Probing Temperature and Size Dependence of Hygroscopic Properties of Atmospheric Aerosols using ATR-IR Spectroscopy

YONG LIU, Mark dela Rosa, Dong Fu

University of Colorado Denver

     Abstract Number: 191
     Last modified: April 28, 2010

Abstract
Phase and hygroscopic properties of atmospheric particles affect global radiation budget and atmospheric composition by changing light scattering and absorption abilities and reactivity of aerosols. Atmospheric aerosols have typical diameters ranging over four orders of magnitude, from a few nanometers to a few tens of micrometers. Likewise, temperature in the troposphere spans nearly 80 degrees from 217K in the tropopause to close to room temperature at sea level. At present, our knowledge about temperature and size effects on hygroscopic properties including deliquescence, efflorescence and hygroscopic growth of aerosols is fairly limited. In this work, hygroscopic properties of aerosols and their temperature and size dependences were investigated using a newly developed environmentally controlled flow reactor coupled to ATR-IR spectrometer. Substrate deposited and size selected water soluble inorganic and organic particles of atmospheric importance, such as NaCl, NaNO$_3, (NH$_4)$_2SO4, Ca(NO$_3)$_2 and CH$_3COONa etc, were used in the present case study to demonstrate feasibility of the approach. The deliquescence relative humidities (DRH) and efflorescence relative humidities (ERH) of aforementioned particles were determined by integration of absorbance of H$_2O bands from infrared spectra. Water to solute molar ratios of hydrating and dehydrating particles were quantified by comparing the integrated absorbances of H2O band and corresponding infrared active bands (e.g. NO$_3$^-, SO$_4$^-) based on measured cross section values. Our DRH, ERH and hygroscopic growth data are in good agreement with previously reported data. Studies of temperature and size effects on hygroscopic properties of aerosols show they are dependent on the compositions. Results here have demonstrated a new approach which is capable of providing improved aerosol hygroscopic data for our current atmospheric models.