Measurement of OH, H2SO4, MSA and HNO3 Aboard the P-3B Aircraft

F.L. Eisele, R. L. Mauldin, M. Zondlo, and E. Kosciuch

            The proposed research involves the use of a multi-channel selected ion chemical ionization mass spectrometer (SICIMS) instrument to perform several difficult, but important chemical measurements.  This instrument was developed as part of a recent NASA downsizing initiative to both reduce the size of a SICIMS OH instrument and to allow several new SICIMS measurement techniques to be developed and used on future NASA missions.  Chemical ionization mass spectrometry (CIMS) and SICIMS have made great strides in the past several years, particularly in the area of atmospheric chemistry, providing rapid yet highly specific and sensitive measurements of several important species.  Each measurement instrument, however, typically requires its own vacuum system, pumps, RF electronics, control electronics, data collection system, inlets, calibration systems, etc., and as such takes up one measurement station on the aircraft.  The hydroxyl radical, H2SO4, and MSA, all of which are typically present in the atmosphere in the sub ppt (parts per trillion) range, require the use of a relatively large and very fast pumped vacuum system to achieve the sensitivity needed for their measurement.  Compounds such as DMSO, HNO3, HO2, RO2 are typically present in the atmosphere at 101-104 times greater concentrations and do not have such extreme detection requirements.  The present instrument uses a small fraction (10 to 20%) of the vacuum pumping required to measure OH, H2SO4, and MSA, and makes it available for the independent measurement of other compounds. 

            For TRACE-P the measurement of OH, H2SO4, MSA and HNO3 are proposed on two of the instruments channels.  It is also proposed that a third independent channel be made available for measuring HO2 and RO2 if these measurements are funded.  The hydroxyl radical will be measured for about 8 seconds once each 30 seconds, and an OH background will be measured for an additional 8 seconds during the same time period.  During the 7 seconds between each OH measurement and background, H2SO4 and MSA will be measured.   This provides a detection limit (2s) of better than 1 ´ 105 OH cm-3 for a 5 minute integration time (ten 30 second measurement periods).  Sulfuric acid will be measured for about 4 seconds once each 15 seconds, providing a detection limit of better than 2 ´ 105 for a 1 minute integration time.  MSA will be measured for about 3 seconds once each 15 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 campaigns, including:  ACE I, PEM Tropics A, PEM Tropics B and TOPSE. 

            Two types of nitric acid measurements are being proposed:  the first is a fast relative measurement of HNO3 in which measurements of gas phase HNO3 with a detection limit of about 5-10 pptv would be provided once every 15 seconds throughout the campaign.  The second measurement would be a comparison between an essentially ambient temperature relative measurement of HNO3 which should detect only gas phase HNO3 and a similar measurement with a heated ion source in which particles could be volatilized.  The goal of this effort is to provide absolute nitric acid measurements for TRACE P on the P-3B  which presently appears quite probable.  There are still enough problems with this measurement, however, that we do not feel comfortable fully committing to a goal of absolute nitric acid measurements at this time.