Studies of Chamber Organic Aerosol Using an Aerodyne High-Resolution Time-of-Flight Aerosol Mass Spectrometer

Abstract

Secondary organic aerosol (SOA) contributes a substantial fraction to total ambient particulate mass. SOA is a complex mixture of different organic species formed via many gas- and particle-phase reaction pathways. The Aerodyne aerosol mass spectrometer (AMS) has become a standard tool in measuring the bulk chemical composition of SOA in realtime. In addition to acquiring mass spectra of SOA, the high-resolution time-of-flight AMS, or HR-ToF-AMS, can distinguish and quantify ions with the same nominal mass but different elemental compositions. This thesis presents results from several studies in which the HR-ToF-AMS is used to chemically characterize SOA generated in chamber experiments. Glyoxal is a common oxidation product of both biogenic and anthropogenic volatile organic compounds (VOCs) and is known to partition into wet inorganic aerosol. Chamber studies of glyoxal uptake onto ammonium sulfate aerosol are conducted to better understand the mechanisms controlling glyoxal uptake onto ambient aerosol. Organic growth due to glyoxal uptake was found to be reversible under dark conditions. HR-ToF-AMS spectra provide evidence for glyoxal dimers and trimers existing in the particle phase. HR-ToF-AMS spectra indicate the irreversible formation of carbon-nitrogen compounds in the aerosol. Organosulfates are not detected under dark conditions; however, active photochemistry was found to occur within aerosol during irradiated experiments. Carboxylic acids and organic esters are identified within the aerosol. An organosulfate, which had been previously assigned as glyoxal sulfate in ambient samples and chamber studies of isoprene oxidation, is observed only in the irradiated experiments. Comparison with a laboratory-synthesized standard and chemical considerations strongly suggest that this organosulfate is glycolic acid sulfate, an isomer of the previously proposed glyoxal sulfate. Developments in HR-ToF-AMS data analysis have allowed for the measurement of the elemental composition of SOA. Additional graphical representations of AMS spectra and elemental composition have been developed to explain the oxidative and aging processes of SOA. It has been shown previously that oxygenated organic aerosol (OOA) components from ambient and laboratory data fall within a triangular region in the f44 vs. f43 space, where f44 and f43 are the ratios of the organic signal at m/z 44 and 43 to the total organic signal, respectively; we refer to this model as the plot. Alternatively, the Van Krevelen diagram has been used to the elemental composition of SOA and describe the evolution of functional groups in SOA. The variability of SOA formed in chamber experiments from twelve different precursors in both plot and Van Krevelen domains are investigated. Spectral and elemental data from the high-resolution Aerodyne aerosol mass spectrometer are compared to offline species identification analysis and FTIR filter analysis to better understand the changes in functional and elemental composition inherent in SOA formation and aging. SOA formed under high- and low-NOx conditions occupy similar areas in the plot and Van Krevelen diagram, and SOA generated from already-oxidized precursors starts higher on the plot. The most oxidized SOA come from the photooxidation of methoxyphenol precursors which yielded SOA O/C ratios near unity. ∝-pinene ozonolysis and naphthalene photooxidation SOA systems have had the highest degree of mass closure in previous chemical characterization studies and also show the best agreement between AMS elemental composition measurements and elemental composition of identified species. In general the elemental composition of chamber SOA follows a slope shallower than -1 on the Van Krevelen diagram. From the spectra of SOA studied, the triangular region originally constructed with ambient OOA components with chamber aerosol can be reproduced. Ambient data in the middle of the triangle represent the ensemble average of many different SOA precursors, ages, and oxidative processes.