Sandia combustion researchers Craig Taatjes, left, and David Osborn discuss data found from the detection and measurement of Criegee intermediate reactions. The apparatus was used to make the measurements, which researchers believe will substantially impact existing atmospheric chemistry. (Photo by Dino Vournas)

In a breakthrough paper published in this week’s issue of Science
magazine, researchers from Sandia’s Combustion Research Facility, the
University of Manchester and Bristol University report direct
measurements of reactions of a gas-phase Criegee intermediate using
photoionization mass spectrometry.

Criegee
intermediates—carbonyl oxides—are implicated in autoignition chemistry
and are pivotal atmospheric reactants, but only indirect knowledge of
their reaction kinetics had previously been available. 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 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.

Ozonolysis,
or 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. In the 1950s, Rudolf Criegee proposed that ozonolysis of
alkenes occurs via the carbonyl oxide biradicals, now called Criegee
intermediates. Criegee intermediates  also have been calculated to be
markers of critical chain-branching steps in hydrocarbon autoignition
chemistry.

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.

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.

This
capability breakthrough was funded by the Office of Basic Energy
Sciences (BES) within the Office of Science in the U.S. Department of
Energy, and conducted using the Advanced Light Source, a scientific user
facility supported by BES.

Sandia Combustion Research Facility

SOURCE