concentration were-15%, 5%, and 8%, respectively. Mineral aerosol contributed-16% to the PM2.5 aerosol mass. These data show that combustion-related particles rather than wind-blown dust dominated the light extinction budget during June 1999.

In ambient air, organic chemicals are common constituents of aerosol particles, and many of them are surfaceactive. If these compounds are present as surface films on aqueous droplets, mass transfer of gaseous molecules into or out of the droplets could be impeded, and the hygroscopic growth of acidic droplets could be reduced. In this work, the effects of organic films on the hygroscopic growth rate of ultrafine H 2 SO 4 aerosols (40-120 nm) were investigated under relative humidity conditions of 20-85% and over time scales of 6 and 10 s. Lauric (C 12 , saturated), stearic (C 18 , saturated), and oleic (C 18, unsaturated) acids were chosen as the film coating materials. The experimental results showed that the presence of monolayer films of some fatty acids retards the hygroscopic growth rate of H 2 SO 4 aerosol due to the obstructive effect of the films on the permeation of water vapor. The retardation effects depend on the original film coating thickness. Molecular structure of the film-forming compounds is an important factor for determining the effectiveness of the film. Linear molecules, such as lauric and stearic acids, have stronger effects on mass transport of water vapor than those of nonlinear molecules, such as oleic acid.

The effect of a single electric charge on the efficiency with which ultrafine particles deposit in human airways has been investigated. When inhaled short-lived radon progeny are attached to electrically neutral particles their deposition efficiency is controlled by diffusion. But most ambient particles carry one, or a few, charges. We measured and compared the deposition (DE) of singly charged, charge-neutralized, and zero-charge 20-nm and 125-nm particles in hollow-cast models of human airways. These particle sizes were selected because they are about where modal peaks occur for the activity of the short-lived radon progeny in indoor air. For singly charged 20-nm particles deposition (+/- standard error) in the casts was 3.4 +/- 0.3 times that for charge neutralized aerosols and 5.3 +/- 0.3 times the amount deposited for zero-charged particles. Corresponding ratios for the 125-nm particles were 2.3 +/- 0.3 and 6.2 +/- 0.7. Since most ambient particles are charged this effect must be considered when models are used to predict dose from inhaled ultrafine particles.

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