Vacuum FTIR study on the hygroscopicity of magnesium acetate aerosols

Hygroscopicity and volatility of secondary organic aerosol (SOA) are two important properties, which determine the composition, concentration, size, phase state of SOA and thus chemical and optical properties for SOA. In this work, magnesium acetate (Mg(Ac)₂) aerosol was used as a simple SOA model in order to reveal relationship between hygroscopicity and volatility. A novel approach was set up based on a combination of a vacuum FTIR spectrometer and a home-made relative humidity (RH) controlling system. The striking advantage of this approach was that the RH and the compositions of aerosols could be obtained from a same IR spectrum, which guaranteed the synchronism between RH and spectral features on a sub-second scale. At the constant RH of 90% and 80% for 3000s, the water content within Mg(Ac)₂ aerosol particles decreased about 19.0% and 9.4% while there were 13.4% and 6.0% of acetate loss. This was attributed to a cooperation between volatile of acetic acid and Mg²⁺ hydrolysis in Mg(Ac)₂ aerosols, which greatly suppressed the hygroscopicity of Mg(Ac)₂ aerosols. When the RH changed with pulsed mode between ~70% and ~90%, hygroscopicity relaxation was observed for Mg(Ac)₂ aerosols. Diffuse coefficient of water in the relaxation process was estimated to be ~5×10^-12m²·s^-1 for the Mg(Ac)₂ aerosols. Combining the IR spectra analysis, the decrease in the diffuse coefficient of water was due to the formation of magnesium hydroxide accompanying acetic acid evaporation in the aerosols.