Atmospheric Reactive Trace Gas Observations in Field and Chamber Studies Using CF₃O⁻ Chemical Ionization Mass Spectrometry

Citation

Crounse, John D. (2011) Atmospheric Reactive Trace Gas Observations in Field and Chamber Studies Using CF₃O⁻ Chemical Ionization Mass Spectrometry. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/JMSN-NF98. https://resolver.caltech.edu/CaltechTHESIS:05172011-160342650

Abstract

This dissertation describes the development of chemical ionization mass spectrometry (CIMS) instrumentation and methods utilizing the CF₃O^- reagent ion for the sensitive, specific, and direct detection of many oxygenated volatile organic compounds (OVOC) and inorganic reactive trace gases in the atmosphere. These species include HNO₃, HONO, HO₂NO₂, SO₂, HCN, H₂O₂, CH₃OOH, CH₃(O)OOH, HC(O)OH, CH₃C(O)OH, HC(O)CH₂OH, CH₃C(O)CH2OH, organic hydroperoxides (ROOH), and many additional multifunctional species (e.g., hydroxynitrates, hydroxycarbonyls, hydroxyhydroperoxides, carbonylnitrates, carbonylhydroperoxides, etc.).

CF₃O^--tandem mass spectrometry (MSMS) is demonstrated to be useful for distinguishing and individual quantification of certain isobaric compounds, as well as solving instrumental background problems for certain species. This technology is applied in field studies conducted from aircraft and ground-based platforms and to chamber studies investigating VOC oxidation and organic aerosol formation mechanisms. Comparisons with simultaneous observations from other instrumentation for several species show good agreement with CIMS observations. CF₃O^--CIMS observations of HCN (a biomass burning tracer) from aircraft are used to quantify the impact of biomass burning emissions to the Mexico City region in March 2006. Biomass burning emissions are shown to contribute significantly to a number of gas and aerosol phase pollutants even in the midst of the large anthropogenic pollution emissions from Mexico City. The analysis of the photochemical aging of a fire plume over the Yucatan Peninsula (March 2006) is also reported. Observations indicate intense chemistry occurring within the fire plume evidenced by high OH levels, fast production of H₂O₂ and conversion of NO and NO₂ (NO_x) into peroxyacetylnitrate (PAN) and aerosol nitrate. This rapid chemistry is likely driven by photolysis of HONO, which is observed to be emitted in high amounts from these fires.

The CIMS methods are applied to studies of VOC oxidation and organic aerosol formation conducted in chamber experiments. Specifically, new insights gained from the study of isoprene oxidation under high and low NO_x conditions are reported. We quantify the formation of small carboxylic acids as well as C₅-hydroxynitrates from the oxidation of isoprene under high NO_x conditions. Under low NO_x conditions, we show that C₅-hydroxyhydroperoxides are formed in high yield. Subsequent oxidation of these hydroxyhydroperoxides is shown to occur through a unique HO_x neutral mechanism that generates C₅-epoxydiols, a likely precursor to organic aerosol.

We utilize the high sensitivity and specificity of CF₃O^- -CIMS to study novel intermolecular hydrogen-shift isomerization processes in peroxy radicals formed during isoprene oxidation. We find these rates to be substantially slower than recent theoretical predictions; however, we find these isomerization rates to be fast enough that they are important in atmospheric isoprene oxidation in regions where lifetimes become long. Globally, we estimate 8-11% of isoprene peroxy radicals react through 1,6-H-shift isomerization reactions.

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