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Measurement techniques for atmospheric nitrogen oxides and observational constraints on alkyl nitrate chemistry


NOx (NO + NO2) molecules act as a control over atmospheric oxidation rates. The chemical lifetime of NOx is controlled by daytime OH-initiated photochemical reactions and nighttime NO3-initiated reactions. One class of products of this chemistry, alkyl nitrates (denoted by the general formula RONO2), is formed by both daytime and nighttime processes, but the balance between these processes is not well understood. In order to investigate mechanisms of RONO2 production and thus shed light on this balance, measurements of reactive nitrogen and other relevant species were taken during three field campaigns, two in Colorado during summer 2014, the Front Range Air Pollution and Photochemistry Experiment (FRAPPE) and Deriving Information on Surface Conditions from COlumn and VERtically Resolved Observations Relevant to Air Quality (Discover-AQ), and one in the northeast US in winter 2015, Wintertime INvestigation of Transport, Emissions, and Reactivity (WINTER). Evidence is presented showing that the nighttime pathway for RONO2 formation, often considered a negligible source compared to daytime production, results in concentrations that are up to half of observed daytime concentrations. High RONO2 concentrations observed in the morning at constant ozone that cannot be explained by loss processes, mixing, or other sources point to rapid nighttime production via NO3 chemistry. This result is surprising because, while nighttime NO3 chemistry has often been shown to be a significant source of organic aerosol, especially in rural regions dominated by biogenic emissions, it has not been shown to be a significant source of alkyl nitrates in an urban area. In addition, measurement comparisons are presented in order to evaluate the accuracy of the observations used in this analysis and to evaluate the winter and nighttime performance of instruments that have been previously well characterized for summer and daytime operation. For the WINTER campaign, comparisons between measurements of the same species using different operating principles showed agreement to better than 20%. The measurements from WINTER show a comprehensive observation-based view of the partitioning of nitrogen oxides under winter and nighttime conditions not previously demonstrated. For FRAPPE and Discover-AQ, a method is presented for comparing measurements between instruments flown on two different aircraft with different flight paths. This comparison shows agreement to within expected accuracies for all 13 species compared except for the total RONO2 measurement (ΣRONO2), a measurement of the sum of all species with the RONO2 general formula. A comparison of ΣRONO2 with measurements of individual RONO2 species scaled to approximate ΣRONO2 reveals a systematic error in the Discover-AQ ΣRONO2 measurements; reasons for this error are discussed.

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