A Novel Methodology for Assessing the Hygroscopicity of Aerosol Filter Samples

Abstract. Due to US regulations, concentrations of hygroscopic inorganic sulfate and nitrate have declined in recent years, leading to an increased importance in the hygroscopic nature of organic matter (OM). The hygroscopicity of OM is poorly characterized because only a fraction of the multitude of organic compounds in the atmosphere are readily measured and there is limited information on their hygroscopic behaviours. Hygroscopicity of aerosol is traditionally measured using Humidified Tandem Differential Mobility Analyzer (HTDMA) or Electrodynamic Balance (EDB). EDB measures water uptake by a single particle. For ambient and chamber studies, HTDMA measurements provide water uptake and particle size information but not chemical composition. To fill in this information gap, we have developed a novel methodology to assess the water uptake of particle collected on Teflon filters, thereby providing an opportunity to link the measured hygroscopicity with ambient particle composition. To test the method, hygroscopic measurements were conducted in the laboratory for ammonium sulfate, sodium chloride, glucose, and malonic acid, which were collected on 25 mm Teflon filters using an aerosol generator and sampler. Constant humidity solutions (CHS) were employed to maintain the relative humidity (RH) at approximately 84 %, 90 %, and 97 % in small chambers. Hygroscopic parameters, including the water-to-solute (W/S) ratio, molality, mass fraction solute (mfs), and growth factors (GF), were calculated from the measurements. The results obtained are consistent with those reported by the E-AIM model and previous studies utilizing HTDMA and EDB for these compounds, highlighting the accuracy of this new methodology. This new approach enables the hygroscopicity and chemical composition of individual filter samples to be assessed so that in complex mixtures such as chamber and ambient samples, the total water uptake can be parsed between the inorganic and organic components of the aerosol.