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.