Sorption of vapor phase octanoic acid onto deliquescent salt particles

Single NaCl and (NH4)(2)SO4 particles (D-p = 30-50 mu m) have been suspended and exposed to airstreams containing octanoic acid vapor and controlled amounts of water vapor. Sorption of vapor phase organic onto the coated droplets was found to occur at a rate that was independent of the relative humidity, the particle size, and the phase (solid or droplet) of the salt. These results are only comparable with atmospheric studies to a limited extent, however, since both the organic vapor concentrations and the particle sizes in this research were generally larger than their atmospheric counterparts. Because of the relatively large coating scale of the experiment, the measured sorption rate is thought to correspond to octanoic acid sorbing onto bulk octanoic coatings around the salt particles, and the average observed uptake coefficient was found to be gamma(avg) = (6.7 +/- 3.0) X 10(-2). Imperfect surface accommodation was hypothesized as a source for the nonunity uptake coefficient, and the accommodation coefficient in this case was found to be alpha(avg) = (3.5(-2.0)(+8.2)) X 10(-4). Formation of organic surfactant layers did not cause the hydrophilic salts to become completely hydrophobic, The organic tended to slow the salts' deliquescence rates, however, which suggests that organic components may effectively limit water uptake by salt aerosols in competitive atmospheric conditions.