CARBOXYLIC-ACIDS IN CLOUDS AT A HIGH-ELEVATION FORESTED SITE IN CENTRAL VIRGINIA

During September 1990 we sampled coarse (> 18-mu m diameter) and fine (18- to 5.5-mu m diameter) droplets and liquid-water content (LWC) in cloud from a tower on a forested ridge top in Shenandoah National Park , Virginia. Cloud-water pH and aqueous- and vapor-phase concentrations of carboxylic acids (HCOOH and CH3COOH) and formaldehyde (HCHO) were measured in parallel over 1- to 1.5-hour intervals. Both size fraction s of cloud droplets contained similar concentrations of carboxylic spe cies and H+ during most sampling; most cloud water was in coarse dropl ets, The pH of coarse (3.27-4.76) and fine (3.22-4.70) droplets couple d with total LWC of 0.04-0.56 g m(-3) STP (standard m(3) at 0 degrees C and 1 atm) resulted in the partitioning of carboxylic acids primaril y in the vapor phase. The observed phase partitioning for CH3COOH was within the uncertainty range of thermodynamic data. However, HCOOH exh ibited significant phase disequilibria, which could not be explained b y artifacts from variable LWC or from mixing droplets of different aci dities. We hypothesize that the large volume of liquid water deposited on the forest canopy interacted with the near-surface cloud leading t o apparent disequilibria based on time-integrated samples. HCOOH was s electively depleted relative to CH3COOH in cloud, particularly at high er pH, suggesting rapid removal of HCOOH by cloud-water deposition. We saw no evidence for significant production of HCOOH from the aqueous- phase oxidation of HCHO.