Detection of Sulfate Esters as a Function of Particle Composition using Single Particle Mass Spectrometry

Secondary organic aerosols (SOA) are of increasing importance due to their effects on atmospheric chemistry. The formation of SOA is acid catalyzed, and since sulfuric acid is readily formed in the atmosphere, it is important to find useful methods to measure SOA particles under atmospheric conditions. It is challenging to differentiate particles containing organosulfates from those containing organics plus sulfuric acid. The Aerosol Time-of-Flight Mass Spectrometer (ATOFMS) can measure single particles in real-time, and has been used in a number of studies of SOA. In order to interpret the ATOFMS results from SOA particles, known particle types need to be analyzed under real-world conditions to understand how ATOFMS signal depends on the particle composition. This work focuses on sulfate esters, one chemical component of SOA that has been proposed to form by acid-catalysis. We investigate the characteristic peaks seen in the single-particle mass spectra when sulfate-ester containing particles are ionized in the ATOFMS to determine whether the intensity of the peaks changes as a function of particle composition. Solutions of a sulfate ester with different concentrations of XSO4 (X = H2, (H)(NH4), or (NH4)2) were nebulized and sampled into an ATOFMS. The measured spectra were analyzed for significant ion peaks and ion intensities. The peak intensities of the detected ions were found to depend on the composition of the particle, with certain characteristic sulfate-ester peaks increasing in intensity as the concentration of XSO4 increases. We will present these trends for a number of model compounds as a function of particle composition.