Measurement of OH, H2SO4,
MSA, and HNO3 Aboard
the P-3B Aircraft
Dr. Fred L. Eisele (Principal Investigator)
Georgia Institute of Technology and National Center for Atmospheric Research
Dr. R. Lee Mauldin (Co-Principal Investigator)
National Center for Atmospheric Research
Research
Summary
The measurement of OH, H2SO4,
MSA and HNO3 will
be made from the P-3B aircraft as part of TRACE-P. These
measurements will be performed by a multi-channel selected ion chemical ionization
mass spectrometer system. The primary
channel of the mass spectrometer will be used for the measurement of OH, H2SO4 and MSA. These
measurements provide a detection limit (2s)
of better than 1 ´ 105 OH
cm-3 for
a 5 minute integration time (17 second measurements each 30 second for 10 measurement
periods). Sulfuric acid will be
measured for about 4 seconds once each 15 seconds, providing a detection limit
of better than 2 ´ 105 cm-3 for a 1 minute integration time. MSA
will be measured for about 4 seconds once each 30 seconds, providing a detection
limit of about 2 ´ 105 cm-3 for
a 1 minute integration time. All
three of these measurements using the same or similar instruments, have been
performed as part of several previous aircraft and ground based campaigns, including: ACE
1, PEM Tropics A, PEM Tropics B and most recently on TOPSE. It has
also participated in two informal ground based OH intercomparison/photochemistry
studies in Colorado,1,2 a photochemistry study at
Mauna Loa Observatory MLOPEX II, and in two photochemistry studies in Antarctica,
i.e., SCATE (Sulfur Chemistry of the Antarctic Troposphere Experiment) and ISCAT
(Investigation of Sulfur Chemistry in the Antarctic Troposphere).
The OH measurement
is accomplished by the rapid titration of OH into H2SO4 via the addition of isotopically labeled
SO2 on
the millisecond time scale and the subsequent measurement of H234SO4 by
SICIMS using the ratio of HSO4 product ions to NO3- reactant
ions. Calibration of this
instrument is accomplished in-flight and under ambient sampling conditions
by photolyzing
a known amount of ambient H2O by a known 184.9 nm UV photon
flux. The inlet used for these
measurements has also undergone extensive wind tunnel testing and some
flow modeling.3 The measurements of ambient H2SO4 and
MSA are accomplished in a similar manner using SICIMS and observing the
HSO4-/NO3- and
CH3SO3-/NO3- ion
ratios respectively.
In addition to deploying the relatively well-established
series of measurements described above (OH, H2SO4,
and MSA), there is also a need to continue testing and development of
new measurement techniques that offer advantages over existing methods,
particularly
for compounds
as important as nitric acid.
Nitric
acid will be measured with a separate mass spectrometer channel using
a selected ion chemical ionization technique which uses MSA to form the
initial
reactant
ion. The concentration of
HNO3 is
then determined from the mass spectrometrically determined ratio of the
HNO3-reactant
ion cluster concentration to the reactant ion concentration. Only a relative measurement of gas phase HNO3 is
being proposed with a detection limit of about 5-10 pptv for a 15 second
sample time. It is hoped,
however, that an absolute measurement capability for HNO3 will be available by the time
TRACE-P takes place. Therefore,
an H15NO3 calibration
source and dilution system will be included on the aircraft for inflight
HNO3 calibrations.
The final item being proposed is the operation of
a third mass spectrometer analytical channel and the associated data
collection for that channel. This
would make possible the measurement of HO2 and HO2/RO2 by
another investigator (Dr. Chris Cantrell) without requiring a separate
measurement station and the much larger weight of an independent instrument.
References
F.L.
Eisele, G.H. Mount, F.C. Fehsenfeld, J. Harder, E. Marovich, J. Roberts, D.J.
Tanner and M. Trainer. An intercomparison
of tropospheric OH and ancillary trace gas measurements at Fritz Peak Observatory,
Colorado, Journal of Geophysical Research, 99, 18,605, 1994.
G.H.
Mount, F.L. Eisele, D.J. Tanner, J.W. Brault, P.V. Johnston, J.W. Harder,
E.J. Williams, A. Fried and R. Shetter. An
intercomparison of spectroscopic laser long-path and ion-assisted in situ
measurements of hydroxyl concentrations during the tropospheric OH photochemistry
experiment,
Fall 1993, Journal of Geophysical Research, 102, 6437-6455,
1997.
F.L. Eisele, R.L. Mauldin III, D.J. Tanner, J.R. Fox, T. Mouch and
T. Scully. An
inlet sampling duct for airborne OH and sulfuric acid measurements, Journal
of Geophysical Research, 102, 27,993-28,002, 1997.
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