Chemical Ionization Mass Spectrometry-Based Measurements
of HO2 and RO2
Christopher A. Cantrell,
NCAR Boulder, CO
The peroxy radical CIMS (PerCIMS) is generally based on the selected
ion CIMS measurement of hydroxyl radicals.In the PerCIMS method,
reagent gases NO and SO2 are added to ambient air, leading
to the conversion of ambient peroxy radicals (RO2 and/or
HO2) to gas-phase sulfuric acid. The H2SO4 product
is detected first by ionization through a proton transfer reaction
with NO3-, followed by introduction into
a vacuum system with ion optics, differential pumping, quadrupole
mass filter and channel electron multiplier for ion counting. A
schematic diagram of the instrument is shown below.
The PerCIMS instrument is unique in that we
have developed methods to yield measurement of total peroxy radical
levels ([HO2] + [RO2]), and measurement of
[HO2] by adjusting the reagent gas concentrations in
the inlet region. These
two measurements together yield additional information about atmospheric
processing of hydrocarbons and the branched chain reactions that
lead to partitioning of peroxy radicals between the hydro- and
organic-peroxy radical forms.
Calibration of the instrument is accomplished
through photolysis of water vapor using 184.9 nm radiation from
a low-pressure mercury lamp. Radical
concentrations in the range of ambient concentrations (and higher)
are easily generated by adjusting the lamp distance, slit width
and water vapor concentration. The lamp flux is determined through
N2O actinometry. Relative
sensitivity of various peroxy radicals are measured using the photolysis
of Cl2 to yield chlorine atoms that react with hydrogen
and various hydrocarbon precursors.
These measurements are accomplished with an
estimated ±30%
accuracy, typical detection limits of 1 x 107 radicals-cm-3 in
15 seconds, and typical 2 precision
of 10% at 1 x 108 radicals-cm-3. Lower
detection limits with better precision are possible with appropriate
averaging.
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