Measurement of OH, H2SO4, MSA, NH3, and DMSO Aboard the P-3B Aircraft

Dr. Fred L. Eisele (Principal Investigator)

Georgia Institute of Technology and National Center for Atmospheric Research

(1) Dr. R. Lee Mauldin (Co-Principal Investigator)

(2) National Center for Atmospheric Research

Research Summary

The measurement of OH, H2SO4, MSA, NH3, and DMSO will be made from the P-3B aircraft as part of PEM-Tropics B. 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 three compounds will be measured on a near simultaneous basis using previously developed methodologies.1,3 The measurement of OH, H2SO4, and MSA, would be performed on a time shared basis for 17, 9, 4 seconds respectively, once each 30 seconds. This corresponds to a detection sensitivity (2s) for each 30 second measurement period of about 2x105, 1x105, and 2x105 molecule cm-3 for OH, H2SO4, and MSA respectively. A calibration/test source for OH and H2SO4 will also be operated inside the aircraft inlet during flight. The calibration coefficient varies with altitude even with a constant volumetric flow, however the calibration remains constant for a given altitude.

The measurement of NH3 and DMSO will be carried out with a secondary channel of the mass spectrometer system. Measurement of DMSO will use the techniques previously developed in ground based studies.3 For each 30 second period, NH3 and DMSO will be measured on a time shared basis. This approach leads to a detection limit (2s) of 1 pptv for both species in a single one minute integration. The NH3 and DMSO measurements will be calibrated in flight using isotopically labeled standards produced from a permeation device. The isotopically labeled standards will be used to distinguish between the standard and the respective naturally occurring species. These calibration sources are designed to minimize influences from abrubt changes in altitude, pressure and temperature, and will be tested both prior to and shortly after the flight campaign.

The measurement techniques and much of the instrumentation to be used for this proposed study have been involved in several very successful field campaigns, both ground based and airborne. The OH, H2SO4, and MSA measurement apparatus to be used on the P-3B is the same basic instrumentation used in 2 previous airborne studies with the exception of modifications to the vacuum housing and size reduction of the electronics. Modifications to the vacuum housing allow for four measurement channels, two of which will be used in this proposed work (one for OH, H2SO4, and MSA; the other for NH3 and DMSO). The remaining two channels of this instrument will be used for experimentation. Once the first five measurements are functioning properly, it is hoped that additional measurements, several of which have been tested on the ground, could be included during some portion of the PEM-Tropics B campaign. These additional measurements might include, HNO3, HCl, HO2,
HO2/RO2, DMSO2, methane sulfinic acid, and acetone. There would be no cost to NASA associated with these tests, and we would not expect to be obligated to provide NASA with formal data for any of these compounds. These tests would simply be a means of expanding our measurement capabilities for future campaigns. If some of these measurements are successful and well calibrated (which we expect will be the case) then this data could be made available to NASA and/or the science team if desired.

The inlet/inlet design to be used in this study has been the emphasis of a great deal of study, including laboratory tests and tests in two separate wind tunnels.4 These tests have shown that the inlet duct assembly can slow the sampled air flow by approximately a factor of 16 while allowing essentially no contact between the inlet surfaces and the air to be sampled. Figure 1 includes a schematic diagram of the inlet/duct assembly used for the measurement of OH, H2SO4, and MSA during the ACE-1 and PEM-Tropics A studies. As can be seen it incorporates a nested restricted flow design. Also designed into the assembly is the calibration source for OH and H2SO4 measurements. The testing and operation of this inlet duct assembly is described in detail in a recent publication.4

While ammonia is important, it is also difficult to measure, in part because it is typically present only at concentrations in the pptv range in the remote marine environment, and also because it is extremely sticky. The former concern is not really applicable to the SICIMS technique proposed. The 1-1000 pptv concentration range is an ideal range for applying this technique under the ion reaction conditions typically used in our instruments (not necessarily the case for commercial chemical ionization mass spectrometer, CIMS, instruments). While the detection of ammonia using the present instrument is relatively simple, the introduction of sampled air into the instrument, and instrument calibration can pose significant problems on any instrument. We have successfully dealt with these aircraft inlet/sampling duct problems for OH, H2SO4, and MSA and will use similar inlet/sampling duct designs for NH3. The difference between sampling NH3 and OH or H2SO4 is that NH3 will not only stick to the walls but also readily evaporate back off. Since the air sampled from the center of the inlet/sampling duct used for the OH/H2SO4 measurement can be shown to be free from wall or boundary layer contact,4 a similar duct design can be used for NH3. While the inlet ducts will be similar to those previously used, they will not be identical. Therefore additional wind tunnel testing and/or modeling will be required. What may not work quite as well for NH3 is the short curved transfer tube that brings OH and H2SO4 from the center to the edge of the main sampling duct. This brief opportunity for wall contact only represents a minor loss in sensitivity for OH and H2SO4, and results in essentially no measurement error because any small loss in this tube is included in the instrument calibration. For NH3 however, even such a short connecting tube could slow the response of the instrument due to NH3 evaporation from the tube walls. For the NH3/DMSO measurements, a new inlet will be used which has extremely little surface area. This inlet will be combined with a transverse sampling ion source installed directly in the center of the inlet/sampling duct. Such a source was designed for and has been used in laboratory ion/aerosol related studies by our group for more than one year. This combination should provide a response time of well under a second, but because it has not been fully tested, only a 1 minute response time is proposed. This longer time will allow, in the case of major ion source difficulties, ion sources more similar to those used in the laboratory or for HNO3 measurements to be employed. It is anticipated that the same ion source will be used for the measurement both of NH3 and DMSO. Using either source, NH3 and DMSO can still be measured once every 30 seconds.

In September 1998, the instrumentation described above will be installed aboard the NASA P-3B, test flown, and finally participate in the PEM-Tropics B study in 1999. In the second year, system operation will be rechecked and systems/system components re-calibrated. Data analysis and submission will also take place during this time period as well as manuscript preparation and submission. In the third year, after the initial round of manuscript submission and modeling has been completed, some of the more elusive results will be written up and published (as is presently the case for PEM-Tropics A data).

The above objectives are all very supportive of the overall PEM-Tropics B mission and all have only recently become possible due to the development of new measurement techniques. The application of the above measurements to the photochemistry and aerosol formation processes is nearly certain to significantly improve present understanding, but will also help define future measurement strategies.

References

1. F. L. Eisele and D. J. Tanner, "Ion Assisted Tropospheric OH Measurement," Journal of Geophysical Research, 6, 9295, 1991.

1. F. L. Eisele and D. J. Tanner, “Measurement of the Gas Phase Concentration of H2SO4 and Methane Sulfonic Acid and Estimates of H2SO4 Production and Loss,” Journal of Geophysical Research, 98, 9001, 1993.

2. H. Berresheim, D. J. Tanner, and F. L. Eisele, “Real-Time Measurement of Dimethylsulfoxide in Air," Analytical Chemistry, 65, 84, 1993.

3. Eisele, F.L., R.L. Mauldin III, D.J. Tanner, J. Fox, T. Mouch, and T. Scully, An inlet/sampling duct for airborne OH and sulfuric acid measurements, J. Geophys. Res., 102, 27993, 1997.

Table 1. Summary of measurement time, detection limit, accuracy and precision for the compounds to be measured.

Figure 1. Schematic diagram of the inlet duct assembly and single mass spectrometer channel used for OH, H2SO4, and MSA measurements during the ACE-1 and PEM-Tropics A studies. The inlet consists of a shroud constructed with elliptical front surfaces which is attached to two nested restricted flow ducts. Also shown is the assembly used for in flight calibrations. (ref. 4)