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Tracing Atmospheric Oxidation Through Organic Nitrates

  • Author(s): Lee, Chiu-Lin
  • Advisor(s): Cohen, Ronald C
  • et al.

Organic nitrates, chemicals with the formula RONO2, are products of atmospheric oxidation of organic molecules in the presence of nitric oxide (NO) and of the oxidation of alkenes by the nitrate radical (NO3). The formation of these organic nitrates results in removal of NOx (NO + NO2) from the atmosphere and consequently suppresses the oxidative capacity of the atmosphere by terminating OH-initiated free radical chain propagation reactions and shifting the HOx radical balance towards HO2 and away from OH. In addition, NOx removal lowers the NO3 concentration in the evening and thus suppresses the nighttime formation of low vapor pressure compounds that condense to give secondary organic aerosol.

In this dissertation, I present the acquisition and analyses of novel laboratory measurements and field observations of the chemistry of RONO2 species. I begin with a chamber experiment of the oxidation of three isomers of isoprene-derived hydroxy nitrates detected using chemical ionization mass spectrometry and thermal dissociation laser induced fluorescence. This study provides a direct experimental constraint on the lifetime and product yield of the single most important class of atmospheric organic nitrates. Guided by these laboratory measurements, I suggest a parameterization for use in global models of the oxidation rates and product yields of the 8 first-generation isoprene nitrate isomers formed in the atmosphere. I then turn to the role of condensed phase RONO2 in constraining the composition and lifetime of atmospheric aerosol. I describe a laboratory study of the oxidation of a model system for atmospheric aerosol by NO3 radical using ~100 nm particles of pure squalane or of pure squalene in a flow tube reactor coupled to a tunable VUV vacuum UV photoionization aerosol mass spectrometer. The results show that oxidation of squalane (a model for saturated organic aerosol) is too slow to be atmospherically relevant but that oxidation of squalene (a model for multi-unsatuarated aerosol) proceeds at a very rapid rate--too rapid for it to be limited by diffusion in solid aerosol particles. The results indicate rapid movement of radical sites within the aerosol matrix and are suggestive that the aerosol is liquid rather than being solid or glassy, as some have suggested.

Organic nitrate formation is strongly affected by temperature, proceeding more quickly at low temperatures, in contrast to many other atmospheric reactions that slow when it is cold. To investigate the role of organic nitrate chemistry at low temperature and to understand its role in wintertime production of ozone (O3) and organic aerosol, I measured the concentration of a variety of nitrogen oxides, including gas- and aerosol-phase RONO2, during the Uintah Basin Winter Ozone Study in Utah, USA, in 2012. I describe how the observations support that at low temperatures (~0°C) organic nitrate formation suppresses ozone formation and that the particulate organic nitrate budget can be closed using reactions of gas phase precursors.

I conclude with a detailed description of a novel instrument combining gas chromatography with thermal-dissociation laser induced fluorescence for detecting specific individual nitrates, which were used in the preparation of isoprene-derived hydroxy nitrates used in oxidation experiments mentioned above, and with some thoughts on the most interesting questions for future research on the links between RONO2, the oxidative chemistry of the atmosphere, and organic aerosol.

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