Sampling at controlled relative humidity with a cascade impactor

The design and function of a device that regulates the relative humidity of an ambient aerosol sample is described. We use this RH controller upstream of MOUDI impactors to permit sampling at relative humidities in the 70-80% range. Humidity control is achieved by allowing the aerosol to approach equilibrium with a saturated salt solution. Benefits to sampling with impactors in this relative humidity range include greatly reduced bounce of fine, hygroscopic particles, minimal flow-induced sizing errors, and minimization of uncertainties in measured size distributions due to diurnal variations in relative humidity during sampling. Data from field measurements in a humid environment (Look Rock, TN) and arid environments (Las Vegas, NV and Meadview, AZ) are discussed.