Water uptake on humic and fulvic acids: Aerosol and thin film measurements
ABSTRACT The water uptake of humic and fulvic acid aerosols was determined by hygroscopic tandem differential
mobility analysis (hTDMA) and extinction Fourier transform infrared (FTIR) spectroscopy. Water uptake on humic and fulvic acid thin films was also investigated using attenuated total reflectance (ATR) FTIR spectroscopy. The hygroscopic growth of monodisperse, 100-nm (dry) Suwannee River fulvic acid (SRFA) and humic acid sodium salt (NaHA) aerosols was determined and modelled using Köhler theory. A single parameter, the ionic density (ρion), which contains physical properties that are not well established for these substances, was determined for SRFA and NaHA to be 2.1×10−3 and 7.0×10−3 mol cm−3 respectively. The hygroscopic growth was then modelled using the ρion-Köhler equation and the critical parameters determined. The critical percent supersaturation of SRFA and NaHA was determined to be 0.60 and 0.33% respectively using the surface tension of water; and 0.35 and 0.19% respectively using the surface tension of aqueous HULIS. κ-Köhler theory, was also used to calculate the critical supersaturation and was found to be in good agreement with the ρion representation. Both extinction FTIR of aerosols and ATR-FTIR absorption measurements of thin films confirm enhanced water uptake with increasing relative humidity (RH).