Journal of Atmospheric Chemistry 22: 285-302, 1995. 285 @ 1995 Kluwer Academic Publishers. Printed in the Netherlands. Solubilities of Pyruvic Acid and the Lower (C1-C6) Carboxylic Acids. Experimental Determination of Equilibrium Vapour Pressures Above Pure Aqueous and Salt Solutions I. KHAN t, R BRIMBLECOMBE 2 and S. L. CLEGG 2 1Department of Chemistry, University of Peshawar, Peshawar, Pakistan 2School of Environmental Sciences, University of East Angtia, Norwich NR4 7TJ, U.K. (Received: 25 April 1994; in final form: 4 October 1994) Abstract. Henry's law constants K~ (tool kg -1 arm-1) have been determined at 298.15 K for the followingorganic acids: formic acid (5.53-t-0.27 × 103); acetic acid (5.504-0.29 × 103);propionic acid (5.71 4-0.34 × 103); n-butyric acid (4.73 +0.18 x 103); isobutyric acid (1.13 4-0.12 x 103); isovaleric acid (1.204-0.11 × 103)and neovaleric acid (0.353:t:0.04 x 103).They have also been determinedfrom T = 278.15 K to T = 308.15 K for n-valeric acid (In (KJ¢) = - 14.3371 + 6582.96/T ); n-caproic acid (ln (K~r) = -13.9424 + 6303.73/T) and pyruvic acid (in (K~z) = -4.41706 + 5087.92/T). The influence of 9 salts on the solubility of pyruvic acid at 298.t5 K has been measured. Pyruvic acid is soluble enough to partition strongly into aqueous atmospheric aerosols. Other acids require around 1 g of liquid water m -3 (typical of clouds) to partition' significantly into the aqueous phase. The degree of partitioning is sensitive to temperature. Considering solubilityand dissociation (to formate) alone, the ratio of formic acid to acetic acid in liquid water in the atmosphere (at equilibrium with the gas phase acids) is expected to increase with rising pH, but show little variation with temperature. Key words: Henry's law, solubility, removal, salt effect, formic acid, acetic acid, propionic acid, n- butyric acid, isobutyric acid, n-valeric acid, isovaleric acid, neovaleric acid, n-caproic acid, pyruvic acid. 1. Introduction Carboxylic acids are common organic constitutents of aerosols and rain drops (Graedel and Weschler, 1981), with formic and acetic acids contributing signif- icantly to natural acidity in precipitation and cloud water (Keene and Galloway, 1988). Propionic and pyruvic acids are also widespread (Kawamura et al., 1985; Talbot et al., 1990). In addition, a wide variety of carboxytic acids with C > 3 is found in urban aerosols (Schuetzle et al., 1975; Cronn et al., 1977; Grosjean et al., 1978), in marine aerosols (Burger and Garrett, 1976; Kawamura and Gagosian, t 990) and in precipitation (Keene and Galloway, 1984; Lunde et al., 1977; Levsen et at., 1990). Most of the carboxylic acids in the atmosphere are stable under ambient condi- tions. The acids are found in aerosols and rain water as well as in the gas phase,