s0 is the surface tension of the solution droplet without surfactants, T is temperature and C is the carbon molar concentration. [11] As can be seen from Figure 1a, the total concentration of MG in the droplets is enhanced by more than an order of magnitude when surface partitioning is taken into account. The surface area‐to‐volume ratio is higher for small droplets, and thus the effect of surface partitioning increases with decreasing droplet size. The total concentration averaged over the size distribution in these conditions was 4.2 × 10−5M and is denoted as a diamond. It has to be noted that the averaged value depends on the shape of the size distribution, but remains always on the black solid curve of Figure 1. This increase in the total concentration might explain in part why measured in‐particle concentrations of MG in PM2.5 samples are often much higher, even 7 mM at the highest [Ho et al., 2007; Kawamura and Yasui, 2005], than those estimated based on its bulk Henry’s law coefficient.

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Figure 2. Concentration of methylglyoxal and the reaction products as a function of droplet radius in cases with and without surface partitioning. [12] As shown, the amount of surfactant in aqueous aerosol particles can be determined if the surface tension of the solution is known as a function of composition. We have conducted sensitivity tests by varying the constants A and B in equation (1). Figure 1b shows the ratio of total concentration as calculated with surface partitioning to that calculated without when the Henry’s law coefficient of MG is assumed. It can be seen that for strong surfactants the enhancement in total concentration can be three orders of magnitude. This would be the case, for example, for a surfactant with surface tension properties of HULIS. The values A and B in equation (1) have been reported to be up to 10 mN m−1 and 106 kg H2O (mol C)−1, respectively for some HULIS and fatty acids [Sorjamaa and Laaksonen, 2006]. 3.1. Implications for Atmospheric Chemistry [13] The increase in total concentration of MG carries implications for the aqueous‐phase chemistry of MG [Sareen et al., 2010; Tan et al., 2010]. To study this, we made a simulation using the same size distribution and ambient conditions as given above, but including a gas‐phase OH concentration of 106 cm−3 and allowing MG to react in the aqueous phase with OH, H3O+, and NH+4 . [14] When modeling aqueous‐phase chemistry in a system in which the organic reactants are not distributed homogeneously throughout the particle, we must carefully consider what concentrations to use. For ammonium and hydronium ions, we use their bulk concentration without accounting for their depletion close to surface due to reactions with MG. It is likely that OH will not be homogeneously distributed through the particle, since it enters from the gas phase and can then rapidly reacts with MG in the near‐surface region. The reactive diffusive length is calculated as [Hanson et al., 1994] rffiffiffiffiffiffiffiffi Daq ¼ : k′

ð2Þ

Daq is the diffusion coefficient for OH in water (∼10−5 cm2 s−1 [Wilke and Chang, 1955]), and k′ is the rate coefficient for the OH and MG aqueous reaction (1.1 × 109 M−1 s−1

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[Ervens et al., 2003]) multiplied by the MG concentration, and has a value of 1–10 mM depending on how the surface thickness is defined. For example, a surface layer of 5 nm (or 2 nm), leads to a concentration of 7 mM in the surface layer, and d ∼ 11 nm (7 nm). Based on this, we conclude that a high fraction of MG+OH reactions occur in the near‐ surface region and the MG concentration within this reactive zone is the relevant quantity for modeling the formation of SOA via the MG+OH reaction. Also, as the reactive diffusive length is longer than the assumed surface thickness, it can be estimated that a large amount of OH penetrates through the surface layer and is in equilibrium inside the drop. [15] As a first approximation for the concentration of each species in our chemistry calculations, we assumed a uniform distribution throughout the entire droplet for OH, H3O+, and NH+4 , while MG is in equilibrium between the surface and bulk as given by Gibbs adsorption isotherm. We also assumed that OH only reacts with MG. The first approximation underestimates the reaction rate between OH and MG as the OH concentration is probably higher at the surface than in the bulk. The second approximation overestimates the reaction rate as it is likely that OH could react again with products from the first reaction. These are quite crude approximations, but the purpose is only to illustrate how much aerosol phase chemistry could be affected by surface partitioning. [16] In Figure 2, the amount of products from chemical reactions is presented as a function of particle size after 15 minutes and 4 hours into the simulation both with (solid curves) and without (dashed curves) surface partitioning. The enhancement in reaction rate is lower than enhancement in the total concentration due to surface partitioning, and this is because the condensation of OH from the gas phase is not fast enough to maintain equilibrium. As the surface area‐to‐volume ratio increases with decreasing droplet size, the chemical production is more efficient in the smallest droplets. The concentration of the reaction products reaches the concentration of MG by 15 minutes into the simulation in droplets with radius smaller than 0.3 mm, when including partitioning. After 4 hours, the concentration of the reaction products is already an order of magnitude higher than the concentration of MG, indicating that surface partitioning can significantly enhance the production of secondary organic material already in aerosols which are subsaturated with respect to water. [17] We further quantified the effect of relative humidity (RH) on production rates. For ammonium and hydronium reactions the effect was small, since higher RH leads to lower concentration, which negates the increased production rate which could be associated with an increase in reactive volume. For the OH reactions, increased volume leads to increased product formation, since both OH and MG enter from the gas phase. This RH dependence is demonstrated in Figure 3, where the production rate from MG chemistry is shown as a function of water saturation ratio (and also wet particle radius) for a single droplet having an initial dry radius of ∼300 nm. Simulations started from the saturation ratio of 0.85 were taken to supersaturation where the particle was activated to a cloud droplet and growth was simulated to the size of ten micrometers. For simplicity, OH concentration in the gas phase is the same in sub‐ and supersaturated conditions. It can be seen that SOA production is faster

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surface tension is less than 3% in the smallest droplets. Furthermore, at critical supersaturation, the droplets become more dilute and surface area increases, and thus the surface tension depression at the point of cloud droplet activation is negligible.

4. Conclusions

Figure 3. Effect of relative humidity on SOA formation.

in the cloud droplet, but only by a factor of ∼20 although the volume increases by a factor of ∼1000 when compared to subsaturated conditions. The reason for this can be inferred from Figure 1a. The surface‐to‐volume ratio decreases for the large droplets and thus, the surface‐bulk partitioning becomes less important, as also shown by Djikaev and Tabazadeh [2003] for cloud droplets. In the atmosphere, an air parcel spends more time in subsaturated conditions than in a cloud. Furthermore, only a fraction of the total aerosol population is capable of forming cloud droplets. Surface‐enhanced SOA formation in aqueous aerosols may therefore be on the same order as that in clouds and should be taken into account in SOA models. In our model, the condensation of OH from the gas phase is a limiting factor in the MG+OH chemistry. With increasing droplet size the surface‐to‐volume ratio decreases and gas‐phase diffusion is not fast enough to keep concentrations close to Henry’s law equilibrium. 3.2. Surface Tension [18] In previous studies, it has been shown that in many cases the surface tension depression due to non‐volatile surfactants is almost negligible at the point of cloud droplet activation due to surface‐bulk partitioning [Kokkola et al., 2006]. However, in those studies it was not necessary to take into account the partitioning between the gas and aqueous phases. In the simulations presented in this study, the concentration of MG in the bulk phase alone is not high enough to decrease surface tension according to equation (1), as the Henry’s law coefficient for MG is too low. However, if we suppose the reaction products to be similarly surface active as the MG‐product mixture of Sareen et al. [2010], the amount of surfactant can be increased by an order of magnitude or more within a few hours in the atmosphere. If we use the sum of the concentrations of MG and its reaction products after 4 hours (Figure 3) to calculate the surface tension of droplets according to equation (1), the decrease in

[19] Based on our modeling results, the surface partitioning of organics notably affects the formation of SOA from aqueous‐phase chemistry. We have shown that the total uptake of MG on aqueous aerosol might be increased by a factor of ten compared to values given by Henry’s law for bulk liquid water. This leads to an enhancement in SOA formation. However, assuming that bulk surface tension behaviour translates to the aerosol system, the amount of formed surfactant is still too low to affect surface tension during the formation of cloud droplets, and SOA product formation by the MG‐OH reaction in cloud or fog droplets is faster than in the aqueous aerosol drops. Even though the effect of the surface tension reduction caused by MG on cloud activation is negligible according to our calculations, it is vital to have surface tension as well as reaction rate data for atmospherically relevant surfactants, to be able to take into account the enhancement of aqueous phase chemistry of semi‐volatile surface‐active compounds due to surface‐bulk partitioning. [20] Acknowledgments. This work was supported by the Academy of Finland (projects 123466, 1118615). VFM and ANS acknowledge the NASA Tropospheric Chemistry program (grant NNX09AF26G), NLP acknowledge the Carlsberg foundation (grant 2009_01_0366) and JNS acknowledge the Saastamoinen Foundation for funding. The National Center for Atmospheric Research is sponsored by the U.S. National Science Foundation.

References Altieri, K. E., S. P. Seitzinger, A. G. Carlton, B. J. Turpin, G. C. Klein, and A. G. Marshall (2008), Oligomers formed through in‐cloud methylglyoxal reactions: Chemical composition, properties, and mechanism investigated by ultra‐high resolution FT‐ICR mass spectrometry, Atmos. Environ., 42, 1476–1490, doi:10.1016/j.atmosenv.2007.11.015. Carlton, A. G., B. J. Turpin, H.‐J. Lim, K. E. Altieri, and S. Seitzinger (2006), Link between isoprene and secondary organic aerosol (SOA): Pyruvic acid oxidation yields low volatility organic acids in clouds, Geophys. Res. Lett., 33, L06822, doi:10.1029/2005GL025374. Djikaev, Y. S., and A. Tabazadeh (2003), Effect of adsorption on the uptake of organic trace gas by cloud droplets, J. Geophys. Res., 108(D22), 4689, doi:10.1029/2003JD003741. Donaldson, D. J. (1999), Adsorption of atmospheric gases at the air— Water interface. I. NH3, J. Phys. Chem. A, 103(1), 62–70, doi:10.1021/ jp9833247. Ervens, B., et al. (2003), CAPRAM 2.4 (MODAC mechanism): An extended and condensed tropospheric aqueous phase mechanism and its application, J. Geophys. Res., 108(D14), 4426, doi:10.1029/ 2002JD002202. Facchini, M. N. C., M. Mircea, S. Fuzzi, and R. J. Charlson (1999), Cloud albedo enhancement by surface‐active organic solutes in growing droplets, Nature, 401, 257–259, doi:10.1038/45758. Hallquist, M., et al. (2009), The formation, properties and impact of secondary organic aerosol: Current and emerging issues, Atmos. Chem. Phys., 9, 5155–5236, doi:10.5194/acp-9-5155-2009. Hanson, D. R., A. R. Ravishankara, and S. Solomon (1994), Heterogeneous reactions in sulfuric acid aerosols: A framework for model calculations, J. Geophys. Res., 99, 3615–3629, doi:10.1029/93JD02932. Ho, K. F., J. J. Cao, S. C. Lee, K. Kawamura, R. J. Zhang, J. C. Chow, and J. G. Watson (2007), Dicarboxylic acids, ketocarboxylic acids, and dicarbonyls in the urban atmosphere of China, J. Geophys. Res., 112, D22S27, doi:10.1029/2006JD008011. Jang, M., and R. M. Kamens (2001), Atmospheric secondary aerosol formation by heterogeneous reactions of aldehydes in the presence of a

4 of 5

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sulfuric acid aerosol catalyst, Environ. Sci. Technol., 35, 4758–4766, doi:10.1021/es010790s. Kanakidou, M., et al. (2005), Organic aerosol and global climate modelling: A review, Atmos. Chem. Phys., 5, 1053–1123, doi:10.5194/acp-51053-2005. Kawamura, K., and O. Yasui (2005), Diurnal changes in the distribution of dicarboxylic acids, ketocarboxylic acids and dicarbonyls in the urban Tokyo atmosphere, Atmos. Environ., 39, 1945–1960, doi:10.1016/j. atmosenv.2004.12.014. Köhler, H. (1936), The nucleus in the growth of hygroscopic droplets, Trans. Faraday Soc., 32, 1152–1161, doi:10.1039/tf9363201152. Kokkola, H., R. Sorjamaa, A. Peräniemi, T. Raatikainen, and A. Laaksonen (2006), Cloud formation of particles containing humic‐like substances, Geophys. Res. Lett., 33, L10816, doi:10.1029/2006GL026107. Kroll, J. H., and J. H. Seinfeld (2008), Chemistry of secondary organic aerosol: Formation and evolution of low‐volatility organics in the atmosphere, Atmos. Environ., 42, 3593–3624, doi:10.1016/j.atmosenv. 2008.01.003. Kroll, J. H., N. L. Ng, S. M. Murphy, V. Varutbangkul, R. C. Flagan, and J. H. Seinfeld (2005), Chamber studies of secondary organic aerosol growth by reactive uptake of simple carbonyl compounds, J. Geophys. Res., 110, D23207, doi:10.1029/2005JD006004. McFiggans, G., D. O. Topping, and M. H. Barley (2010), The sensitivity of secondary organic aerosol component partitioning to the predictions of component properties—Part 1: A systematic evaluation of some available estimation techniques, Atmos. Chem. Phys., 10, 10,255–10,272, doi:10.5194/acp-10-10255-2010. Prisle, N., T. Raatikainen, R. Sorjamaa, B. Svenningsson, A. Laaksonen, and M. Bilde (2008), Surfactant partitioning in cloud droplet activation: A study of C8, C10, C12 and C14 normal fatty acid sodium salts, Tellus, Ser. B, 60, 416–431, doi:10.1111/j.1600-0889.2008.00352.x. Sareen, N., A. N. Schwier, E. L. Shapiro, D. Mitroo, and V. F. McNeill (2010), Secondary organic material formed by methylglyoxal in aqueous aerosol mimics, Atmos. Chem. Phys., 10, 997–1016, doi:10.5194/acp-10997-2010.

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Schwier, A. N., N. Sareen, D. Mitroo, E. L. Shapiro, and V. F. McNeill (2010), Glyoxal‐methylglyoxal cross‐reactions in secondary organic aerosol formation, Environ. Sci. Technol., 44, 6174–6182, doi:10.1021/ es101225q. Seinfeld, J. H., and J. F. Pankow (2003), Organic atmospheric particulate material, Annu. Rev. Phys. Chem., 54, 121–140, doi:10.1146/annurev. physchem.54.011002.103756. Sorjamaa, R., and A. Laaksonen (2006), The influence of surfactant properties on critical supersaturations of cloud condensation nuclei, J. Aerosol Sci., 37, 1730–1736, doi:10.1016/j.jaerosci.2006.07.004. Sorjamaa, R., B. Svenningsson, T. Raatikainen, S. Henning, M. Bilde, and A. Laaksonen (2004), The role of surfactants in Köhler theory reconsidered, Atmos. Chem. Phys., 4, 2107–2117, doi:10.5194/acp-4-2107-2004. Tan, Y., G. Annmarie, S. P. Carlton, B. Seitzinger, and J. Turpin (2010), SOA from methylglyoxal in clouds and wet aerosols: Measurement and prediction of key products, Atmos. Environ., 44, 5218–5226, doi:10.1016/j.atmosenv.2010.08.045. Wilke, C. R., and P. Chang (1955), Correlation of diffusion coefficients in dilute solutions, AIChE J., 1, 264–270, doi:10.1002/aic.690010222. Zhou, X. L., and K. Mopper (1990), Apparent partition‐coefficients of 15 carbonyl compounds between air and seawater and between air and fresh‐water—Implications for the air sea exchange, Environ. Sci. Technol., 24, 1864–1869, doi:10.1021/es00082a013. H. Kokkola, Kuopio Unit, Finnish Meteorological Institute, PO BOX 1627, FI‐70211 Kuopio, Finland. A. Laaksonen and S. Romakkaniemi, Department of Applied Physics, University of Eastern Finland, PO BOX 1627, FI‐70211 Kuopio, Finland. ([email protected]) V. F. McNeill and A. N. Schwier, Department of Chemical Engineering, Columbia University, New York, NY 10027, USA. N. L. Prisle, Department of Physics, University of Helsinki, PO Box 48, FI‐00014 Helsinki, Finland. J. N. Smith, National Center for Atmospheric Research, PO Box 3000, Boulder, CO 80307, USA.

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