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 sus pended and exposed to airstreams containing octanoic acid vapor and co ntrolled 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 d roplet) of the salt. These results are only comparable with atmospheri c studies to a limited extent, however, since both the organic vapor c oncentrations and the particle sizes in this research were generally l arger than their atmospheric counterparts. Because of the relatively l arge coating scale of the experiment, the measured sorption rate is th ought to correspond to octanoic acid sorbing onto bulk octanoic coatin gs around the salt particles, and the average observed uptake coeffici ent was found to be gamma(avg) = (6.7 +/- 3.0) X 10(-2). Imperfect sur face accommodation was hypothesized as a source for the nonunity uptak e coefficient, and the accommodation coefficient in this case was foun d 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 comple tely 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 condit ions.