Parameterization of Aerosol Particle Growth Factors as a Function of Relative Humidity

Data collected as part of BAQS-Met in Southern Ontario, Canada during summer 2007 was used to study particle growth factors as a function of relative humidity using a Hygroscopic Tandem Differential Mobility Analyzer (HTDMA). Dry particle diameters of 50 nm, 100 nm, and 150 nm were used. The HTDMA was scanned between 50% and 85% RH over one hour cycles. A simple model was used to fit particle growth factors as a function of RH. We considered both a two component case in which particles were assumed to consist of organic material and ammonium sulfate, and a three component case in which ammonium bisulfate was added. The fits make it possible to retrieve the relative amounts of the three phases and the hygroscopicity parameter of the organic matter. Aerosol mass spectrometer (AMS) data was used to independently estimate the organic and sulfate fractions. In most cases the organic fraction from the fits was in a reasonable agreement with the AMS data. However, when the AMS organic fraction was low, there was a tendency for the fits to overestimate it and to assign the organic fraction a very high hygroscopicity parameter. Including a small amount of ammonium bisulfate significantly improved the results.