Atmos. Chem. Phys., 7, 4601–4611, 2007 www.atmos-chem-phys.net/7/4601/2007/ Atmospheric © Author(s) 2007. This work is licensed Chemistry under a Creative Commons License. and Physics Modelling the cloud condensation nucleus activity of organic acids on the basis of surface tension and osmolality measurements Z. Varga1, G. Kiss2, and H.-C. Hansson3 1Department of Earth and Environmental Sciences at University of Pannonia, Egyetem u. 10, 8200 Veszprem,´ Hungary 2Air Chemistry Group of the Hungarian Academy of Sciences, University of Pannonia, P.O.Box 158, 8201 Veszprem,´ Hungary 3Institute of Applied Environmental Research, Stockholm University, Stockholm, 10691, Sweden Received: 8 March 2007 – Published in Atmos. Chem. Phys. Discuss.: 19 April 2007 Revised: 15 August 2007 – Accepted: 3 September 2007 – Published: 7 September 2007 Abstract. In this study vapour pressure osmometry was cases good agreement was found. For modelling purposes used to determine water activity in the solutions of organic Sc vs. ddry plots are given and the dependence of water activ- acids. The surface tension of the solutions was also moni- ity and surface tension on concentration are also formulated. tored in parallel and then Kohler¨ curves were calculated for nine organic acids (oxalic, malonic, succinic, glutaric, adipic, maleic, malic, citric and cis-pinonic). Surface tension de- pression is negligible for most of the organic acids in di- 1 Introduction lute (≤1 w/w%) solutions. Therefore, these compounds af- fect equilibrium vapour pressure only in the beginning phase The role of organic compounds in cloud formation (and of droplet formation when the droplet solution is more con- thereby in indirect aerosol forcing) has received particular centrated but not necessarily at the critical size. An exception attention in the past few years. It has been shown that in addi- is cis-pinonic acid which remarkably depress surface tension tion to inorganic salts organic aerosol particles can also act as also in dilute (0.1 w/w%) solution and hence at the critical cloud condensation nuclei. Kohler¨ theory is often used to cal- point. The surface tension of organic acid solutions is influ- culate the critical supersaturation corresponding to an aerosol enced by the solubility of the compound, the length of the particle of known dry size and composition. However, the carbon chain and also by the polar functional groups present reliability of the calculation strongly depends on the input in the molecule. Similarly to surface tension solubility plays parameters of the model, namely the surface tension and the an important role also in water activity: compounds with water activity of the solution formed on the aerosol particle. higher solubility (e.g. malonic, maleic and glutaric acid) re- Inorganic salts of atmospheric relevance do not change the duce water activity significantly in the early phase of droplet surface tension of water significantly, so in the model cal- formation while less soluble acids (e.g. succinic and adipic culations the surface tension of water can be used. This is acid) are saturated in small droplets and the solution starts not the case for organic aerosol components, some of which diluting only in bigger droplets. As a consequence, com- are surfactants, e.g. humic-like substances or other organic pounds with lower solubility have a minor effect on water acids. In the past few years surface tension of cloud water activity in the early phase of droplet formation. To deduce (Hitzenberger et al., 2002; Decesari et al., 2004), fog wa- the total effect Kohler¨ curves were calculated and critical su- ter and aerosol extract (Seidl and Hanel,¨ 1983; Capel, 1990; persaturations (Sc) were determined for the organic acids us- Facchini et al., 2000; Decesari et al., 2004; Kiss et al., 2005) ing measured surface tension and water activity. It was found as well as that of solution of individual organic compounds that critical supersaturation grew with growing carbon num- (Ervens at al., 2004; Shulman et al., 1996; Tuckerman and ber. Oxalic acid had the lowest critical supersaturation in the Cammenga, 2004) were studied leading to a basic knowledge size range studied and it was comparable to the activation of on their surface tension effect. However, hardly any infor- ammonium sulphate. The Sc values obtained in this study mation is available on the water activity of such atmospheric were compared to data from CCNC experiments. In most samples or model solutions of organic compounds. Conse- quently, the critical supersaturation of particles consisting of Correspondence to: G. Kiss a pure organic compound or mixture of different components ([email protected]) can be estimated only with high uncertainty. Published by Copernicus Publications on behalf of the European Geosciences Union. 4602 Z. Varga et al.: Modelling the effect of organic acids Kohler¨ theory (Kohler,¨ 1936) predicts the supersaturation rived from the dissociation constants and the actual concen- (S) of water vapour over a solution droplet of a given radius tration of oxalic acid in the droplet. Thus, reasonably good (r): agreement between theoretical calculations and experimental results were found. While this approach gives more realistic S = p/p0 − 1 = aw exp(2σ Mw/(rρRT ) − 1 (1) results for strong acids than a fixed van’t Hoff factor, it can- not be used for mixtures of unknown compounds (e.g. atmo- Where p is the water vapour pressure over the droplet solu- spheric samples) since the dissociation constants, the molec- tion, p is the water vapour pressure over a flat water sur- 0 ular weight and the concentration of the aerosol constituents face, a is the water activity in the droplet solution, σ is the w are not known. In such a case, of course, also the fixed van’t surface tension of the droplet solution, M is the molecular w Hoff factor approach leads to high uncertainty. weight of water, ρ is the density of water, R is the universal In this paper we applied a new approach for the calculation gas constant and T is the temperature. The vapour pressure is of the Kohler¨ curves. The osmolality and surface tension of decreased by the dissolved matter (Raoult effect, represented a series of solution were measured then water activity was by a ) while the curvature of the droplet exerts an opposite w derived from osmolality. Finally, the original form of the effect (Kelvin effect, accounted for by the exponential term Kohler¨ equation (Eq. 1) was used to calculate critical super- which is dependent on the surface tension of the solution.) saturation. Thus, uncertainties originating from the use of the Water activity data for organic compounds are rare, es- simplified equation (Eq. 2) were eliminated. The applicabil- pecially for mixtures. So, often the simplified form of the ity of the method was demonstrated on organic acids, since Kohler¨ equation is used to calculate the critical supersatura- these compounds are usually found in highest concentration tion: in aerosol samples as individual compounds. Furthermore, S = A/r − B/r3 (2) model calculations and measurements with cloud condensa- tion nucleus counters (CCNC) are available for organic acids, In this equation A/r represents the Kelvin effect, where so the results of our model calculations can be compared to A=2σ Mw/ρRT . The second term stands for the Raoult ef- these data. fect and B=3i*ms/Ms*Mw/4πρ, where ms is the mass of the solute in the droplet, Ms is the molecular weight of the solute in the droplet, i is the van’t Hoff factor of the so- 2 Experimental lute in the droplet. It can be shown that i∼ν8 (Kreiden- weis et al., 2005) where ν is the number of ions produced In this study nine organic acids were included: ox- from one solute molecule and 8 is the molal osmotic coeffi- alic, malonic, succinic, glutaric, adipic acid (C2–C6 cient of the solute in solution. Often 8=1 is assumed and i α, ω-dicarboxylic acids), maleic acid (unsaturated C4 dicar- is considered to reflect the dissociation ability of the solute. boxylic acid), malic acid (hydroxy C4 dicarboxylic acid), Thus, the van’t Hoff factor is about 1 for non-electrolytes citric acid (hydroxy C6 tricarboxylic acid) and pinonic acid and typically higher than 1 for electrolytes. This form of the (monocarboxylic C10 acid, oxidation product of terpene). Kohler¨ equation contains several mathematical and physico- Basic properties of the investigated acids are summarized in chemical simplifications, which can lead to uncertainty, es- Table 1. First, a solution corresponding to a defined growth pecially in the early phase of droplet formation when the factor (GF) was prepared. Then the osmolality was mea- concentration of the solute is high. The van’t Hoff factor sured with a KNAUER K –7000 vapour pressure osmome- is a critical parameter in the Raoult term since it depends on ter (VPO) at 40◦C with equilibrium time of 3.5 min. The the concentration of the organic compound. Furthermore, in working principle of the VPO is the measurement of tem- case of mixtures other organic and inorganic species present perature difference arising from the vapour pressure differ- in the solution can also influence it. Due to the lack of in- ence between a solution droplet and a pure water droplet. formation on concentration dependence, fixed value of van’t The advantage of the method is that the osmometer accounts Hoff factor is applied in most investigations to calculate the for all phenomena (e.g. dissociation of electrolytes, electro- critical supersaturation. Since organic acids are weak elec- static interactions in the solution, etc.) that influence the trolytes, van’t Hoff factor of unity is often used (Hori et al., vapour pressure of the solution. Furthermore, the wide work- 1996; Cruz and Pandis, 1997, 1998; Corrigan and Novakov, ing range (0.005–10 mol/kg water) of the instrument allows 1999; Prenni et al., 2001; Giebl et al., 2002; Raymond and activity measurement in the 0.85–1 activity range where the Pandis, 2002; Kumar et al., 2003; Bilde and Svenningsson, electrodynamic balance is less applicable.