Measuring Aerosol Phase Changes and Hygroscopicity with a Microresonator Mass Sensor

The interaction between atmospheric aerosol particles and water vapor influences aerosol size, phase, and composition, parameters which critically influence their impacts in the atmosphere. Methods to accurately measure aerosol water uptake for a wide range of particle types are therefore merited. We present here a new method for characterizing aerosol hygroscopicity, an impaction stage containing a microelectromechanical systems (MEMS) microresonator. We find that deliquescence and efflorescence relative humidities (RHs) of sodium chloride and ammonium sulfate are easily diagnosed via changes in resonant frequency and peak sharpness. These agree well with literature values and thermodynamic models. Furthermore, we demonstrate that, unlike other resonator-based techniques, full hygroscopic growth curves can be derived, including for an inorganic-organic mixture (sodium chloride and malonic acid) which remains liquid at all RHs. The response of the microresonator frequency to temperature and particle mechanical properties and the resulting limitations when measuring hygroscopicity are discussed. MEMS resonators show great potential as miniaturized ambient aerosol mass monitors, and future work will consider the applicability of our approach to complex ambient samples. The technique also offers an alternative to established methods for accurate thermodynamic measurements in the laboratory.