US and UK researchers report direct measurement of key atmospheric reactant; more rapid formation of secondary aerosols
Molecules called Criegee intermediates—carbonyl oxides—are important atmospheric reactants, but only indirect knowledge of their reaction kinetics has been available. Now, researchers from Sandia National Laboratory’s Combustion Research Facility, the University of Manchester and the University of Bristol report in a paper in Science the first direct kinetics measurements made of reactions of any gas-phase Criegee intermediate, in this case formaldehyde oxide (CH2OO).
The researchers believe the measurements will substantially impact the understanding of atmospheric chemistry mechanisms—in particular, in the more rapid formation of secondary aerosols, important in the formation of local smog as well as in global climate change.
Dr. Carl Percival, Reader in Atmospheric Chemistry at The University of Manchester and one of the authors of the paper, believes there could be significant research possibilities arising from the measurement of the Criegee biradicals.
Criegee radicals have been impossible to measure until this work carried out at the Advanced Light Source. We have been able to quantify how fast Criegee radicals react for the first time. Our results will have a significant impact on our understanding of the oxidizing capacity of the atmosphere and have wide ranging implications for pollution and climate change.—Dr. Carl Percival
A significant ingredient [alkenes] required for the production of these Criegee biradicals comes from chemicals released quite naturally by plants, so natural ecosystems could be playing a significant role in off-setting warming.—Professor Dudley Shallcross, Professor in Atmospheric Chemistry at The University of Bristol
Ozonolysis—the cleavage of carbon-carbon double bonds through reaction with ozone—is a reaction that plays a key role in a number of fields, including synthetic chemistry and tropospheric removal of unsaturated hydrocarbons. More than six decades ago, Rudolf Criegee proposed that ozonolysis of alkenes occurs via carbonyl oxide biradicals (Criegee intermediates). Criegee intermediates also have been calculated to be markers of critical chain-branching steps in hydrocarbon autoignition chemistry. (Hence, Sandia’s interest.) Alkenes can be biogenic or anthropogenic.
In 1949, Rudolf Criegee proposed that ozonolysis of alkenes proceeds via carbonyl oxide biradicals, in an ozonolysis mechanism that is now generally accepted. Because a large fraction of the tropospheric oxidation of unsaturated hydrocarbons is initiated by reaction with ozone, these biradical “Criegee intermediates” play a substantial role in the tropospheric budgets of secondary organic aerosols (SOAs), ozone, NOx, NOy, and HOx.—Welz et al.
In a commentary accompanying the main paper by Welz et al., Dr. George Marston from the University of Reading (UK) notes that:
The Criegee intermediates (CIs) are central to understanding the reactions of ozone with unsaturated compounds. These reactions contribute directly to the oxidation of hydrocarbons in the lower atmosphere, are important sources of hydroxyl radicals, atmospheric organic acids, and carbonyl compounds, and can lead to the generation of secondary aerosols. On the local scale, these secondary aerosols contribute to the low visibility and health problems associated with photochemical smog, while on the global scale, their formation has crucial implications for climate change. Yet despite their central importance in these processes, little is known about the reactivity of CIs.—Marston (2012)
Welz et al. report 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 was 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 present case, CH2OO can be distinguished from its more stable isomer, formic acid (HCOOH), because of their differing thresholds for photoionization. The Manchester and Bristol researchers recognized that this apparatus could elucidate not only combustion reactions but also important tropospheric oxidation processes, such as ozonolysis.
However, until 2008 no gas-phase Criegee intermediate had been observed, and rate coefficients derived from indirect measurements spanned orders of magnitude.
In the Science publication, Sandia 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.
|Sandia combustion researchers Craig Taatjes and David Osborn discuss data found from the detection and measurement of Criegee intermediate reactions.
In particular, the present measurements show that the reactions of CH2OO with SO2 and NO2 are far more rapid than previously thought. Moreover, the Bristol and Manchester investigators demonstrated that 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 Criegee intermediate is much faster than modelers assumed, Criegee reactions may be a major tropospheric sulfate source, changing predictions of tropospheric aerosol formation.
The capability breakthrough was funded by the Office of Basic Energy Sciences (BES) within the Office of Science in the US Department of Energy, and conducted using the Advanced Light Source, a scientific user facility supported by BES.
Oliver Welz, John D. Savee, David L. Osborn, Subith S. Vasu, Carl J. Percival, Dudley E. Shallcross, and Craig A. Taatjes (2012) Direct Kinetic Measurements of Criegee Intermediate (CH2OO) Formed by Reaction of CH2I with O2. Science 335 (6065), 204-207. doi: 10.1126/science.1213229
George Marston (2012) An Elusive Intermediate Gets Caught. Science 335 (6065), 178-179. doi: 10.1126/science.1217165