Simultaneously Characterizing the Volatility Distribution and Phase State of Laboratory-Generated and Ambient Aerosol Particles with a Vocus 2R Chemical Ionization Mass Spectrometer

Sining Niu, Jordan Krechmer, Harald Stark, YUE ZHANG, Texas A&M University

     Abstract Number: 371
     Working Group: Aerosol Chemistry

Abstract
Aerosols are ubiquitous in the atmosphere, and they play critical roles in both climate and human health. Recent studies have shown that the physicochemical properties of aerosols, including the volatility and phase state, have significant implications on their growth, gas-particle partitioning, chemical reactivity, and climate impacts. Nevertheless, obtaining the online volatility and phase state information of atmospheric particles has been challenging, limited our abilities to accurately predict their evolution and climate implications.

This study establishes a novel online method for the first time to obtain both the volatility distribution and phase state with the Vocus 2R Chemical Ionization Mass Spectrometer and Vocus Inlet for Aerosols (Vocus 2R/VIA). A calibration line linking thermal desorption temperature to known saturation mass concentration (C*) is first established using six types of pure organic aerosols, covering a range from 0.001 – 1000 μg m^-3. The volatility distribution of secondary organic aerosols (SOA) is determined by using multi-linear regression fitting on the desorption curve with calibration data.

The viscosity value, a parameter to characterize aerosol phase state, is subsequently quantified by combining the volatility distribution, a volatility-glass transition parameterization, and the Vogel-Tammann-Fulcher (VTF) equation at any given temperature and relative humidity (RH). This method is further validated using lab-generated α-pinene and β-caryophyllene SOA from ozonolysis reactions, with the derived viscosity values agreeing with previous literature findings.

This above method is applied during a mobile laboratory field campaign to determine the real-time volatility and viscosity of ambient aerosols. Combined with meteorological conditions (such as RH and temperature) and inorganic chemical composition from an high resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS), the phase state of complex ambient particles were determined. The results demonstrates that the phase state of the complex aerosols in Houston are mostly liquid or semi-solid. Such techniques provide a unique way to measure the phase state of the aerosol particles with high time resolution. The results can help further constraining the formation, reaction, and climate effects of aerosol particles.