In a breakthrough paper published in this week’s issue of Science magazine, researchers from Sandia’s Combustion Research Facility and the University of Manchester and Bristol University report the potentially revolutionary effects of Criegee biradicals.
These chemical intermediates are powerful oxidizers of pollutants produced by combustion such as nitrogen dioxide and sulfur dioxide. Although Criegee intermediates — carbonyl oxides — were hypothesized in the 1950s, it is only recently that they have been detected. Previously, scientists only had indirect knowledge of their reaction kinetics.
“This is a breakthrough discovery, but really only the opening of the door on this work. The results from this study, and the doors this work opens for future studies of Criegee intermediates, will make models of the troposphere more accurate, providing a better understanding of how anthropogenic and biogenic chemistry together shape our planet’s environment,” says David Osborn (8353).
The article, titled Direct Kinetic Measurements of Criegee Intermediate (CH2OO) Formed by Reaction of CH2I with O2, reports the first direct kinetics measurements made of reactions of any Criegee species, in this case formaldehyde oxide (CH2OO). These measurements determine rate coefficients with key species, such as sulfur dioxide (SO2) and nitrogen dioxide (NO2), and provide new insight into the reactivity of these transient molecules.
The detection and measurement of the Criegee intermediate reactions were made possible by a unique apparatus, designed by Sandia researchers, that uses light from a third-generation synchrotron user facility, Lawrence Berkeley National Laboratory’s Advanced Light Source, to investigate chemical reactions that are critical in hydrocarbon oxidation. The intense tunable light from the synchrotron allows researchers to discern the formation and removal of different isomeric species — molecules that contain the same atoms but are arranged in different combinations.
In the Science publication, the researchers reported a new means of producing gas-phase Criegee intermediates and used this method to prepare enough CH2OO to measure its reactions with water, SO2, nitric oxide (NO), and NO2. The ability to reliably produce Criegee intermediates will facilitate studies of their role in ignition and other oxidation systems.
The researchers found that the Criegee biradicals react more rapidly than first thought. Moreover, the British and Manchester investigators demonstrated these kinetics results imply a much greater role of carbonyl oxides in tropospheric sulfate and nitrate chemistry than models had assumed, a conclusion that will substantially impact existing atmospheric chemistry mechanisms.
For example, SO2 oxidation is the source of sulfate species that nucleate atmospheric aerosols. Because the oxidation of SO2 by the Criegee intermediate is much faster than previously assumed, Criegee reactions may be a major tropospheric sulfate source, changing our picture of tropospheric aerosol formation.
This capability breakthrough was funded by the Office of Basic Energy Sciences (BES) in DOE’s Office of Science, and conducted using the Advanced Light Source at Lawrence Berkeley National Laboratory, a scientific user facility supported by BES.