A New Application of the Fundamental Physics of Atmospheric Pressure Ionization Mass Spectrometry to Ozone and Fine Particulate Formation MechanismsEPA Grant Number: R828179
Title: A New Application of the Fundamental Physics of Atmospheric Pressure Ionization Mass Spectrometry to Ozone and Fine Particulate Formation Mechanisms
Investigators: O'Brien, Robert J. , Atkinson, Dean B.
Current Investigators: O'Brien, Robert J. , Atkinson, Dean B. , Hard, Thomas M.
Institution: Portland State University
EPA Project Officer: Shapiro, Paul
Project Period: July 1, 2000 through June 30, 2002
Project Amount: $223,574
RFA: Exploratory Research - Engineering, Chemistry, and Physics) (1999) RFA Text | Recipients Lists
Research Category: Air Quality and Air Toxics , Water , Land and Waste Management , Air , Engineering and Environmental Chemistry
Atmospheric VOC oxidation mechanisms, especially of aromatic and biogenic hydrocarbons, are dominant contributors to oxidant and particulate formation in polluted air. In spite of much progress, a thorough understanding of aromatic/biogenic VOC oxidation mechanisms has long eluded researchers, due to the complexity of the mechanisms, the wide variety of oxidation products, and their immunity to standard chemical analysis procedures. Current photochemical mechanisms for control of atmospheric oxidants and particulates do not adequately portray the effects of aromatic and biogenic hydrocarbons on air quality.
Although the fundamental physics of atmospheric pressure ionization (API) was described by Kebarle as long ago as 1970, the API process has never been fully exploited for the study of atmospheric reactivity. The proposed study employs the fundamental physics of the ion molecule reactions occurring in atmospheric pressure ionization mass spectrometry in a entirely novel way to study these reactions. Although the role of hydronium ions (H3O+) as protonating agents in APIMS is well known, the presence of co-produced hydroxyl radicals (OH) is usually overlooked: 2 H2O H3O+ + OH.+ e-. In planned work, these hydroxyl radicals can quickly react with an introduced trace organic to initiate the formation of oxidation products which mimic those formed in the ambient atmosphere. The oxidation products are then efficiently protonated by the reagent hydronium ions and mass analyzed. In a proof-of-principle study using a high resolution API mass spectrometer, we have used these source reactions to analyze toluene's oxidation products, which are shown by their high resolution masses to be identical to those found in previous experiments using other approaches. In the experiments described here, the products will be unambiguously identified by selective ion fragmentation using the MSn capabilities afforded by the ion trap mass analyzer. VOC oxidation products will be quantified and reaction yields determined using semi-empirical relationships for gas-phase basicities which we have previously demonstrated. These calculated API sensitivities will overcome a major hurdle of all previous studies which require calibration standards, few of which are available for these oxidation products. Reaction studies carried out directly in the ion source will circumvent analytical difficulties and/or wall loss of the reaction products. Addition of chemical reagents (e.g CO, NO) to the API source will verify that the reactions are initiated by OH and will document that product yields shift in ways predictable from characteristic atmospheric oxidation mechanisms.
This work will identify atmospheric oxidation products and reaction mechanisms which operate under real world conditions. These products, only partially understood at this time, cause buildup of urban and rural oxidants and fine particulates. More cost effect control strategies for ozone and particulates will result from this study.